Cysteine Enhanced Ferrous Iron/persulfate System For The Remediation Of Monochlorobenzene-contaminated Groundwater

Over the past few decades,environmental pollution has become increasingly prominent with rapid industrialization.The groundwater of a pharmaceutical factory located in the lower reaches of Hanjiang River was seriously polluted by monochlorobenzene(MCB),having the characteristics of a large pollution area and deep burial.The exploration of groundwater remediation technology in this contaminated site should have theoretical and practical significance for groundwater remediation work in China.At present,in situ chemical oxidation reaction zone technology has a short remediation period and good remediation effect,making it a good application prospect in the field of groundwater remediation in contaminated sites.As one of the common oxidants,sodium persulfate(PS)has the merits of high stability,low cost,and small impact on the environment.However,the oxidation capacity of PS is not particularly strong.Therefore,PS is usually activated by some methods to generate strong oxidizing free radicals during its application in environmental remediation.Thereinto,ferrous ion(Fe²⁺)is frequently used for the PS activation due to the advantages of environmental friendliness,good activation effect,simple operation,etc.However,the Fe²⁺activated PS reaction system(Fe²⁺/PS)still has some drawbacks,such as the slow transformation of Fe³⁺to Fe²⁺and the narrow efficient p H range,leading to its low removal efficiency of pollutants.In this regard,many scholars began to explore the method of strengthening the oxidation efficiency of the Fe²⁺/PS system.These methods mainly include applying energy to the reaction system,adding complexing agents,and introducing reducing agents.However,these methods have some defects such as complex operation,extra energy consumption,and secondary pollution to the environment.It was observed that the introduction of environmentally friendly cysteine(Cys)into the Fe²⁺/PS system could enhance the oxidation efficiency of the reaction system.The study of Cys enhanced Fe²⁺/PS system for in-situ remediation of contaminants in groundwater is conducive to improve the PS-based advanced oxidation processes theory system and provides a technical reserve for the improvement of the remediation process of organic pollution in groundwater in China.In this study,Cys was introduced into the Fe²⁺/PS system to build a Fe²⁺/Cys/PS system with stronger oxidation performance.The efficiency,mechanism,influencing factors of its degradation of MCB and its simulation application in the in-situ chemical oxidation reaction zone were systematically studied,to provide a reference for its implementation in the actual site.The main research contents and results are as follows:(1)The degradation efficiency of MCB in groundwater by Cys enhanced Fe²⁺/PS systemThe comparison of the Fe²⁺/Cys/PS system with other systems such as the Fe²⁺/PS system for MCB degradation showed that the addition of Cys could improve the oxidation efficiency of the Fe²⁺/PS system.The optimal dosage of Cys,Fe²⁺,and PS for degradation of MCB was determined to be 0.5 m M,5 m M,and 15 m M,respectively,through the single variable method.The MCB degradation efficiency increased by 22.26%after adding Cys into the Fe²⁺/PS system under the optimal dosage.MCB degraded rapidly in the first minute of the reaction,and then the degradation rate became slower,which was in accordance with the Behnajady-Modirshahla-Ghanbery(BMG)model.Increased the p H of water slightly promoted the MCB degradation in the Fe²⁺/Cys/PS system.Nevertheless,the degradation rate of MCB would decrease when the initial p H of the reaction solution was too high or too low.The reusability of Fe²⁺/Cys in Fe²⁺/Cys/PS system was investigated.It was found that the removal efficiency of Rh B was still more than 70%in the third run.The MCB degradation experiments also focused on the treatment of the actual MCB-polluted groundwater from contaminated site.It was observed that the degradation efficiencies of MCB in groundwater were 52.74%,34.29%,and 3.35%by Fe²⁺/Cys/PS,Fe²⁺/PS,and Cys/PS processes,respectively,indicating the effect of the Fe²⁺/Cys/PS system on MCB degradation in actual groundwater.Moreover,sequential addition of Cys(6 times)also improved the degradation efficiency of MCB than the single addition of the same amount of Cys in the Fe²⁺/Cys/PS process.(2)Degradation mechanism of MCB by Cys enhanced Fe²⁺/PS systemWhen comparing the degradation efficiency of MCB in Fe²⁺/PS system,Fe²⁺/Cys/PS system,Fe³⁺/Cys/PS system,and Fe³⁺/Cys/PS system,we found that the degradation efficiency of MCB was significantly improved by adding Cys,along with the increased concentration of Fe²⁺due to the reduction by Cys.By adding compounds similar in structure to Cys,the sulfhydryl group(-SH)was regarded as the main functional group for Fe³⁺reduction.Both the reduction and complexation effects of Cys on Fe²⁺were observed through the contrast of the Fe²⁺/Cys/PS system with the Fe²⁺/hydroxylamine/PS system.The combining effects of initial solution p H and Fe²⁺(or Cys)concentration on MCB degradation in Fe²⁺/Cys/PS indicated an increased degradation efficiency of MCB after Cys addition with elevated p H,which was mainly ascribed to the stronger complexation ratio of Fe²⁺/Cys in a higher p H.The radical scavenger tests using methanol and tert-butanol as quenchers showed that both SO₄^·-and HO^·played an important role in Fe²⁺/Cys/PS system.Combined with the quencher experiment and electron paramagnetic resonance analysis,the dominant oxidant species to the degradation of MCB was HO^·.The strengthening mechanism of Cys on the Fe²⁺/PS system was mainly the formation of Fe³⁺/Fe²⁺and Cys/Cys-Cys cycles in the reaction system,and the complexation of Cys on Fe²⁺and Fe³⁺inhibited the formation of Fe³⁺.Major intermediates of MCB degradation in the Fe²⁺/Cys/PS system were detected by GC/MS,and the degradation pathway was deduced.MCB was first degraded to small organic molecules and finally mineralized to CO₂ and H₂O.(3)The effect of groundwater matrix on Cys enhanced Fe²⁺/PS systemIn the absence of matrix species,the BMG kinetic model well described the degradation of MCB in Fe²⁺/Cys/PS and Fe²⁺/PS systems.The changes of p H and ORP during the reaction occurred primarily in the initial stage of the reaction.We systematically explored the impacts of 10 common anions(CO₃^2-,HCO₃^-,SO₄^2-,NO₃^-,NO₂^-,PO₄^3-,HPO₄^2-,H₂PO₄^-,Cl^-,Br^-),5 cations(NH₄⁺,Mg²⁺,Al³⁺,Mn²⁺,Cu²⁺)and natural organic matter(NOM)on the degradation kinetics of MCB by the Fe²⁺/Cys/PS system.The results indicated that all of the anions had inhibitory impacts on MCB oxidation,among which SO₄^2-and NO₃^-had weaker inhibitory effects.NH⁴⁺,Mg²⁺,and Al³⁺had little effect on the degradation of MCB,Mn²⁺had an inhibitory effect on MCB degradation,whereas Cu²⁺had a promoting effect on MCB degradation at low concentration and an inhibitory effect at high concentration.Similar to Cu²⁺,NOM also had a dual effect on MCB degradation.The toxicity assessment of the transformation products(TPs)in the presence of Cl^-and Br^-based on the quantitative structure-activity relationships(QSAR)model showed some TPs may possess certain toxicity.(4)Simulation experiment on remediation of the polluted aquifer by Cys enhanced Fe²⁺/PS systemThe simulation experiment of the remediation process of in situ chemical oxidation reaction zone in a contaminated aquifer was conducted using a sand-filled rectangular tank with injection wells.Tank?,as the reaction tank,was injected with oxidation reagents,whereas tank?without oxidant injection was a control tank.Under the same conditions,the Fe²⁺/Cys/PS in situ reaction zone for remediation of MCB polluted aquifer was explored by comparing tank?and tank?.The results suggested that the concentration of MCB had been effectively controlled in tank?due to the oxidant injection.Migration of oxidant to the bottom of the tank?resulted in a low concentration of MCB in the bottom and a relatively high concentration in the top.The highest concentration of PS was detected at the bottom of the tank?near the injection well,but it also migrated along the groundwater flow direction.The distribution of p H and ORP in the tank?was strongly correlated with the distribution of PS.The values of p H were negatively correlated with the values of PS concentration,whereas the values of ORP were positively correlated with PS concentration.The MCB concentration in the tank?gradually increased followed by a steady trend with a slight variation of p H and ORP.The removal rate of MCB in the aquifer by Fe²⁺/Cys/PS in situ reaction zone was calculated by MCB concentration of water samples near the outlet of tank?and tank?.It was found that the removal rate of MCB at the bottom of the tank was close to 100%,and even at the top of the tank with the lowest removal rate was close to 75%,indicating the implementation of Fe²⁺/Cys/PS in-situ reaction zone can effectively remediate the contaminated aquifer.