Effectiveness and mechanisms of phosphate adsorption on iron-modified biochars derived from waste activated sludge

[Display omitted] •The maximum P adsorption capacity of FeCl3-modified biochar was 111.0mg/g.•The P removal efficiency of Fe3+-attached biochar was better than Fe3O4.•Fe3+-modified biochar showed good P removal ability in wider concentration range.•P removal rate by Fe3+-modified biochar kept over 60% after 5 times reuse. Excessive discharge of phosphate (P) into the surface water is the key factor to cause the eutrophication, so its removal has aroused much attention in recent years. In this study, different iron modification (chemical co-precipitation of Fe3+/Fe2+ or FeCl3 impregnation) was used to improve the phosphate adsorption capacity of waste activated sludge (WAS)-based biochar. Comparative tests demonstrated that the FeCl3-impregnated WAS-based biochar exhibited much superior phosphate adsorption capacity (111.0mg/g) in all as-prepared samples and performed well even under the interferences with pH and coexisting ions. X-ray diffraction (XRD) analyzes indicated that the iron in FeCl3-impregnated WAS-based biochar existed mainly in amorphous phase, as hematite and amorphous hydroxides forms, which was of great benefit to the phosphate adsorption. Besides, ligand exchange plays important role in the adsorption of phosphate. The WAS-based biochar kept over 60% phosphate removal efficiency after five recycles.