Copyrolysis of Biomass with Phosphate Fertilizers To Improve Biochar Carbon Retention, Slow Nutrient Release, and Stabilize Heavy Metals in Soil

Two phosphate fertilizers, triple superphosphate (TSP) and bone meal (BM), were premixed with sawdust and switchgrass biomass for pyrolytic biochar formation. Carbon retention, P release kinetics, and capacity of biochar for stabilizing heavy metals in soil were evaluated. Results show that TSP and BM pretreatment increased carbon retention from 53.5–55.0% to 68.4–74.7% and 58.5–59.2%, respectively. The rate constants (k2) of P release from the TSP- and BM-composite biochars are 0.0012–0.0024 and 0.89–0.91, respectively, being much lower than TSP and BM themselves (0.012 and 1.79, respectively). Copyrolysis with phosphate fertilizers enhanced biochar capability for stabilizing metals in soil significantly, especially the BM-composite biochar which increased Pb, Cu, and Cd stabilization rates by up to about 4, 2, and 1 times, compared to the pristine biochars. During the pyrolysis process, Ca(H2PO4)2 in TSP converted to Ca2P2O7 and reacted with biomass carbon to form C–O–PO3 or C–P, leading to greater carbon retention and lower P release. PO43– in both composite biochars could precipitate with heavy metals, resulting heavy metal immobilization in soil. This study indicates that copyrolysis of biomass with P-containing fertilizers could obtain multiple environmental benefits.