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

S2405-8440(24)12038-5
10.1016/j.heliyon.2024.e36007
e36007
Research Article
Solid organic acid, aminopolycarboxylic acid and alkali salt in deliming to reduce nitrogenous pollution in tannery wastewater
Maoya Modinatul
Hashem Md Abul hashem_518@yahoo.com
mahashem@le.kuet.ac.bd
⁎
Miem Md Mukimujjaman
Enan Eshtiyaq Tauhid
Islam Sakina
Hasan Mehedi
Department of Leather Engineering, Khulna University of Engineering & Technology, Khulna 9203, Bangladesh
⁎ Corresponding author. hashem_518@yahoo.commahashem@le.kuet.ac.bd
08 8 2024
15 9 2024
08 8 2024
10 17 e360077 7 2024
7 8 2024
© 2024 The Authors
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/).
In leather manufacturing, ammonium-free deliming is gaining popularity because deliming with traditionally used ammonium salts contributes ammonia-nitrogen (NH3–N) into wastewater. Wastewater contains NH3–N effect on the treatment facility. In this study, an ammonia-free deliming agent-a combination of citric acid (CA), sodium hexametaphosphate (SHMP), and ethylenediaminetetraacetic acid (EDTA) is utilized against the traditional deliming agent to limit the release of NH3–N. Physicomechanical properties have been assessed in the produced effluent, delimed pelt, and crust leather. In contrast to traditional deliming, the amount of NH3–N and Total Kjeldahl Nitrogen (TKN) is estimated to have decreased by 98.1 % and 85.26 %, respectively. Deliming with CA + SHMP + EDTA can significantly reduce the amount of TKN in the deliming float. A notable reduction of biochemical oxygen demand (34.8 %), chemical oxygen demand (85.7 %), total dissolved solids (56.4 %), electrical conductivity (54.5 %), and turbidity (65.2 %) was achieved. The physical and organoleptic characteristics of crust leathers produced with the CA + SHMP + EDTA deliming process are comparable to the traditional deliming technique. Similar fiber structure is observed in SEM images of crust leather, wet blue leather, limed pelt, and delimed pelt. A pilot-scale production is carried out; the outcomes are similar to the experimental results. For all experimental and control investigations, the Pseudo Second Order kinetic model fits the reaction kinetics. Deliming with CA + SHMP + EDTA offers a simpler wastewater treatment process since it reduces nitrogen contamination in the wastewater.

Graphical abstract

Image 1

Highlights

• Developed a cleaner deliming method replacing conventional ammonium salt.

• Achieved reduction of NH3–N (98.1 %), TKN (85.3 %), BOD (34.8 %), and COD (85.7 %).

• Physicomechanical properties of leather were comparable to conventional one.

• Identical fiber orientation was observed for both delimed pelt and crust leather.

Keywords

Physicomechanical
Ammonia-nitrogen
Ammonium sulfate
Organoleptic
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pmc1 Introduction

In the process of making leather, a number of unit procedures are needed to convert putrescible raw hides or skins into non-putrescible leathers [1]. As a result, enormous volumes of liquid (35–40 m3/t) and solid (700 kg/t) waste are generated [2]. The beamhouse, tanning, post-tanning, and finishing are four steps comprised in leather manufacturing. In beamhouse, deliming is a process where alkaline limed pelt is neutralized. Moreover, it prevents the alkaline swelling of limed pelt and prepared it for bating (enzymatic treatment) at pH 7.0–9.0 to yield superior leather quality to meet the customer demand that fulfills consumer expectation [3,4]. Leather manufacturing processes involve a series of chemicals that emit extremely contaminated wastewater. The liquid waste is emitted from the various operations; deliming is one of them, which is mostly dependent on ammonium salt. Ammonium salt (ammonium sulfate and ammonium chloride) is used as a traditional deliming agent as it permeates the pelt rapidly and acts as a buffer to maintain the pH at the desired level [5]. A deliming agent should be acidic so that it can penetrate into inner layer of limed pelt to reduce the effect of alkaline swelling.

In conventional deliming, both ammonia gas (NH3) and dissolved ammonia in the form of NH3–N are released into the atmosphere and wastewater, respectively. The NH3 gas blocks oxygenation in tissue causing the nervous system disorders (brain and spinal cord) [6]. The high quantity of ammonia in the discharged tannery effluent raises the Biochemical Oxygen Demand (BOD), which is detrimental to fish and other aquatic life [5]. During deliming with traditional deliming agents, the level of NH3–N in wastewater emissions varies between 2000 and 4000 mg/L [7,8]. In beamhouse processes, NH3–N (40 %) and organic nitrogen (60 %) accounted for the total contamination with nitrogen [8]. The emitted excess nitrogen triggers hypertrophic and dystrophic situations, resulting in eutrophication [9].

Thus, the primary goal of researchers for many years is investigating ammonia-free deliming to regulate the emission of NH3–N. There have been attempts to replace the traditional deliming agents (ammonium sulfate and ammonium chloride) with various compounds, but each has pros and cons of its own. For example, boric acid, formic acid, citric acid, lactic acid [7,10], leaf extracts [11], carbon dioxide [6,12,13], glycolic acid [5], and sulfosalicylic acid, nylon acid, gluconic acid [14], and sulfonated phthalic magnesium salt [15] are used. The enzymatic beamhouse process has been the focus on several studies but its industrial scale application is rare due to various challenges, including collagen destruction [16], costly [17], and ineffectively manually operated [18]. An effective deliming agent with reduced NH3–N emissions is amino acid-based glycine. However, it is ineffective in controlling total nitrogen [19]. Minimizing NH3–N and TKN emission in deliming, supercritical carbon dioxide (SCCD) has been used; a major barrier of this method is to use a specific high-pressure resistance apparatus for CO2 storage. Ariram et al. [20] reported that eliminating water usage, the SCCD could have a positive deliming impact. Furthermore, organic acids have little buffering power, tend to cause acid swelling and penetrate the limed pelt very slowly, but they can stop pollution during the deliming process [19]. Organic acid-based deliming agents, lactic and acetic acid cause swelling when used in excess amounts [10]. Although weak organic acids with minimal buffering capacity such as, formic acid, carbonate base and glycolic acid can have a certain deliming effect but can also cause acid swelling [21]. Boric acid can improve the buffering action and permeability making it as an effective deliming agent [10], but it has been classified as a substance of extreme concern being toxic for reproduction [19].

Among other deliming agents, citric acid (CA) acts as a chelating agent, for being excellent in binding metals by solubilizing [22]. The ethylenediaminetetraacetic acid (EDTA) possesses a buffering effect in the deliming bath between pH 8.0–9.0 that can break down the lime scale and capture metal ions like Ca2+ and Fe3+ which could effectively increase the Ca removal from the limed pelt [23]. It is expected that sodium hexametaphosphate (SHMP) can be used in deliming, it is very good at chelating with alkaline earth metals e.g., Ca and Mg. Furthermore, it can supply phosphorus; a necessary nutrient for the biological treatment of wastewater [10]. In this study, the combination of CA, SHMP, and EDTA is employed as a deliming agent and the process parameter i.e., the dose of the mixtures has been optimized. The effect of newly developed deliming technique on overall appearance of the leather has been assessed. The suitability and benefits associated with the biological treatment of the tannery effluent generated from this new deliming process has also been investigated.

2 Materials and methods

2.1 Collection of raw materials

For the deliming trials, limed pelts were produced from the raw goatskin using the conventional soaking, liming, and fleshing operations.

2.2 Chemicals and reagents

Analytical grade reagents were purchased for the analysis of BOD, NH3–N, COD, and TKN. The citric acid (CA) (Pallav Chemicals and Solvents, India), sodium hexametaphosphate (SHMP) (Pallav Chemicals and Solvents, India), and ethylenediaminetetraacetic acid (EDTA) (LOBA Chemie, India) were procured from Khulna, Bangladesh. The procedure of preparing leather involved the use of traditional chemicals. The structure of citric acid (CA), sodium hexametaphosphate (SHMP) and ethylenediaminetetraacetic acid (EDTA) are shown in Supplementary Figure (SF1).

2.3 Initial experiment with different ratios of deliming agent

The buffering effect of CA and SHMP was not sufficient for deliming, so there might be a chance of acid swelling of the pelt. A small amount of EDTA was used for the required buffering action. One goat-limed pelt was cut into four pieces and the mixture of deliming agents at different ratios was applied to the cut pieces separately. Four different ratios of deliming agents were used: (i) 0.5 % CA + 1.0 % SHMP + 0.1 % EDTA, (ii) 0.5 % CA + 1.25 % SHMP + 0.1 % EDTA, (iii) 0.5 % CA + 1.50 % SHMP + 0.1 % EDTA, (iv) 0.5 % CA + 1.75 % SHMP + 0.1 % EDTA. The weight % of chemicals were calculated based on the pelt weight.

2.4 Optimization of the ratio of deliming agents

The ratio of deliming agents in optimized combination was determined by monitoring the pH of the solution and the amount of NH3–N, and TKN in deliming wastewater. The optimized ratio of deliming agents was applied for final experiment.

2.5 Experimental, control, and pilot scale experiment

One goatskin was used for experimental and control studies. This goatskin was divided into two halves along with the backbone line. Between the two pieces of goatskin, the left side was used for the control study and the right side for the experimental study. On the experimental piece, the optimized combination of deliming agent was applied based on the pelt's weight and for the control the conventional deliming agent, ammonium sulfate (AS), was followed. A total of 10 goatskins had been collected for pilot scale deliming. During deliming, shrinkage temperature, NH3–N, and TKN analyses were conducted. The discharged wastewater was also analyzed to determine the water quality.

2.6 Leather tanning process after deliming

After deliming, both the experimental and control pelts were tanned using the conventional chrome tanning processes followed by retanning. The strategies for the beamhouse, tanning, and retanning operations as represented in Supplementary Table (ST1 and ST2).

2.7 Wastewater characterization

The electrical conductivity (EC), total dissolved solids (TDS), and salinity of the delimed wastewater were assessed via a conductivity meter (CT-676, BOECO, Germany). A pH meter (BT-675, BOECO, Germany) was used to measure the pH. The calculation of COD and BOD was done via the standard methods of APHA-5220C [24] as well as APHA-5210 B [25]. For all measurements, triplicate measures were taken, and the standard deviation and the mean were taken into account when representing the findings.

2.7.1 Determination of TKN concentration

Kjeldahl apparatus (VELP SCIENTIFICA UDK 129) and APHA [26] technique were used to calculate the TKN of the deliming effluent. The digestion temperature was maintained between 375 °C and 385 °C with potassium sulfate, sulfuric acid, and copper sulfate.

2.7.2 Determination of NH3–N concentration

The APHA [26] standard strategy was adopted for measuring the NH3–N content of the delimed effluent. A portion of delimed wastewater was obtained and diluted to make 50 mL in a borosilicate glass flask. To reduce cyanates and the hydrolysis of organic nitrogen, 20 mL of borate buffer was added. The pH of the mixture was adjusted to 9.5 by 6N NaOH solution. To steam out the liquid employed in the distillation device, some glass beads were added. The distillate, containing 50 mL of boric acid until the quantity reached 200 mL, was collected in an Erlenmeyer flask. Up until the indicator changed into (pale) lavender, the resulting solution was titrated using a sulfuric acid standard of 0.02N. Every step of the way, a blank distillation was filled in to make the outcome understandable. The computation was performed using equation (i).(1) mgNH3−N/L=(A−B)×280mLsample

‘A’ denotes the required volume of H2SO4 titrated in mL for wastewater sample and ‘B’ denotes the required volume of H2SO4 titrated in mL for blank sample.

2.8 Assessment of leather samples

The worldwide standard techniques of ISO 2418 [27], ISO 2589 [28], and ISO 2419 [29] were followed for performing the physical test, where the official sampling position was taken into consideration for the leather samples, using the EN ISO 2418 [30] process. The samples were collected from both perpendicular as well as parallel orientations to the backbone after conditioning. The organoleptic qualities e.g., smoothness, fullness, grain tightness, softness as well as the overall look of the traditional, experimental, and pilot-scale samples were assessed by a panel of experts through manual as well as visual assessment. The experts approved the specimen on a ranking system of 0–10 for the aforementioned properties with a higher rating denoting an improved property.

2.8.1 Shrinkage temperature

The ISO 3380 [31] standard was used in evaluating the shrinkage temperature (Ts) of the Cr-tanned leather, pickled pelt, bated pelt, delimed pelt, limed pelt, and raw goatskin using a shrinkage tester apparatus manufactured by SATRA STD, 114, UK. A leather sample (80 × 10 mm) was cut and placed in the test meter. The specimens were subsequently immersed in the distilled water. By observing the temperature, when the leather sample started to shrink, the Ts of this specific specimen were ascertained. As mentioned in the previous literature, the Ts of the Cr-tanned leather samples were calculated by substituting a glycerol-water (70:30) solution for the distilled water [[32], [33], [34]].

2.8.2 Determination of thickness of pelts

For the experiment and traditional pelts, the thickness of the limed (after fleshing operation) as well as delimed pelt was measured. The limed pelt depicted in Fig. 1 is an indicator with seven points, including the head (H), back (B), shoulder (S), belly (Be), hind shank (HS), shell (Sh), and fore shank (FS). Using the ASTM D1814-70 [35] standard, a thickness gauge was used to measure the limed pelt's thickness to the closest 0.1 mm at the seven points. After completing deliming, same way thickness of the delimed pelt was measured. The readings at each point were taken three times for validation of the measurement process. Besides, the delimed pelt's cross-section was examined for acid penetration using phenolphthalein as an indicator. Finally, the thickness variation of the delimed pelts was estimated using the subsequent equation (ii).(2) ΔT(%)=(T1−T2)T1×100

where ‘T1’ represented the limed pelt's thickness (mm); ‘T2’ depicted the delimed pelt's thickness (mm)Fig. 1 Limed pelt indication for thickness measurement-head (H), shoulder (S), back (B), shell (Sh), hind shank (HS), belly (Be), and fore shank (FS).

Fig. 1

2.8.3 Determination of tensile strength and percentage of elongation

The leather sample was split into a dumbbell-shaped pattern with a dimension of 25 mm × 110 mm in parallel and perpendicular orientations. With a SATRA STD simple tensile tester, the percentage of elongation at break and tensile strength (N/mm2) were calculated following EN ISO 3376 [36]. The dumbbell-shaped sample is clamped at both ends by the two jaws of the tensile tester. Until the sample splits in two, it is dragged at a steady speed in the opposite direction. After recording the break load and distance between the jaws at both the beginning and final points of the breakage phase, values were obtained using equations (iii) and (iv). Six samples were taken from both parallel and perpendicular orientations for each leather specimen. Furthermore, the experiment's mean and standard deviation were calculated.(3) Tensilestrength(N/mm2)=Breakingload(N)Thickness(mm)×width(mm)

(4) Elongationatbreak(%)=Lengthatbreak(mm)−Initiallength(mm)Initiallength(mm)

2.8.4 Determination of ball bursting strength

The entire strength of the leather is measured by the ball bursting strength, which was ascertained by applying the ASTM D2207-00 [37] standard technique. The specimens were clamped onto a digital lactometer, SATRA STM 463. The force was pushed upward from the leather's bottom through a 6.25 mm-sized steel ball. When the leather sample cracked and ruptured, the distension (mm) and force (kg) requirements were recorded. The grain strength at multiple directions of stress is represented by the distension and load values.

2.8.5 Determination of calcium (Ca) removal from limed pelt

Around 10 g delimed and limed pelts were carried out according to the process IUP 2 [38] and the obtained pelts were divided into pieces, then dried at103 ± 2 °C for 12 h. After that, it was burned at 700 °C for 8 h using a muffle furnace. The resultant burned material was combined with 10 mL of 2 mol/L nitric acid as well as heated for 5 h at 110 °C on a heated plate. The Whatman filter paper 1 (Cat No. 1001-125, 11 μm) was used to filter the mixture [7,10,39]. The AAS (Spectra240FS AA, Agilent, USA) was used to determine the amount of calcium in the filtrate. The following equation (v) and (vi) was used to determine the amount of Ca and percentage of Ca removal from the pelt.(5) Cacontentinpelt(%)=wcwp×100

(6) Caremoval(%)=c1−c2c1×100

wc denotes the ‘Ca’ weight of limed pelt; ‘wp’ is the weight of limed pelt in dry condition; ‘c1’ represents the limed pelt's ‘Ca’ content and ‘c2’ denotes the delimed pelt's ‘Ca’ content.

2.8.6 Measurement of Cr content

To measure Cr content, crust leathers were digested through an acid digestion process. Approximately 2.0 g of the crust leather fragments were chopped for acid digestion using nitric acid. After heating on a hot plate, the resulting mixture was permitted to reflux and then heated once more. The process was terminated after adding more nitric acid and heating the mixture until the brown vapors vanished. After the mixture was cooled, 30 % of the hydrogen peroxide was incorporated. After heating the mixture once more, it was allowed to reflux. The mixture's structure either was unchanged or produced fewer bubbles after additional hydrogen peroxide was included. The resulting combination was heated further until it had a volume of 5 mL. Subsequently, 30 mL of deionized water was incorporated, as well as the heating process proceeded for an additional hour. Following cooling, the entire combination was passed through the filter paper (Whatman No. 1, Cat No. 1001-125, 11 μm)). After that the deionized water was used to get the filtrate's volume down to 50 ml. For Cr analysis, the filtrate was stored at 4 °C in an HDPE (high-density polyethylene) sample container [10]. A quantitative assessment of Cr was performed on the acid-digested aliquot using AAS (Spectra240FS AA, Agilent, USA). The Cr was detected at 357.9 nm wavelengths on the air-acetylene direct flame. Every measurement was done three times, and then the mean and standard deviation were computed.

2.8.7 Morphological assessment

The official sampling placement was used to cut samples of identical thicknesses from the experimental and conventional delimed pelt. Following the recommended dehydration procedure, the experimental and conventional delimed pelts were progressively dried out [10,40]. On the contrary, the crust leather samples were prepared with no additional dehydration process and conditioned in a desiccator. The fiber structure of the limed pelt, delimed pelt, wet blue leather, and crust leather was captured in SEM images where SEM (JEOL JSM-6490, USA) was used to investigate the samples.

2.8.8 Lime removal

Pre-weighed limed pelt underwent treatment with developed CA + SHMP + EDTA ratio (left half) and AS (right half) for the purpose of analyzing the lime removal efficiency and its mechanism of action. The amount of lime in the wastewater and pelt was calculated using the method outlined by Sarkar [41] and Sathish et al. [6]. At various time intervals (10, 20, 30 40, 50, 70 and 90 min), the wastewater and pelt samples were collected from the control and experimental procedure, and their lime concentration was determined. For estimating, the amount of lime in the wastewater, a predetermined quantity of liquor was poured out into a conical flask, and phenolphthalein was used as an indicator to titrate against 0.1N HCl until the pinkish color disappeared. Similarly, after precisely weighing 5 g of the pelt samples, the substance was put in a 250 mL container with 150 mL distilled water. The aforementioned pelt was titrated until the pinkish tint disappeared using 0.1N HCl and phenolphthalein indicator. The limed pelt container was shaken vigorously and then rested for 10 min. The inner layer of the pelt's lime dissolved throughout the mixture, creating the impression of a pinkish color. The titration and shaking were repeated until the pink color of the pelt was disappeared. The titration provides a measurement of the lime content of liquor and pelt. The following equation was used to determine the effectiveness of lime removal:(7) Limeremoval(%)=L0−LtL0×100

‘L0’ is the quantity of lime in the pelt. ‘Lt’ denotes the quantity of lime in the float and pelt at different sampling time (t).

2.9 Kinetics study

For simplify the reaction rate, linear form of pseudo first-order (PFO) and pseudo second-order (PSO) kinetics was used for the experimental data. PFO kinetic can be described using the Lagergren equation [42], and PSO kinetic can also be described by Ho and Mckay [43].

The following equation denotes PFO kinetics:(8) ln(qe−qt)=lnqe−k1t

The PSO model was evaluated on the basis of the following equation (vi):(9) tqt=1k2qe2+(1qe)t

‘qe’ denotes the amount of lime removal (mg/g) at equilibrium condition, ‘qt’ represents lime removal content (mg/g) at ‘t’ time, ‘k1’ and ‘k2’ represent the rate constant for PFO and PSO reaction, respectively.

3 Results and discussion

3.1 Impact of CA + SHMP + EDTA mixing ratio on deliming performance

Table 1 depicts the effect of the mixing ratio of CA + SHMP + EDTA deliming agent. The suitability of each ratio of the combined deliming agent was determined by monitoring the pH of solution and quantifying NH3–N, and TKN in deliming wastewater. After the deliming process, the final pH was attained at 8.4, 8.4, 8.5, and 8.6 for the trial (i), (ii), (iii), and (iv) mentioned in Section 2.3. To stop the release of H2S gas and acid swelling of the pelt surface, the pH of the delimed effluent typically needs to be greater than 5 during deliming [44]. The presence of total NH3–N in deliming wastewater from experimental samples were measured 41.1, 28.9, 28.0, and 36.4 mg/L where, TKN were 279.1, 270.7, 250.4, and 214.7 mg/L, respectively. On the contrary, the NH3–N and TKN in traditional deliming wastewater were 1735.4 and 1857.7 mg/L, respectively. Considering the pH, cost, and the presence of NH3–N and TKN in wastewater, second composition (0.5 % CA + 1.25 % SHMP + 0.1 % EDTA) was selected as an optimum percentage of deliming agents to be used for the final application. The optimization of the deliming performance is illustrated in Supplementary Figure (SF2).Table 1 Impact of the mixing ratio of CA + SHMP + EDTA on deliming wastewater.

Table 1Sample	TKN	NH3–N	
CA (0.5 %) + SHMP (1.0 %) + EDTA (0.1 %)	279.06 ± 4.62	41.06 ± 3.23	
CA (0.5 %) + SHMP (1.25 %) + EDTA (0.1 %)	270.67 ± 7.93	28.93 ± 1.62	
CA (0.5 %) + SHMP (1.50 %) + EDTA (0.1 %)	250.41 ± 5.12	28.01 ± 2.9	
CA (0.5 %) + SHMP (1.75 %) + EDTA (0.1 %)	214.67 ± 6.42	36.4 ± 4.84	
AS (2.25 %)	1857.67 ± 6.36	1735.41 ± 7.61	

3.2 Characteristics of deliming and bating wastewater

The parameters of the deliming wastewater from conventional, experimental, and pilot-scale samples are illustrated in Table 2. The pH differences appeared not to be that significant, such as conventional (8.37), experimental (8.43), and pilot-scale (8.33), accordingly. The salinity of the traditional deliming effluent was 2.4 and 2.2 times greater than that of experimental and pilot-scale samples processing effluents, respectively. However, the value was almost the same for the experimental and pilot-scale manufacturing. Additionally, the turbidity of experimental and pilot-scale deliming effluents was almost similar, where the traditional deliming effluent had a 2.9 and 2.8times greater level of contamination, respectively. EC and TDS levels for the experimental and pilot-scale samples generated effluents were lower, compared to the conventional delimed wastewater. All these data suggest that the experimental deliming agents generate wastewater that contains less contaminant in comparison to that of traditional deliming which will result in a significant reduction in the cost of treating wastewater [45]. The COD and BOD of traditional deliming effluent were measured as 3733.33 and 724.32 mg/L, accordingly with 85.71 % and 34.78 % reductions compared to the experimental, 34.3 %, and 80.0 % compared to the pilot-scale operation. Additionally, Table 2 shows, CA + SHMP + EDTA generated wastewater has a high biodegradability (BOD/COD ratio of 0.89), indicating that the effluent degrades easily and thus offers cleaner deliming [46].Table 2 Pollution load in experimental and conventional deliming wastewater.

Table 2Parameters	Experimental	Conventional	Pilot	Unit	
pH	8.4 ± 0.06	8.4 ± 0.06	8.3 ± 0.06	–	
Salinity	6.3 ± 0.1	15.2 ± 0.06	6.8 ± 0.06	g/kg	
TDS	5.0 ± 0.01	11.4 ± 0.04	5.5 ± 0.01	g/L	
Turbidity	435.8 ± 1.9	1251.7 ± 11.6	441.8 ± 1.3	NTU	
BOD	472.4 ± 6.9	724.3 ± 6.9	476.0 ± 13.9	mg/L	
COD	533.3 ± 13.9	3733.3 ± 3.0	746.7 ± 3.9	mg/L	
EC	12.8 ± 0.01	28.0 ± 0.08	13.1 ± 0.10	mS/cm	
NH3–N	32.7 ± 5.8	1726.7 ± 10.6	31.7 ± 6.5	mg/L	
TKN	274.4 ± 4.9	1861.07 ± 12.6	298.7 ± 10.6	mg/L	

Table 3 shows the properties of the bating effluents of manufacturing on experimental, traditional, and pilot-scale samples. The pH of the bated effluents of conventional, pilot-scale, and experimental operations were 8.1, 8.2, and 8.1, respectively-no significant difference. The salinity of traditional bating wastewater was 1.5 times and 1.7 times higher than that of pilot-scale and experimental bating wastewaters. TDS and EC levels were observed comparable for all the effluents.Table 3 Pollution load in experimental and conventional bating wastewater.

Table 3Parameters	Experimental	Conventional	Pilot	Unit	
pH	8.1 ± 0.0.1	8.07 ± 0.06	8.2 ± 0.06	–	
Salinity	21.3 ± 0.1	32.1 ± 0.17	18.6 ± 0.1	g/kg	
TDS	15.6 ± 0.02	22.7 ± 0.03	13.8 ± 0.01	g/L	
EC	37.5 ± 0.03	54.1 ± 0.1	33.2 ± 0.01	mS/cm	

3.2.1 NH3–N and TKN concentration

The findings revealed that the NH3–N in deliming effluent of manufacturing on conventional, experimental, and pilot-scale were 1726.7, 32.7, and 31.7 mg/L, respectively (Table 2). The results showed that using CA + SHMP + EDTA as a deliming agent significantly lessened the NH3–N level in deliming effluent by 98.1–98.2 %. It could be possible that the tiny amount NH3–N (32.7 mg/L) in the CA + SHMP + EDTA deliming effluent was caused by the disintegration of the collagen in the limed pelt, which was released from the pelt during deliming [8,15]. In traditional delimed effluent, the TKN was released around 1861.1 mg/L. The TKN values for the manufacturing on the pilot-scale and experimental samples were 298.7 and 274.4 mg/L (Table 2), which corresponds to 85.3 %–84.0 % reduction, respectively compared to traditional operation. The breakdown of pelt proteins is most likely the source of the nitrogen in non-ammonia deliming effluent [10]. Nonetheless, there is no noticeable distinction between leather manufacturing in experimental and pilot-scale.

3.3 Influence of CA + SHMP + EDTA deliming on crust leather properties

3.3.1 SEM

Physical strength is heavily dependent on the arrangement of fibers, making it a crucial aspect in evaluating the quality of leather [47]. The impact of CA + SHMP + EDTA deliming on the leather qualities was analyzed comparing the morphology and physical characteristics of the control (AS) and experimental limed pelts, delimed pelts, wet blue, and crust leathers. Fig. 2 (a, b, c, d, e, f, g, h) show the SEM pictures of the limed pelts, delimed pelts, wet blue, and crust leathers produced from the traditional and experimental samples.Fig. 2 SEM micrographs-limed pelt for conventional (a) limed pelt for experimental (b) delimed pelt with AS (c) delimed pelt with CA + SHMP + EDTA (d) wet blue delimed with AS (e) wet blue delimed with CA + SHMP + EDTA (f) crust leather delimed with AS (g) and crust leather delimed with CA + SHMP + EDTA (h).

Fig. 2

The cross-sections of the limed pelts are displayed in Fig. 2(a and b). The fiber alignment and cross-sectional area of limed pelts appear to be comparable to the conventional one; there was no noticeable difference. Wang et al. [7] reported that the sodium sulfide is used in liming to separate the hair and hide. Some hair roots and epidermis may remain present on the pelt. Thus, the remaining epidermis and hair roots could affect appearance. The cross-section of the delimed pelts was investigated to confirm the removal of Ca salt from the delimed pelt of the experimental sample; the outcomes are presented in Fig. 2(c and d). According to the observations, it suggests the cross-section and fiber orientation of delimed pelts for the experimental and conventional pelts are comparable. Actually, the pelt delimed with CA + SHMP + EDTA had a relatively clearer texture. This is because using a traditional deliming agent (AS) results in the production of calcium sulfate, which is weakly soluble and may stay inside the pelt. Sathish et al. [6] reported that the cross-section of delimed pelts may become deposited with insoluble calcium carbonate/sulfate during the deliming process. On the contrary, excellent deliming efficiency is further confirmed by the fact that the delimed pelt from CA + SHMP + EDTA had no deposits of salt in the cross-section [20]. According to Fig. 2(e and f), collagen fibers in the wet blue cross-section from each method had opened up to a satisfactory degree. The findings suggest that post-tanning reagents could effectively permeate the fibers of the leather [7].

After post-tanning, the cross-section of the crust leather was examined to determine the configuration of the fibrils. The traditional and experimental crust leathers had similar fiber networks, as demonstrated by the SEM images in Fig. 2(g and h). The adhesion between the fibers is not densely blocky or visible. The leathers delimed with AS and CA + SHMP + EDTA have the same degree of "opening up" in their fiber bundles. These outcomes indicate that the inner layers are all properly bated and tanned [4]. Furthermore, the experimental leather's physical characteristics, including its softness, bursting strength, tear strength, as well as tensile strength were similar to those of the control crust leather [19]. These findings suggested that the final leather's appearance was unaffected by the CA + SHMP + EDTA deliming process. The earlier literature reported the same outcome [20]. In Fig. 2(g), control image of crust leather displays fiber separation where, the experimental outcome depicts a partially distributed fiber arrangement indicating an enhancement in the fullness of the leather. This clarifies that the experimental trial showed firmer, stronger, and longer fiber structure than the control [5].

3.3.2 Shrinkage temperature

The material shrinks when heated in water, which indicates the structural integrity of collagen-based substances under hydrothermal conditions. The shrinkage temperature (Ts) is the temperature at which a substance is fully hydrated and specific requirements are fulfilled [48]. Collagen-based materials shrink hydrothermally because the intermolecular and intramolecular forces (i.e., hydrophobic bonding, hydrogen bonding, and crosslink bridges) break down during heating, weakening or dislocating the triple helix shape [49].

The Ts for the chrome-tanned leather, bating pelt, pickled pelt, delimed pelt, limed pelt, and raw goatskin are shown in Fig. 3. The Ts was found to be 63.8 °C for conventionally and 63.8 °C for experimentally delimed pelts. The Ts of the raw goatskin for pilot-scale experiment, 1 to 10 were: 63.7 °C, 63.2 °C, 63.7 °C, 63.3 °C, 62.8 °C, 63.5 °C, 62.7 °C, 63.2 °C, 63.5 °C and 64.2 °C. The increased Ts in raw goatskin is caused by the cross-linking of collagen fiber with other components. As a result, the raw goatskin's Ts on the pilot scale varied between 62.67 °C and 64.17 °C [50]. During liming, hydrated lime and a sharpening agent (sodium hydrosulfide or sodium sulfide) are employed, which increases the dermis's porosity by eliminating cementing and protein materials [44,51]. For the pilot-scale, the Ts of limed pelts ranged from 48.2 °C to 49.8 °C. Notably, the Ts of the limed pelts did not appear to differ much from one another.Similarly, the Ts of traditional and experimental delimed pelts were 57.7 °C and 59.7 °C, respectively. The Ts varied from 57.5 °C to 59 °C for the pilot scale. The lime is usually removed using deliming chemicals in the limed pelt. This indicates the formation of a salt (such as calcium sulfate) that blocks chemical penetration to get improved tanning [13,52].

Fig. 3 Shrinkage temperature for raw goatskin, limed pelt, delimed pelt, pickled pelt, and chrome tanned leather of laboratory scale and pilot scale experiment.

Fig. 3

After pickling and bating, the Ts progressively dropped. The reason can be that there is a significant and noticeable impact of the short link or hydrogen bond structure when an alkali or acid is present in a very high or very low PH range. A significant degree of swelling develops in the pH range of 2.0–3.0, which surely impacts the structure of CO–NH's capacity to sustain itself and consequently leads to structural instability [53]. Lischuk et al. [54] also reported that the Ts increased as a result of the decrease in porosity. One possible explanation for these outcomes is the opening up and closer proximity of minor structural components during the enzymatic pickling process. Protease enzyme is utilized in bating to break up collagen fiber and extract nonstructural proteins [47,55]. Furthermore, fibrous protein is eliminated at this stage to achieve the right grain softness and texture [51].

Budrugeac et al. [49] reported that the collagens are untanned materials. During the chrome-tanning operation, basic chromium sulfate is incorporated, which permeates the pelts as well as blocks the collagen fiber's empty area by cross-linking and Ts increased as the cross-linking rate increased [56]. The Ts of experimental and conventional wet-blue leather were 109.2 °C and 109.2 °C, respectively. However, deliming with CA + SHMP + EDTA demonstrated comparable Ts to traditional wet-blue. It may be concluded that deliming with CA + SHMP + EDTA did not obstruct chrome tanning penetration or cross-link.

3.3.3 Calcium (Ca) removal of limed pelt

The ideal deliming agent can minimize collagen damage caused by friction between pelts and drum rotation while quickly penetrating, deswelling, and softening the limed pelt [46]. The essential goal of the deliming procedure is the removal of lime from the pelt, lowering the pelt's pH to a range of 8–9 to assist bating [47]. For instance, leftover Ca salts from the delimed pelts could affect the efficiency of the trypsin during bating, resulting in impermeable calcium sulfate precipitating in the pelts throughout the pickling, as well as negatively impact the uniformity of fat-liquoring [[8], [57]]. Moreover, a high concentration of Ca salts precipitating on the surface or in pelts would cause cracked grain, inadequate strength and also reduces color homogeneity in wet blue and crust leather [10,[44], [57]].

A crucial factor in assessing a deliming agent's quality is how much calcium has been removed from the pelt. In the CA + SHMP + EDTA and AS deliming procedures, the percentage of Ca removed from limed pelt was 70.6 and 66.2 %, respectively. It appears that CA + SHMP + EDTA extract Ca even more effectively. According to these findings, the combination of CA + SHMP + EDTA may be able to eliminate uneven dyeing and fatliquoring as well as lime. SHMP may be able to form water-soluble complexes with Ca2+ due to its strong chelating activity, which may account for its great calcium-removing tendency [58]. Additionally, CA is a calcium complexing agent [22]. Furthermore, it can be seen that the delimed with CA + SHMP + EDTA had a clear SEM picture after Ca removal, as shown in Fig. 2 (d). Based on this study, the capacity of CA + SHMP + EDTA, Ca removal is similar to that of AS. The Ca removal through the AS and super-critical carbon dioxide deliming processes was 63 % and 87 %, respectively [20] and Wang et al. [7] achieved 70 % Ca removal with 3 % AS.

3.3.4 Cr content in crust leather

The Cr absorption rate for produced crust leathers is shown in Fig. 4. Basic Chromium Sulfate is used more frequently during chrome tanning because of its pleasant feel and heat stability. Hashem et al. [59,60] reported that the crust leather contains Cr about 60 %. In both the experimental and conventional crust leather, the Cr uptake was 57.1 ± 12.2 g/kg and 65.4 ± 16.0 g/kg, respectively. A slight fluctuation in Cr uptake was observed, which can be caused by the functional group interacting with Cr [61]. Due to biological reasons and feeding strategies, animals’ functional group decreases at a certain period of their lives [62].Fig. 4 Chromium content in crust leather for laboratory scale and pilot scale experiment.

Fig. 4

3.3.5 Physical properties of leather

The product preparation plays a major role in the quality assessment, which also affects its value based on its physical characteristics [63]. Table 4 depicts the ball bursting strength, tensile strength, and % of elongation. The tensile strength and % of elongation of experimental leather 327.1 to 318.7 kg/cm2 and 54.0–75.9 %, respectively. On the other hand, the tensile strength and % of elongation for the conventional leather found were 252.9 to 240.7 kg/cm2, 45.0 %–73.7 %, respectively. The experimental leather exhibited a ball bursting strength and distension of 45.3 kg and 10.3 mm, respectively, while the conventional leather demonstrated almost comparable values of 40.5 kg and 10.2 mm, respectively. Similarly, the ball bursting strength, tensile strength, as well as % of elongation of the pilot scale's leather fulfilled the required specifications. A comparison of the experimental, conventional, and pilot data revealed minimal differences, according to the evaluation of the test values. The physical characteristics of the leather were influenced by several significant factors, such as collagen flexibility; weave pattern, tanning technique, and fiber orientation [64,65]. Overall, it can be concluded that the physical qualities of leather were not affected by the deliming with CA + SHMP + EDTA.Table 4 Tensile strength, percentage of elongation at break and ball bursting strength.

Table 4Sample ID	Tensile strength (kg/cm2)	Elongation (%) at break	Ball bursting strength	
Parallel	Perpendicular	Parallel	Perpendicular	Load (kg)	Distension (mm)	
Conventional	252.9 ± 4.6	238.6 ± 4.6	45.0 ± 1.0	73.7 ± 0.2	40.5 ± 5.7	10.2 ± 0.3	
Experimental	327.1 ± 3.9	317.7 ± 4.6	54.0 ± 1.58	75.9 ± 2.5	45.3 ± 2.3	10.3 ± 0.3	
P1	301.9 ± 3.7	260.1 ± 2.6	60.7 ± 1.0	67.0 ± 0.5	47.0 ± 1.7	9.6 ± 0.1	
P2	297.5 ± 3.9	252.8 ± 1.4	61.9 ± 2.0	56.0 ± 0.9	29.8 ± 0.3	8.9 ± 0.4	
P3	284.5 ± 4.7	241.7 ± 4.5	55.5 ± 1.6	53.4 ± 0.4	43.0 ± 1.4	9.1 ± 0.1	
P4	269.5 ± 0.2	227.1 ± 0.2	71.0 ± 1.8	56.9 ± 1.9	47.4 ± 1.2	9.2 ± 0.1	
P5	254.2 ± 2.4	231.6 ± 4.1	63.7 ± 1.7	42.6 ± 0.3	51.1 ± 1.7	9.6 ± 0.1	
P6	239.2 ± 2.4	221.4 ± 1.7	63.9 ± 1.1	45.0 ± 0.6	58.3 ± 0.9	9.8 ± 0.1	
P7	232.3 ± 0.3	237.5 ± 0.1	61.9 ± 0.5	47.2 ± 0.3	53.4 ± 1.1	9.4 ± 0.1	
P8	259.3 ± 0.1	243.7 ± 2.5	50.3 ± 1.3	43.8 ± 0.6	48.5 ± 0.5	9.4 ± 0.1	
P9	319.2 ± 0.7	263.8 ± 0.2	69.0 ± 0.6	59.2 ± 1.0	53.3 ± 1.4	9.4 ± 0.01	
P10	306.4 ± 2.6	260.7 ± 2.9	66.0 ± 0.5	53.6 ± 0.3	42.3 ± 0.3	9.3 ± 0.02	
Standard	200 kg/cm2	40–65 %	20 kg	7 mm	
References	ISO 3376:2020	ISO 3376:2020	ISO 3379:2015	

Fig. 5 shows the organoleptic qualities of the pilot-scale, conventional, and experimental leather, including color uniformity, fullness, smoothness, grain tightness, softness, and overall appearance. Folding, rolling, tugging, pressing, stretching, and other movements were among the tactile qualities used to identify the aforementioned parameters. The summarized findings by the experts indicated that traditional and experimental leathers didn't significantly differ in their organoleptic characteristics. Using CA + SHMP + EDTA as a deliming agent did not affect the organoleptic qualities of the pilot scale leather as well.Fig. 5 Organoleptic properties of experimental, conventional, and pilot scale production leather.

Fig. 5

The crust leather's primary mechanical and organoleptic characteristics are comparable to one another. These results showed that only the pelt surface developed calcium precipitates for its restriction dose by CA + SHMP + EDTA deliming, which helped to prevent the damage of enzymatic hydrolyzation degradation to the grain structure. Simultaneously, while neutralizing the alkali successfully, CA + SHMP + EDTA penetrated into the pelt without producing acid swelling, and thus created the necessary circumstances for the enzyme involved in the bating process [4].

3.3.6 Impact on thickness variation of limed pelt with deliming agents

Materials have to penetrate to neutralize alkali without producing swelling during the deliming procedure. The plasticity of delimed pelts was assessed measuring the rates at which their thickness changed. A larger plasticity is indicated by a larger thickness change rate. The delimed pelts using CA + SHMP + EDTA had a similar plasticity to those of AS [19]. The percentage of experimental thickness changes ranged from 45.8 % to 59.9 %, while the conventional ranged from 38.4 % to 53.0 %. The thickness changes for both deliming agents were aligned, as shown in Fig. 6. Nevertheless, there appeared to be a minor variation in the thickness change of the belly section. The explanation may be because their bellies are the smallest and weakest parts of their bodies due to their open structure [66]. The combination of CA + SHMP + EDTA had good results, enabling minimal acid swelling and a steady decrease in thickness; therefore, the overall change in thickness was satisfactory. The AS and CA + SHMP + EDTA deliming agents differed in percentage of thickness by 6.9 %–7.4 %. A comparison between the experimental and conventional deliming agents reveals that the CA + SHMP + EDTA reduced thickness more than the AS. Perhaps for this reason, the experimental deliming agent (CA + SHMP + EDTA) generated an additional permeable matrix in the deliming process.Fig. 6 Thickness changes of the delimed pelt with AS and (CA + SHMP + EDTA).

Fig. 6

3.3.7 Lime removal efficiency

Fig. 7 represents the percentage of lime removal efficiency for AS and CA + SHMP + EDTA in deliming operation. After 20 min, the lime removal for AS and CA + SHMP + EDTA was 34.6 % and 53.3 %, respectively. In comparison to AS deliming, Sathish et al. [6] reported a similar pattern for CO2 deliming. In this study, more effectively lime removal was followed in the initial stage of the deliming process. For lime removal, AS needs a longer period to run than CA + SHMP + EDTA. The highest percentage of lime removal was 72.8 % for CA + SHMP + EDTA and 66.3 % for AS. After that, the condition was stabilized with slower removal efficiency. Sathish et al. [67] and Flowers [68] research groups observed the similar outcomes as stated in section 3.3.3.Fig. 7 Percentage of lime removal for AS and (CA + SHMP+) in deliming operation at different time intervals.

Fig. 7

3.4 Kinetics study

The results of the PFO and PSO kinetics for AS deliming procedure is illustrated in Fig. 8 (a, b) and for CA + SHMP + EDTA deliming process in Fig. 8(c and d). For PFO kinetic, the calculated amount of k1, qe and R2 for AS deliming were 0.0011 min−1, 0.0486 mg/g and 0.8991; and for CA + SHMP + EDTA deliming were 0.0012 min−1, 0.0503 mg/g and 0.8946, respectively. Consequently, for PSO kinetic, R2, qe and k2 were computed to be 0.9995, 0.0533 mg/g, and 10.7082 g mg−1 min−1 for AS deliming; and 0.9997, 0.0556 mg/g, and 11.1387 g mg−1 min−1, respectively for CA + SHMP + EDTA deliming. The value of R2 and qe for PSO kinetic is greater than the PFO kinetic for both control and experimental study. The results clearly suggest that the reaction might be fitted to the PSO equation more precisely. Comparable findings were reported by Sathish et al. [67]. It is clear from the results that the CA + SHMP + EDTA deliming's rate constant (k2: 11.1387 g mg−1 min−1) and qe (0.0556 mg/g) exceeds than AS deliming's (k2: 10.7082 g mg−1 min−1; qe: 0.0533 mg/g). Thus, the results also confirm that the CA + SHMP + EDTA deliming procedure performs at higher speeds than the AS deliming. The ST3 provides the data of the PFO and PSO kinetics.Fig. 8 PFO and PSO kinetics for AS and (CA + SHMP + EDTA) deliming operations.

Fig. 8

3.5 Cost assessment of CA + SHMP + EDTA deliming technique

The expenses of AS and CA + SHMP + EDTA were compared. Table 5 clearly shows that CA + SHMP + EDTA appears to be slightly more expensive than AS. In comparison to AS, 1.24 times less CA + SHMP + EDTA is required to complete the procedure. In addition, the number of pollutants in the delimed liquor and ETP load of tanneries made with CA + SHMP + EDTA deliming agents is lower than with the traditional method, which would result in a large reduction in the expense of eliminating ammonia and other associated contaminants from wastewater. Therefore, compared to AS, the total cost of deliming based on CA + SHMP + EDTA is lower. Considering the cost of the chemicals and the results of the other physical tests, CA + SHMP + EDTA would be a better investment. When wastewater treatability and environmental benefits are combined, the advantage of the developed deliming agent becomes even more appealing.Table 5 Cost assessment of (CA + SHMP + EDTA) deliming technique.

Table 5Chemicals	Unit price (U$/kg)	Dose (%)	Cost (U$)	
Citric acid	0.87	0.50	0.44	
Sodium hexametaphosphate	1.50	1.25	1.88	
Ethylenediaminetetraacetic Acid	2.50	0.10	0.25	
Ammonium sulfate	1.00	2.25	2.25	

3.6 Implications of the study

In the leather industry, only the conventional deliming process is being used, although a high amount of NH3–N (1726.7 mgL−1), TKN (1861.1 mgL−1), BOD, COD, and TDS are produced by the conventional deliming. The main drawback of using ammonium salt in deliming is that it results in a high NH3–N level in the deliming effluents, which lowers the wastewater's C:N ratio. The CA + SHMP + EDTA deliming may result in significant energy savings by replacing ammonium salt. The CA + SHMP + EDTA deliming process produces high-quality leather and lessens the amount of pollutant in wastewater. Deliming by CA + SHMP + EDTA decreases in the amounts of BOD, COD, TDS, TKN (274.4 mgL−1) and NH3–N (32.7 mgL−1) in the deliming float, with increasing the C:N ratio in the tannery wastewater. Zeng et al. [10] and Ammary [69] reported that the use of SHMP will introduce phosphorus into the wastewater, a kind of necessary nutrient to favor the biological treatment of total wastewater. In the present study, the effectiveness of CA + SHMP + EDTA as a deliming agent was only investigated under a laboratory scale. In future work, this new deliming agent should be applied on different animal hides and skin collected from different geographical locations to further investigates its efficiency.

4 Conclusion

The nitrogen-containing contamination in tannery wastewater can be dramatically reduced using the mixture of CA, SHMP, and EDTA in the deliming process. As a deliming agent, CA + SHMP + EDTA exhibits a good buffering action as well as it can keep pH at 8.0–9.0, which can reduce the possibility of sudden acid swelling. The CA and SHMP are effective in removing calcium salts from the limed pelts because of their good chelating ability with calcium. Compared to the traditional deliming agent, the wastewater generated from the experimental deliming process has lower levels of BOD, COD, EC, and TDS. These findings demonstrate that a considerable reduction in pollutants throughout the leather manufacturing process can be achieved. Furthermore, this procedure does not produce ammonia, which is important for controlling the environmental pollution. The reduction in the amount of NH3–N and TKN by CA + SHMP + EDTA are 98.1 % and 85.26 %, respectively, in contrast to conventional deliming. The physical characteristics, shrinkage temperature, and Cr uptake of the produced leather meet the standard criteria. Similar outcomes are also found in the pilot scale experiments. Therefore, this study suggests, CA + SHMP + EDTA can be an effective replacement for ammonium salts to delime the pelt making the leather production more environmentally friendly.

5 Statement & declarations

The authors declare that the submitted manuscript is original. Authors also acknowledge that the current research has been conducted ethically and the final shape of the research has been agreed by all authors. The authors declared that this manuscript does not involve researching about humans or animals.

Funding

Authors are grateful to University Grants Commission (10.13039/100015747 UGC ) for providing financial support approved by the Committee for the Advanced Studies & Research (CASR), 10.13039/501100008677 Khulna University of 10.13039/100000084 Engineering & Technology (KUET), Khulna, Bangladesh.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

CRediT authorship contribution statement

Modinatul Maoya: Conceptualization, Formal analysis, Investigation, Methodology, Writing – original draft. Md Abul Hashem: Investigation, Methodology, Software, Supervision, Visualization, Writing – review & editing. Md Mukimujjaman Miem: Formal analysis, Methodology, Validation. Eshtiyaq Tauhid Enan: Data curation, Methodology. Sakina Islam: Investigation, Methodology, Software. Mehedi Hasan: Methodology, Software.

Declaration of competing interest

All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.

This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.

The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript.

Appendix A Supplementary data

The following is the Supplementary data to this article.Multimedia component 1

Multimedia component 1

Appendix A Supplementary data to this article can be found online at https://doi.org/10.1016/j.heliyon.2024.e36007.
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