Effects of nitrite on the degradation of carbamazepine by sulfate radical oxidation

Activated persulfate oxidation has been demonstrated to be a viable approach to degrade carbamazepine (CBZ). However, the influence of nitrite (NO2) commonly found in wastewaters and groundwaters on the degradation remains unclear. Herein, we found that the presence of NO2 not only reduced the rate but substantially changed the pathway of CBZ degradation in a heat activated peroxydisulfate (heat/PDS) oxidation process. This was because of the scavenging of sulfate radical (SO4-) by NO2 and the formation of nitrogen dioxide radical (NO2-). Acridine was identified as a key intermediate during CBZ degradation. In the presence of NO2, acridine could be further transformed to nitrated derivatives. During this process, an N-centered acridine radical is the key intermediate which coupled with the NO2- originated from the oxidation of NO2. Besides nitroacridines, several nitrophenols were identified as well. In contrast to the nitroacridines, the nitro group in these nitrophenols was derived from oxidation of the amine in the parent CBZ. Increasing the NO2 concentration increased the formation of nitroacridines but inhibited the formation of nitrophenols. Since nitroaromatic compounds are usually toxic and persistent, this study highlights the potential environmental risks when SO4- is applied to degrade CBZ in the presence of NO2.