Removal of 4-Chloro-3-Methylphenol (p-Chlorocresol) from Tannery Effluent, from Lab to Pilot Scale

Abstract

This study determined the effectiveness of ozone and Fenton’s reagents, and the ozone delivery system, for the degradation of the persistent organic compound p-chlorocresol (4-chloro-3-methylphenol) in tannery wastewater. Laboratory trials demonstrated that removal of p-chlorocresol from wastewater at pH 4.1 using ozone was at least 70% within 2–3 h. Fenton’s reagent did improve the p-chlorocresol removal, but only by 10%. When we normalize the % removal data to account for reaction time and ozone dose, the degradation rates of p-chlorocresol in the ozone only and ozone plus Fenton’s treatments were similar, ~76 mg p-chlorocresol g⁻ O₃ h⁻¹. In the pilot trials, the addition of ozone via a venturi delivery system with improved circulation achieved up to 46% removal of the p-chlorocresol in wastewater at pH 4.1 within 24 h; this equated to a removal rate of 3.8 mg p-chlorocresol g⁻ O₃ h⁻¹. The large difference in removal rate between the pilot and lab trials was attributed to higher organic load in the wastewater. The addition of Fenton’s reagent increased the removal of the p-chlorocresol to 66% in the venturi injection trial and almost doubled the removal rate (6.7 mg chlorocresol g⁻ O₃ h⁻¹). The addition of ozone via a direct blower system was not as effective as the venturi system because of the large loss of ozone from the tank reactor.

1. Introduction

The tannery industry generates wastewater containing ammonium, sulfides, and persistent organic compounds (POCs) including phenolic compounds (i.e., cresols) [1,2,3]. Phenolic compounds such as p-chlorocresol (4-chloro-3-methylphenol) are commonly used in tanneries as an antimicrobial agent. However, p-chlorocresol is considered a priority pollutant by the US Environmental Protection Agency (EPA) due to its toxicity to aquatic life [4,5].

In Australia, during tannery wastewater treatment, dissolved air flotation (DAF) is used to remove solids, which are disposed of in landfills while the clarified wastewater is applied to land. However, both the solid and liquid streams from tannery DAF systems contain p-chlorocresol. If p-chlorocresol can be removed from wastewater prior to DAF treatment, then the cost of landfill disposal would decrease significantly, and the potential contamination of water bodies from land application of liquid waste disposal could be alleviated.

Many of the organic compounds applied in leather tanning processes resist conventional chemical and biological wastewater treatment processes. For this reason, other methods need to be applied as an alternative to biological and classical physico-chemical processes [2,6,7,8]. An effective means of degrading p-chlorocresol into more benign compounds is the use of advanced oxidation procedures (AOPs) such as ozonation, ultraviolet (UV) oxidation, the Fenton process (addition of hydrogen peroxide—H₂O₂ and iron—Fe²⁺), and combined processes [8,9,10,11,12]. The high turbidity of tannery wastewater may exclude UV treatment, and Fenton’s reagents can be a major cost barrier, particularly H₂O₂ [6,8,13].

Another approach for POC degradation is to use ozone (O₃) [10,14,15], a powerful oxidizing agent that can be produced onsite relatively cheaply. Ozone is known to degrade phenolic compounds [16], but studies to date have mostly focused on the degradation of pure p-chlorocresol rather than p-chlorocresol in an industrial wastewater environment. Wastewater has a much higher organic load than pure compound; therefore, the efficiency of POC breakdown by O₃ is expected to be much lower. In such case, it may be possible to hybridize the oxidation process, for instance by combining O₃ oxidation and the Fenton reaction [10,13]. The Fenton reaction is essentially an oxidation reaction in which H₂O₂ forms highly reactive hydroxyl radicals (•OH) in the presence of Fe²⁺ [7,17]. Again, the combination of O₃ oxidation and the Fenton reaction has only been applied to pure phenolic compounds including pure p-chlorocresol, not industrial wastewater [8,10,14,18,19,20].

In addition to the choice of AOP, the delivery method into the wastewater stream is important from both an efficiency and operational perspective. For example, O₃ can be delivered to the wastewater stream via dissolution in water, or through direct injection. The dissolution option involves drawing O₃ into a water stream with a venturi and delivering that water stream into a wastewater holding tank [8]. Venturi injection allows O₃ to dissolve in the delivery water, giving it excellent contact with the organic material in the wastewater. However, the delivery water must be relatively free of solids to avoid clogging of either the pump or venturi orifice. Direct injection involves pumping O₃ into the wastewater. While this technique avoids issues of clogging, the transfer of O₃ into the liquid phase is low; hence, the efficiency of degradation can be compromised [13].

To the best of our knowledge, no study has evaluated the degradation of p-chlorocresol in tannery wastewater environment. In this study, we evaluated the suitability of O₃ oxidation, both alone and in combination with Fenton’s reagent, as a means for degrading p-chlorocresol in tannery wastewater. We also compared dissolved and direct techniques for O₃ delivery for removal of p-chlorocresol. All trials were conducted at both laboratory and pilot scales. We designed and constructed a pilot scale system and investigated the optimal operational parameters for the removal of p-chlorocresol from tannery wastewater. The main objective of this work was to develop and propose an effective technology for p-chlorocresol removal suitable for implementation in tannery processing plants.

2. Materials and Methods

2.1. Lab-Scale Trials

2.1.1. Sample Preparation and Tannery Wastewater Collection

Pure p-chlorocresol (4-chloro-3-methylphenol) and surfactant TritonX were purchased from Sigma-Aldrich (USA). According to the manufacturer’s data sheet, solubility of pure p-chlorocresol in water varies from 3.3 g L⁻¹ at pH 5 to 4.2 g L⁻¹ at pH 9. It is highly soluble in organic solvents such as hexane, ethanol, dichloromethane, and acetone. Pure p-chlorocresol was first dissolved in water with the assistance of surfactant TritonX (1 g L⁻¹) to prepare an 850 mg L⁻¹ test solution.

Wastewater samples for the laboratory experiments were collected from the balance tank from a commercial tannery (NSW, Australia) for lab trials. The balance tank is located between the wastewater sump and dissolved air flotation system, where the wastewater is temporally stored for pH adjustment after the removal of large solids. For the pilot trials at the site, wastewater from the main sump was used before any primary treatments. The elemental composition of the wastewater sample was analysed by the Environmental Analysis Laboratory at Southern Cross University, NSW, Australia.

2.1.2. Establish Basic Ozone Treatment Conditions

In the initial trials, 1 L of pure p-chlorocresol test solution (850 mg L⁻¹) and tannery wastewater sample (from balance tank) were treated separately at an O₃ dose rate of 0.5 g h⁻¹ using a small O₃ generator (1 g h⁻¹) using bottled O₂ (Figure 1a) under laboratory conditions at room temperature (≈22 °C), and samples were collected every hour for 3 h. The collected samples were analyzed for p-chlorocresol using the gas chromatographic method described below (Section 2.3). There was a noticeable loss of O₃ from the reaction vessel in the first trials, prompting the use of a taller reaction vessel.

Figure 1. Schematic diagram of lab setup (a) showing O₃ generator (left), 2 L capacity reaction vessel with magnetic stirrer (center), and residual O₃ trap (right); pilot system set up at Hide Tanners (b) for venturi reactor system with recirculation pump and 1000 L reaction tank and (c) for blower reactor system and 1000 L reaction tank with a fine bubbler. The black arrows show the direction of flow.

A second series of experiments with wastewater was carried out using a new elongated reaction vessel. The column was 90 mm (Φ) × 640 mm (height) and held 4 L of liquid, allowing for greater contact time between O₃ and the wastewater. The liquid sample was agitated using a magnetic stirrer, and the O₃ was introduced using a sintered glass bubbler. For this experiment, a series of O₃ dose rates (0.4, 0.5, 0.65, and 0.75 g h⁻¹) were tested over a 3 h incubation period.

2.1.3. Combined Ozone Treatment with Fenton’s Oxidation

To improve p-chlorocresol breakdown without increasing the incubation time, we tested the effects of Fenton’s reagent (Fe²⁺ and H₂O₂) in combination with O₃ on the degradation of p-chlorocresol using an elongated reactor column. The O₃ dose rate was fixed at 0.5 g h⁻¹, as this was the most efficient delivery rate from the previous experiment. An Fe²⁺ concentration of 2 mM was chosen; this was kept constant through all the experiments. The concentrations of H₂O₂ tested were 1, 10, 50, and 100 mM. In addition, an experiment at 50 mM H₂O₂ without O₃ was also performed. All experiments were run over 3 h.

2.2. Pilot-Scale Field Trials

2.2.1. Dissolved Ozone Delivery by Venturi

In the first field trial, O₃ was delivered to the wastewater through a venturi system. Wastewater from the main drainage sump was manually added to a 1 m³ holding tank and then recirculated at 140 L min⁻¹ via an external centrifugal pump (Figure 1b). The venturi drew O₃ from a 5 g h⁻¹ O₃ generator, which was mixed with the recirculating wastewater and returned to the holding tank via a 4.8 m reaction column (50 mm PVC pipe). A series of trials with O₃ and Fenton’s reagents were carried out (Table 1). The ambient temperature during the field trials was approximately 24 °C.

Table 1. Details of the pilot-scale trials with tannery wastewater using both venturi-based dissolved O₃ delivery and direct O₃ injection. Concentrations of the Fenton’s reagents are also shown.

Delivery System	Treatment	Wastewater (L)	O3 Application (g h−1)	H2O2 (mM)	Fe2+ (mM)
Venturi delivery with centrifugal pump	O3	400	5	-	-
	O3 + Fenton’s	400	5	50	0.25
	O3 + Fenton’s	400	5	50	0.50
	Fenton’s only	400	-	50	0.125
Venturi delivery with screw pump	O3	700	20	-	-
	O3 + Fenton’s	700	20	50	0.5
Direct delivery with blower	O3	700	20	-	-
	O3 + Fenton’s	700	20	50	0.5

Wastewater samples were collected prior to the treated wastewater returning to the holding tank. The final treated wastewater samples were collected after 24 h. The p-chlorocresol levels in the samples were analyzed. The COD (chemical oxygen demand), TOC (total organic carbon), and DOC (dissolved organic carbon) were also determined in the samples (Environmental Analysis Laboratory at Southern Cross University, NSW).

Samples were collected every 2 and 4 h for 24 h. The p-chlorocresol in the samples were analyzed.

2.2.2. Direct Ozone Delivery by Air Blower

The delivery of O₃ via a side channel air blower was tested (Figure 1c). The O₃ system was also upgraded to a 20 g h⁻¹ generator. To increase contact time, we used 700 L rather than 400 L. The series of trials were carried out in duplicate (Table 1). The samples were collected every 2 or 4 h for 24 h. The p-chlorocresol levels in the samples were analyzed.

2.3. Method for p-Chlorocresol Determination

Due to low molecular weight of p-chlorocresol (4-chloro-3-methylphenol), we focused on a gas chromatographic method. Pure p-chlorocresol (Sigma-Aldrich) was purchased, and a series of standards were prepared in ethanol. Pure compound, which is relatively insoluble, was first dissolved in water with the assistance of the surfactant TritonX (1 g L⁻¹). The developed method relies on gas chromatography with a flame ionization detector (GCFID). The method takes 30 min per sample, with a retention time of 19.75 min. A linear response was found with a limit of detection of approximately 50 µg L⁻¹ (50 ppb). To extract the p-chlorocresol, 50 mL of sample was partitioned against dichloromethane (DCM) and salted with a saturated CaCl₂ solution to drive the compound into the organic phase. The DCM solutions were taken to dryness on a rotary evaporator, made up quantitatively with 10 mL of ethanol, and analyzed via GCFID against standards of known p-chlorocresol concentrations. The same procedure was followed for all wastewater samples to identify the p-chlorocresol levels.

2.4. Kinetic Models

The time series data of p-chlorocresol removal in wastewater were fitted to mathematical equations using Pseudo-first-order, Pseudo-second-order (PSO), and Elvoich kinetic models. The linear forms of Pseudo-first-order, Pseudo-second-order, and Elvoich kinetic models can be represented by Equations (1)–(3), respectively:

$$q_t = q_e\left(1 -exp\left(-K_{1}t\right)\right)$$

(1)

$$q_t=K_2{q_e}^{2}t/1+K_2{q}_{e}t$$

(2)

$$q_t=a ln\left(t\right)+b$$

(3)

where q_t (mg L⁻¹) and q_e (mg L⁻¹) are the amount of p-chlorocresol removed from the wastewater at time t and at equilibrium, respectively; k₁ (h⁻¹) and k₂ (L mg⁻¹ h⁻¹) are the rate constant of the pseudo-first-order and PSO models, respectively; a (mg L⁻¹ h⁻¹) and b are Elovich constants. The Excel Solver is used to calculate the models’ parameters.

3. Results

The elemental composition of untreated tannery wastewater is presented in Table 2. The average pH of the tannery wastewater was 4.09. It contained an average concentration of 175 mg L⁻¹ of p-chlorocresol, along with high levels of total organic carbon. Significant concentrations of chromium and iron were also found in the tannery wastewater (Table 2).

Table 2. Elemental composition of untreated wastewater collected from tannery wastewater sump.

Parameter	Concentration
pH	4.09
Conductivity (EC) (mg L−1)	32.6
Total organic carbon (mg L−1)	2244
* Total organic carbon (mg L−1)	1558
p-chlorocresol (mg L−1)	175.0 ± 12.14
Iron (mg L−1)	16.0
Aluminum (mg L−1)	7.77
Chromium (mg L−1)	457
Manganese (mg L−1)	0.82
Copper (mg L−1)	<0.001
Zinc (mg L−1)	<0.001
Nickel (mg L−1)	0.55
Lead (mg L−1)	<0.001
Selenium (mg L−1)	0.02

Note: * Wastewater collected from balance tank.

3.1. Lab-Scale Optimization

3.1.1. Basic Ozone Treatment

The removal of pure p-chlorocresol was 95% (Figure 2). The concentration of p-chlorocresol in the wastewater (pH 4.09) reduced by 75% within 3 h of treatment at an O₃ dose rate of 0.5 g h⁻¹ (Figure 2). This was slower than the pure compound trial at the same O₃ dose rate (Figure 2), most likely due to the presence of other organic compounds in the tannery wastewater that were consuming O₃.

Figure 2. Concentration of pure p-chlorocresol and p-chlorocresol in tannery wastewater over time at an O₃ dose rate of 0.5 g h⁻¹.

Higher O₃ dose rates did not induce a linear increase in the p-chlorocresol degradation because there was a significant loss of O₃ observed from the small reactor vessel (Figure 1a). Hence, another set of experiments was performed on tannery wastewater in a lab-scale 4 L trial using a modified reactor set up with a taller reaction vessel.

For the modified lab reactor, all O₃ dose rates showed a reduction in the p-chlorocresol concentration in wastewater over time (Figure 3a), with the quickest consumption of p-chlorocresol measured at the highest dose rate (0.75 g h⁻¹; Figure 3a). There is a significance difference (p > 0.05; Table S1 in Supplementary Materials) in the p-chlorocresol degradation observed within the O₃ dose rates. However, at the two highest dose rates (0.65 and 0.75 g h⁻¹), there was a notable discharge of O₃, indicating that the contact time could be improved. At a dose rate of 0.4 g h⁻¹, 42% of the p-chlorocresol in 4 L of wastewater was consumed in 3 h, whereas at 0.75 g h⁻¹ 70% of the test POC was consumed within 2 h (Figure 3a). Because different O₃ doses, reactor volumes, and treatment times were used, it can be difficult to compare the p-chlorocresol removal between different experiments. To make this comparison easier, we calculated a POC consumption rate which accounts for different experimental conditions, where units are mg p-chlorocresol g⁻ O₃ h⁻¹. At the highest O₃ dose rate, the consumption of p-chlorocresol was 121 mg chlorocresol g⁻ O₃ h⁻¹ (Figure 3a).

Figure 3. (a) Degradation of p-chlorocresol in 4 L tannery wastewater at different O₃ dosing rates of 0.4, 0.5, 0.65, and 0.75 g h⁻¹ over 3 h. (b) Degradation of p-chlorocresol in 4 L tannery wastewater at different H₂O₂ dosing rates (1, 10, 50, and 100 mM) over 3 h. The O₃ dosing rate was 0.5 g h⁻¹, and the Fe²⁺ concentration was kept constant at 2 mM. A Fenton’s-only (50 mM) and O₃-only treatment (0.5 g h⁻¹) are also shown. Percent reduction is also shown, as well as the p-chlorocresol consumption rate, in units of mg p-chlorocresol g⁻ O₃ h⁻¹ (bold black numbers).

3.1.2. Combined Ozone and Fenton’s Reagent Treatment

A significance difference (p > 0.05; Table S1) in the p-chlorocresol degradation is observed in the O₃ plus Fenton dose rates. The Fenton’s reagent alone (50 mM H₂O₂) showed a 37% reduction in p-chlorocresol over three hours, roughly half of the removal achieved with the 0.5 g h⁻¹ O₃ dose (Figure 3b). This rules out the use of the Fenton’s reaction as a replacement for O₃. When Fenton’s reagent and O₃ were combined, no increase in p-chlorocresol removal was found at a H₂O₂ dose concentration of 1 mM compared to the 0.5 g h⁻¹ O₃ dose (Figure 3b). However, at 10, 50, and 100 mM of H₂O₂, the removal of p-chlorocresol was generally faster, with a lower final concentration. At the highest H₂O₂ dose rate (100 mM), there was a 10% improvement in the overall removal of p-chlorocresol relative to the O₃ only treatment. At this highest H₂O₂ dose rate, the consumption of p-chlorocresol was 84 mg POC g⁻ O₃ h⁻¹, which was higher than the consumption rate with the 0.5 g h⁻¹ O₃ alone dose rate (72 mg p-chlorocresol g⁻ O₃ h⁻¹; note that if we calculate this at 2 h, the removal rate is around 120 mg p-chlorocresol g⁻ O₃ h⁻¹, Figure 3b). Interestingly, the effect of combining Fenton’s reagent and O₃ was not additive. In other words, the % removal with the combined O₃ and Fenton’s reagent (68%) was less than expected if we were to add the individual consumption rates together (37% and 63% for the 50 mM Fenton’s-only and 0.5 g h⁻¹ O₃-only treatments, respectively).

3.2. Pilot Field Trials

The effects of different advanced oxidation processes and delivery systems on the removal of p-chlorocresol in tannery wastewater during 24 h field trials are presented in Table 3. The studies compare O₃ treatment alone, O₃ combined with Fenton’s reagent, and Fenton’s reagent alone across varying O₃ delivery approaches, reactor volumes, and circulation systems.

Table 3. Removal of p-chlorocresol during the 24 h field trials.

Treatment	O3 (g h−1)	H2O2 (mM)	Fe2+ (mM)	Removal *
				(%)	mg p-Chlorocresol g− O3 h−1
Venturi delivery with centrifugal pump
O3	5	-	-	33.5	7.5
O3 + Fenton’s	5	50	0.25	39.8	9.0
O3 + Fenton’s	5	50	0.50	47.9	11.5
Fenton’s only	-	50	0.125	32.3	-
Venturi delivery with screw pump
O3	20	-	-	46 ± 3.5	3.8 ± 1.5
O3 + Fenton’s	20	50	0.5	66 ± 2.1	6.7 ± 1.4
Direct delivery with blower
O3	20	-	-	37 ± 3.1	4.1 ± 0.42
O3 + Fenton’s	20	50	0.5	55 ± 7.1	5.3 ± 0.28

Note: * Data represent the mean of duplicate trials ± standard deviation.

3.2.1. Dissolved Ozone Delivery

Without Fenton’s reagent, dissolved O₃ delivered by venturi using a centrifugal pump achieved 33% removal of p-chlorocresol; however, the removal rate (7.5 mg p-chlorocresol g⁻ O₃ h⁻¹; Table 3) was much lower than in the lab-based trials (72 mg p-chlorocresol g⁻ O₃ h⁻¹; Figure 3b). This is because the O₃ dose in the lab trial (Figure 3) per L of wastewater was much higher than for the field trial (0.5 g h⁻¹ for 4 L vs. 5 g h⁻¹ for 400 L).

In addition, for the lab trials, the wastewater was collected from the balance tank where the TOC was less 30% of the TOC in the main sump (Table 2), which is where wastewater was drawn for the pilot trials. The higher organic load would undoubtedly consume O₃. With the addition of Fenton’s reagent, an increase in the percent removal of p-chlorocresol was observed (Table 3). Maximum removal was 48% (11.5 mg p-chlorocresol g⁻ O₃ h⁻¹; Table 3) at a dose rate of 0.50 mM Fe²⁺ and 50 mM of H₂O₂. The combination treatment system appears to be at least additive, in that the % removal with O₃ + Fenton’s (48%) was higher than the sum of individual % removals (33.5% for O₃ only and 32.3% for Fenton’s only).

Figure 4 shows the comparative removal efficiencies of COD, TOC, DOC, and p-chlorocresol under different treatment conditions in the venturi delivery trials with a centrifugal pump. A significance difference (p > 0.05; Table S2) in the removal efficiencies is observed within the parameters under different treatments. O₃ only produced modest removals, with COD, TOC, and DOC each reduced by about 26%, 25%, and 16%, respectively, and p-chlorocresol showed the highest removal at ~33.5. For O₃ plus Fenton’s reagent at a lower iron dose (0.25 mM Fe), COD removal remained at ~24%, but mineralization and micropollutant breakdown improved, with TOC, DOC, and p-chlorocresol removals all increasing to ~38–40%. At the higher iron dose (0.50 mM Fe), COD removal rose to ~41%, TOC removal was moderate at ~31%, DOC dropped sharply to ~12%, and p-chlorocresol removal reached the highest level observed (~48%). Overall, these results indicate that the combined O₃ and Fenton process is more effective than O₃ only for removing p-chlorocresol and bulk organics. However, optimization of Fe and H₂O₂ dosages is critical, since excessive Fe loading can hinder the mineralization efficiency despite improving target pollutant removal. In addition, the p-chlorocresol accounts for about 8% of the total carbon content of the wastewater (Table 2), and yet the total organic carbon removal was well over 20% for all treatments (Figure 4).

Figure 4. Removal of COD (chemical oxygen demand), TOC (total organic carbon), DOC (dissolved organic carbon), and p-chlorocresol in the venturi delivery trials with centrifugal pump.

There were no issues observed with clogging of the improved circulation pump (screw pump) due to the high solid contents in the tannery wastewater. By using only O₃ through a 20 g/h O₃ generator and venturi delivery system with the improved circulation pump unit, the p-chlorocresol was removed by 46% (Table 3), which was higher than that of the venturi delivery system with a centrifugal pump (33.5%; Table 3). The addition of Fenton’s reagents increased the percent removal of p-chlorocresol by 43% compared to O₃ only using the improved circulation pump. However, the overall p-chlorocresol removal rate (expressed as mg p-chlorocresol g⁻¹ O₃ h⁻¹) decreased by 42–49% in the improved circulation pump system with a 20 g/h O₃ generator compared to the centrifugal pump system using a 5 g/h O₃ generator (Table 3). This reduction in the p-chlorocresol removal rate was attributed to the larger wastewater volume used (700 L instead of 400 L) in the improved circulation pump system, which also resulted in greater O₃ losses with the 20 g/h O₃ generator.

3.2.2. Direct Ozone Delivery

The direct delivery system with a 20 g/h O₃ unit reduced p-chlorocresol removal by 20% (O₃ only) and 17% (O₃ plus Fenton’s reagents) compared to the venturi system with an improved circulation pump, which achieved 37% and 55% removal, respectively (Table 3). In contrast, under direct delivery trial, p-chlorocresol removal increased from 33.5% to 37% with O₃ alone, and from 48% to 55% with O₃ plus Fenton’s reagents, compared to venturi trials using a centrifugal pump. The p-chlorocresol removal rate for O₃ alone was nearly the same (~4 mg p-chlorocresol g⁻¹ O₃ h⁻¹) using direct delivery and the venturi system with an improved circulation pump. However, with O₃ plus Fenton’s reagents, the removal rate decreased from 6.7 to 5.3 mg p-chlorocresol g⁻¹ O₃ h⁻¹ (Table 3).

3.3. Kinetics of p-Chlorocresol Removals

The kinetic data of p-chlorocresol removal was fitted with pseudo-first-order, pseu-do-second-order, and Elovich kinetic models. The estimated kinetic models’ parameters are presented in Table 4. The curve fitting of the kinetic models for the removal of the p-chlorocresol are presented in Figure 5.

Table 4. Estimated kinetic model parameters for removal of p-chlorocresol in wastewater.

Treatment	Pseudo-First Order			Pseudo-Second Order			Elovich
	K1 (h−1)	R2	qe (mg L−1)	K2 (L mg−1 h−1)	R2	qe (mg L−1)	R2	qe (mg L−1)
Venturi delivery *
O3	0.0767	0.9468	78.96	0.0006	0.7505	98.8	0.9369	61.3
O3 + Fenton’s	0.3433	0.8388	109.95	0.0101	0.9955	115.0	0.7288	106.5
Direct delivery
O3	0.0618	0.8503	71.45	0.0005	0.4607	88.0	0.9311	56.3
O3 + Fenton’s	0.3102	0.8683	94.63	0.0266	0.9988	93.0	0.4117	87.0

Note: * With screw pump for improved circulation.

Figure 5. Removal of the p-chlorocresol in 700 L of tannery wastewater at a 20 g h⁻¹ O₃ dosing rate over 24 h using (a) a venturi delivery system with improved circulation pump (screw pump), and (b) direct delivery system with blower in pilot trials. Two replications of each of trial were used. Error bars reflect the variability among different batches of wastewater, but the overall trends remain clear. A Fenton’s dose rate of 0.50 mM Fe²⁺ and 50 mM of H₂O₂ was added. Curves are fitted using pseudo-first-order model, pseudo-second-order (PSO) model, and Elovich kinetic models.

A significance difference (p > 0.05; Table S3) in the removal of p-chlorocresol is observed under different treatment conditions (O_3, and O₃ plus Fenton’s reagent), and delivery systems. The O₃ plus Fenton’s reagent showed a significantly higher removal capacity (qe) of p-chlorocresol in wastewater for both the venturi delivery (with an improved circulation pump) and direct delivery (with blower) system compared to O₃ only (Table 4; Figure 5). The p-chlorocresol removal using O₃ plus Fenton’s reagent was better described by the pseudo-second-order model, with relatively greater qe values and correlation coefficient (R²) (Table 4; Figure 5). The pseudo-first-order and Elovich models exhibited relatively higher correlation coefficients (R²) for O₃ only, suggesting a better fit for describing its removal behavior (Table 4; Figure 5).

It was estimated from curve fitting using the PSO equation that a maximum removal of p-chlorocresol in wastewater could be reached up to ≈64% (112 mg/L) within 24 h using O₃ plus Fenton’s reagent via venturi delivery (with an improved circulation pump). There could be little to no further removal of p-chlorocresol in wastewater beyond 24 h under the same conditions (Figure 5a). Using O₃ only could remove the p-chlorocresol in wastewater up to ≈42% (74 mg/L) in 24 h. However, there could be further removal of the p-chlorocresol in wastewater after 24 h using O₃ only (Figure 5a). The p-chlorocresol could be removed up to 17% (31 mg/L) in 5 h by O₃ only. However, when the Fenton’s reagents were added with O₃, the removal of p-chlorocresol was ≈57% (99 mg/L) within 5 h (Figure 5a).

The PSO model estimated that a maximum ≈ 55% (87 mg/L) removal of p-chlorocresol in wastewater was achieved in 24 h using O₃ plus Fenton’s regent with the direct delivery system (Figure 5b). O₃ only could remove the p-chlorocresol in wastewater up to ≈38% (72 mg/L) in 24 h. However, there could be further removal of the p-chlorocresol after 24 h using O₃ only. O₃ only can remove the p-chlorocresol by only 12% (22 mg/L) in 5 h. The removal of p-chlorocresol can be achieved up to 52% (82 mg/L) within 5 h using the Fenton’s reagents plus with O₃ (Figure 5b).

4. Discussion

4.1. Influence of Treatment Type on p-Chlorocresol Removal

Ozonation, and the combination of ozonation with Fenton’s reagents, are effective means of destroying p-chlorocresol in pure solutions [10,16,21]. Combining O₃ treatment with Fenton’s oxidation reaction is also a promising technique for degrading phenolic compounds in wastewaters [3,7,13,14,19,22]. Our results clearly demonstrate that O₃ is a highly effective oxidizing agent for degrading the p-chlorocresol in real tannery wastewater (Figure 5; Table 3). Furthermore, the addition of Fenton’s reagent was clearly beneficial for the p-chlorocresol breakdown, almost doubling the percent removal of our p-chlorocresol compared to O₃ only in both the venturi delivery and direct delivery trials (Table 3). For example, the addition of Fenton’s reagent with O₃ increased the removal of p-chlorocresol in wastewater up to 66% (Table 3), which indicates a synergistic effect of the O₃ plus Fenton process. These results align with the literature showing that hybrid O₃/Fenton’s processes are promising for treating phenolic pollutants [23,24]. This is also consistent with the reports that hybrid AOPs provide synergistic effects by simultaneously generating hydroxyl radicals (•OH) through ozone decomposition and Fenton’s reaction [25,26]. Our data (Table 3) shows that p-chlorocresol removal increased with higher H₂O₂ and Fe²⁺ within the tested window (e.g., at 400 L, increasing Fe²⁺ from 0.25 to 0.50 mM, with H₂O₂ 50 mM increasing removal from 39.8% to 47.9%). This is consistent with higher transient hydroxyl radical production when the H₂O₂:Fe²⁺ ratio is favorable.

4.2. Influence of Treatment Type on COD, TOC, and DOC Removal

When O₃ only was applied, moderate removal of COD (≈26%) and TOC (≈27%) was achieved (Figure 4). However, DOC removal remained low (≈17%; Figure 4), indicating that ozonation alone was insufficient for mineralization of dissolved organics. This is consistent with previous findings that O₃ only primarily acts through selective oxidation of phenolic compounds rather than complete degradation of organic matter [26,27]. The addition of Fenton’s reagent substantially improved overall removal. The COD, TOC, and DOC removals increased to ≈38–41% (Figure 4), suggesting a synergistic effect between O₃ and hydroxyl radicals generated by the Fenton’s reaction. Hydroxyl radicals are non-selective oxidants that attack a broader range of organic molecules, enhancing both degradation and mineralization [22,28]. In contrast, increasing the Fe concentration (from 0.25 to 0.50 mM) while keeping H₂O₂ constant resulted in higher COD (≈42%) removal, but TOC (≈28%) and DOC (≈12%) removals decreased significantly. This may be attributed to the scavenging of hydroxyl radicals by excess Fe²⁺, which reduces the availability of reactive species for mineralization of bulk organic matter [29,30].

Multiple studies, including olive-mill and pharmaceutical wastewater tests, reported an improved COD/TOC removal and faster treatment when O₃ and Fenton are integrated vs. either alone. Optimizing the treatment sequence, reagent dosage, and pH is critical [31]. Ozonation often breaks down refractory molecules into smaller, more biodegradable intermediates, thereby increasing the BOD/COD ratio. A subsequent Fenton step can then achieve further mineralization. Sequential or combined O₃–Fenton processes are designed to exploit this synergy [31].

The very high organic load in the tannery wastewater means that much of the O₃ is directed towards the oxidation of other organic material, which was reflected in the removal of COD and TOC (over 25% using O₃; over 24–41% using O₃ plus Fenton; Figure 4). Similar findings were reported in other studies with ozonation and Fenton oxidation used for the treatment of industrial wastewater [6,8,13]. Nevertheless, we were still able to remove appreciable p-chlorocresol from the tannery wastewater.

4.3. Influence of Wastewater pH on p-Chlorocresol Removal

The pH of our wastewater (4.09; Table 2) may be a limiting factor for the removal of p-chlorocresol, as pH is likely the most critical parameter given that ozone and Fenton’s reagent processes operate optimally in different pH ranges. Ozone can react directly with electron-rich groups (such as double bonds and activated phenols). Under near-neutral to alkaline conditions (pH > 7), it also decomposes to generate hydroxyl radicals (•OH), which are less selective but far more powerful oxidants [10,32]. The homogeneous Fenton’s reaction (Fe²⁺ + H₂O₂ → Fe³⁺ + •OH + OH⁻), which involves the use of hydrogen peroxide (H₂O₂) and iron (Fe²⁺), produces hydroxyl radical most efficiently at pH ≈ 2.5–3.5. At very low pH values (< 2), H₂O₂ is stabilized and side reactions occur, while at pH ≈ 3.5–4, iron precipitates as hydroxides, reducing catalytic activity [33,34]. Therefore, combining O₃ and Fenton’s reagent typically increases the overall hydroxyl radical flux, leading to faster oxidation, higher mineralization (TOC removal), and shorter treatment times compared with either process alone [10]. The oxidation pathway likely favored the degradation of p-chlorocresol, a phenolic compound highly susceptible to hydroxyl radical attack [35].

In our study, as the pH (4.09) lies between the optimal windows for ozonation and Fenton processes, p-chlorocresol is oxidized by both molecular O₃ and hydroxyl radicals. Under this condition, ozone contributes through direct electrophilic attack and limited hydroxyl radical generation, while Fenton’s regent activity persists (though weaker than at pH ≈ 3) because some Fe²⁺ remains soluble (less precipitation than at pH > 5.5) [34,36]. A combined O₃ plus Fenton sequence or hybrid (simultaneous) treatment can produce synergies (enhanced hydroxyl radical availability, faster ring-opening/mineralization), but this requires careful control of reagent dosing, pH, and scavengers because partial oxidation can produce more toxic intermediates if mineralization is incomplete [37,38]. Cresol isomers have been shown to reach high removal (e.g., >80% degradation within 2 h under optimized Fenton conditions in bench studies), but mineralization (TOC) is lower and takes longer [22].

4.4. Influence of Delivery System and Reaction Volume on p-Chlorocresol Removal

Improved circulation, such as better mixing, smaller bubbles, and stronger mass transfer, correlates with higher removals and sometimes improved ozone-specific efficiency. At pH 4.5, where radical chemistry is important, transport of O₃ into the liquid and rapid mixing to distribute H₂O₂/Fe²⁺ are rate-limiting. The type of O₃ delivery strongly influenced treatment efficiency. The venturi delivery system with a screw pump (improved circulation) consistently outperformed the direct delivery system with a blower. For instance, O₃ plus Fenton’s at 700 L with venturi achieved 66% removal compared to 55% with the blower system (Table 3). Similarly, O₃ utilization (7.5 vs. 5.3 mg p-chlorocresol g⁻¹ O₃ h⁻¹; Table 3) was higher in the venturi system. This clearly indicates that an increase in the removal rate of p-chlorocresol is due to efficient utilization of O₃ by the venturi delivery system with improved circulation. This likely improved O₃ dispersion and mixing, enhancing hydroxyl radical production. The improved contact between O₃ and water through fine bubble generation in venturi injectors likely explains this performance [39]. There was more significant loss of O₃ from the direct delivery system which contributed to the low rates of the p-chlorocresol breakdown compared to venturi delivery (Table 3). The fine ceramic diffusers for O₃ and a tall reaction column may be used to improve the direct delivery system. However, this requires an increase in the pressure of the blower, which could in turn require more air to be pushed through the system, diluting the O₃ and reducing its effectiveness. Therefore, reactor design, such as bubble size, contact time, and recirculation, substantially affects apparent synergy. This observation matches reactor-scale studies showing that hydrodynamics strongly influence AOP performance [40].

At a higher working volume (700 L) and higher ozone doses (20 g h⁻¹), removal efficiency did not proportionally increase. For example, O₃ alone gave 46% removal at 700 L compared to 33.5% at 400 L, but the corresponding values of removal rate were lower (3.8 vs. 7.5 mg p-chlorocresol g⁻¹ O₃ h⁻¹). This indicates a decrease in ozone utilization efficiency at larger volumes, likely due to reduced ozone mass transfer and increased scavenging losses [41]. These results highlight diminishing returns in ozone utilization at higher O₃ supply if mass transfer, contact time, or radical coupling are limiting. Efficient radical generation (e.g., via Fenton’s reagent) and effective gas–liquid contact are therefore critical to make each gram of O₃ count [36].

4.5. Kinetics of p-Chlorocresol Removal

Venturi delivery (with improved circulation) is more effective than direct delivery, providing higher equilibrium p-chlorocresol removal (q_e) and better fits to the kinetic models. O₃ plus Fenton’s reagent greatly enhances p-chlorocresol removal rate compared to O₃ only, regardless of delivery method. The pseudo-second-order model provides the best fit for O₃ plus Fenton’s reagent (high R² ≈ 0.99), suggesting that surface reaction mechanisms dominate in p-chlorocresol degradation. O₃ only fits better with the pseudo-first-order or Elovich models, indicating a slower process in p-chlorocresol degradation.

Curve fitting with the PSO equation shows that the removal rate of p-chlorocresol in the wastewater was rapid early on by O₃ plus Fenton’s reagents but was then followed by gradual or somewhat slower removal (Figure 5) because ring cleavage and mineralization require successive oxidations. This pattern appears for both Fenton and ozonation [40]. It is observed that venturi delivery (with improved circulation using a screw pump) can remove the p-chlorocresol up to ≈42% in 24 h with only O₃ (Figure 5). When Fenton’s reagent was combined with O₃, the removal of p-chlorocresol from wastewater increased significantly, reaching 57% within 5 h and ≈64% within 24 h (Figure 5). This improvement is likely due to enhanced mass transfer achieved through venturi delivery and additional radical generation from the Fenton’s reaction, both of which accelerated and extended the removal process. Removal appears to plateau, suggesting some sort of limit on the reduction of the p-chlorocresol, probably caused by competition with other organics in the wastewater that were also consuming O₃. This is why it was seen that the TOC removal using O₃ only, and O₃ plus Fenton’s reagent, was well over 25% in wastewater, where the p-chlorocresol accounts for about 8% of the TOC in the wastewater (Table 3; Figure 4). It was also observed that the p-chlorocresol removal rate in the wastewater was slower than the pure p-chlorocresol compound trial at the same O₃ dose (Figure 2), most likely due to the presence of other organic compounds in the tannery wastewater that were consuming O₃. There could be other factors, i.e., the available fraction of p-chlorocresol in the wastewater, that limited its reduction. Most of the p-chlorocresol in the wastewater was removed within 5 h when combining the O₃ and Fenton’s treatments (Figure 5), which was probably from the freely available fraction of p-chlorocresol. Some fraction of p-chlorocresol may be strongly bound up in the fats and other solids in the wastewater, which could contribute to the slower removal of p-chlorocresol after 5 h in the O₃ plus Fenton’s reagent treatments (Figure 5).

Similar trends in the removal of the p-chlorocresol were observed in the direct delivery system, but the removal was less compared to the venturi delivery system (Figure 5). It is now clear that the addition of Fenton’s reagent with O₃ can potentially reduce the time for the removal of p-chlorocresol in tannery wastewater. Other studies also reported that the reaction time for ozonation processes could be shorter to degrade phenolic compounds in wastewater if Fenton or H₂O₂ is added to the ozonation process [3,14,16,21,22].

4.6. Intermediates During p-Chlorocresol Degradation

One important aspect regarding the possible toxicity of the reaction or transformation products during the oxidation process needs to be considered. Partial oxidation of chlorophenols can produce chlorocatechols, quinones, and short-chain carboxylic acids. Some intermediates (e.g., certain chlorinated quinones) can be more toxic than the parent compound; therefore, monitoring intermediate formation and TOC or AOX (adsorbable organic halogens) is important. Ozonation alone in some cases increases intermediate toxicity unless radical mineralization proceeds further; combined processes tend to lower overall toxicity when properly optimized [32].

In general, all of the aromatic intermediate products (e.g., 4-Methylcatechol, dihydroxyfumaric acid, artaric acid, citric acid, malic acid, etc.) formed during oxidation of phenolic compounds (e.g., cresols) are less toxic compared to cresols [42,43]. The intermediate products degrade to shorter chains of carboxylic acids with further oxidation. The major degradation pathway of cresol is benzene ring oxidation through electrophilic aromatic substitution reaction by molecular O₃ and hydroxyl radicals [42].

Other aspects require consideration, such as oxidative treatment of tannery wastewater with O₃, which may lead to re-oxidation of chromium (Cr(III) to the Cr(VI). In tannery wastewater, Cr(III) may present as the chromium salt used in the tanning process. Cr(III) in most cases reacts to form insoluble compounds and is commonly considered to be nontoxic [44]. In our study, Cr(VI) formation was not found during the ozonation of tannery wastewater. Another study reported that effluent ozonation only leads to toxicologically insignificant Cr(VI) concentrations of Cr(III) in wastewater [45].

5. Conclusions

In this study, ozonation and ozonation plus Fenton’s reagent were applied to remove the p-chlorocresol from tannery wastewater. Laboratory trials demonstrated that removal of p-chlorocresol from wastewater at pH 4.1 using ozone was over 70% (76 mg p-chlorocresol g⁻ O₃ h⁻¹). Fenton’s reagent did improve the p-chlorocresol removal, but only by 10%. A relatively high percentage of p-chlorocresol removal (66%; 6.7 mg p-chlorocresol g⁻ O₃ h⁻¹) in the tannery wastewater was found in the ozonation plus Fenton’s reagent process compared to the ozonation-only process (46% removal; 4.8 mg p-chlorocresol g⁻ O₃ h⁻¹). At the pH of our tannery wastewater (≈4.5), the O₃ plus Fenton process operates in a mixed regime, where direct ozone oxidation occurs in parallel with Fenton-driven hydroxyl radical production. Achieving true synergy and sustaining high ozone-specific removal efficiency require both effective hydrodynamics and optimized reagent stoichiometry.

In our pilot-scale studies, dissolved O₃ delivery into wastewater using a venturi with improved circulation was more efficient than direct delivery via an air blower system. The combined use of dissolved O₃ and Fenton’s reagent via venturi delivery is recommended as an efficient, effective, and environmentally friendly technique to break down the p-chlorocresol in wastewater. However, ozone utilization efficiency declines at larger volumes, highlighting the importance of optimizing gas–liquid mass transfer for scale-up. For a full-scale tannery effluent treatment, the combination of a high-efficiency gas transfer device with advanced oxidation (O₃ plus Fenton) is promising, as it enhances pollutant removal rates and allows for shorter contact times and smaller reactor volumes. This has real operational advantages (such as smaller reactors, lower residence times) for high-load industrial streams. The findings of this study are very useful to guide the implementation of a full-scale commercial system and optimize the operational parameters for p-chlorocresol removal in wastewater.