The sorption mechanism of arsenate [As(V)] on schwertmannite was investigated by means of batch sorption experiments as a function of As(V) concentration in acidic solution at 25 °C. Structural simulation indicated that the surface sites of schwertmannite comprised various O atom (or hydroxyl) and SO4 groups. Sorption experiments showed that the reactive sites for As(V) sorption are surface-coordinated SO4 groups rather than surface hydroxyl groups, as reported in earlier studies. The As(V) sorption mechanism involves ligand exchange with surface-adsorbed and structural SO4. The results of the sorption experiments also suggested monodentate As(V) coordination at the surface-adsorbed SO4 sites [(Fe1)2SO4] and bidentate As(V) coordination at the structural SO4 sites [(Fe3)2SO4]. The overall ligand-exchange reaction was

\[0.61\ (Fe_{1})_{2}SO_{4}\ +\ 0.39\ (Fe_{3})_{2}SO_{4}\ +\ 1.61\ H_{2}AsO_{4}^{{-}}\ {\rightarrow}\ 1.22\ Fe_{1}H_{2}AsO_{4}\ +\ 0.39\ (Fe_{3})_{2}HAsO_{4}+\ 0.39\ H^{+}\ +\ SO_{4}^{2{-}}\]

where the 1 and 3 in Fe1 and Fe3 are coordination numbers. The equilibrium constant derived for the exchange reaction, log KEX = 4.96, describes the observed As(V) sorption behavior. Nanocrystalline materials like schwertmannite are widespread in nature and typically contain significant amounts of anionic impurities, such as sulfate and silicate. Our results indicate that the effects of impurities can be significant and should be considered in order to gain a realistic understanding of sorption processes in natural systems.

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