**1. Introduction**

New trends in photocatalysis aim to identify niche applications for emerging semiconductor materials within a sustainable context. Titanium dioxide is the most studied photocatalyst and photoelectrocatalyst for water treatment [1,2] and water splitting applications [3,4]. However, recent research efforts have explored alternative semiconductor materials to overcome the most stringent barrier for the implementation of TiO2 in developing countries: its low activation under visible light irradiation [5,6]. Enabling the use of the natural sunlight irradiation would reduce operational expenditure, providing the opportunity for technology adaptation by regions with high solar radiation according to solar maps. For example, the northeastern region of Brazil receives >22 MJ m2 of sunlight irradiation annually due to its proximity to the equatorial line, which corresponds to approximately 10 h of sunlight every day [7].

Niobium-based oxide semiconductors have promising characteristics for environmental applications due to their hypoallergenic character, low cytotoxicity, and physiological and chemical inertness, along with their high thermodynamic stability [8,9]. However, the most remarkable aspect for their application in Brazil is their wide availability in the country, which is the major producer of niobium worldwide, with 99.0% of the world's niobium being located in Brazil [10]. Sustainable exploitation of this resource for environmental applications may positively affect socio-economic aspects, since few applications of niobium have been identified.

Brazil is the fourth biggest cotton textile exporter worldwide and the fifth biggest global manufacturer. Textile fiber dyeing process requires large volumes of water, resulting in the release of large amounts of colored wastewater effluent [11,12]. Green and sustainable manufacturing approaches require the minimization of the usage of water resources. Removal of organic dyes can incentivize water reuse in dyeing baths [13]. Photocatalytic treatment could be a suitable low-cost alternative for small/mid-sized Brazilian textile facilities [14,15]. Photocatalytic treatment is classified as an Advanced Oxidation Process, since it allows the in situ generation of highly oxidant species such as hydroxyl radicals (-OH) [16,17]. The light irradiation of a semiconductor with photons with a higher energy than its band gap enables the photo-excitation of an electron from the filled valence band of the semiconductor to the empty conduction band (ecb−), leaving a positively charged vacancy or hole (hvb<sup>+</sup>) following Reaction (1) [18,19]. Organic pollutants (i.e., dyes) can then be oxidized by the photogenerated hole, as well as by heterogeneous -OH formed on the photocatalyst surface from the water oxidation by the hvb<sup>+</sup> according to Reaction (2) [20,21].

$$\text{Semiconductor} + l\nu \to \text{h}\_{\text{vb}}\text{ }^{+} + \text{e}\_{\text{cb}}\text{ }^{-}\tag{1}$$

$$\rm H\_{vb}^{+} + H\_{2}O \to \rm \textsuperscript{0}OH + H^{+} \tag{2}$$

The recombination of ecb<sup>−</sup> with unreacted hvb<sup>+</sup> through Reaction (3) is responsible for the oxidative power loss in photocatalytic systems [22,23]. Dissolved oxygen can react with ecb<sup>−</sup>, yielding superoxide radicals (O2 -−) through Reaction (4), which contributes to slowing down the recombination reaction [24,25]. However, the use of ecb<sup>−</sup> scavengers can further minimize the extent of this undesirable reaction while enhancing performance. For example, the weak oxidant H2O2 can react with ecb<sup>−</sup> and O2 -<sup>−</sup>, producing additional -OH from Reactions (5) and (6), respectively [26].

$$\text{Fe}\_{\text{cb}}^{-} + \text{h}\_{\text{vb}}^{-} \rightarrow \text{heat} \tag{3}$$

$$\text{Fe}\_{\text{cb}}\text{}^- + \text{O}\_2 \rightarrow \text{O}\_2\text{}^{\bullet -} \tag{4}$$

$$\text{Fe}\_{\text{cb}}\text{}^{-} + \text{H}\_{2}\text{O}\_{2} \rightarrow \overset{\bullet}{\text{OH}} + \text{OH}^{-} \tag{5}$$

$$\rm O\_2\rm O^- + H\_2O\_2 \rightarrow \rm OOH + OH^- + O\_2 \tag{6}$$

Dye bath effluent decolorization enables water reuse for additional textile dyeing processes, minimizing the environmental footprint and the capital costs associated with water usage. This work studies the applicability of novel niobium oxide photocatalysts (Nb2O5) in the efficient decolorization of wastewaters containing azo-dyes. The synthesized materials were characterized in accordance with calcination methodologies. Decolorization capabilities were evaluated, and operational variables were optimized. This innovative alternative for water treatment processes, which is emerging as a new trend in photocatalytic technologies, meets sustainable technology manufacturing needs through the use of regionally abundant natural resources.
