Effect of water extract of compost on the adsorption of arsenate by two calcareous soils

The increasing mobility of arsenate will increase its leaching potential to groundwater and uptake by plants. The mobility of arsenate in soils is related to the competitive adsorption with other substances. The effect of organic substances on the adsorption of arsenate by soils was evaluated using the water extract of compost (WEC) as a complex anion source in a batch experiment. Two calcareous slate alluvial soils, Chiwulan and Shuipientou, with higher arsenic contents of 23.7 and 12.9 mg kg(-1), respectively, were used. The Langmuir equation has been used to describe successfully the As adsorption isotherm for the two soils. The maximum adsorption of As was 6.098 and 4.785 mumol g(-1) for Chiwulan and Shuipientou soils, respectively. There was competitive adsorption for binding sites on the soils between arsenate oxyanions and organic anions derived from the dissolved organic carbon (DOC) of WEC. Different critical pH values were for arsenate addition related to arsenate adsorption on both soils in the absence of DOC of WEC but not in the presence of DOC of WEC. The soil properties related to arsenate adsorption by the two soils may govern the critical pH values.