Temperature Effect on Per- and Polyfluoroalkyl Substance Degradation by Ultraviolet/Sulfite: Insights on Lamp Heat, Molecular Transformation, and Photochemical Principles

The UV/sulfite treatment has become a promising approach for degrading per- and polyfluoroalkyl substances (PFAS) in concentrated waste streams. Previously, most benchtop studies controlled the water temperature at around 20 °C. In this study, we found that the heat from UV lamps elevated the water temperature and significantly enhanced PFAS degradation. Unexpectedly, the best PFAS degradation performance was shown at 50 °C, whereas further heating to 90 °C led to even poorer performance than at 20 °C. Transformation product analysis revealed that a higher temperature not only accelerated the reaction but also promoted the cleavage of relatively strong C-F bonds. Notably, temperature elevation increased the UV absorbance by aqueous sulfite but decreased the photon flux from the lamp, leading to reduced efficiency of sulfite activation. The balance between enhanced C-F bond cleavage and lowered sulfite activation at elevated temperatures resulted in the optimal performance at around 50 °C. The same temperature effect applied to nitrate reduction by UV/sulfite and the iodide-enhanced UV/sulfite system. Overall, the thermal energy produced by UV lamps could be harnessed to accelerate and intensify PFAS defluorination and further optimize the energy metrics for practical application.