Sludge-derived biochar and its application in soil fixation

Pyrolysis of sewage sludge into biochar is a particularly promising disposal approach for municipal sewage sludge because the pyrolysis process can effectively stabilize heavy metals in sludge to reduce the ecological risk. The by-product sludge-derived biochar (SDBC) can provide considerable active sites for metal complexation and alkalinity for precipitation. Hence, the SDBC is considered as an efficient sorbent for immobilization of cationic Pb(II), oxyanionic Cr(VI), and As(III) species, as well as atrazine. The pyrolysis conditions can influence the yield and properties of the produced SDBC. The SDBC prepared at 400°C for 2 h or 500°C for 1 or 1.5 h seems to own an excellent sorption capacity for contaminants with high Brunauer-Emmett-Teller surface area and low extractable metal levels in itself, as many organic functional groups and phosphorous components still remain. When SDBC is applied in the metal contaminated soil, it can effectively reduce the mobility of the cationic metals, such as Pb(II), Cu(II), Ni(II), and Cd(II), as well as the oxyanionic Cr(VI) and As(III). The long contact time favors their stabilization. When SDBC is exposed in the air or flushed by soil solution, surface oxidation and acidification occur to change the properties of SDBC, influencing the metal stabilization by SDBC. Further investigation on the long-term interactions among soil components, metal species, and SDBC should be performed.