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 (As3 + ) and arsenate (As5 + ) by RH-Fe-MCM-41 at the molar ratio of Si/Fe 10 gave removal efficiencies of 47% and 99% for As3 +  and As5 + , 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 As3 +  and 2 h at pH 4 for As5 + . The adsorption capacity by the Langmuir constants (Qm) for As3 +  and As5 +  were 416.67 μg g−1 and 277.78 μg g−1, respectively, while the Freundlich constants (KF) were 0.583 μg g−1 for As3 +  and 112.87 μg g−1 for As5 + . The fit for As5 +  was good with both the Langmuir and Freundlich isotherms by considering the R2 values, which were quite similar (0.916 and 0.912); As3 +  fit well only with the Langmuir isotherm. The impregnation of iron (Fe3 + ) into the RH-Fe-MCM-41 structure increases the adsorption capacity of As3 +  and As5 +  both by oxidizing As3 +  to As5 +  and by co-precipitation with As5 + .