Copyrolysis of Recycled Plastics and Biomass Reduces Biochar Bioavailable Silicon Production and Cadmium Phytotoxicity

Plastic-biomass copyrolysis is often used to recycle solid waste and obtain highly aromatic biochar, which can be used for pollutant adsorption. Bioavailable silicon in derived biochar is of great importance to stabilize and alleviate phytotoxicity of soil heavy metal. However, the exact role of plastic-biomass copyrolysis in biochar bioavailable silicon is not well understood. Copyrolysis of different plastics with silicon-containing biomass (such as rice straw) was carried to investigate transformation of silicon in biochar. It was found that biomass inherent alkaline carbonate (such as K2CO3) reacted with silicon to form K2SiO3, and its dissolution led to a high amount of bioavailable silicon in biochar. In the meantime, copyrolysis of heteroatom-containing plastics (such as polyvinyl chloride and triphenyl phosphate) with biomass inhibited K2SiO3 formation in a dose-dependent manner, which mainly involves the capture of biomass inherent K2CO3 to drive new biochar mineral (KCl and K3PO4) formation. Lack of K2SiO3 formation reduced the bioavailable silicon content of copyrolyzed biochar and simultaneously decreased the Brunauer–Emmett–Teller surface area because of consumption of K2CO3 (porogen). It was also revealed that plastic-biomass copyrolysis reduced Cd stabilization capacity of its derived biochar. Moreover, according to wheat seed germination experiments, copyrolyzed biochar exhibited weak capacity to reduce Cd phytotoxicity. These crucial findings highlight the complex nature of using copyrolyzed biochar.