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[3311-24-12] Removal mechanism of Cr(VI) with green rust based on pH in aqueous solution

^○Abdullah Al Mamun¹, Aina Onoguchi¹, Masao Morita¹, Mitsuaki Matsuoka¹, Chiharu Tokoro¹ (1. Waseda University)

司会: 和嶋隆昌（千葉大学）

Keywords:Green rust, pH, Chromate

Hexavalent chromium is toxic to the ecosystem and difficult to remove in solution. Though several iron oxides found to be effective for chromate removal but it failed to prove high removal efficiency. Green rust (GR) is comprised with layered double hydroxide (LDH) intercalated with anionic species and water molecules. This is famous for its rapid immobilization of toxic metal such as CrO₄^2-, SeO₄^2- and UO₂^2-due to its strong redox potential. Though sulfate containing GR structures are among the most studied forms, however, sorption mechanism with chromate is not well understood. The objective of this study was to reveal sorption mechanism of chromate with GR based on pH. To accomplish the objectives, series of the experiment of a wide range of pH from 5 to 11 involving with various analytic tools were conducted. GR proved more efficient in removing chromate in acidic condition (pH 5). In addition, its removal ability was six times higher than that of ferrihydrite. Usually, GR dissolute in acidic solution but chromate suppressed the dissolution at lower pH thus reduce the potentiality of sulfate release from the GR and promote solid state oxidation. Sulfate replacement by the chromate was not the main mechanism for chromate removal by GR because less sulfate was released during the experiment at pH 5. On the contrary, Fe(II) in GR sparingly dissolute over pH 9, release comparatively more sulfate into solution and finally precipitate as Fe(III) through a redox reaction with chromate. X-ray diffraction result suggested that goethite (α-FeOOH) was dominant as more oxidation occurs at pH 5 whereas magnetite (Fe₃O₄) was dominant over pH 9. Absorption spectrometry suggested that Fe(II)/Fe_tot ratio at pH 9 dropped from 0.72 to 0.46 in one hour which might marching towards pure magnetite (0.33). XRD result of them also resembled the mixture of magnetite and goethite.