Simultaneous recovery of nitrogen and phosphorus from biogas slurry by Fe-modified biochar

In this study, corn straw (CS) impregnated with FeCl3 solution was pyrolyzed at different temperatures to obtain the Fe modified biochar (Fe-BC) for nitrogen and phosphorus recovery from biogas slurry. Biochar pyrolyzed at 690 °C for 80 min showed the highest NH4+-N (N) and PO43−-P (P) removal efficiencies. The characterization techniques indicated that Fe element in Fe-BC mainly existed in forms of Fe0 and Fe3O4, and Fe(OH)3 and FePO4 after adsorption. The adsorption process was mainly controlled by the chemical process and could be well fitted to the pseudo-second-order kinetic equation and the Langmuir isotherm model. Based on the Langmuir model, the maximum adsorption capacities of Fe-BC were 11.68 mg.g−1 (for N) and 26.14 mg.g−1 (for P), which were increased by 131.7% and 253.2%, respectively, compared with unmodified biochar. N and P in biogas slurry are removed mainly by electrostatic attraction, intermolecular force, ion exchange, coprecipitation, flocculation, and functional group reactions. The Fe-BC can be used as a soil fertilizer since it gradually released about 10% of the adsorbed nutrients.