Enhanced vacuum ultraviolet/ultraviolet process for advanced water treatment by using air micro/nano bubbles

The key constraint of vacuum ultraviolet (VUV)/ultraviolet (UV) process is the short VUV penetration resulting in a relatively high local concentration of hydroxyl radicals (HO^●) and associated mass transfer limitation. This study investigated the enhancement of organic contaminant degradation by combining VUV/UV process with air micro/nano bubbles (MNBs) (VUV/UV/MNBs). Using sulfamethazine (SMN) as a model contaminant, the degradation rate in the VUV/UV/MNBs process (0.219 min^-1) was twice that of the VUV/UV process (0.108 min^-1). This enhancement was attributed to three factors: (1) air MNBs improved mixing and mass transfer within solution, suppressing HO^● self-recombination, as evidenced by a decrease in H₂O₂ concentration; (2) air MNBs optimized the light behavior in solution, enabling longer VUV penetration; (3) elevated dissolved oxygen levels facilitated additional HO^● generation and inhibited the recombination of carbon-centered radicals. The contributions of these factors were quantitatively evaluated. Additionally, the enhancement by air MNBs persisted in the presence of dissolved organic matter and coexisting anions (i.e., Cl^-, NO₃^-, SO₄^2-), and the effects of photoreactor inner diameter and initial SMN concentration were explored for potential practical applications. This study demonstrates the novel VUV/UV/MNBs as a high-efficiency and chemical-free advanced water treatment process.