Removing arsenic from water with an original and modified natural manganese oxide ore: batch kinetic and equilibrium...

Arsenic contamination of drinking water is a serious water quality problem in many parts of the world. In this study, a low-cost manganese oxide ore from Vietnam (Vietnamese manganese oxide (VMO)) was firstly evaluated for its performance in arsenate (As(V)) removal from water. This material contains both Mn (25.6%) and Fe (16.1%) mainly in the form of cryptomelane and goethite minerals. At the initial As(V) concentration of 0.5 mg/L, the adsorption capacity of original VMO determined using the Langmuir model was 0.11 mg/g. The modified VMOs produced by coating VMO with iron oxide (Fe^a-VMO) and zirconium oxide (Zr^a-VMO) at 110 °C and 550 °C achieved the highest As(V) adsorption capacity when compared to three other methods of VMO modifications. Langmuir maximum adsorption capacities of Fe^a-VMO and Zr^a-VMO at pH 7.0 were 2.19 mg/g and 1.94 mg/g, respectively, nearly twenty times higher than that of the original VMO. Batch equilibrium adsorption data fitted well to the Langmuir, Freundlich, and Temkin models and batch kinetics adsorption data to pseudo-first order, pseudo-second order, and Elovich models. The increase of pH progressively from 3 to 10 reduced As(V) adsorption with a maximum reduction of 50-60% at pH 10 for both original and modified VMOs. The co-existing oxyanions considerably weakened the As(V) removal efficiency because they competed with As(V) anions. The competition order was PO₄^3- > SiO₃^2- > CO₃^2- > SO₄^2-. The characteristics of the original and modified VMOs evaluated using SEM, FTIR, XRD, XRF, surface area, and zeta potential explained the As(V) adsorption behaviour.