*3.2. Synthesis of Microparticulated Nb2O5 Photocatalyst*

The ammonium salt of the niobium oxalate complex was calcined using a temperature ramp of 10 ◦C min−<sup>1</sup> to 500 ◦C, where it remained for 4 h under ambient atmosphere. Under these conditions, the oxalate was completely incinerated, leading to the formation of amorphous Nb2O5 photocatalyst.

### *3.3. Solar Photocatalytic Experiences*

The photochemical reactor used during photocatalytic experiments consisted of an undivided open cell directly exposed to sunlight. The photocatalyst was suspended in the solution in slurry and the tests were carried out under vigorous stirring with a magnetic bar at 700 rpm to ensure the

homogeneous distribution of the catalyst in the bulk, while also favoring the transport of reactants to/from the catalyst surface. The photochemical cell had a double jacket in which water was circulated to maintain the solution temperature at 25 ◦C using a LAUDA A100 thermostat, avoiding the evaporation of the treated solution by solar irradiation heating. Prior to the photocatalytic experiments, the solutions were maintained at the defined pH, dye and catalyst concentration conditions for 30 min in the dark.

### *3.4. Apparatus and Analytical Procedures*

Scanning electron microscopy (SEM) micrographies of the synthesized Nb2O5 photocatalyst were obtained using a Hitachi TM-3000 system with a frequency of 50/60 Hz and a magnification capacity of up to 3000x. The X-ray diffractogram (XRD) patterns were recorded on a Bruker D2 Phaser diffractometer with Cu-K<sup>α</sup> (λ = 1.54 Å) irradiation source using a Ni filter and a Lynxeye detector, performing the analysis in the 2θ range from 2◦ to 70◦. The average crystallite sizes were estimated by applying Scherrer's Formula (13) to the identified crystal planes [50,51]:

$$
\pi = \mathbf{K} \,\lambda / \beta \,\cos \Theta \tag{13}
$$

where τ refers to the mean size of the ordered crystalline domains, K is the Scherrer constant, a dimensionless shape factor that usually has values close to unity, λ is the X-ray wavelength, β is the line width at half maximum in radians and θ is the Bragg angle in degrees.

Fourier transform infrared (FTIR) spectra were recorded from a Shimadzu-8400S FTIR spectrometer in the medium IR region (4000–400 cm<sup>−</sup>1) after 30 scans with a resolution of 4 cm−1. The UV-vis diffuse reflectance spectra between 200–600 nm, used to determine the band gap from solid samples on the basis of Kubelka-Munk and Tauc plots [52], was obtained with a UV-vis spectrometer Cary 500 Scan using the barium sulfate pattern as a reference material. The specific surface area was determined on the basis of nitrogen adsorption-desorption isotherms registered on a Micrometrics ASAP 2020. The Nb2O5 catalyst samples were previously degassed at 300 ◦C for 3 h and subsequently submitted to a nitrogen atmosphere at 77 K. The nitrogen adsorption-desorption curve made it possible to determine the specific surface by area using BET method in the region of low relative pressure (p/p0 = 0.1–1.0). The Barret–Joyner–Halenda method (BJH) was used to determine the pore size distribution.

The pH of the treated solutions was adjusted using a pH-meter Tecnopon mPA-210. The percentage of color removal for the solution during the photocatalytic treatment was estimated using Equation (14) [12,53]:

$$\% \text{Color Removal} = (\text{A}\_0 - \text{A}\_l) / \text{A}\_0 \times 100 \tag{14}$$

where A0 is the initial absorbance and At the absorbance at the treatment time *t*. The absorbance was determined at the maximum absorptivity of MO (λmax = 642 nm) using a UV-vis spectrophotometer Analytikjena SPECORD 210 PLUS. The point of zero charge (PZC) of Nb2O5 was determined by the pH drift method, as described by Hashemzadeh et al. [40]. Solutions of 50 mL of 0.01 M of NaCl where adjusted to different pH between 1 and 11 by adding HCl or NaOH. After achieving the defined initial pH, 0.05 g of Nb2O5 photocatalyst was added to the solution, which was maintained at 25 ◦C for 48 h under constant stirring at 700 rpm before measuring the solution pH to determine the pHfinal. The pHPZC was determined from the intersection of the curve pHfinal vs. pHinitial with the straight line pHfinal = pHinitial.

### **4. Conclusions**

The potential application of Nb2O5 for photocatalytic decontamination and decolorization of wastewater containing azo dyes was proved on basis of the efficient removal of a model pollutant: MO. A novel Nb2O5 photocatalyst was successfully synthesized using a facile calcination method from a natural precursor extracted as a natural resource in Brazil and characterized. The photocatalytic assays demonstrated high removal efficiency of MO azo dye in the presence of H2O2, which was used as an ecb- scavenger, and oxidants, which acted as a photogeneration enhancer. The effects of different

control parameters were analyzed and optimized to enable the faster decolorization under the mildest conditions. Thereby, a concentration of Nb2O5 catalyst of 1.0 g L−<sup>1</sup> in slurry was identified as the optimal conditions for the complete removal of color at lower catalyst dosages. The tests carried out also defined as optimal the mild conditions of pH 5.0 and 0.20 M of H2O2, with treatable concentrations of MO ranging up to 15 mg L−1. It should be noted that the study was developed in order to find potential applications for niobium materials, which represent a key material produced extensively in Brazil. Our results prove the promising applicability of these innocuous and highly re-utilizable photocatalysts in AOPs for wastewater treatment. It is worth mentioning that this technique is emerging as suitable approach for depollution treatment of textile effluent in mid-sized industry in the Northeast region of Brazil, which receives approximately 10 h/day of sunlight irradiation for more than 350 days a year due to its proximity to the equatorial line.

**Author Contributions:** Conceptualization: A.J.d.S. and S.G.-S.; methodology: A.J.d.S., L.M.B.B. and S.G.-S.; validation: A.J.d.S., C.A.M.-H. and A.P.d.M.A.; formal analysis: A.J.d.S., S.G.-S. and C.A.M.-H.; investigation: A.J.d.S., L.M.B.B. and S.G.-S.; resources: C.A.M.-H. and A.P.d.M.A.; data curation: A.J.d.S., L.M.B.B. and S.G.-S.; writing—original draft preparation: A.J.d.S. and S.G.-S.; writing—review and editing: C.A.M.-H., A.P.d.M.A. and S.G.-S.; visualization; A.J.d.S. and L.M.B.B.; supervision: C.A.M.-H., A.P.d.M.A. and S.G.-S.; project administration: C.A.M.-H. and A.P.d.M.A.; funding acquisition: C.A.M.-H., A.P.d.M.A. and S.G.-S.

**Funding:** Financial supports from National Council for Scientific and Technological Development (CNPq— 465571/2014-0; CNPq—446846/2014-7 and CNPq—401519/2014-7) and FAPESP (2014/50945-4) are gratefully acknowledged. A.J. dos Santos and L.M.B. Batista gratefully acknowledge the grants awarded from CAPES.

**Conflicts of Interest:** The authors declare no conflict of interest.
