Adsorption of Oxyanions and Post-Adsorption Behavior of Schwertmannite

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  • シュベルトマナイトへの様々なオキシ陰イオンの吸着とその後の安定性
  • シュベルトマナイト エノ サマザマナ オキシ インイオン ノ キュウチャク ト ソノゴ ノ アンテイセイ

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Abstract

Adsorption and post-adsorption characteristics of different oxyanions such as P, Cr, As and Se onto schwertmannite were investigated in order to understand the role of schwertmannite in acid mine drainage, and its performance and safety assessment in remediation system. Synthetic schwertmannite was used for the adsorption and post-adsorption experiments as a starting material. In the adsorption experiments, the starting material was reacted with various oxyanion solutions such as Na_2HPO_4, Na_2CrO_4・4H_2O, Na_2HAsO_4・7H_2O and Na_2SeO_4 with concentration up to 2.0mM at around pH 3.9. Dominant chemical species of the oxyanions in the experimental condition were thermodynamically calculated as H_2PO_4^-, HCrO_4^-, H_2AsO_4^- and SeO_4^<2-> by Geochemist's Workbench. The obtained adsorption isotherms showed that the schwertmannite has selectivity for oxyanions during adsorption with the affinity sequence being H_2AsO_4^->H_2PO_4^->HCrO_4^->SeO_4^<2->. This sequence corresponds to the sequence of decreasing ionic potential of the anions, which suggest that the observed selectivity of schwertmannite could not be attributed to the electrostatic interaction between the oxyanions and schwertmannite. It is known that oxyanions with low ionic potential have a tendency to form inner-sphere complexation with mineral surfaces. Therefore, the observed order of oxyanion retention may correspond to the sequence in affinity for the inner-sphere complexation on schwertmannite. This is also supported by the results of zeta potential measurement, which showed greater decrease of zeta potential after H_2AsO_4^- and H_2PO_4^- adsorption compared to other onyanions. In post-adsorption experiments (aging experiments), transformation to goethite was observed in X-ray diffraction patterns for the schwertmannite containing oxyanions with lower selectivity such as SeO-4^<2-> and SO_4^<2-> while there is no change in X-ray diffraction patterns of schwertmannite with H_2AsO_4^- and H_2PO_4^- even after 556 hours. Desorption of SeO_4^<2-> and SO_4^<2-> was observed prior to the transformation, although most of H_2AsO_4^- and H_2PO_4^- were retained onto schwertmannite. The desorption of oxyanions therefore resulted in the transformation of schwertmannite to goethite. Oxyanions with higher adsorption affinity, i.e., lower ionic potential, have a greater stabilizing effect on schwertmannite structure, although ordinary schwertmannite with SO_4^<2-> is metastable and easily transforms to goethite.

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