Phosphate sorption by iron-activated biochars derived from dephosphatized poultry litter and swine manure

The removal of phosphorus (P) from animal manure (dephosphatation) produces a low-P manure byproduct. In this work, we aimed to upcycle dephosphatized poultry litter (PL) and swine manure (SM) into biochar-based P sorbents via pyrolysis and chemical activation. We evaluated different pre-and post-pyrolysis magnesium and iron (Fe) activation methods and determined that post-pyrolysis Fe-activation was the most effective at increasing biochar's affinity for soluble P. Phosphorus sorption by Fe-activated, dephosphatized poultry litter (FewPL) and swine manure (FewSM) was strongly pH-dependent, with higher P sorption under more acidic conditions. Experimental and modeling results showed that FewPL had a higher affinity for P than FewSM, although both biochars removed ≥95% of added P from solution at pH < 5. Attenuated Total Reflectance-Fourier Transform Infrared spectroscopic analysis of biochar pre- and post-P sorption suggested P association with Fe(III)-OH groups as the dominant adsorption mechanism for both biochars. Measured maximum P sorption capacities (QM, obtained at pH 6.2) from isotherm experiments were 21.4 and 14.1 mg P g−1 for FewPL and FewSM, respectively. To obtain estimates of QM under environmentally relevant P concentrations, we used an alternative batch experimental method whereby biochar was repeatedly equilibrated with 2.5 mg P L−1 (pH 6.2). Under these conditions, QM values for FewPL and FewSM were 1.8 and 1.1 mg P g−1, respectively. Overall, our results demonstrate that pyrolysis and Fe-activation are viable for converting dephosphatized PL and SM into biochar-based P sorbents as an approach for utilizing animal waste byproducts to mitigate P contamination in soil and water.