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

The potential photocatalytic activity of Mt (montmorillonite) after Fe(III) doping was measured for the photo catalytic remediation of hexavalent chromium (Cr(VI)). Mt was modified with Fe(III) at pH = 2 with IAP <= 1.5 center dot 10(-38) < Ksp((Fe(OH)3)) = 6.0 center dot 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.

Automated summary

This study investigates the photocatalytic activity of Fe(III)-doped montmorillonite (Mt) for the remediation of hexavalent chromium (Cr(VI)). Fe(III) doping enhances the photocatalytic activity of Mt, and the transformation of Cr(VI) to Cr(III) is mainly facilitated by Fe(III) doping on the interlayer and aluminosilicate layers of Mt. Higher Fe(III) doping leads to higher photocatalytic efficiency, but excessive Fe(III) reduces the efficiency due to electron trapping and overlapping with active sites. The mechanism of the photocatalytic property of Fe(III)-doped Mt is revealed for the first time, which involves the formation of a new Fermi level and narrowing of the band gap of Mt.