Degradation Mechanisms And Toxicity Estimation Of Fluorinated Organics In Typical Environmental Transformation Processes

Recently, fluorinated organics have been widely used in numerous areas worldwide due to their unique physico-chemical characteristics. Given their continuous release into the environment during their applications, studying their degradation mechanisms in typical environmental processes and toxicity changes could contribute to evaluate their environmental behaviors and fate. In this investigation, perfluorosulfonic acid membrane Nafion N117 and antibiotic flumequine (FLU) were selected as the test chemicals, and several typical environmental transformation processes, including thermolysis, ozone and persulfate (PS)-based advanced oxidation processes, and an emerging water treatment reagent ferrate(VI) (Fe(VI)), were individually applied to study their degradation products and pathways. Additionally, the possible toxicity in these processes was evaluated. Detailed studies consist of the following five aspects:(1) The thermolysis of Nafion N117, a typical perfluorosulfonic acid membrane that is widely used in numerous chemical technologies, was investigated. Structural identification of the thermolysis products in water and methanol was performed using liquid chromatography-mass spectrometry (LC-MS). F release was studied using an ion-chromatography system, and the membrane thermal stability was characterized by thermogravimetric analysis. Notably, several types of perfluorinated compounds (PFCs) including perfluorocarboxylic acids were detected and identified. Based on these data, a thermolysis mechanism was proposed involving cleavage of both the polymer backbone and its side chains by attack of radical species. The results showed that Nafion thermolysis was a potential environmental source of PFCs, which have recently attracted growing concern. Additionally, this study provided an analytical justification of the LC-MS method for characterizing the degradation products of polymer electrolyte membranes. These identifications can substantially facilitate an understanding of their decomposition mechanisms and offer insight into the proper utilization and effective management on these membranes.(2) The toxicity of thermolysis products of N117 was evaluated by using two animal models, i.e., freshwater fish and mouse. Firstly, we investigated the antioxidant response and Na+, K+-ATPase activity in liver of Carassius auratus exposed to different thermolysis products of N117 for 5,15, and 30 days. The results showed that these products have the capability to induce oxidative stress and suppress Na+, K+-ATPase activity, as indicated by some significant changes on these toxicity end-points in fish liver. According to the integrated biomarker response (IBR) index, the toxicity intensity of these experimental treatments was tentatively ranked. On the other hand, we studied the histopathological alterations, oxidative stress biomarker responses, and transcriptome profiles in the liver of male mice exposed to N117 and its thermolysis products for 24 days. The results indicated that these transformation products could induce histopathological changes and oxidative stress in mice liver. The transcriptomics analysis identified some significantly altered molecular pathways, including the metabolism of xenobiotics, carbohydrates and lipids; signal transduction; cellular processes; immune system; and signaling molecules and interaction. These studies provide preliminary data for the potential toxicity of N117 and its thermolysis products on living species, and may fill the information gaps in the toxicity databases for current-use PEMs in typical environmental transformation processes.(3) The kinetics, transformation products, mechanisms and toxicity variations of the ozonation process for FLU were systematically determined. The possible effects of solution pH, inorganic ions, dissolved organic matters, and tert-butyl-alcohol (OH· scavenger), as well as the type of water matrices on FLU removal by ozonation, were studied from the perspective of the degradation kinetics. The data showed that ozone can be used as an effective oxidant for the fast removal of FLU from natural waters. Using LC-MS, a total of thirteen ozonation products of FLU were identified, and their specific formation mechanisms were proposed. The oxidation pathways involving the hydroxylation, decarboxylation and defluorination were tentatively proposed. Meanwhile, the generation of three low-molecular-weight carboxylic acids was detected, while the potential toxicity of the degradation mixtures of FLU by ozone was assessed. Overall, this paper can be a unique contribution to the systematic elucidation of the ozonation process of this antibiotic in water.(4) The kinetics, degradation mechanisms and pathways of aqueous FLU by PS were measured. Three common activation methods, including heat, Fe2+ and Cu2+, and a novel heterogeneous catalyst, namely, polyhydroquinone-coated magnetite/multi-walled carbon nanotubes (Fe3O4/MWCNTs/PHQ), were investigated to activate PS for FLU removal. Three common activators enhanced FLU degradation obviously, and certain influencing factors, such as solution pH, inorganic ions and dissolved organic matters, exerted their different effects. The catalysts were characterized, and an efficient catalytic degradation performance, high stability and excellent reusability were observed. The total organic carbon levels suggested that FLU can be effectively mineralized by the catalysts. Radical mechanism was studied by the quenching tests and electron paramagnetic resonance analysis. It was assumed that SO4-· predominated in the activation of PS with Fe3O4/MWCNTs/PHQ for FLU removal, while OH· also contributed to the catalytic oxidation process. In addition, a total of fifteen transformation intermediates of FLU were identified, from which two possible pathways were proposed involving hydroxylation, decarbonylation and ring opening. Overall, this study represented a systematical evaluation regarding the transformation process of FLU by PS, and showed that the heterogeneous catalysts can efficiently activate PS for aqueous removal of FLU.(5) FLU oxidation by aqueous Fe(VI) was studied by performing the experiments related to kinetics, influencing factors, transformation products and pathways, and toxicity estimation. FLU in the concentration range of μg/L-mg/L can be efficiently removed by Fe(VI), whereas coexisting constituents such as humic acid and N4+ could potentially affect the degradation efficiency. This study indicated that its combined use with peroxymonosulfate (PMS) exhibited more powerful capability for FLU removal and may be considered as the innovative chemical oxidation technology in future water treatments. Additionally, twenty-three products of FLU by Fe(VI) with or without PMS were identified using the solid phase extraction-liquid chromatography-high-resolution mass spectrometry, and three reaction pathways were proposed. Meanwhile, the aquatic toxicity of FLU and its transformation products was predicted by ECOSAR program, from which some products of higher toxicity than FLU were detected. Overall, these results provided unique contributions to the practical application of Fe(VI) for FQs removal during water treatment.This study investigated the individual transformation processes of Nafion and FLU during waste disposal and water treatment, elucidating their degradation mechanisms and toxicity changes in these typical environmental transformation processes. These studies could provide some valuable data for the understanding of their behaviors and fates in the environment, and fill the information gaps of the current research areas.