Degradation Of Trichloroethylene In Groundwater Using Iron And Modified Iron Catalyzed Calcium Peroxide Systems

With the rapid economic development and industrialization progress in China,the environmental situation around this country is becoming more and more serious.The contamination of soil and groundwater has become an important environmental issue.The widespread applications of chlorinated hydrocarbon solvents and improper operation in their disposal make them the major organic pollutants in soil and groundwater in contaminated sites.The remediation of chlorinated hydrocarbons contaminated groundwater via in situ chemical oxidation technologies has received widespread attentions.In this study,trichloroethylene(TCE),a common chlorinated hydrocarbon in contaminated groundwater,was chosen as the target contaminant,and the Fe(?)/Fe(?)catalyzed oxidation systems based on calcium peroxide(CP),a novel oxidant,were developed for TCE degradation in aqueous solution.Two kinds of chelating agents,(S,S)-ethylenediamine-(N,N')-disuccinic acid trisodium(EDDS)and citric acid(CA),and one kind of reducing agent,ascorbic acid(AA),were used for process modification.The effects of reaction parameters and groundwater quality conditions on TCE degradation were investigated.The TCE degradation mechanism was elucidated by analyzing the generation of free radicals and their contributions to TCE degradation.The feasibility of applying iron catalyzed calcium peroxide system in actual groundwater remediation was also validated.The main conclusions are as follows:(1)TCE could be readily degraded in Fe(?)catalyzed CP system.TCE removal efficiency reached 99,8%after 15 min reaction with an initial TCE concentration of 0.15 mM and a CP/Fe(?)/TCE molar ratio of 4/8/1.Excessive or deficient dosages of CP and Fe(?)decreased TCE degradation efficiency.The results of free-radical probe tests confirmed the generation of hydroxyl radical(HO·)and superoxide anion(O2·-)in Fe(?)catalyzed CP system,in which HO· was responsible for TCE degradation process while O2·-played a supporting role.TCE degraded under HO· attack could achieve complete mineralization.The increase of solution pH inhibited TCE degradation,and Cl" and HCO3-also had significant inhibition effects.Although TCE degradation efficiency was significantly lower when applying Fe(?)catalyzed CP system in the actual groundwater,desired TCE treatment could be achieved with more oxidant and catalyst doses.(2)The modification with EDDS chelation enhanced TCE degradation performance in Fe(?)catalyzed CP system at near-neutral pH condition.TCE degradation process in Fe(?)-EDDS catalyzed CP system could be divided into two stages,i.e.,rapid stage and slow stage.The Fe(?)/EDDS molar ratio controlled the existing form and reactivity of the catalysts,and the optimal Fe(?)/EDDS molar ratio was found to be 1/1.The precipitation of ferric ions was prevented effectively by replacing free Fe(?)with Fe(?)-EDDS complex,and the utilization of oxidants was hence improved.The increase of solution pH decreased TCE removal efficiency while HCO3-and humic acid had apparent inhibition effects.Fe(?)-EDDS catalyzed CP system had better adaptation to the impact of actual groundwater quality conditions and better performance in the treatment of TCE contaminated groundwater than Fe(?)catalyzed CP system.(3)TCE could be efficiently degraded in Fe(?)catalyzed CP system.TCE removal efficiency reached 96.1%after 180 min reaction with an initial TCE concentration of 0.15 mM and a CP/Fe(?)/TCE molar ratio of 20/40/1.TCE degradation in CP/Fe(?)system well fitted the pseudo-first-order kinetic model.Excessive CP was detrimental to TCE degradation,whereas excessive Fe(?)has no inhibition effect.Both HO· and O2·-were formed in Fe(?)catalyzed CP system.HO· played a major role in TCE degradation which was a complete mineralization process.Several aqueous anions had inhibition effects on TCE degradation and the effects diminished in the following order:HCO3->Cl-?SO42->NO3-.The presence of humic acid promoted TCE degradation.(4)The addition of CA into Fe(?)catalyzed CP system significantly increased the pseudo-first-order reaction kinetic constants of TCE degradation.The presence of CA lowered solution pH,and CA chelated with Fe(?),thus the precipitation of Fe(?)was inhibited,HO· generation and TCE degradation were promoted simultaneously.However,excessive CA would scavenge HO· and inhibited TCE degradation.The optimum CP/Fe(?)/CA/TCE molar ratio was found to be 4/8/2/1.The elevation of solution pH destroyed the chelating coordination in Fe(?)-CA complex,which was unfavorable to TCE degradation.The presence of Cl' and HCO3-inhibited TCE degradation.TCE degradation efficiency in Fe(?)catalyzed CP system was improved with the addition of CA when implemented in actual groundwater.(5)The addition of AA to Fe(?)catalyzed CP system significantly enhanced HO·generation and TCE degradation.TCE removal efficiency reached 97.7%after 90 min reaction with an initial TCE concentration of 0.15 mM and a CP/Fe(?)/AA/TCE molar ratio of 4/4/2/1.In the initial reaction stage,AA reduced Fe(?)to Fe(?)effectively,and AA oxidation products could coordinate with iron ions to maintain sufficient catalyst concentration in the reaction system.Excessive AA would decrease TCE degradation efficiency as the inhibition effect of HO· scavenging exceeded the promotion effect of reducing Fe(?).Although intermediate products such as dichloroacetic acid,formic acid and oxalic acid were produced in TCE degradation process,complete TCE degradation was finally achieved without residual chlorinated intermediates.By increasing chemical dosages and adjusting chemical ratios appropriately,TCE degradation performance could be improved in AA enhanced system effectively in actual contaminated groundwater remediation.