Nature-inspired bio-composites for phosphate sequestration and recovery: Green synthesis, mechanistic insights, and multi-stage optimization

Abstract

Phosphorus-induced eutrophication poses major environmental and economic threats. This study used iron-chitosan composite bead (FeCSB) as an eco-friendly adsorbent for aqueous phosphate removal. The Taguchi method identified pH, contact time, and initial phosphate concentration as key factors. Response surface methodology showed interaction effects and optimal removal conditions. The optimal FeCSB removed 90–97 % phosphate within 120–180 min from waters containing 100 μg P/L to 5 mg P/L. The maximum adsorption capacity of the beads was 57.75 mg/g at neutral pH, and that increased to 117.66 mg/g at pH 2. The adsorption process followed the Freundlich isotherm and the pseudo-second-order kinetic model. The point-of-zero-charge was determined to be 6.8. Electrostatic interaction, complexation, and intra-particle diffusion were identified as the main mechanisms for phosphate removal. XPS analysis revealed both inner-sphere Fe–O–P bonding and outer-sphere interactions involving Fe–OH and amine groups. The FTIR data supported these observations with spectra showing P–O stretching vibrations and shifts in -OH and -NH bands. EDX mapping confirmed the successful incorporation of phosphate into FeCSB. The dried form of the beads showed 27.3 % higher removal relative to the wet form. The use of glutaraldehyde (GA) crosslinker enhanced the mechanical integrity of the adsorbent and reusability over multiple cycles. When applied to municipal wastewater, FeCSB removed >95 % of phosphate. The beads developed here are robust and versatile with potential use for eutrophic water restoration and wastewater treatment.