Isotherms, kinetics and mechanism analysis of phosphorus recovery from aqueous solution by calcium-rich biochar produced from biosolids via microwave pyrolysis

Phosphorus (P) scarcity and eutrophication have triggered the development of new approaches for phosphorus recovery. This study investigated the impact of calcium-doped biochar, produced from biosolids via microwave pyrolysis at 700 °C for 20 min, on phosphorus recovery. The phosphorus removal isotherms, removal kinetics and the impact of initial pH of phosphorus solution on phosphorus recovery were studied. The phosphorus recovery was proportional to the calcium content in biochar, leading predominantly to the production of brushite. Precipitation was the main mechanism of phosphorus removal by calcium-doped biochar. Phosphorus removal capacity of biochar reached equilibrium after 8 h of contact time and was described by a pseudo-second-order kinetic model. The Langmuir isotherm model fitted the experimental data well with a maximum adsorption capacity of 147 mg-P/g for biochar BC20 (20 wt% Ca(OH)2). The results of phosphorus removal from a real wastewater demonstrated that using Ca-doped biochar for phosphorus removal from contaminated streams is a promising alternative for phosphorus recovery as well as for biosolids management. Heavy metal leaching from Ca-doped biochar was much lower than from undoped biochar, and decreased as the amount of calcium in biochar increased.