Magnesium activation affects the properties and phosphate sorption capacity of poultry litter biochar
Joshua T. Padilla, D. W. Watts, Jeffrey M. Novak, Vasile Cerven, James A. Ippolito, Ariel A. Szögi, Mark G. Johnson
Abstract
Abstract Biochars with a high affinity for phosphorus (P) are promising soil amendments for reducing P in agricultural runoff. Poultry litter (PL) is an abundant biochar feedstock. However, PL-derived biochars are typically high in soluble P and therefore require chemical modification to become effective P sorbents. This study investigated the effect of magnesium (Mg) activation on extractable P (EP) and P sorption capacities of PL-derived biochars. Biochar was produced at 500–900 °C from PL activated with 0–1 M Mg. Three differentially aged PL feedstocks were evaluated (1-, 3–5-, and 7–9-year-old). Increased Mg activation level and pyrolysis temperature both resulted in EP reductions from the biochars. Specifically, biochars produced at temperatures ≥ 700 °C from PL activated with ≥ 0.25 M Mg had negligible EP. X-ray diffractograms indicated that increased Mg loading favored the formation of stable Mg 3 (PO 4 ) 2 phases while increasing temperature favored the formation of both Mg 3 (PO 4 ) 2 and Ca 5 (PO 4 ) 3 OH. Maximum P sorption capacities (P max ) of the biochars were estimated by fitting Langmuir isotherms to batch sorption data and ranged from 0.66–10.35 mg g −1 . Average P max values were not affected by PL age or pyrolysis temperature; however, biochars produced from 1 M Mg-activated PL did have significantly higher average P max values ( p < 0.05), likely due to a greater abundance of MgO. Overall, the results demonstrated that Mg activation is an effective strategy for producing PL-derived biochars with the potential ability to reduce P loading into environmentally sensitive ecosystems.