| Summary: | Titanium dioxide (TiO<sub>2</sub>) nanoparticles have been extensively studied for catalyzing the photo-degradation of organic pollutants, the photocatalyst being nonselective to the substrate. We, however, found that TiO<sub>2</sub> nanoparticles prepared via the sol-gel and hydrothermal synthetic routes each possess a definite specificity to the charge of the substrate for photodegradation. The nanoparticles were characterized by SEM, FTIR, XRD, TGA, and UV-visible spectra, and the photocatalytic degradation under UV-B (285 nm) irradiation of two model compounds, anionic methyl Orange (MO) and cationic methylene blue (MB) was monitored by a UV-visible spectrophotometer. Untreated sol-gel TiO<sub>2</sub> nanoparticles (T<sub>sg</sub>) preferentially degraded MO over MB (90% versus 40% in two hours), while after calcination at 400 °C for two hours (T<sub>sgc</sub>) they showed reversed specificity (50% MO versus 90% MB in one hour). The as-prepared hydrothermal TiO<sub>2</sub> nanoparticles (T<sub>ht</sub>) behaved in the opposite sense of T<sub>sg</sub> (41% MO versus 91% MB degraded in one and a half hours); calcination at 400 °C (T<sub>htc</sub>) did not reverse the trend but enhanced the efficiency of degradation. The study indicates that TiO<sub>2</sub> nanoparticles can be made to degrade a specific class of organic pollutants from an effluent facilitating the recycling of a specific class of pollutants for cost-effective effluent management.
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