Humic Substances from Sludge Compost with Biochar Addition: An Effective Washing Agent for Arsenic Removal from Contaminated Soil

Matured sewage sludge compost is a rich source of humic substances (HS) with reduction capacity. To improve the reduction performance of sludge compost and to obtain a mature compost, the effects of moisture content, aeration rate and biochar addition on sludge compost were analyzed. HS with optimal reduction performance was selected for the removal of arsenic (As) in soil to investigate the removal efficiency and environmental risk. The result showed that the aeration rate and the addition of biochar had a significant effect on the total reduction capacity (TRC) of the compost. The compost with a moisture content of 65%, an aeration rate of 0.2 L<middle dot>min-1<middle dot>kg-1 and biochar addition possessed the highest TRC (2062 mu mol/g). A removal efficiency of 25% was obtained by washing As-contaminated soil with extracted HS. There was no effect on the concentration of metals, physicochemical properties and phase composition of the soil after washing with HS. Further EEM studies revealed that quinoid functional groups of HS were involved in the washing process and converted between hydroquinone, benzoquinone and quinone oxide after the reaction. Reducing functional groups separated and released arsenic and iron minerals by reductive dissolution. Recycle experiments showed that HS could be used three times. HS from sewage sludge compost is a green and economical washing agent.