A novel Ca/Mn-modified biochar recycles P from solution: mechanisms and phosphate efficiency

The excessive use of phosphate leads to severe environmental issues, such as a shortage of phosphorus resources and water eutrophication. In this study, a novel Ca-Mn-impregnated biochar (CMBC) composite was synthesized by co-pyrolysis at 600 °C for P recovery from solution. The efficiency of P-sorbed CMBC as a fertilizer was assessed via pot experiments. Kinetic experiments exhibited a higher phosphate sorption efficiency for the CM₁BC and CM₂BC composites. Pot experiments revealed that P-sorbed CMBC treatment significantly improved plant growth and yield. Compared with those of the control, the dry weight, fresh weight, and P content of rape leaf (root) P₅₀-CMBC_1.0% treatment increased by 108.6% (350%), 88.7% (396%), and 132.8% (109.3%), respectively. This may be due to the porous surface structures that develop during the treatment and Ca-P precipitation on P-sorbed biochar partly dissolved in the slightly acidic soil (pH: 6.25). The maximum P adsorption values fitted by the Langmuir isotherm model were up to 9.15, 3.19, 15.58, and 20.84 mg g^-1 for CBC, MBC, CM₁BC, and CM₂BC, respectively. The mechanism of phosphate recovery mainly includes electrostatic attraction, surface precipitation, and the formation of inner-sphere complexes with hydroxyl groups that were assessed using BET, zeta potential, SEM-EDS, FTIR, and XPS analyses. These mechanisms suggest that phosphate may be desorbed from the P-laden sorbent, which was consistent with the results of the pot experiments and desorption experiments in aqueous media. These results imply that P-sorbed modified biochar has the potential to be a promising P fertilizer in soil.