Photodegradation Of Environmentally Persistent Perfluorooctane Sulfonate:Mechanism And Improvement Measures

Perfluorinated chemicals (PFCs) are a class of toxic and highly-persistent pollutants widely distributed in various aqueous environments. One promising way of PFC elimination is photodegradation, which not only enables a high degree of mineralization (i.e., high defluorination ratio) but is also environmentally friendly. However, the practical implementation of such processes has been limited by a low decomposition rate and a poor understanding of the degradation mechanism.In this thesis, the mechanism of photodegradation of perfluorooctane sulfonate (PFOS), one major category of PFCs, and possible measures to improve this process were investigated. In particular, we revealed an important role of hydrated electrons in ultraviolet-driven degradation of PFOS and identified the major influential factors. The following mechanism of PFOS photodegradation was proposed:PFOS is first reduced by photo-generated hydrated electrons and then undergoes a series of hydrolysis and redox reactions to achieve a high degree of mineralization. Based on this improved knowledge, we identified the optimal pH and solution temperature conditions for this process, under which an efficient chemical-free photodegradation of PFOS in aquatic solution was achieved. The highest PFOS decomposition rate achieved in our study was over109-times higher than the reported maximum values in previous studies. In addition, we demonstrated that this chemical-free photodegradation of PFOS in environmental aqueous matrices, such as wastewater treatment plant effluent and lake water, is also feasible. An acceptable decomposition rate, which is comparable to that achieved in simple, buffered aqueous solution, was obtained. The effects of some commonly co-existing substances in the water environment on the photodegradation process were also investigated. The suitable pH and ionic strength were identified to account for the acceptable decomposition of PFOS in environmental matrices. Humic acid was found to play a complex role in this process.Owing to the pivotal role of hydrated electrons in this photodegradation, introducing another substance, even some pollutants, which increase the quantum yield or steady state concentration of hydrated electrons, was able to enhance the efficiencies of PFOS photodegradation.On the basis of the reaction properties of hydrated electrons and the surface active nature of PFOS, an advanced reduction process (under ultraviolet irradiation and boiling conditions) was also proposed to promote the removal efficiencies of PFOS. Under the boiling conditions, the decomposition of PFOS was significantly enhanced even at a low temperature where boiling was obtained by reduced pressure. Based on the influence of temperature, boiling intensity, hydronium and oxygenating, the mechanism of this advanced reduction process was proposed. In summary, this thesis offers new insights into the photodegradation processes of PFOS, proposes new enhancement technologies for improving PFOS removal, and may provide valuable implications for the elimination of PFCs and other persistent water contaminants with similar properties.