Trace Metals in Biochars from Biodegradable By-products of Industrial Processes

Three biodegradable materials (i.e. wood chips (WC), digested sewage sludge (DSS), and lignin (LG)) obtained as by-products of industrial processes were selected for biochar (BC) production under slow pyrolysis conditions at 450 and 700 °C. At 450 °C, the analysed trace metals (Cd, Cr, Cu, Ni, Pb, and Zn) showed the same temperature trend with concentrations which varied depending on the feedstock, increasing in BCs from WC (by as much as 191 %) and DSS (by as much as 288 %) and decreasing in BC from LG (by as much as 46 %). At 700 °C, in all the BCs, the total concentration of Ni, Pb, and Zn increased (by as much as 135, 248, 283 %, respectively) and Cr decreased (by as much as 69 %) whereas the total concentration of Cd and Cu increased or decreased depending on the feedstock. The most suitable pyrolysis temperature for reducing trace metal leachability and bioavailability (450 or 700 °C) depends on the trace metal considered. The temperature of 450 °C was effective in stabilising Cr and Ni in the analysed BCs as these trace metals were not prone to leaching or present in bioavailable forms. In all the BCs, an increase in pyrolysis temperature made trace metals such as Zn and Cu more stable in the char matrix, decreasing in the bioavailable fractions, hindering leachability of Zn, and decreasing leachability of Cu to less than 1 % of the total Cu concentration. Trace metals such as Cd and Pb did not show a clear temperature trend, increasing or decreasing in the bioavailable or leachable fractions depending on the feedstock.