Adsorption of arsenic in aqueous solution by RH-Fe-MCM-41

RH-Fe-MCM-41 adsorption efficiency and capacity were examined with arsenic in lower concentrations in aqueous solution under varying pH values. The adsorption of arsenite (As^3 +) and arsenate (As^5 +) by RH-Fe-MCM-41 at the molar ratio of Si/Fe 10 gave removal efficiencies of 47% and 99% for As^3 + and As^5 +, respectively, in the 100–1,000 μg l⁻¹ concentration range. For the adsorption test of arsenic the best conditions were found to be an equilibrium time of 12 h at pH 7 for As^3 + and 2 h at pH 4 for As^5 +. The adsorption capacity by the Langmuir constants (Q_m) for As^3 + and As^5 + were 416.67 μg g⁻¹ and 277.78 μg g⁻¹, respectively, while the Freundlich constants (K_F) were 0.583 μg g⁻¹ for As^3 + and 112.87 μg g⁻¹ for As^5 +. The fit for As^5 + was good with both the Langmuir and Freundlich isotherms by considering the R² values, which were quite similar (0.916 and 0.912); As^3 + fit well only with the Langmuir isotherm. The impregnation of iron (Fe^3 +) into the RH-Fe-MCM-41 structure increases the adsorption capacity of As^3 + and As^5 + both by oxidizing As^3 + to As^5 + and by co-precipitation with As^5 +.