Comparação dos processos UV-C, UV-C/H2O2, UV-C/S2O82- e UV-C/HSO5- para a degradação de micropoluentes orgânicos em efluente sanitário terciário

Abstract

The presence of micropollutants in environmental aqueous matrices has generated concern due to their recalcitrance to conventional treatments in sewage treatment plants (WWTP). Thus, the advanced oxidation processes (AOPs) arise as a complementary treatment alternative in STP aiming at degrading the contaminants of emerging concern. In the present work, the degradation of the mixture of the compounds fipronil, oxybenzone and propylparaben (100 µg L-1 of each) in tertiary sanitary effluent through the UV-C process alone and/or combined with different peroxide sources (H2O2, S2O82- e HSO5-) was evaluated. The efficiency of the processes was monitored by the decay of the average concentration of the target compounds and by the consumption of the oxidants H2O2, S2O82- e HSO5-. At pH 7.0, direct photolysis resulted in 47% degradation of the mixture after 20 min of irradiation. When combining the irradiation with the oxidants, there was a synergism in the degradation rate of the mixture and the degradation efficiency improved when increasing the concentration of each oxidant from 100 to 400 µmol L-1, reaching above 75% degradation and higher performance was presented by the UV-C/H2O2 process (88%). The degradation is also dependent on the target compounds structure of the and radical species. Thus, the degradation profile of the mixture was compared to that of naproxen (NAP) as it is a compound with high electronic density sites, and a sequence of the degradation efficiency of NAP (UV-C/S2O82- > UV-C/HSO5- > UV-C/H2O2) inverse to that of the mixture was obtained. The influence of pH (3.0, 7.0 (natural matrix) and 11.0) was also evaluated and the difference in degradation efficiency ranged from 92% (pH 3.0) to 78% (pH 11.0), 94% (pH 3.0) to 55% (pH 11.0) and 100% (pH 3.0) to 76% (pH 7.0) in the UV-C/H2O2, UV-C/S2O82- and UV-C/HSO5- processes, respectively. The radical species responsible for the majority degradation is pH dependent. Majority degradation occurred via sulphate radicals (SO4●–) at pH 3.0 in the UV-C/S2O82- process, while hydroxyl radicals (HO•) were responsible for majority degradation at pH 7.0 and 11.0 in the UV-C/S2O82- and UV-C/HSO5- processes, respectively. Both species were co-responsible for the majority degradation at pH 3.0 and 7.0 in the UV-C/HSO5- process. On the other hand, the majority degradation in UV-C/S2O82- process at pH 11.0 occurred via photolysis. Therefore, this work suggests that the mixture of H2O2 and S2O82- is a better option to increase the efficiency of degradation, avoiding the use of HSO5- that besides having a high cost, demands more energy for its activation and can reduce the pH of the medium.