Heterogeneous photocatalysis using TiO₂ modified with hydrotalcite and iron oxide under UV-visible irradiation for...

The objective of this study was to evaluate ADMI color removal from a biologically treated textile mill effluent by heterogeneous photocatalysis with UV-visible irradiation (UV-vis) using a novel catalyst composed of TiO₂ supported on hydrotalcite and doped with iron oxide (HT/Fe/TiO₂). 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 TiO₂ and HT/Fe/TiO₂ of Fe:Ti molar ratios of 0.25, 0.5, 0.75 and 1, with varying catalyst doses (0-3 mg/L), initial pH values (4-10) and UV-vis times (0-6 h). The highest ADMI color removal with unmodified TiO₂ 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/TiO₂ 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 TiO₂, 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/TiO₂ 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 UV-vis, because of the low initial COD (78 mg/L). Secondary effluent toxicity to Daphnia similis (EC₅₀ = 70.7%) was reduced by photocatalysis with TiO₂ (EC₅₀ = 95.0%) and the HT/Fe/TiO₂ 1 composite (EC₅₀ = 78.6%). HT/Fe/TiO₂ 1 was reused five times and still lowered secondary effluent ADMI color below local discharge limits. Benefits of the HT/Fe/TiO₂ 1 catalyst compared to TiO₂ 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.