Hexavalent Chromium Reduction In The Complicated System Of Nontronite-organic Acids-chromium And Its Possible Reaction Mechanisms

Chromium is a common pollutant in natural soil and groundwater environments.The mobility and toxicity of chromium are closely related with its oxidation state.Therefore,the transformation of chromium in environments has received extensive attention from different kinds of fields.Previous researches has shown that aqueous Fe²⁺,zero-valent iron,and iron-bearing minerals（such as pyrite and clay mineral）,played important roles both in the natural chromium cycle and heavy-metal remediation projects.In the above-mentioned iron forms,iron-bearing clay minerals are widespread in natural soil and sediments,and structural iron in clays accounts for about half of the iron pool in soils and sediments.The structural iron in clay minerals can be reduced as structural Fe（II）through biological reduction or chemical reduction.The formed structural Fe（II）displayed high reactivity to many inorganic and organic pollutants.The critical role of clay mineral in the transformation of chromium has arouse widespread attention few decades ago.The interactions between clay minerals and chromium have been reported extensively so far.However,the components of contaminated soils and sediments are very complicated.The transformation of chromium in the complicated system affected by various environmental factors such as organic matter,minerals,or microorganisms especially under near-neutral pH condition has not been studied well to date.The aim of this paper is to investigate the effects of various organic acids on the hexavalent chromium（Cr（VI））reduction by structural Fe（II）in nontronite（an iron-rich smectite）at dark and near-neutral pH condition（pH⁶）.Our results demonstrated that（1）The influences of organic acids on the Cr（VI）reduction rate by structural Fe（II）in nontronite greatly depended on the species of organic acids.For example,the presence of citrate decreased the Cr（VI）reduction rate by structural Fe（II）in nontronite.However,tartrate and malate greatly increased the Cr（VI）reduction rate by structural Fe（II）in nontronite.（2）Organic acids containingα-hydroxy orα-carbonyl group could provide additional electrons to reduce Cr（VI）at near-neutral pH condition in the complicated system of chemically reduced nontronite-organic acids-Cr（VI）,and thus increased the total amount of reduced Cr（VI）.However,these organic acids could not reduce Cr（VI）alone or in the presence of oxidized nontronite.（3）Our TEM direct observations revealed that the reduced Cr（VI）product by structural Fe（II）in nontronite alone was nanometer-sized Cr containing particles accumulating on the surface or edge of nontronite in the form of Cr₂O₃/Cr（OH）₃.But the in the presence of organic acids,the form of reduced Cr（VI）product was different determined by the nature of organic acids.For instance,in the presence of tartrate,malate,or citrate,the reduced Cr（VI）products by structural Fe（II）in nontronite was in the form of soluble Cr（III）-organics complexes.Whereas in systems containing other organics,such as succinate,propionate,the reduced Cr（VI）products were in the form of solid Cr（III）-oxides or hydroxides.Our study highlights the importance of organic acids on Cr（VI）reduction and immobilization when iron-rich smectite was applied to treat Cr（VI）contaminant in dissolved organic matter（DOM）rich environments.