Synthesis of an Alginate-Based Fe3O4-MnO2 Xerogel and Its Application for the Concurrent Elimination of Cr(VI) and Cd(II) from Aqueous Solution.

In this study, magnetite-manganese oxide (Fe3O4-MnO2) nanoparticles were synthesized and immobilized on alginate, producing a magnetite-manganese oxide xerogel (mMOX). This eco-friendly xerogel was used as an adsorbent of Cr(VI) and Cd(II). It was mesoporous and thermally stable, as determined by Brunauer-Emmett-Teller and thermogravimetric analysis. A scanning electron microscope coupled with an energy-dispersive X-ray system, Zetasizer, and attenuated total reflectance-Fourier transform infrared were used for characterization of adsorbents. The performance of the mMOX was investigated for the simultaneous adsorption of Cr(VI) and Cd(II) at different temperatures, pH values, contact times, initial concentrations of the adsorbate, and adsorbent doses. The developed xerogel (mMOX) showed high adsorption capacities of 3.86 mg/g for Cr(VI) and 3.95 mg/g for Cd(II) on 120 min of contact time with 5 ppm Cr(VI) and Cd(II) solution. The kinetic data fitted well with the pseudo-second order, while the Freundlich isotherm model was found to be fit for adsorption data. Thermodynamic study revealed the adsorption to be spontaneous and exothermic. The adsorbent showed useful application for real water samples by more than 75% uptake of Cr and Cd with low adsorption of Na, K, and Mg. The regeneration study indicated that the mMOX could be reused up to six cycles with more than 50% removal of Cr(VI) and Cd(II) ions from aqueous solution with minimal leaching of metal ions (Fe, Ca, Na, K, and Mn) into the solution.