Metal chloride-loaded biochar for phosphorus recovery: Noteworthy roles of inherent minerals in precursor

Phosphorus (P) is a valuable resource, while it is vastly lost with wastewater causing eutrophication. In this study, to recover P, composite biochars were prepared by pyrolyzing biowaste impregnated with FeCl3 or MgCl2. It was found that inherent mineral profiles in the biowastes played important roles in interacting with metal chlorides and determined P sorption and precipitation. Specifically, two biowastes containing distinct mineral contents, sawdust and sediment, were selected as model components, being alone or mixed at 1:1 (w/w) to prepare biochars with low, moderate and high mineral contents. Results showed that biochar itself could not absorb P, while loading FeCl3 or MgCl2 achieved P recovery rates of approximate 60–100% and 50–100%, respectively, via electrostatic attraction or ligand exchange of PO43− with –OH/-COOH, which was attributed to the enhanced positive charges and –OH/-COOH on the materials by these metal chlorides. Inherent minerals inhibited FeCl3 transforming into Fe3O4 in pyrolysis and promoted generation of Fe4(PO4)3(OH)3 in P sorption, thus high-mineral content was more appropriate for FeCl3 loading; however, precursors with low-mineral content was suitable for MgCl2 loading, since the bulk-C in biochar acted as porous structure to support MgO crystals with high superficial area (∼255.85 m2 g−1). Besides, FeCl3 and MgCl2 both drove dissolution of inherent minerals significantly, while inherent minerals inhibited release of soluble Fe and Mg2+ into solution, which minimized secondary pollution. This study implied that in constructing composite biochar for catching P, the type of metal chloride should match the inherent minerals in biowastes to maximize P recovery and minimize secondary pollution. [Display omitted] •Composite biochar with incorporation of FeCl3/MgCl2 achieved 50–100% P sorption.•Selection of FeCl3 or MgCl2 as loading agents should consider minerals in biowaste.•High minerals content was appropriate to load FeCl3 for Fe4(PO4)3(OH)3 formation.•Low minerals content was appropriate to load MgCl2 for providing more bulk-C frame.•Minerals inhibited pyrolytic conversion of FeCl3 to Fe3O4, C supported MgO crystals.