Surfactant-assisted removal of 2,4-dichlorophenol from soil by zerovalent Fe/Cu activated persulfate

Ling Xu,Ji Li,Wenbin Zeng,Kai Liu,Yibing Ma,Liping Fang,*,Chenlu Shi,*

1 Faculty of Material Science and Chemistry,China University of Geosciences,Wuhan 430074,China

2 Guangdong Key Laboratory of Integrated Agro-environmental Pollution Control and Management,Institute of Eco-environmental and Soil Sciences,Guangdong Academy of Sciences,Guangzhou 510650,China

3 National-Regional Joint Engineering Research Center for Soil Pollution Control and Remediation in South China,Guangzhou 510650,China

4 Macau Environmental Research Institute,Macau University of Science and Technology,Taipa,Macau,China

Keywords:Zero-valent iron and copper Advanced oxidation process Persulfate Chlorinated organic pollutants Surfactant

ABSTRACT The organic compounds contaminated soil substantially threatens the growth of plants and food safety.In this study,we synthesis zero-valent bimetallic Fe/Cu catalysts for the degradation of 2,4-dichlorophenol(DCP)in soils with persulfate(PS)in combination of organic surfactants and exploring the main environmental impact factors.The kinetic experiments show that the 5%(mass)dosage of Fe/Cu exhibits a higher degradation efficiency (86%) of DCP in soils,and the degradation efficiency of DCP increases with the increase of the initial PS concentration.Acidic conditions are favorable for the DCP degradation in soils.More importantly,the addition of Tween-80,and Triton-100 can obviously desorb DCP from the soil surface,which enhances the degradation efficiency of DCP in soils by Fe/Cu and PS reaction system.Furthermore,the Quenching experiments demonstrate that?and ·OH are the predominant radicals for the degradation of DCP during the Fe/Cu and PS reaction system as well as non-radical also exist.The findings of this work provide an effective method for remediating DCP from soils.

1.Introduction

Chlorophenol (e.g.2,4-dichlorophenol;DCP) is a group of the persistent toxic pollutants,and has been widely used in petrochemical industries,including coatings,synthetic plastics,rubber,textile industries,cosmetics,pesticides herbicides,insecticides and wood preservatives [1–4].Due to its extensive use,this inevitably leads to the discharge of chlorophenol-containing wastewaters,polluting waters and soils,especially near the industrial regions [5].Once DCP enter soils,they can substantially threaten the growth of plants and food safety.In order to address this serious environmental problem,there is an urgent demand for developing efficient solution to remove DCP in soils.

In general,the strategy for removing DCP can be divided into three groups:chemical,physical,and biological-based technologies [6,7].Of these technologies,advanced oxidation process(AOP) is an emerging chemical technology and receiving intense interest in recent years [8,9].It has been widely used to degrade organic pollutants through generating reactive oxygen species such as hydroxyl radicals (·OH) and sulfate radicals[10,11].In addition,AOP has multiple advantages such as less energy and chemicals input and less second pollution output comparing with other technologies.Niu and coworkers reported that the ·OH produced by the heterogeneous Fenton-like reaction exhibited highly efficient to degrade chlorophenols[12].However,the main disadvantage of ·OH radical are its strong pH-dependence and non-selective towards organic compounds[13],which become a major obstacle for treating organic pollutants in complicated matrices such as soils.In contrast,?is highly selective towards chlorophenol compounds with a redox potential up to 2.60 V [14],and can be generated at a wide range of pH[15,16].The basic principle of?generation is through activating persulfateby transition metal ions [17,18].Therefore,?based AOP have become a hotpot in the degradation of DCP.

Previous studies suggest that transition metal ions such as Ag,Ce,Mn,Co,and Fe have strong capacities of activating persulfate[16,19,20],while most of them are relatively expensive and rather difficult to be extensively applied in practice.Recent studies have shown that binary metallic oxides such as Mn/Fe,Fe/Ni,and Cu/Fe can significantly enhance PS/PMS activation compared to their single metal form [21,22].As a result,these binary oxides (e.g.CuFeO2) exhibit a great capacity in degrading organic pollutant through activating PS [23].Our recent work suggests that there is a strong synergistic effect occurring between Cu and Fe atoms for promoting the degradation of DCP from waters,which can be attributed to the acceleration of electron cycling between the two metals[21].Despite the great success of binary metals applied in organic pollutants removal from waters,few attentions have been paid to organic pollutants removal from soils.

It is well known that soils are generally composed of complex matrices such as minerals and organic matters,which have diverse capacities of adsorbing/complexing organic pollutants like DCP,and then reducing their bioavailability and degradability [24,25].Hence,it is expectable that this process can significantly affect the degradation performance of PS activation by binary metals.As an effective solution to tackle the above issue,the addition of surfactants such as Triton,Tween-80,and nonionic surfactants into soils to desorb organic pollutions from soils has been previous proposed,it has been attracted great interests in combining with soil washing and Fenton’like reaction-based technologies in soil remediations [26,27].In contrast to the positive effects of surfactants towards desorption of organic pollutants,their organic nature leads to significant quenching reactive radicals such as·OH radicals[1].However,it is unknown whether such quenching effects still remain towards?,in particular,?radicals are more selective to electron-deficient organic pollutants like DCP.

Herein,this study aims to study the effects of binary zero-valent Fe/Cu nanoparticles in DCP degradation in soils with PS in combination of organic surfactants (e.g.Triton-100 and Tween-80).The performance of Fe/Cu activated PS in the degradation of DCP from soil was systematically investigated,and the effects of PS concentration,pH,and Fe/Cu dosage were analyzed.In addition,two different surfactants were examined and compared for their performance of assisting DCP degradation by Fe/Cu with PS under varying conditions.Finally,quenching experiments were applied to reveal the possible reactive oxygen species responsible for DCP degradation,and the mechanism of DCP degradation was proposed.

2.Materials and Methods

2.1.Regents

All chemicals were of analytical reagent grade and used without further purifications.Phosphoric acid,acetonitrile,Tween-80(CP),Triton-100(CP),sodium borohydride (NaBH4),ferrous sulfate heptahydrate(FeSO4?7H2O),copper sulfate pentahydrate(CuSO4?5H2O),ethanol absolute,2,4-dichlorophenol (DCP),sodium persulfate(Na2S2O8),methanol (MeOH),the tert-butanol (TBA) and 5,5-dimethylpyrroline oxide(DMPO)were purchased from Sinopharm Chemical Reagent Co.,Ltd (Shanghai,China).Deionized (DI) water was used the whole experiment.The soil was collected from a locate site in Wuhan.Then the soil was air-dried and passed through a 80 mesh sieve before the batch experiments.Deionized(DI) water was used in all experiments

2.2.Preparation of zero-valent Fe/Cu

The Fe/Cu nanoparticles were synthesized following a modified method [13].In brief,0.70 g FeSO4?7H2O and 1.64 g CuSO4?5H2O were dissolved in 50 ml of DI water,and mixed with 30 ml ethanol absolute,then transfer to 200 ml three-necked flask under nitrogen.DI water was constantly purged with nitrogen for 60 min to remove dissolved O2.The mixed solution was stirred(250 r?min-1)for 30 min under nitrogen.After that,the 20 ml NaBH4solution was added to the system to reduce Fe2+and Cu2+to zero-valent bimetallic Fe/Cu.After 30 min of reaction,the Fe/Cu products were magnetically separated and washed three times with amount of ethanol absolute,and vacuum oven-dried at 70°C for 3 h.The Fe/Cu mass ratio is 1/3 optimized by our previous study [21],and stored in an anerobic glovebox for further use.

2.3.Characterization

Potassium dichromate volumetric method was used to analysis the organic matter content.The contents of the other major elements in soil sample were characterized using a X-ray fluorescence spectrometer(XRF)[28].The morphologies of Fe/Cu were analyzed by scanning electron microscopy (SEM;SU8220,HIT ACHI,Japan)and transmission electron microscopy (TEM;JEM-2100F TEM(JEOL,Japan)) equipped with an oxford INCA Energy TEM 200 EDS device with an accelerating voltage.X-ray diffraction (XRD)pattern of Fe/Cu was determined on a Smartlab (9) X-ray diffractometer (Rigaku,Japan) with a Cu target in the 2θ range from 10°to 90° (9 kW).

2.4.Batch experiments

The degradation experiments of DCP by Fe/Cu with persulfate were carried out to investigate their catalytic performance.All batch experiments were performed in a 100 ml three-necked flask at room temperature.The reaction system was 40 ml and the initial concentrations of PS was 6 mmol?L-1at pH 3.0.5 g of soil and appropriate Fe/Cu particles added to the reaction solution under mechanical stirring.The amount of Fe/Cu was adjusted to achieve a dose of 0,0.13 g?L-1(1% of soil mass),0.63 g?L-1(5% of soil mass) and 1.25 g?L-1(10% of soil mass).To investigate the effects of PS concentration,the initial concentrations of PS in the reaction system at pH 3 ranged from 3 to 9 mmol?L-1,respectively.The effects of pH in reaction systems were also analyzed by adjusting the initial pH at 3,5,7 and 9 with 0.1 mol?L-1NaOH or HCl.The surfactants with different concentrations were also studied by adding a proper amount of Triton-100 or Tween-80 into the above reaction systems.At each interval time,2 ml of suspension solution was collected,and centrifuged at 4000 r?min-1for 2 min and followed by filtering through a syringe filters with a pore size of 0.22 μm.0.1 ml of methanol was added to ensure quenching radicals in filtrates.This is the DCP concentration in aqueous solution.Then,5 ml of methanol added to the remaining soil and ultrasonic for 30 minutes for extraction soil part.After that,the procedure was the same as above.Finally,the DCP concentration was analyzed.

Quenching experiments were set up to identify the possible radicals generated during activating PS by the Fe/Cu catalyst.MeOH and TBA were used as the typical radical scavengers for quenching radicals of·OH and?[29,30],respectively.The concentration of scavengers was added about 500 times of that for DCP to ensure a complete quenching.Electron paramagnetic resonance(ESR)spectroscopy on an X-band spectrometer(A300,Bruker,Germany)experiments were used to further confirmed the production of free radicals.DMPO as a spin-trapping agent was proposed to combine free radicals for the determination.After the addition of the Fe/Cu catalyst to initiate reactions after 5 min,aliquots of the suspensions were added with 1 ml of 0.1 mol?L-1DMPO solution,and then filtered through a 0.22 μm syringe filter for analysis the potentially formed radicals.

2.5.Analysis methods

The concentrations of DCP were determined by high performance liquid chromatograph (HPLC Detector,Agilent 1260) with a C18 reversed-phase column (25 mm×0.20 mm×0.40 μm).The mobile phase was acetonitrile and phosphoric acid (pH=3)with a flow rate of 1 ml?min-1.The UV detector was set at 284 nm for DCP.

3.Results and Discussion

3.1.Characterization

The detailed physicochemical properties of the soil sample are given in Table 1;results suggest that the soil organic matter content is 15.8 mg?kg-1,and the ratio of silica reaches 71.53%followed by 16.43% for Al2O3.More importantly,the contents of transmit metals such as Fe and Cu are found to be 4.40% as Fe2O3and 0.01% for CuO,which are relatively low but crucial components for PS activation and DCP degradation.The morphologies of the as-synthesized Fe/Cu were analyzed by SEM and TEM.Fig.1(a)and (b) clearly show that Fe/Cu nanoparticles are aggregated to chain-like structure,the particle size of Fe/Cu ranges between 60 and 70 nm,which is probably due to the intrinsic magnetic and the van der Waals forces amongst Fe/Cu nanoparticles [31].Elemental analysis confirms that the surface of particles mainly consists of Fe and Cu without the presence of other impurities(Fig.1(d)).In addition,the crystalline structure of Fe/Cu was ana-lyzed by XRD,and Fig.1(c) exhibits notable diffraction peaks at 44.9°,50.4°,and 74.3° that can be assigned to that of Fe0and Cu0[32].Thus,the zero-valent Fe/Cu nanoparticles have been successfully synthesized.

Table 1 Summary statistics of soil physicochemical properties

3.2.Catalytic performance of Fe/Cu for soil DCP degradation

Fig.1.(a) SEM image of Fe/Cu;(b) TEM image of Fe/Cu;(c) XRD pattern of Fe/Cu;(d) EDS pattern of Fe/Cu.

Fig.2.(a)DCP concentration in aqueous phase at pH 3;(b)the degradation of DCP in total reaction system at pH 3;Fitting of kinetic data with linear pseudo-first-order for the degradation of DCP in aqueous phase (c) and the total reaction system (d).PS concentration:6 mmol?L-1.

The catalytic performances of the Fe/Cu catalysts with different dosages[0,0.13 g?L-1(1%of soil mass),0.63 g?L-1(5%of soil mass),and 1.25 g?L-1(10%of soil mass)]for DCP degradation in soils have been investigated and compared.As shown in Fig.2(a) and (b),there is a negligible degradation of the DCP in aqueous phase and whole reaction system without adding Fe/Cu,indicating that the minerals in soils cannot directly degrade DCP.The addition of PS leads to the degradation of DCP,which can be probably attributed to the PS activation by the small fraction of Fe and Cu in soils as indicated in Table 1.However,the degradation efficiency of DCP is considerably low in aqueous phase and whole reaction system(27%)(Fig.2(a)and(b)),indicating a poor capacity of those minerals in activating PS and DCP degradation,similar to previous studies [25].In addition,the only addition of Fe/Cu can decrease the DCP concentration in aqueous phase,while the degradation efficiency is significantly lower than that with the addition of PS and Fe/Cu(Fig.2(a) and(b)).This is because Fe0can also generate part of reactive oxygen species such as ·OH radicals to degrade DCP[33,34].The addition of the Fe/Cu catalyst and PS results in a rapid decline of DCP in aqueous phase and the whole reaction system within 5 min.This result shows that Fe/Cu catalyst exhibits a strong ability to activate PS to degrade DCP in soil,in line with the previous literature in the solution system [21].Furthermore,with the increase of the Fe/Cu dosage(from 0%to 5%),the degradation efficiency of DCP by Fe/Cu catalysts also increases from 12%to 86%.However,it decreases to 77% when the Fe/Cu dosage up to 10%,which is mainly due to the aggregation of Fe/Cu.This result suggests that the 5% (mass) dosage of Fe/Cu is the superiority for the removal of DCP.

Since the reaction reaches equilibrium almost within 5 min,the kinetic data were fitted in two stages.As shown in Fig.2(c)and(d),the DCP degradation kinetic data fit well with the pseudo-firstorder model through two segments.It was observed that a 5%(mass) dosage of Fe/Cu exhibits a significant highly rate constant(kobs) (first stage:0.22 min-1,second stage:0.008 min-1) than other dosages (Table S1 and S2,Supplementary Material ).The kinetic results also suggest that a high performance on the DCP degradation was found in 5% (mass) Fe/Cu and PS.Thus the 5%(mass) dosage of Fe/Cu was selected for the following studies.

3.3.Effect of PS concentration on the DCP degradation in soil

In order to study the effect of the PS initial concentration,different concentrations (3,6,and 9 mmol?L-1) of PS were selected to investigate the performance of DCP degradation.As shown in Fig.3(a) and (b),with the increase of the initial PS concentration,the degradation efficiency of DCP by Fe/Cu catalysts have increased significantly in the aqueous phase and the whole reaction system,nearly 89%of DCP was found to be degraded when the initial concentration of PS is 6 mmol?L-1.This result shows that the higher initial concentration of PS can improve the degradation efficiency of DCP.The reason is that a lower initial concentration of PS does not generate enough radicals to trigger more DCP degradation,which limits the removal rate of DCP.This conclusion is also consistent with the results of previous studying the degradation of other pollutants (naphthalene,nitrobenzene,trichloroethylene,etc.) by PS [18].However,no obvious improvement of DCP degradation efficiency can be found in further increasing PS initial concentration to 9 mmol?L-1,mainly due to the high degradation efficiency of DCP has reached 89% when the initial concentration of PS is 6 mmol?L-1.In addition,the DCP degradation kinetic data were fitted in two stages with the pseudo-first-order model and also calculated kinetic rate constants (Figs.S1(a) and (b)).The results show that thekobsvalues for the different initial PS concentration (3,6,and 9 mmol?L-1) increase from 0.06 to 0.34 min-1in the aqueous phase and whole reaction (first stage) (Fig.3(c) and(d)) (Table S3 and S4).Considering the similar high DCP degradation efficiency when the initial PS concentration is 6 and 9 mmol?L-1,thus,this result suggests that a more moderate 6 mmol?L-1was selected as the subsequent experimental condition.

Fig.3.(a)DCP concentration in aqueous phase with different initial PS concentration(3,6,and 9 mmol?L-1)at pH 3;(b)the degradation of DCP in total reaction system at pH 3;the obtained rate constants (first stage) as a function of the initial PS concentration in the aqueous phase (c) and the total reaction system (d).Cu/Fe dosage:0.63 g?L-1.

3.4.Effect of reaction pH on the degradation of DCP in soil

The pH condition of soil solutions is likely to be an important factor affecting the performance of Fe/Cu in activating PS for DCP degradation from soils.The effect of different pH (3,5,7,and 9)on the degradation of DCP are depicted in Fig.4(a)and(b),the concentration of DCP in reaction system dramatically declines within the first 5 min,and the highest DCP degradation performance in the aqueous and whole reaction (89%) are achieved at pH 3.The kinetic data of DCP degradation are well fitted by pseudo-firstorder kinetic with two stages (Fig.S2(a) and (b)).Thekobsvalues decreases in the aqueous phase and whole reaction (first stage)with the pH increase (Fig.4(c) and (d)).Thekobsvalues with two stages for pH 3 is higher than among other pH conditions(Table S5 and S6).This result shows that the degradation efficiency decreases as the pH increase.However,only 17% of degradation extent decrease at pH 9.Similar observation was also reported in Fe0/PS and Fe/Cu(II)/PS reaction [16,25].On the one hand,PS is mainly dominated as negativelyspecies under acidic conditions and a small fraction ofspecies are presented at alkaline conditions[21],and the positively charged of the Fe/Cu catalysts is favorable to reaction with PS and generation radicals under acidic condition.On the other hand,iron ions would generate iron hydroxide precipitation in the solution at alkaline conditions,which adhered to the surface of the Fe/Cu,forming an electron barrier,then shielding the electron transfer[35].Thus,this inhibits the further activation of PS by Fe/Cu,leading to a decrease in the degradation efficiency of DCP.In addition,Deng and coworkers also found that the pH of the reaction process was more than 4.0,it can affect the activity of Fe0and Cu0[36].Hence,acidic conditions were favorable for the DCP degradation,and pH 3 is the superiority condition for the removal of DCP,which was selected for the whole studies.

3.5.Enhanced DCP degradation by surfactants

Fig.4.(a) DCP concentration in aqueous phase at pH conditions from 3 to 9;(b) the degradation of DCP in total reaction system at pH 3;the obtained rate constants (first stage) as a function of the pH in the aqueous phase (c) and the total reaction system (d).Concentration:6 mmol?L-1,Cu/Fe dosage:0.63 g?L-1.

Considering the DCP is easily adsorbed to the surface of the soil,which may weaken the degradation of DCP from soil.Many previous studies are mainly focused on the degradation process such as increasing the initial concentration of PS,the addition dosage of material [1,25].However,how to desorb DCP as much as possible from the soil,which is more important for the DCP degradation.Therefore,different surfactants such as Tween-80 and Triton-100 were added to the reaction system to explore the performance of DCP degradation from soil.As shown in Fig.5(a)and(b),after addition of different surfactants,the concentration of DCP in the aqueous phase increases immediately within the first 5 min,which indicates that two surfactants can obviously desorb DCP from the soil surface.After reaction,the DCP desorption capacity of Tween-80 is significantly than Triton-100 when the concentration is 0.12 mmol?L-1.In addition,this result shows that the desorption capacity also increases as the concentration of surfactant increases.The main reason is that surfactant has a good hydrophilicity and lipophilicity,it could significantly reduce the surface tension of the solvent,and effectively improve the solubility of hydrophobic organic matter at its critical micelle concentration [37],which can desorb the organic pollutants in the soil [38,39].Tween-80 exhibits a stronger ability in desorption effect at a concentration of 0.012 mmol?L-1and Triton-100 at a concentration of 0.22 mmol?L-1,which were selected for the subsequent experiments.

The DCP degradation performance was further investigated by adding surfactants.The effect of Tween-80 and Triton-100 on the degradation of DCP in Fig.5(c) and (d),Tween-80 and Triton-100 have a great improvement in the degradation efficiency in aqueous compared to without surfactant,and reaches 93% and 95% after 90 min reaction.This result indicates that adding an appropriate amount of surfactant to the system can promote the degradation of pollutants in the aqueous phase.Since without adding the surfactant,the degradation efficiency can reach 89%.Thus,the total degradation efficiency of DCP is not significantly different.However,Fig.4(d) shows that the addition of Tween-80 and Triton-100 can enhance the desorption efficiency of DCP within the first 5 min and reaches reaction equilibrium compared to without adding surfactants,which is mainly because surfactants promote the desorption of DCP in the soil and accelerate DCP degradation efficiency [40,41].Additionally,both Tween-80 and Triton-100 are eco-friendly surfactants which pose less toxicity towards organisms,and inversely,these surfactants can be biodegradable as a part of natural organic carbon in soils [42,43].Moreover,more eco-friendly surfactants such as biosurfactants can be an ideal option to replace chemical agents [44].Alternatively,surfactant recovery from wastewaters after soil washing using biochar or activated carbon is also another solution [45].Anyhow,the findings of this work demonstrate the advantage of combining surfactants with persulfate activation for persistent organic pollutant removal,while additional efforts in seeking proper surfactants or their recovery should be addressed in further work.Considering reducing the influence of catalysts on the soil as much as possible,the recyclability of catalysts will be the focus of our future work such as designing magnetically recyclable catalysts.Therefore,the addition of surfactants can enhance the DCP degradation efficiency by Fe/Cu/PS in soils.

3.6.Radical identification and reaction mechanisms

Fig.5.The effect of different surfactants Tween-80(a)Triton-100(b)for 2,4 DCP desorption.(c)DCP concentration in in aqueous phase at pH 3;(d)the degradation of DCP in total reaction system at pH 3.PS concentration:6 mmol?L-1,Cu/Fe dosage:0.63 g?L-1.

Fig.6.(a)The DCP degradation kinetics by Fe/Cu with PS quenched by methanol and tert-butyl alcohol at pH 3;(b)ESR spectra of DMPO-OH and DMPO- ?formed in the Fe/Cu and PS system at pH 3;PS concentration:6 mmol?L-1,Cu/Fe dosage:0.63 g?L-1.

To further decipher the reaction mechanism and whether and how much the potential radical species (e.g.?and ·OH) can be responsible for the degradation of DCP,MeOH and TBA were used as radical scavengers to quench the reaction system?and ·OH[46].As shown in Fig.6(a),in the presence of TBA,the degradation of DCP slightly decreases from 89% to 79% comparing to without quencher.However,the addition of MeOH significantly inhibits the degradation efficiency of DCP from 89%to 56%,it can be calculated that the contribution of radical species in the Fe/Cu/PS reaction system changes as the following order:33%>10%(·OH).This implies that?is likely the predominant radical for the degradation of DCP during the Fe/Cu activated PS,while the secondary formed ·OH radical may contribute comparably less to the overall DCP degradation.In addition,there is still part of DCP has been removed after adding the two quenchers,which is mainly due to the occurrence of non-free radical reaction[47].The results show that the contribution of non-radicals reaches about 56%.Previous studies also found that non-free radical reaction involved in DCP degradation by Fe/Cu/PS [21].It probably because Fe/Cu adsorbed DCP during the reaction.Furthermore,typical ESR spectra of DMPO/?and ·OH in the reaction were observed (Fig.6(b)),which clearly confirms that the reaction system can directly generate?and ·OH radicals.This is again evidence that?and ·OH is the predominant radical for the degradation of DCP.According to the above results,the mechanism for the degradation of DCP in soils can be proposed as follow.The added surfactant(Tween-80 and Triton-100)firstly promoted the desorption of DCP from the soil surface.Subsequently,Fe0and Cu0directly react with PS to generate?(Eqs.(1)and(2)),and partly Fe0oxidized by O2to produce Fe(II).Then the generated Cu(II)can be reduced to Cu(I)by Fe0and Fe(II)(Eq.(3)),which further reacts with PS to generate?(Eq.(4)).More importantly,our previous study suggested that a significant synergistic effect between Fe and Cu in the catalysts,especially Cu(I) produced by Fe0and Fe(II) reduction predominate in activating PS [48],which improves?generation.Furthermore,the generated Fe(II)also react with H2O2and the conversion of?to form ·OH(Eps.(5)and(6))[49].In addition to free radical processes,part of the DCP is removed by non-free radical processes.Therefore,we can propose that the?and ·OH radicals processes and non-free radical drive the DCP degradation by Fe/Cu activated PS in soils.The possible reaction processes for the Fe/Cu and PS can be proposed as follows [50,51]:

The possible pathway of the DCP degradation is further analyzed.The generated?and ·OH reacts with DCP and occur degradation reaction.Therefore,the possible pathway of the DCP degradation in the soil suspension is similar to the water system.Our previous study found that the intermediate products of the DCP degradation process by Fe/Cu activated PS in water include 2-chlorohydroquinone,2-chlorobenzoquinone,and oxalate ions[21].After reaction,chloride ions (Cl-) have been detected.It suggests that the generated free radicals can firstly attack the chlorine located at the benzene ring of DCP,corresponding to a dechlorination process,which indicates that the chlorine group at the 4-position of the ring is more vulnerable to be replaced [21,52].The followed process is the opening of the rings by free radicals,which can generate multiple secondary intermediates such as 2-chlorphydroquninine,fumaric acid,and acetic acid [53].Finally,these intermediate products can be further mineralized into CO2,H2O,and Cl-.

4.Conclusions

In this study,zero-valent bimetallic Fe/Cu catalysts were synthesized for the degradation of DCP in soils with PS in combination of organic surfactants.The results show that the 5%dosage of Fe/Cu exhibits a higher degradation efficiency of DCP in soils.The degradation efficiency of DCP increases with the increase of PS initial concentration.Acidic conditions are favorable for the DCP degradation in soils compared to alkaline conditions.More importantly,the addition of organic surfactants (e.g.,Tween-80,and Triton-100) obviously desorb DCP from the soil surface,which can enhance the degradation efficiency of DCP in soils by Fe/Cu and PS reaction system.Furthermore,?and ·OH are the predominant radical for the degradation of DCP during the Fe/Cu and PS reaction system,while non-radical also exist.The findings of this work provide an efficient method for the degradation of DCP in soils.

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.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21 876161;41420104007),and the National Key Research and Development Project of China (No.2018YFF0213403),Guangdong Academy of Sciences’ Project(2019GDASYL-0 102006;2019GDASYL-0 301002;2018GDASCX-0501).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.03.031.