Photocatalytic decolourisation of Reactive Red 4 dye by an immobilised TiO2/chitosan layer by layer system

The synergistic photocatalysis–adsorption processes by the immobilized TiO2/chitosan layer by layer system on a glass support (TiO2/CS/glass) were investigated for the decolourisation of Reactive Red 4 (RR4) dye solution. Effects of different reaction parameters such as TiO2 loading, initial pH of the solution, visible light, dissolved oxygen and radical quenchers were studied. The decolourisation rate of RR4 by TiO2/CS/glass was more than 32 times faster than a single layer of TiO2 but was highly dependent on the TiO2 loading and the initial pH of the solution. A thin layer of TiO2 and acidic conditions favoured the adsorption of RR4 at the TiO2/CS interface. The h+/OH species that diffused from the TiO2 layer into the TiO2/CS interface oxidised the chemisorbed RR4 anions at the interface, and the generated electrons were then transferred to the conduction band of TiO2. Excess electrons in the conduction band of TiO2 increased the number of superoxide ions produced and thus improved the photocatalytic decolourisation of RR4. Therefore, apart from the synergistic photocatalysis–adsorption processes, a charge transfer process was also found to be responsible for maintaining the efficiency, sustainability and reusability of the TiO2/CS/glass layer by layer system.

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► Immobilised TiO2/chitosan layer by layer system was examined as photocatalyst.
► TiO2/chitosan layer by layer system was 32 times better than a single layer of TiO2.
► Thin layer of TiO2 and acidic condition were favourable.
► The mechanisms were adsorption, photocatalytic degradation and charge transfer.
► The photocatalyst was highly efficient, sustainable and reusable.