Abstract
Battery grade γ-MnO2 powder was investigated as an oxidant and an adsorbent in combination with Fe/Al coagulants for removal of arsenic from contaminated water. Simultaneous oxidation of As(III) and removal by coprecipitation/adsorption (one step process) was compared with pre-oxidation and subsequent removal by coprecipitation/adsorption (two step process). The rate of As(III) oxidation with MnO2 is completed in two stages: rapid initially followed by a first order reaction. As(III) is oxidised to As(V) by the MnO2 with a release of approximately 1:1 molar Mn(II) into the solution. No significant pH effect on oxidation of As(III) was observed in the pH range 4 - 6. The rate showed a decreasing trend above pH 6. The removal of As(V) by adsorption on the MnO2 decreased significantly with increasing pH from 4 to 8. The adsorption capacity of the γ-MnO2 with particle size 90% passing 10 µm was determined to be 1.5 mg/g at pH 7. MnO2 was found to be more effective as an oxidant for As(III) in the two step process than in the one step process.
Highlights
Arsenic in contaminated groundwater occurs largely as arsenite (As(III)). [1] Effective and complete removal of arsenic by adsorption/coprecipitation methods requires pre-oxidation of As(III) to As(V)
Chen and Fang [28] reported that the oxidation rate of As(III) by MnO2 was rapid initially followed by a firstorder kinetics with respect to As(III) concentration
This paper reports the investigation of MnO2 as an oxidant for As(III) and as adsorbent in combination with Fe/Al coagulants for As(V) removal
Summary
Arsenic in contaminated groundwater occurs largely as arsenite (As(III)). [1] Effective and complete removal of arsenic by adsorption/coprecipitation methods requires pre-oxidation of As(III) to As(V). Oxygen or air is a cheap but kinetically slow oxidant for As(III). Various other oxidants for As(III) have been reported in the literature, including permanganate ( MnO4 ), [2,3,4] ozone (O3), [5] hydrogen peroxide (H2O2), [6] chlorine (Cl2), [7,8,9,10] or hypochlorite (ClO−), [11,12,13] catalyzed sulphite/O2 (air) mixture, [14,15] and UV catalyzed systems. The oxidation process was reported to be limited by diffusion of the reactant As(III) to or the reaction products away from the surface [27,28,29]
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