A comparative study of photocatalytic degradation and mineralisation of an azo dye using supported and suspended nano-TiO2 under UV and sunlight irradiations
Authors: Bahreini Zaker , Heydari Vahid , Hekmat Ali N. , Taheri Minoo , Vahid Behrouz , Moradkhannejhad Leila 
Published: March 2016

Purpose
– This paper aims to introduce two methods for immobilisation of TiO2 nanoparticles on a glass plate by means of silicon resin as a medium. Then, to ensure the effectiveness of these stabilisation methods, the photocatalytic degradation and mineralisation of the dye C.I. Reactive Blue 21 (RB21), as a model organic pollutant, were compared using these immobilised systems and the suspended one utilizing UV and sunlight irradiations individually.

Design/methodology/approach
– TiO2 nanoparticles were supported onto a glass support by silicon resin as an adhesion agent by spraying of TiO2 nanoparticles on the resin surface, which covered the glass plate or brushing the mixture of TiO2 and the resin onto the glass. The characteristics of the applied nano-TiO2 were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Brunauer

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Emmett–Teller. Photocatalytic degradation and mineralisation of C.I. Reactive Blue 21 (RB21) by two immobilised systems were compared with suspended system in a batch mode under UV and sunlight irradiations after 2 h of treatment.

Findings
– The results showed that these immobilised modes had efficiencies, including 82-87 per cent degradation of RB21 and 52-58 per cent decrease in chemical oxygen demand (COD) for the operational time of 120 min, comparable to that of the suspended mode (91 per cent degradation of RB21 and, consequently, COD is decreased by 65 per cent). Comparison between photocatalytic efficiencies of two immobilised systems revealed that coating by spraying method performed better than brushing one due to more available surface area of TiO2. Finally, the results obtained from the mentioned supported systems under sunlight indicated the efficiencies about 87 to 89 per cent in comparison of the suspension system regardless of the reaction time enhancement up to 15 h compared to the UV irradiation.

Research limitations/implications
– In this research, the fixation of TiO2 nanoparticles on a substrate such as normal glass by an easy, inexpensive, durable, repairable and repeatable technique for wastewater treatment was introduced. Due to the simplicity and cheapness of these stabilisation methods and as these stabilisation methods are applicable on other substrates such as concrete, ceramics, etc., you can use these methods in major scales for purification of contaminated water, for example for stabilisation of TiO2 nanoparticles on wall pool utilized for water purification can be used.

Originality/value
– Two introduced immobilisation methods in this study are novel. The photocatalytic efficiency of these immobilised systems in degradation of water contaminants was investigated by using these systems in degradation and mineralisation of the dye C.I. Reactive Blue 21 (RB21), as a model organic pollutant compared with same TiO2 nanoparticles in an aqueous suspension system under UV light. Furthermore, this paper investigated replacing of inexpensive sources of UV light instead of UV lamps, and then the same photocatalytic reactions were carried out under sunlight as a UV source and degradation efficiencies by two UV sources were compared.