Natural chromite ore as an efficient low-cost adsorbent for phosphate removal: mechanistic and kinetic insights

Abstract

Human activities are gradually increasing the PO43− levels in aquatic systems that accelerated eutrophication and degraded freshwater quality. Therefore, searching for low-cost and efficient adsorption treatments is crucial for sustainable PO43− control. This study investigates the potential of natural chromite ore to adsorb PO43− ions from aqueous solution while targeting adsorption kinetics, process performance, and model validation. Despite chromite containing chromium, its stable spinel structure sequesters poorly soluble Cr (III), inhibiting its mobilization in conditions relevant to water treatment. Chromite physicochemical properties were characterized using SEM-EDX, FTIR, XRD, BET and point zero charge to identify surface characteristics. Key parameters, i.e., solution pH (2–12), temperature (298–318 K), initial PO43− concentration (1–5 mg L− 1), adsorbent dosage (0.1–1 g), and contact time (0–80 min), were optimized to improve PO43− removal efficiency. The highest PO43− adsorption was achieved at pH of 2, 298 K temperature, adsorbent dose of 0.6 g, at higher initial PO43− concentration of 5 g L− 1 and longer contact time of 50 min. Equilibrium and kinetic data were examined using Langmuir, Freundlich, Dubinin-Radushkevich, Temkin, and diffusion models. These models were validated by probability distribution function, absolute average relative deviation, and corrected Akaike information criterion. Freundlich isotherm and pseudo-second-order kinetic models showed the best fit to the data. This showed that heterogeneous multilayer adsorption dominated on active non-uniform surfaces. Dubinin-Radushkevich and thermodynamic analyses indicated that the process was spontaneous, exothermic, and dominated by a physisorption mechanism. Diffusion modeling revealed dual-controlled adsorption, with the initial stage governed by external film diffusion, followed by intraparticle and surface diffusion as the rate-determining process. Maximum PO43− adsorption of 4.05 mg g− 1 at adsorption efficiency of 90% was achieved under optimized conditions. These findings emphasize the ability of chromite ore as cost effective and operationally relevant adsorbent for sustainable PO43− ion removal in water treatment technologies.