Aqueous pollutants in water bodies can be photocatalytically reduced by TiO2 nano-particles in the presence of natural organic matters

Abstract

Increasing use of man-made products containing TiO2 nano-particles (TNPs) has resulted in accumulation of TNPs in the environment. Harmful effects of TNPs on aquatic life are well-documented, but little is known about their activity as photocatalyst under sunlight in natural water bodies. The present study investigates photocatalytic activity of TNPs in the presence of some natural organic matters (NOMs) having antioxidant properties (Sapindus mukorossi and Ocimum tenuiflorum, Indian names: Aritha and Tulsi), and aqueous pollutants (azo dyes and Cu(II)) under sunlight, simulating conditions prevailing in a natural water body. Both the NOMs acted as hole-scavengers and promoted photocatalytic reduction (PCR) of pollutants manifested by formation of aromatic amines from azo dyes and disappearance of Cu(II). 81.5 and 85.2% decolourization of initial 50 mgL−1 RB5, and 78.81 and 83.32% decolourization of initial 50 mgL−1 MO were obtained in the presence of 2%(v/v) extracts of Tulsi and Aritha, respectively, at TNP dose of 1 gL−1 and pH 3. Similarly, 53 and 73% of 10 mgL−1 Cu(II) (chelated with EDTA) were removed respectively, at pH 8. Under similar conditions, >90% of unchelated Cu(II) was removed in the presence of both NOMs. PCR of RB5 occurred even at high concentrations of the two most common anions in natural water bodies; chloride and sulphate, but the extent of decolourization and formation of aromatic amines reduced with increasing concentrations of anions. The present study suggests that TNPs in contaminated natural water bodies receiving NOM (e.g. through domestic/industrial discharges) can aid to natural purification process by causing reductive transformation of pollutants over a wide range of pH and anion concentrations.