Enhanced Reclamation of Phosphorus from Biogas Slurry Using Ca/Zn-Modified Biogas Residue Biochars

The utilization of biogas residue and biogas slurry is critical to the sustainable development of the anaerobic digestion technique. Hence, various functional biochars were prepared via pyrolysis of treated biogas residue for the effective sorption of phosphorus (P) from biogas slurry in the present study. The calcium (Ca)- and zinc (Zn)-modified biochars derived from biogas residue achieved a P removal rate of over 99% in aqueous solutions with an initial concentration of 110 mg-P/L. Furthermore, Ca-modified biochar exhibited the highest phosphate sorption capacity (150.7 mg/g), and it reached sorption equilibrium in the shortest time (2 h), which was well described by Langmuir and pseudo-second-order models. As for the actual biogas slurry, 95.6% of P was successfully recovered by Ca-modified biochar after membrane separation. Electrostatic attraction, ligand exchange, hydrogen bond, ion exchange, and surface precipitation were identified as the potential mechanisms for P sorption onto the modified biochar. Density functional theory calculations further confirmed the great P sorption capacity of modified bichar, owing to higher sorption energies (−7.01 to −7.67 eV) than the original biochar (−1.36 to −1.46 eV). The diffusive gradients in the thin-film results showed that the available P content in soil amended with biochar after P sorption (115.77–141.22 μg/L) was much higher than that in soil (23.08 μg/L), highlighting the good potential of modified biochar after P sorption as a promising biochar-based fertilizer for agricultural production.