Surface Reactive Sites and Ion Adsorption Modeling of Schwertmannite

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Surface complexation model of Pb (II), Cu (II), Zn (II), Cd (II), As (V), Cr (VI), F (I), and B (III) ions on the surface of schwertmannite was examined with the fitting program FITEQL. Total numbers of surface sites and their protonation and deprotonation constants were initially determined by acid-base titration data with FITEQL analysis. Then, ion adsorption experiments were performed and the resulting adsorption data were used for modeling of the surface complexation. Results indicate that the cationic ions (Pb (II), Cu (II), Zn (II), Cd (II)) were strongly adsorped at pH>6-8 by complexation with the negatively charged surface sites. Whereas the anionic ions (As (V), Cr (VI), F (I)) were adsorped progressively with decreasing pH due to complexation with the positively charged sites. These adsorption properties on the schwertmannite surface were evaluated by complexation equilibrium constants of the surface species. In addition, effect of adsorped ions on the stability of schwertmannite structure was investigated by aging at 50°C. X-ray diffraction analysis indicated that adsorption of As (V) inhibited strongly phase transition to goethite.
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