Arsenic Fraction And Transformation In Soils Contaminated By Sewage Irrigation

Sewage irrigation causes contamination of agricultural soils,which leads to accumulation of metal/metalloids in agricultural systems.As one of common contaminants,arsenic is poisonous and extremely harmful to human health.Previous studies have mainly focused on arsenic in deep aquifers.However,few studies have been carried out to investigate speciation and migration and transformation of arsenic in contaminated soil.This study took sewage irrigated soils to investigate speciation and migration and transformation of arsenic in contaminated soil with batch experiments and column experiments.Major conclusions are shown as follows.(1)Soils from both sewage-contaminated profiles(QY-51 and QY-53)and the background profile(QY-87)are alkaline.The surface soil(0-20 cm)in the contaminated profiles are polluted by As,Pb,Zn,Cu and other elements.Contents of those elements showed obviously decreasing trends along the depth,indicating that sewage irrigation led to accumulation of metal/metalloids on the surface soils.Arsenic contents in surface soils from the contaminated profiles QY-51 and QY-53 were 25.1 and 50.6 mg/kg,respectively.In the two contaminated profiles,the main fractions of soil As were crystalline Fe oxide mineral bound form(S5)and strongly adsorbed form(S2).(2)The results of experiments on iron oxide reactivity indicated that the reduction conditions were conducive to the release of arsenic in soils.The studied soils had very low initial reduction rates,and Fe(?)-oxide minerals were highly heterogeneous,and had high initial reaction activity.Although Fe-oxides with low reaction rates were difficult to be dissolved,Fe(?)-oxides would be reduced in the soils due to their high activity.(3)Column experiments showed that the contaminants on the surface soil were transported into the deep soil layer with the water,and arsenic would readily be released and migrated from the surface layer to the deep layer.The weakly adsorbed arsenic(S1)and the strongly adsorbed arsenic(S2)were easily dissolved and released,while the arsenic co-precipitated with silicates(S6)and residual arsenic(S7)were difficult to be dissolved and released.(4)Introduction of organic matter greatly increased the release of arsenic,followed by the acid rain infiltration.Organic-mediated microorganisms promoted reduction of Fe(III)oxides,and therefore release of arsenic,while acidic conditions increased the proton-induced dissolution of Fe-oxides.After the increase in water level,the dissolved oxygen content in the column decreased,and consequently the released As,Fe and Mn were increased and migrated into deep layers.