Effects of biochar derived from wheat straw and chicken manure on the immobilization of Cu, Zn and Cd in sludge composting

The increasing production of sludge has added to the sewage treatment burden, making its disposal an important challenge for urban environmental management. Biochar has great potential to effectively reduce the mobility and bioavailability of heavy metals in municipal sludge due to its adsorption capacity. This study conducted a 20-day sewage sludge composting experiment with the addition of wheat straw and chicken manure biochars to test their effects on the sewage sludge physicochemical properties and immobilization of copper (Cu), zinc (Zn), and cadmium (Cd) in the sewage sludge. During composting, the physicochemical properties of the treatments changed to different degrees compared to the original pile. Specifically, pH, CEC, TP, and TK increased, while the content of OM and TN decreased. As a result of the concentration effect, the total contents of Cd, Cu, and Zn increased in both of the C1 and C2 treatments. In the present study, heavy metals were stabilized more effectively by the wheat straw biochar than that derived from the chicken manure. Both types of biochar are effective at immobilizing Cd, Cu, and Zn in the compost, while WSB is more effective. These results are affected by indirect physicochemical properties of the compost, as well as the direct ion exchange, complexation and precipitation. Therefore, these results indicate that it is feasible to use biochar, especially derived from the wheat straw, to immobilize heavy metals in the sludge.