環境・リサイクル

[3401-11-04] Elucidation of pH dependent sorption mechanism of chromate with green rust sulfate through XAFS analysis

○Abdullah Al Mamun¹, Aina Onoguchi¹, Giuseppe Granata¹, Chiharu Tokoro¹ (1. Waseda University)

司会：和嶋 隆昌（千葉大学）

Keywords:Chromate, Green rust, pH, Sulfate replacement, Surface passivation

Green rust (GR) is an iron oxide consisting of Fe^II and Fe^III hydroxide units intercalated with water molecules and anions such as Cl^-, CO₃^2- or SO₄^2-. Since GR has a good ability towards the immobilization toxic metal anions like CrO₄^2- and SeO₄^2-, many researches focused on its formation and reaction kinetics. However, the reaction mechanism and its dependence on the pH requires further elucidation. In this work, we investigated the effect of pH on the preparation of sulfate-GR and its removal properties towards chromate. We prepared two types of GR at two different pH, namely pH 8.75 (GR_8.75) and 7.50 (GR_7.50) and we conducted the chromate removal experiments at pH 5 and 9. During the preparation of GR, increase of pH from 7.50 to 8.75 in confirmed a larger inclusion of SO₄^2- and Na⁺ within the GR interlayer and almost doubled surface area. The higher pH also determined the enlargement of the crystal lattice and the increase of single corner Fe-Fe coordination via oxygen. The chromate removal by GR_8.75 was found to be 1.7 times more efficient than GR_7.50. At pH 5, GR_8.75 oxidized significantly to goethite whereas magnetite was the predominant product at pH 9. The surface passivation of GR_8.75 as a consequence of a surface reduction of the chromate ion confirmed by XPS. Instead, the GR_7.50 removed the chromate mostly via replacement of the sulfate in the interlayer. For both GRs, the EXAFS results confirmed the change of the Cr-Cr bond to chromite-like bidentate mononuclear Cr-Fe bonding. For GR_8.75, the increase of Fe-Fe edge sharing and double corner sharing coordination numbers confirmed the surface-based redox reaction. On the other hand, the increase of single corner sharing Fe-Fe coordination via oxygen for GR_7.50 was a confirmation of the lateral insertion of Cr in the interlayer of GR.