Arsenic removal by manganese-doped mesoporous iron oxides from groundwater: Performance and mechanism

FeMn bimetallic oxides have been widely used in catalytic adsorption due to their large pore size, large specific surface area and mesoporous structure, which have great potential for high As groundwater remediation. In this study, FeMn composite oxide was synthesized by template-free route and forming mesopores through high temperature calcination, and its efficiency and mechanism for As removal were subsequently investigated. The results showed that the different Fe/Mn molar ratios and calcination temperatures have important effect on FeMn composite oxides performance. For all synthesized materials, the largest specific surface area is 388.6 m²/g of Fe₁Mn₁-300. The maximum As absorption capacity was also reached by Fe₁Mn₁-300, which is 59.44 mg/g for As(III) and 31.68 mg/g for As(V), respectively. As removal efficiency was further evaluated through batch adsorption experiments conducted with five variables, initial As concentration, adsorption equilibrium time, pH, solid-to-liquid ratio, and competitive ions. The adsorption capacity of the material reaches to the maximum when the initial As concentration is 40 mg/L, and that for As(III) and As(V) is 74.05 and 38.09 mg/g, respectively. When the pH rises, the adsorption capacity generally shows a decreasing trend, thus acidic conditions are more conducive to the adsorption reaction. The optimum solid-to-liquid ratios for removal 10 mg/L of As(III) and As(V) are 0.3 mg/L and 1 mg/L, respectively. The order of competitive ions effects on As removal is: PO₄^3- > HCO₃^- > SO₄^2- ≈ NO₃^- ≈ Cl^-. The adsorption mechanisms for As by FeMn composite oxides included adsorption, co-precipitation and oxidative chelation, which was a combination of physical and chemical process.