Nitrate and/or nitric acid formation in the presence of different radical scavengers during ozonation of water samples; are scavengers effective?

Abstract

In this study, we investigated the effect of different radical scavengers on the nitrate and/or nitric acid (NO3 and/or HNO3) formation chain in liquid while the dielectric barrier discharge plasma system (DBD) was used for ozone (O-3) generation. The effects of the excess concentration of each scavenger were studied individually. In addition, ultrapure water (UPW), tap water, and surface water samples were examined in the same condition. Due to the absence of scavengers in the UPW, we expected the highest NO3 formation in this experiment because all active species produced by the DBD system should have formed NO3. However, the obtained results were unexpected; the highest NO3 formation was obtained in the tap water at 385 +/- 4.6 mg/L. The results can be explained by some compounds in tap water acting as a trap for radicals involved in chain reactions that form NO3 and/or HNO3. The second highest result was obtained in the sodium hydroxide solution as 371 +/- 4.9 mg/L, since the OH ions accelerated the decomposition of O-3 to its intermediates such as hydroperoxide (HO2), ozonide (O-3), and hydroxyl radical (OH center dot), and, by increasing radicals in the liquid, more chain reactions can be promoted that lead to the formation of NO3 and/or HNO3. On the other hand, the quenching of radicals by scavengers such as carbonate ion and phosphoric acid and/or the long-term stabilization of O-3 as O-3 negatively affected the chain reactions that generate NO3 and/or HNO3.