Rate constants of hydroxyl radicals reaction with different dissociation species of fluoroquinolones and sulfonamides: Combined experimental and QSAR studies

Hydroxyl radicals (center dot OH) initiated degradation is an important process governing fate of aquatic organic micropollutants (OMPs). However, rate constants for aqueous reaction of OMPs with center dot OH (k(OH)) are available only for a limited number of OMPs, which complicates fate assessment of OMPs. Furthermore, molecular structures of many OMPs contain ionizable groups, and the OMPs may dissociate into different anionic/cationic species with different reactivity towards center dot OH. Therefore, it is of importance to determine k(OH) of ionizable OMPs, and to develop quantitative structure-activity relationship (QSAR) models for predicting k(OH) of OMPs at different ionization forms. Herein k(OH) values of 9 fluoroquinolones (FQs) and 11 sulfonamides (SAS) at 3 dissociation forms (FQ(+/-)/FQ(+)/FQ(-), SA(0)/SA(+)/SA(-)) were determined by competition kinetics experiments. A QSAR model using theoretical molecular structural descriptors was subsequently developed. The QSAR model successfully corroborated previous experimental results, exhibited good statistical performance, and is capable to predict k(OH) for FQs and SAS with different dissociation forms at environmentally relevant pH conditions. As organic ions have rarely been included in previous QSAR studies, the newly developed model that covers both neutral molecules and ions is of significance for future QSAR development as well as fate assessment of ionizable OMPs. (C) 2019 Elsevier Ltd. All rights reserved.