Effect And Mechanism Of Typical Reducing Substances On Carbamazepine Degradation By Bromate In Ice

The cryosphere is a unique layer of Earth composed of solid water formed under low-temperature conditions.It engages in direct or indirect exchanges of matter and energy with other spheres,playing a critical role in the response and feedback mechanisms affecting global climate,environmental change,and pollutant transport.Surface waters in the middle and high latitudes exhibit seasonal freezing during winter.The environmental behavior and fate of solutes are inevitably influenced by the chemical reactions occurring within the ice.In recent years,the global prevalence of epilepsy and increased attention to the condition have led to a rise in the use of antiepileptic drugs.Carbamazepine is one of the longest-used and most commonly prescribed antiepileptic drugs,with significant environmental persistence.Consequently,carbamazepine is frequently detected in the effluent from wastewater treatment plants,surface water,groundwater,and even ice worldwide.Bromate,a byproduct of chlorination and ozonation processes during water treatment,is widely distributed in aquatic environments.When waters receiving effluent from wastewater treatment plants freeze during winter in cold regions,carbamazepine and bromate coexist within the ice.In addition,the widespread presence of reducing substances in aquatic environments,such as Cr（Ⅲ）,nitrite,chloride,and bromide can also affect bromate-dominated chemical reactions.However,the effects and mechanisms of reducing substances on the transformation of pharmaceutical compounds and bromate within ice are scarcely reported.In this study,carbamazepine,a typical pharmaceutical emerging contaminant,was selected as a target pharmaceutical compound.Based on batch simulation experiments,modern instrumental analysis,and density functional theory calculations,this study investigated the characteristics and mechanisms of bromate-mediated degradation of carbamazepine in ice.By comparing the degradation characteristics of carbamazepine and its structural analog oxcarbazepine in ice,the effect of molecular structure was analyzed.This study also elucidated the effect and mechanism of reducing substances on the degradation of carbamazepine by bromate.Finally,the degradation products of carbamazepine by reducing substances and bromate in ice were speculated and their toxicity was assessed.The main research results are summarized as follows:The freeze-concentration effect was the primary cause of the enhanced carbamazepine degradation by bromate in ice.The degradation rate constant of carbamazepine by bromate in ice under simulated sunlight was 64.7%higher than that in dark.The production of hypobromic acid（HOBr）accelerated the degradation rate of carbamazepine in ice.Under simulated sunlight,HOBr and hydroxyl radical（·OH）formed during the direct photolysis of bromate enhanced carbamazepine degradation in ice.In the presence of bromate,carbamazepine was mainly degraded by deamidation,decarbonylation,decarboxylation,hydroxylation,molecular rearrangement,and oxidation reactions,with 18.5%of degradation products being less toxic than the parent carbamazepine.Under dark and simulated sunlight conditions,the degradation rate constants of carbamazepine by bromate in ice were 2.6 and 2.1 times higher than those of oxcarbazepine,respectively.Oxcarbazepine was more prone to react with bromate to form brominated degradation products than carbamazepine.Under dark and simulated sunlight conditions,one and eight brominated degradation products of oxcarbazepine were identified,respectively.The proportion maximum of brominated degradation products was 13.9%.Molecular structure influenced the active sites,stability,and isomer conversion,leading to the difference in degradation characteristics between oxcarbazepine and carbamazepine.Different reducing substances variably influenced carbamazepine degradation by bromate in ice,exhibiting both promotive and inhibitory effects.The degradation rate constants of carbamazepine by bromate and Cr（Ⅲ）in ice were 29.4%–60.3%lower than those by bromate alone,which was attributed to the formation of Cr（VI）.The degradation rate constants of carbamazepine by bromate and nitrite,bromate and chloride,and bromate and bromide in ice were 1.7–1.8,2.3–2.5,and 38.3–39.6 times higher than those by bromate alone,respectively,which was attributed to the formation of reactive species.Under simulated sunlight,·OH or reactive nitrogen species（·NO₂and NO₂Br）played a major role in carbamazepine degradation in the bromate/Cr（Ⅲ）/freezing or bromate/nitrite/freezing reaction systems.In the bromate/halide/freezing reaction system,carbamazepine degradation primarily proceeded through direct electron transfer-mediated non-radical pathways,reactive chlorine and bromine species were important reactive species.Bromide and chloride were key reducing substances affecting bromate-dominated chemical reactions in ice in natural waters.In the presence of bromate and Cr（Ⅲ）,carbamazepine was mainly degraded through deamidation,decarboxylation,decarbonylation,hydroxylation,molecular rearrangement,oxidation,and bromination reactions.Notably,the presence of nitrite or chloride led to nitration or chlorination reactions in carbamazepine degradation in ice.In the presence of Cr（Ⅲ）,nitrite,chloride,and bromide,after the interaction of bromate and carbamazepine in ice,50.0%,42.4%,36.3%,and 38.8%of degradation products were less toxic than the parent carbamazepine,respectively.The presence of reducing substances could effectively reduce the toxicity of carbamazepine/bromate system.This study contributes to a profound understanding of the environmental behavior and fate of emerging contaminants in cold regions,providing a scientific reference for the environmental risk prevention and control of such emerging contaminants.