Role of pH in green rust preparation and chromate removal from water

The removal of hexavalent chromium from water by sustainable methods still presents challenging aspects. Green rust (GR) is a mixed Fe(II)-Fe(III) layer double hydroxide intercalated with anions and water molecules that was recently found capable of immobilizing low concentrations of chromate. Nevertheless, the influence of pH on GR preparation and chromate removal mechanism has not been fully clarified yet. This work elucidated the influence of pH on GR preparation and chromate removal by sulfate-GR. Two types of GR were prepared at two different pH, namely pH 8.75 (GR_8.75) and 7.50 (GR_7.50), and were used in chromate removal experiments at pH 5 and 9. XRD, XPS and XAFS analysis were carried out to assess the change of phase composition, surface oxidation state and crystal structure. Increasing the preparation pH from 7.50 to 8.75 produced a larger inclusion of SO₄ ²⁻ and Na⁺ within the GR interlayer and resulted in a larger crystal lattice and more surface area. GR_8.75 was 1.7 times more efficient than GR_7.50 and 6.7 times more efficient than ferrihydrite as Fe required to remove 10 mg/L of chromate from water. While removing chromate, GR_7.50 released a larger amount of SO₄ ²⁻ than GR_8.75 in spite of a lower initial content. At pH 5, GR_8.75 reduced chromate and oxidized mostly to goethite, whereas magnetite was the main oxidation product at pH 9. In contrast, GR_7.50 removed chromate and transformed into Cr-intercalated ferrihydrite. XPS results confirmed the larger passivation of GR_8.75. All results indicated that GR_8.75 removed chromate mainly via surface reduction whilst GR_7.50 removed it mostly via replacement of SO₄ ²⁻ in the interlayer prior to reduction. EXAFS analysis of solid residues highlighted the presence of bidentate mononuclear FeCr₂O₄-like Cr–Fe bonding as well as Cr₂O₃-like Cr–O and Cr–O–Cr bonding under all investigated conditions. The increase of Fe–Fe edge sharing and double corner sharing coordination numbers in the final solid product upon chromate removal by GR_8.75 suggests a surface-based reaction between GR and chromate. In contrast, for GR_7.50 upon chromate removal, the increase of single corner sharing Fe–Fe coordination via oxygen can be resulted from lateral insertion of chromate into GR interlayer.