Photocatalytic reductive applications of C-doped ZrO₂/PANI composite towards Cr(VI)

Carbon doped zirconium oxide polyaniline, C-ZrO₂/PANI was synthesised for application in the photocatalytic reduction of Cr(VI) under visible light irradiation. The carbon doping agent applied was carboxymethyl cellulose. C-ZrO₂ was obtained via the precipitation method. Coupling of the polyaniline to the C-ZrO₂ occurred by in situ oxidative polymerization to obtain the C-ZrO₂/PANI composite. XPS and EDS showed presence of C, Zr, O, N in the C-ZrO₂/PANI sample. The physiochemical characteristics of the photocatalyst such as a large surface area of 31,47 m²/g, good charge separation were important aspects in the adsorption and subsequent Cr(VI) reduction. Band edge positions for the C-ZrO₂/PANI deduced from UV–Vis DRS and Mott-Schottky plot showed that the lowest unoccupied molecular orbital (LUMO) of PANI was below the C-ZrO₂ conduction band (CB). Further, photogenerated electrons from LUMO were transferred to C-ZrO₂ CB and were involved in the generation of superoxide radicals, ^•O₂⁻, which were critical in Cr(VI) reduction to Cr(III). Comparably the photocatalyst, C-ZrO₂/PANI_SDS, prepared in the presence of surfactant sodium dodecyl sulfate (SDS) indicated diminished photocatalytic reduction of Cr(VI). From the photocatalytic results, performances of the photocatalysts were in the order C-ZrO₂/PANI > PANI > C-ZrO₂/PANI_SDS. Photocatalytic results revealed that 100% of 20 mg/L Cr(VI) was reduced by C-ZrO₂/PANI within 70 mins at pH 2. The first order reaction rate constant was 0.0309 min⁻¹. The photocatalyst can be reused for 4 cycles without excessive loss of efficiency and with maintained stability.