Simultaneous Removal Of Cr(Ⅵ) And Carbon Tetrachloride From Groundwater By Stabilized Green Rust

Hexavalent chromium(Cr(Ⅵ))and carbon tetrachloride(CT)are common contaminants in the soil and groundwater,having serious threat to ecological safety and human health.Green rusts(GRs)are of great potential for in-situ remediation of soil and groundwater.However,GRs trend to lose structural Fe(II)and transfer into low reactivity minerals,which restricts its long-distance transport in soil and groundwater media.In this study,green rust chloride(GR)was modified with stabilizer to produce stabilized green rust(GR-X),and these GR-X were primarily used for stability assessment and Cr(Ⅵ)removal tests.And then the effects of co-existing Cr(Ⅵ)on CT dehalogenation by GR-Si were investigated to reveal mechanism of enhanced CT dehalogenation by GR-Si in presence of Cr(Ⅵ).The main research results are summarized as follows:(1)GR-X synthesis and its stability assessment.GR-X was stabilized using silicate(Si),phosphate(P),fulvic acid(FA),bone char(BC)and carboxymethyl cellulose(CMC),respectively.It was demonstrated that the release of Fe(II)from GR was effectively suppressed by above reagents,enabling at least 50% lower Fe(II)leaching from the stabilized GR-X than that from the pristine GR.The intact hexagonal GR plates and crystallinity were also confirmed by the SEM images and XRD patterns after 7 days storage,indicating the stable structure of GR-X was remained.(2)Evaluation of Cr(Ⅵ)removal by GR-X.In the Cr(Ⅵ)removal tests,Cr(Ⅵ)was reduced by GR-X rapidly with a Fe(II)utilization efficiency over 90%.The reaction product analysis demonstrated that the GR-X was able to transfer Cr(Ⅵ)into stable GROX-Cr(III)precipitates(Fe-Mn oxides fraction).The p H monitor of post-remediation systems indicated that GR-X systems are stable,the p H buffering ability could avoid acidification and lower the Cr leaching risk.(3)Simultaneous removal of Cr(Ⅵ)and CT by GR-Si.GR-Si could remove Cr(Ⅵ)and CT at the same time,and the presence of Cr(Ⅵ)was shown to be necessary for CT dehalogenation by GR-Si.The half-life of CT was 14.4 h in the system of GR-Si with Cr(Ⅵ)existence.(4)The mechanism of CT dehalogenation by GR-Si in presence of Cr(Ⅵ).Further analysis on Fe(II)species evolution and solids phase transformation evidenced that Cr(Ⅵ)could initiate structural changes in GR-Si,which enable the release of Fe(II)and generate reactive sites for CT dehalogenation.Adsorption experiments showed that generated GROX-Cr(III)might strongly adsorb Si(qm=22.0mg/g),then opening the reactive sites as Si desorbs from the GR.This results reveal the importance of GROX-Cr(III)precipitates formation during the remediation.The results show that GR-X are of great potential for in-situ remediation of soil and groundwater.This study will would further push forward the application of GRs in in-situ remediation of soil and groundwater,providing theoretical and technical support for the remediation of combined pollution of heavy metals and chlorinated solvents.