
==== Front
Ultrason Sonochem
Ultrason Sonochem
Ultrasonics Sonochemistry
1350-4177
1873-2828
Elsevier

S1350-4177(24)00274-8
10.1016/j.ultsonch.2024.107026
107026
Review
Ultrasonic decomposition of endocrine disrupting Compounds − A review
Merouani Slimane a
Dehane Aissa a
Hamdaoui Oualid ohamdaoui@ksu.edu.sa
ohamdaoui@yahoo.fr
b⁎
a Laboratory of Environmental Process Engineering, Department of Chemical Engineering, Faculty of Process Engineering, University Salah Boubnider Constantine 3, P.O. Box 72, 25000 Constantine, Algeria
b Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, 11421 Riyadh, Saudi Arabia
⁎ Corresponding author. ohamdaoui@ksu.edu.saohamdaoui@yahoo.fr
13 8 2024
11 2024
13 8 2024
110 10702631 5 2024
2 8 2024
11 8 2024
© 2024 The Author(s)
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Endocrine disrupting compounds (EDCs) need to be removed by efficient treatment methods as they are a major concern for both human and environmental health. To reduce the impact of EDCs in water, this review examines the use of ultrasonic degradation processes. Following an overview of EDCs and their origins, the basic concepts of sonochemistry are examined, highlighting the potential of ultrasound in chemical reactions. An in-depth analysis of the variables that affect the ultrasonic degradation of EDCs, such as frequency, intensity/power, temperature and solution chemistry, prepares the reader for a case study investigation focusing on specific EDCs. The study also looks at synergistic methods, emphasizing how hybrid ultrasonic systems can improve removal efficiency. The study provides a comprehensive overview of the use of sonochemistry in the treatment of EDCs by addressing current issues and suggesting future research directions. The aim of this review paper is to provide insightful analysis and useful suggestions for scientists working on EDC remediation projects.

Keywords

Ultrasound
Endocrine Disrupting Compounds (EDCs)
Destruction pathway
Water constituents
==== Body
pmc1 Introduction

Endocrine disrupting compounds (EDCs) are a prominent category of newly discovered environmental toxins that are becoming increasingly dangerous to human health and aquatic life. EDCs are classified into four primary groups: natural estrogens, synthetic estrogens, phyto-estrogens, and various industrial chemicals (e.g., pesticides, alkylphenols, organohalogens, heavy metals, persistent organochlorines). These chemicals can interact with an organism's endocrine system, leading to a variety of reproductive and developmental disorders, as well as feminizing effects [1], [2]. In particular, natural and synthetic estrogens are more estrogenic than phyto- and xenoestrogens [1]. While biodegradation and sorption to microbial flocs are two processes used in typical wastewater treatment plants to partially remove EDCs, the relative contributions of each pathway are not well understood [1]. In addition, several techniques such as sedimentation/filtration, adsorption using granular or powdered activated carbon, and coagulation/flocculation have been investigated for the treatment of EDCs [3]. Furthermore, these compounds (EDCs) exhibit high bioactivity, are ubiquitously distributed, and possess a recalcitrant and persistent nature, thereby constituting a critical contaminant. It should be noted that while some common organic pollutants (which include a wide range of substances such as volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), and persistent organic pollutants (POPs)) can also be EDCs, not all have endocrine-disrupting properties. Therefore, the key difference lies in the specific mechanisms through which EDCs exert their effects, primarily by interfering with the endocrine system, in contrast to common organic pollutants, which can cause various health problems (such as acute toxicity, cancer, neurotoxicity, and damage to the liver and kidneys) and ecological damage (such as air, water, and soil pollution, and harm to plant and animal life, and habitat alteration). Additionally, the enormous diversity of EDCs (including natural and synthetic hormones, phytoestrogens, pesticides, and a variety of industrial chemicals and byproducts) means that it is not possible to define a 'typical' EDC. Each chemical or mixture must be carefully evaluated regarding its response to remediation techniques.

The ability of advanced oxidation processes (AOPs) to generate hydroxyl radicals (•OH) in situ makes them unique and provides a flexible way to react with a wide variety of organic compounds in water [4]. Due to the special properties of ultrasound, which not only accelerate chemical reaction rates but also efficiently modify surface properties, ultrasound-based approaches have attracted much interest among the many AOP techniques [5]. The removal of EDCs from water has prompted research on traditional AOPs, including photolysis, UV/H2O2, photocatalysis, Fenton, and ozonation [6]. The use of ultrasound to degrade EDCs is a promising development. This is particularly noteworthy because ultrasound-based methods adhere to clean technology principles and operate without the need for additional chemical inputs. In addition, the ease with which ultrasound can be combined with other processes allows for synergistic treatment options [7], [8]. Because sonochemistry is a part of chemistry, it provides a complete approach to improving the overall efficiency of water treatment by addressing all of the chemical effects of ultrasound. The many advantages of ultrasound in AOPs make it an effective tool for addressing the problems associated with EDC removal from water systems.

Acoustic cavitation, which is the creation, expansion, and implosive collapse of acoustic cavitation bubbles resulting in extremely high pressures and temperatures at “local hot spots” in the liquid, is the basis of sonochemistry [9]. Despite the advancements in research related to the sonochemical field, much work is still required to improve our understanding of the intricate interplays between the acoustic cavitation and its surrounding medium. In aqueous solutions, ultrasound acts as an AOP when water pyrolysis produces hydroxyl radicals [10]. Some of these radicals are carried into the bulk solution to oxidize water impurities, while the majority combine to form water and hydrogen peroxide [10]. In addition, oxidation at the gas/liquid interface and/or thermal destruction of contaminants in collapsing bubbles can decontaminate water [10]. The type of ultrasonic equipment used, the chemical and physical characteristics of the contaminants, and the composition of the water all affect the amount of decontamination required [10]. It should be indicated that despite the fact that OH radicals exhibit exceptionally high chemical reactivity, these radicals are characterized by an extremely short lifespan, approximately 10-9 s. Additionally, it is estimated that about 10 % of H and OH radicals generated in the bubble can diffuse in to the bulk solution. Therefore, in the absence of scavenger at the bubble–liquid interface, hydroxyl radicals recombine. It is worth mentioning that all these processes are complicated by the lifetime (<100 µs) and velocity (>5 m/s) of the acoustic bubble which impact the diffusion as well as the recombination or the scavenging mechanisms of the hydroxyl radicals at the bubble-solution interface.

This paper provides a comprehensive analysis of recent developments in the use of ultrasound for EDC degradation. The review begins with an introduction to EDCs and their sources, before delving into the fundamentals of sonochemistry and emphasizing how ultrasound can be used to induce chemical reactions. A thorough analysis of the factors that influence the ultrasonic degradation of EDCs is provided. Case studies focusing on specific EDCs are reviewed in detail, and the effects of various water matrix elements and water quality on ultrasonic degradation are discussed. In addition, the possibility of synergistic techniques such as hybrid ultrasonic systems to improve removal efficiency is highlighted. The review concludes with a discussion of the issues at hand, suggestions for future lines of investigation, and a thorough analysis of the use of sonochemistry to treat EDC contamination. The goal of the review is to increase the knowledge of sonochemistry as a viable and long-term method of water treatment by synthesizing current developments in ultrasound-mediated EDC degradation.

2 Nature and origins of endocrine disrupting compounds (EDCs)

2.1 Definition of EDC

According to Fuhrman et al. [11], the Environmental Protection Agency (EPA) defines an exogenous drug carrier (EDC) as “any substance that interferes with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body that are responsible for maintaining homeostasis, reproduction, development, and/or behavior”. Despite the well-documented adverse effects of Endocrine Disrupting Compounds (EDCs), a definitive risk assessment tool for this broad category of substances remains elusive. While certain decision-making bodies have endeavored to develop methodological frameworks for evaluating potential effects, risk assessors continue to encounter substantial uncertainties and knowledge gaps. The quality of the interference is not specifically addressed in this definition. Similar to this definition, the Endocrine Society states that the duration of exposure affects the nature of the chemical interference with hormone activity and deliberately omits any mention of ”adverse“ effects [12]. ”An endocrine disruptor is an exogenous substance or mixture that alters the function(s) of the endocrine system and consequently causes adverse health effects in an intact organism or its progeny or (sub)populations,“ according to the most recent working definition of EDCs provided by the World Health Organization (WHO) and the International Program on Chemical Safety (IPCS) [13]. This definition is widely accepted and functional in the context of environmental and human risk assessment [14], [15]. Consequently, the scope of chemicals included is limited. This WHO definition has been endorsed by the European Food Safety Authority and the European Commission [16]. These substances, which may be synthetic or natural, have the ability to mimic and/or disrupt endocrine system processes, thereby altering the ability of an organism to grow, develop or reproduce [17], [18]. They have also been shown to be mutagenic, genotoxic, and carcinogenic in both humans and animals [18], [19].

2.2 Common origins and classification of EDCs

While several compounds have been identified as endocrine disruptors, a significant proportion of chemicals currently in use have not been tested [11]. EPA plans to test approximately 87,000 chemicals for endocrine potential [20], and many new EDCs continue to be identified and studied each year [11]. For example, a partnership of federal agencies in the United States, Tox21, is in the process of testing over 10,000 chemicals for endocrine disruption, which is expected to reveal a large number of new potential EDCs [21], [22]. The public can already access information on chemicals with established EDC effects or suspected effects awaiting testing through a number of databases [23], [24], [25], [26], [27].

Due to increasing competition in the marketplace, the number of newly discovered compounds with the potential to cause endocrine disruption is constantly increasing [19]. A list of some EDCs is provided in Table 1, along with information on their classes and sources (where available). According to research, some of the most commonly found EDCs [19] are: bisphenol A (BPA), dioxins, perchlorate, organophosphates, phytoestrogens, phthalates, polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyls (PCBs), triclosan, agrochemicals, pharmaceuticals, steroid hormones, and a variety of other persistent pollutants.Table 1 List of some compounds classified as EDCs, with their class and source [6].

EDCs class	Compound detected	Source/use	
Pesticides	DDT, DDE, deltamethrin, carbofuran, atrazine, lindane, vinclozolin, carbendazim and tributylin	Extensively used in agriculture. Herbicides, insecticides and fungicides are included.	
Bisphenols	Bisphenol A	Bisphenol A is used in the manufacture of polymers (polycarbonates and epoxy resins), rubber chemicals and flame retardants	
Parabens	Methyl, ethyl, propyl and butylparabens	Used as preservatives in most personal care products and cosmetics	
Phthalates	Butylbenzylphthalate, di-(2 ethylhexyl)phthalate anddi-n-butylphthalate	Found in resins, detergents, some addictive and monomers used in the plastics’ production	
Alkylphenols (surfactants)	Nonylphenol, nonylphenol etoxylate, octylphenol, octylphenol etoxylate	Used during the production of phenol resins, as plastic additives, emulsifiers, in agricultural and industrial application	
Dioxins and furans	Dibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxinand 2,3,7,8-tetrachlorodibenzofuran	Produced during the incineration of paper, chlorinated aromatic compounds and in the production of PVC plastic	
Organotin compounds	Tributyltin and triphenyltin	Compounds used in antifouling paints on ships	
Polychlorinated biphenyls (PCBs)	2,2′,4,4′-Tetrabrominated diphenyl ether, 2,5-dichloro-4, hydroxybiphenyl	They have been used as lubricants and coolants in transformers, capacitors and other electrical equipment.	
Polycyclic aromatic hydrocarbons	Fluorene, phenanthrene, fluoranthene, anthracene, pyrene, and naphthalene	Generated during incomplete combustion processes of oil, coal and wood	
Pharmaceuticals (synthetic steroids)	Diethylstilbestrol and 17-ethinylestradiol	Pharmaceuticals mainly consists of oral contraceptives as well as steroids used for substitution therapy during menopause	
Phytoestrogens	Daidzen and genistein, matairesol, enterodiol andenterolactone	Natural substances found in many food plants such as grains, cereals, vegetables, fruits and others	
Natural hormones	Estrone, 17-estradiol	Estrogens naturally and daily excreted in the human urine and animals	
heavy metals	Cadmium, mercury and lead	Industrial mining and metallurgy	
Triclosan	5-chloro-2-{2,4-dichlorophenoxy}phenol	Commonly used as an antiseptic agent in personal care and consumer products	
Atrazine	Atrazine	s-triazine herbicides used in agriculture	

The hydrophobic nature of most EDCs is demonstrated by their low KOW and Henry's constant values (Table 2). The chemical structures of these substances, which often contain hydrophobic moieties or nonpolar groups that make them poorly soluble in water, are responsible for their hydrophobic character. Because of their beneficial hydrophobic properties, EDCs, especially those derived from industrial processes, are more stable, resistant to degradation and have higher bioavailability in lipophilic environments. Many synthetic EDCs are designed and synthesized with hydrophobicity as a top priority, which ensures their longevity in the environment and facilitates their interaction with lipid membranes and hormone receptors of the endocrine system. In addition, because EDCs are hydrophobic, they can more easily adsorb to particulate matter and accumulate in soils and sediments, increasing their environmental persistence and potential ecological effects [28].Table 2 Properties of certain compounds with endocrine-disrupting properties.

Compound	Molecular weight (g/mol)	Log KOW	Solubility
(mg/L)	Henry’s constant (atm m3/mol)	pKa	Ref.	
17α-estradiol	272.4	4.01	3.90	3.64 × 10-11	10.46	[69]	
17β-estradiol	272.4	4.01	3.60	3.64 × 10-11	10.23	
Ethinyl estradiol	296.4	3.67	11.3	7.94 × 10-12	10.21	
Estriol	288.4	2.45	441	1.30 × 10-12	10.05	
Estrone	270.4	3.13	30.0	3.80 × 10-10	10.40	
Equilin	268.4	3.35	50.1	3.94 × 10-10	10.26	
Bisphenol A	228.3	3.32	102	1.1 × 10-11	10.2	[77]	
Isopropylphenol	136.19	2.9	1100	1.09 × 10-6	10.2	[133], [134]	
4-cumylphenol	212.29	4.1	43.3	8.8 × 10-8	10.1	[75]	
4-n-nonylphenol	202.4	4.48	5.43	/	10.7	[82]	
Dicofol	370,48	5.02	0.80	2.4 × 10-7	10.7	[88]	
2-phenoxyethanol	138.166	1.16	26,000	/	15.1	[85]	
Fluoxetine	309.33	4.0	1.70	0.84 × 10-7	1.05	[79]	
propylparaben	180.02	3.04	463	6.37 × 10-9	8.24	[80]	
Benzophenone-3	288.2	3.8	/	1.5 × 10-8	7.56	[87]	
Dimethyl phthalate	194.18	1.6	4000	2.0 × 10-7	/	[89]	
Atrazine	215.683	2.6	33.0–34.7	2.6 × 10-9	14.84	[135]	

2.3 Environmental and health effects

Environmental concerns have been raised by the presence of EDCs in many environmental matrices, including the atmosphere, soil, and aquatic compartments, as well as in food sources, personal care products, and manufactured goods. The fact that many of these chemicals are being released into the environment without a full understanding of their potential adverse effects only adds to this concern [19], [29]. In addition, most wastewater treatment plants are unable to effectively remove EDCs [30].

The presence of these compounds in various environmental compartments poses significant ecological risks, mainly due to their ability to accumulate over time [31], [32]. In particular, EDCs have a reputation for causing changes that may affect subsequent generations or occur in an organism during its early life stages [33]. It is noteworthy that these compounds can have adverse effects on animal and human health even at very low doses (μg/L to ng/L) [19]. As a result, the hazards associated with the introduction of EDCs into the environment continue and are evident as a large number of EDCs have been found worldwide in a variety of taxa [34].

A wide range of health problems have been implicated as adverse human health effects, including infertility, heart disease, metabolic disorders, respiratory problems, degenerative diseases, mental disorders, and various forms of cancer [19], [35], [36]. Comparably significant adverse effects have also been reported in animals, including reduced egg hatching rates, changes in the reproductive systems of reptiles, and phenomena such as imposex, masculinization, hermaphroditism, and feminization, to name a few [19], [37], [38]. In addition, these substances have the capacity for easy entry into the environment, significant biomagnification and bioaccumulation, all of which enhance their ecological effects [19], [39], [40]. In other cases, these effects may be a factor in the extinction of a particular species [19].

3 Sonochemistry

The phenomenon of acoustic cavitation (the microscale generation, development, and implosive collapse of small bubbles in solution exposed to ultrasonic frequencies in the range of 20 to 1000 kHz) is the source of the chemical and physical effects of ultrasonic waves. These bubbles can be inert, developing for one acoustic cycle (or less) before collapsing during the compression phase, or stable, bouncing around their equilibrium radius for many cycles [41]. The abrupt collapse of the initial bubbles, which can generate pressures and temperatures as high as 5200 K and 500 atm, enables a wide range of ultrasonic applications [42], [43]. These applications include pollutant destruction [44], [45], cleaning [46], polymer synthesis [47], nanomaterial production [48], [49], food science [50], [51], and biomedical applications [52], [53], [54].

During the collapse phase, intense chemical reactions occur that produce highly reactive species, including ●OH, H●, HO2●, O3, and others. These reactions include trapped water vapor, gases, and volatile substrates undergoing degradation (pyrolysis or combustion reaction in the presence of O2) [55], [56]. Water sonolysis (reaction (1) is the main reaction that propagates when additional bubble components are involved. Depending on the saturating gas and operating conditions, other radicals/products (such as reactions (2), (3), (4), (5) are formed [57], [58]. The hydroxyl radical (●OH), one of several reactive species produced, is essential to sonochemistry. To act as an aqueous oxidant, radicals can recombine within the bubble, react with gaseous species, or diffuse out of the bubble [10]. Radical-radical recombination can also occur at the bubble/solution interface, producing H2O2 in the case of ●OH radicals (reactions (16), (17).(1) H2O → ●OH+H●

(2) O2 → 2O

(3) N2 → 2 N

(4) •OH→O+H•

(5) O+H2O → •OH + •OH

(6) H• + O2 → O + •OH

(7) H• + O2 → O + •OH

(8) H• + O2 → HO2•

(9) O2 + O→O3

(10) O2 + N→O+NO

(11) O2 + N→O+NO

(12) O2 + NO→O+NO2

(13) •OH+NO→HNO2

(14) •OH+NO2 → HNO3

(15) H• + H• → H2

(16) HO2• + HO2• → H2O2 + O2

(17) •OH + •OH→H2O2

The physical properties of aqueous contaminants determine their reactivity: non-volatile compounds are oxidized by free radicals (primarily ●OH) either at the bubble/solution interface (for hydrophobic compounds) or within the liquid bulk (for hydrophilic compounds) [59], [60]. Volatile compounds, whether organic or inorganic, vaporized within the bubble can be degraded by pyrolysis/incineration in the extremely hot cavity. As a result, sonochemical reactions occur in the bulk of the solution, at the bubble/solution interface, and in the bubble gas phase. Only 10 % of the radicals reach the solution bulk, with a higher concentration of ●OH near the liquid shell of the bubble [61], [62], [63].

The three main processes that contribute to the physical consequences of ultrasonication are the shock wave, the microjet, and the acoustic microstreaming field, all of which are generated when the bubbles burst [59]. Severe cavity collapse causes a self-reinforcing bubble to deform, creating a microjet (a rapidly moving stream of liquid directed at the surface of a solid). This microjet causes localized erosion and pitting on the surface (increasing the surface area of the solid and producing new fine surface material) [64], with a velocity of about 100 m/s [65].

In addition, shock waves generated by cavity collapse [64] separate weakly agglutinated grains by exploiting pre-existing cracks and increasing the surface area. These shock waves can also be used to dislodge particles that are only loosely attached to a solid surface [64]. Sonoluminescence (SL), which is used to measure cavity temperature, pressure, and active bubble size, is also produced by the powerful implosion of bubbles [66], [67]. By analyzing SL spectra, Professor Suslick's group observed temperatures of about 5200 K and pressures up to 1700 atm [66].

4 Ultrasonic destruction of EDCs

The particular ultrasonic and operational parameters used have a significant impact on how well EDCs respond to ultrasonic treatment. Operational conditions include elements such as temperature, pH, saturating gas, initial concentration of EDCs, reactor shape and volume, and sonoreactor power and frequency. Ultrasonic conditions include additional variables such as these. A summary of the studies on EDCs is provided in Table 3, along with the factors examined in each study. The following sections provide an in-depth analysis of several studies, organized by type of research. The focus is on describing the conditions used in each study and highlighting key findings. In addition, an attempt is made to examine studies that evaluate the same EDC regardless of publication order.Table 3 Key studies on the ultrasonic destruction of EDCs.

Entry	EDC	Investigated parameters	Treatment’s effectiveness	Ref.	
1	17-estradiol, 17-estradiol, equilin, 17-dihydroequilin, 17-ethinyl estradiol, estrone, estriol and norgestrel	Power (0.6–4.0 kW) for single frequency (20 kHz)	Highly effective degradation of all estrogens, especially at lower power (0.6 kW)	Suri et al. [68]	
2	17α-estradiol, ethinyl estradiol, 17β-estradiol, estrone, gestodene, equilin, norgestrel and levonorgestrel	Batch and continuous mode reactions (both with 20 kHz), solution pH, liquid temperature and pressure	Effective degradation of estrogens’ mixture (50 µg/L), particularly at pH 3	Fu et al. [69]	
3	Bisphenol A (BPA)	BPA concentration (0.15–460 µmol /L), Saturating gas (O2, Ar and air), applied power (20–80 W) and ultrasonic frequency (300– 800 kHz)	Highly effective treatment, particularly when conducted under an O2 atmosphere, with higher applied power and at a frequency of 300 kHz.	Torres et al. [70]	
4	Bisphenol A (BPA)	Acoustic intensity (3.0, 9.0 and 12.9 kW/m2) for a single BPA concentration (0.5 mM)	Total BPA removals at 2.0, 3.0, and 10 h, corresponding to power intensities of 12.9, 9.0 and 3.5 kW/m2, respectively.	Inoue et al. [71]	
5	Bisphenol A (BPA)	Acoustic intensity (20–80 W/cm2), as well as US/CCl4 and US/O3 combined systems, were all applied under 20 kHz horn ultrasonication.	Effective removal of BPA was achieved with both US and US/CCl4 systems, while a synergistic effect was observed with the US/O3 system.	Guo and Feng [72]	
6	4-cumylphenol (4-CyP)	Initial 4-CyP concentration (0.05–30 mg/L), pH (2, 6.5 and 10), power (20–100 W), temperature (20–50 °C) and saturation gases (Ar, N2 and air) and frequency (300 and 600 kHz)	4-CyP’s effective degradation was ensured, especially under optimized conditions.	Chiha et al. [75]	
7	4-Isopropylphenol	Initial 4-IPP concentration (0.05–200 mg/L), for single ultrasonication system (278 kHz, 80 W)	Effective treatment was achieved, particularly at low EDC concentrations in the presence of bicarbonate ions	Chiha et al. [75]	
8	17β-estradiol (E2) and 17a-ethinylestradiol (EE2)	Frequency: (40, 380, 850, 1000 kHz) in Capocelli et al. and (28, 580, and 1000 kHz) in Park et al.	All systems degraded both EDCs, with an optimum frequency of 580 kHz	Capocelli et al. [76] and Park et al. [77]	
9	17β-estradiol (E2) and 17a-ethinylestradiol (EE2)	Initial concentration (1–3 mg/L), pH (3–9), power (6.76–27.82 W acoustic), temperature (10–30 °C), for a single frequency of 850 kHz	Ultrasonication efficiently removed both estrogens	Ifelebuegue et al. [78]	
10	Fluoxetin (FLX)	Initial FLX concentration (2.9–162.0 μM), pH (3, 6.5, 11), power (20–60 W), and saturation gases (Ar, air and He), using a single frequency 600 kHz	The US treatment efficiently remove FLX, especially in the presence of FeSO4 (90 µM) salt.	Serna-Galvis et al. [79]	
11	propylparaben (PPB)	pH, water quality and initial PPB concentration (25–100 mg/L), all for single ultrasonication system (352 kHz/80 W)	Effective removal of PPB in various matrices (including natural and seawaters), especially at low initial PPB concentrations (<25 mg/L)	Boutemedjet et al. [80]2016	
12	2-phenoxyethanol (PhE)	Initial PhE concentration, pH (2–10), power (20–120 W), temperature (15–45 °C) and nitrogen saturation, all for 600 kHz plate-type sonicator.	Effective removal of PhE was achieved, particularly with applying higher power to lower pollutant concentration	Boutamine et al. [85] 2018	
13	Benzophenone-3 (BP-3)	Frequency (215–1134 kHz), power density (40–200 W/L).	Ultrasonication efficiency degraded BP-3, especially when applying higher power density (140 W/L)	Vega and Penuela [87]2018	
14	4-n-nonylphenol (NP)	Initial NB concentration (0.5–24.3 µM), pH (3–10.5), power (20–100 W), all for a single frequency (278 kHz) and air-equilibrium medium.	Effective removal was achieved, especially when applying higher power on lower NB concentration	Delhatou et al. [81]	
15	4-n-nonylphenol (NP)	Initial NP concentration (20–480 µM), pH (3, 6, 1.8), gas (O2, Ar, air), all for a single frequency (20 kHz) and power (0.46 W/L).	NP effective degradation was ensured, especially in basic medium	Ince et al. [82] and Gultekin et al. [83]	
16	4-n-nonylphenol (NP)	gas (O2, Ar, air), for a single frequency (200 kHz), power intensity (6 W/cm2) and initial concentration (30 µM).	Effective removal was achieved, and many pyrolysis products are identified and quantified.	Kim et al. [84].	
17	Dicofol	Initial concentration (5.4–54 µM), power (150–450 W), pH (3, 5 7), temperature (10–30 °C), for a single frequency of 20 kHz.	86 % of Dicofol degraded within 1 h, indicating improved remediation performance	Debabrata et al. [88]	
18	Dimethyl phthalate (DMP)	Initial concentration (0.005–0.1 mM), frequency (20, 200 and 400 kHz), pH (∼3–9)	Effective treatment; more than 50 % of DMP was removed within 12 to 20 min.	Xu et al. [89]	
19	Ethyl paraben (EP)	Initial concentration (250–1250 µg/L), power density (20–60 W/L), pH (3–8), for a single frequency of 20 kHz.	Significant degradation, reaching up to 98 %, was achieved through the utilization of diverse combined parameters.	Papadopoulos et al. [129]	
20	Atrazine	Pétrier et al.: 500 kHz/18.5 W, 70 µM.
Hiskia et al.: 850 kHz/120 W, 70 µM.
Koskinen et al.: 20 kHz/70–80 W, 3.1 µM	All obtained results affirm the efficacy of ultrasound in mineralizing this compound.	Pétrier et al. [90], Hiskia et al. [91], Koskinen et al. [92]	

A number of estrogen hormones were specifically targeted for degradation in aqueous solutions by Suri et al. [68], focusing on norgestrel, 17β-estradiol, 17α-estradiol, estrone, equilin, and estriol. Within 10 to 15 min, almost 50 % of the compounds were eliminated using 0.6 kW of 20 kHz ultrasonication. Furthermore, in a very short time, i.e., 40 to 60 min, a significant clearance of each estrogen (80 to 90 % at an initial concentration of 10 μg/L) was achieved. With increasing power intensity (0.6 − 4 kW), the first-order degradation rate constant of each estrogen showed an increase. Interestingly, the energy efficiency of the reactor was found to be greater at lower power densities (0.6 kW). This underscores the importance of carefully selecting the reactor type and ultrasound power to achieve the best possible kinetics and energy efficiency. Pseudo-first order rate constants were reported for 2.0 kW systems operated at different pH values (3.0, 7.0 and 9.0) and with different initial concentrations of 1 μg/L and 10 μg/L. It was found that, under the conditions studied, 17α-dihydroequilin > 17α-estradiol > equilin > estrone > 17β-estradiol > ethinyl estradiol > gestodene > levonorgestrel/norgestrel was the order in which estrogens were susceptible to sonolytic degradation in a pH 7.0 mixed solution. Estrogens were shown to be more susceptible to destruction at lower solution temperatures, but reaction efficiency was negatively affected by higher fluid pressure. Under an ultrasound density of 2.1 W/mL at pH 7 (10 µM of each estrogen in the mixture), the effect of estrogen degradation on physicochemical parameters, including molecular weight, solubility, Kow, and Henry's constant, was evaluated. A satisfactory relationship was found between the rate constant and the molecular weight of the estrogens, although no significant correlation was found between the rate constants and solubility, Kow, or Henry's constant.

Fu et al. [69] used a 1.10 W/mL batch reactor and a 2.10 W/mL continuous flow reactor, both operating under a 20 kHz probe system, to sonolyze a mixture of estrogens (17α-estradiol, ethinyl estradiol, 17β-estradiol, estrone, gestodene, equilin, norgestrel, and levonorgestrel) in aqueous solutions (10 µg/L each). The effects of process factors such as pressure, pH, and temperature were systematically investigated. Remarkably, the total estrogen content decreased more rapidly at pH 3 than at pH 7 or 9. The decrease in total estrogens was comparable at pH 7 and 9, and the final efficiencies converged after approximately 20 min. Pseudo-first-order rate constants were reported for 2.0 kW systems operated at different pH values (3.0, 7.0, and 9.0) and with different initial concentrations of 1 and 10 μg/L. It was found that, under the conditions studied, 17α-dihydroequilin > 17α-estradiol > equilin > estrone > 17β-estradiol > ethinyl estradiol > gestodene > levonorgestrel/norgestrel was the order in which estrogens were susceptible to sonolytic degradation in a pH 7.0 mixed solution. Estrogens were shown to be more readily destroyed at lower solution temperatures, but reaction efficiency was negatively affected by higher fluid pressure. Under an ultrasound density of 2.1 W/mL at pH 7 (10 µM of each estrogen in the mixture), the effect of estrogen degradation on physicochemical parameters, including molecular weight, solubility, Kow, and Henry's constant, was evaluated. A satisfactory relationship was found between the rate constant and the molecular weight of the estrogens, although no significant correlation was found between the rate constants and the solubility, Kow, or Henry's constant. Specifically, a negative correlation was observed between the rate constant and molecular weight, suggesting that lower molecular weight estrogen molecules exhibit greater diffusivity into the interfacial region, thereby undergoing more rapid ultrasound-induced degradation.

In a thorough investigation of the sonolytic degradation of bisphenol A (BPA), Torres et al. [70] evaluated a number of operational factors. These included initial concentrations of BPA (15.0 − 460 μM), frequency (300 − 800 kHz), electrical power (20 − 80 W), and saturating gases (O2, Ar, and air). Under certain conditions, including 300 kHz frequency, 80 W power, and an O2 environment, BPA (118 μM) was effectively removed in approximately 90 min. Even after prolonged ultrasonic treatment (barely 9 h), the solution still contained more than half of the initial COD and 80 % of the TOC. The primary ultrasonic BPA degradation pathway involved the reaction of BPA with •OH radicals at the bubble-solution interface. HPLC-MS analysis revealed several hydroxylated intermediates, including monohydroxylated BPA, quinone of monohydroxylated BPA, 4-isopropenylphenol, quinone of dihydroxylated BPA, monohydroxylated 4-isopropenylphenol, dihydroxylated BPA, and 4-hydroxyacetophenone. Examination of H2O2 generation in the presence and absence of BPA supported this. Interestingly, the rate of H2O2 generation decreased with increasing BPA content, indicating that BPA was able to scavenge more hydroxyl radicals at the bubble interface. The initial BPA products were converted to biodegradable aliphatic acids after 2 h. In agreement with Torres et al. [70], the BPA degradation rate showed a decrease with frequency (initial degradation rate of 3.5 µM/min at 300 kHz vs. 2.8 µM/min at 500 kHz, 2.2 µM/min at 600 kHz, and 2.1 µM/min at 800 kHz) and an increase with increasing power (initial degradation rate of 0.8 µM/min at 20 W vs. 1.9 µM/min at 40 W, 2.7 µM/min at 60 W, and 3.5 µM/min at 80 W). These findings are mainly attributed to the enhancement of number density as well as the improvement of the distribution of acoustic pressure either by the increase of wave frequency (up to an optimal value) or the increase of acoustic pressure in the sonicated solution. Under O2 saturation, a higher degradation rate was observed (initial degradation rate of 4 µM/min for O2, 3.5 µM/min for air, and 3.4 µM/min for argon). Despite its relatively low polytropic index (1.41) compared to argon gas (1.66), the outperformance of oxygen (compared to the other gases) could be attributed to its participation in the chemical mechanism, thus, the yield of hydroxyl radicals (and BPA degradation) is enhanced. On the other hand, the removal efficiency showed an inverse relationship with the initial degradation rate and became more significant as the initial BPA content decreased. The tendency of degradation efficiency is mainly ascribed to the fact that the generated amount of OH radicals is constant independently of the initial concentration of BPA, thus, the rise in the concentration of this later will be accompanied with a decrease (the opposite behavior for the initial decomposition rate) in its degradation efficacy.

Inoue et al. [71] conducted a separate study and found that at 404 kHz, a concentration of 0.5 mM BPA showed complete degradation after 2.0, 3.0, and 10 h at power intensities of 12.9, 9.0, and 3.5 kW/m2, respectively. Organic acids, formaldehyde, and 3-hydroxy-BPA were among the intermediates found. The production yield of H2O2 and HNO3 was quantified to be higher in the absence of BPA than in its presence, indicating support for the •OH radical process. Using 20 kHz ultrasound, Guo and Feng [72] consistently found that the main mechanism for sonolysis of BPA (100 µg/L) at room temperature (25 °C, pH 6.5) was •OH radical-induced oxidation. Compound elimination increased with increasing power intensity applied to the solution in the range of 20 − 80 W/cm2 (calculated on a kilogram basis). The addition of 2 mM Fe2+ ions (as FeSO4 salt) did not increase the rate of BPA degradation at pH 3 according to Inoue et al. [71]. Instead, higher doses of FeSO4 were found to cause an increase in TOC reduction; this increase reached 4.0 mM at 404 kHz and 9.0 kW/m2. The initial amount of H2O2 in the reaction process was insufficient to react with all the iron ions generated by the Fenton reaction to form the reactive radicals that lead to BPA destruction, according to the author, who explained why iron addition was inefficient in eliminating BPA. However, this explanation seems unreasonable since Ricardo et al. [73] confirmed the improvement of BPA (and also TOC) removal by iron addition at low concentrations (100 µM). Inoue's result could be attributed to the high concentration of iron ions, which could act as scavengers instead of activators of the Fenton reaction. This scenario was confirmed by Merouani et al. [74], who investigated the effect of Fe(II) concentration on the sonolytic degradation of Rhodamine B at 300 kHz and 60 W. They found that a lower Fe(II) dosage (0.018 − 0.09 mM) enhanced dye removal, while a higher dosage (above 0.09 mM) inhibited the degradation rate.

Chiha et al. [75] investigated the variables affecting the degradation of 4-cumylphenol (4-CyP). 40 min was found to be sufficient for complete degradation of low levels of the contaminant (less than 1 mg/L) at 300 kHz and 100 W. However, more time was required as the substrate concentration increased, and the rate constant showed a steady decrease with increasing 4-CyP initial concentration. The main reaction zone was identified as the bubble interface based on the best fit of the data with the Langmuir model. The influence of tert-butanol quenching on the sonolytic destruction of 4-CyP validated the •OH oxidation pathway. The oxidation of EDC was decreased by nitrogen saturation, while there was little variation in argon and air atmospheres. Better results were obtained with a heated solution (50 °C) subjected to 300 kHz and 100 W. The sonolytic process was improved by the addition of KBr and NaHCO3 salts, but only at low 4-CyP concentrations (5 and 0.05 mg/L for the KBr and NaHCO3 effects, respectively).

In another paper, Chiha et al. [75] used 278 kHz and 80 W to study the ultrasonic degradation of 4-isopropylphenol in aerated solutions over a wide concentration range from 5 to 200 mg/L. Degradation was observed and the power was directly related to the initial concentration of the compound. Higher degradation rates were obtained at lower concentrations. Since carbonate radicals were present in the bulk solution under these circumstances, bicarbonate ions had a stimulating effect (see section 6.7 for more information).

Capocelli et al. [76] used ultrasound at different frequencies (40, 380, 850, and 1000 kHz) to study the degradation of 17b-estradiol (E2) and 17a-ethinylestradiol (EE2). Higher frequencies were shown to have a greater effect on the degradation of EDCs, with 850 kHz being the most effective at 0.9 mg/kW h for E2 and 0.68 mg/kW h for EE2 at baseline levels of 1 mg/L. The same estrogens (E2 and EE2) and bisphenol A (BPA) were evaluated for degradation using ultrasonic reactors at 28, 580, and 1000 kHz in a study by Park et al. [77]. EE2 > BPA>E2 was the degradation order for EDCs, which followed pseudo-first order rate kinetics. The degradation sequence at an initial concentration of 1 mM for each drug and a contact time of 30 min was 580 kHz (92 − 97 %) > 1000 kHz (90 − 94 %) > 28 kHz (62 − 67 %). The addition of tert-butanol slowed down the degradation process, demonstrating the existence of an •OH oxidation mechanism. On the other hand, CCl4 enhanced the degradation of all three EDCs. Furthermore, Ifelebuegu et al. [78] reported that the degradation rate of EE2 at 850 kHz decreased with increasing acoustic power (6.76 − 27.82 W) and pH in the range of 3 − 9.

Serna-Galvis et al. [79] used a biological system in combination with ultrasonic treatment to degrade fluoxetine (FLX). Even under ideal pH and temperature conditions, the biological process alone was unable to remove the contaminant. However, removal of FLX was effectively achieved by ultrasonic treatment (600 kHz, 60 W). The treatment took 60 min to remove approximately 80 % of the FLX (40.5 μM). Furthermore, using the same initial contaminant concentration (162.0 μM) as in the biological test, FLX was completely degraded by the ultrasonic process in 240 min. At 15 % mineralization, ultrasonication was completed in 360 min. The ultrasonic process converts the contaminant into biodegradable compounds that can be mineralized in a future biological treatment, according to biodegradability studies.

The effect of water quality on the sonolytic degradation of propyl paraben (PPB) at 352 kHz and 80 W was investigated by Boutemedjet et al. [80]. The main objective was to evaluate the effect of initial concentration (25 − 100 mg/L) on the reactivity of the compound in different water matrices (deionized water, natural water and salt water). Lower concentrations of PPB (10 and 25 mg/L) could be completely removed in 90 min, while higher concentrations required more time. Interestingly, changes in solution pH from 3 to 8 in deionized water had no effect on PPB degradation kinetics. Excellent data fitting using heterogeneous Langmuir-type kinetic models indicated that PPB elimination was caused by interaction with •OH at the interfacial reaction zone.

Four different studies have investigated the degradation of 4-nonylphenol (NP): Delhatou et al. (278 KHz) [81], Ince et al. [82], Gultekin et al. (both at 20 KHz) [83], and Kim et al. (200 kHz, 6 W/cm2) [84]. In each case, ultrasound was found to be a successful technique for NP degradation. After 120 min of ultrasonication, Delhatou et al. [11] reported that NP was completely removed over a concentration range of 0.5 − 24.5 µM, especially at higher power applications. The destruction process involved •OH radicals at the bubble/solution interface, which is consistent with the findings of Ince et al. [82]. Furthermore, as shown in the study by Delhatou et al., the pH range (3 − 5) had no discernible effect on the removal kinetics. Ince et al.'s [82] pattern data at 20 kHz were consistent with Delhatou et al.'s results at 278 kHz; however, the amount of NB removal at 278 kHz was much higher than at 20 kHz. In addition, the NB removal rate in the 20 kHz sonolysis was significantly slower at pH 6, 2.2 times faster at pH 3.0, and 4 times faster at pH 10.8 (compared to pH 6) [82]. The difference in the initial concentration of NB used (8.1 µM in Delhatou's study and 100 µM in Ince's study) may be the reason for the discrepancy between Delhatou et al. [81] and Ince et al. [82]. Previous research has shown that the initial substrate concentration, in addition to other ultrasonication parameters such as frequency, operating temperature, and power, has a significant effect on the pH effect. In contrast to Delhatou et al. [81], Ince et al. [82] found that the addition of a small amount of tert-butanol increased the degradation rate constant of NB by 2.14 times. This increase was attributed to the NB degradation process involving CH3• radicals (resulting from tert-butanol pyrolysis). During sonolytic degradation of NP under argon, Kim et al. [84] found a 50 % reduction in TOC of the original NP solution (5.4 mg/L) in 60 min at 200 kHz under argon saturation. In addition, gaseous products (CO, CO2, CH4, C2H2, C2H4, and C2H6) and hydroxyhydroquinone were formed. The addition of a hydroxyl radical after cleavage of the alkyl chain from the aromatic ring was demonstrated by the formation of hydroxyhydroquinone. After 100 min of ultrasonication, the pH decreased from 7.1 to 4.2 due to the increase of organic acids and carbon dioxide in the solution.

The degradation of 2-phenoxyethanol (PhE) over a range of operating conditions, including changes in initial PhE concentration, pH (2 − 10), power (20 − 120 W), temperature (15 − 45 °C), and nitrogen saturation, was achieved by Boutamine et al. [85] using 600 kHz ultrasonication. At a concentration of 10 mg/L, the pollutant was completely removed in 100 min, and at 120 W applied power, the COD was reduced by about 60 %. At lower initial PhE concentrations and higher power levels (120 W), the removal efficiency was much higher. The investigation revealed that •OH was the main oxidant, as evidenced by observable variations in H2O2 generation rates in the presence and absence of the chemical in different degradation situations. The radical probing approach [86] validated the •OH oxidation pathway using specific scavengers such as sucrose, Triton X-100, and 2-propanol.

Vega and Penuela [87] used variable frequency ultrasound (215 − 1134 kHz) and power density (40 − 200 W/L) to demonstrate the efficient degradation of benzophenone-3 (BP-3). Approximately 50 % of BP-3 (1 mg/L) was eliminated at 40 W/L in 30 to 60 min using frequencies of 215, 373, 574, 856, and 1134 kHz; 574 kHz was the most effective frequency. It took less than 1 h to achieve 100 % removal at 140 W/L at frequencies of 574, 856, or 1134 kHz. The discovery of hydroxylation intermediates, such as 1-(2-hydroxy-4-methoyphenyl)propan-1-one, and the best fit of the degradation rate using the Langmuir interfacial model verified the •OH radical oxidation pathway at the bubble solution interface. Toxicity testing using the Microtox method yielded an EC50 of 1.70 mg/L for 5 min and 2.07 mg/L for 15 min. At 30 % BP-3 degradation, the toxicity profile showed an initial decrease and then an increase throughout the process.

The study by Debabrata et al. [88] investigated the degradation of dicofol at a frequency of 20 kHz under a range of operating conditions. After 60 min of treatment, 86 % of the dicofol was degraded with a pseudo-first order degradation rate constant of 0.032 min−1, which was found to be the ideal parameter. The degradation process was mainly characterized by thermal degradation along with radical attacks at the bubble-vapor interface, according to the analysis performed by HPLC and GC–MS.

In the absence and presence of peroxymonosulfate, the study by Xu et al. [89] investigated the degradation of dimethyl phthalate (DMP) using ultrasound at different frequencies (20, 200, and 400 kHz). After 30 min of ultrasonication at 200 and 400 kHz, approximately 70 % of DMP (10 µM) was removed, demonstrating the high effectiveness of the process in removing contaminants. The addition of peroxymonosulfate further increased the degradation rate, especially at higher frequencies, demonstrating a synergistic effect of the treatment.

Atrazine is a known endocrine disruptor. It is a common agricultural pesticide, and research has been conducted on how it affects the endocrine system, namely the reproductive and hormonal systems of humans and animals. Numerous research organizations have studied the sonolytic degradation of atrazine [90], [91], [92]. The usefulness of ultrasound in mineralizing this chemical has been confirmed by all the data found. According to Pétrier et al. [90], 500 kHz ultrasound (18.5 W acoustic power) completely removed atrazine (70 µM) in 70 min, while 20 kHz ultrasound removed only 30 %. Using 850 kHz and 120 W, Hiskia et al. [91] made similar observations at pH 5 in an oxygenated atrazine solution (70 µM). During the first one to two hours of sonolysis, a number of intermediates, including CAAT, OIET, CIAT, CAET, and CDIT, were formed and degraded, while the final product (∼70 µM) was chloride ions.

Patricia Vega et al. [93] investigated the effect of different power levels (40 − 200 W/L) and frequencies (213, 373, 574, 856 and 1134 kHz) on the degradation of triclosan (TCS: 1.7–11 µM), a potential EDC. At both pH 6.9 and pH 10, 574 kHz (140 W/L) ultrasonication produced the best results in terms of the highest degradation rate of the contaminant, with 1 mg/L of triclosan being completely eliminated in 30 min. The degradation rate of TCS was in good agreement with an interfacial model, supporting a process involving degradation of •OH radicals. At 40 % and 90 % of TCS degradation, 2,7/2,8-dibenzodichloro-p-dioxin was found during the process, while naphthalene was found at 40 % of TCS degradation. At various reaction times, 2,4-dichlorophenol was also detected, along with acetic acid and 4-chloro-3-(4-chlorophenoxy)phenol. The above discussion clearly underscored the importance of an accurate selection of the operating conditions, where most of EDCs were degraded in the frequency range from 200 to 600 kHz. The selection of these conditions should be optimized with respect to the main affecting parameters (ultrasound frequency, acoustic density, reactor design, number and distribution of transducers…etc.) controlling the energy efficiency of the sonochemical degradation of EDCs.

5 Degradation mechanism

The ultrasonic oxidation of almost all EDCs has been attributed to their reactivity with acoustically generated •OH, mainly at the bubble-solution interface, according to the review analysis presented in Section 4. The reason for this phenomenon is that most EDCs are hydrophobic, as evidenced by their low KOW values and Henry's constants (Table 2), which prevents them from penetrating into the gas bubble phase. Instead, they accumulate near the bubble/solution interface where they are exposed to a strong flux of hydroxyl radicals. The following techniques have been used to validate this mechanism:• Finding the hydroxylation intermediates, as shown by the reaction pathway of bisphenol A [70]. Monohydroxylated BPA, quinone of monohydroxylated BPA, 4-isopropenylphenol, quinone of dihydroxylated BPA, monohydroxylated 4-isopropenylphenol, dihydroxylated BPA, and 4-hydroxyacetophenone are among the intermediates or by-products,

• H2O2 production measured by direct •OH dosimetry. To evaluate the change due to EDC scavenging, H2O2 levels must be measured both with and without the chemical. Studies of bisphenol A [70], 2-phenoxyethanol [85], 4-n-nonylphenol [81], and others have used this approach,

• The use of heterogeneous interfacial kinetic models and the Langmuir equations (Eqs. (8), (19) has been successful. Under a variety of ultrasonication settings, these models have been shown to accurately describe the degradation pathways of several EDCs, such as triclosan [93], propyl paraben [80], fluoxetine [79], 2-phynoxyethanol [85], 4-cumylphenol [75], benzophenone-3 [87], and bisphenol A [70].

(18) r0=kKC01+KC0

(19) r0=Kb+kKC01+KC0

C0, k, K, Kb, and r0 where, respectively, the initial contaminant concentration, the pseudo-rate constant, the equilibrium constant, constant indicating the bulk degradation rate, and the initial degradation rate.

The use of radical probing methods has proven to be a useful strategy. For example, Boutamine et al. [86] investigated the reaction zone and oxidation pathway of 2-phenoxyethanol (PhE) in a 600 kHz/120 W reaction system using specific scavengers, namely 2-propanol, Triton X-100, and sucrose. Due to its high volatility (log KOW=4.86), 2-propanol acted as an •OH scavenger mainly in the gaseous regions of the study (inside the bubble). Triton X-100 surfactant was used to study the interfacial reaction zone because it preferentially adsorbs near the interface of the collapsing bubble where it competes with impurities for •OH radical reactions. When reacting with hydroxyl radicals, Triton X-100 exhibited a high rate constant of (8.8–9.6) × 109 M−1 s−1. In addition, the •OH reaction in bulk solution was studied using sucrose, a highly hydrophilic substrate (log Kow = -3.69) (k●OH-sucrose = 3.12 × 109 M−1 s−1). Degradation results showed that sucrose had no effect on PhE removal, whereas very little PhE was removed (about 7 %) in the presence of 100 mM 2-propanol. PhE was significantly reduced (72 %) in the presence of Triton X-100 (100 mM). These results showed that hydroxyl radical attack leads to the major degradation of PhE at the bubble/solution interface. Similarly, Serna-Galvis et al. [79] investigated the •OH pathway for fluoxetine using 1-hexanol and 2-propanol as scavengers; both scavengers dramatically hindered the ultrasonic effect.

6 Parameters impacting ultrasonic destruction of EDCs

6.1 Ultrasonication frequency

As Table 3 shows, a number of studies have investigated the effect of ultrasound frequency on the sonolytic degradation of EDCs. The results highlight the important role of ultrasound frequency and it is worth noting that very few studies have investigated a wide range of frequencies, usually reporting two or three. Even with this limitation, and considering the entire frequency range used in sonochemistry (20 − 1000 kHz), 200 − 600 kHz has been repeatedly recognized as the ideal range. For example, the ultrasonic degradation of bisphenol A (BPA: 118 µM) in the 300 − 800 kHz frequency range showed that the degradation rate decreased with increasing frequency [70]. As the ultrasonication frequency increased to 300, 500, 600, and 800 kHz, the initial BPA degradation rate decreased to 3.5, 2.8, 2.2, and 2.1 µM/min, respectively. Analysis of H2O2 production, which showed a continuous decreasing trend in both the presence and absence of BPA, provided additional evidence of this pattern. The initial rates of H2O2 production in pure water were 4.8 µM/min at 300 kHz, 2.6 µM/min at 500 kHz, 2.1 µM/min at 600 kHz, and 1.7 µM/min at 800 kHz. The addition of BPA was shown to result in reduced production yields. The 4-cumylphenol EDC decreased exponentially according to Chiha et al. [75], reaching 80 % removal in 25 min at 300 kHz and 37 min at 600 kHz. According to Pétrier et al. [90], atrazine was degraded 7.8 times faster at 500 kHz than at 20 kHz. Vega and Penuela [87], [93] used variable frequency ultrasound (215, 373, 574, 856 and 1134 kHz) to show how well benzophenone-3 and triclosan were degraded. The highest performance was shown at 574 kHz. Remarkably, regardless of the type of water (deionized, brackish, or seawater), Park et al. [94] showed significantly greater removal of BPA and 17α-ethinylestradiol at 580 kHz compared to 28 kHz.

The effect of frequency can be explained by how it affects the rate at which oxidants, especially •OH, are produced. Two unique scales of phenomena (microscopic and macroscopic) control this effect. At the microscopic level, the effect of frequency on the dynamic behavior and response of individual bubbles is significant. A thorough simulation study of single bubble sonochemistry was performed by Merouani et al. [57], [58] and yielded enlightening results. In the 20 − 1000 kHz frequency range, the bubble resonance size and expansion time decrease with increasing frequency. As a result, there is less water vapor trapped inside the bubble and its temperature decreases because the subsequent collapse is less effective. Reaction efficiency decreases under these circumstances, resulting in lower chemical bubble yields (oxidants). The number of active bubbles increases significantly as the frequency increases on a macroscopic scale. While bubble coalescence is reduced, mass transfer can be enhanced at higher frequencies by turbulence, acoustic streaming, and other physical phenomena. These circumstances may result in a higher rate of hydroxyl radical production. Merouani et al. [95] and Dehane et al. [96], [97], [98] have provided numerical data indicating that an increase in frequency in the 20 − 1000 kHz range is associated with a greater number of active bubbles. As a result, increasing frequency results in two opposite trends: a steady increase in the number of active bubbles (a macroscopic event) and a steady decrease in the yield of a single bubble (a microscopic event). This suggests that an ideal frequency is likely to result from a balance between the microscopic and macroscopic events associated with an increase in frequency. As noted above, this ideal frequency is regularly found to be between 200 and 600 kHz for a variety of EDCs and several organic contaminants [62], [99], [100], [101], [102].

6.2 Ultrasonication intensity/power

Table 3 summarizes the number of research papers that have investigated the effect of reactor power input, commonly characterized as electrical power (W), acoustic power (W), power density (W/L), or power intensity (W/cm2), on the sonolytic degradation of EDCs. The degradation rate of BPA at 300 kHz showed an increase with increasing power, as reported by Torres et al. [70]; initial degradation rate of 0.8 µM/min at 20 W, 1.9 µM/min at 40 W, 2.7 µM/min at 60 W, and 3.5 µM/min at 80 W. This pattern was also observed in the H2O2 measurements, which showed values of 1 µM/min at 20 W, 2. 6 µM/min at 40 W, 4 µM/min at 60 W, and 4.8 µM/min at 80 W. In parallel, Inoue et al. [71] found a significant increase in BPA degradation at 404 kHz, with pseudo-first-order rate constants increasing to 0.84 × 10-4, 2.48 × 10-4, and 4.50 × 10-4 s−1 for 3.0, 9.0, and 12.9 kW/m2, in that order. After 2 h of ultrasonication, 33, 45, 51, and 55.0 % of BPA (100 µg/L) was degraded using a 20 KHz titanium horn system by applying 20, 40, 60, and 80 W/cm2 of acoustic intensity [72]. As the power applied to the reactor increased between 20 and 100 W, Chiha et al. [75] observed a significant increase in the removal of 4-cumylphenol at 300 kHz. The removal of 4-n-nonylphenol (278 kHz: 20–100 W) [81], triclosan (40–200 W/L) [93], 2-phenoxyethanol (600 kHz: 40–120 W) [85], fluoxetine (600: 20–60 W) [79], and dicofol (20 kHz: 150–450 W) [88] showed a similar trend; an increase in power within the ranges studied significantly increased the removal rate. Vega and Penuela [87] reported a significant improvement in the removal of benzophenone-3 at 574 kHz by increasing the power density between 40 and 200 W/L.

As a result, increasing power accelerates the ultrasonic degradation of EDCs, especially in the range of values between the cavitation threshold and the upper limit where a gas layer forms near the transducer surface and blocks the flow of radiation into the solution [103], [104]. The amplitude of the bubble oscillation increases with increasing acoustic power. As a result, the resonance size of the bubble increases, allowing more water vapor to evaporate and a more violent collapse that raises the temperature of the bubble. As the applied power increases, the single bubble yield (oxidant production) increases. These observations are supported by some numerical evidence [105], [106]. At higher applied power, there are more active bubbles in the sonicated solution as a result of the simultaneous expansion of the active bubble size range [107], [108]. Ultimately, a more effective production of hydroxyl radicals at higher applied power is the result of the simultaneous increase in single bubble yield and bubble number, which effectively removes EDCs.

6.3 Saturating gas

Overall, the degradation results showed that EDCs can be treated very well with argon saturation, with nitrogen and air being the next most effective di- or polyatomic gases. However, there are some documented exceptions. For example, Torres et al. [70] found that O2 saturation gave better performance for bisphenol A degradation than air and argon. With ultrasonication at 300 kHz, the initial BPA degradation rates were 4 µM/min for O2, 3.5 µM/min for air, and 3.4 µM/min for argon. Similarly, at 20 kHz and 0.46 W/mL, Gultekin et al. [83] reported that O2 > Ar > air was the order in which 4-n-nonylphenol was degraded. In contrast, air and argon performed better than nitrogen in the degradation of 4-cumylphenol at 300 kHz and 80 W [75]. However, using N2 saturation instead of air at 600 kHz, Boutamine et al. [85] observed a reduction in the ultrasonic degradation of 2-phenoxyethanol. Similarly, Ar > air > He was the sequence reported for the degradation of fluoxetine (40.5 µM) at 600 kHz and 60 W [79].

Gases play an important role in sonochemistry. They can be divided into two groups: inert gases, which include monoatomic gases such as argon and helium, and active gases that actively participate in the bubble reaction (such as air and O2). A fascinating interaction occurs when argon, although inert, has a better ability to reach higher bubble temperatures during collapse. This is due to its lower thermal conductivity (λAr = 179 × 10-4 W/m K, λair = 259 × 10-4 W/m K and λO2 = 265 × 10-4 W/m K) and higher specific heat ratio (γAr = 1.66, γair = 1.40, γO2 = 1.33). Despite the inert nature of argon, the addition of O2, air, and N2 to the bubble reaction, either directly or via pyrolysis, offsets the decrease in radical generation caused by the lower bubble temperature these gases produce. In radical generation, the competition between argon and polyatomic gases becomes a dynamic process affected by operating parameters such as pH, temperature, power, and frequency. These factors have a significant effect on gas solubility; higher frequencies and powers result in greater degassing effects and lower concentrations of dissolved gases. The complex interaction does not end there: lower gas concentrations result in fewer cavitation nucleation sites, which in turn reduces the number of active bubbles. In particular, argon tends to produce more bubbles than oxygen due to its greater solubility. An important finding is that the ratio of argon to polyatomic gases in radical generation depends on the complex interactions of several variables.

6.4 pH

For comparable overall reaction efficiencies, the 20 kHz ultrasonication method for the degradation of estrogens performed better in acidic (pH 3) than in neutral (pH 7) or basic (pH 9) conditions [69]. Park et al. [94] found that the degradation rate of bisphenol A and 17α-ethinyl estradiol in different water matrices decreased as the pH increased from 4 to 7 and 11. Under 352 kHz/80 W ultrasonication, the degradation of propylparaben was not affected by pH values between 3 and 8, although it decreased when the pH reached 10.5 [80]. Minimal variations were observed in the degradation of 2-phenoxyethanol at 600 kHz [85] and 4-cumylphenol at 300 kHz [75] over the pH range of 2 − 10. According to Serna-Galvis et al. [79], fluoxetine was degraded more efficiently in acidic (pH 3.0) or basic (pH 11.0) environments than at a natural pH of 6.2. Compared to the initial degradation rates at natural pH, both examples showed a 45 % increase. Similar trends were found by Gultekin et al. [83] for the oxidation of 4-n-nonylphenol at 20 kHz; pH 6 performed worse, while pH 10.8 was the most efficient (better than pH 3).

Whether a contaminant is an ionic species or a molecule determines the kinetics of ultrasonic degradation at different pH values [109]. The influence of pH on the degradation rate depends mainly on the pKa value, a critical physicochemical property of ionizable organic contaminants. The protonated form of the contaminant predominates when the pH is less than the pKa, causing it to accumulate at the bubble interface (gas/liquid region). This accumulation accelerates the rate of degradation by making the contaminant more susceptible to highly reactive hydroxyl radicals. In addition, faster degradation rates under acidic conditions are caused by electrostatic attraction between oppositely charged hydrophobic chemicals and the charged bubble-water interface at lower pH values [110]. In contrast, at pH values above the pKa, the ionic form of the contaminant predominates, leading to an increase in repulsive forces and a corresponding increase in the distance of the molecules from the bubble interface. This can cause the degradation process to shift to the bulk, which has a lower concentration of hydroxyl radicals. The following equation [75] can be used to determine the ionic fraction of EDC molecules:(20) φ=11+10(pKa-pH)

Since most EDCs have pKa values between 7 and 10 (Table 2), the reactive accumulation of the protonated form of the compounds at the bubble interface is thought to be responsible for the improved performance of most EDCs in acidic environments (and even up to neutral pH). However, poorer degradation performance was observed when chemicals were ionized and subsequently accumulated in the liquid bulk. There was minimal variation in degradation rate (the molecule remained mostly protonated) for compounds without ionizable groups or when the pH range tested was lower than the pKa [85]. Therefore, it is important to keep the pH of the solution below the pKa to achieve a faster rate of EDC degradation during ultrasonication.

6.5 Initial concentration

Fu et al. [69] found that the degradation rate constant of several estrogens (in the combination of sonicated estrogens) decreased with increasing initial concentration from 1 to 10 µg/L. The kinetic pattern shown here is comparable to that of other EDCs such as propyl paraben [80], 2-phynoxyethanol [85], 4-cumylphenol [75], bisphenol A [70], fluoxetine [79], and 4-n-nonylphenol [81]. However, in each of these studies, the initial degradation rate of the target EDCs increased with increasing initial concentration and plateaued at higher doses. The concentrations studied ranged up to 100 mg/L. This broad pattern was largely attributed to increased substrate concentration at the bubble-surface interface, which is the reactive zone for EDCs. This enhanced the scavenging of •OH radicals until the substrate molecules completely reoccupied the bubble surface. Beyond this threshold, there was no further improvement in degradation rate. The H2O2 study, which showed the opposite trend of the contaminant, i.e., a steady decrease in the H2O2 production rate with increasing pollutant concentration, provided direct support for this interpretation. As a result, when the EDC content increased, a competition occurred at the bubble-solution interface between self-•OH recombination (to generate H2O2) and its reactivity with the target contaminant. Therefore, for better treatment performance, it is recommended to perform ultrasonic degradation of EDCs at lower concentrations (below the indicated upper concentration limit). The effectiveness of the ultrasonic treatment will be inhibited by the increased contaminant concentration.

6.6 Temperature

Some estrogens were found to be more susceptible to degradation at lower solution temperatures [69]. However, the degradation of 2-phenoxyethanol at 600 kHz showed no discernible effect over a temperature range of 15 − 45 °C [85]. In contrast, the degradation efficiency of 17α-ethinylestradiol at 850 kHz (10 − 30 °C) [78] and 4-cumylphenol at 300 kHz (20 − 50 °C) [75] was increased by increasing the temperature. In addition, Debabrata et al. [88] found that within the temperature range of 10 − 30 °C studied, 20 °C was the ideal temperature for the elimination of dicofol at 20 kHz. Thus, it appears that the effect of liquid temperature has a complicated behavior that depends on the particular operating conditions used.

Through macroscopic and microscopic scales, fluid temperature affects the sonolytic process. Single bubble sonochemistry has been referred to as the tiny scale. The Merouani research group provided a thorough review of this topic [111], [112]. They discovered that the ultrasonic generation of •OH radicals during the implosion of a single bubble occurs best at a liquid temperature between 20 and 30 °C. This optimal state results from the interplay between the temperature drop of the bubble (which lowers the polytropic index of the gas mixture inside the bubble) and the increase in water vapor content (which is a source of radicals), both related to the process of liquid heating [113]. You can get numerical data on both parameters with a single bubble yield in the provided reference [113]. The same research group developed a method to calculate the macroscopic scale, or the number of active bubbles, and evaluated how frequency and liquid temperature changes affected it [95], [111], [114]. They found that the number of bubbles increased monotonically with increasing liquid temperature. They attributed these results to the heating-induced decrease in the Black threshold, as well as the viscosity and surface tension of the liquid. Therefore, the influence of liquid temperature on the total activity (solution) may vary depending on the ultrasonication parameters (frequency, power, type of saturating gas, etc.):• Increased progressively as the temperature of the solution rose steadily. In this case, the number of bubbles regulates the sonochemical activity (a decrease in the yield of a single bubble does not significantly affect the number of growing bubbles).

• Display an optimum temperature. In this case, the increase in the number of bubbles controls the effect of liquid temperature below the ideal temperature, while the superposition of individual bubble yield occurs above the ideal temperature.

• steadily decreases as the solution heats up. In this case, the yield of individual bubbles could control the overall temperature effect.

In particular, the ultrasonic degradation of several EDCs was obtained for the three scenarios presented. Merouani et al. [111], [114] calculated the total generation rate of •OH in the aqueous solution (in the range of 25 − 55 °C) under specific conditions (300 and 1700 kHz) by multiplying the single bubble yield by the number of bubbles. As the liquid temperature increased from 25 to 50 °C, the results showed a steady increase in the total sonochemical activity, which supported their degrading action on a number of synthetic dyes. However, a thorough review of the literature indicates that ultrasonication can be used to induce a significant degree of degradation in the 20 − 40 °C range.

6.7 Constituents of water matrices

Torres et al. [115] found that ultrasonication at 300 kHz and 80 W significantly improved the removal of low concentration bisphenol A (0.15 µM) in natural water at pH 7.6 compared to deionized water. Na+ (9.10 mg/L), Ca2+ (486 mg/L), SO42− (1187 mg/L), Cl− (10 mg/L) and HCO3– (402 mg/L) were the elements present in the natural water composition. The cause of the first effect was not clear until Pétrier et al. [116] addressed the matter in a later paper. Their investigation highlighted the intriguing effect of bicarbonate ions (HCO3–), which is closely related to the initial concentration of bisphenol A. Of the other mineral anions (Cl−, SO42−, HPO42−), only HCO3– showed any kind of benefit, especially at low initial concentrations of bisphenol A. According to this process, the carbonate radical is formed when •OH radicals combine with bicarbonate ions (Eqs. (21), (22).(21) HCO3– + ●OH→CO3●– + H2O k = 5.8 × 106 M−1 s−1

(22) CO32– + ●OH→CO3●– + OH– k = 3.9 × 108 M−1 s−1

Because CO3●– has a longer lifetime than ●OH, it can migrate into the bulk of the solution and cause micropollutant degradation [117]. With rate constants ranging from 102 to 109 M−1 s−1, the carbonate radical interacts with both organic and inorganic molecules by electron or hydrogen transfer. As a result, the addition of bicarbonate (50 − 500 mg/L) at pH 8.3 increased the degradation rate of BPA by a factor of 3.2 and 2 at low BPA concentrations of 0.022 and 0.112 µM, respectively [115]. When elevated levels of contaminants were reached, the bicarbonate effect was completely inhibited. The accumulation of a significant amount of BPA at the bubble interface limited the availability of •OH radicals to react with HCO3−, which is why the carbonate radical formed more slowly at higher concentrations. It's important to note that carbonate ions (CO32−) showed a comparable boosting effect, with findings more intense at pH 11.1 for Rhodamine B dye [117].

For other EDCs, particularly 4-cumylphenol (5 µg/L, 300 kHz, 80 W) [75], 4-isopropylphenol (10–1000 µg/L, 278 kHz, 80 W) [118], and 4-n-nonylphenol (0.5–24.6 µM, 278 kHz, 100 W) [81], other study groups also demonstrated the previously indicated beneficial effect of bicarbonate. Since bicarbonate ions (∼400 mg/L) make up the majority of the composition of natural mineral water, the degradation of 4-cumylphenol (50 µg/L) in this water also showed an acceleration (by about 20 %) compared to deionized water. This is mainly due to CO3●–. However, in the case of fluoxetine (600 KHz, 60 W), this effect disappears due to the high initial concentration of fluoxetine (40.5 µM).

The enhancing effect of bicarbonate on the degradation of 4-isopropylphenol (4-IPP: 10 µg/L) was evaluated in the presence of 0.1 − 1.0 g/L sucrose, a hydrophilic substrate with a radical quenching effect in the bulk liquid, to verify the availability of CO3●– in the bulk liquid [118]. In the presence of sucrose, the enhancing effect of bicarbonate on the sonolytic degradation of the substrate was reduced, although it was still faster than in the absence of bicarbonate (control). Thus, by scavenging carbonate radicals in the liquid bulk, sucrose slows down the destruction by carbonate radicals that occurs in the solution bulk. Finally, the degree of sucrose scavenging increased significantly in the presence of carbonate radicals compared to hydroxyl radicals. This indicates the importance of interfacial processes in the hydroxyl radical-mediated degradation pathway of 4-IPP, while carbonate radical-mediated degradation occurs in the liquid bulk.

According to Chiha et al. [75], the sonolytic degradation of 4-cumylphenol (5 µg/L, 300 kHz, 80 W) was enhanced in the presence of bromide ions (5 g/L). The enhancement was explained by the presence of bromide-reactive species, particularly Br2●–, which are produced at the bubble interface when ●OH reacts with bromide ions (Eqs. (23), (24). Lower contaminant concentrations can potentially have a stronger bromide-enhancing effect in this case [119].(23) Br– + ●OH→Br● + OH–

(24) Br● + Br– → Br2●–

(25) Br2●– + Br2●– → Br– + Br3–

At the surface of the cavitation bubbles, Br2●– radicals undergo more restricted radical–radical recombination (reaction (25), k = 2 × 109 M−1 s−1) in contrast to ●OH (2●OH→H2O2, k = 5.5 × 109 M−1 s−1). Br2●– may therefore be more accessible than ●OH to react with 4-CyP and thus promote its degradation.

Boutemedjet et al. [80] used a range of PPB concentrations (25 − 100 mg/L) to investigate the effects of water quality, including deionized water, natural water, and seawater, on the sonolytic degradation of propylparaben (PPB) at 352 kHz and 80 W. The degradation process was shown to be enhanced at low PPB concentrations (5 mg/L) in both natural and seawater. Degradation was only slightly reduced at higher PPB concentrations (25 − 100 mg/L), especially in natural water. All tested media showed comparable degradation rates at intermediate PPB concentrations (10 mg/L). The results highlight the importance of ultrasonic EDC elimination in complex matrices, which is a unique advantage not found in many other advanced oxidation processes (AOPs). The effectiveness of the ultrasonic process was not significantly affected by the high salinity of the seawater. Park et al. [94] observed similar data for the degradation of bisphenol A and 17α-ethinyl estradiol in artificial brackish water and artificial saline water at pH 4 and pH 7.5.

7 Comparative treatments and synergies in EDCs removal

As shown in Table 4, ultrasound has been successfully used with a variety of augmentation strategies for a number of EDCs. In the following sections, many hybrid methods that have demonstrated significant advantages in the treatment of various EDCs were discussed.Table 4 Sono-hybrid systems for the destruction of EDCs.

Entry	EDC	Investigated system	Treatment’s effectiveness	Ref.	
1	4n-nonylphenol, Bisphenol A, Fluoxetine	US/Fe(II),US/Fe(III),US/Cu
(II)	Appreciable enhancement was reported, in either pollutants removal or COT reduction.	Yim et al. [84], Serna-Galvis et al. [79], Torres et al. [73], Gultekin et al. [83]	
2	Bisphenol A	US/CCl4	Significant improvement was reported in compound removal.	Guo and Feng [72]	
3	Dimethyl Phthalate	US/HSO5−,
US/S2O82−	Synergy observed.	Xu et al. [89], Papadopoulos et al. [129]	
4	Bisphenol A	US/particles (inert)	Improvement was recorded	Park et al. [94]	
5	Bisphenol A, 2-phenoxyethanol	US/UV (254 nm)	Improvement in TOC/COD was reported	Torres et al. [73],
Boutamine et al. [85]	
6	Bisphenol A	US/UV/Fe2+	Total mineralization was achieved	Torres et al. [73]	
7	Bisphenol A	US/UV/TiO2	Synergy reported	Torres et al. [126]	
8	Bisphenol A	Fe2+/US/TiO2-photocatalysis	A significant synergistic effect was retrieved	Torres et al. [127]	
9	Bisphenol A	US/O3	Synergy reported.	Guo and Feng [72]	
10	Methyl paraben	US/Electrochemical	The hybrid technique demonstrated a higher mineralization efficiency	Steter et al. [128]	

Guo and Feng [72] investigated the use of CCl4 or O3 as radical scavengers in aqueous solutions in combination with ultrasound (US: 20 kHz probe). When 25 µg/L CCl4 was added to a 100 µg/L BPA solution, the elimination of BPA increased by 15 to 20 % at different applied ultrasonication intensities (20 − 80 W/cm2). In addition, the synergistic index of the US/O3 system was determined to be 2.26. Two processes were identified as responsible for the effect of carbon tetrachloride: (i) the pyrolysis of volatile CCl4 within the acoustic bubble, producing chlorine-reactive species (HOCl, Cl2, •Cl, •CCl3,:CCl2) that can diffuse into the liquid and react with BPA, and (ii) a greater number of hydroxyl radicals were released into the BPA solution as a result of the significant scavenging of H atoms by CCl4 within the bubble. In particular, Dehane et al. [120], [121], [122] provided theoretical support for both processes. Additional evidence for the increase in active bubbles in the presence of CCl4 was provided by Dehane et al. [98], [123]. Because of these elements, US/CCl4 is a promising technology for accelerating the degradation of micropollutants, especially in industrial wastewater [124]. On the other hand, two variables have been identified as responsible for the synergistic effect of US/O3: (i) the sonolysis of O3 in the bubbles and the increased yield of ●OH generation; and (ii) the increased number of reactive bubbles due to O3 dissolution, which in turn increases the nucleation sites for cavitation. A detailed discussion of both techniques can be found in the review by Merouani and Hamdaoui [125].

The effectiveness of 300 kHz and 80 W sonolysis for BPA degradation was compared to Fenton's reagent (100 µM ferrous sulfate and continuous H2O2 injection at 119 µM/h from 35 mM stock H2O2 solution) at pH 3 in a preliminary study by Torres et al. [115]. In less than 19 min, both methods achieved complete BPA removal, produced the same major intermediates, and showed the same BPA removal rates. The most frequently proposed chemical pathways included interactions with ●OH radicals. The Fenton method was slightly more successful than ultrasonic treatment in removing BPA metabolites from deionized water, according to COD and TOC tests. On the other hand, quenching of the Fenton process was observed in naturally occurring water at pH 7.6, while the ultrasonic process continued to function normally.

Torres et al. [73] have done a lot of work to improve the efficacy of BPA ultrasound treatment by combining ultrasound with Fe2+ (100 µM) and/or UV light (254 nm). This led to the creation of three treatment combinations: US/UV, US/Fe2+ and US/UV/Fe2+. Hydrogen peroxide dissociation was enhanced by both UV and Fe2+ (Eqs. (26), (27), resulting in more •OH radicals and a more significant decrease in COD and TOC removal.(26) H2O2→UV2•OH

(27) Fe2+ + H2O2 → Fe3+ + ●OH + OH–

In fact, ultrasonic treatment of BPA at 300 kHz and 80 W with pH 3 and O2 saturation completely removed BPA (118 µM) in only 90 min, but only 15 % of TOC and 50 % of COD in 500 min [73]. The addition of UV (254 nm) or 100 µM FeSO4 to the US treatment had no significant effect on BPA removal, but steadily increased the reduction of COD and TOC. 20 % for UV, 50 % for US, ∼60 % for US/UV, ∼70 % for US/Fe2+, and 98 % for US/UV/Fe2+ were removed from the sample after 200 min. 0 % for UV, 5 % for US, ∼32 % for US/UV, ∼25 % for US/Fe2+, and 100 % for US/UV/Fe2+ were the corresponding TOC reductions. These results indicate that US/UV, US/Fe2+, and US/UV/Fe2+ are all energetic mechanisms for destroying BPA under the conditions used. Due to their hydrophilic character, sonolytic products of BPA prefer to remain in the bulk solution, away from the reactive interfacial zone where the hydroxyl radical concentration is higher. When US treatment is combined with UV and/or Fe2+, the catalytic and dissociative effects of Fe(II) and 254 nm UV, respectively, on the continuously produced H2O2 (4.8 µM/min) from the acoustic bubble could result in a significant increase in the bulk concentration of hydroxyl radicals. As a result, the hydrophilic BPA by-products may be directly exposed to a greater flow of •OH, which would cause them to mineralize in addition to degradation.

When Fe2+ (90 µM) was added, Serna-Galvis et al. [79] saw an identically enhanced effect on the sonolytic elimination of fluoxetine (40.5 μM) at 600 kHz and 60 W. In the presence of Fe2+ ions, the t1/2 value decreased from 30 min to only 10 min. Interestingly, the initial rate of hydrogen peroxide formation decreased to 0.33 μM/min in the presence of ferrous ions, while the rate in the absence of the ions was 1.29 μM/min. This suggests that an in-situ Fenton reaction between the sonochemically produced hydrogen peroxide and ferrous ions is responsible for the beneficial effect of ferrous ions (Equation (27). Under these conditions, more hydroxyl radicals are generated in the bulk of the solution, which accelerates the degradation of fluoxetine.

Yim et al. [84] investigated the effect of adding Fe(II) and Fe(III) (100 µM) on the degradation of 4n-nonylphenol (5.4 mg/L) at 200 kHz and 6 W/cm2 in an argon and oxygen environment. After 30 min, the TOC data showed a reduction of 27 − 30 % with ultrasound treatment alone, 42 % and 50 % with argon and O2, and 60 % and 50 % with argon and O2 on the US/Fe(III) system. The reduction of Fe(III) to Fe(II) by H2O2 and the acoustically generated HO2● (O+O2 → HO2●, which is more pronounced in an O2 atmosphere [57]) were the main causes of the greater enhancement of sonolytic mineralization of nonylphenol by Fe(III).(28) Fe(III) + H2O2 → Fe(II)–OOH2+ + H+

(29) Fe(II)–OOH2+ → Fe(II) + HO2●

(30) Fe(III) + HO2● → Fe(II) + O2 + H+

A similar procedure was proposed by Gultekin et al. [83] using Cu(II) (10 µM) instead of Fe(II). Compared to ultrasound alone, the degradation of nonylphenol (100 µM) was more than doubled at 20 kHz and 0.46 W/cm. Cu2+ and Fe2+ can both activate H2O2 at acidic pH (Eqs. (31), (32), (33), (34). However, the US/Cu2+ system showed no further increase upon addition of external H2O2, most likely due to H2O2 quenching of the hydroxyl radical (Eq. (34) [83].(31) Cu2+ + H2O2 → Cu+ + HO2●+ H+

(32) Cu+ + H2O2 → Cu2+ + ●OH+OH–

(33) Cu2+ + HO2●→ Cu+ + O2 + H+

(34) ●OH+H2O2 → HO2● + H2O

Dimethyl phthalate (DMP) was degraded by Xu et al. [89] using a synergistic method combining ultrasound and peroxymonosulfate (HSO5−) at different ultrasonication settings. At HSO5− concentration of 2 mM, the combined treatment showed synergy, eliminating more than 70 % of DMP (10 µM) in 30 min of ultrasonication at frequencies of 200 and 400 kHz. However, US/HSO5− treatment at 20 kHz was unsuccessful and may need to be administered for a longer period of time, which would increase the cost of treatment. According to the scheme below [89], the observed increase was attributed to the ultrasonic activation of HSO5−, which resulted in the formation of sulfate radicals and enhanced hydroxyl radical production.(35) HSO5-→USSO4•-+•OH

(36) SO4●– + H2O→SO42– + ●OH+H+

(37) ●OH+HSO5− → SO5●– + H2O

(38) SO4●– + HSO5− → HSO5– + SO5●–

In the study by Park et al. [94], inert glass beads (0.1 mm, 25 g) were added to aqueous samples containing bisphenol A or 17α-ethinylestradiol. The samples were then exposed to ultrasonication at frequencies of 28 and 580 kHz in sono-reactors at pH 7.5. The degradation rates of BPA and 17α-ethinylestradiol were positively affected by the presence of glass beads, with k = 0.018 − 0.107 min−1 without beads and 0.052 − 0.142 min−1 with beads; 0.021 − 0.111 min−1 without beads and 0.054 − 0.136 min−1 with beads. This improvement was attributed to the glass beads acting as additional cavitation nucleation sites, which increased the cavitation bubble flow and enhanced the pollutant degradation efficiency.

Boutamine et al. [85] used a combination of UV 254 nm irradiation and ultrasound (600 kHz, 120 W) to destroy 10 mg/L 2-phenoxyethanol (PhE) in aerated solutions at 25 °C at natural pH. The study showed that there was no significant difference in PhE removal between the separate processes and the combined treatment. However, after 2 h of treatment, there was a significant improvement in the oxidation of organic matter, with COD reduction reaching 94 % for US/UV, 76 % for UV alone, and 65 % for ultrasound alone. No synergistic effect was observed, although the combined procedure removed COD more effectively.

In a batch loop system coupling a 300 kHz and 80 W standing wave ultrasonic reactor with a photochemical reactor consisting of a series of three Pyrex glass vessels (50 mL each), Torres et al. [126] investigated the synergy of ultrasound and photocatalysis. A CPS Suntest solar lamp system with an irradiance of 830 W/m2 (0.5 % of photons shorter than 300 nm and about 7 % between 300 and 400 nm) was used to externally illuminate the photochemical reactor. Solutions were recirculated from the sonoreactor to the photochemical reactors at a flow rate of 230 mL/min using a peristaltic pump. Although each isolated system produced the same BPA byproducts, the two processes worked well together: ultrasonication showed greater efficacy in removing the target contaminant, but the photocatalysis showed superior efficiency in achieving mineralization. For BPA mineralization, the combined system showed an interesting synergistic effect that depended on the TiO2 loading. The most synergistic effect was observed at low catalyst loading. After 4 h of combined treatment with 0.05 g/L TiO2, 62 % of the COD was removed. However, only 6 % and 12 % of COD were removed by photocatalysis and ultrasound alone, respectively. US/photocatalysis or photocatalysis removed 68 % and 50 % of the COD using 1 g/L catalyst, respectively. The inhibitory effect of the catalyst nanoparticles on cavitation activity was identified as the cause of the limited synergistic effect at this catalyst loading.

Using the same batch loop approach as in their previous work [126], Torres et al. [127] investigated a ternary system, Fe2+/US/TiO2 photocatalysis, with the goal of improving the mineralization of BPA. For both the removal and mineralization of BPA at pH 3, the effects of different amounts of Fe2+ (0.56 and 5.6 mg/L) and TiO2 (10 and 50 mg/L) were investigated. Even at modest catalyst loadings, a remarkable synergistic effect was observed, resulting in rapid and complete removal of COD. After 4 h, nearly 93 % was achieved with 10 mg/L TiO2 and 5.6 mg/L Fe2+, respectively. In contrast, only 5.0 %, 6.0 %, and 22 % of COD were removed by UV/TiO2, ultrasound, and photo-Fenton treatments, respectively, in the same time period. The removal of the initial substrate and the supply of H2O2 to the photocatalytic devices were largely enabled by ultrasound. Meanwhile, the intermediates were mostly converted to CO2 and H2O by photo-Fenton and photocatalysis.

The sonoelectrochemical degradation of methyl paraben (100 mg/L in 0.05 M K2SO4 at pH 5.7, boron-doped diamond anode, horn: 20 kHz and 523 W/cm2) was investigated by Steter et al. [128]. Constant current densities of 10.80 and 21.60 mA/cm2 were used for electrolysis at 25, 30 and 35 °C. Compared to the electrochemical approach, the hybrid method showed a higher mineralization efficiency (about 60 %) with about 50 % TOC removal, demonstrating a synergistic treatment. At a lower applied current density of 1.8 mA/cm2, the degree of sonoelectrochemical mineralization increased (34 − 52 %) with increasing temperature (20 − 35 °C).

The ultrasonic degradation of ethyl paraben (EP) was investigated in the work of Papadopoulos et al. [129]. The studies were conducted under variable conditions, including EP concentration (250 − 1250 µg/L), ultrasound density (20 − 60 W/L), reaction time (up to 120 min), and initial pH (3 − 8), at a fixed ultrasound frequency of 20 kHz and liquid bulk temperature of 30 °C. The studies, conducted in secondary treated wastewater and ultrapure water, showed efficient degradation with higher ratios up to 98 % achieved by using different combination systems.

8 Future challenges and prospects

One of the current challenges is the toxicity of the treated wastewater. It is unacceptable for the effluent to be more or equally hazardous as the original substrate. The temporal evolution of toxicity during the ultrasonic degradation of benzophenone-3 was studied by Vega and Penuela [87]. 30 % of BP-3 degradation was followed by an increase in the toxicity profile. The breakdown products of many EDCs can be more harmful than the parent material. Because these by-products are primarily hydrophobic and may not be near the reactive bubble interface to react with ●OH, this problem persists, especially as treatment time is increased. To achieve higher levels of mineralization, it is usually necessary to combine ultrasonic treatment with other methods to ensure elimination of residual toxicity. Section 7 reviews and discusses a number of situations with encouraging results. For example, in the case of bisphenol A, 100 % or close to 100 % mineralization was achieved by combining ultrasonic treatment with either UV 254 and/or iron salt or photocatalysis [72], [73], [126], [127].

Another difficulty is the economics of ultrasonic treatment. Diffraction and other processes cause more than half of the energy applied to the transducer to be wasted, and less of the remaining 50 % is used to produce active cavitation. In Pétrier's economic study of bisphenol A removal [73], the ultrasonic approach was found to be the least expensive of all combinations. The electrical energy per order of contaminant removal (EE/O) for various systems (we only considered processes that produced more than 60 % of the TOC) were calculated. The EE/O values for US/Fe(II), US/UV, and US/Fe(II)/UV are 6010, 3735, and 1033 kWh/m, respectively. Compared to the US/UV and US/Fe(II) treatments, the EE/O of the ternary system, 1033 kWh/m, is 4 and 6 times lower, respectively. Thus, the US/UV/Fe(II) system provides the most economical method for BPA mineralization of all the processes investigated in this study.

Only a few studies have shown pilot scale applications for the ultrasonic approach; otherwise, it has only been used at the laboratory scale (smaller volumes). Since it is difficult to scale up significant ultrasonic processes, no true industrial scale has been used. In such a design, a uniform distribution of the cavitation activity zone is required. On the other hand, if the treated volume is increased, the ultrasonic wave may be attenuated and the sonochemical reactive zone may contract. Many studies have investigated the use of a multi-transducer technique, but its use is also associated with higher costs [130], [131], [132]. Therefore, further knowledge of the physics and chemistry of cavitation in large-scale ultrasonic reactors is needed.

In order to improve the efficacy and sustainability of the treatment, future studies on the application of sonochemical techniques for the elimination of EDCs should focus on the following important areas:• With respect to real wastewater, a review of the scientific literature indicates that most attention has been paid to low concentrations of EDCs in artificial samples. However, wastewater containing EDCs is usually highly concentrated and contains a variety of harmful contaminants. For a reliable assessment of the effectiveness of the removal process, natural water samples must be used. This allows studies to be conducted in environments that are as close as possible to real conditions and makes it easier to think about process scale-up.

• Synergistic Treatment Methods: Sonochemistry can be used with other (AOPs) or biological remediation techniques to explore and create more synergistic treatment approaches. This can improve removal efficiencies in general and address specific problems associated with particular EDCs.

• Mechanistic Understanding: Expand your knowledge of the fundamental processes underlying the ultrasonic degradation of specific EDCs. Examine reaction pathways, identify key reactive species, and clarify how elements of the water matrix affect the process. This information will help to develop targeted and effective treatment plans.

• Real system application and scale-up: By focusing on the scale-up of sonochemical reactors, you can bridge the gap between laboratory-scale research and practical applications. Investigate the performance and practicality of ultrasonic processes in different water matrices and at higher volumes to determine their economic and practical utility.

• To analyze the potential negative impact of ultrasonic treatment on aquatic ecosystems, thorough toxicity studies will be conducted to determine how the treatment affects the formation of by-products. This will ensure that the entire treatment process produces environmentally benign products in addition to EDC removal.

• Energy efficiency and cost effectiveness: Develop plans to make ultrasonic processes more energy efficient and evaluate their cost effectiveness compared to other advanced water treatment technologies. This includes investigating novel reactor configurations, alternative energy sources, and environmentally friendly materials for ultrasonic applications.

• Continuous flow systems: Investigate the feasibility of continuous-flow ultrasonic systems for the continuous and sustained removal of EDCs. This includes the construction and improvement of sonochemical reactors that can handle varying flow rates and continue to treat efficiently for extended periods of time.

• Extend modeling studies for a complete understanding: Modeling studies can be extended to quantify additional operating characteristics beyond the pressure field distribution. It is possible to accurately quantify variables such as mass transfer coefficient, temperature distribution, and fluid flow. These factors are essential for controlling the dynamics of the entire process, and it is especially important to quantify them specifically for applications such as EDC removal.

• Application to Emerging Contaminants: Use sonochemical methods to treat emerging contaminants in addition to traditional EDCs. Investigate how well sonochemistry works to remove personal care products, pharmaceuticals, and other emerging contaminants while accounting for their unique chemical properties.

Future research can help ultrasonic processes become more effective and sustainable solutions for removing emerging contaminants and EDCs from water sources by addressing these study topics.

9 Conclusion

This review has provided a thorough examination of the use of ultrasound to accelerate the degradation of EDCs. The relevance of EDCs as newly discovered environmental toxins that threaten human health and aquatic life was explained at the beginning of the discussion. After reviewing the basics of sonochemistry and highlighting the unique ability of ultrasound to catalyze chemical reactions, a detailed examination of the variables that affect ultrasonic degradation, such as temperature, power, frequency, and solution chemistry, was conducted.

The review included case studies highlighting the effectiveness of ultrasonics in degrading specific EDCs. The effects of various water matrix elements and water quality on ultrasonic degradation were examined, and the possibility of greatly increasing removal efficiency through the use of synergistic techniques, such as hybrid ultrasonic systems, was explored.

The report also suggests future research opportunities to address current issues such as the need for continuous flow reactors and the treatment of actual effluents containing high levels of EDCs. By consolidating recent advances in ultrasound-mediated degradation of EDCs, this study advances the understanding of sonochemistry as a viable and long-term method for water treatment. The wide range of applications, from medicine and food science to purification and contaminant remediation, highlights the adaptability and promise of sonochemistry in addressing today's environmental challenges. Future developments in ultrasonic process research and innovation will be essential to improve water treatment technologies and mitigate the effects of EDC contamination.

CRediT authorship contribution statement

Slimane Merouani: Conceptualization, Methodology, Formal analysis, Project administration, Supervision, Visualization, Writing – original draft, Writing – review & editing. Aissa Dehane: Conceptualization, Methodology, Formal analysis, Visualization, Writing – review & editing. Oualid Hamdaoui: Conceptualization, Methodology, Formal analysis, Supervision, Funding acquisition, Visualization, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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