Silica from rice husk for sludge-based biochar modification: As a novel adsorbent for lead

Converting municipal sludge into biochar through pyrolysis is an ideal method to realize waste utilization, but the pristine biochar usually has the disadvantage of relatively low adsorption efficiency. Modification with silicon is an effective means to improve adsorption capacity. Different from the general silica source, this study creatively extracted silica from rice husk waste for biochar modification to prepare a novel silica-rich biochar (SiBC). Adsorption experiments showed that the maximum adsorption capacity of Pb2+ reached 230.47 mg/g when the ratio of rice husk ash (RHA)/carbon was 1:2, which was 6.43 times higher than that of the pristine biochar. Compared with pristine biochar, SiBC has a higher specific surface area and porosity and also has a new functional group structure on the surface. The main mechanisms of Pb2+ adsorption by SiBC were electrostatic adsorption, ion exchange, functional group complexation, and precipitation. In addition, SiBC demonstrated good interference resistance in coexisting cation interference experiments and also showed certain adsorption capacity for Pb in real water bodies. In conclusion, this study exhibited an innovative approach to recycling rice husk and sludge for resource utilization and provided a new method for the removal of Pb2+ from an aqueous solution.