Complexation Reactions of Oxyanions on Brucite Surfaces

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  • ブルーサイト表面におけるオキシ陰イオンの錯形成反応
  • ブルーサイト ヒョウメン ニ オケル オキシ インイオン ノ サクケイセイ ハンノウ

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Abstract

Complexation reactions of environmentally important oxyanions such as nitrate (NO_3), sulfate (SO_4), phosphate (PO_4), and arsenate (AsO_4) on brucite (Mg(OH)_2) surfaces were investigated to understand the role of brucite in immobilization of hazardous oxyanions at hyperalkaline environment. The impacts by surface complexation reactions on solid state properties of brucite were also evaluated. Synthetic brucite was used for the adsorption and post-adsorption experiments as a starting material. In the adsorption experiments, the synthesized brucite was reacted with various oxyanion solutions with concentration up to 100μmol/L for 12 hours at 25 degrees Celsius. The obtained adsorption isotherms showed that brucite had the selectivity for oxyanions during adsorption with the affinity sequence being NO_3<SO_4≪PO_4<AsO_4. To distinguish the adsorption mode (inner-sphere complexes or outer-sphere complexes) of these oxyanions, zeta potential measurements were performed. For the brucite with NO_3 and SO_4, there is little change in the zeta potential in the pH range 10.5 to 10.7. However, the zeta potential of brucite with adsorbed PO_4 decreased at pH 10.6 when the adsorbed amounts increased. The zeta potential of brucite with adsorbed AsO_4 also decreased with increasing amounts adsorbed. This shows that PO_4 and AsO_4 were attached to the brucite surfaces by a strong interaction additional to the electrostatic balance. Isoelectric point (pH_<IEP>) of the brucite was also determined by zeta potential measurements. The pH_<IEP> of brucite with NO_3 and SO_4 was close to 13.0. On the other hand, pH_<IEP> of the brucite with PO_4 was approximately 10.7 and that of the brucite with AsO_4 was close to 10.2. These results indicate that NO_3 and SO_4 presented as diffuse ions on the brucite surfaces. In contrast, PO_4 and AsO_4 formed inner-sphere complexes by a ligand-substitution reaction on the brucite surfaces. It took notice of influences on the solubility of brucite caused by the formation of inner-sphere complexes. The saturation indexes (SI) of brucite with PO_4 and AsO_4 have a range from 0.24 through 0.39, and were lower than that of sample without the oxyanions. The decreasing in SI indicates that the solubility of brucite is decreasing by the inner-sphere complexes with PO_4 and AsO_4. Therefore, it implies that the adsorption reactions of PO_4 or AsO_4 induced the notable decreasing in the solubility of brucite itself. It is revealed that the formation of inner-sphere complexes on the brucite surfaces contributes to the stabilization of brucite as well as the decreasing mobility of these oxyanions. Consequently, the adsorption mode of anions on brucite should be determined by chemical properties inherent in the elements; oxyanions with low ionic potential have a tendency to form inner-sphere complexes with brucite surfaces.

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