Preparation of ball-milled phosphorus-loaded biochar and its highly effective remediation for Cd- and Pb-contaminated alkaline soil

Pyrolytic biochar is a good material for remediating soils contaminated with heavy metals; however, it exhibits strong alkalinity, which easily causes soil alkalization and fertility reduction. Herein, a series of novel biochar materials (BPBCs) were prepared by combined ball milling and phosphorus (P)-loading. The optimized BPBC were fabricated in the basis of Cd and Pb adsorption capacities of the biochar, with pyrolysis at 700 °C, ball milling for 12 h and the addition of 5% red P (BPBC700). Ball milling could effectively grind pristine biochar into submicron particles and nanoscale P particles could be uniformly loaded on BPBC700. Moreover, the oxidative conversion of red P into phosphorus oxides, phosphoric acid and (hydro)phosphates was promoted due to reactions with the carbonates, alkaline minerals and O-containing functional groups of biochar. These reactions also decreased the biochar and soil pH to nearly neutral by acid-base neutralization. Pot experiments showed that BPBC700 had better effects than the pristine or ball-milled biochar in improving soil properties (e.g., cation exchange capacity and organic carbon), increasing the concentrations of soil nutrients (e.g., N and P), promoting alkaline phosphatase, catalase and urease activities, decreasing soil mobility and plant accumulation of Cd and Pb, and alleviating Cd and Pb stress on maize plants. Thus, BPBC is a promising and ecofriendly amendment to enhance its adsorption ability on Cd and Pb, soil quality and plant productivity in contaminated soil.