Highly efficient phosphate extraction from water using bio-composites of nano zero valent iron supported on orange peel powder (nZVI@OPP): performance evaluation and mechanistic insights

In recent times, nZVI composites have been developed as environmentally friendly adsorbents to tackle the issue of eutrophication in freshwater bodies. Herein, we synthesized nano zero valent iron loaded orange peel powder (nZVI@OPP) in different proportions (1:1, 1:3, 1:5, and 1:10) and investigated its PO₄³⁻ elimination potential from water. Among them, nZVI@OPP (1:5) composite presented excellent PO₄³⁻ removal performance (93.3%) comparable to that of 1:1 (100.0%) and 1:3 (98.9%), and therefore was selected for further analysis. The physicochemical properties of nZVI@OPP (1:5) also showed porous and irregular surface with more available sorption sites and reactive functional groups than planar and crystal surface of raw OPP, as revealed by SEM–EDX, XRD, FT-IR, and elemental mapping. The optimum conditions (nZVI@OPP (1:5) dosage: 2 g/L, contact time: 60 min, pH: 7, initial PO₄³⁻ concentration: 10 mg/L, and temperature: 298 K) indicated 93.3% PO₄³⁻ removal from simulated water samples. Based on higher R² values, PSO kinetic and Langmuir isotherm models showed better fitting with PO₄³⁻ sorption data. Moreover, various coexisting anions posed a negative impact on PO₄³⁻ removal in the given order: NO₃⁻ < SO₄² < Cl⁻ < mixed anions, while no significant impact of thermal variations on PO₄³⁻ removal was observed. The spent nZVI@OPP (1:5) also showed reasonable reusability potential when removing PO₄³⁻ from aqueous solution. The dominant PO₄³⁻ removal mechanisms including physisorption, chemisorption, ligand exchange, and complexation reactions were identified. In general, the current study provides new insights into the importance of selecting appropriate mixing proportion of nZVI and OPP, with the potential of extracting maximum PO₄³⁻ content from water considering economic and waste management perspective.

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