VUV/UV oxidation performance for the elimination of recalcitrant aldehydes in water and its variation along the light-path.

Vacuum ultraviolet/ultraviolet (VUV/UV) oxidation using a low-pressure mercury lamp emitting dual wavelengths (185 nm (VUV) and 254 nm (UV)) significantly varies in performance along the light-path (lP), which has not been fully characterized. Therefore, VUV/UV oxidation in solution was investigated at various lP in terms of the degradation kinetics and mineralization pathway of representative aldehydes with various alkyl-chain lengths. Oxidative degradation of parent aldehydes with shorter alkyl chains was less efficient, specifically the pseudo-zero-order rate constant (kobs) of formaldehyde was only 51% of that of propionaldehyde (kobs = 0.078 μM s-1). In contrast, the mineralization of aldehydes with longer alkyl chains was less efficient because these aldehydes underwent mineralization into more refractory carboxylic byproducts, e.g., oxalic acid. VUV was mainly absorbed by superficial water (lP < 0.55 cm), which resulted in highly heterogeneous oxidation in homogeneous water. Thus, kobs of acetaldehyde dramatically decreased from 0.13 to 0.033 μM s-1 as the total lP of solution increased from 1.0 to 3.0 cm. On the basis of mineralization pathways proposed above, an iterative kinetic model was developed to characterize the degradation of parent aldehydes and the formation of carboxylic acids along lP. This model predicted the VUV/UV oxidaton for the first time by considering the fast diffusion of pollutants by limited diffusion of transient radical species. Thus, it realized the prediction of •OH concentration at specific water solution and byproduct evolution within specific water solution in turbulent flow regime, wherein •OH was predominantly formed in superficial water-layers wherein •OH in water-layers of lP <0.16 cm and <0.81 cm contributed to 50% and 90% of the total oxidation performance, respectively. This result would help to improve the VUV-UV-reactor design in terms of optimizing the thickness of water-layer and turbulence of water-flow.