Effect of ionic Fe(III) doping on montmorillonite for photocatalytic reduction of Cr(VI) in wastewater

Abstract

The potential photocatalytic activity of Mt (montmorillonite) after Fe(III) doping was measured for the photocatalytic remediation of hexavalent chromium (Cr(VI)). Mt was modified with Fe(III) at pH = 2 with IAP ≤ 1.5 ∙ 10–38 <Ksp(Fe(OH)3) = 6.0 ∙ 10 -38 to prevent the precipitation of Fe(OH)3. Doped ionic Fe(III) was located mainly in the interlayer space but presented partially on the Mt layers. XRD, FTIR, XPS and XANES/EXAFS analyses suggested that Fe(III) should exist in the cationic form. The photocatalytic characterization of the obtained Fe/ Mt suggests that Mt gained photocatalytic activity after Fe(III) ion doping. The photocatalytic transformation of Cr(VI) to harmless trivalent chromium (Cr(III)) on Fe/ Mt was also dominated by Fe(III) doping on the Mt interlayer and Mt aluminosilicate layers. Higher Fe(III) doping correlated with higher photocatalytic efficiency with the Fe(III) adsorbed on Mt at a ratio lower than 0.274 mmol/g. A further intercalation of Fe(III) reduced the photocatalytic efficiency, which could be attributed to the excess Fe(III) providing a recombination state for the electrons, trapping the electrons from the Cr(VI) reduction, and the extra Fe(III) overlapping with the active sites receiving irradiation. Based on the characterization and photocatalytic activity test, the mechanism of the photocatalytic property of Fe(III)-doped Mt was revealed for the first time. The new Fermi level formed between the forbidden bands narrowed the band gap of Mt, endowing the Mt with photocatalytic activity. This finding may contribute to explaining the role of Fe/ Mt in Fe/ Mt-based photocatalytic composites.