Influence of the iron-oxide mass fractions of magnetic powdered activated carbon on its hexavalent chromium adsorption performance in water

Abstract

Magnetic powdered activated carbon (Mag-PAC) is an effective adsorbent to remove hexavalent chromium (Cr(VI)) from water and can be recovered for reuse. However, the tradeoff between the adsorption performance of Cr(VI) and magnetic properties of Mag-PAC remains unclear. Herein, we prepared a series of Mag-PAC adsorbents containing various iron-oxide mass fractions with FeSO4·7H2O as the precursor, using a facile wet-chemical precipitation route and conducted batch experiments to evaluate the Cr(VI) adsorption performance. Results revealed that Mag-PAC was functionalized by magnetic iron oxide comprising crystalline goethite and magnetite structures. Furthermore, its adsorption performance was highly dependent on pH and was most effective at an initial solution pH of 2. Both the sorption rate constant and Cr(VI) adsorption capacity were greatly influenced by magnetization, and they gradually decreased as the iron-oxide mass fraction increased. Among the prepared adsorbents, Mag-PAC-75 (∼32% wt iron) exhibited not only an excellent Cr(VI) adsorption performance (Langmuir adsorption capacity: 75.76 mg/g) but also effective magnetic properties (saturation magnetization: 9.66 emu/g). Coexisting anions had a negligible competitive effect on Cr(VI) removal by Mag-PAC-75 at an initial pH of 2, whereas the presence of tannic acid markedly improved the Cr(VI) elimination. The presence of trivalent chromium on the surface of Mag-PAC-75 confirmed via X-ray photoelectron spectroscopy indicated that some synergistic redox reactions may occur during the sorption process. After five regeneration cycles using NaOH, Mag-PAC-75 continued to exhibit a high Cr(VI) removal efficiency and magnetic stability. These findings indicate that optimizing the adsorption performance and magnetic properties is a key factor for realizing the practical application of Mag-PAC for Cr(VI) removal. Overall, Mag-PAC may have been a promising application prospect for Cr(VI) removal from water due to its high adsorption capacity and magnetic properties, coupled with its good reusability and magnetic stability after regeneration cycles.