Journal of Hazardous Materials 416 (2021) 125721

Available online 25 March 20210304-3894/© 2021 Published by Elsevier B.V.

Research Paper

Chlorophenols in textile dyeing sludge: Pollution characteristics and environmental risk control

Xiaohui Chen , Xun-an Ning *, Xiaojun Lai , Yi Wang , Yaping Zhang , Yao He Guangdong Key Laboratory of Environmental Catalysis and Health Risk Control, School of Environmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou 510006, China

A R T I C L E I N F O

Editor: Dr. C. LingXin

Keywords: Chlorophenols (CPs) Textile dyeing sludge Risk assessment Fenton

A B S T R A C T

Chlorophenols (CPs) are toxic contaminants that tend to accumulate in textile dyeing sludge and pose a threat to the environment through the disposal process. To comprehensively evaluate CPs in sludge, the characteristics and risks of CPs from five textile dyeing plants (TDPs) were investigated in this study. The total concentration of 19 CPs (Σ19 CPs) varied from 170.90 to 6290.30 ng g− 1 dry weight (dw), among which high-chlorine phenols accounted for the greatest proportion. The ecological screening level (ESL) of CPs was used to judge their pollution levels, while the risk quotient (RQ) value and dioxin conversion rate were used to analyze their po-tential risk. The results indicated that CPs may pose a moderate to high risk to the environment. The Fenton process was used to condition the hazardous sludge, and a higher content of CPs was found after conditioning. A lower rate of CP increase was achieved with a reagent dose of 180 mmol/L, H2O2:Fe2+ = 1:1, pH of 3–4 and reaction time of 30 min. In summary, the work helps to address the general knowledge gap in the textile dyeing industry and provides a reference for further research.

1. Introduction

With the rapid development of the global economy, the production scale of the textile dyeing industry has shown an increasing trend and environmental problems caused by this industry have become increas-ingly serious, which has attracted widespread attention (Chen et al., 2015; Zhou et al., 2019; Dai et al., 2019). As one of the pillar industries in China, the textile dyeing industry ranks third in the country in terms of total industrial wastewater discharge, accounting for 10.1% (CN MEE, 2015). According to statistics from the National Bureau of Statistics of China in 2018 (CN NBS, 2018), the total discharge of textile dyeing wastewater was 1.9 billion tons and sludge production was as high as 4.758 million tons (with a moisture content of 80%). Wastewater from the textile dyeing industry has a high content of pollutants, such as dyes, surfactants, detergents, solvents, heavy metal ions and recalcitrant compounds (Vanhulle et al., 2008; Wang et al., 2019), which are hard to treat, and some refractory substances can accumulate in sludge. Large amounts of textile dyeing sludge are mainly landfilled and incinerated in subsequent disposal (Ren et al., 2017). Thus, in Guangdong Province, China, one of the most highly urbanized and industrialized provinces where TDPs are densely distributed, textile dyeing sludge has been listed

as a strictly controlled waste (CN GDEE, 2011). Among the organic contaminants in sludge, chlorophenols (CPs),

which include 19 different congeners, deserve more attention because of their hazardous risks to the environment and human health. CPs have chemical structures with easy oxidation characteristics and are versatile intermediates in chemical synthesis. For example, 2-chlorophenol (2- CP), 2,4-dichlorophenol (2,4-DCP) and 2,4,6-trichlorophenol (2,4,6- TCP) are used as intermediates in the production of higher CPs, such as tetrachlorophenol (TeCP) and pentachlorophenol (PCP). Meanwhile, TCP, TeCP and PCP are primarily applied in the textile, pulp and paper, leather tanning and wood bleaching industries due to their antimicrobial properties (Van Aken et al., 2019). However, CPs have low solubility in water because of their relatively high octanol water partition coefficient (Kow), which makes them more likely to accumulate in sludge (Faludi et al., 2015). It is worth noting that CPs not only have direct toxicity but also have the potential for carcinogenic, teratogenic and mutagenic activities (Wei et al., 2019). Studies have shown that the content of CPs accounted for the largest proportion among phenolic pollutants at 64.55% in sludge (Liu et al., 2012). Moreover, the toxic effect of CPs in the environment is higher than that of other phenolic compounds listed as priority pollutants in the United States and China, such as phenol,

* Corresponding author. E-mail address: ningxunan666@126.com (X.-a. Ning).

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https://doi.org/10.1016/j.jhazmat.2021.125721 Received 8 November 2020; Received in revised form 15 March 2021; Accepted 18 March 2021

Journal of Hazardous Materials 416 (2021) 125721

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cresol and nitrophenol (Aruoja et al., 2011), and even low-content CPs at ppb level can cause harm to the environment (Faludi et al., 2015). Due to the negative impact of CPs, three kinds of CPs, 2-CP, 2,4-DCP and 2,4, 6-TCP, have been listed as priority control persistent organic pollutants by the United States Environmental Protection Agency (US EPA) and European Union (EU), while limit value standards for 2,4-DCP, 2,4, 6-TCP and PCP have been set in “surface water environmental quality standards” (GB 3838—2002) in China.

As the two main disposal methods for sludge, landfills and inciner-ation can both alleviate the massive accumulation of sludge. The landfill method poses a threat to the terrestrial environment due to CPs in textile dyeing sludge. CPs will dissociate in soil and exist as neutral phenols and anionic phenates (Zhao et al., 2017), and they show hydrophobicity and adhesion in soil, which increases the difficulty of soil restoration. In addition, some ecotoxicological studies have found that CPs might be cytotoxic to animals and plants (Ge et al., 2017; Suzuki and Shoji, 2020; Kucuk and Liman, 2018). Incineration is an effective method for reducing and stabilizing textile dyeing sludge from the perspective of environmental supervision (Cieslik and Konieczka, 2017); however, harmful gases are inevitably produced and released into the atmosphere as chemical reactions occur during this process. Many studies have proven that CPs are important precursors of dioxins during the incin-eration process (Zhou et al., 2015; Zhan et al., 2019). Therefore, textile dyeing sludge needs to be pretreated before disposal to reduce envi-ronmental hazards. However, comprehensive research on the charac-teristics and risk of organic contaminants in actual textile dyeing sludge that can provide basic data with reference value is lacking.

In recent years, advanced oxidation processes (AOPs) have been widely studied to address organic pollutants, such as Fenton and Fenton- like processes, UV/H2O2, UV photolysis, catalysis and ozonation. Among various AOPs, Fenton is the most popular in scientific research and practical application due to its exciting advantages such as simple operation, strong anti-interference ability, wide application range, rapid degradation and acceptable cost (Zhang et al., 2019). Fenton process is capable of oxidizing organic pollutants quickly and nonselectively through producing highly oxidative hydroxyl radical (•OH) during the reaction of H2O2 with Fe2+ under acid condition. But when Fenton is applied to actual sludge conditioning, macromolecular organic com-pounds will compete for more •OH than CPs and produce CP in-termediates during the degradation process (Li et al., 2017; Bae et al., 2013), resulting in the changes in the total amount of CPs and their component distribution. These changes will affect the degree of harm to the environment. However, most studies are carried out on a single or only a few types of CPs listed as priority control indicators through simulation experiments (Boruah et al., 2017; Li et al., 2017; Liu et al., 2020). Such work is not sufficient to determine the total emissions of CPs or demonstrate their degradation effect in actual sludge that contains more complex organic contaminants than CPs. Therefore, monitoring the changes in CP levels and the distribution of CPs is of great importance.

In the actual sludge conditioning process with Fenton’s reagent, although CPs may be produced as intermediate products of degradation of macromolecular organic compounds (Li et al., 2017; Bae et al., 2013) and cause potential harm to the environment, the effect of CPs on the overall toxicity of sludge is relatively weak compared to that of other larger and more toxic organic substances, and the changes in their content do not play a decisive role in the overall toxicity of sludge. Thus, from the perspective of cost and overall effect (Xiang et al., 2020; Audino et al., 2019), the focus in practical applications is not the high degradation of CPs but the reduction of overall sludge toxicity. There-fore, the changes in CPs should be taken into consideration under the premise of ensuring the total organic degradation efficiency to minimize environmental risk.

To supplement the basic data on the textile dyeing industry and expose possible environmental problems, a small-scale study was initially conducted on five typical TDPs located in the Pearl River Delta

region of Guangdong Province. The content and component distribution of CPs from five TDPs were investigated and the predominant CPs in textile dyeing sludge were listed. The inherent risk and potential risk of CPs were also assessed in this study to judge the degree of harm to the environment. Fenton oxidation was used for initial research on the degradation efficiency of CPs in actual sludge, and different reaction conditions were compared to achieve a final condition with a better overall effect.

2. Materials and methods

2.1. Sampling

The sludge samples used in this study were collected from five representative textile dyeing wastewater treatment plants (TD-WWTPs) located in three different cities in the Pearl River Delta region of Guangdong Province, China (Fig. 1) from June to July 2020. To reduce the influence of light, the inner layer of the stacked sludge after dehy-dration stored in the final storage container was taken and wrapped with aluminum before being packed in a plastic sealed bag. All samples were collected as described above and stored in a refrigerator at − 20 ℃ until analysis. Basic information of the TDPs, including the wastewater treatment capacity, textile material, dye type and wastewater treatment process, as well as sludge moisture content and sludge organic matter content of the collected sludge are shown in Table 1.

2.2. Standards and chemicals

The mixed standard solution (100 mg/L in methylene chloride) of 19 CPs (mono through penta) was purchased from O2si Smart Solutions (Charleston, SC, USA). Chromatographic pure organic solvents, including dichloromethane, n-hexane and ethyl acetate, were obtained from ANPEL Laboratory Technologies (Shanghai, China). Hydrochloric acid solution (3 mol/L) and sodium hydroxide solution (5 mol/L) were prepared to adjust the pH. Anhydrous sodium sulfate was baked at 450 ℃ for 4 h and then stored in a desiccator. The water used in the experiment was obtained from a Unique-R20 multifunctional ultrapure water system (Xiamen, China).

2.3. Sample preparation

All sludge samples were freeze-dried for 24 h and then manually ground and sieved (by 100 mesh) in a dark environment before

Fig. 1. Location of the sampling site of five textile dyeing plants distributed in Guangzhou, Dongguan and Zhongshan, Guangdong Province, China.

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subsequent extraction, purification and enrichment. Approximately 10 g dry weight (dw) of each sample was mixed with diatomite evenly and extracted with a pressurized dichloromethane/n-hexane (2:1, v/v) fluid using an accelerated solvent extractor (ASE350, Thermo Fisher, USA) at an extract temperature of 100 ℃, pressure of 1500 psi, static extraction time of 5 min, rinse volume of 60% of the pool volume, and nitrogen purge time of 60 s, with two extraction cycles. Subsequently, the extract solution was purified by separating acidic analytes from neutral and basic analytes according to the acid-base distribution method (SL 391.8–2007) as follows. In the alkaline distribution purification process, twice as much ultrapure water (adjusted to pH > 12 and refrigerated at 4 ℃ in advance) was added to the extract solution and fully shaken in the separatory funnel for 2 min. The same process was repeated three times, and the water phases were combined for further purification. In the acid distribution purification process, 40 mL of dichloromethane/ ethyl acetate (4:1, v/v) was added to the above water phase and fully shaken in the separatory funnel for 2 min. Then, the organic phase was dehydrated with anhydrous sodium sulfate, collected in pear-shaped bottles, and finally combined with dichloromethane/ethyl acetate (4:1, v/v) to wash the anhydrous sodium sulfate. The final organic phase was concentrated to approximately 1 mL by rotary vacuum evaporation and then concentrated to 1 mL under a gentle flow of nitrogen with an N-EVAP 112 system (Organomation, USA). The final extract solution was injected into the GC-MS for quantification.

2.4. Fenton conditioning experiments

Five grams of sludge in dry weight was added to a certain amount of water to a 98% moisture content and then stirred with a blender at 250 rpm to reach a homogeneous mixing state. Fenton conditioning experiments were carried out under different conditions based on our previous studies (Lin et al., 2016; Liang et al., 2016). Factors including the reaction time (15 min, 30 min and 45 min), pH (pH < 2, pH 3–4 and pH 5–7), reagent dose (100 mmol/L, 140 mmol/L and 180 mmol/L) and reagent ratio of H2O2:Fe2+ (1:1.5, 1:1 and 2.5:1) were studied. The re-action was started under different Fenton conditions and stopped by adding 0.2 mL isopropanol. Afterwards, the sludge was centrifuged, and the sedimentation was freeze-dried. Extraction, purification and enrichment steps were routinely performed in the same way as the sample preparation (2.3) before analysis.

2.5. Instrumental analysis

Analysis of the concentration of CPs was performed using a gas chromatograph-mass spectrometer (GC-MS, Agilent, USA) equipped with an HP-5MS column (30 m × 0.25 mm, 0.25 µm film thickness,

Agilent, USA). The operating conditions for GC were set as follows: oven temperature program initiated at 80 ℃ (held for 1.0 min) and ramped to 250 ℃ at a rate of 10 ℃/min (held for 4.0 min). One microliter samples were injected using an automatic sampler in splitless mode, and the time for solvent delay was set to 3 min. MS was operated under the following conditions: ion source temperature of 230 ℃, transfer line temperature of 250 ℃, and electron impact ionization mode of 70 eV. Full scan mode was used for substance identification, and SIM mode was used for quantification, with the base peak ion and two additional qualified ions used for confirmation.

2.6. Quality assurance and quality control (QA/QC)

The procedural blanks, spiked blanks and sample duplicates were routinely performed with sludge samples, and no interference was detected. The method detection limit of CPs was 2–5 ng g− 1. The linear range (correlation coefficient ≥0.995) was 0–4000 ng g− 1 for CPs. The recoveries of the compounds were acceptable and ranged from 88.3% to 134.0%, and the relative deviation of parallel sample determination results was within 30%. All concentrations were normalized to dry sample weight.

The chromatographic peaks of most CPs can be well separated under the method described here, whereas 2,3-DCP and 2,4-DCP cannot be separated into different peaks due to their extremely similar properties; thus, their contents are summarized in this article. A chromatograph of the standard solution at 1 mg/L is presented in Fig. S1.

3. Results and discussion

3.1. Distribution characteristics of CPs

The content of all nineteen CPs was determined in five textile dyeing sludge samples. According to the results, the concentration of total CPs (Σ19 CPs) varied greatly, ranging from 170.90 to 6290.30 ng g− 1 dw, with a mean value of 1682.45 ng g− 1 dw, which is much higher than that in marine sediment (Sim et al., 2009) and general industrial wastewater (Ju et al., 2009). The complete results for the individual CPs are provided in Table S1.

To determine the relationship between the content of CPs and the characteristics of TDPs, a cluster analysis and principal component analysis (PCA) were performed for five TDPs. As shown in Fig. 2(a), the five TDPs can be divided into three clusters, which is consistent with the results of principal component analysis presented in Fig. 2(b). Linking the results with the information of TDPs (Table 1), the clustering of TDPs may be attributed to their differences in wastewater sources, dye ma-terial, and/or treatment processes. TDP 5, as a separate category, has the

Table 1 Basic information for five textile dyeing wastewater treatment plants (TD-WWTPs).

TD- WWTP

Wastewater treatment capacity (m3

d− 1) Textile material

Dye type

Wastewater treatment process

Sludge moisture content (%)

Sludge organic matter content (%)

TDP 1 16,000 CF Mix dyes

Flocculation, A/O 63.97 ± 0.81 50.83 ± 1.33

TDP 2 11,000 C+CF Mix dyes

A/O, Flocculation 56.02 ± 1.48 45.59 ± 1.57

TDP 3 10,000 C+CF Mix dyes

Flocculation, A/O 80.56 ± 0.84 42.51 ± 1.50

TDP 4 30,000 C+CF Mix dyes

Flocculation, A/O 69.36 ± 0.59 35.67 ± 0.92

TDP 5 20,000 C+CF Mix dyes

(A/O[N], A2O[A]), Flocculation

68.95 ± 1.27 49.90 ± 1.23

TD-WWTP: Textile dyeing wastewater treatment plants. C: cotton; CF: Chemical fiber. Mix dyes: Mixture of ionic dye, acid dye, disperse dye and reactive dye. A/O: Anaerobic/Aerobic; A2O: Anaerobic/Anoxic/Aerobic. [N] Treatment for neutral textile dyeing wastewater from the entire industrial zone. [A] Treatment for alkaline textile dyeing wastewater from the entire industrial zone.

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following salient features. First, the source of wastewater is more gen-eral than that of other TDPs and covers the entire textile dyeing indus-trial zone, resulting in a higher organic content and a more complex composition. Second, the wastewater is treated separately based on the acid-base properties of the wastewater instead of mixing all wastewater and treating it in a unified manner, thereby weakening the interaction of acid and alkaline substances during the treatment process. However, through comparisons among TDP 2, TDP 3, and TDP 4 in cluster 2, the difference in the order of A/O and flocculation during the treatment process appears to have little effect on the clustering results. For cluster 3, the most obvious feature compared to other clusters is that its textile material is only chemical fiber, which makes the raw materials and chemical substances of TDP 1 relatively simpler than those of other TDPs (Robinson et al., 2001).

In terms of the nineteen individual CPs of five TDPs, a PCA was performed on the data after dimensionality reduction, and the results show that three CPs, namely 2,3,5,6-TeCP, 2,3,4,6-TeCP and 2,3,4-TCP, are particularly characteristic in the evaluation of textile dyeing sludge, and the mean values are in the following order: 2,3,5, 6-TeCP (665.89 ng g− 1) > 2,3,4,6-TeCP (454.97 ng g− 1) > 2,3,4-TCP (156.25 ng g− 1) (Table S1). They are all trichloro- or tetrachloro- substituted CPs, which are more likely to accumulate in sludge for several reasons. First, TCP and TeCP are the main chemical in-termediates for the synthesis of dyes, and they are also used in the

sterilization process of textile dyeing (Ju et al., 2009). Second, due to insufficient oxidation in the treatment process, they become degradation intermediate products of some polymer compounds. In addition, the concentrations of PCP (92.00 ng g− 1) and 4-CP (46.49 ng g− 1) were below that of the three characteristic CPs (Table S1) mainly because they are used only for sterilization and dye synthesis. In addition, their symmetrical structures make them more stable and hard to eliminate in the treatment process. The five CPs emphasized here are regarded as textile dyeing predominated chlorophenols (TD-CPs) in this study.

To our knowledge, the normative standards for CPs in sludge are very limited (Van Aken et al., 2019). Among the nineteen CPs, the EPA screening level (ESL) has proposed content indexes for 2-CP, 2,4-DCP, 2, 4,6-TCP, 2,3,4,6-TeCP and PCP. Moreover, 2-CP, 2,4-DCP and 2,4,6-TCP showed extremely low concentrations or were below the detected limit in this study; therefore, they have almost no contribution to the total amount of CPs and are not enough to cause further environmental hazards; thus, they do not need to be included as a control focus for textile dyeing sludge.

Although ESL-CPs have a certain reference value for the control of CPs, they lack pertinence in the textile dyeing industry. TD-CPs are probably more likely to provide a reference value for the textile dyeing industry than ESL-CPs. Among the five TD-CPs, 2,3,4,6-TeCP and PCP need to be emphasized because they are both included in the TD-CP and ESL lists. However, 2,3,5,6-TeCP, 2,3,4-TCP and 4-CP, as newly

Fig. 2. Cluster analysis and principal component analysis (PCA) corresponding to the CP fractions. (a) Dendrogram of the cluster analysis of TDPs. (b) Plot of the PCA of TDPs. (c) Plot of the PCA of CPs species.

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proposed characteristic CPs, need to be considered in future research as well.

3.2. Component distribution of CPs

The properties of nineteen CPs are different due to differences in the structure of the compound molecule (Table S1), including the number of chlorine atoms and their position in the molecule, which can affect the behavior of CPs in the natural environment (Czaplicka, 2004). There-fore, in addition to the total amount of CPs, the distribution of the components has also been studied.

The component distributions of nineteen CPs in five textile dyeing plants are shown in Fig. 3(a). TeCPs accounted for the highest propor-tion (19.1%~84.8%), followed by TCPs (7.1%~60.6%) and PCP (4.6% ~13.1%) among the nineteen CPs, while monochlorophenols and DCPs accounted for 0~12.6% and 0~10.6%, respectively. High-chlorine phenols (Cl ≥ 3) showed a higher concentration in sludge than low- chlorine phenols (Cl < 3) because the fate and transport of chemical compounds are strictly dependent on the value of the dissociation con-stant (Ka) and octanol water partition coefficient (Kow) (Table S2). The values of Ka and Kow theoretically increase with the degree of chlorine substitution, and a higher value refers to a stronger ability to remain in sludge (Van Aken et al., 2019). A similar phenomenon was found in the distribution of five TD-CPs: high-chlorine phenols occupied the domi-nant position and reached 81.6~100% while low-chlorine phenols only accounted for 0~18.4% (Fig. 3(b)). Actually, the concentration of CPs in textile dyeing sludge also depends on their content level in the whole process of textile dyeing production and wastewater treatment. For example, in the production process, TCP and TeCP are used for both dye synthesis and sterilization while PCP is mainly used for sterilization. In the wastewater treatment process, due to insufficient oxidation, PCP may dechlorinate and be converted into TCP and TeCP intermediates. Therefore, PCP accounts for a lower proportion than TCPs and TeCPs in this study.

It is worth mentioning that the toxic effects of CPs are directly affected by the degree of chlorination and the position of the chlorine atom (Czaplicka, 2004). The higher the degree of chlorination, the stronger the toxicity (Liu et al., 2006). For the chlorine atom position, toxicity increases when chlorines are substituted at the 3-, 4- and 5-po-sitions. In contrast, the lower toxicity is due to the simultaneous occurrence of chlorine atoms substituted at the 2- and 6-positions or only at the 2-position (Czaplicka, 2004). Among the five TD-CPs, most have chlorine atoms substituted at the 2-, 3- and 4-positions, which has an unignorable toxic effect on the environment. Thus, five TD-CPs need to be given more attention in the textile dyeing industry.

3.3. Environmental risk assessment

3.3.1. Inherent risk assessments of CPs based on ESL Since the control limit of CPs in sludge has not been determined, the

environmental risk can only be roughly assessed by referring to the value of sediment or soil (Ning et al., 2014). Inherent risk assessments of CPs in textile dyeing sludge were conducted by comparison with sedi-ment ecological screening levels (ESLs) developed by the EPA. Table 2 shows that nineteen kinds of CPs could be divided into 5 series according to the number of chlorine substituents and each series has a chlor-ophenol selected to give the screening level from EPA. Due to the lack of a screening level for all nineteen CPs, the given ESL was marked as the reference ESL for the series CPs. Values exceeding the reference ESL are marked with an asterisk in the table. Four of the five TD-CPs exceeded the reference screening level. PCP is the only TD-CP below the reference standard limit, while 4-CP is nearly two times higher than the reference limit. 2,3,4,6-TeCP and 2,3,5,6-TeCP are both an order of magnitude higher than the reference limit, and 2,3,4-TCP slightly exceeds the standard. These findings indicate that TD-CPs are the main contributor to the inherent toxicity of CPs in textile dyeing sludge and pose an un-ignorable risk to the environment, which needs to be taken seriously in future research.

3.3.2. Potential risk assessment of CPs . Landfills and incineration are

Fig. 3. (a) Component distribution of 19 chlorophenols (CPs) in five textile dyeing plants. (b) Component distribution of 5 textile dyeing predominated chlor-ophenols (TD-CPs) in five textile dyeing plants.

Table 2 ESL risk assessments of CPs.

Series CPs ESL (ng g− 1) Measured value (ng g− 1)

Mean Maximum

2-CP 31.9 1.6 8.2 Monochlorophenol 3-CP — N.D. N.D.

4-CP — 46.5a 91.6a

2,3-DCP — 7.6b 38.0b

2,4-DCP 81.7 Dichlorophenol 2,5-DCP — 2.4 12.1

2,6-DCP — 18.4 48.1 3,4-DCP — 36.8 59.6 3,5-DCP — 0.2 1.0 2,3,4-TCP — 156.2 223.8a

2,3,5-TCP — 45.7 119.9 2,3,6-TCP — 26.3 49.7

Trichlorophenol 2,4,5-TCP — 28.8 47.0 2,4,6-TCP 208 29.8 68.5 3,4,5-TCP — 27.6 52.9 2,3,4,5-TeCP — 41.7 117.8

Tetrachlorophenol 2,3,4,6-TeCP 129 455.0a 2176.5a

2,3,5,6-TeCP — 665.9a 3038.4a

Pentachlorophenol PCP 504 92.0 289.2

a Values exceeding thereference ESL. b Summation of 2,3-DCP and 2,4-DCP.

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the main destinations for dehydrated textile dyeing sludge. During these processes, chemical reactions will inevitably occur and the toxicity originating from CPs will change correspondingly, causing potential harm to the environment. Therefore, the potential risks based on these two treatment directions are discussed.

For landfills, studies have found that CPs adversely affect soil fertility and plant growth because they reduce soil microbial diversity and affect soil enzymes that drive organic matter transformation (Singh et al., 2020). Intermediate products that are more toxic than CPs (e.g., hy-droquinone and catechol) were detected according to studies on the degradation of CPs (Dong et al., 2017). CP residues in soil can easily accumulate in organs, tissues and cells via food chains (Liu et al., 2006; Peng et al., 2016a, 2016b). Thus, toxicity assessments of CPs acting on different trophic levels were studied through a computer fugacity model (EPI Suite) (Kates et al., 2014). The corresponding PNEC and RQ were calculated and are shown in Table 3. RQ values exceeding 1 are marked with an asterisk, which indicates a high risk, while 0.1 < RQ < 1 in-dicates moderate risk and RQ < 0.1 indicates low risk. For low trophic level green algae, TeCP presented a moderate risk and other TD-CPs presented a low risk. However, when the toxicity endpoint increases to higher trophic level Daphnia and fish, the toxicity of TeCP shifts from a moderate-risk state to a high-risk state and other CPs shifts from a low-risk state to a moderate-risk state, thus illustrating that the toxicity of CPs to organisms of different nutritional levels may be cumulative and gradually amplified along the food chain. Therefore, the source control of CPs is particularly important for controlling environmental risk.

In terms of incineration, CPs can be converted into dioxins as the main precursor at a high rate due to the high interactivity of phenolic hydroxyl groups with phenolic chloride (Peng et al., 2016a, 2016b). Among the nineteen CPs, 2,3,4,6-TeCP ranked highest in terms of the five textile dyeing characteristic CPs and had the highest formation rate of dioxins (Altarawneh et al., 2009). Thus, 2,3,4,6-TeCP was selected as the representative for analyzing the possible environmental hazards caused by incineration by estimating its conversion rate. As reported by Peng et al. (2016a, 2016b), the formation rate of PCDD from TeCP as the precursor can reach 13.41 g PCDD/g min− 1. The selective conversion efficiency of 2,3,4,6-TeCP to PCDD reaches 4%. Therefore, the yield of conversion from 2,3,4,6-TeCP into PCDD ranged from 0.28 to 87.06 ng PCDD/g dw, with a mean value of 18.20 ng PCDD/g dw under the assumption that all sludge is incinerated (conversion yield of PCDD/Fs under different proportions of sludge used for incineration was pre-sented in Table S3).

3.4. Fenton treatment and potential risk control Since CPs pose a nonignorable risk to the environment, it is necessary

to study the changes in CPs based on Fenton process, which is commonly used to reduce the overall toxicity of sludge. In practical applications, considering the cost and overall effect, organic pollutants will not be fully mineralized and CPs as mid-downstream products may increase. That is, potential risk control is not focused on the high degradation of CPs in the sludge but rather on the increase in CPs. Hence, the influence of the Fenton process on Σ5 TD-CPs and Σ19 CPs under different condi-tions was taken into consideration in this part through a single-factor experiment.

According to the results shown in Fig. 4, Σ5 TD-CPs and Σ19 CPs both increased after the Fenton process under the given conditions. Changes in content can be explained by the relative rate of increase and decrease of CPs in the Fenton process. The increase in CPs can be explained as follows: (1) CPs in the sludge may be further released by the Fenton reaction (Lin et al., 2016); (2) CPs may be generated as intermediates during the degradation of macromolecular compounds (Li et al., 2017; Bae et al., 2013); and (3) substances with benzene ring structures existing in the system may combine with chlorine radicals to resynthe-size CPs (Xiang et al., 2020). The decrease in CPs can be explained as follows: (1) •OH undergoes electrophilic addition directly on the ben-zene ring, and CPs are converted into chlorophenol intermediates Ta

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—

—

—

—

—

—

4-CP

13

8.58

89

.33

96

8.86

N

.D.

91.6

1 46

.49

—

0.66

0.

34

—

1.03

* 0.

52

—

0.09

0.

05

2,3-

DCP

15

4.01

11

1.78

11

61.2

6 N

.D.b

38.0

4b 7.

61b

—

0.25

b 0.

05b

—

0.34

b 0.

07b

—

0.03

b 0.

01b

2,4-

DCP

14

6.03

11

1.27

11

35.2

2 2,

5-D

CP

151.

50

111.

72

1152

.88

N.D

. 12

.12

2.42

—

0.

08

0.02

—

0.

11

0.02

—

0.

01

0.00

2,

6-D

CP

165.

70

112.

68

1197

.99

N.D

. 48

.08

18.4

3 —

0.

29

0.11

—

0.

43

0.16

—

0.

04

0.02

3,

4-D

CP

135.

41

110.

27

1099

.05

N.D

. 59

.06

36.8

1 —

0.

44

0.27

—

0.

54

0.33

—

0.

05

0.03

3,

5-D

CP

130.

72

109.

90

1082

.00

N.D

. 1.

02

0.20

—

0.

01

0.00

—

0.

01

0.00

—

0.

00

0.00

2,

3,4-

TCP

19

8.08

18

4.17

17

44.2

8 16

.22

223.

80

156.

25

0.08

1.

13*

0.79

0.

09

1.22

* 0.

85

0.01

0.

13

0.09

2,

3,5-

TCP

19

5.59

18

3.69

17

37.3

8 14

.24

119.

94

45.6

7 0.

07

0.61

0.

23

0.08

0.

65

0.25

0.

01

0.07

0.

03

2,3,

6-TC

P

220.

36

185.

96

1826

.80

6.28

49

.74

26.2

8 0.

03

0.23

0.

12

0.03

0.

27

0.14

0.

00

0.03

0.

01

2,4,

5-TC

P

194.

55

183.

21

1732

.60

2.53

47

.03

28.7

5 0.

01

0.24

0.

15

0.01

0.

26

0.16

0.

00

0.03

0.

02

2,4,

6-TC

P

203.

24

184.

02

1764

.12

6.59

68

.51

29.7

9 0.

03

0.34

0.

15

0.04

0.

37

0.16

0.

00

0.04

0.

02

3,4,

5-TC

P

176.

11

181.

29

1658

.79

4.81

52

.93

27.5

5 0.

03

0.30

0.

16

0.03

0.

29

0.15

0.

00

0.03

0.

02

2,3,

4,5-

TeCP

18

2.51

21

1.54

18

47.8

7 3.

99

117.

85

41.7

0 0.

02

0.65

0.

23

0.02

0.

56

0.20

0.

00

0.06

0.

02

2,3,

4,6-

TeCP

19

5.93

21

1.61

19

07.6

0 6.

88

2176

.51

454.

97

0.04

11

.11*

2.

32*

0.03

10

.29*

2.

15*

0.00

1.

14*

0.24

2,

3,5,

6-Te

CP

202.

98

212.

51

1935

.78

41.7

5 30

38.4

4 66

5.89

0.

21

14.9

7*

3.28

* 0.

20

14.3

0*

3.13

* 0.

02

1.57

* 0.

34

PCP

18

4.05

23

8.59

20

10.9

3 22

.42

289.

23

92.0

0 0.

12

1.57

* 0.

50

0.09

1.

21*

0.39

0.

01

0.14

0.

05

Rela

ted

raw

par

amet

ers

are

show

n in

Tab

le S

2.

*Va

lues

indi

catin

g a

high

ris

k.

aPN

EC s

ludg

e w

as c

onve

rted

from

PN

EC w

ater

and

pre

dict

ed b

y EC

OSA

R Ve

rsio

n 1.

11 w

ith th

e fo

rmul

a: P

NEC

slu

dge=

PNEC

wat

er ×

Kd.

bSu

mm

atio

n of

2,3

-DCP

and

2,4

-DCP

.

X. Chen et al.

Journal of Hazardous Materials 416 (2021) 125721

7

(Ghaly et al., 2001); and (2) •OH attacks the carbon atoms on the benzene ring, leading to the dechlorination of CPs (Karci, 2014).

As shown in Fig. 4(a), at a reagent dose of 100 mmol/L, Σ19 CPs increased significantly compared with that in raw sludge while Σ5 TD- CPs increased relatively moderately. These findings indicate that more CPs other than TD-CPs were formed during the process because high- molecular-weight organic matter in sludge competes for insufficient •OH, degrades first and produces low-chlorophenol-based chlorophenol intermediates (Bae et al., 2013). As the dosage increased, the content of •OH increases correspondingly so that more CPs could compete for •OH, making more unstable CPs able to be degraded. Here, TD-CPs still increased slightly but the total CPs started to decline, showing that TD-CPs are particularly difficult to degrade. When the dose reached 180 mmol/L, the proportion of TD-CPs in the total CPs decreased but the total CPs did not change much, which means that TD-CPs began to be effectively degraded.

Regarding the ratio of reaction reagents of H2O2 and Fe2+, Fig. 4(b) shows that when the ratio was 1:1.5, Σ5 TD-CPs and Σ19 CPs both slightly increased, which may be related to the ability of excessive Fe2+ to remove •OH (Ozdemir et al., 2011) and reduce the overall oxidation degree of the system, resulting in a low degradation rate of the whole system and little increase in CP intermediates. When the ratio was 1:1, macromolecular organics began to degrade and produced CP in-termediates; thus, the content of CPs increased. When the ratio shifted to 2.5:1, the total amount of CPs was slightly lower than that at 1:1, which may be related to the ability of excessive H2O2 to generate O2 through self-decomposition (J.S. Kim et al., 2001; I.W.C. Lau et al., 2001), bring the sludge to the surface of the water, and remove •OH to a certain degree (Gogate and Pandit, 2004). The overall oxidation degree of the system was reduced, and the production of CPs was reduced as well.

The results in Fig. 4(c) show that the content of CPs is higher under low pH conditions than under high pH conditions because high pH

conditions are not conducive to the progress of the Fenton reaction (Kwon et al., 1999), thus causing a reduction in the degradation rate of organic matter and the production of CPs. Fig. 4(d) indicates that the content of CPs decreased from 15 min to 30 min, then increased again at 45 min and maintained a relatively stable content level until the reac-tion time was extended to 120 min, which may be related to the short existence time of •OH (Bissey et al., 2006); thus, the concentration in the system after 45 min was too low to continue oxidative degradation of organic matter. The increase in CPs at 45 min was mainly due to the resynthesis of CPs by the combination of benzene rings and chlorine radicals, resulting in a slight increase in Σ5 TD-CPs and a relatively greater increase in Σ19 CPs.

In summary, reaction conditions with a reagent dose of 180 mmol/L, H2O2:Fe2+ = 1:1, pH 3–4 and reaction time of 30 min could reach a lower rate of increase of Σ5 TD-CPs and Σ19 CPs under the premise of ensuring the oxidation efficiency of the Fenton process. The Fenton oxidation efficiencies for characteristic organic matter in textile dyeing sludge, such as PAHs and AAs, are listed in Figs. S2 and S3.

This part of the study demonstrated and analyzed the changes in CPs before and after Fenton conditioning. In follow-up studies, the genera-tion and degradation pathways of CPs must be further explored and confirmed. Treatment methods other than the Fenton method (e.g., adsorption, membrane, electrocatalysis, photocatalysis, etc., and their combination) are also expected to be developed to achieve better co-ordination between the reduction of overall sludge toxicity and the control of chlorophenol increase. The management and control stan-dards for characteristic CPs in textile dyeing sludge are proposed as well.

4. Conclusions

The pollution characteristics of 19 CPs were investigated in this study, and the mean total concentration was 1682.45 ng g− 1 dw, among

Fig. 4. Concentrations of five textile dyeing predominant chlorophenols (Σ5 TD-CPs) and nineteen total chlorophenols (Σ19 CPs) in raw sludge and conditioned sludge under different conditions. (a) Reaction reagent dose. (b) Reagent ratio of H2O2:Fe2+ based on the dose of H2O2 is 140 mmol/L. (c) Initial reaction pH. (d) Reaction time.

X. Chen et al.

Journal of Hazardous Materials 416 (2021) 125721

8

which high-chlorine phenols accounted for the greatest proportion. 2,3,5,6-TeCP, 2,3,4,6-TeCP, 2,3,4-TCP, PCP and 4-CP were defined as TD-CPs with reference significance to the textile dyeing industry. An assessment of the environmental risk showed that CPs posed a moderate to high risk to the environment and the toxicity of CPs may be gradually amplified along the food chain. Preliminary exploration of the influence of the Fenton process on CPs in actual sludge identified a noteworthy phenomenon in which the content of CPs increased significantly after Fenton conditioning. To minimize environmental risks, optimal condi-tions (reagent dose of 180 mmol/L, H2O2:Fe2+ = 1:1, pH 3–4 and re-action time of 30 min) for controlling the rate of increase of CPs were also initially determined.

CRediT authorship contribution statement

Xiaohui Chen: Conceptualization, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Xun- an Ning: Conceptualization, Supervision, Resources, Project adminis-tration, Funding acquisition. Xiaojun Lai: Writing - review & editing. Yi Wang: Writing - review & editing. Yaping Zhang: Writing - review & editing. Yao He: Writing - review & editing.

Declaration of Competing Interest

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

Acknowledgments

This work was supported by the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program [No. 2017BT01Z032], Natural Science Foundation of China [No. 21577027], and the 2017 Central Special Fund for Soil, Preliminary Study on Harmless Treatment and Comprehensive Utilization of Tailings in Dabao Mountain [18HK0108].

Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jhazmat.2021.125721.

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