Shinjiro Sato, Graduate School of Engineering, Soka University, Hachioji-shi, JAPAN and Shinjiro Sato, Department of Environmental Engineering for Symbiosis, Soka University, Hachioji, Japan
Sewage sludge biochar (SSB) is expected to provide essential plant nutrients when applied to soils. However, heavy metals in sewage sludge may be concentrated during carbonization process, therefore soil pollutions by heavy metals from SSB application may be of great concern. In this study, absorption of heavy metals (Cu, Zn, Cd, Pb) in a plant grown in soils applied with SSB and their fractionation in postharvest soils were investigated. SSBs (< 300 µm) pyrolysed by two different temperatures: low (300oC: SSB-L) and high (800oC: SSB-H) were applied to two different soils (< 2 mm): low-humic Andisol and decomposed granite (DG) soil at rates of 10% (v/v) in 1 L pot. A bioassay experiment was conducted using Japanese mustard spinach (Brassica rapa). Fractionation of the heavy metals was performed using modified BCR four-step sequential extraction procedure (water-soluble, acid-soluble, reduced, and oxidized and residual fractions). The bioassay experiment showed no symptoms of excess absorption of any heavy metals. The spinach dry matter in Andisol was 2.91 g in the control, 2.37 g in SSB-H, and 0.16 g in SSB-L treatments. The spinach in DG soil did not grow with SSB-L treatment, and the dry matter weights under control and SSB-H were not significantly different. The dry matter weights were positively related to soil pH with correlation coefficient being 0.849 (p<0.01). The heavy metals concentrations in plant in Andisol SSB-L treatment were higher compared with those in control and SSB-H treatments. The dry matter weights in Andisol SSB-L treatment showed a negative correlation with 4 heavy metals concentrations in plant (r2= –0.62~–0.77; p<0.01), which suggested negative effect of heavy metals from SSB-L on plant growth. Therefore, the effects of SSB application to soil on heavy metal absorption in and growth of plant may vary depending on the characteristics of SSB and soils, which need further clarification.