Formation,Transport And Transformation Of NOM-Cr(Ⅲ)Colloid In Groundwater Environment

Chromium（Cr）is widely used in industrial production and metal processing.Among heavey metal pollution,the pollution caused by Cr is particularly serious.The poisonousness and solubility of Cr is related to the speciation.The valence state of Cr in nature are mainly Cr（Ⅵ）and Cr（Ⅲ）.in the environment.Cr（Ⅵ）is generally considered as mobile in the environment,whereas Cr（Ⅲ）predominantly exist as precipitation.Cr（Ⅵ）is more toxic than Cr（Ⅲ）.Recently,field studies have reported that NOM-Cr（Ⅲ）colloid exist in the pore water and underground water.Formation of NOM-Cr（Ⅲ）colloid could increase the solubility of Cr（Ⅲ）and may lead to a concern of Cr（Ⅲ）reoxidation.Therefore,it is important to investigate the new insight of NOM mediate NOM-Cr（Ⅲ）colloid formation and the transport or reoxidation of NOM-Cr（Ⅲ）colloid in groundwater environment.The thesis combines the geochemical process with colloidal chemistry.Behaviors of NOM-Cr（Ⅲ）colloid were investigated in groundwater environment containing rich organic carbon.The research results may provide foundamental data help to understand and predict the formation,transport and transformation behaviors of Cr.The first part of this thesis focuses on the mechanism of NOM-Cr（Ⅲ）colloid formation from the reduction of dissolved Cr（Ⅵ）source.Batch experiments were used to investigate Cr（Ⅵ）reduced by reduced natural organic matter(NOM_red),in an unoxic groundwater environment and NOM-Cr（Ⅲ）colloid formation by natral organic matter（NOM）complex with reduced Cr（Ⅲ）product.This study indictes that Cr（Ⅵ）can be repaidly reducted by NOM_red,and the rates of reduction depend on the sources of NOM_red,concentrations of NOM_red,pH,ion strength,spieciation of ion.The reducted production is predominately NOM-Cr（Ⅲ）colloid.This research provides a new opproach for NOM-Cr（Ⅲ）colloid formation in an unoxic groundwater environment.The second part of this thesis focuses on the mechanism of NOM-Cr（Ⅲ）colloid formation from the dissolve of Cr（Ⅲ）precipitation source.Batch experiments were used to investigate the formation of NOM-Cr（Ⅲ）colloid by trivalent iron and Cr（Ⅲ）copercipitate(Cr_xFe_1-x（OH）₃（0<x≤1）)dissolving in a rich NOM condition.This study indictes that Cr_xFe_1-x（OH）₃ can be dissolved by NOM in both oxic and unoxic condition.The dissolving rates of Cr_xFe_1-x（OH）₃ depend on redox state of NOM and the initial Cr/Fe molar ratio.Aquaous Cr（Ⅲ）predominately exists as NOM-Cr（Ⅲ）colloid.This research provides another new opproach of NOM-Cr（Ⅲ）colloid formation in both unoxic and oxic groundwater environment.The third part of this thesis focuses on the stability of NOM-Cr（Ⅲ）colloid in groundwater environment.Batch experiment and one dimentional transport column were conducted to investigate the mechanism of stability of NOM-Cr（Ⅲ）colloid and transport of NOM-Cr（Ⅲ）colloid porous media.The results suggest that the stability of NOM-Cr（Ⅲ）colloid depends on source of NOM,initial C/Cr molar ratio,speciation of ion and ion strength.NOM-Cr（Ⅲ）colloid can stable exist in part of natral water over a wide rage of related water condition.And NOM-Cr（Ⅲ）colloid may probably transport in porous media.The research indictes that formation of NOM-Cr（Ⅲ）colloid may increase the transport ability of Cr（Ⅲ）.The last part of this thesis focuses on the oxidation effect of NOM-Cr（Ⅲ）colloid in groundwater environment.Bath experiment were conducted to investigate the mechanism of NOM-Cr（Ⅲ）colloid oxidized by hydrogen peroxize（H₂O₂）.The results suggest that NOM-Cr（Ⅲ）colloid is likely to be oxidized repaidly by H₂O₂ and soluble Cr（Ⅵ）is produced in the end.Both oxidation rates and portion of soluble Cr（Ⅵ）production depended on the concentrations of H₂O₂,concentrations of ferrous（Fe（II））,pH and Cr（Ⅲ）species.This study indictes that NOM-Cr（Ⅲ）colloid has potential reoxidation properties and may release toxic Cr（Ⅵ）to groundwater environment.Research of the doctoral thesis provides a new insight of NOM-Cr（Ⅲ）colloid formation in a rich NOM groundwater environment instead of existing as precipitation as we thought before,（i.e.formation of NOM-Cr（Ⅲ）colloid form soluble Cr（Ⅵ）reduction or Cr（Ⅲ）precipitation dissolution）.Formation of NOM-Cr（Ⅲ）colloid increase the transport ability and reoxidation risk of Cr（Ⅲ）.This thesis takes into new insight from colloidal science to understand the mechanisms of formation,transport and transformation of NOM-Cr（Ⅲ）colloid over a wide range of environmental conditions.The results may further prediction and treatment of Cr in groundwater environment.