Effects of biochar and zero valent iron on the bioavailability and potential toxicity of heavy metals in contaminated soil at the field scale.

Heavy metals (HMs) such as copper, nickel and chromium are toxic, so soil contaminated with these metals is of great concern. In situ HM immobilization by adding amendments can decrease the risk of contaminants being released. A five-month field-scale study was performed to assess how different doses of biochar and zero valent iron (ZVI) affect HM bioavailability, mobility, and toxicity in contaminated soil. The bioavailabilities of HMs were determined and ecotoxicological assays were performed. Adding 5 % biochar, 10 % ZVI, 2 % biochar + 1 % ZVI, and 5 % biochar + 10 % ZVI to soil decreased Cu, Ni and Cr bioavailability. Metals were most effectively immobilized by adding 5 % biochar + 10 % ZVI, and the extractable Cu, Ni, and Cr contents were 60.9 %, 66.1 % and 38.9 % lower, respectively, for soil with 5 % biochar + 10 % ZVI added than unamended soil. The extractable Cu, Ni, and Cr contents were 64.2 %, 59.7 % and 16.7 % lower, respectively, for soil with 2 % biochar + 1 % ZVI added than unamended soil. Experiments using wheat, pak choi and beet seedlings were performed to assess the remediated soil toxicity. Growth was markedly inhibited in seedlings grown in extracts of soil with 5 % biochar, 10 % ZVI, or 5 % biochar + 10 % ZVI added. More growth occurred in wheat and beet seedlings after 2 % biochar + 1 % ZVI treatment than the control, possibly because 2 % biochar + 1 % ZVI simultaneously decreased the extractable HM content and increased the soluble nutrient (carbon and Fe) content of the soil. A comprehensive risk assessment indicated that adding 2 % biochar + 1 % ZVI gave optimal remediation at the field scale. Using ecotoxicological methods and determining the bioavailabilities of HMs can allow remediation methods to be identified to efficiently and cost-effectively decrease the risks posed by multiple metals in soil at contaminated sites.