In-situ Mobilization And Transformation Of Iron Oxides- Adsorbed Arsenate In High Arsenic Groundwater

The geogenic high-arsenic groundwater is one of the serious public environmental geologic problems,which threatens the health of millions of residents globally including China,particularly in the Hetao Basin,Inner Mongolia.Reductive dissolution of Fe???oxides and As?V?dissimilatory reductive have been well accepted as key mechanisms for As mobilization in alluvial aquifers.However,the release mechanisms of adsorbed arsenate into groundwater under geogenic conditions remain unclear.This investigation has been focused on high arsenic groundwater from the Hetao Basin.In-situ batch and column experiments were conducted with different types of iron oxides?e.g.,ferrihydrite,goethite,hematite?to elucidate the release and mechanisms of Fe oxides-adsorbed arsenate into groundwater as well as the factors controlling As release.This study shows its significance in both theory and practice.Results of in-situ batch experiments and column experiments indicated that Fe???-oxide minerals controlled the patterns of arsenic release to groundwater.For ferrihydrite-sand,arsenic was initially desorbed,followed by reductive dissolution,while As desorption occurred predominantly on goethite and hematite sand.The percentage of released As was higher for ferrihydrite and goethite-coated sand,followed by hematite-coated sand.Redox conditions were the main controls on the mobilization of As.Anoxic environments promoted reductive dissolution of ferrihydrite and dissimilative reduction of As?V?,enhancing the release of As into groundwater.Arsenic desorption was promoted by dissolved HCO₃^-,DOC,phosphate and silicate.Mineral transformation of ferrihydrite to lepidocrocite and goethite/or mackinawite also contributed to As release.The presence of organic matter favored As release to groundwater.Natural organic matter coated Fe???oxide minerals and thus led to the smaller specific surface areas.This promoted the formation of As-Fe-NOM complex,which enhanced As mobility.However,dissolved acetate suppressed As???adsorption onto Fe???oxides,and resulted in elevated As???concentrations in groundwater.Microbial community structure and diversity were controlled by groundwater geochemistry and organic matters.The richness and diversity of microbial communities were higher in high arsenic groundwater as compared to those in low arsenic groundwater.Along the experimental column,the microbial community diversity decreased from inlet to outlet,and the percentage of Hydrogenophaga,Comamonadaceae,Pseudomonas reduced,while the percentage of Flavobacterium increased.Sequence analysis of 16S rDNA demonstrated that Pseudomonas and Flavobacterium played a significant role in reductive dissolution of Fe oxide minerals.The presence of organic matter enriched the diversity of microorganism and altered its structure,which promoted the formation of anoxic conditions and the release of adsorbed arsenic from sediments into groundwater.