Valorization of olive pomace into gadolinium-modified biochar for phosphate removal from wastewater

Phosphate (PO 4 3− ) discharge from municipal wastewater (MWW) is a major driver of eutrophication, necessitating efficient and sustainable removal technologies. This study aims to develop a gadolinium-modified biochar (Gd-BC) derived from olive pomace waste for enhanced PO 4 3− removal. Gd-BC was synthesized via co-pyrolysis and assessed for PO 4 3− uptake performance using synthetic solutions and real municipal wastewater (MWW). Physicochemical characterization confirmed the successful Gd incorporation and a marked increase in surface area from 1.2 to 20 m 2 /g. Batch experiments demonstrated removal efficiencies of up to 96.5% at 1 g/L using 40 mgPO 4 3− /L, a maximum uptake capacity of 108.11 mg/g, and optimal removal of 97.2% at pH 4. The Gd-BC composite further exhibited high selectivity, maintaining 70–90% PO 4 3− removal in the presence of common anions and cations. In real MWW, removal efficiencies exceeded 98% at an initial PO 4 3− concentration of 37.1 mg/L using dosages of 1.25–2.0 g/L, confirming its effectiveness under complex conditions. Mechanistic analyses, supported by kinetic, isotherm, and thermodynamic modeling together with material characterization, revealed that PO 4 3− removal is mainly associated with chemisorption via inner-sphere complexation and surface coordination, in addition to physisorption via electrostatic attractions. Overall, these findings demonstrate that Gd-BC is a promising rare-earth-based biochar for efficient PO 4 3− remediation in wastewater treatment applications.