Reactivity of Fe-amended biochar for phosphorus removal and recycling from wastewater

Abstract

Using biochar to remove phosphorus (P) from wastewater has the potential to improve surface water quality and recycle recovered P as a fertilizer. In this research, effects of iron modification on P sorption behavior and molecular characterization on two different biochars and an activated carbon were studied. A biochar produced from cow manure anaerobic digest fibers (AD) pyrolyzed under NH3 gas had the greatest phosphate sorption capacity (2300 mg/kg), followed by the activated carbon (AC) (1500 mg/kg), and then the biochar produced from coniferous forest biomass (BN) (300 mg/kg). Modifying the biochars and AC with 2% iron by mass increased sorption capacities of the BN biochar to 2000 mg/kg and the AC to 2300 mg/kg, but decreased sorption capacity of the AD biochar to 1700 mg/kg. Molecular analysis of the biochars using P K-edge X-ray absorption near edge structure (XANES) spectroscopy indicated that calcium phosphate minerals were the predominant species in the unmodified biochar. However, in the Fe-modified biochars, XANES data suggest that P was sorbed as P-Fe-biochar ternary complexes. Phosphorus sorbed on unmodified BN biochar was more available for release (greater than 35% of total P released) than the AD biochar (less than 1%). Iron modification of the BN biochar decreased P release to 3% of its total P content, but in the AD biochar, P release increased from 1% of total P in the unmodified biochar to 3% after Fe modification. Results provide fundamental information needed to advance the use of biochar in wastewater treatment processes and recover it for recycling as a slow-release soil fertilizer.