Mieko Yachigo and Shinjiro Sato, Department of Environmental Engineering for Symbiosis, Soka University, Hachioji-shi, Tokyo, Japan
Sewage sludge-derived biochar (SSB) is expected to provide essential plant nutrients when applied to soils. However, soil pollution by heavy metals from SSB application can also occur and its dynamics are not fully understood. In this study, effects of SSB application to soil on leachability of heavy metals (Cu, Zn, Pb, Cd) from SSB and their absorption by plants were investigated. SSB produced by two different pyrolysis temperatures: low (300oC: SSB-L) and high (800oC: SSB-H) were applied to soil at rates of 5%, 15%, 25% (v/v) and 25%, 50%, 75% (v/v) in 1 L pot, respectively. A bioassay experiment was conducted using Japanese mustard spinach (Brassica rapa) and common bean (Phaseolus vulgaris). Dry matter yields of both plants with 5% SSBC-L rate were significantly increased compared with controls with no SSB or chemical fertilizers applied and other rates including SSB-H application. Dry matter yield decreased with increasing the application rates of SSB-H. Heavy metals concentrations in belowground part were higher than those in aboveground part of both plants. However, none of treatment caused plants to show symptoms of excess absorption of heavy metals because of root’s biochemical mechanism for heavy metal tolerance. Heavy metals left in SSB decreased after the bioassay experiment, implying leachability of heavy metals from SSB. Leachability of heavy metals from SSB to soil was confirmed by differences of 0.1 M HCl-extractable heavy metals in soil samples and the total amounts of heavy metals in SSB analyzed before and after the pot experiment. Cu and Zn concentrations in plants at 75% SSB-H rate were higher than those at other rates. The results in this experiment suggested that almost all heavy metals investigated leached from SSB were adsorbed to soil in a stable form.