Enhanced degradation mechanism of tetracycline by MnO2 with the presence of organic acids

In this study, we constructed MnO 2 /organic acid (OA) systems using MnO 2 colloid, the most reactive phase of Mn(IV), and two kinds of OA (oxalic acid and l -tartaric acid). We investigated the effect of OA on tetracycline (TC) degradation by MnO 2 . The results show that both OA obviously accelerate TC degradation by MnO 2 . Mn(III) formed during the reaction lead to the acceleration. Mn(III)-oxalate complex formed in oxalic acid system resulted in the lower degradation efficiency than that in l -tartaric acid system. The acceleration of oxalic acid was decreased when the concentration was more than 75 μM, and even completely disappeared with the concentration of 500 μM, owning to the fact that excess oxalic acid decreased the pH and some MnO 2 was fast reduced to Mn 2+ by oxalic acid and unable to react with TC. The impact of pH on TC degradation resulted from the influences of H + on MnO 2 redox potentials and TC deprotonation. And acidic conditions accelerated TC degradation. The addition of Mg 2+ , Ca 2+ , Fe 3+ and Zn 2+ exhibited an inhibitory effect in both systems for their occupying reactive sites on MnO 2 surface and blocking the access of TC to MnO 2 . Similar intermediates in the two systems were detected, indicating a similar TC degradation mechanism including a series of reactions like dehydration, hydroxylation and oxidation. The MnO 2 /OA system provides an efficient treatment of TC in wastewater. And it is also noticeable that MnO 2 /OA system should also have an important effect on the fate of pollutants in environment, from our results.