Correlation between Adsorption and Photocatalysis in the Aqueous System Cr(VI)-TiO₂

Abstract

The photocatalytic removal of Cr(VI) (0.80 mM, pH 2) using various commercially available photocatalysts (P25, UV100, PC50) was revisited, with particular attention given to Cr(VI) adsorption (as a Cr(VI)-TiO₂ surface complex) and the formation of a Cr(III) hydroxide layer during the photocatalytic reduction. Cr(VI) adsorption followed a quasi-Langmuir-type isotherm, and the spectra of the Cr(VI)-TiO₂ surface complex were deconvoluted into two Gaussian peaks, red-shifted when a rutile phase was present. Cr(VI) photoreduction exhibited nearly pseudo first-order kinetics, with P25 showing the highest reaction rate. Adsorbed Cr(VI) was reduced by e_CB⁻, and the formed Cr(III) was retained over the TiO₂ surface under non-equilibrium conditions, acting as a new adsorption site for Cr(VI). At longer reaction times, partial dissolution of the Cr(III) layer was observed. These findings suggest that the photoreduction kinetics are primarily governed by the slow adsorption of Cr(VI) onto the Cr(III) deposition layer. As an important conclusion, three consecutive processes never mentioned before take place: (1) reduction of adsorbed Cr(VI), (2) formation of Cr(III) over the photocatalyst and (3) adsorption of Cr(VI) over the deposited Cr(III) layer, together with partial Cr(III) redissolution. This insight provides a deeper understanding of the underlying photocatalytic mechanism.

Keywords

Photocatalysis / Titanium dioxide / Cr(VI) / Cr(VI)-TiO₂ surface complex / Cr(III) deposition

Supplementary Materials

The following supporting information can be found at: https://www.sciepublish.com/article/pii/602. Section S1: Diffraction spectrum of the rutile sample (UV100 calcined at 650 °C). Figure S1: XRD spectrum of the rutile sample. Section S2: Diffuse reflectance spectra of the Cr(VI)-TiO₂ complex formed using different samples. Figure S2: (a) Diffuse reflectance spectra of pure UV100 and UV100 after adsorption equilibrium with solutions containing different [Cr(VI)]₀; (b) remission function spectra of the same samples after subtraction of the remission function of pure UV100. Figure S3: (a) Diffuse reflectance spectra of pure PC50and PC50 after adsorption equilibrium with solutions containing different [Cr(VI)]₀; (b) remission function spectra of the same samples after subtraction of the remission function of pure PC50. Figure S4: (a) Diffuse reflectance spectra of a pure rutile sample and a rutile sample after adsorption equilibrium with solutions containing different [Cr(VI)]₀; (b) remission function spectra of the same samples after subtraction of the remission function of pure rutile support. Figure S5: Normalized remission function spectra of Cr(VI) adsorbed on P25, UV100, PC50, and rutile supports, compared with the absorption spectrum of 250 μM Cr(VI) at pH 2. Section S3: Equations used for multiple Gaussian fittings. Table S1: Multiple Gaussian fitting parameters for initial Cr(VI) concentrations above 250 μM. Figure S6: Tauc plots obtained from diffuse reflectance spectra and calculated optical band gaps for the naked supports. Section S4: Discussion about the Cr(VI)-TiO₂ absorption spectra. Section S5: Cr(VI) adsorption equilibria over the different TiO₂ samples. Figure S7: Compared calculated saturation degrees (θ) of Cr(VI) adsorption equilibrium on (a) P25, (b) UV100, (c) PC50, and (d) rutile. Inset: same data in the 0-20 μM range for P25, UV100, and PC50, and 0-1.5 μM range for rutile. Section S6: Comparison between the pseudo first-order and the mixed zero- + first-order models for the photocatalytic reduction of Cr(VI) over the different TiO₂ samples. Figure S8: Temporal profile of normalized Cr(VI) concentration for the photocatalytic reduction over P25, UV100, and PC50. Table S2: Fitting parameters of Equations (S4) and (S5) taken from Figure S8.

Author Contributions

Conceptualization: M.A.G., E.S.R. and M.I.L.; Methodology: J.M.M., H.B.R., E.S.R. and M.I.L.; Software: J.M.M. and H.B.R.; Validation: J.M.M. and H.B.R.; Investigation: M.A.G., E.S.R. and M.I.L.; Writing—Original Draft Preparation: J.M.M., H.B.R., E.S.R. and M.I.L.; Writing—Review & Editing: J.M.M., H.B.R., E.S.R. and M.I.L.; Supervision: M.A.G., E.S.R. and M.I.L.; Funding Acquisition: M.I.L.

Ethics Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article and its Supplementary Materials. Raw data are available from the authors, upon reasonable request.

Funding

This research was funded by Agencia Nacional de la Promoción de la Ciencia y Tecnología de Argentina (ANPCyT), PICT-06 512, 2011-0463 and PICT-015-0208 projects.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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