Kinetics and equilibrium adsorption studies of chromium (VI) and iron (III) from aqueous solution systems using hydroxyapatite, activated carbon and their composites

Abstract

Industrial effluents have become an environmental issue harming the ecosystem, remediation of these effluents is critical in order to mitigate some of this issue. Three adsorbents, activated carbon from sugarcane bagasse (ASB), hydroxyapatite (HAP), and their composites (ncpA), were prepared for the adsorption of Cr (VI) and Fe (III) from wastewater in this work. The hydroxyapatite was synthesized using the wet precipitation method, and the activated carbon was derived from sugarcane bagasse, resulting in a composite with a hydroxyapatite to activated carbon ratio of 1:1. The adsorbents surface and chemical properties were determined by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller 2 2 -1 (BET), and X-ray diffraction (XRD). The BET surface areas were 1.34 ± 0.04 m²/g and 26.4 ± 0.4 m²g⁴ for HAP and ASB respectively. The influence of initial concentration of metal ions, adsorbent dosage, pH, contact time and temperature on the adsorption process were investigated. Two isotherm models and different kinetic models were used in fitting the experimental data. The adsorption of Cr (VI) and Fe (III) fitted well into the Langmuir isotherm model with maximum monolayer adsorption capacities of 19.92 mg/g, 16.69 mg/g and 10.33 mg/g respectively for Cr (VI) and 113.64 mg/g, 113.64 mg/g and 107.54 mg/g respectively for Fe (III) removal onto HAP, ASB and ncpA respectively. The pseudo-second-order model that best suited the kinetic data was chemisorption-controlled, and this is referred to as the mechanism of the adsorption. Sum of square 2 error (SSE) and non-linear chi-square (ꭓ² ) were used to further validate the mechanism.