A Mechanism of Molybdate Adsorption on αFe2O3

Abstract

The adsorption of molybdate on αFe2O3 (hematite) was studied at pH 4.0 and 7.75 by equilibrium dialysis at 22.0 and 40.0C. Two distinct adsorption reactions, both following the Langmuir equation, occurred at pH 4.0. The first reaction was insensitive to a change in temperature and appeared saturated at an equilibrium concentration of about 10 ppm Mo. The second reaction was saturated at 55 ppm Mo and was endothermic. The reactions were ascribed to the adsorption of monomeric molybdate ions at two different sites on the iron oxide. The endothermic nature of the adsorption at the higher concentration was considered dues to the formation of polymolybdate from monomeric condensation. At pH 7.75, the adsorption of molybdate on the oxide surface was about 5 times less than at pH 4.0. The reaction in the alkaline system was not sensitive to temperature change. A comparison of the surface areas of three molybdate forms with nitrogen surface areas of the molybdated oxides indicated that the monomeric, HxMoO2x−2, form was principally adsorbed. It was concluded that the condensation of the paramolybdate form, Mo7O24−6, was initiated as the temperature was increased at the high Mo concentrations. The behavior of the equilibrium function with increasing surface coverage for the second adsorption reaction supported the suggested mechanism.