Degradation Efficiencies And Mechanisms Of Trichloroethylene In Groundwater By Two Novel In Situ Chemical Oxidation Systems

Trichloroethylene(TCE),as one of the most frequently detected organic contaminants in groundwater,which has strong toxicity and potential carcinogenicity and is threatening the safety of groundwater and human health.Therefore,it is urgent to develop effective technique for the remediation of TCE-contaminated groundwater.In the past decades,various biological,chemical and physical methods have been adopted to remediate TCEcontaminated groundwater in situ or ex situ.Among them,in situ chemical oxidation(ISCO)has been considered as one of the most attractive options.In the present study,the degradation efficiency and mechanism of TCE in groundwater by FeⅢ-L/H2O2 system and Fe-M/Oxone system were investigated under circumneutral pH conditions.The main results obtained in this study were summarized as follows:Firstly,the degradation efficiencies of TCE in simulated groundwater by six kinds of FeⅢ-L(i.e.,EDTA,EDDS,NTA,CA,MA,TA)complexes-catalyzed H2O2 were investigated under circumneutral pH conditions.The results showed that FeⅢ-NTA/H2O2 led to more efficient degradation of TCE in comparison with those obtained by other FeⅢL/H2O2 systems.In addition,the effects of NTA:FeⅢ molar ratio,FeⅢ-NTA complex and H2O2 dosages on the degradation kinetics of TCE were investigated,the optimal conditions were obtained as NTA:FeⅢ molar ratio at 1.5:1,0.1 mM FeⅢ-NTA complex,5.0 mM H2O2 and initial pH 8.2,more than 98%degradation efficiency of TCE in the actual groundwater were achieved after 60 min of the reaction time.The results of electron spin resonance(ESR)detection and radical scavenging tests indicated that both hydroxyl radical(HO’)and superoxide radical(O2·-)were generated from FeⅢ-NTA/H2O2,and the degradation of TCE was mainly ascribable to the attack of HO’.Moreover,the degradation efficiency of TCE in sand columns by FeⅢ-NTA/H2O2 in situ was further evaluated under continuous-flow mode.The results showed that more than 90%degradation efficiency of TCE(20 mg L-1)was Oxidation Systems obtained at the loading dosages of FeⅢ-NTA(1:1.5)complex and H2O2 at 0.5 mM and 4.0 mM,respectively.It was found that TCE was decomposed into formic acid and Cl-(the resulting stoichiometry ratio of ΔCl-/ΔTCE was close to 3:1).The reaction stoichiometry efficiencies of ΔTCE/ΔH2O2 achieved 4-8%.Furthermore,the presence of HCO3-in groundwater can enhance the degradation efficiency of TCE by FeⅢ-NTA/H2O2,whereas natural organic matters such as fulvic and humic acids led to inhibitory effects on the degradation rate of TCE.Secondly,two kinds of iron-based bimetallic composites(i.e.,Fe-Cu and Fe-Zn bimetallic oxides)were prepared via the co-precipitation method,which were employed to activate Oxone for the remediation of TCE-contaminated groundwater in situ.The characterization results showed that the prepared Fe-Cu and Fe-Zn catalysts had good crystal structure and plenty of pores,and the distributions of metals were quite uniform.The results of ESR detection indicated that the prepared Fe-Cu/Fe-Zn catalysts were capable of effectively activating Oxone to generate both sulfate radical(SO4·-)and HO·.In particular,the degradation efficiencies of TCE in sand columns by Fe-Cu/Oxone and Fe-Zn/Oxone in situ were evaluated under continuous-flow mode(110 days).The results showed that more than 98%degradation efficiencies of TCE(20 mg L-1)were obtained at the loading dosage of Oxone at～7.0 mM.TCE was decomposed into formic acid and Cl-(the resulting stoichiometry ratio of ΔCI-/ΔTCE was close to 3:1).The reaction stoichiometry efficiencies of ΔTCE/ΔOxone achieved 3-4%.It was noteworthy that the stability of the prepared FeCu/Fe-Zn catalysts were not good enough,resulting in significant leaching of Cu2+and Zn2+,which should be taken into account in practical applications.In summary,both FeⅢ-NTA/H2O2 and Fe-Cu/Fe-Zn-activated Oxone systems can effectively degrade TCE in groundwater in situ.The obtained results in this study can provide guidance for in situ remediation of the TCE-contaminated groundwater,which has great significance in both theoretical and practical application.