Adsorption of hexavalent chromium from water using manganese-aluminum coated sand: Kinetics, equilibrium, effect of pH and ionic strength.

Removal of hexavalent chromium [Cr(VI)] from water was evaluated using a low-cost coated sand adsorbent for potential application in a flow-through filter system using permeable adsorption media. Manganese-aluminum coated sand (MACS) was investigated as a hybrid metal oxide based adsorbent due to the potential efficacy of manganese oxide and aluminum oxide for adsorption of Cr(VI) from water. Adsorbent characterization was performed using XRD, SEM/EDX, XPS and BET. Adsorption experiments were performed to determine adsorption capacity and kinetics. The effect of pH, common co-existing ions found in natural water, and the recyclability of adsorbent were investigated. Adsorbent characterization showed that the MACS sorbent contained aluminum oxide, manganese (III) oxide and manganese (IV) oxide. Adsorption followed the Langmuir and Freundlich adsorption equations, indicating favorable adsorption of Cr(VI) onto the MACS sorbent, while results from the Dubinin-Radushkevich equation were suggestive of physical adsorption of Cr(VI). Cr(VI) adsorption onto the MACS sorbent followed pseudo-second order kinetics. The adsorbent was effective in removing Cr(VI) over a broad pH range from 3 to 9.5, while surface charge analysis confirmed the adsorption of Cr(VI) onto the acidic surface of the MACS sorbent with a pHPZC of 9.72. The presence of co-existing ions bicarbonate, sulfate and phosphate in water resulted in a decrease in Cr(VI) uptake in the following order: phosphate > bicarbonate > sulfate. The presence of calcium resulted in a slight increase in Cr(VI) uptake. The MACS sorbent is a recyclable sorbent for adsorption and removal of Cr(VI) from water within 30 minutes of contact time.