Effect of dissolved organic carbon from sludge, Rice straw and spent coffee ground biochar on the mobility of arsenic in soil

Abstract

To date, studies on the mobility of arsenic (As) in soil amended with biochar have primarily relied on broad empirical observations, resulting in a gap between the behavior of As in amended soil and the chemical mechanisms controlling that behavior. This study focuses on the influence of abiotic factors in As mobility in As-contaminated soils amended with biochar. In order to understand the leaching of DOC and phosphate across a range of biomass feed-stock and pyrolysis temperature, rice straw and granular sludge from an anaerobic digester were pyrolyzed at 300, 550, and 700 degrees C, and subjected to leaching studies by mixing air dried soil with 10 wt% of biochar at a soil: water ratio of 1:1(w/v). The concentration of DOC in the presence of granular sludge biochar and rice straw biochar increased from 190 mg L-1 to 2605 mg L-1 and 1192 mg L-1, respectively, which considerable accelerated the mobilization of Fe and As. More specifically, DOC drove the reduction of Fe (III) to Fe (II). Our results suggest enhanced release of As via the reductive dissolution of iron oxides, including by the chelating-enhanced dissolution of Fe oxides, and competitive desorption by DOC and phosphate from biochar. The influence of DOC and phosphate was further evaluated using realistic application amounts (1, 3, and 5wt%) of biochars derived from pyrolysis of granular sludge, rice straw and spent coffee ground at 300 and 550 degrees C. The results from these experiments further confirm that DOC is a key factor for influencing the mobility of As in the amendment of biochar to As-contaminated soil, which indicates that biochar having low levels of leachable carbon should be amended to As-contaminated soils, and with caution.