Adsorption performance of Cr(vi) onto Al-free and Al-substituted ferrihydrites

Abstract

Metal (hydr)oxides are ubiquitous in subsurface sediments. Therefore, investigating the adsorption and desorption of Cr(VI) on ferrihydrite (Fh) and Al-substituted ferrihydrite (FAh) is highly important in understanding the fate and transport of heavy metals in subsediments at their initial sources. In the present paper, both Fh and FAh were prepared on the basis of the abundance of Al and Fe elements in the surface environment. The adsorption of Cr(VI) onto Fh and FAh samples was also studied. The influence of pH, initial concentration, temperature, and some familiar anions such as SO₄²⁻, H₂PO₄⁻, C₂O₄²⁻, CO₃²⁻, and F⁻ on the adsorption of Cr(VI) onto Fh was studied by batch techniques. Results show that FAh exhibited a higher adsorption capacity for Cr(VI) than did Fh. The adsorption capacity for Cr(VI) was found to be 12.97 and 39.79 mg g⁻¹ for Fh and FAh, respectively. The adsorption kinetics in the two systems followed a pseudo-second-order equation and the Langmuir isotherm model. The adsorbed Cr(VI) species mainly existed in the forms *Fe(wk)-OHCrO₄²⁻ and *Fe(wk)CrO₄⁻ on the sample surfaces. Thermodynamic parameters, including the Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°), indicated that the adsorption of Cr(VI) ions onto Fh and FAh was feasible, spontaneous, and endothermic at the temperature range 20–50 °C. Thus, both Fh and FAh can be used as effective, low-cost adsorbents for the treatment of industrial wastewaters contaminated with Cr(VI) ions.