Removal of sulfide from aqueous solutions using novel alginate–iron oxide magnetic hydrogel composites

Magnetic hydrogel composite beads were prepared by impregnating iron oxide nanoparticles (NP) into macroporous calcium alginate gel (Alg) and used as an adsorbent (Alg/iron oxide-NP) for the removal of sulfide from aqueous solutions. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray diffraction (XRD) and Fourier transform infra-red (FTIR) spectroscopic analyses were performed to characterize the iron oxide nanoparticles and the Alg/iron oxide-NP for exploring the mechanism involved in sulfide removal. The experimental results showed that adsorption of sulfide on Alg/iron oxide-NP beads reached equilibrium within 3 h. At pH 7.5 and 25 °C, Alg/iron oxide-NP was capable of reducing sulfide concentration from 64.3 to 1.1 mg/L, thus achieving more than 98% removal. The removal of sulfide decreased with the increase in solution ionic strength. The equilibrium isotherm studies obeyed Langmuir model better than Freundlich model. The maximum uptake capacity was found to be 136.9 mg/g. Pseudo-second-order model described the kinetics of sulfide adsorption having rate constant 0.004 g mg⁻¹ min⁻¹. The proposed mechanism and the experimental data evidenced that chemical interaction was involved in adsorption wherein elemental sulfur and sulfates were produced as byproducts. The adsorbent was regenerated with calcium chloride solution. In five adsorption–desorption cycles, the adsorbent lost less than 5% of its removal efficiency. The magnetic property of the iron oxide nanoparticles enabled easy and rapid separation of the adsorbent from the solution via magnetic field. The synthesized magnetic composite had comparable if not superior adsorption capacity in comparison with carbon-based adsorbents.