Comparison of adsorption characteristics of Se(IV) and Se(VI) onto hematite: effects of reaction time, initial concentration, pH, and ionic strength

In this study, the adsorption of selenite [Se(IV)] and selenate [Se(VI)] onto synthesized hematite is investigated under various conditions of reaction time, initial concentration, pH, and ionic strength. The adsorption of 50 µM Se(IV) and Se(VI) reaches steady-state within 5 h and 5 min, respectively, for the condition of 0.8 g/L hematite, 0.01 M NaCl and pH 5.5 ± 0.2. It is found that the parabolic diffusion model is the best to represent the adsorption kinetics of the two Se species onto hematite. On the contrary to the kinetic results, more Se(IV) is adsorbed than Se(VI) at equilibrium by approximately three times under the same physicochemical condition. The maximum adsorption capacities of Se(IV) and Se(VI) onto hematite are 4.04 × 10⁻⁶ and 1.15 × 10⁻⁶ mol/m², respectively, for 10–400 µM initial Se species concentration, 0.8 g/L hematite, 0.01 M NaCl, and pH 5.5 ± 0.2. The Langmuir model is the best to describe the isotherm data of both Se(IV) and Se(VI) adsorption. The adsorption of both Se(IV) and Se(VI) is maximum at acidic pH, showing considerable gap of adsorption capacity between the two Se species. As pH increases, the gap decreases and at pH higher than 10 it becomes negligible. The adsorption envelope of the Se(IV) is insensitive to ionic strength but that of the Se(VI) decreases with increasing ionic strength, indicating the formation of inner-sphere complex and outer-sphere complex for Se(IV) and for Se(VI), respectively.