Assessing the Chemical-Free Oxidation of Trace Organic Chemicals by VUV/UV as an Alternative to Conventional UV/H₂O₂

Low-pressure mercury lamps with high-purity quartz can emit both vacuum-UV (VUV, 185 nm) and UV (254 nm) and are commercially available and promising for eliminating recalcitrant organic pollutants. The feasibility of VUV/UV as a chemical-free oxidation process was verified and quantitatively assessed by the concept of H₂O₂ equivalence (EQ_H2O2), at which UV/H₂O₂ showed the same performance as VUV/UV for the degradation of trace organic contaminants (TOrCs). Although VUV showed superior H₂O activation and oxidation performance, its performance highly varied as a function of light path length (L_p) in water, while that of UV/H₂O₂ proportionally decreased with decreasing H₂O₂ dose regardless of L_p. On increasing L_p from 1.0 to 3.0 cm, the EQ_H2O2 of VUV/UV decreased from 0.81 to 0.22 mM H₂O₂. Chloride and nitrate hardly influenced UV/H₂O₂, but they dramatically inhibited VUV/UV. The competitive absorbance of VUV by chloride and nitrate was verified as the main reason. The inhibitory effect was partially compensated by ^•OH formation from the propagation reactions of chloride or nitrate VUV photolysis, which was verified by kinetic modeling in Kintecus. In water with an L_p of 2.0 cm, the EQ_H2O2 of VUV/UV decreased from 0.43 to 0.17 mM (60.8% decrease) on increasing the chloride concentration from 0 to 15 mM and to 0.20 mM (53.5% decrease) at 4 mM nitrate. The results of this study provide a comprehensive understanding of VUV/UV oxidation in comparison to UV/H₂O₂, which underscores the suitability and efficiency of chemical-free oxidation with VUV/UV.