Removal of chromium from water using manganese (II, III) oxides coated sand: adsorption and transformation of Cr(VI) and Cr(III)

A manganese coated sand (MCS) sorbent containing manganese (II,III) oxides was developed for adsorption and transformation of chromium [Cr(VI) and Cr(III)] with potential application in flow-through permeable media adsorption filters. Characterization of the MCS sorbent using XRD and XPS showed that the oxides of manganese (II) and manganese (III) were present on the MCS sorbent surface. Adsorption of both Cr(VI) and Cr(III) onto the MCS sorbent occurred over a broad pH range from 3 to 10. Surface charge analysis of the MCS sorbent determined a pHPZC of 7.8, which may facilitate the uptake of both oxy-anionic Cr(VI) species and cationic Cr(III) species. Favorable adsorption of Cr(VI) and Cr(III) onto the MCS sorbent occurred according to the Langmuir and the Freundlich adsorption equations, with a higher adsorption capacity for Cr(III) than Cr(VI). Adsorption parameters from the Langmuir, the Freundlich and the Temkin adsorption equations showed a stronger binding of Cr(VI) than Cr(III). Adsorption of Cr(III) decreased with increasing calcium concentration while adsorption of Cr(VI) decreased with increasing concentration of common anions found in natural water in the following order: phosphate > sulfate> bicarbonate. Transformation of chromium occurred on the surface of the MCS sorbent due to the partial reduction of Cr(VI) and the partial oxidation of Cr(III), which may be attributed to the role of surface manganese (II,III) oxides as either reducing or oxidizing agents. The MCS sorbent is a recyclable and sustainable adsorbent for removal of chromium from water with an environmental impact comparable to ion-exchange technology.