*2.5. E*ff*ect of Initial Dye Concentration on Nb2O5 Performance*

The initial dye concentration is a variable of interest that gives invaluable information about the range of pollutant concentration that is efficiently treatable within a reasonable timeframe. For this reason, MO solutions of 5, 10, 15 and 30 mg L−<sup>1</sup> were treated under the optimum conditions of pH 5.0 with 0.20 M of H2O2 and 1.0 g L−<sup>1</sup> of Nb2O5 photocatalyst. Figure 9 depicts the abatement of

dye concentration as a function of photocatalytic treatment time for different initial concentrations of MO. The estimation of kdec exhibits variation over an order of magnitude when increasing the dye concentration. Decreasing kdec values from 1.07·10−<sup>3</sup> <sup>s</sup>−<sup>1</sup> (R<sup>2</sup> <sup>=</sup> 0.998) for 5.0 mg L−<sup>1</sup> of MO, 4.37·<sup>10</sup> <sup>4</sup> <sup>s</sup>−<sup>1</sup> (R2 <sup>=</sup> 0.999) for 10.0 mg L−<sup>1</sup> of MO, 2.17·10−<sup>4</sup> <sup>s</sup>−<sup>1</sup> (R2 <sup>=</sup> 0.996) for 15.0 mg L−<sup>1</sup> of MO, down to 1.01·10−<sup>4</sup> s−<sup>1</sup> (R2 = 0.993) for 30.0 mg L−<sup>1</sup> of MO were estimated as depicted in the inset panel of Figure 9. The greater azo dye concentration implies a greater colorization of the water being treated, thus limiting the light penetration into the solution and diminishing photocatalytic Reactions (1) and (2), which generate oxidant species. Furthermore, the greater accumulation of organic intermediates susceptible adsorption onto the Nb2O5 active sites could inhibit the photocatalytic generation of vacancies and the production of other oxidants, decreasing the organic events and, consequently, the decolorization rate [9,23,43]. Therefore, longer treatment times would be required to completely decolorize MO at higher pollutant loads.

**Figure 9.** Methyl Orange azo dye abatement vs photoelectrocatalytic treatment time of 100 mL of solution with 1.0 g L−<sup>1</sup> of Nb2O5, 0.20 M of H2O2 at pH 5.0 and dye concentration of: (-) 5 mg L<sup>−</sup>1, (-) 10 mg L<sup>−</sup>1, () 15 mg L−<sup>1</sup> and () 30 mg L<sup>−</sup>1. The corresponding kinetic analysis assuming a pseudo-first-order reaction for MO is given in the inset panel.
