Visible light activation of TiO2 nanoparticles via dye sensitization for wastewater treatment

Water is fundamental for all forms of life, making it one of the essential resources of nature. The unregulated discharge of processed water from industries, such as textiles, pharmaceuticals, food and chemical, sugar, paper mills, beverages, etc., containing various recalcitrant organic pollutants leads to the deterioration of natural water bodies [1,2]. About 5 million people die each year by drinking contaminated water, and approximately 2 billion people on the planet lacks access to safe drinking water, because of the increasing contamination of it, even in developed countries [3,4]. Many efforts have been made over the last few decades to improve methods for pollutant removal by developing suitable and sustainable treatments. However, the available conventional biological and physico-chemical processes for wastewater treatment are inefficient in removing these recalcitrant organic pollutants. In this context, various advanced oxidation processes (AOPs) relying on the generation of highly reactive hydroxyl and superoxide radicals have shown potential for achieving efficient degradation of these compounds [5,6]. Amongst the available AOPs, photocatalysis is being actively investigated because it can potentially mineralize the organic pollutant into CO2 and H2O with less sludge production [7] . Various catalysts, viz. TiO2, ZnO, ZnS, Graphene composites, and Fe2O3, etc., are being used as photocatalysts, amongst which TiO2 is one of the widely used photocatalysts due to its availability, high oxidizing power, non-toxicity, photostability, chemical stability, low cost and easy preparation [8,9]. However, due to its large bandgap, pure TiO2 is only activated under UV irradiation (< 5% of the solar spectrum) limiting its application with visible light (~45% of the solar spectrum). Hence, it is essential to develop visible light active photocatalyst, having a relatively smaller bandgap [10,11]. Various surface modification techniques for achieving such visible light activity include doping with metals [12] and non-metals [11], heterogeneous composites [13,14], and dye- sensitization with synthetic [15], and natural dyes [16]. Among these, dye-sensitization is a promising technique in various fields including solar cell fabrication [17,18] and removal of pollutants from real wastewater [19]. A dye adsorbed onto the catalyst surface is excited with visible radiation, then an electron passes from its HOMO (highest occupied molecular orbital) to its LUMO (lowest unoccupied molecular orbital), and subsequently to the conduction band of TiO2.

Several studies are available in which synthetic dyes, such as eosin-Y [20], thionine [21], Nile blue-A, safranine-O, rhodamine-B and methylene blue [15], porphyrin [22], and ruthenium complexes [23], are being used as a sensitizer in semiconductors for photodegradation of various dyes. The major problems with synthetic organic dyes include their complex synthetic routes, environmental toxicity, and low yield. Besides, transition metal based dyes are the best compounds to achieve good electron transfer to the semiconductor, but are expensive and not environment friendly [24]. On the other hand, natural organic dyes and pigments, such as anthocyanin, chlorophylls, etc., found in flowers, leaves, and fruits of plants, can be extracted by simple methods. These natural sources are cost-effective, non-toxic, abundant, and environment-friendly [25,26].

Sensitization of commercial TiO2 nanoparticles with the dyes extracted from red water lily flower and water hyacinth leaves to degrade the water pollutants have not been reported. Therefore, the present study focuses on the activation of TiO2 nanoparticles as a photocatalyst at visible light using the natural organic dyes (anthocyanin and chlorophyll) of red water lily flower and water hyacinth leaves, and comprehensive investigation on its visible light activity with the degradation of toxic organic dyes. The effect of the real wastewater matrix on degradation will be also investigated.

• Research Project