Sorption of uranium(VI) onto hydrous ferric oxide-modified zeolite: Assessment of the effect of pH, contact time, temperature, selected cations and anions on sorbent interactions

Zeolites are commonly used as adsorbents for metal removal in most applications e.g. in wastewater. However, the ubiquity of iron in such systems may, in the long-term, distort the true interactions and mechanisms of contaminant removal as a result of modification of the zeolite surface. In this study, this potential phenomenon was assessed for the removal of uranium(VI) from aqueous solution by hydrous ferric oxide-modified zeolite (HFOMZ). This was prepared by precipitating iron hydroxide (the common precipitate of iron in aqueous systems) onto zeolite. The prepared HFOMZ was characterised by the scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR). Batch adsorption experiments were performed to assess the effect of: pH, initial uranium(VI) concentration, adsorbent dosage, contact time, temperature, presence of cations (Pb2+, Cr3+, Cu2+, Mn2+ and Co2+) and anions (SO42-, CO32- and HPO42-) on the adsorption of uranium(VI). Kinetic studies under these conditions indicated that the pseudo second-order kinetic model (R-2 > 0.99) best described the adsorption behaviour, implying that this could be proceeding through a chemisorption process. The experimental data was best described by the Freundlich isotherm model (R-2 > 0.93), an implication that the adsorption surface was heterogeneous. The thermodynamic parameters calculated from the experimental data suggested that the adsorption of U(VI) onto HFOMZ was spontaneous and exothermic in nature. The adsorption of U(VI) onto HFOMZ was dominated by complexation with strong ionizable hydroxyl sites on the hydrous ferric oxide surfaces and the edge sites of the zeolite. At pH values from 2 to 6, increased adsorption was observed and this decreased at higher pH values (above 6). This corresponded with the changes in speciation as determined by the PHREEQC modelling code. The presence of the cations (Pb2+, Cr3+, Cu2+, Mn2+ and Co2+) and anions (CO32- and HPO42-) resulted in a significant decrease in the adsorption capacity of U(VI) by HFOMZ, implying that in a system where these anions and cations are present in high concentrations over time, U(VI) will adsorb less onto the material. (C) 2017 Elsevier Ltd. All rights reserved.