Adsorptive recovery of phosphate using iron functionalized biochar prepared via co-pyrolysis of wheat straw and sewage sludge

In recent years, the removal and recovery of phosphate (PO₄³⁻) from freshwater reservoirs using carbonaceous adsorbents has received much attention to address eutrophication issues and plant phosphate requirements. The viability of FeCl₃ impregnated biochar (Fe@CBC) synthesized via co-pyrolysis of wheat straw (WS) and sewage sludge (SS) for phosphate removal from water under systematically designed sorption experiments and its subsequent potential as phosphatic fertilizer for improving plant growth, was thoroughly investigated in this study. The relatively higher PO₄³⁻ sorption performance of Fe@CBC (5.23 mg/g) compared to FeCl₃ impregnated biochars (Fe@WBC: 4.16 mg/g and Fe@SBC: 5.14 mg/g) synthesized via separate pyrolysis of WS and SS were primarily ascribed to the nano porous structure, higher point of zero charge (pH_pzc) and enriched iron complexes on its surface (Fe-OH and FeC). Consequently, dominant sorption mechanism of PO₄³⁻ ions towards Fe@WBC was associated to ligand exchange and chemisorption whereas that of Fe@SBC and Fe@CBC was identified as electrostatic surface complexation coupled with reduction. In comparison to Fe@WBC and Fe@SBC, the surface properties and identified phenomenon allowed Fe@CBC to efficiently recover PO₄³⁻ ions under optimal water chemistry conditions and coexisting interfering species environment. Additionally, PO₄^3- -sorbed Fe@CBC effectively improved the physical growth (root length: 2 cm, shoot length: 9 cm, fresh weight: 79 mg and dry weight: 8.3 mg) of mustard plants. Economic analysis suggested profit of PO₄^3- removal and recovery by Fe@CBC was $1.5 per kg. Therefore, PO₄^3- -sorbed Fe@CBC could be a promising phosphatic fertilizer for improving plant growth and may have agricultural applications.