Numerous applications of TiO2 nano-particles (TNPs) has resulted in their accumulation in various compartments of environment. Photocatalytic oxidation of a hazardous compound may lead to formation of some potentially harmful/toxic intermediates, and therefore, the presence of TNPs in the natural environment may be harmful. Effort has been made in the present report to investigate the reductive photocatalytic (RPC) property of TNPs in the presence of some natural anti-oxidants widely used in India, such as, Sapindus mukorossi (Indian name: aritha), Ocimum tenuiflorum (Indian name: tulsi), and Cymbopogon citratus (common name: lemon grass), which may exist in the water bodies. Reductive decolorization of azo dyes, Reactive Black 5 (RB5) and Methyl Orange (MO), was studied using P25 (Evonik) TNPs in the presence of UV radiation and extract of one of the anti-oxidants as hole-scavenger (HS). Formation of aromatic amines, vinyl sulfone (VS, 4- aminophenylsulfonyl-hydroxy ethyl sulfato ester), and sulphanilic acid, respectively from RB5 and MO, was considered a proof of reduction of dyes. More than 96% of decolourization of initial 50 mg/L of RB5 was obtained after 1h of irradiation and TNP dose of 1 g/L when both tulsi and aritha were used as HS, individually. Decolorization was comparable with the synthetic HS like oxalic acid (95.84%) and formic acid (94.32%) but the rate of reaction was lower. Tulsi and aritha gave higher decolourization and higher rate of reaction at initial pH of 3 as compared to that at neutral pH. At ph 3, K (first order rate constant) was found to follow: Koxalic&gt;Kformic&gt; Ktulsi&gt;Karitha. 94.7 and 90.6% removal of initially 20 mg/L MO was observed in the presence of aritha and lemon-grass, respectively, at pH 3 at the end of 1h, as compared to 97.8% in &lt;15 min. using oxalic acid. The MO decolorization in the presence of aritha was consistent at pH 7 too (96.6%); however, other HS could achieve only 44.4% (oxalic acid), and 40% (lemon-grass) decolorization at the end of 1h. Formation of sulfanilic acid was observed in the presence of aritha and lemon-grass, irrespective of the initial pH; while, in the presence of oxalic acid, it was observed at acidic pH only. The results indicate that TNPs can interact with natural anti-oxidants in natural environment in the presence of sun light, and lead to photocatalytic reduction of pollutants.

Reductive photocatalysis of azo dyes using TiO2 nano-particles in the presence of some natural anti-oxidants as Hole scavengers

251st National Meeting (2016)

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