Efficient degradation of tannic acid in water by UV/H 2 O 2 process

Abstract

In this work, the degradation of tannic acid (TA) by UV/H2O2 process has been investigated. UV/H2O2 oxidation experiments were performed by UV irradiation at 254 nm of TA aqueous solutions in a photo-reactor. Firstly, the effects of certain parameters including pH, H2O2 content, and temperature on the decomposition of H2O2 during UV/H2O2 process were evaluated. Results obtained showed that UV-irradiation of 1 g H2O2 L−1 aqueous solution achieved almost complete decomposition of H2O2 (> 98%) at pH = 5 and 28 °C. H2O2 decomposition follows pseudo-first-order kinetics with a rate constant of 0.0086 min−1 under the optimal conditions. Secondly, the degradation of TA by UV/H2O2 process was investigated. The effects of certain operating conditions including initial pH, temperature, H2O2 concentration and initial total organic carbon (TOC) on the efficiency of degradation and mineralization of TA by UV/H2O2 were evaluated. The results showed that UV/H2O2 process achieved almost complete disappearance of the absorbance at 276 nm (> 96%) and high TOC removal (94%) from TA aqueous solutions containing 82 mg C L−1 under pH = 9 using 6 g H2O2 L−1 at 28 °C. The results confirmed that the degradation and mineralization of TA molecules in water occur via chemical oxidation with hydroxyl radicals produced by photo-decomposition of H2O2 by UV light. Chromatography analysis revealed that the mechanism of TA degradation involves several steps including the formation of gallic acid and pyrogallol. These aromatic intermediates undergo further oxidative ring opening to form carboxylic acids (oxalic and maleic acids) that end up to be completely mineralized. The electrical energy requirement consumed under the optimal conditions was 7.9 kWh g−1 TOC which is highly competitive with other advanced oxidation technologies such as ozonation and UV/O3 processes. Based on these results, it can be concluded that UV/H2O2 can be considered as a cost-effective method to treat wastewaters contaminated with natural organic matter.