
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12834-4
10.1016/j.heliyon.2024.e36803
e36803
Research Article
Efficient electrochemical removal of ammoniacal nitrogen from livestock wastewater: The role of the electrode material
Galoppo Simona a
Fenti Angelo angelo.fenti@unicampania.it
a⁎
Falco Giovanni b
Huang Qingguo c
Chianese Simeone a
Musmarra Dino a
Iovino Pasquale b
a Department of Engineering, University of Campania “Luigi Vanvitelli”, Via Roma 29, 81031, Aversa, Italy
b Department of Environmental, Biological and Pharmaceutical Science and Technologies, University of Campania “Luigi Vanvitelli”, Via Vivaldi 43, 81100, Caserta, Italy
c College of Agricultural and Environmental Sciences, University of Georgia, Griffin, GA, United States
⁎ Corresponding author. angelo.fenti@unicampania.it
23 8 2024
15 9 2024
23 8 2024
10 17 e368035 2 2024
29 7 2024
22 8 2024
© 2024 The Authors. Published by Elsevier Ltd.
2024

https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Wastewater from livestock farms contains high concentrations of suspended solids, organic contaminants, and nitrogen compounds, such as ammoniacal nitrogen. Discharging livestock effluents into water bodies without appropriate treatment leads to severe environmental pollution. Compared to conventional treatment methods, electrochemical oxidation exhibits higher nitrogen removal efficiencies. In the present work, the electrochemical removal of ammoniacal nitrogen from real livestock wastewater was investigated through a lab-scale reactor. Preliminary experiments were carried out to investigate the effects of different anode materials, including boron-doped diamond and iridium/ruthenium-coated titanium, on the total nitrogen removal efficiency using synthetic wastewater. Boron-doped diamond, a well-known non-active electrode, allowed to obtain 63.7 ± 1.21 % of total nitrogen degradation efficiency. However, the iridium/ruthenium-coated titanium electrode, belonging to the class of active anodes, showed a higher performance, achieving 78.8 ± 0.76 % contaminant degradation. Coupling iridium/ruthenium-coated titanium anode with a stainless-steel cathode improved the performance of the system, achieving even 96.2 ± 2.73 % of total nitrogen removal. The optimized cell configuration was used to treat livestock wastewater, resulting in the degradation of 67.0 ± 2.25 % of total nitrogen and 37.3 ± 0.68 % of total organic carbon when sodium chloride was added. At the end of the process, the ammonium content was completely removed, and only 17.7 ± 0.51 % of the initial nitrogen turned into nitrate. The results show that the proposed system is a promising approach to treating livestock wastewater by coupling high contaminant removal efficiencies with low operational costs. Anyway, further studies on process optimization with an emphasis on power requirements and electrode costs need to be carried out.

Graphical abstract

Image 1

Highlights

• Electrochemical oxidation is effective for nitrogen removal from livestock wastewater.

• Ti/Ru-Ir active anode shows the highest nitrogen compound degradation.

• Chlorine reactive species are mainly involved in the degradation mechanism.

• The use of a stainless-steel cathode enhances the removal efficiency.

• 100 % removal of NH4+-NH3 was achieved at the end of the treatment.

Keywords

Livestock wastewater treatment
Ammoniacal nitrogen pollution
Electrochemical oxidation
Electrode material comparison
Chlorine reactive species
Cost assessment
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pmc1 Introduction

In the last decades, the continued industrialization and the massive growth of the global population have drastically increased food requirements. Consequently, the worldwide diet structure has progressively changed from plant-based to animal-based [1]. Intensive livestock farming has rapidly increased to address this high food demand, enhancing the production of dairy and meat goods. Along with the growth of these systems, the generation of large volumes of livestock wastewater has become a prominent environmental problem [[2], [3], [4]].

Livestock wastewater contains high contents of nutrients, suspended solids, pathogenic organisms, and refractory contaminants, including veterinary drugs, heavy metals, and nitrogen compounds, among others [[5], [6], [7], [8]]. The discharge of livestock wastewater into water bodies without effective treatments can cause negative consequences for human health and ecosystems. Ammoniacal nitrogen (NH4+-NH3) is a major livestock wastewater nitrogen form that cannot be degraded by traditional treatments; thus, the development of advanced removal processes is essential for further water purification [9,10]. The excess nitrogen compounds leach into surface water and groundwater, stimulating the primary production of plankton communities and causing eutrophication [11,12]. High concentrations of nitrogen compounds impact human health, increasing the risk of blood methemoglobinemia, recognized as “blue baby syndrome”, cancer, Alzheimer's, and diabetes [[13], [14], [15]]. Moreover, NH4+-NH3 can be converted into nitrite and nitrate by nitrification reactions, involving further toxic effects. Some countries have already issued standard values for NH4+-NH3 in water [16,17]. The European Environment Agency (EEA) defined “highly critical2” the impacts of nitrogen from agricultural activities and provided two specific Key Type Measures (KTMs) to address the problem, which are “Reduction of agricultural nutrient pollution (KTM2)” and “Upgrading or improvement of industrial wastewater treatment plants, including farms (KTM16)” [18].

Various technologies have been applied to remove different nitrogenous forms from wastewater, including biological, physical and chemical methods, such as biodegradation [19], ammonium oxidizing (anammox) bacteria-mediated nitrogen removal [20], reverse osmosis [21], ion exchange [22], adsorption [23], chemical precipitation, break-point chlorination and ammonia stripping [24,25]. They have shown promising removal percentages of NH4+-NH3 from impacted water, but some limitations and drawbacks remain. For instance, biological treatments require high energy and economic costs, ion exchange and adsorption are affected by the capacity of the adsorbent and pH, chemical precipitation can produce new pollutants, break-point chlorination requires high chlorine concentration under a high amount of organic matter [24,26,27].

Among the advanced oxidation processes (AOPs), electrochemical oxidation (EO) has recently gained much attention to degrade NH4+-NH3 and other contaminants from water because of less sludge production, simple operation and high efficiency [[27], [28], [29], [30], [31]]. The NH4+-NH3 can be oxidized by two main pathways: (i) direct EO based on direct electron transfer (DET) between the anode and the molecules of contaminants adsorbed on the anode surface or (ii) indirect EO via adsorbed hydroxyl radicals (•OH), and other oxidasing mediators (Cl2, HOCl, OCl−, S2O82−, H2O2) in-situ electrogenerated [3,[32], [33], [34]].

EO efficiency is often related to the electrode materials used for treatment, especially the anode. In general, the anode materials are classified into “active” and “non-active” groups based on the bond type established with •OH [35]. Active anodes can partially oxidise pollutants; conversely, non-active anodes can provide complete contaminant mineralization [36].

Therefore, the choice of electrode material influences the efficiency and selectivity of the EO process, the potential generation of intermediates [28,37], as well as the overall costs of the treatment. Thus, identifying cost-effective electrode types capable of degrading NH4+-NH3 is becoming crucial. Suitable materials must have high oxygen evolution potential (OEP), stability, long life, cost-effectiveness, and the ability to generate chlorine reactive species (CRS). Previous works have reported the effectiveness of EO in degrading NH4+-NH3 using various anode materials, including noble metals, mixed metal oxide, graphite, and BDD, among others [38,39]. He et al. [40] studied the electrochemical oxidation of NH4+-NH3 using dimensionally stable anodes (DSA) belonging to the class of active anodes, achieving 95 % pollutant removal in 60 min of treatment. Recently, DSA electrodes such as Ti/Ru-Ir, Ti/RuO2, Ti/IrO2, and Ti/IrO2–Ta2O5–Pt have gained interest due to their stability [3,40]. This group of electrodes showed higher abilities to effectively generate CRS than others (such as graphite and non-active materials) [38,41]. Among them, the Ti/Ru-Ir group is preferred over platinum-based materials due to its lower cost [42]. Regarding non-active electrodes, Cabeza et al. [43] completely removed NH4+-NH3 from landfill leachate after 360 min of processing using a BDD anode, but almost half of the initial nitrogen load was oxidized to nitrate. Although other non-active anodes, such as those based on SnO2, PbO2, or SbO2, showed high OEP and current efficiency for •OH generation, they also exhibited poor conductivity, stability, and toxicity due to the presence of Sb, Pb, or Sb [42,44]. On the other hand, BDD showcased several remarkable characteristics, such as a wide operational potential range, a low voltammetric background current, significant overpotential for oxygen and hydrogen evolution, corrosion resistance, and long service life. Additionally, diamond films exhibit remarkably uniform behaviour over prolonged periods, even in harsh environments like fluoride solutions. These favourable attributes position BDD as an excellent choice for numerous emerging applications, including the electrochemical treatment of wastewater containing high levels of NH4+-NH3 [33,45,46]. Among the disadvantages of BDD electrodes is their high cost, which could limit their use [[47], [48], [49]].

According to the Council Directive 91/676/EEC (Nitrates Directive) [50] regarding the protection of water sources against nitrogen contamination from agricultural activities, the maximum limit of nitrogen pollution from livestock effluents is 170 kg of N hectare−1 year−1 in vulnerable zones. Campania Region (southern Italy) has been identified as a vulnerable zone strongly impacted by nitrogen water pollution due to almost two hundred thousand head of cattle [51] spread across roughly ten thousand livestock farming activities [52,53]. For this reason, the development of innovative and effective removal treatments of nitrogen compounds from water is urgently needed.

This study investigated the EO of nitrogen compounds in water by testing different anodes, such as BDD and Ti/Ir-Ru anodes. Then, a stainless-steel cathode was used to evaluate the impact of different cathode materials on the treatment. Although some previous studies have already examined the EO of ammoniacal nitrogen and other nitrogen species from livestock wastewater using different types of electrodes, to our knowledge, none of them have performed experimental tests to compare the degradation performance between active and non-active anodes. Furthermore, contrary to other studies, since the cost of the electrodes is one of the major factors influencing the total cost of the electrochemical process, a further effort was carried out to identify a less expensive but equally effective cathode material [[54], [55], [56]]. The best cell configuration, in terms of electrode material, was applied to remove nitrogen compounds from real livestock wastewater from a farm in the Campania Region. The experimental plan was developed to confirm or deny our formulated hypotheses: i) the electrochemical oxidation of ammoniacal nitrogen occurs mainly through an indirect oxidation mechanism via electrogenerated in-situ chlorine reactive species (CRS) in acidic conditions; ii) active anodes, such as Ti/Ru-Ir, result in higher efficiencies in the degradation of ammoniacal nitrogen; iii) the presence of CRS triggers some undesirable side reactions, such as the formation of chlorate and perchlorate, which pose health risks and environmental concerns.

2 Materials and methods

2.1 Reagents

Ammonium chloride (NH4Cl) and benzoic acid (BA) were provided by Sigma-Aldrich (St. Louis, MO, USA). Sodium chloride (NaCl) and sodium perchlorate (NaClO4) were used as received from various chemical suppliers. All solutions were prepared using Milli-Q deionized water (18.2 MΩ cm resistivity, 25 °C) from an Elix ® Essential 10 UV water purification system (Merck, Darmstadt, Germany).

2.2 Livestock wastewater (LWW)

The LWW used in this study was kindly provided by a livestock farm located in the city of Villa di Briano (Fig. S1, Caserta, Campania, southern Italy – supplementary material) in the middle of a nitrate-vulnerable zone [52]. The sample was collected in an HDPE bottle refrigerated at 4 °C after a travel time of 30 min. Table S1 (supplementary material) shows an initial physicochemical analysis of the LWW sample performed after a filtration step on 45 μm Whatman filter paper. The Hanna Instrument HI99301 EC meter, equipped with a HI 76306 sensor, was used for measuring conductivity. TSS and moisture were analyzed according to IRSA-CNR 2090, 1984 [57].

2.3 Electrochemical oxidation experiments

EO tests were performed in a lab-scale batch reactor of 0.5 L, treating 250 mL of an NH4Cl solution with an initial concentration of 50 mg L−1, equal to an initial total nitrogen (TN) concentration of 15 mg L−1. The baseline experimental conditions optimized by Iovino et al. [33] in a previous work have been applied to this experimental set. The experiments were carried out at room temperature with a 250 mg L−1 NaCl solution at a constant current density of 30 A m−2. Each of the experimental tests was conducted in triplicate.

The effect of the electrode material on the removal of NH4+-NH3 was investigated by varying the electrode type, as reported in Table 1. The experiments initially focused on comparing the TN degradation performance of two different commercial electrode materials: (1) Ti/Ru-Ir (Chemical Newtech, Italy) (2) BDD (Neocoat, Switzerland). This initial cell configuration allowed for identifying the most effective anode in degrading ammoniacal nitrogen since the EO of the contaminant relies on an indirect oxidation mechanism that involves electrogenerated reactive species on the anode surface [58]. Then, the effect of a different commercial cathode material on the degradation process was examined using a commercial SSL plate (provided by a local supplier) as a cathode (run 3).Table 1 EO experimental plan.

Table 1Experimental run	Anode	Cathode	
1	Ti/Ru-Ir	Ti/Ru-Ir	
2	BDD	BDD	
3	Ti/Ru-Ir	SSL	

A previous study by Salvestrini et al. [59] reports the schematization of the electrochemical reactor. The electrodes (gap equal to 1 cm) were connected to a DC power supply BPS-305 (Lavolta, London, UK), allowing it to operate in amperostatic conditions. The pH of the solution was continuously monitored using a HI 5221 Hanna Instruments pH meter, equipped with a HI1131 glass body electrode and a HI7662-W temperature probe. The best operative conditions obtained from the experimental set were used to carry out the electrochemical oxidation treatment of the LWW sample.

2.4 Estimation of the concentration of hydroxyl radicals

Specific experiments were conducted to estimate the concentration •OH produced by each anode tested in the study (BDD and Ti/Ru-Ir). The tests were carried out with 5 mg L−1 of BA in 250 mL of milliQ, using NaClO4 0.25 M as an inert electrolyte, and applying a current density of 30 A m−2 for a treatment time of 180 min. At selected interval times, BA solution samples were withdrawn from the system and analyzed by high-performance liquid chromatography (HPLC, Shimadzu, model CBM-20A) equipped with a UV diode array detector (DAD). A C-18 column (150 mm–2.1 mm, 5 μm) was used as a stationary phase. The mobile phase consisted of acetonitrile and Milli-Q water at a 70:30 (v/v) ratio, with a flow rate of 1.0 mL min−1. The analyses were carried out under isocratic conditions at room temperature, with an injection volume of 20 μL. Quantitative BA analyses were performed using an absorbance–concentration calibration curve at 220 nm, which showed linearity in the 0.5–10.0 mg L−1 range.

2.5 Chemical analysis

The initial solution and the samples at selected times during the EO treatments were analyzed by the TN module of a TOC-L CSH/CSN analyzer (Shimadzu, Tokyo, Japan) equipped with a chemiluminescence gas analyzer for TN detection. The assessment of EO performance was carried out in terms of TN removal %:(1) TNremoval(%)=TN(t=0)−TN(t)TN(t=0)

Since the LWW sample included a high initial amount of total organic carbon (TOC), the evaluation of EO efficiency in this case was also estimated in terms of TOC removal %:(2) TOCremoval(%)=TOC(t=0)−TOC(t)TOC(t=0)

The TOC measurement was performed using the TOC module of the TOC-L CSH/CSN analyzer (Shimadzu, Tokyo, Japan) equipped with a Non-Dispersive Infrared Sensor (NDIR) for CO2 detection. In addition, ammonium (NH4+), nitrite (NO2−), nitrate (NO3−), chloride (Cl−), chlorate (ClO3−) and perchlorate (ClO4−) measurements were performed by an Ion Chromatograph 930 Compact IC Flex (Metrohm, Origgio, Italy), equipped with an IC Column Metrosep A Supp 5–250/4.0 for the anionic analysis, and an IC Column Metrosep C4 – 250/4.0 for the cationic analysis, respectively. The eluent solutions consisted of HNO3 2.5 mM/C2H2O4 0.5 mM for the cationic analysis and Na2CO3 3.2 mM/NaHCO3 1.0 mM for the anionic analysis, respectively.

All analyses were conducted in triplicate, and statistical data comprised mean and standard deviation calculated for each measured value.

Measures for quality assurance and control were implemented to prevent sample contamination. Prior to usage, all laboratory glassware and equipment (including glass beakers, spatulas, vials etc.) underwent meticulous rinsing with milliQ water. The procedures were conducted within a laminar flow cabinet to minimize the risk of contamination. To ensure the absence of contamination in the systems, a total of n = 10 instrumental blanks (milliQ water) were analyzed prior to chemical analyses. Additionally, procedural blanks (n = 3), each consisting of 250 mL of milliQ water, were treated under the same operational conditions as the real samples. This process was performed to confirm the absence of cross-contamination, release of interferences by electrodes, background response, or potential system contamination. Before further data processing, all data were subjected to blank subtraction.

Finally, the electric energy cost (EEC), based on theoretical electric energy consumption (Ec), to assess the energy requirements of the EO treatment of LWW, was calculated by the following equation:(3) EEC[€m3]=Ec*EnergyUnitcost

where the energy unit cost was considered equal to 0.2 € kWh [60], and the Ec was assessed with the following equation:(4) Ec[kWhm3]=PtV

where P is the applied power at the electrochemical cell [kW], t is the treatment time [h], and V is the treated water volume [m3] [61].

3 Results and discussion

3.1 Electrode material effect

3.1.1 Estimation of the concentration of hydroxyl radicals

A preliminary investigation on •OH production for each anode tested, BDD and Ti/Ru-Ir, was carried out using BA, which is a well-known recalcitrant compound to direct oxidation [62] (i.e. its degradation is to be attributed only to the generation of •OH, in the absence of other oxidants species [63,64]. The results of this experiment are shown in Fig. 1.Fig. 1 Effect of the anode material on the BA degradation through BDD (red points), Ti/Ru-Ir (black points); treatment time = 180 min; [BA]0 = 5 mg L−1; electrolyte = NaClO4 0.25 M; applied voltage = 3.3 V (Ti/Ru-Ir) and 3.6 V (BDD).

Fig. 1

No BA degradation occurred using Ti/Ru-Ir as anode material under the adopted operative conditions, implying that no •OH are generated during the process. Conversely, 99.2 ± 0.23 % of BA degradation was encountered using BDD after 180 min of treatment, respectively. The results are reasonable since Ti/Ru-Ir materials belong to the class of active anodes [65]. On the other hand, BDD non-active anodes are well-known for their high production of physiosorbed •OH, which allows higher contaminant degradation rates [35,36,64]. According to Wang et al. [66], an estimation of the steady-state hydroxyl radical concentration ([•OH]ss) for each anode type during the process was performed by using the following equations (5), (6)):(5) −d[BA]dt=kobs,BA=k•OH,BA[BA][•OH]ss

(6) [•OH]ss=kobs,BAk•OHBA

The second-order rate constant for •OH reacting with BA (k•OH,BA) is 4.3*109 M−1s−1 [64]. The pseudo-first-order rate constants of BA (kobs,BA) were measured by BA degradation experiments reported in Fig. 1. The steady-state hydroxyl radical concentration can be calculated via Eq. (6). The findings indicated that the [•OH]ss were 5.35 × 10−12 M and 1.39 × 10−14 M, operating with BDD and Ti/Ru-Ir as anodes, respectively, confirming the results reported in a recent study carried out by Dwhale et al. [64].

3.1.2 Anode material effect on TN removal

TN removal through the two electrodes was evaluated by performing EO experiments on an NH4Cl solution containing an initial TN concentration of 15 mg L−1 (experimental runs 1–3, Table 1) in the presence of 250 mg L−1 of NaCl as electrolyte. The results of this experimental set are displayed in Fig. 2.Fig. 2 Effect of different electrode materials on the TN removal %: NaCl = 250 mg L−1, J = 30 A m−2, [TN]0 = 15 mg L−1, initial pH = 6.4; conductivity (mS/cm) = 0.96; applied voltage = 19.2 V (Ti/Ru-Ir) and 20.7 V (BDD).

Fig. 2

After 3 h of treatment, TN removals of 78.8 ± 0.76 % and 63.7 ± 1.21 % were obtained utilizing Ti/Ru-Ir and BDD, respectively. Since Ti/Ru-Ir anode was not capable of generating •OH (section 3.1.1), the TN removal is likely attributed to other reactive species generated on the electrode surface [67]. When NaCl is used as an electrolyte, the generation of chlorine reactive species (CRS), HOCl, OCl−, Cl2, on the anode surface is accomplished [33,58,68,69]:(7) 2 Cl− → Cl2 + 2 e− E = −1.36 V

(8) Cl2 + H2O → HOCl + H+ + Cl−

(9) HOCl → OCl− + H+

Although the EO mechanism of NH4+-NH3 is complex, it can be generally mediated by both direct and indirect EO [70]. When the solution pH is higher than 9.25, the direct EO constitutes the prevalent pathway of NH4+-NH3 oxidation [71]. Conversely, under acidic pH conditions, the indirect EO prevails, involving the action of free radicals like •OH, ClO•, HOCl, and other CRS generated during the electrochemical process. However, CRS exhibited superior oxidation capacity and selectivity for NH4+-NH3 compared to other oxidizing agents, including •OH, making them the main species responsible for the oxidation of ammoniacal nitrogen [72,73]. Jiang et al. [74] demonstrated that CRS can react more rapidly with electron-rich moieties, such as NH4+-NH3, compared to •OH. In the present work, since the pH of the treated solution ranged between 6.4 and 7.3 under all the experimental conditions examined, the main CRS present in the system was HOCl [75].

The generation of CRS occurs through the Volmer-Heyrovský (V-H) mechanism, involving the adsorption of Cl− onto the anode surface due to electrostatic forces, leading to the loss of an electron and the formation of Cl•. Then, Cl• can either combine pairwise or with Cl− to produce Cl2. Subsequently, Cl2 diffuses into the solution and reacts with water, further generating HOCl or ClO− [76,77]. As known, DSA active electrodes, including Ti/Ru-Ir, can facilitate the generation of high amounts of CRS concerning others, such as graphite or non-active anodes like BDD [61,78,79].

Therefore, even if the BDD anode demonstrated to form a higher concentration of •OH with respect to Ti/Ru-Ir electrode (3.1.1 section), the lower TN degradation efficiency showed in Fig. 2 confirmed that the degradation of NH4+-NH3 was primarily governed by indirect EO through CRS, particularly HOCl.

3.1.3 Cathode material effect on TN removal

Ti/Ru-Ir demonstrated a higher performance towards TN removal %. The industrial scale-up applicability of the EO technology is affected by the total operational cost required to treat the contaminants. Although EO is an energy-consuming process, the fabrication and mechanical stability of electrode materials are important economic cost drivers in the treatment [80]. If suitable anode materials are necessary to achieve high contaminant degradation efficiency, the application of different and cheaper cathodes during the process should at least be investigated to identify more economically convenient operative conditions. For instance, DSA electrodes coated with oxides are expensive, and their service lifetimes are short [81]. In this context, the effect of the combination of stainless-steel (SSL) cathode and Ti/Ru-Ir anode, which demonstrated the highest performance towards TN removal %, was investigated. Previous studies have demonstrated the effectiveness of applying SSL as a cathode for removing NH4+-NH3 from impacted water [43,67]. Moreover, SSL is characterized by low cost, availability, low overpotential, and high stability in EO applications [58,82]. The effect of the cathode material on TN removal is shown in Fig. 3.Fig. 3 Effect of cathode material type on the TN removal %: NaCl = 250 mg L−1, J = 30 A m−2, [NH4Cl] = 15 mg L−1, initial pH = 6.4; conductivity (mS/cm) = 0.95; applied voltage = 19.2 V (Ti/Ru-Ir as cathode) and 9.9 V (SSL as cathode).

Fig. 3

At the end of the treatment, operating with SSL as the cathode, a higher TN removal (96.2 ± 2.73 %) was achieved with respect to the removal obtained using Ti/Ru-Ir as the cathode. The SSL cathode allowed to operate with a lower applied voltage (9.9 V) than Ti/Ru-Ir cathode (19.2 V). Operating at high applied voltages can reduce the overall oxidation efficiency due to the formation of unreacted chlorine by-products, evaporation of chlorine gas, and limitation of ammonium mass transfer [38]. Consequently, a lower applied voltage resulted in an enhancement of the overall TN removal efficiency. In addition, the outcome could also be ascribed to the ability of SSL to produce H2O2 [83], a weak and eco-friendly oxidant that can be involved in the NH4+-NH3 oxidation [84,85]. H2O2 generation is generally possible through two routes:(10) O2 + 2 H+ + 2 e− → H2O2

(11) 2H2O → H2O2 + 2 H+ + 2 e−

In the presence of SSL as the cathode material, the first pathway (Eq. (10)) is favored since the second route (Eq. (11)) can only occur when an anode with a high overpotential for the oxygen evolution reaction (OER) is present [86,87]. The process is rather benign as it occurs at low temperatures, pressures, and pH levels [87,88].

3.2 Electrochemical treatment of livestock wastewater

Based on the higher contaminant removal obtained using Ti/Ru-Ir and SSL as electrode pair (section 3.1.2), EO experiments for treating livestock wastewater (LWW) were carried out with this electrochemical cell arrangement, exploring the effect of the variation of the concentration of Cl−. In particular, two tests were carried out: the first with the raw LWW, which spontaneously contains 31.2 ± 1.55 mg L−1 of Cl− (Table S1); the second with the addition of 250 mg L−1 of NaCl to the raw LWW, resulting in an initial Cl− concentration of 181.0 ± 3.3 mg L−1. A specific amount of an inert electrolyte (NaClO4) was added to the raw LWW to ensure the same initial conductivity conditions. The effect of Cl− concentration on TN removal is shown in Fig. 4.Fig. 4 TN removal % for EO of livestock wastewater operating at different initial [Cl−]; J = 30 A m−2, [TN]0 = 15 mg L−1, initial pH = 7.31; conductivity (mS/cm) = 1.06; applied voltage = 19.7 V (Cl− 31.2 mg L−1) and 11.4 V (Cl− 181.0 mg L−1).

Fig. 4

The higher the initial Cl− concentration, the higher the TN removal, achieving 67.0 ± 2.25 % with Cl− = 181.0 mg L−1 after 180 min, thanks to the higher amount of CRS generated [89]. Since the pH measured during the treatment was slightly acidic (5.5–6.5), the main CRS present in the solution was HOCl [90,91]:(12) 2 NH4+ + 3 HOCl → N2 + 3H2O + 5 H+ + 3 Cl-

Table S1 shows an initial total organic carbon (TOC) content of 74.42 ± 0.88 mg L−1. For this reason, the EO treatment of the LWW was also evaluated in terms of TOC removal %. The results are reported in Fig. S2. TOC degradation increased by decreasing the Cl− initial concentration, showing an opposite behaviour with respect to the TN removal. In detail, 37.3 ± 0.68 % and 25.1 ± 1.99 % of TOC removal were obtained, operating with 31.2 and 181.0 mg L−1 of Cl−, respectively. The findings are in line with previous studies that have demonstrated the competition between ammoniacal nitrogen and organic matter during EO treatments [92,93]. In the presence of a lower concentration of Cl−, the higher degradation of TOC can be attributed to DET reactions since less CRS can compete with organic contaminant molecules for the adsorption on the anode surface [94]. Urtiaga et al. [89] reported that CRSs electrogenerated in solution have a relevant role in the indirect EO of ammoniacal nitrogen and organics. The findings are in line with those reported by Deng et al. [95], who demonstrated that the removal of NH4+-NH3 is greater than that of TOC when indirect EO is dominant, while the removal rate of TOC takes priority under direct EO.

3.2.1 Fate of nitrogen and treatment cost evaluation

Considering the LWW pH of 7.3 ± 0.1, more than 98 ± 1.0 % of TN is present as NH4+. Consequently, the overall main reaction occurring in solution is shown in Eq. (12) [69,96]. Anyway, as reported by a recent study carried out by Iovino et al. [33], the presence of HClO can also lead to the formation of nitrate as a side product:(13) NH4+ + 4 HOCl → NO3− + H2O + 6 H+ + 4 Cl−

Fig. 5 shows the discrimination of the different nitrogen forms present in the solution before and after the electrochemical treatment of the LWW.Fig. 5 Nitrogen compounds discrimination in LWW before the EO treatment (dashed bars) and after the EO treatment at 180 min (blank bars); Cl− = 181 mg L−1; J = 30 A m−2, initial pH = 7.3; conductivity = 1.13 mS/cm.

Fig. 5

In the LWW before the treatment, 79.1 ± 0.37 % (8.73 ± 0.41 mg L−1) of total nitrogen (11.04 ± 0.52 mg L−1) was present in the form of NH4+, while the remaining amount (2.3 ± 0.05 mg L−1, 20.9 ± 0.69 %) is likely present as organic nitrogen. After 3 h of EO treatment, about 55.4 % of TN removal occurred, decreasing from 11.04 ± 0.52 mg L−1 to 4.92 ± 0.16 mg L−1, but no ammonium was detected. In addition, the 17.7 ± 0.51 % (1.96 ± 0.07 mg L−1) of the initial nitrogen turned into NO3−. The findings demonstrate that the EO treatment removed the ammoniacal nitrogen from the raw LWW. Moreover, no nitrite formation was detected at the end of the process, which is in line with previous studies [97]. Several authors stated that N2,gas and NO3− were the main products generated from the EO of ammoniacal nitrogen [33,67,98]. Furthermore, direct oxidation of organic nitrogen species on the anode surface could also be implicated in NO3− generation [99,100]. Although the possible generation of other nitrogen forms, including chloramines and hydroxylamine (NH2OH) cannot be excluded during the EO process, their further oxidation and treatment time are still reasonable. Mandal et al. [98] reported that ammoniacal nitrogen in solution might react with free chlorine species, generating mono, di-, and tri-chloramine, following typical break-point chlorination reactions. However, these by-products can further be oxidized to form N2,gas at the end of the process. In other previous studies, it is reported that after the beginning of electrolysis, Cl•, ClO•, and HClO species can react with NH4+-NH3 directly to form NH2•, which eventually reacts with •OH, generating NH2OH. The latter compound then oxidized to form nitrate as the final product [38,70].

The electric energy cost (EEC), calculated according to Eq. (4), was assessed to evaluate the economic feasibility of the process. The results indicated that EEC of 3.3 € m−3 was required to achieve the 55.4 % TN removal, operating under the conditions used during the treatment. The energy cost was in line with those reported by other authors who have investigated the application of EO for treating wastewater, pointing out that the proposed approach for LWW treatment is also cost-feasible [80,101,102]. Electrode supply must be considered as another major capital cost driver in an EO process [91]. The initial purchasing cost of the materials can highly impact the difference in operating costs. The soaring price of iridium hampers the spread of practical application of this material [92]. However, the use of an SSL cathode, instead of more expensive materials such as BDD or mixed metal oxides (MMO), can result in a significant cost-saving since the price of SSL is affordable (around 5.5 € kg−1), guaranteeing high availability, stability, and low overpotential [93]. The total expenses (energy and electrode costs) incurred to achieve 55.4 % of TN removal were estimated to be 6.0 € m−3 of processed livestock wastewater.

Considering other NH4+-NH3 removal techniques, Zarebska et al. [103] provided a comprehensive review of various methods for removing ammoniacal nitrogen from manure, such as ultra/nanofiltration (UF/NF), reverse osmosis (RO), membrane distillation (MD), chemical precipitation (CP), and ultrafiltration/ion exchange (UF/IE), among others. Despite the energy costs of these techniques ranging from 0.3 to 3.0 € m−3 of manure (energy unit cost uniformed to 0.2 € kWh), the overall expenses, which include chemicals and maintenance, exceeded 10 € m−3 for CP and UF/IE. Hence, while other techniques may entail lower electric energy costs than EO, they also incur higher operational expenses, which ultimately affect the overall feasibility of the treatments. Furthermore, filtration techniques necessitate pretreatment of the manure, leading to additional costs.

3.2.2 Formation of chloride oxyanions

To further delve into the environmental performance of the electrode pair successfully used to treat the LWW, Fig. 6 depicts the concentration of chloride oxyanions, as chlorate and perchlorate, detected at the end of the treatment.Fig. 6 Chloride oxyanions evolution over the EO treatment time of the LWW sample; treatment time = 180 min; initial Cl− = 181 mg L−1; J = 30 A m−2, initial pH = 7.3; conductivity = 1.13 mS/cm.

Fig. 6

The presence of chloride can trigger some undesirable side reactions, such as the formation of chlorate and perchlorate, which confers health risks [96,[104], [105], [106]]:(14) 6 HClO + 3H2O → 2 ClO3− + 4 Cl− + 12 H+ + 3/2 O2 + 6 e−

(15) ClO3− + H2O → ClO4− + 2H+ + 2 e−

The results showed that only 3.96 ± 0.62 % (7.24 ± 0.17 mg L−1) of initial chloride oxidized to chlorate. In agreement with previous studies, the lower formation of chlorine oxyanions on mixed metal oxide anodes, such as Ti/Ru-Ir, is ascribed to the low oxygen evolution potential (OEP) of about 1.5 V vs SHE [80,107]. Moreover, since the ammoniacal nitrogen reactions with HOCl occur close to the anode surface, the generation of oxyanions is hindered [108]. Zhang et al. [109] confirmed that the moderate oxidizing ability of active anodes leads to the formation of chlorate as the final chloride oxidation product. However, the undesired formation of chloride oxyanions can be hampered by operating with lower initial chloride contents or by using more suitable anode materials with a lower OEP [80].

4 Conclusions

Electrochemical oxidation is an efficient process of removing nitrogen compounds from wastewater. This study has highlighted that TN removal is strongly affected by the material of the electrodes used during the treatment. Ti/Ru-Ir anode, belonging to the class of active anodes, showed higher TN removal % (78.8 ± 0.76 %) than BDD (63.7 ± 1.21 %). In addition, a stainless-steel electrode cathode, cheaper than the Ti/Ru-Ir one, was tested, resulting in an enhanced TN removal (≅ 100 %). The results confirmed the first study hypothesis, demonstrating that the indirect electrochemical oxidation of ammoniacal nitrogen is primarily governed by electrogenerated chlorine reactive species, particularly HOCl. Moreover, the Ti/Ru-Ir anode demonstrated the ability to generate more CRS than the non-active BDD anode, resulting in higher efficiencies in ammoniacal nitrogen degradation. This outcome confirmed the second hypothesis of the study. The optimized cell configuration was applied to treat real livestock wastewater from farming in the Campania Region. This area is deeply affected by nitrogen water pollution and is identified as a vulnerable zone. After the EO process, a total ammonium removal was achieved, and only 17.7 ± 0.51 % of initial TN turned into nitrate. After treatment, wastewater met the limit requirement from the Italian legislation (D.M. 260/2010) for discharging in water bodies in terms of ammoniacal nitrogen (≤0.5 mg L−1) and nitrate (≤50 mg L−1). Furthermore, the third working hypothesis was partially refuted, as the analysis of by-product formation from chloride revealed that only 3.96 ± 0.62 % of the initial chloride converted to chlorate, with no detection of perchlorate formation. Finally, the energy cost assessment highlighted the cost-effectiveness of EO treatment for treating livestock wastewater. Moreover, the use of an SSL cathode can result in a significant cost-saving.

Further investigations are required to optimize the process in terms of electrochemical cell configuration, such as operating in continuous mode. Although the results demonstrated the efficiency and economic feasibility of using a stainless-steel electrode as cathode material, new studies and the development of novel electrodes for EO applications are imperative. By exploring diverse avenues such as doping with alternative elements, augmenting the intermediate layer, employing novel substrate materials, and investigating electrode surface modification methods, the adoption of EO technology to treat complex wastewater will be facilitated. Additionally, the potential application of EO to disinfect pathogens-rich wastewater can be considered. The in-situ electrogeneration of well-known disinfectants such as chlorine reactive species can provide a crucial advantage over chemical chlorination methods typically used in water purification treatments. Nevertheless, improving the long-term stability and electrocatalytic performance, as well as reducing the cost of electrode materials, are challenges for the future industrial scale-up of the process.

Data availability statement

The data that support the findings of this study are available from the corresponding author, upon reasonable request.

Ethics declarations

This study did not involve any human samples and was approved by all authors.

CRediT authorship contribution statement

Simona Galoppo: Investigation, Formal analysis. Angelo Fenti: Writing – original draft, Investigation, Formal analysis, Data curation. Giovanni Falco: Investigation, Formal analysis. Qingguo Huang: Writing – review & editing, Validation. Simeone Chianese: Writing – original draft, Software, Data curation. Dino Musmarra: Writing – review & editing, Validation, Supervision. Pasquale Iovino: Writing – review & editing, Resources, Project administration, Methodology, Conceptualization.

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.

Appendix A Supplementary data

The following are the Supplementary data to this article:Multimedia component 1

Multimedia component 1

Multimedia component 2

Multimedia component 2

Acknowledgements

This work was supported by the Campania Region through Regional Law dated June 29, 2021, No. 5-DRD No. 410/2021, Action B, under the Project “RiduciN”. The authors would like to thank Nicola Franzese's buffalo company for providing the opportunity to sample the wastewater, and Valentino Canale for assisting with the graphical abstract editing.

Appendix A Supplementary data to this article can be found online at https://doi.org/10.1016/j.heliyon.2024.e36803.
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