Molar absorption coefficients of water components under vacuum ultraviolet light, implications for pollutant degradation: a review

Vacuum ultraviolet light produces reactive species that can degrade aqueous organic pollutants. Here we review the methods for measuring the molar absorption coefficients at 185 nm, the absorption coefficients of water components, and the effect of wavelength, water pH, temperature, and concentration. We present absorption coefficients of water, dissolved gases, ions, organic compounds, oxidants and reductants. We observe that absorption of vacuum ultraviolet photons by trace-level pollutants is negligible. By contrast, absorption of vacuum ultraviolet photons by inorganic compounds such as nitrate and chloride, and by oxidants or reductants, e.g. chlorine and sulfite, are pronounced and often overlooked. Increasing the temperature favors water cleavage but disfavors vacuum ultraviolet absorption by other substances, hence diminishing direct photolysis. Unexpectedly, the molar absorption coefficients of many compounds such as potassium exhibit high variability among published reports.