Adsorption and oxidation of arsenic by ultra-long α-MnO2 nanowires with the (1 1 0) surface

Abstract

There are many areas in the world where the ground water has been contaminated by arsenic. MnO₂ is one of the most cheap materials which can both adsorb arsenic and oxide arsenite [As(III)] to arsenate [As(V)]. In this study, the Ultra-long α-MnO₂ nanowires with the (1 1 0) surface are used to remove As(III) and As(V) in solution under different conditions. The adsorption kinetics for both As(III) and As(V) are fitted well with the pseudo second-order kinetic model. For example, the pseudo second-order adsorption kinetic constants are 0.076×10³ g μg⁻¹ min⁻¹ and 0.077×10³ g μg⁻¹ min⁻¹ at pH 7 for As(III) and As(V), respectively. At the same time, the Langmuir model is better than the Freundlich model to describe the adsorption equilibrium isotherms. For example, the maximum adsorption capacity values of the α-MnO₂ nanowires obtained from the Langmuir model are 1.012×10³ μg g⁻¹ and 1.158×10³ μg g⁻¹ at pH 7 for As(III) and As(V), respectively. The adsorption capacities of the α-MnO₂ nanowires are affected by the pH values and increased with the arsenic initial concentration. Furthermore, the As(III) oxidation rate can be denoted by the pseudo zero-order kinetic model and is related with the O₂ concentrations, the different light sources, the initial concentrations of the As(III) and the pH values. For example, the pseudo zero-order kinetic equation is y = −0.90x+163.6 with 4 L min⁻¹ of O₂, dark condition, 0.2×10³ μg L⁻¹ of As(III) and at pH 7. At last, the oxidation mechanism is investigated and the oxidant in this reaction should be related with O₂ not the Mn(II). Besides its special structure and easy separation ability, it is a promising material to remove arsenic in water.