ADMI color and toxicity reductions in raw textile mill effluent and dye mixtures by TiO2/UV is limited by presence of vat dyes

Full-scale application of heterogeneous photocatalysis for industrial wastewater treatment remains a challenge because of the complex nature of these matrices and the potential to form toxic by-products during treatment. A recent unsuccessful attempt to find adequate conditions for TiO2/UV treatment of a cotton dyeing textile mill led to this study on the treatability of mixtures of the dyes used in the greatest amounts at the mill and therefore most likely to be present in mill effluent. Four reactive and three vat dyes were mixed in different combinations and treated (10 mg/L of each dye, 0.5 mg/L TiO2, pH 4) to evaluate the influence of the different dyes on ADMI color, chemical oxygen demand (COD), and acute toxicity. While ADMI color removal was similar in all dye mixtures, COD removal was higher when vat dyes were absent. When treated individually, vat dyes exhibited greater recalcitrance, with no ADMI color removal and COD removals of less than 30%. Toxicity to Daphnia similis was decreased or eliminated from dye mixtures that exhibited the highest COD removals and corresponded to those in which reactive dyes were partially degraded. For raw textile mill effluent, photocatalysis reduced but did not eliminate treated effluent toxicity (EC50 = 26.8%)

Heterogeneous photocatalysis using TiO2 modified with hydrotalcite and iron oxide under UV–visible irradiation for color and toxicity reduction in secondary textile mill effluent

The objective of this study was to evaluate ADMI color removal from a biologically treated textile mill effluent by heterogeneous photocatalysis with UVevisible irradiation (UVevis) using a novel catalyst composed of TiO 2 supported on hydrotalcite and doped with iron oxide (HT/Fe/TiO 2 ). Simulated biological treatment of solutions of the dyes (50 mg/L) used in the greatest amounts at the mill where the textile effluent was collected resulted in no color removal in reactive dye solutions and about 50% color removal in vat dye solutions, after 96 h, indicating that the secondary effluent still contained a large proportion of anionic reactive dyes. Photocatalytic treatments were carried out with TiO 2 and HT/Fe/TiO 2 of Fe:Ti molar ratios of 0.25, 0.5, 0.75 and 1, with varying catalyst doses (0e3 mg/L), initial pH values (4 e10) and UVevis times (0e6 h). The highest ADMI color removal with unmodified TiO 2 was found at a dose of 2 g/L and pH 4, an impractical pH value for industrial application. The most efficient composite was HT/Fe/TiO 2 1 at pH 10, also at a dose of 2 g/L, which provided more complete ADMI color removal, from 303 to 9 ADMI color units (96%), than unmodified TiO 2 , from 303 to 37 ADMI color units (88%), under the same conditions. Hydroxyl radicals were responsible for the color reduction, since when 2- propanol, an OH scavenger, was added color removal was very low. For this reason, the HT/Fe/TiO 2 1 composite performed better at pH 10, because the higher concentration of hydroxide ions present at higher pH favored hydroxyl radical formation. COD reductions were relatively low and similar, approximately 20% for both catalysts after 6 h under UVevis, because of the low initial COD (78 mg/L). Secondary effluent toxicity to Daphnia similis (EC 50 1⁄4 70.7%) was reduced by photocatalysis with TiO 2 (EC 50 1⁄4 95.0%) and the HT/Fe/TiO 2 1 composite (EC 50 1⁄4 78.6%). HT/Fe/TiO 2 1 was reused five times and still lowered secondary effluent ADMI color below local discharge limits. Benefits of the HT/Fe/TiO 2 1 catalyst compared to TiO 2 include its lower bandgap energy (2.34 eV vs 3.25 eV), higher ADMI color removal and its magnetic nature that facilitated its recovery and would reduce treatment costs

ADMI color and toxicity reductions in raw textile mill effluent and dye mixtures by TiO2/UV is limited by presence of vat dyes

Full-scale application of heterogeneous photocatalysis for industrial wastewater treatment remains a challenge because of the complex nature of these matrices and the potential to form toxic by-products during treatment. A recent unsuccessful attempt to find adequate conditions for TiO2/UV treatment of a cotton dyeing textile mill led to this study on the treatability of mixtures of the dyes used in the greatest amounts at the mill and therefore most likely to be present in mill effluent. Four reactive and three vat dyes were mixed in different combinations and treated (10 mg/L of each dye, 0.5 mg/L TiO2, pH 4) to evaluate the influence of the different dyes on ADMI color, chemical oxygen demand (COD), and acute toxicity. While ADMI color removal was similar in all dye mixtures, COD removal was higher when vat dyes were absent. When treated individually, vat dyes exhibited greater recalcitrance, with no ADMI color removal and COD removals of less than 30%. Toxicity to Daphnia similis was decreased or eliminated from dye mixtures that exhibited the highest COD removals and corresponded to those in which reactive dyes were partially degraded. For raw textile mill effluent, photocatalysis reduced but did not eliminate treated effluent toxicity (EC50 = 26.8%)