Degradation performance of carbamazepine by ferrous-activated sodium hypochlorite: Mechanism and impacts on the soil system

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Degradation performance of carbamazepine by ferrous-activated sodium hypochlorite: Mechanism and impacts on the soil system ⁎

⁎

Xuan Zoua,b,c, Xiaoming Lia,b, , Can Chenc, , Xiaofei Zhua,b, Xiaoding Huanga,b, You Wua,b, Zhoujie Pia,b, Zhuo Chena,b, Ziletao Taoa,b, Dongbo Wanga,b, Qi Yanga,b a

College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, PR China c Hunan Research Academy of Environmental Sciences, Key Laboratory of Water Pollution Control Technology, Changsha 410018, PR China b

H I GH L IG H T S

-activated NaOCl could effectively degrade Carbamazepine (CBZ) in the soil system. • FeThe effects of operation factors on the Fe /NaOCl process were investigated. • Reaction mechanisms CBZ degradation in soil were proposed. • Microbial community inforsoil system was negatively affected by Fe /NaOCl treatment. • 2+

2+

2+

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbamazepine Chemical oxidation Ferrous-activated NaOCl Soil remediation

The existence of carbamazepine (CBZ) in soil environment has been continuously reported and efficient elimination techniques are imperative. This study investigated the feasibility and the optimal conditions of CBZ degradation in soil by using Fe2+-activated hypochlorite (NaOCl). The Fe2+/H2O2, Fe2+/persulfate and Fe2+/ NaOCl systems were applied to degrade CBZ under different initial soil pH conditions, of which Fe2+/NaOCl system achieved a higher CBZ removal rate over a wider pH range. The results indicated that 94.5% of CBZ could be degraded after 4 h when NaOCl concentration was 75 mmol kg−1 at Fe2+/NaOCl molar ratio of 1:1, and the reaction followed pseudo-first-order model. The CBZ removal efficiency could be improved with the increasing of NaOCl dosage. The reduction of initial CBZ concentration had a dual effect on CBZ removal efficiency at a fixed NaOCl dosage. Raising the temperature could considerably speed up the reaction. The high liquid to soil ratio, excessive humic acid and inorganic anions (Cl−, HCO3−) were not conducive to CBZ removal. Electron Spin Resonance (ESR) confirmed that the Fe2+/NaOCl system yield more hydroxyl radicals (HO%) than that of NaOCl alone for CBZ degradation. The transformation pathways were proposed for CBZ degradation based on the identified intermediates. The investigation with soil surface morphology and mineral composition (Fe, Mn, Cu and Zn) showed no significant differences between the original and the treated soil samples, while the soil microbial community was negatively affected.

1. Introduction Pharmaceuticals and personal care products (PPCPs) as the newly organic pollutants in soil have attracted worldwide attention. Generally, the PPCPs could not be removed completely due to their recalcitrant during the wastewater treatment process, thereby, soils become the final reservoirs for these chemicals when irrigating with these wastewater effluents or fertilizing with the biosolids from the wastewater treatment plant [1]. Carbamazepine (CBZ), as an

⁎

antiepileptic drug and one of the most persistent PPCPs, has been widely used for the treatment of seizures and other mental disorders [2,3], its annual consumption in the world was estimated to be at 1014 tons [4]. The half-lives of CBZ ranged from 355 to 1624 days due to its persistence in the soil [5]. What is more, CBZ showed a tendency in accumulation in soil. It has been reported that the concentration of CBZ in sludge and sewage irrigated soil has reached 20.9 mg kg−1 [6] and 8.38 μg kg−1 [7], respectively. The presence of CBZ in agricultural soils might contaminate food via plant uptake, resulting in bioaccumulation

Corresponding authors at: College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China (X. Li). E-mail addresses: [email protected] (X. Zou), [email protected] (X. Li), [email protected] (C. Chen).

https://doi.org/10.1016/j.cej.2019.123451 Received 15 August 2019; Received in revised form 8 November 2019; Accepted 9 November 2019 1385-8947/ © 2019 Published by Elsevier B.V.

Please cite this article as: Xuan Zou, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123451

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and potential human exposure [8]. For example, CBZ could be detected in common vegetables sold in the market in California, Israel and Irvine, and Jerusalem [9]. In fact, CBZ has been listed as a list of chemicals with high bioaccumulation factors in plant roots, leaves and stems [10], and CBZ-related teratogenic effects have been clinically discovered [11]. Given those, it is necessary to develop feasible control and elimination techniques for CBZ in soil. There are several innovative technologies including physical, chemical, physicochemical and biological processes for the organic pollutants removal in soil [12,13]. Previous research interests and efforts were mainly focused on the application of bioremediation technology for the degradation of CBZ in soil. However, the use of biotechnology for the treatment of CBZ was limited by its low degradation efficiency and long degradation time. Thelusmond et al. [1] observed that less than 50% of CBZ was removed by biodegradation after 80 days of incubation. Shao et al. [14] reported that the biodegradation rate of CBZ after incubation for 120 days was only 5.82–21.43%. With the rigor of industrial standards and the improvement of land economic value, efficient and rapid soil remediation methods are imperative. Chemical oxidation, as an efficient and promising technique for the remediation of soil organic contamination [15], has advantages of relatively short remediation time, low cost and effectiveness against multiple contaminants [16]. Na2S2O8, H2O2, KMnO4 and ozone are frequently used and investigated oxidants in the chemical oxidation of organic contaminants. Hypochlorite, as a relatively weak oxidant, was often used in water treatment, but its application in soil remediation was rarely reported. Previous studies have elucidated the feasibility of using hypochlorite as a powerful oxidant for soil decontamination [17,18]. There were several advantages to use NaOCl for the removal of organic pollutants. First of all, NaOCl is a very inexpensive reagent that facilitates its large-scale use in the industry. Secondly, NaOCl could produce strong oxidizing hydroxyl radicals (HO%) after activation by Fe2+ [19] (Eqs. (1) and (2)), which was a reaction parallel to the Fenton initiation reaction [20].

NaOCl + H2 O ⇋ HOCl + NaOH

(1)

Fe 2 + + HOCl → HO·+ Fe3 + + Cl−

(2)

Table 1 Basic properties of carbamazepine. Name

Molecular formula

Carbamazepine

C15H12N2O

Molecular structure

Molecular weight 236.27

N

O

NH2

2. Materials and methods 2.1. Chemicals CBZ (≥99%) was purchased from Sigma-Aldrich (Saint-Louis, MO, USA) and its basic properties were listed in Table 1. Methanol (≥99%) and acetonitrile (≥99%) were purchased from Merck (KgaA, Darmstadt, Germany). Sodium hypochlorite (≥5.2%), ferrous sulfate heptahydrate (FeSO4%7H2O, > 99%) and other chemicals were purchased from Sinopharm Chemical Reagent Company (Shanghai, China). The ferrous solution was newly prepared before utilization. Deionized (DI) water (Millipore, Massachusetts, USA) was used for all experiments. All chemicals used were of analytical grade. 2.2. Soil samples The surface soil sample was collected from the top layer (0–20 cm) of a place near the Xiangjiang River, Hunan. The soil was air-dried and grinded, followed by passing through a 2 mm nylon sieve to remove rocks and roots, and then stored in brown wide-mouth glass vials for further determination or oxidation experiments. Physicochemical characteristics of the soil, such as pH and organic matter content, were determined by the national standard method and are listed in Table 2 [24]. The soil was classified as sandy clay following the United States Department of Agriculture (USDA) Soil Textural Triangle. Soil water content was determined by drying soil at 105 ± 2 °C for hours until the weight stabilized. The metal contents in the soil were measured according to EPA 3050 B method [25]. No CBZ was detected in the original soil sample.

Nevertheless, the reaction rate between Fe2+ and H2O2 (k = 0.22 dm3 mol−1 s−1) was about three orders of magnitude less than that between Fe2+ and HOCl (k = 220 ± 15 dm3 mol−1 s−1) [19]. NaOCl could also be used at a temperature of 25 °C, which effectively prevented heat-induced changes in soil minerals [21]. Although the Fe2+/NaOCl process has made substantial progress in the field of wastewater treatment and sludge dewatering [22,23], its feasibility in the removal of soil organic pollutants has not been discussed. The performance and mechanism of Fe2+/NaOCl in degrading organic pollutants are still unclear, and its impacts on the soil system remain unknown. In this work, the remediation of organic polluted soil by the Fe2+activated NaOCl process was systematically investigated with CBZ as the model organic pollutant. First, the influence of main factors such as NaOCl dosage, Fe2+/NaOCl molar ratio, pH, inorganic anions, etc. on the CBZ degradation was investigated. Then, the underlying transformation pathways of CBZ in soil were proposed based on the identified intermediates. Furthermore, the impacts of the Fe2+/NaOCl oxidation process on the soil property were evaluated according to the variation of soil surface morphology, mineral composition (Fe, Mn, Cu and Zn) and microbial community. This work verified the feasibility of Fe2+/ NaOCl process to effectively degrade CBZ in soil and the findings obtained might contribute to the future development of soil remediation techniques.

2.3. Preparation of spiked soils The desired mass of CBZ was dissolved in methanol and mixed with a specified volume of soil, and then more methanol was added to submerge the soil completely. The entire slurry was stirred constantly to achieve homogeneity. Afterwards, the soil sample was placed in a Table 2 The properties of the soil sample.

2

Parameter

Data

pH Water content (%) Organic matter (%) cation exchange capacity (cmol kg−1) Sand (%) Silt (%) Clay (%) Bulk density (mg m−3) Fe (g kg−1) Mn (g kg−1) Cu (mg kg−1) Zn (mg kg−1)

8.14 ± 0.14 2.04 ± 0.12 2.88 ± 0.35 12.15 ± 0.55 57 8.3 34.7 1.52 ± 0.19 37.8 0.52 22.06 90.14

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fume hood to evaporate methanol until the weight was constant. The CBZ concentration in spiked soil was around 20 mg kg−1. The prepared soil samples were kept in the dark before use. 2.4. Experimental setup The degradation experiments were conducted in a water bath shaker with thermostatic equipment (180 rpm, ambient temperature). After the shaker was preheated to the desired temperature (i.e., 30–60 °C), the samples were placed in to start the experiment. The water-to-soil ratio was set to 1:1 unless otherwise noted. A series of vials were created at each sampling time (0, 15, 30, 60, 90, 120 and 240 min). The NaOCl stock solution was prepared by diluting a specific amount of concentrated NaOCl solution in DI water. The prepared soil sample (4 g) was added to the vials before each experiment. Then, 4 mL of DI water, 1 mL FeSO4 stock solution and 1 mL NaOCl stock solution were added and thoroughly mixed with the soil sample. Besides, to eliminate the CBZ reduction caused by soil adsorption when optimizing the doses of NaOCl, a set of control treatments was employed without adding Fe2+ and NaOCl, and the other operations were the same as the experimental group. The reaction was quenched by adding 2 mL of 1 M sodium thiosulfate solution at predetermined time intervals. In the pH experiment, 8 mL of DI water was first added to the soil sample, and then the desired pH conditions were adjusted with a certain volume of dilute NaOH or HCl solution before the addition of Fe2+ solution and NaOCl solution. Before the start of the humic acid (HA) experiment, a certain amount of HA was added and uniformly mixed with the soil sample. The slurry was combined extracted using 20 mL methanol, first sonicated for 30 min and then shaken at 220 rpm for 30 min (ambient temperature). Afterwards, the soil suspension was centrifuged for 10 min (6000 rpm). The supernatant was pipetted and filtered through 0.22 μm polytetrafluoroethylene (PTFE) membrane for analyzing immediately or refrigerated at −20 °C until analysis. The extraction recovery test of CBZ in this study was carried out to evaluate the feasibility of the extraction method. The recovery rate of CBZ from the soil ranged between 91% and 97%.

Fig. 1. CBZ degradation in soil system under different NaOCl dosages. Experimental conditions: [CBZ]0 =20 mg kg-1, pH = initial soil pH, [NaOCl] = 0-100 mmol kg−1, Fe2+: NaOCl = 2:1.

E.Z.N.ATM Mag-Bind Soil DNA Kit (Omega Bio-tek, Inc., Norcross, GA, USA). Sequencing was accomplished on the Illumina Hiseq platform in Shanghai Sangon Biotech Co. Ltd. The PCR amplification of the bacterial 16S rDNA gene was performed with the bacterial primers, 805R and 341Fbac, targeting the V3-V4 regions. 3. Results and discussion 3.1. Factors affecting the degradation of CBZ in Fe2+/NaOCl system 3.1.1. Effect of NaOCl dosage The effect of NaOCl dosages on CBZ degradation by the Fe2+/NaOCl process was investigated (Fig. 1). The control test confirmed that CBZ could not be degraded in the absence of NaOCl. As shown in Fig. 1, the degradation efficiency of CBZ increased substantially as the NaOCl dosage increased from 0 to 100 mmol kg−1. After 4 h of the reaction, the removal rate of CBZ increased from 41.3% to 85.1% with the NaOCl dosage increasing from 25 to 75 mmol kg−1 at Fe2+/NaOCl molar ratio of 2:1. The result indicated that increasing the NaOCl dosage could critically facilitate the removal rate of CBZ. This could be contributed to the enhanced generation of reactive radicals (HO%) at relatively high NaOCl concentrations, which was essential for CBZ to overcome the competition from other components in soil. However, a further increase in NaOCl concentration (100 mmol kg−1) produced less improvement in CBZ degradation efficiency (89.6%). The reason may be that excessive NaOCl could react with HO% to form ClO% radicals, while the latter was less reactive and could not promote the elimination of CBZ [26]. Considering the potential adverse effects and excessive oxidant doses may aggravate the burden on soil, NaOCl dosage was set to 75 mmol kg−1 for subsequent experiments to maximize the removal efficiency while minimizing side reactions at the same time. However, since the oxidants are generally not selective, and it will be consumed by reacting with other components of the soil organic matter, the dose of the oxidant may vary significantly in practical use. The pseudo-first-order kinetic model could be used to fit the CBZ degradation curve at different NaOCl doses (Eq. (3)):

2.5. Soil microbial community analyses Soil samples before and after Fe2+/NaOCl oxidation were collected for analysis of bacterial community composition. The experimental conditions were set at the NaOCl dosage was 75 mmol kg−1 and the Fe2+/NaOCl molar ratio was 1:1. The reaction time was four hours and the pH was not adjusted. After the Fe2+/NaOCl treatment, the treated soil sample was centrifuged to remove the supernatant and freeze-dried for the next Illumina Hiseq sequencing analysis. 2.6. Analytical methods The concentration of CBZ was determined by High-Performance Liquid Chromatography (HPLC) (Shimadzu SPD-10A, Japan) equipped with Shim-pack VP-ODS C18 column (5 μm, 4.6 × 150 mm) coupled with a UV detector at a wavelength of 284 nm, the injection volume was 10 μl, and the column temperature was 25 °C. The mobile phase consisted of 55% DI water with 0.5% acetic acid and 45% acetonitrile at a flow rate of 0.8 mL min−1. The intermediates formed in the reaction were characterized by liquid chromatography-triple quadrupole mass spectrometry (LC-MS/MS) (SCIEX QTRAP 6500, USA). The mass spectra were recorded in full scan mode (m/z 100–300) for qualitative analysis. Electron Spin Resonance (ESR) measurement of radicals was carried out on JES FA200 (JEOL, Japan), the center field was 326 mT with a range of 10 mT. Soil surface morphology was analyzed using a scanning electron microscope (SEM) (TESCAN MIRA3, Czech Republic). The mineral composition was analyzed by inductively coupled plasmaatomic emission spectroscopy (ICP; Thermo Fisher 7400, USA) using standard procedures. DNA was extracted from the soil samples using

C ln ⎛ t ⎞ = −kobs t ⎝ C0 ⎠ ⎜

⎟

(3) −1

where kobs reflects the pseudo-first-order rate constants (min ) of CBZ by Fe2+/NaOCl process, C0 and Ct are the concentrations of CBZ at time zero and t. The kobs value increased positively with increasing NaOCl concentration (The kobs values were 0.0043, 0.0110, 0.0168 and 3

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Fig. 2. (a) CBZ degradation in soil system under different Fe2+/NaOCl molar ratios. (b) Effect of Fe2+/NaOCl molar ratio on CBZ degradation rate. Experimental conditions: [CBZ]0 = 20 mg kg−1, pH = initial soil pH, [NaOCl] = 75 mmol kg−1, Fe2+: NaOCl = 5:1-1:10, individual: NaOCl alone without Fe2+ involvement, reaction time = 4 h. (c) Intensity of HO% radicals at three conditions: Fe2+/NaOCl system, NaOCl alone, Fe2+ alone. Experimental conditions: [Fe2+] = [NaOCl] = 75 mmol kg−1, Fe2+: NaOCl = 1:1, pH = initial soil pH, T = ambient temperature.

0.0174 min−1 at NaOCl concentrations of 25, 50, 75, 100 mmol kg−1, respectively) (Fig. S2). Since the reaction approached equilibrium after 120 min, the data from the first 120 min was used for data fitting.

[32] reported that HO% in the NaOCl system could be produced by the catalysis of soluble inorganic Mn salts. Although the results indicate that the addition of iron may be unnecessary in soils rich in mineral iron content, there are several advantages to use mineral iron as the catalyst, like wider pH working range and the promotion of oxidant mobility in the soil [33]. It can be seen from Fig. 2b that the degradation of CBZ under different Fe2+/NaOCl molar ratios conformed to the pseudo-first-order kinetic model. In the presence of NaOCl, the removal rate of CBZ was decreased as the proportion of Fe2+ increased from 1:1 to 5:1. Meanwhile, decreasing the proportion of Fe2+ (from 1:1 to 1:10) also reduced the removal rate, which might be attributed to insufficient Fe2+ concentration could not activate NaOCl effectively. ESR technique was used to determine the intensity of HO% radicals in three systems (Fe2+/NaOCl, NaOCl alone and Fe2+ alone) (Fig. 2c). The result demonstrated that the intensity of HO% was enhanced dramatically in the Fe2+/NaOCl process. According to thermodynamic arguments, HO% were generated between HOCl and Fe2+ due to a single electron-transfer reaction [19] (Eq. (2)). The produced HO% are highly reactive, simultaneously with the organic radical-induced chain reaction, ultimately resulting in the degradation of organic compounds [34]. In the process of OCl− being reduced, it was easy to get electrons and has strong oxidation ability.

3.1.2. Effect of Fe2+/NaOCl molar ratio Ferrous ions can act as a catalyst to promote the production of HO% and have a positive effect on the formation of oxidizing species [22], but excess amounts of Fe2+ can serve as HO% scavenger [27]. Therefore, the molar ratio of Fe2+ to NaOCl was investigated to obtain optimal oxidation conditions (Fig. 2a). At a fixed NaOCl dosage of 75 mmol kg−1, the degradation efficiency of CBZ under several Fe2+ to NaOCl molar ratios (5:1, 2.5:1, 2:1, 1:1, 1:2, 1:2.5, 1:5, 1:10) and the NaOCl alone was studied. As presented in Fig. 2a, the optimal CBZ degradation efficiency could be obtained at the Fe2+/NaOCl molar ratio of 1:1 (94.5%). However, the removal rate of CBZ was greatly reduced as the proportion of Fe2+ increased. For example, the CBZ removal efficiency dropped to 12.8% at the Fe2+/NaOCl molar ratio of 5:1. The result indicated that excess of ferrous ions was not conducive to the degradation of CBZ. This could be attributed to the competition between Fe2+ and HO% that reduced the concentration of reactive radicals [28]. It is worth mentioning that the removal efficiency of CBZ by NaOCl alone was considerable, reached 84.4%. The reaction rate constant of HOCl with the ionized form of CBZ was estimated to be 1.8 × 106 M−1 s−1 [29]. This may indicate that CBZ may be selectively oxidized by NaOCl [30]. Besides, NaOCl may be partially activated by naturally occurring species (i.e., Fe/Mn oxides, pyrite) present in the soil, therefore improving the reactivity of the oxidant [31]. Liu et al.

3.1.3. Comparison of CBZ degradation in different systems under various pH conditions To further evaluate the oxidation performance of the Fe2+/NaOCl system, the ability of Fe2+/NaOCl to degrade CBZ under different soil 4

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Fig. 4. Effect of initial CBZ concentration on CBZ degradation and corresponding kinetic curves (inset). Experimental conditions: [CBZ]0 =535 mg kg−1, pH = initial soil pH, [NaOCl] = 75 mmol kg−1, Fe2+: NaOCl = 1:1.

Fig. 3. Degradation of CBZ at six pH levels in three chemical oxidation systems. Experimental conditions: (a) Fe2+/H2O2 process: [H2O2] = 75 mmol kg−1, Fe2+: H2O2 = 1:10. (b) Fe2+/NaOCl process: [NaOCl] = 75 mmol kg−1, Fe2+: NaOCl = 1:1. (c) Fe2+/Na2S2O8 process: [Na2S2O8] = 75 mmol kg-1, Fe2+: Na2S2O8 = 1:1. [CBZ]0 = 20 mg kg−1, reaction time = 4 h.

The Fe2+/NaOCl process achieved the ideal result without adjusting the pH, which means it has a broader range of applications and good utilization potential. Taking into account the impact of potential pH changes on soil quality, all degradation experiments were conducted at the original pH of the soil.

2+

pH conditions (3, 5, 7, 8.14, 9, 11) was compared with Fe /H2O2 and Fe2+/Na2S2O8 processes (Fig. 3). In the Fe2+/H2O2 process, the highest CBZ degradation efficiency (82.6%) could be obtained at pH 3, which was consistent with the previous study [35]. Persulfate is a competitive oxidant with good reactivity, but the result showed that the degradation of CBZ by Fe2+/NaOCl process was superior to that of Fe2+/ Na2S2O8 process in the pH range of 3–11. The optimal degradation efficiency of CBZ for Fe2+/Na2S2O8 at pH 5 (85.0%) was still lower than that of Fe2+/NaOCl at pH 7 (90.3%). The result indicated that the effect of acidic or alkaline environment on the elimination of CBZ in soil by Fe2+/NaOCl process was negligible. In the Fe2+/H2O2 process, the decomposition of H2O2 requires the presence of H+ (Eq. (4)) [36], which indicates that an acidic environment is required to yield the maximum amount of HO%. Meanwhile, the increase in pH promotes the precipitation of Fe(OH)3, which further leads to the decrease of HO% produced in the Fe2+/H2O2 process.

2Fe 2 + + H2 O2 + 2H+ → 2Fe3 + + 2H2 O

3.1.4. Effect of initial CBZ concentration The effect of initial CBZ concentration on the CBZ removal rate was studied at a fixed Fe2+ and NaOCl dosage (75 mmol kg−1). As depicted in Fig. 4, the removal rate of CBZ increased first and then decreased as initial CBZ concentration increased from 5 to 35 mg kg−1. When the initial CBZ concentration was reduced from 20 to 5 mg kg−1, the CBZ removal efficiency dropped from 88.2% to 82.1%. Khan et al. [37] reported the similar result that the removal rate of atrazine increased as the initial atrazine concentration increased from 1.16 to 9.82 μM in two cases (when the molar ratio of [atrazine]0/[PMS]0 was fixed at 1:10 or when PMS concentration was fixed at 46.4 μM). A possible explanation for this may be that the increase in initial CBZ concentration increased the number of CBZ molecules exposed to free radicals for decomposition. This means that higher initial CBZ concentrations could enhance the collision frequency between CBZ molecules and the oxidants, thereby stimulating the degradation of CBZ. Another possible explanation is the effect of soil adsorption on CBZ molecules, which could weaken the reactivity of CBZ molecules with the oxidants during the reaction. As a result, a decrease in degradation efficiency at lower initial CBZ concentrations was observed (considering the fact that the adsorbed part of CBZ molecules could be extracted from the soil at the end of the reaction). However, the removal efficiency decreased significantly as the initial CBZ concentration increased from 20 to 35 mg kg−1, where 76.7% and 60.1% of CBZ were degraded at an initial CBZ concentration of 30 and 35 mg kg−1, respectively. This was probably due to the increased demand for reactive radicals at higher initial CBZ concentrations. A higher concentration of CBZ also led to the buildup of intermediates, which in turn reduced the ratio between the reactive radicals and CBZ molecules. Since the NaOCl concentration was fixed when increasing the initial CBZ concentration, the inhibition was more pronounced.

(4)

2+

Different from the Fe /H2O2 system, which is strongly pH dependent, Fe2+/NaOCl can react over a wide range of pH values. This may be attributed to the chemical species of HOCl. According to Eq. (5), two major acid-base species of chlorine could be observed when 5 < pH < 11, and the good reactivity of CBZ with chlorine might be attributed to the presence of Cl2 and Cl2O when pH > 6 (Eqs. (6) and (7)) [29].

HOCl ⇋ H+ + OCl− 2HOCl ⇋ H2 O + Cl2 O

HOCl +

H+

+

Cl−

K a HOCl = 10−7.54

(5)

K a Cl2 O = 8.7 × 10−3

⇋ Cl2 + H2 O

K a Cl2 = 5.1 ×

(6)

10−4

(7)

Under neutral and basic conditions, CBZ degradation could be attributed to direct ClO− and HOCl reactivity (in addition to reaction with Cl2 and Cl2O) [29]. However, the chloride concentration was expected to have no significant influence on the degradation of CBZ when pH > 8 [29]. Hence, the degradation of CBZ at this pH range (> 8) might be mainly attributed to hydroxyl radical reactions. As for the strong pH adaptability of the Fe2+/Na2S2O8 process, more SO4%− could convert to HO% at higher pH values, which changed the reaction mechanism of the degradation of organic compounds (Eq. (8)).

SO4 ·− + OH− → HO·+SO4 2 −

3.1.5. Effect of temperature The removal trend of CBZ under different temperature conditions (30, 40, 50, 60 °C) was shown in Fig. 5. During the first 15 min of the reaction, the removal rate of CBZ increased sharply with the increasing temperature. After 5 min of the reaction, the degradation rates of CBZ at

(8) 5

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Fig. 5. Effect of temperature on CBZ degradation. Experimental conditions: [CBZ]0 = 20 mg kg−1, pH = initial soil pH, [NaOCl] = 75 mmol kg−1, Fe2+: NaOCl=1:1, temperature = 30–60 ℃.

Fig. 6. Effect of L/S ratio on CBZ degradation. Experimental conditions: [CBZ]0 =20 mg kg−1, pH = initial soil pH, NaOCl dosage was fixed at 75 mmol kg−1, Fe2+: NaOCl = 1:1, L/S ratio = 1:1-5:1.

30, 40, 50 and 60 °C were 37.7%, 40.0%, 45.3% and 70.1%, respectively. The result showed that the increase of temperature could greatly shorten the time for the reaction to reach equilibrium, from 4 h at ambient temperature to 1 h after heating. Increasing the temperature could enhance the effective collision frequencies between NaOCl and CBZ molecules, which might promote the degradation of CBZ by Fe2+/ NaOCl process. Research has reported that hypochlorite could generate reactive radicals when exposed to heat or UV light [17], the ClO%, Cl%, and HO% generated from the process could proceed to polymerize, degrade, hydrolyze or halogenate the existing organic compounds [38]. However, when the temperature was further raised to 60 °C, the removal efficiency was lower than that of 50 °C after 4 h of the reaction. This may be contributed to the instability of the NaOCl molecule. Higher temperatures induced the self-decomposition of NaOCl, which greatly weakened the oxidation ability of NaOCl. Moreover, rising the temperature was unrealistic in the in-situ chemical oxidation treatment, and there was only a slight difference in CBZ degradation efficiency between 50 °C and ambient temperature. Hence, although the reaction achieved maximum efficiency at 50 °C, it is more appropriate to select ambient temperature conditions in practical.

3.1.7. Effect of HA Natural organic matter has an important influence on the presence of organic pollutants in the soil environment. To determine the effect of natural organic matter on the removal of CBZ, HA, as an important constituent of soil organic matter, was introduced into the soil sample at concentrations of 0, 5, 10 and 20 g kg−1. Fig. 7 showed changes in CBZ residual concentration at different HA dosages. When the initial HA dose increased from 0 to 20 g kg−1, the degradation efficiency was reduced from 91.9% to 20.8%. Compared to the degradation efficiency without HA addition, considerable inhibition was observed. Previous research suggested that HA molecules were rich in functional groups like hydroxyl, carboxyl, and quinine, which could be easily attacked by HO% [40]. The rate constants of HA reacting with Cl% and HO% were 1.3 × 104 and 2.5 × 104 (mg/L)−1 s−1, respectively [41], suggesting that the scavenging effect of HA on HO% was more pronounced compared to that of Cl%. Therefore, excessive HA could react with oxidants directly to compete with the substrate for free radicals, causing the degradation of CBZ molecules to be inhibited. The above results revealed that the radicals generated by the Fe2+/NaOCl process were preferentially consumed by HA rather than the CBZ molecules. Therefore, soil organic matter content is a key factor that cannot be ignored

3.1.6. Effect of L/S ratio Fig. 6 showed the important effect of the L/S ratio on the chemical oxidation process of soil organic pollutants. The study was performed under different L/S ratio conditions (1:1, 2:1, 5:1) with the fixed oxidant dosage (75 mmol kg−1). It can be seen from Fig. 6 that the CBZ removal rates in the Fe2+/NaOCl system varied significantly at three L/ S ratio levels. The degradation efficiency decreased sharply with increasing L/S ratio. In a complex soil environment, due to the non-selective characteristics of chemical oxidation reagent, several issues affected the ability of oxidant to remove target contaminants from the soil effectively. Previous research reported that the L/S ratio affected the oxidant concentration, which was the greatest contributor to soil oxidant demand (the amount of oxidant consumed by the soil oxidizable matter) [39]. Therefore, a higher L/S ratio led to higher soil oxidant demand, which significantly affected the ability of the NaOCl to degrade CBZ. In fact, excessive L/S ratio diluted the concentration of the oxidant, reducing the collision probability of hydroxyl radicals and the contaminant, resulting in a severe reduction in degradation efficiency. The highest CBZ removal rate was observed at the L/S ratio of 1:1 (91.8%). It was also practical for field projects without increasing the cost and difficulty of subsequent wastewater treatment.

Fig. 7. Effect of HA on CBZ degradation. Experimental conditions: [CBZ]0 =20 mg kg−1, pH = initial soil pH, [NaOCl] = 75 mmol kg−1, Fe2+: NaOCl=1:1, [HA]0=0-20 g kg-1. 6

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suppression was observed after the addition of HCO3−, indicating that HO% might play a leading role in the decomposition of CBZ by Fe2+/ NaOCl process. Eqs. (9), (10), and (11) show the mechanism [44].

HCO3− → H+ + CO32 −

(9)

HCO3− + HO·→CO3·− + H2 O

(10)

CO32 − + HO·→CO3·− + OH−

(11)

Based on the above results, it could be concluded that the inorganic anions presented in the soil system have an adverse effect on the removal of organic pollutants. 3.2. Reaction products and proposed degradation pathways The above results indicated that CBZ in soil could be effectively degraded by the Fe2+/NaOCl process. To further investigate how the Fe2+/NaOCl oxidation process advanced, the reaction intermediates were characterized by LC-MS/MS technology. Based on the obtained mass-to-charge ratio (m/z) and MS/MS fragmentation patterns, molecular structures of the CBZ transformation intermediates were characterized and three probable degradation pathways were tentatively proposed in Fig. 9 (detailed information of the products were provided in supporting information). Previous studies suggested that the mechanism of HO% participated in CBZ degradation was high-boundary electron density preferential hydroxylation [45]. The olefinic double bond of the central heterocyclic ring of CBZ was first attacked by HO% to form hydroxylated product P8, and then the seven-membered ring was opened, resulting in unstable intermediates facing ring contraction to form P4 [46]. Then, P8 was converted to a heterocyclic radical, and P9 was subsequently generated by electron transfer [47]. Through intramolecular cyclization, P5 was formed and it could be further oxidized to corresponding carboxylic acid P6 as reported in the hydroxylation process [48]. Through further oxidation and hydrogen rearrangement reaction, the transformation products P12 and P11 were formed. The product P13 was identified as acridine formed by the decarbonylation of P12 and the elimination of carboxaldehyde from P11. It could be transformed into ring-opening compounds after further attacked by HO%, such as tartaric acid, malonic acid and catechol [49]. Furthermore, those transformation products might be degraded to compounds with low molecular weight by further oxidation, and eventually mineralized to CO2 and H2O. Besides, Cl% might attack the olefinic double bond of the central heterocyclic ring of CBZ to generate P3, or substituted by chlorine to product P2. Because the fragment ions showed that the tricyclic structure of CBZ remained intact, this indicated that chlorine directly substituted the hydrogen on CBZ [45]. During the degradation of CBZ by Fe2+/NaOCl process, the chlorinated products could be formed by electrophilic substitution of chlorine and electron transfer pathways. However, chlorinated products might cause greater toxicity than the mother compound. It should be noted that more unknown intermediates may be formed during the degradation of CBZ and the risk of soil salinization was also worthy of attention. Further researches and improvements are required for the Fe2+/NaOCl technique to minify the toxicity of transformation products and potential environmental risks.

Fig. 8. Effect of inorganic anions on CBZ degradation. (a) Cl− ions. (b) HCO3− ions. Experimental conditions: [CBZ]0 = 20 mg kg−1, pH = initial soil pH, [NaOCl] = 75 mmol kg−1, Fe2+: NaOCl=1:1, Cl-: NaOCl = 0:1-2:1, HCO3−: NaOCl = 0:1-2:1.

in the chemical oxidation process of organic contaminated soil. 3.1.8. Effect of inorganic anions Various inorganic anions in the soil system might co-exist with organic pollutants thus affecting the free radical-induced reactions in the Fe2+/NaOCl system. To investigate the effect of inorganic anions on the Fe2+/NaOCl oxidation process of CBZ, two common anions, Cl− and HCO3− were chosen. The Cl−/HCO3− to NaOCl molar ratio was set at several levels (0:1, 0.5:1, 1:1 and 2:1). As can be seen from Fig. 8a, lower concentrations of Cl− (0.5:1, 1:1) hardly affect the oxidation process, the removal efficiency at this Cl− concentration was comparable to that without the addition of Cl− ions. However, the removal rate of CBZ decreased from 94.0% to 88.4% at higher Cl− concentration (2:1). The inhibitory effect of Cl− on CBZ degradation was probably due to its influence on hydroxyl radicals and hypochlorite ions, which were supposed to responsible for the CBZ removal [42]. However, the reaction experienced a distinctive inhibition effect after the introduction of HCO3− ions. Fig. 8b showed that the removal efficiency of CBZ decreased continuously as the molar ratio of HCO3− to NaOCl increased from 0:1 to 2:1. The removal efficiency of CBZ at HCO3− to NaOCl molar ratio of 0.5:1, 1:1, 2:1 were 86.0%, 75.8%, 58.1%, respectively. The reason behind the result may be that the hydroxyl radicals were highly reactive, and it could be easily consumed by HCO3−/CO32− which could work as a scavenger, leading to the production of less reactive radicals (i.e., CO3%−) [43]. Significant

3.3. Impacts of Fe2+/NaOCl treatment on the soil system 3.3.1. Changes in soil characteristics before and after oxidation The presence of NaOCl played a vital role in the decomposition of CBZ, while the addition of NaOCl might change the properties of the soil. To determine the differences in soil microstructure and surface morphology between the original and treated soil samples, SEM characterization was conducted. As shown in Fig. 10, the flaky structure of the soil before and after treatment was clear, but the oxidized soil particles tend to agglomerate and the debris was reduced. In addition to 7

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3.3.2. Soil mineral composition analysis The partitioning of the metals (Fe, Mn, Cu and Zn) that might be affected by the oxidation process was shown in Fig. 11. Four fractions (exchangeable, iron and manganese oxides, carbonates and residual) were measured to determine the partitioning of metals. It can be observed from Fig. 11 that these metals in the soil samples were mostly presented as residual fractions before or after oxidation, except that the addition of ferrous ions resulted in the accumulation of Fe/Mn oxides fraction of Fe (which suggested the presence of iron oxides after treatment). And Fe was particularly abundant in the soil studied. Different from the acidic conditions that might cause metal dissolution [39], the effect of Fe2+/NaOCl treatment on the soil mineral composition might be mild. Research has reported that the persulfate process irreversibly changed the soil subsurface due to the oxidation of reduced minerals [50]. Compared with the background value of iron in the soil, the effect of iron added in the experiment on the iron content in the soil environment was almost negligible. Moreover, the residual metals might mainly contain primary and secondary minerals, which could hold metals within their crystal structure [51]. They did not tend to release into solution under the natural conditions generally encountered, or the experimental conditions conducted in this study over the reaction period. Moreover, in in-situ chemical oxidation, the limited migration of heavy metals means lower environmental risks.

HO

N

O NH2

O

+·OH

N NH2

O

N NH2

O

+H2O

O

+·Cl HO

Cl

OH

N

ring contraction

N

+·OH

NH 2

O

NH 2

O

N

NH2

O

-NH

O

Cl

HO

N

N NH 2

O

OH

O

+·OH

-NH

O

N

NH

O

rearrangement

O

3.3.3. Impacts of Fe2+/NaOCl treatment on soil microbial community The Fe2+/NaOCl treatment reduced the organic pollution load in soil. At the same time, it might exert an adverse influence on soil microbial communities. The distribution of microbial community structure at the genus level in the S0 (original contaminated soil) and S1 (after the Fe2+/NaOCl treatment) samples were determined to further evaluate the impact (Fig. 12). The Fe2+/NaOCl treatment caused loss to the dominant genera in the original microbial community, such as Solirubrobacter (5.48–0.04%), Thermoleophilum (2.89–0.02%), Conexibacter (1.91–0.02%), Rubrobacter (1.77–0.01%), leaving Pseudomonas (0.08–21.36%) became the new dominant genus in the S1 sample. The result might indicate that the Rubrobacteridae in indigenous soil microorganisms was not resistant to the oxidant NaOCl. However, the relative phylotype frequency of the genus that did not prevail in primitive soil microorganisms was greatly strengthened. Particularly, the Hydrogenophaga (0.04–11.85%), Moheibacter (0.01–6.84%), Brevundimonas (0.04–2.42%), Ancylobacter (0.002–2.01%), Shinella (0.15–4.44%), Bacillus (0.51–2.93%) prevailed in the S1 sample. This might indicate that the new prevalent genera were more tolerant to NaOCl than other genera, or they might benefit from the Fe2+/NaOCl degradation process of CBZ. The latter possibility could be a sign of coexisting chemical and biological degradation, because these genera (e.g., Pseudomonas sp.) have been reported to play an important role in the biodegradation of refractory organic pollutants (e.g., pesticide remediation), as they could use complex hydrocarbons as nitrogen,

O

N O

NH

N

N

O

rearrangement O

-O N

N H

Fig. 9. Proposed transformation pathways for CBZ degradation in the Fe2+/ NaOCl process in soil.

changes in soil particle size before and after oxidation, no distinct differences in soil surface structure were observed. Therefore, the possibility of clogging the aquifer pore spaces thus reducing the hydraulic conductivity after the Fe2+/NaOCl treatment was slight [39].

Fig. 10. SEM images of (a) original soil sample, (b) Fe2+/NaOCl treated soil sample. Experimental conditions: [NaOCl] = 75 mmol kg−1, Fe2+: NaOCl =1:1, pH = initial soil pH, reaction time = 4 h. 8

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Fig. 11. Distribution of Fe, Mn, Cu and Zn in sequentially extracted fractions of the soil samples (before and after Fe2+/NaOCl oxidation). Experimental conditions: [NaOCl] = 75 mmol kg−1, Fe2+: NaOCl =1:1, pH = initial soil pH, reaction time = 4 h.

Fig. 12. Barplot diagrams illustrating the distribution of microbial community structure at the genus level in two soil samples. S0: the original contaminated soil sample before Fe2+/NaOCl treatment. S1: the Fe2+/NaOCl treated soil sample. 9

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References

carbon or energy sources [12]. The result suggested that changes in species abundance could be induced by pollutants or nutrients produced during the Fe2+/NaOCl oxidation. Moreover, genus like Phyllobacterium (0.03% to 8.66%) is considered to be a plant probiotic that ensures soil fertility and promotes crop growth [52]. Although there were no reports on the biodegradation of CBZ by these specific genera (e.g., Hydrogenophaga), the current results supported the possibility of coupled bioremediation after the Fe2+/NaOCl oxidation treatment. The above results suggested that the soil microbial community was negatively affected by Fe2+/NaOCl treatment in the short term. However, since the treated soil was not sampled after a longer recovery time, it was difficult to estimate the extent of microbial regeneration after the Fe2+/NaOCl treatment. In many cases, the microcosms recovered with time, and most of the interference of chemical oxidation on microbial community parameters were mitigated within months [53], even the greatest reduction in organic pollutants degraders caused by ozone (compare to modified Fenton chemistry, and iron-activated persulfate) [54]. Furthermore, time and inflow of groundwater will help the recovery process in actual application. Further researches are required to determine the mechanisms and time frame for the complete recovery of the bacterial activity after the Fe2+/NaOCl treatment.

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4. Conclusions In this work, the degradation of CBZ in soil by Fe2+-activated NaOCl and the optimal conditions were investigated. The results indicated that 94.5% of CBZ could be degraded when NaOCl concentration was 75 mmol kg−1 at Fe2+/NaOCl molar ratio of 1:1. The degradation of CBZ followed the pseudo-first-order kinetic model. Compared to Fe2+/H2O2 and Fe2+/persulfate processes, the Fe2+/ NaOCl process achieved higher CBZ removal efficiency over a wide pH range of 3–11. Higher temperatures could considerably speed up the reaction within 60 °C, but above 50 °C negatively affected CBZ degradation. The initial CBZ concentration exhibited a concentrationbased dual effect on CBZ removal efficiency at the fixed NaOCl dosage. The higher L/S ratio diluted the concentration of the oxidant, thus weakening its oxidation ability. The presence of HCO3− ions and HA significantly suppressed the degradation of CBZ, while Cl− ions slightly reduced the removal rate of CBZ. ESR test demonstrated that the intensity of HO% was dramatically enhanced in the Fe2+/NaOCl process. Through LC-MS analysis, several transformation intermediates were identified and the degradation mechanism was preliminarily proposed. Additionally, the soil microorganism was disturbed by Fe2+/NaOCl treatment, but specific organism degrading genus benefited from the CBZ degradation process. Further optimization studies will be needed for the Fe2+/NaOCl process to reduce the potential environmental risks and to assess the feasibility of this process at field sites. 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 research was financially supported by National Natural Science Foundation of China (NSFC) (NO. 51478170, 51779088), National Key R&D Program of China (2018YFC1800400), and Planned Science and Technology Project of Hunan Province, China (No.2017WK2091, 2017SK2352, 2017SK2420). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123451. 10

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