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Activation of persulfate with biochar for degradation of bisphenol A in soil
Chemical Engineering Journal 381 (2020) 122637
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Activation of persulfate with biochar for degradation of bisphenol A in soil a
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Junguang Liu , Shaojun Jiang , Dongdong Chen , Guangling Dai , Dongyang Wei , ⁎ Yuehong Shua, a b c
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China School of Environment and Energy, South China University of Technology, Guangzhou 510006, China South China Institute of Environmental Sciences, Ministry of Environmental Protection, Guangzhou 510530, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
could effectively activate • Biochar persulfate to degrade BPA high efficiently.
products were identified • Degradation and the degradation pathways were proposed.
dosage of biochar or per• Appropriate sulfate was important for soil BPA degradation.
could countered pH drop • Biochar during the remediation of BPA contaminated soil.
A R T I C LE I N FO
A B S T R A C T
Keywords: BC Persulfate BPA Efficient removal Soil remediation
In this study, a new alternative activator, biochar (BC) pyrolyzed by waste biomass—lychee branch, was applied in persulfate (PS) -based remediation for bisphenol A (BPA) in soils. Radical species, solution pH, dose of PS and BC were studied to evaluate the performance of PS activation by BC to degrade BPA in aqueous solution. The results show that BC can efficiently activate PS to generate sulfate radicals and hydroxyl radicals to degrade BPA high efficiently. The increase of PS and BC dosage can increase BPA removal rate, and a lower pH is benefit for the BPA degradation in aqueous solution. In addition, the degradation intermediates of BPA were characterized and the degradation pathways were proposed. Furthermore, the PS/BC system is most efficient for BPA degradation compared with other oxidants. For soil spiked with 31.93 mg kg−1 BPA, BC can also activate PS and degrade BPA effectively. The increase of BC and PS dose cannot always increase BPA degradation because of the competition of readily oxidizable matter introduced by BC itself and/or quenching of sulfate radicals. During BPA degradation in soil, the pH value drops less than those in literatures on PS oxidation of organic contaminants in soil by other activation methods, which can alleviate soil acidification in the remediation process. The results of this study suggest a novel technique for potential application in in-situ remediation of organic contaminated sites.
1. Introduction Endocrine disrupting chemicals (EDCs), which are recognized as emerging contaminants, are found in soil in recent years [1,2]. They ⁎
have received increasing interest due to the characteristic activity of estrogens or androgenic activity. Among the EDCs, bisphenol A (BPA) is one of the most widely used industrial raw materials, it has been used in various daily consumer products, including water bottle, PVC, food
Corresponding author. E-mail address: [email protected] (Y. Shu).
https://doi.org/10.1016/j.cej.2019.122637 Received 12 May 2019; Received in revised form 29 July 2019; Accepted 26 August 2019 Available online 27 August 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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can enhance the sorption of pollutants by π-electron interactions and hydrogen bond interaction. The persistent free radicals (PFRs) were also identified, the g-factor of BC used in the study is 2.0048, suggesting the PFRs exist and the species of PFRs is oxygen-center free radicals [23], which could activate PS to generate sulfate radicals [21]. Soil for the experiments was collected from non-polluted agricultural fields in Harbin, China at a depth of 0–20 cm, which contained 71% sand, 21.60% silt and 7.40% clay. After air-drying, the soil was passed through a 2 mm sieve and then stored for the experiments. The soil pH was 7.63 and the organic carbon content was 16.14 g kg−1.
packaging, sport equipment, dental sealants, etc. [3]. Until now, more and more reports have shown that BPA were detected in soil. For example, 32–147 μg kg−1 in agricultural fields of Midwestern United States [4] and 50–145 μg kg−1 in estuarine sediments of Auckland [5]. Because it was demonstrated that BPA can pose potential risk to human reproductive system and child development [6], it has attracted widely concern to the public and academic. Conventional treatments of BPA contaminated soil include chemical leaching [7], bioremediation [8], electrochemical remediation [9], etc. However, there are also some shortcomings with the above methods, such as unobvious removal, longer degradation period and so on. Therefore, an economical and effective method for BPA removal in soil is needed. Advanced oxidation processes based on persulfate (PS) has been increasingly applied for water and soil treatment, but PS must be activated to generate sulfate radicals by activation methods, such as heat [10], soluble transition metals [11], UV [12], zero-valent iron [13], ultrasound [14], phenols [15] and even alkaline conditions [16]. As an environment-friendly material, the application of biochar (BC) may improve soil fertility, mitigate greenhouse gas and sequester carbon [17], furthermore, BC can also be used as an effective sorbent for contaminants. Until now, a number of studies have shown that it has the potential to adsorb BPA in soil [18,19]. However, the ability of BC to catalytically degrade contaminants was neglected in studies. Previous researches reported that the persistent free radicals (PFRs) existed in BC may activate hydrogen peroxide and PS to generate hydroxyl radical [20] and sulfate radical [21], respectively, which could degrade polychlorinated biphenyls (PCBs) efficiently in water. Similarly, Kemmou, Frontistis, Vakros, Manariotis and Mantzavinos [22] reported the antibiotic sulfamethoxazole was degraded by BC-acivated PS. Despite there were evidences demonstrating that BC can activate PS for some organic contaminant degradation in aqueous solution, its potential degradation effects on BPA in soil are less clear. Therefore, the aims of this work are as follows: 1) to investigate the performance of PS activation by BC to degrade BPA in solution; 2) to evaluate the feasibility of BC to activate PS to degrade BPA in soil.
2.2. Preparation of soil samples spiked with BPA One kilogram of soil was spiked with 40 mg BPA dissolved in acetone, and then the soil was thoroughly mixed on a rotary shaker, followed by the evaporation of acetone for 2 days under a fume hood. Finally, the spiked soil sample was aged for two months in the fume hood before use. The initial BPA concentration of the spiked soil sample was determined to be 31.93 mg kg−1 following the method: 5.0 g spiked soil was placed in a 40-mL vials with 20 ml acetone/hexane (1:1, v/v). After shaking for 1 hour, the mixture was extracted by ultrasoundassisted extraction for 30 min and centrifugated for 10 min, then 10 mL upper organic layer was transferred to a 20 mL vial and dried under a gentle N2 stream, finally dissolved again with 5 mL methanol and determined by high performance liquid chromatography (HPLC). 2.3. BC-activated PS oxidation of BPA in aqueous solution Aqueous reactions were performed in 250 mL comical flasks with ground glass stoppers, placed on magnetic stirrers (CJJ78-1, China) operated at 25 °C. Predetermined amount of BC was quickly added to the reaction solution (200 mL), which contained desired concentrations of BPA, PS and phosphate buffer, then the residual BPA was detected at preset time intervals. The pH of reaction solution was adjusted by phosphate buffer to 7 in the study except for the experiments of initial pH impacts on BPA degradation, in which the BPA solution was firstly mixed with PS solution and adjusted to the desired pH condition by NaOH and H2SO4 before BC addition to the solution. In addition, optimal concentrations of other oxidants were used in the comparison of the common oxidants, and biochar was added as initiator in the reaction as well as the experiments method of PS. Initial experiments were conducted to observe the feasibility of activating PS by BC and the oxidation–reduction potential (ORP) in PS solution with BC addition, and the BC dose was 1 g L−1. Biochar was mixed with PS solution and 5,5-dimethyl-1-pyrroline N-oxide (DMPO), then the radical species generated in the mixed solution was analyzed by an electron paramagnetic resonace (EPR) spectrometer (A300-10/12, Bruker), in which DMPO was used as spin-trapping agent. To determine the effect of BC dosages on BPA degradation, BC dose varied from 0.25 g L−1 to 2.0 g L−1. To identify the effect of PS concentration on BPA degradation, the concentration of PS varied from 2 mM to 15 mM. Periodically, 2 mL samples were transferred to 20 mL vials and filtered through 0.22 μm fiberglass membrane before the addition of 0.2 mL 2 M Na2S2O3 to quench the reaction, and then, the residual BPA was analyzed by HPLC. Each experiment was run in triplicates and blank control experiments without BC and PS were also under the same condition.
2. Materials and methods 2.1. Materials BPA (C15H16O2, ≥99%) was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China). Sodium persulfate (Na2S2O8, 99%), sodium thiosulfate (Na2S2O3·5H2O, 99.5%), disodium hydrogen phosphate (Na2HPO4, 99%), sodium dihydrogen phosphate (NaH2PO4, 99%), and potassium iodide (KI, ≥99%) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Peroxymonosulfate (PMS) in the form of Oxone (2KHSO5·K2SO4·K2SO4), sodium chloride (NaCl, ≥99.5%), sodium bicarbonate (NaHCO3, ≥99.8%) and Humic acid (HA, fulvic acid ≥ 90%) were supplied by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). 30% Hydrogen peroxide (H2O2) was purchased by Guangzhou chemical regent factory (Guangzhou, China). Methanol, acetone and hexane were all high-performance liquid chromatography (HPLC) grade and supplied by Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Milli-Q ultra pure water prepared from a Millipore system (Bedford, USA) was used to prepare solution in the experiments. BC used in the study was manufactured by lychee branch in muffle furnace with a temperature of 600 °C, then gently ground and passed through a 0.15 mm sieve. The characteristics of the BC displayed were described in the Supporting Information (Table S1; Fig. S1; Fig. S2; Fig.S3), the main characteristics of BC were: oxygen element content 16.66 wt%, hydrogen element content 3.136 wt%, nitrogen element content 0.72 wt%, ash content 6.3 wt%, pH 8.94 in water, specific surface area 187.76 m2 g−1, average pore size 2.17 nm, many aromatic and hydroxyl functional groups exist in the biochar. These structures
2.4. BC-activated PS oxidation of BPA in soil Five grams of spiked soil and predetermined BC dosage were added to a 40 mL vial, followed by the addition of 20 mL sodium PS solution at three concentration levels, 3.68 g L−1, 18.40 g L−1, 36.80 g L−1, respectively. Thereafter, the vails were placed in a thermostatic rotary shaker (SPH-200B, China) operated at 25 °C in the dark and shaken at 100 rpm. Control experiments in the absence of PS or BC were also 2
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conducted under the identical conditions. All experiments of BPA degradation in soil were prepared in triplicates. At several time intervals, the sample vials were immediately put into ice water bath and mixed with excess potassium iodide to quench the reaction [24]. Thereafter, the samples were centrifuged to separate the aqueous phase from the slurry, followed by transferring the aqueous phase to a 20 mL vials and recording the pH of the aqueous phase with a pH meter (PHS-3C, China). The concentrations of residual PS and BPA in the aqueous phase were determined by a modified iodometric titration method and HPLC, respectively. On the other hand, the slurry remained in the vials was freeze-dried and the residual BPA in soil was extracted according to the methods described in Section 2.2. 2.5. BPA analysis BPA was quantified using a LC-16A HPLC system coupled with a UV detector (Shimadzu, Japan). The solution was filtered with a 0.22 μm membrane (Keyilong Lab Equipment Co., Ltd, Tianjin, China) and separation was carried out on an Athena C18-WP reversed-phase column (ANPEL Laboratory Technologies Inc., Shanghai, China). The mobile phase consisted of 70 % methanol and 30% water at a flow rate of 1.0 mL min−1. The UV wavelength for the detection of BPA was set at a signal wavelength of 228 nm and the injection volume was 20 μL. The detection limit of BPA on HPLC in this study is 6.6 μg L−1. 2.6. GC/MS analysis of BPA oxidation products Agilent GC/MS (7890A-5975C) was used to analyze the potential degradation intermediates. The GC/MS system was equipped with a capillary column (0.25 mm × 30 m) and helium was used as carrier gas with a flow rate of 1.0 mL min−1. The initial column temperature was maintained at 60 °C for 2 min, then increased to 300 °C at 7.0 °C min−1 and held for 5 min. The temperature of injector and detector were set at 280 °C and 250 °C, respectively. 20 mL aqueous solution from section 2.3 at specified time intervals was quenched by the addition of sodium thiosulfate, and 1 mol L−1 sulfuric acid was added to ensure all carboxylic acid in the unionized forms, freeze-dried and dissolved with 10 mL acetone, then the upper acetone layer was moved to 2 mL vial and dryness by a gentle N2 stream, followed by 200 μL BSTF-TMCS added for derivatization and adjusted to 500 μL by the addition of hexane. Thereafter, the derivatized samples were analyzed through GC/MS. 3. Results and discussion 3.1. Feasibility of PS activated by BC to degrade BPA in aqueous solution Previous studies showed that BC can induce PS to generate sulfate radicals. On one hand, PFRs in BC activate the PS to form sulfate radicals directly [21]. On the other hand, PFRs may act as electron acceptors [25] and induce the formation of reactive oxygen species [26], then the reactive oxygen species could reduce PS to form sulfate radicals. In addition, the g-factor of BC used in the study is 2.0048 (Fig. S2), suggesting the PFRs exist and the species of PFRs is oxygen-center free radicals [23]. So it is possible for BC used in the study to activate PS. Firstly, the feasibility of BC-activated PS for BPA degradation was explored in aqueous solution in this research. 3.1.1. Effect of BC dosage As shown in Fig. 1a, no remarkable removal of BPA is founded in PS solution without BC addition, suggesting the oxidation activity of PS at ambient temperature is weak and the activation of PS is needed. The similar result was also found in previous studies that the degradation of BPA was no appreciable with PS only in the experiment [27]. BPA removal rate increases substantially as BC dosage increases from 0.25 g L−1 to 2.00 g L−1. For example, the removal rate of BPA is
(caption on next page)
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Fig. 1. (a) Effect of the BC dosage on BPA removal; (b) The proportion of adsorption and degradation in BPA removal; (c) Variations of ORP in PS solution added with BC. (d) EPR of PS/BC in the presence of DMPO at 10 min. Experimental conditions: [PS]0 = 8 mM; [BPA]0 = 5 mg L−1; T = 25 °C. Black square: DMPO-OH; grey circle: DMPO-SO4.
37.04% with BC dosage of 0.25 g L−1 in 120 min. In contrast, BPA is completely removed with BC dosage of 2.00 g L−1 in the same period. Comparing with the blank adsorption setups, all the removal rates increase with the addition of BC, and the order of the adsorption capacity is: 2.0 g L−1 > 1.0 g L−1 > 0.50 g L−1 > 0.25 g L−1. At a relatively high PS concentration, the increase of BC dosage is crucial in generating sulfate radicals to degrade BPA. While, in the system with BC only, a significant portion of BPA is removed, which is even exceeds BPA removed in PS/BC system with 0.25 g L−1 and 0.50 g L−1 BC added. It is well known that BC can adsorb significant amount of BPA [28,29]. In order to identify the proportion of degradation in the BPA removal, the residual BC in the experiment was filtered and extracted to quantify the amount of BPA adsorbed by the BC. The results in Fig. 1b shows that BPA removed by adsorption only accounts for a small part of the total BPA removal in different BC dosage systems, and the highest proportion is 28.76%, suggesting BC-activated PS for BPA degradation plays a major role in BPA removal. With the increase of BC dosage, the proportion of BPA degradation first decreases and then increases. We assume that there is a certain amount of readily degradable organic matter in BC, which would first react with sulfate radicals. The sulfate radicals may be consumed at a lower BC dosage, resulting in inhibiting BPA oxidization. As the dosage of BC increases, the sulfate radicals generated by PS are sufficient to degrade more BPA in solution. In addition, the possible mechanism is that BPA may be preferentially adsorbed into BC, then the PFRs in BC activate PS decomposition to form sulfate radicals and hydroxyl radicals to degrade BPA. Fig. 1c shows variation of OPR in different solution system. Although the ORP values in PS system and PS/BC system are larger than those in CK, the oxidation capacity of PS is still weak in PS system, which can be seen from Fig. 1a. With the addition of BC, the ORP in PS/ BC system attains 395 mv, indicating the oxidation capacity is enhanced. To confirm the existence of reactive radicals, the EPR measurement was conducted. Fig. 1d shows that the signals of DMPO-SO4 (1:1:1:1:1:1) and DMPO-OH (1:2:2:1), indicating that the addition of BC can activate PS to generate sulfate radicals and hydroxyl radicals. PFRs in biochar act as electron shuttles to mediate electron transfer reaction, and these PFRs induced S2O82− to produce sulfate radicals [21], and the hydroxyl radicals could be yielded by the reaction between sulfate radicals and H2O or OH–. Above results demonstrate that BC can activate PS for BPA degradation in aqueous solution.
Fig. 2. Effect of PS concentration on BPA removal. Experimental conditions: [BPA]0 = 5 mg L−1; [BC]0 = 1 g L−1; T = 25 °C.
3.1.3. Effect of initial pH The solution pH may affect the process of the PS activation. Some literatures reported that hydroxyl ion may react with sulfate radical to produce hydroxyl radical under alkaline conditions, which have higher redox potential than sulfate radicals [33]. While, other researchers stated that the generating of PS sulfate radical will be accelerated by acid catalysis at relatively strong acidic conditions [34]. Thus, pH experiments were conducted to evaluate the effect of initial pH values on BPA degradation, and the pH values during the degradation process were also monitored. As shown in Fig. 3a, the removal rates of BPA are in the order of pH = 3＞ pH = 5＞pH = 7 ＞ pH = 9. The zeta potential for the BC is negative in the pH range studied and increases with the decrease of pH values (Fig. S3). This indicates the adsorption of BPA on BC would be enhanced at lower pH conditions, which is also confirmed in the adsorption experiments (Fig. S4). Another reason may be that PS is activated by acid catalysis at the solution condition of pH = 3, and then the sulfate radicals and hydroxyl radicals degrade BPA in the reaction solution [34]. From Fig. 3b, in the different pH systems, the pH values of the solution all rise with the degradation process of BPA, except initial pH = 9, in which pH values drop a little. Two factors can result in pH drop in the process of BPA oxidized by PS. The one is that the decomposition of one molar persulfate would yield two molar equiv. of protons [35]. The other is the acid products of BPA degradation. As the pH of BC used in the study is 8.94 (Table S1), which could counter the pH drop from the BPA degradation process, so the pH of the four systems was not dropped significantly. Fig. 3c shows the proportion of adsorption and degradation in the BPA removal at different pH values. From Fig. 3c, the proportion of adsorption increases with the increase of pH, which may be attributed to the activation of PS by acid catalysis at lower pH values [34]. So, the solution pH can affect the magnitude of BPA adsorption onto BC, and thus the degradation performance on the surface of BC. This phenomenon confirms the possible degradation mechanism proposed in Section 3.1.1, namely, BPA is preferentially adsorbed onto BC, and then the sulfate radicals and hydroxyl radicals generated by PS activation degrade BPA on the BC surface. Preliminary aqueous experiments demonstrate that BC is capable to activate PS to generate sulfate radicals and hydroxyl radicals for degrading BPA. In aqueous experiments, nearly complete BPA degradation can be achieved with PS of 15 mM and BC dosage of 1.0 g L−1. However, in soil remediation, there are more factors affecting the PS activation by BC in BPA degradation process, such as adsorption of BPA into soil, competitive oxidation with other oxidizable matters [32] and decrease of BC activation sites by inorganic or organic coating in soil,
3.1.2. Effect of PS concentration The impact of PS concentration on the BPA removal is illustrated in Fig. 2. The removal rate of BPA increases correspondingly as the initial PS concentration increases. At an initial PS concentration of 2 mM, the removal rate of BPA is low and only 53.28% of BPA is removed over 120 min. When PS concentration increases from 4 mM, 8 mM to 15 mM, the removal rate increases from 69.79%, 82.68% to 93.46%, respectively. The results suggest that as the initial PS concentration rises in some degree, more sulfate radicals may be generated, which leads to more BPA degradation. Interestingly, the increments of removal rate become slower with the PS concentration increasing in the study. Similar results have been reported in previous papers [30,31] that the increment of organic contaminants removal rate was not proportional to the increase of PS concentration. The results may be attributed to the limited BC dosage which could not provide sufficient activation sites, competition by degradation intermediates of BPA, or potential quenching of sulfate radical by residual PS and sulfate radicals [32]. 4
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3.1.4. The mechanism of BPA degraded in BC/PS system After the reaction solutions were freeze-dried and derivatized, the derivatized samples were analyzed by GC/MS. The identification of the products was performed by searching the NIST library with Agilent MassHunter Workstation software and comparing the MS spectra with BPA degradation intermediates observed in other researches [27,36]. The identified intermediates or products are show in Table 1. Based on the three main degradation mechanism (detachment of hydrogen, addition at multiple bonds and one electron transfer process [37]), the possible pathways and mechanism for BPA removal in the PS/BC system were proposed in Fig. 4. Firstly, BPA in the solution may suffer one electron transfer process to generate radical cations, and then the radical cation would react with H2O to generate a hydroxyl group at the ortho-position of the hydroxyl group of BPA, while the direct addition to aromatic rings occurs in the presence of hydroxyl radicals [38]. Similar result has been reported by Du et al. for the formation of phenolic radicals in PMS/Mn-MGO system [39]. Then, the sulfate radicals and hydroxyl radicals could attack the para-positioned C–C bond between isopropyl and benzene rings [36,39], which is due to the higher frontier electron density [40]. Thus, the phenol radical and isopropenylphenol cation generate. With the stepwise oxidation, the isopropenylphenol cation can yield 3,4-Dihydroxybenzoic acid and 3,4Dihydroxyacetophenone. And further oxidation would cleave the aromatic rings and generate oxalic acid, which would be ultimately mineralized to carbon dioxide and water. The results of GC/MS show that the BPA could be mineralized by sulfate radicals and hydroxyl radicals in PS/BC system, and the possible pathway is proposed according to the degradation intermediates. Although the concentration of BPA in the environment is relatively low, it also poses potential risk to ecosystems and human beings through environmental exposure due to its low dose effects on non-target organisms [41,42]. It is important that the BPA could be mineralized completely. Therefore, the method of PS activation by BC may be a feasible solution to remove the BPA in the environment. 3.1.5. Comparison of different oxidants activated by BC The other common oxidants, such as hydrogen peroxide and PMS, were also founded in papers that studied BPA degradation in aqueous solution [43,44], and both of the two oxidants needed to be activated by activator. In addition to hydrogen peroxide being shown to be activated by BC [20], no article has reported whether BC could activate PMS. In the following studies, the research also explored the feasibilities of activation of the two oxidants by lychee branch BC. Fig. 5 shows the comparison of BPA removal efficiency in PS/BC, PMS/BC and H2O2/BC systems. With the absence of BC, any of the three oxidants could hardly degrade BPA. The presence of BC significantly increases the removal rate of BPA with the order: PS/BC > PMS/BC > H2O2/ BC, suggesting BC can activate the three common oxidants and effectively promote the degradation of BPA in aqueous solution. The results of EPR measurement of PMS/BC and H2O2/BC systems are shown in Fig. S5, which indicates that BC can activate PMS to generate sulfate radicals and hydroxyl radicals, while only generate hydroxyl radical in activating H2O2. It have been reported that PMS could yield sulfate radicals [45,46], and then the sulfate radicals could also react with water to generate hydroxyl radicals [47]. The above results indicate that the BC used in the study could activate the three common oxidants and enhance the removal rate of BPA efficiently. While, PS/BC system was the most efficient for the removal of BPA in aqueous solution under the optimal oxidant concentration.
Fig. 3. (a)Effect of initial solution pH on BPA removal; (b) The monitor of reaction solution; (c) The proportion of adsorption and degradation in BPA removal at different solution pH. Experimental conditions: [PS]0 = 8 mM; [BPA]0 = 5 mg L−1; [BC]0 = 1 g L−1; T = 25 °C.
3.2. Effect of BC dosage on BPA removal in soil The effect of BC dosage on the BPA degradation in soil is depicted in Fig. 6a. There is almost no BPA removal in the control experiments in absence of PS and BC, and only 10.64% of BPA is removed with BC absent (PS only) in 160 min. When BC dosage increases from 1 wt% to 4
etc. Therefore, the degradation of BPA in soil is performed subsequently to confirm whether the BC/PS system is capable of degrading BPA in spiked soil. 5
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Table 1 Possible BPA degradation intermediates identified by GC–MS. Peak No.
MW
Identified structure
Name
A
370
3,4-Dihydroxybenzoic acid
B
296
3,4-Dihydroxyacetophenone
C
234
Oxalic acid
D
372
BPA
Molecular structure
Fig. 5. Comparison of PS, PMS and H2O2 activated by BC. Experimental con[PMS] [H2O2] ditions: [PS]0 = 8 mM; 0 = 1 mM; 0 = 10 mM; [BPA]0 = 5 mg L−1; [BC]0 = 1 g L−1; pH = 7; T = 25 °C.
Fig. 4. Possible degradation pathways of BPA in the PS/BC system.
wt%, BPA removal is enhanced up to 98.4%. While further increase of BC dose (8%) results in the decrease of BPA removal. The reason may be that, with the increase of BC dosage, more readily oxidizable matters from BC also increases, resulting in the consumption of sulfate radicals and then the decrease of BPA degradation. The other possible reason is that higher dosage of BC accelerates PS to produce more sulfate radicals and increases the collision of sulfate radicals [48], thus the degradation rate decreases with the quenching of sulfate radicals. In situ chemical oxidation (ISCO) based on PS activation has been increasing applied in the remediation of soil and water contaminated by organic contaminants [49–51]. In order to evaluate the efficiency of PS in degrading BPA, the variation of residual PS in the degradation experiments is tested (Fig. 6b). With the increase of BC dosage, the remaining PS in the solution decreases. While BPA removal decreases instead in 8 wt% BC system (Fig. 6a), the reasons has been explicated in the above paragraph. Another potential issue about the degradation based on PS activation is the changes of soil pH in remediation. Although buffering capacity of soil may alleviate the impact of pH drop caused by the degradation process, the soil pH may be also affected by the oxidant concentration or other acidic intermediates. As shown in Fig. 6c, pH values of all the experiments with BC addition drop rapidly in the first
15 minutes when most BPA degrades, and then levels off gradually. The pH value declines by 2.24 pH units in 4 wt% BC system, which is followed by 8 wt% and 1 wt%, and the trend is in accordance with the respective BPA removal rates of the three degradation tests. The reason can be attributed to the pH drop from protons generated from the decomposition of PS. Moreover, although the consumption of PS is the most in 8 wt% BC system(Fig. 6b), the pH value drops less than that in 4 wt% BC system. This is probably because buffering capacity is enhanced with more addition of BC in soil [52].
3.3. Effect of PS concentration on BPA removal in soil Fig. 7a shows the removal profiles of BPA under various PS concentrations. The results show that no appreciable removal of BPA is observed with PS absent (BC only), and BPA removal rate increases substantially as PS concentration increases from 3.68 to 36.8 g L−1. At an initial PS concentration of 3.68 g L−1, 18.40 g L−1 and 36.80 g L−1, 63.83%, 98.39% and 98.55% of BPA is removed, respectively. Obviously, the removal efficiency is only slightly enhanced at PS concentration of 36.80 g L−1 compared with that of 18.40 g L−1. It is 6
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Fig. 6. (a) The effect of BC dosage on BPA degradation in soil; (b) PS decomposition profiles in different dosage of BC setups; (c) Changes of pH in different dosage of BC setups. Experimental conditions: 5.0 g BPA spiked soil; 20 mL 18.40 g L−1 PS solution; T = 25 °C.
Fig. 7. (a) The effect PS concentration on BPA degradation in soil; (b) PS decomposition profiles in different concentrations of PS setups; (c) Changes of pH in different concentrations of PS setups. Experimental conditions: 5.0 g BPA spiked soil; 4 wt% BC; 20 mL PS solution; T = 25 °C.
necessary to increase the PS concentration to generate more sulfate radicals, which could overcome the consumption of other oxidizable matters from soil [53] and BC. While the quenching effects of residual PS and sulfate radicals themselves would occur at higher PS concentration [54,55], in addition, the desorption of organic contaminations may be affected by salting-out effect at higher PS concentration
[56], resulting in inhibiting the degradation of BPA. So, selecting appropriate PS dose in the degradation system is necessary. The residual PS monitoring data are shown in Fig. 7b. PS is rapidly consumed at the first 15 min and BPA is also removed quickly over the same period (Fig. 5a). The results imply that the initial rapid decomposition of PS is closely related with the degradation of BPA and some 7
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were found in the experiments, the degradation rate of BPA was not significantly decreased in the study, suggesting that the PS/BC system has relatively strong tolerance to background electrolytes. 3.5. Environmental implication The results from the study demonstrate that BPA contaminated soil can be efficiently remediated by BC-activated PS. Furthermore, the method of PS activation shows great potential for in situ remediation. As a novelty activator for PS, BC can offer some benefits for soil remediation based on PS activation. On one hand, because of the abundance of feedstock materials, BC is low-cost and available. On the other hand, compared with other activators, high degradation rate can be also achieved by BC-activated PS. What’s more, BC can alleviate soil acidification in the remediation process based on PS activation and the PS/ BC system can show tolerance to the effect of background electrolytes. 4. Conclusions Fig. 8. Effect of HCO3–, Cl−, and HA on BPA degradation in soil. Experimental conditions: 5.0 g BPA spiked soil; 4 wt% BC; 20 mL PS solution; T = 25 °C.
In the present study, the feasibility of BPA removal by BC-activated PS is investigated in aqueous solution. The results of this study illustrate that BPA can be efficiently removed in aqueous solution, dosage of PS and BC play important role in BPA removal, lower initial solution pH has positive effect on BPA degradation in aqueous experiments, besides, results also suggest that degradation is dominant in the BPA removal process. Additionally, the possible degradation pathways are proposed, and the BC used in the study can activate PMS and H2O2 as well. Then, BC/PS system is applied in remediating soil spiked with BPA, results demonstrate that the method is capable of degrading BPA in spiked soil. To some degree, It is useful to increase BC and PS dosage to accelerate the degradation in soil, but higher dosage of BC or PS may inhibite BPA degradation because of the excessive consumption of sulfate radicals and hydroxyl radicals in the spiked soil remediation. Results also demonstrate that the addition of BC could counter the pH drop in soil, thus avoiding soil acidification in the remediation process. Meanwhile, the common background electrolytes exhibit negligible effects on BPA degradation. Pilot studies should be run to obtain more detailed data on the adaptability for in situ field application. In addition, further studies should be performed to manipulate more quantity of persistent free radicals in BC to activate PS for the degradation of more refractory organic pollutants.
other readily oxidizable matters [57]. Most PS is consumed in 36.80 g L−1 system, but BPA removal is only slightly enhanced compared with 18.40 g L−1 system, suggesting the quenching effect of residual PS is very likely to occur in the experiment. Considering the costeffective remediation of contaminated soil, the optimal concentration of BC-activated PS for BPA degradation is 18.40 g L−1. The variation of pH is presented in Fig. 7c. The slurry pH values drop with the increase of PS dose, and the lowest is 3.25. The drop of pH coincides with the consumption PS, as two molar equiv. of protons were produced through the decomposition of PS. From other similar researches about the degradation of organic contaminants by PS in soil, the pH values dropped to 1.60 and 1.36 in the degradation process [14,35], which was much lower than that in our study. The results indicate that BC can not only activate PS to generate sulfate radicals for BPA degradation, but alleviate the drop of pH in the remediation of soils contaminated by organic pollutants. As a result, utilizing BC as PS activator can avoid the excessive acidification of soil during the in situ remediation in the future. 3.4. Effect of HCO3–, Cl− and HA on BPA degradation in soil
Acknowledgements It has been reported that sulfate radicals and hydroxyl radicals could be scavenged by inorganic anions [58] and HA [59]. Considering the PS/BC system may be affected by some natural existing substances in soil such as HCO3–, Cl− and HA. Therefore, the effect of HCO3–, Cl−, and HA on BPA degradation was investigated. The concentrations of HCO3– and Cl− were set at different molar ratio concentrations to persulfate viz. 0:1, 0.5:1, 1:1 and 2:1, and the concentration of HA ranged from 0 to 50 mg kg−1. As shown in Fig. 8, the three factors exhibit similar effect on BPA degradation in soil. In the presence of HCO3–, Cl−, and HA, the reactivity of persulfate is all inhibited, suggesting the scavenging effect existed in the reaction. In addition, with the molar ratio increasing, the scavenging effect is enhanced in all the reactions, and the relatively significant changes occur in PS/BC system with the addition of HA. HCO3– can compete with the target organic contaminants as an electron donator and react with sulfate radicals and hydroxyl radicals easily [60]. Cl− could react with sulfate radicals as a result of it reducing property, and it also could react rapidly with hydroxyl radicals to form less reactive species [61,62]. The reasons of the negative effect induced by HA on degradation are as follows: on one hand, HA can strongly react with sulfate radicals and hydroxyl radicals; on the other hand, HA may be adsorbed onto the surface of biochar and cover the active sites to inhibit the capacity of activation. Although the scavenging effects
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Activation of persulfate with biochar for degradation of bisphenol A in soil a
a
b
a
T
c
Junguang Liu , Shaojun Jiang , Dongdong Chen , Guangling Dai , Dongyang Wei , ⁎ Yuehong Shua, a b c
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China School of Environment and Energy, South China University of Technology, Guangzhou 510006, China South China Institute of Environmental Sciences, Ministry of Environmental Protection, Guangzhou 510530, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
could effectively activate • Biochar persulfate to degrade BPA high efficiently.
products were identified • Degradation and the degradation pathways were proposed.
dosage of biochar or per• Appropriate sulfate was important for soil BPA degradation.
could countered pH drop • Biochar during the remediation of BPA contaminated soil.
A R T I C LE I N FO
A B S T R A C T
Keywords: BC Persulfate BPA Efficient removal Soil remediation
In this study, a new alternative activator, biochar (BC) pyrolyzed by waste biomass—lychee branch, was applied in persulfate (PS) -based remediation for bisphenol A (BPA) in soils. Radical species, solution pH, dose of PS and BC were studied to evaluate the performance of PS activation by BC to degrade BPA in aqueous solution. The results show that BC can efficiently activate PS to generate sulfate radicals and hydroxyl radicals to degrade BPA high efficiently. The increase of PS and BC dosage can increase BPA removal rate, and a lower pH is benefit for the BPA degradation in aqueous solution. In addition, the degradation intermediates of BPA were characterized and the degradation pathways were proposed. Furthermore, the PS/BC system is most efficient for BPA degradation compared with other oxidants. For soil spiked with 31.93 mg kg−1 BPA, BC can also activate PS and degrade BPA effectively. The increase of BC and PS dose cannot always increase BPA degradation because of the competition of readily oxidizable matter introduced by BC itself and/or quenching of sulfate radicals. During BPA degradation in soil, the pH value drops less than those in literatures on PS oxidation of organic contaminants in soil by other activation methods, which can alleviate soil acidification in the remediation process. The results of this study suggest a novel technique for potential application in in-situ remediation of organic contaminated sites.
1. Introduction Endocrine disrupting chemicals (EDCs), which are recognized as emerging contaminants, are found in soil in recent years [1,2]. They ⁎
have received increasing interest due to the characteristic activity of estrogens or androgenic activity. Among the EDCs, bisphenol A (BPA) is one of the most widely used industrial raw materials, it has been used in various daily consumer products, including water bottle, PVC, food
Corresponding author. E-mail address: [email protected] (Y. Shu).
https://doi.org/10.1016/j.cej.2019.122637 Received 12 May 2019; Received in revised form 29 July 2019; Accepted 26 August 2019 Available online 27 August 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 381 (2020) 122637
J. Liu, et al.
can enhance the sorption of pollutants by π-electron interactions and hydrogen bond interaction. The persistent free radicals (PFRs) were also identified, the g-factor of BC used in the study is 2.0048, suggesting the PFRs exist and the species of PFRs is oxygen-center free radicals [23], which could activate PS to generate sulfate radicals [21]. Soil for the experiments was collected from non-polluted agricultural fields in Harbin, China at a depth of 0–20 cm, which contained 71% sand, 21.60% silt and 7.40% clay. After air-drying, the soil was passed through a 2 mm sieve and then stored for the experiments. The soil pH was 7.63 and the organic carbon content was 16.14 g kg−1.
packaging, sport equipment, dental sealants, etc. [3]. Until now, more and more reports have shown that BPA were detected in soil. For example, 32–147 μg kg−1 in agricultural fields of Midwestern United States [4] and 50–145 μg kg−1 in estuarine sediments of Auckland [5]. Because it was demonstrated that BPA can pose potential risk to human reproductive system and child development [6], it has attracted widely concern to the public and academic. Conventional treatments of BPA contaminated soil include chemical leaching [7], bioremediation [8], electrochemical remediation [9], etc. However, there are also some shortcomings with the above methods, such as unobvious removal, longer degradation period and so on. Therefore, an economical and effective method for BPA removal in soil is needed. Advanced oxidation processes based on persulfate (PS) has been increasingly applied for water and soil treatment, but PS must be activated to generate sulfate radicals by activation methods, such as heat [10], soluble transition metals [11], UV [12], zero-valent iron [13], ultrasound [14], phenols [15] and even alkaline conditions [16]. As an environment-friendly material, the application of biochar (BC) may improve soil fertility, mitigate greenhouse gas and sequester carbon [17], furthermore, BC can also be used as an effective sorbent for contaminants. Until now, a number of studies have shown that it has the potential to adsorb BPA in soil [18,19]. However, the ability of BC to catalytically degrade contaminants was neglected in studies. Previous researches reported that the persistent free radicals (PFRs) existed in BC may activate hydrogen peroxide and PS to generate hydroxyl radical [20] and sulfate radical [21], respectively, which could degrade polychlorinated biphenyls (PCBs) efficiently in water. Similarly, Kemmou, Frontistis, Vakros, Manariotis and Mantzavinos [22] reported the antibiotic sulfamethoxazole was degraded by BC-acivated PS. Despite there were evidences demonstrating that BC can activate PS for some organic contaminant degradation in aqueous solution, its potential degradation effects on BPA in soil are less clear. Therefore, the aims of this work are as follows: 1) to investigate the performance of PS activation by BC to degrade BPA in solution; 2) to evaluate the feasibility of BC to activate PS to degrade BPA in soil.
2.2. Preparation of soil samples spiked with BPA One kilogram of soil was spiked with 40 mg BPA dissolved in acetone, and then the soil was thoroughly mixed on a rotary shaker, followed by the evaporation of acetone for 2 days under a fume hood. Finally, the spiked soil sample was aged for two months in the fume hood before use. The initial BPA concentration of the spiked soil sample was determined to be 31.93 mg kg−1 following the method: 5.0 g spiked soil was placed in a 40-mL vials with 20 ml acetone/hexane (1:1, v/v). After shaking for 1 hour, the mixture was extracted by ultrasoundassisted extraction for 30 min and centrifugated for 10 min, then 10 mL upper organic layer was transferred to a 20 mL vial and dried under a gentle N2 stream, finally dissolved again with 5 mL methanol and determined by high performance liquid chromatography (HPLC). 2.3. BC-activated PS oxidation of BPA in aqueous solution Aqueous reactions were performed in 250 mL comical flasks with ground glass stoppers, placed on magnetic stirrers (CJJ78-1, China) operated at 25 °C. Predetermined amount of BC was quickly added to the reaction solution (200 mL), which contained desired concentrations of BPA, PS and phosphate buffer, then the residual BPA was detected at preset time intervals. The pH of reaction solution was adjusted by phosphate buffer to 7 in the study except for the experiments of initial pH impacts on BPA degradation, in which the BPA solution was firstly mixed with PS solution and adjusted to the desired pH condition by NaOH and H2SO4 before BC addition to the solution. In addition, optimal concentrations of other oxidants were used in the comparison of the common oxidants, and biochar was added as initiator in the reaction as well as the experiments method of PS. Initial experiments were conducted to observe the feasibility of activating PS by BC and the oxidation–reduction potential (ORP) in PS solution with BC addition, and the BC dose was 1 g L−1. Biochar was mixed with PS solution and 5,5-dimethyl-1-pyrroline N-oxide (DMPO), then the radical species generated in the mixed solution was analyzed by an electron paramagnetic resonace (EPR) spectrometer (A300-10/12, Bruker), in which DMPO was used as spin-trapping agent. To determine the effect of BC dosages on BPA degradation, BC dose varied from 0.25 g L−1 to 2.0 g L−1. To identify the effect of PS concentration on BPA degradation, the concentration of PS varied from 2 mM to 15 mM. Periodically, 2 mL samples were transferred to 20 mL vials and filtered through 0.22 μm fiberglass membrane before the addition of 0.2 mL 2 M Na2S2O3 to quench the reaction, and then, the residual BPA was analyzed by HPLC. Each experiment was run in triplicates and blank control experiments without BC and PS were also under the same condition.
2. Materials and methods 2.1. Materials BPA (C15H16O2, ≥99%) was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China). Sodium persulfate (Na2S2O8, 99%), sodium thiosulfate (Na2S2O3·5H2O, 99.5%), disodium hydrogen phosphate (Na2HPO4, 99%), sodium dihydrogen phosphate (NaH2PO4, 99%), and potassium iodide (KI, ≥99%) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Peroxymonosulfate (PMS) in the form of Oxone (2KHSO5·K2SO4·K2SO4), sodium chloride (NaCl, ≥99.5%), sodium bicarbonate (NaHCO3, ≥99.8%) and Humic acid (HA, fulvic acid ≥ 90%) were supplied by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). 30% Hydrogen peroxide (H2O2) was purchased by Guangzhou chemical regent factory (Guangzhou, China). Methanol, acetone and hexane were all high-performance liquid chromatography (HPLC) grade and supplied by Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Milli-Q ultra pure water prepared from a Millipore system (Bedford, USA) was used to prepare solution in the experiments. BC used in the study was manufactured by lychee branch in muffle furnace with a temperature of 600 °C, then gently ground and passed through a 0.15 mm sieve. The characteristics of the BC displayed were described in the Supporting Information (Table S1; Fig. S1; Fig. S2; Fig.S3), the main characteristics of BC were: oxygen element content 16.66 wt%, hydrogen element content 3.136 wt%, nitrogen element content 0.72 wt%, ash content 6.3 wt%, pH 8.94 in water, specific surface area 187.76 m2 g−1, average pore size 2.17 nm, many aromatic and hydroxyl functional groups exist in the biochar. These structures
2.4. BC-activated PS oxidation of BPA in soil Five grams of spiked soil and predetermined BC dosage were added to a 40 mL vial, followed by the addition of 20 mL sodium PS solution at three concentration levels, 3.68 g L−1, 18.40 g L−1, 36.80 g L−1, respectively. Thereafter, the vails were placed in a thermostatic rotary shaker (SPH-200B, China) operated at 25 °C in the dark and shaken at 100 rpm. Control experiments in the absence of PS or BC were also 2
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conducted under the identical conditions. All experiments of BPA degradation in soil were prepared in triplicates. At several time intervals, the sample vials were immediately put into ice water bath and mixed with excess potassium iodide to quench the reaction [24]. Thereafter, the samples were centrifuged to separate the aqueous phase from the slurry, followed by transferring the aqueous phase to a 20 mL vials and recording the pH of the aqueous phase with a pH meter (PHS-3C, China). The concentrations of residual PS and BPA in the aqueous phase were determined by a modified iodometric titration method and HPLC, respectively. On the other hand, the slurry remained in the vials was freeze-dried and the residual BPA in soil was extracted according to the methods described in Section 2.2. 2.5. BPA analysis BPA was quantified using a LC-16A HPLC system coupled with a UV detector (Shimadzu, Japan). The solution was filtered with a 0.22 μm membrane (Keyilong Lab Equipment Co., Ltd, Tianjin, China) and separation was carried out on an Athena C18-WP reversed-phase column (ANPEL Laboratory Technologies Inc., Shanghai, China). The mobile phase consisted of 70 % methanol and 30% water at a flow rate of 1.0 mL min−1. The UV wavelength for the detection of BPA was set at a signal wavelength of 228 nm and the injection volume was 20 μL. The detection limit of BPA on HPLC in this study is 6.6 μg L−1. 2.6. GC/MS analysis of BPA oxidation products Agilent GC/MS (7890A-5975C) was used to analyze the potential degradation intermediates. The GC/MS system was equipped with a capillary column (0.25 mm × 30 m) and helium was used as carrier gas with a flow rate of 1.0 mL min−1. The initial column temperature was maintained at 60 °C for 2 min, then increased to 300 °C at 7.0 °C min−1 and held for 5 min. The temperature of injector and detector were set at 280 °C and 250 °C, respectively. 20 mL aqueous solution from section 2.3 at specified time intervals was quenched by the addition of sodium thiosulfate, and 1 mol L−1 sulfuric acid was added to ensure all carboxylic acid in the unionized forms, freeze-dried and dissolved with 10 mL acetone, then the upper acetone layer was moved to 2 mL vial and dryness by a gentle N2 stream, followed by 200 μL BSTF-TMCS added for derivatization and adjusted to 500 μL by the addition of hexane. Thereafter, the derivatized samples were analyzed through GC/MS. 3. Results and discussion 3.1. Feasibility of PS activated by BC to degrade BPA in aqueous solution Previous studies showed that BC can induce PS to generate sulfate radicals. On one hand, PFRs in BC activate the PS to form sulfate radicals directly [21]. On the other hand, PFRs may act as electron acceptors [25] and induce the formation of reactive oxygen species [26], then the reactive oxygen species could reduce PS to form sulfate radicals. In addition, the g-factor of BC used in the study is 2.0048 (Fig. S2), suggesting the PFRs exist and the species of PFRs is oxygen-center free radicals [23]. So it is possible for BC used in the study to activate PS. Firstly, the feasibility of BC-activated PS for BPA degradation was explored in aqueous solution in this research. 3.1.1. Effect of BC dosage As shown in Fig. 1a, no remarkable removal of BPA is founded in PS solution without BC addition, suggesting the oxidation activity of PS at ambient temperature is weak and the activation of PS is needed. The similar result was also found in previous studies that the degradation of BPA was no appreciable with PS only in the experiment [27]. BPA removal rate increases substantially as BC dosage increases from 0.25 g L−1 to 2.00 g L−1. For example, the removal rate of BPA is
(caption on next page)
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Fig. 1. (a) Effect of the BC dosage on BPA removal; (b) The proportion of adsorption and degradation in BPA removal; (c) Variations of ORP in PS solution added with BC. (d) EPR of PS/BC in the presence of DMPO at 10 min. Experimental conditions: [PS]0 = 8 mM; [BPA]0 = 5 mg L−1; T = 25 °C. Black square: DMPO-OH; grey circle: DMPO-SO4.
37.04% with BC dosage of 0.25 g L−1 in 120 min. In contrast, BPA is completely removed with BC dosage of 2.00 g L−1 in the same period. Comparing with the blank adsorption setups, all the removal rates increase with the addition of BC, and the order of the adsorption capacity is: 2.0 g L−1 > 1.0 g L−1 > 0.50 g L−1 > 0.25 g L−1. At a relatively high PS concentration, the increase of BC dosage is crucial in generating sulfate radicals to degrade BPA. While, in the system with BC only, a significant portion of BPA is removed, which is even exceeds BPA removed in PS/BC system with 0.25 g L−1 and 0.50 g L−1 BC added. It is well known that BC can adsorb significant amount of BPA [28,29]. In order to identify the proportion of degradation in the BPA removal, the residual BC in the experiment was filtered and extracted to quantify the amount of BPA adsorbed by the BC. The results in Fig. 1b shows that BPA removed by adsorption only accounts for a small part of the total BPA removal in different BC dosage systems, and the highest proportion is 28.76%, suggesting BC-activated PS for BPA degradation plays a major role in BPA removal. With the increase of BC dosage, the proportion of BPA degradation first decreases and then increases. We assume that there is a certain amount of readily degradable organic matter in BC, which would first react with sulfate radicals. The sulfate radicals may be consumed at a lower BC dosage, resulting in inhibiting BPA oxidization. As the dosage of BC increases, the sulfate radicals generated by PS are sufficient to degrade more BPA in solution. In addition, the possible mechanism is that BPA may be preferentially adsorbed into BC, then the PFRs in BC activate PS decomposition to form sulfate radicals and hydroxyl radicals to degrade BPA. Fig. 1c shows variation of OPR in different solution system. Although the ORP values in PS system and PS/BC system are larger than those in CK, the oxidation capacity of PS is still weak in PS system, which can be seen from Fig. 1a. With the addition of BC, the ORP in PS/ BC system attains 395 mv, indicating the oxidation capacity is enhanced. To confirm the existence of reactive radicals, the EPR measurement was conducted. Fig. 1d shows that the signals of DMPO-SO4 (1:1:1:1:1:1) and DMPO-OH (1:2:2:1), indicating that the addition of BC can activate PS to generate sulfate radicals and hydroxyl radicals. PFRs in biochar act as electron shuttles to mediate electron transfer reaction, and these PFRs induced S2O82− to produce sulfate radicals [21], and the hydroxyl radicals could be yielded by the reaction between sulfate radicals and H2O or OH–. Above results demonstrate that BC can activate PS for BPA degradation in aqueous solution.
Fig. 2. Effect of PS concentration on BPA removal. Experimental conditions: [BPA]0 = 5 mg L−1; [BC]0 = 1 g L−1; T = 25 °C.
3.1.3. Effect of initial pH The solution pH may affect the process of the PS activation. Some literatures reported that hydroxyl ion may react with sulfate radical to produce hydroxyl radical under alkaline conditions, which have higher redox potential than sulfate radicals [33]. While, other researchers stated that the generating of PS sulfate radical will be accelerated by acid catalysis at relatively strong acidic conditions [34]. Thus, pH experiments were conducted to evaluate the effect of initial pH values on BPA degradation, and the pH values during the degradation process were also monitored. As shown in Fig. 3a, the removal rates of BPA are in the order of pH = 3＞ pH = 5＞pH = 7 ＞ pH = 9. The zeta potential for the BC is negative in the pH range studied and increases with the decrease of pH values (Fig. S3). This indicates the adsorption of BPA on BC would be enhanced at lower pH conditions, which is also confirmed in the adsorption experiments (Fig. S4). Another reason may be that PS is activated by acid catalysis at the solution condition of pH = 3, and then the sulfate radicals and hydroxyl radicals degrade BPA in the reaction solution [34]. From Fig. 3b, in the different pH systems, the pH values of the solution all rise with the degradation process of BPA, except initial pH = 9, in which pH values drop a little. Two factors can result in pH drop in the process of BPA oxidized by PS. The one is that the decomposition of one molar persulfate would yield two molar equiv. of protons [35]. The other is the acid products of BPA degradation. As the pH of BC used in the study is 8.94 (Table S1), which could counter the pH drop from the BPA degradation process, so the pH of the four systems was not dropped significantly. Fig. 3c shows the proportion of adsorption and degradation in the BPA removal at different pH values. From Fig. 3c, the proportion of adsorption increases with the increase of pH, which may be attributed to the activation of PS by acid catalysis at lower pH values [34]. So, the solution pH can affect the magnitude of BPA adsorption onto BC, and thus the degradation performance on the surface of BC. This phenomenon confirms the possible degradation mechanism proposed in Section 3.1.1, namely, BPA is preferentially adsorbed onto BC, and then the sulfate radicals and hydroxyl radicals generated by PS activation degrade BPA on the BC surface. Preliminary aqueous experiments demonstrate that BC is capable to activate PS to generate sulfate radicals and hydroxyl radicals for degrading BPA. In aqueous experiments, nearly complete BPA degradation can be achieved with PS of 15 mM and BC dosage of 1.0 g L−1. However, in soil remediation, there are more factors affecting the PS activation by BC in BPA degradation process, such as adsorption of BPA into soil, competitive oxidation with other oxidizable matters [32] and decrease of BC activation sites by inorganic or organic coating in soil,
3.1.2. Effect of PS concentration The impact of PS concentration on the BPA removal is illustrated in Fig. 2. The removal rate of BPA increases correspondingly as the initial PS concentration increases. At an initial PS concentration of 2 mM, the removal rate of BPA is low and only 53.28% of BPA is removed over 120 min. When PS concentration increases from 4 mM, 8 mM to 15 mM, the removal rate increases from 69.79%, 82.68% to 93.46%, respectively. The results suggest that as the initial PS concentration rises in some degree, more sulfate radicals may be generated, which leads to more BPA degradation. Interestingly, the increments of removal rate become slower with the PS concentration increasing in the study. Similar results have been reported in previous papers [30,31] that the increment of organic contaminants removal rate was not proportional to the increase of PS concentration. The results may be attributed to the limited BC dosage which could not provide sufficient activation sites, competition by degradation intermediates of BPA, or potential quenching of sulfate radical by residual PS and sulfate radicals [32]. 4
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3.1.4. The mechanism of BPA degraded in BC/PS system After the reaction solutions were freeze-dried and derivatized, the derivatized samples were analyzed by GC/MS. The identification of the products was performed by searching the NIST library with Agilent MassHunter Workstation software and comparing the MS spectra with BPA degradation intermediates observed in other researches [27,36]. The identified intermediates or products are show in Table 1. Based on the three main degradation mechanism (detachment of hydrogen, addition at multiple bonds and one electron transfer process [37]), the possible pathways and mechanism for BPA removal in the PS/BC system were proposed in Fig. 4. Firstly, BPA in the solution may suffer one electron transfer process to generate radical cations, and then the radical cation would react with H2O to generate a hydroxyl group at the ortho-position of the hydroxyl group of BPA, while the direct addition to aromatic rings occurs in the presence of hydroxyl radicals [38]. Similar result has been reported by Du et al. for the formation of phenolic radicals in PMS/Mn-MGO system [39]. Then, the sulfate radicals and hydroxyl radicals could attack the para-positioned C–C bond between isopropyl and benzene rings [36,39], which is due to the higher frontier electron density [40]. Thus, the phenol radical and isopropenylphenol cation generate. With the stepwise oxidation, the isopropenylphenol cation can yield 3,4-Dihydroxybenzoic acid and 3,4Dihydroxyacetophenone. And further oxidation would cleave the aromatic rings and generate oxalic acid, which would be ultimately mineralized to carbon dioxide and water. The results of GC/MS show that the BPA could be mineralized by sulfate radicals and hydroxyl radicals in PS/BC system, and the possible pathway is proposed according to the degradation intermediates. Although the concentration of BPA in the environment is relatively low, it also poses potential risk to ecosystems and human beings through environmental exposure due to its low dose effects on non-target organisms [41,42]. It is important that the BPA could be mineralized completely. Therefore, the method of PS activation by BC may be a feasible solution to remove the BPA in the environment. 3.1.5. Comparison of different oxidants activated by BC The other common oxidants, such as hydrogen peroxide and PMS, were also founded in papers that studied BPA degradation in aqueous solution [43,44], and both of the two oxidants needed to be activated by activator. In addition to hydrogen peroxide being shown to be activated by BC [20], no article has reported whether BC could activate PMS. In the following studies, the research also explored the feasibilities of activation of the two oxidants by lychee branch BC. Fig. 5 shows the comparison of BPA removal efficiency in PS/BC, PMS/BC and H2O2/BC systems. With the absence of BC, any of the three oxidants could hardly degrade BPA. The presence of BC significantly increases the removal rate of BPA with the order: PS/BC > PMS/BC > H2O2/ BC, suggesting BC can activate the three common oxidants and effectively promote the degradation of BPA in aqueous solution. The results of EPR measurement of PMS/BC and H2O2/BC systems are shown in Fig. S5, which indicates that BC can activate PMS to generate sulfate radicals and hydroxyl radicals, while only generate hydroxyl radical in activating H2O2. It have been reported that PMS could yield sulfate radicals [45,46], and then the sulfate radicals could also react with water to generate hydroxyl radicals [47]. The above results indicate that the BC used in the study could activate the three common oxidants and enhance the removal rate of BPA efficiently. While, PS/BC system was the most efficient for the removal of BPA in aqueous solution under the optimal oxidant concentration.
Fig. 3. (a)Effect of initial solution pH on BPA removal; (b) The monitor of reaction solution; (c) The proportion of adsorption and degradation in BPA removal at different solution pH. Experimental conditions: [PS]0 = 8 mM; [BPA]0 = 5 mg L−1; [BC]0 = 1 g L−1; T = 25 °C.
3.2. Effect of BC dosage on BPA removal in soil The effect of BC dosage on the BPA degradation in soil is depicted in Fig. 6a. There is almost no BPA removal in the control experiments in absence of PS and BC, and only 10.64% of BPA is removed with BC absent (PS only) in 160 min. When BC dosage increases from 1 wt% to 4
etc. Therefore, the degradation of BPA in soil is performed subsequently to confirm whether the BC/PS system is capable of degrading BPA in spiked soil. 5
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Table 1 Possible BPA degradation intermediates identified by GC–MS. Peak No.
MW
Identified structure
Name
A
370
3,4-Dihydroxybenzoic acid
B
296
3,4-Dihydroxyacetophenone
C
234
Oxalic acid
D
372
BPA
Molecular structure
Fig. 5. Comparison of PS, PMS and H2O2 activated by BC. Experimental con[PMS] [H2O2] ditions: [PS]0 = 8 mM; 0 = 1 mM; 0 = 10 mM; [BPA]0 = 5 mg L−1; [BC]0 = 1 g L−1; pH = 7; T = 25 °C.
Fig. 4. Possible degradation pathways of BPA in the PS/BC system.
wt%, BPA removal is enhanced up to 98.4%. While further increase of BC dose (8%) results in the decrease of BPA removal. The reason may be that, with the increase of BC dosage, more readily oxidizable matters from BC also increases, resulting in the consumption of sulfate radicals and then the decrease of BPA degradation. The other possible reason is that higher dosage of BC accelerates PS to produce more sulfate radicals and increases the collision of sulfate radicals [48], thus the degradation rate decreases with the quenching of sulfate radicals. In situ chemical oxidation (ISCO) based on PS activation has been increasing applied in the remediation of soil and water contaminated by organic contaminants [49–51]. In order to evaluate the efficiency of PS in degrading BPA, the variation of residual PS in the degradation experiments is tested (Fig. 6b). With the increase of BC dosage, the remaining PS in the solution decreases. While BPA removal decreases instead in 8 wt% BC system (Fig. 6a), the reasons has been explicated in the above paragraph. Another potential issue about the degradation based on PS activation is the changes of soil pH in remediation. Although buffering capacity of soil may alleviate the impact of pH drop caused by the degradation process, the soil pH may be also affected by the oxidant concentration or other acidic intermediates. As shown in Fig. 6c, pH values of all the experiments with BC addition drop rapidly in the first
15 minutes when most BPA degrades, and then levels off gradually. The pH value declines by 2.24 pH units in 4 wt% BC system, which is followed by 8 wt% and 1 wt%, and the trend is in accordance with the respective BPA removal rates of the three degradation tests. The reason can be attributed to the pH drop from protons generated from the decomposition of PS. Moreover, although the consumption of PS is the most in 8 wt% BC system(Fig. 6b), the pH value drops less than that in 4 wt% BC system. This is probably because buffering capacity is enhanced with more addition of BC in soil [52].
3.3. Effect of PS concentration on BPA removal in soil Fig. 7a shows the removal profiles of BPA under various PS concentrations. The results show that no appreciable removal of BPA is observed with PS absent (BC only), and BPA removal rate increases substantially as PS concentration increases from 3.68 to 36.8 g L−1. At an initial PS concentration of 3.68 g L−1, 18.40 g L−1 and 36.80 g L−1, 63.83%, 98.39% and 98.55% of BPA is removed, respectively. Obviously, the removal efficiency is only slightly enhanced at PS concentration of 36.80 g L−1 compared with that of 18.40 g L−1. It is 6
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Fig. 6. (a) The effect of BC dosage on BPA degradation in soil; (b) PS decomposition profiles in different dosage of BC setups; (c) Changes of pH in different dosage of BC setups. Experimental conditions: 5.0 g BPA spiked soil; 20 mL 18.40 g L−1 PS solution; T = 25 °C.
Fig. 7. (a) The effect PS concentration on BPA degradation in soil; (b) PS decomposition profiles in different concentrations of PS setups; (c) Changes of pH in different concentrations of PS setups. Experimental conditions: 5.0 g BPA spiked soil; 4 wt% BC; 20 mL PS solution; T = 25 °C.
necessary to increase the PS concentration to generate more sulfate radicals, which could overcome the consumption of other oxidizable matters from soil [53] and BC. While the quenching effects of residual PS and sulfate radicals themselves would occur at higher PS concentration [54,55], in addition, the desorption of organic contaminations may be affected by salting-out effect at higher PS concentration
[56], resulting in inhibiting the degradation of BPA. So, selecting appropriate PS dose in the degradation system is necessary. The residual PS monitoring data are shown in Fig. 7b. PS is rapidly consumed at the first 15 min and BPA is also removed quickly over the same period (Fig. 5a). The results imply that the initial rapid decomposition of PS is closely related with the degradation of BPA and some 7
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were found in the experiments, the degradation rate of BPA was not significantly decreased in the study, suggesting that the PS/BC system has relatively strong tolerance to background electrolytes. 3.5. Environmental implication The results from the study demonstrate that BPA contaminated soil can be efficiently remediated by BC-activated PS. Furthermore, the method of PS activation shows great potential for in situ remediation. As a novelty activator for PS, BC can offer some benefits for soil remediation based on PS activation. On one hand, because of the abundance of feedstock materials, BC is low-cost and available. On the other hand, compared with other activators, high degradation rate can be also achieved by BC-activated PS. What’s more, BC can alleviate soil acidification in the remediation process based on PS activation and the PS/ BC system can show tolerance to the effect of background electrolytes. 4. Conclusions Fig. 8. Effect of HCO3–, Cl−, and HA on BPA degradation in soil. Experimental conditions: 5.0 g BPA spiked soil; 4 wt% BC; 20 mL PS solution; T = 25 °C.
In the present study, the feasibility of BPA removal by BC-activated PS is investigated in aqueous solution. The results of this study illustrate that BPA can be efficiently removed in aqueous solution, dosage of PS and BC play important role in BPA removal, lower initial solution pH has positive effect on BPA degradation in aqueous experiments, besides, results also suggest that degradation is dominant in the BPA removal process. Additionally, the possible degradation pathways are proposed, and the BC used in the study can activate PMS and H2O2 as well. Then, BC/PS system is applied in remediating soil spiked with BPA, results demonstrate that the method is capable of degrading BPA in spiked soil. To some degree, It is useful to increase BC and PS dosage to accelerate the degradation in soil, but higher dosage of BC or PS may inhibite BPA degradation because of the excessive consumption of sulfate radicals and hydroxyl radicals in the spiked soil remediation. Results also demonstrate that the addition of BC could counter the pH drop in soil, thus avoiding soil acidification in the remediation process. Meanwhile, the common background electrolytes exhibit negligible effects on BPA degradation. Pilot studies should be run to obtain more detailed data on the adaptability for in situ field application. In addition, further studies should be performed to manipulate more quantity of persistent free radicals in BC to activate PS for the degradation of more refractory organic pollutants.
other readily oxidizable matters [57]. Most PS is consumed in 36.80 g L−1 system, but BPA removal is only slightly enhanced compared with 18.40 g L−1 system, suggesting the quenching effect of residual PS is very likely to occur in the experiment. Considering the costeffective remediation of contaminated soil, the optimal concentration of BC-activated PS for BPA degradation is 18.40 g L−1. The variation of pH is presented in Fig. 7c. The slurry pH values drop with the increase of PS dose, and the lowest is 3.25. The drop of pH coincides with the consumption PS, as two molar equiv. of protons were produced through the decomposition of PS. From other similar researches about the degradation of organic contaminants by PS in soil, the pH values dropped to 1.60 and 1.36 in the degradation process [14,35], which was much lower than that in our study. The results indicate that BC can not only activate PS to generate sulfate radicals for BPA degradation, but alleviate the drop of pH in the remediation of soils contaminated by organic pollutants. As a result, utilizing BC as PS activator can avoid the excessive acidification of soil during the in situ remediation in the future. 3.4. Effect of HCO3–, Cl− and HA on BPA degradation in soil
Acknowledgements It has been reported that sulfate radicals and hydroxyl radicals could be scavenged by inorganic anions [58] and HA [59]. Considering the PS/BC system may be affected by some natural existing substances in soil such as HCO3–, Cl− and HA. Therefore, the effect of HCO3–, Cl−, and HA on BPA degradation was investigated. The concentrations of HCO3– and Cl− were set at different molar ratio concentrations to persulfate viz. 0:1, 0.5:1, 1:1 and 2:1, and the concentration of HA ranged from 0 to 50 mg kg−1. As shown in Fig. 8, the three factors exhibit similar effect on BPA degradation in soil. In the presence of HCO3–, Cl−, and HA, the reactivity of persulfate is all inhibited, suggesting the scavenging effect existed in the reaction. In addition, with the molar ratio increasing, the scavenging effect is enhanced in all the reactions, and the relatively significant changes occur in PS/BC system with the addition of HA. HCO3– can compete with the target organic contaminants as an electron donator and react with sulfate radicals and hydroxyl radicals easily [60]. Cl− could react with sulfate radicals as a result of it reducing property, and it also could react rapidly with hydroxyl radicals to form less reactive species [61,62]. The reasons of the negative effect induced by HA on degradation are as follows: on one hand, HA can strongly react with sulfate radicals and hydroxyl radicals; on the other hand, HA may be adsorbed onto the surface of biochar and cover the active sites to inhibit the capacity of activation. Although the scavenging effects
This study was funded by the National Natural Science Foundation of China (NO.21347033), Joint Key Funds of the National and Natural Science Foundation of Guangdong Province, China (No.U1201234) and the Natural Science Foundation of Guangdong Province, China (No. 2016A030313021). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122637. References [1] J. Michałowicz, Bisphenol A – sources, toxicity and biotransformation, Environ. Toxicol. Phar 37 (2014) 738–758. [2] R. Gibson, J.C. Durán-Álvarez, K.L. Estrada, A. Chávez, B. Jiménez Cisneros, Accumulation and leaching potential of some pharmaceuticals and potential endocrine disruptors in soils irrigated with wastewater in the Tula Valley, Mexico, Chemosphere 81 (2010) 1437–1445. [3] J.R. Rochester, Bisphenol A and human health: a review of the literature, Reprod. Toxicol. 42 (2013) 132–155. [4] C.A. Kinney, E.T. Furlong, D.W. Kolpin, M.R. Burkhardt, S.D. Zaugg, S.L. Werner, J.P. Bossio, M.J. Benotti, Bioaccumulation of pharmaceuticals and other anthropogenic waste indicators in earthworms from agricultural soil amended with
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