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

In this study, we constructed MnO2/organic acid (OA) systems using MnO2 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 MnO2. The results show that both OA obviously accelerate TC degradation by MnO2. 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 MnO2 was fast reduced to Mn2+ by oxalic acid and unable to react with TC. The impact of pH on TC degradation resulted from the influences of H+ on MnO2 redox potentials and TC deprotonation. And acidic conditions accelerated TC degradation. The addition of Mg2+, Ca2+, Fe3+ and Zn2+ exhibited an inhibitory effect in both systems for their occupying reactive sites on MnO2 surface and blocking the access of TC to MnO2. 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 MnO2/OA system provides an efficient treatment of TC in wastewater. And it is also noticeable that MnO2/OA system should also have an important effect on the fate of pollutants in environment, from our results.