Degradation of micropollutants in flow-through VUV/UV reactors: Impact of internal diameter and baffle allocation

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-, SO42-), 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.

Determining wavelength-dependent quantum yields of photodegradation: importance of experimental setup and reference values for actinometers

Accurate quantum yields are crucial for modeling photochemical reactions in natural and engineered treatment systems. Quantum yields are usually determined using a single representative light source such as xenon lamps to mimic sunlight or UVC light for water treatment. However, photodegradation modeling can be improved by understanding the wavelength dependence of quantum yields and the potential errors introduced by the experimental setup. In this study, we investigated the effects of experimental setup on measured quantum yields using four photoreactor systems and up to 11 different light sources. When using a calibrated spectroradiometer to measure incident irradiance on an open solution surface, apparent quantum yields were up to two times higher if light reflection and light screening were not accounted for in the experimental setup. When the experimental setup was optimized to allow for accurate irradiance measurements, quantum yields were reproducible across photoreactors. The optimized experimental setup was then used to determine quantum yields of uridine, atrazine, p-nitroanisole (PNA), sulfamethoxazole, and diclofenac across the UV spectrum. No significant wavelength dependence of quantum yields was observed for sulfamethoxazole and diclofenac, in contrast to wavelength-dependent quantum yields for uridine, atrazine, and PNA. These reference values can be used for determining wavelength-dependent quantum yields of other compounds of interest. Additionally, more accurate results can be obtained when using (1) an actinometer with similar light absorption and photoreactivity compared to that of the target chemical, (2) optically transparent actinometer solutions that can account for light reflection within reaction vessels, and (3) a quantum yield that corresponds to the spectrum of the selected light source.

What is the role of nitrate/nitrite in trace organic contaminants degradation and transformation during UV-based advanced oxidation processes?

The effectiveness of UV-based advanced oxidation processes (UV-AOPs) in degrading trace organic contaminants (TrOCs) can be significantly influenced by the ubiquitous presence of nitrate (NO3-) and nitrite (NO2-) in water and wastewater. Indeed, NO3-/NO2- can play multiple roles of NO3-/NO2- in UV-AOPs, leading to complexities and conflicting results observed in existing research. They can inhibit the degradation of TrOCs by scavenging reactive species and/or competitively absorbing UV light. Conversely, they can also enhance the elimination of TrOCs by generating additional •OH and reactive nitrogen species (RNS). Furthermore, the presence of NO3-/NO2- during UV-AOP treatment can affect the transformation pathways of TrOCs, potentially resulting in the nitration/nitrosation of TrOCs. The resulting nitro(so)-products are generally more toxic than the parent TrOCs and may become precursors of nitrogenous disinfection byproducts (N-DBPs) upon chlorination. Particularly, since the impact of NO3-/NO2- in UV-AOPs is largely due to the generation of RNS from NO3-/NO2- including NO•, NO2•, and peroxynitrite (ONOO-/ONOOH), this review covers the generation, properties, and detection methods of these RNS. From kinetic, mechanistic, and toxicologic perspectives, future research needs are proposed to advance the understanding of how NO3-/NO2- can be exploited to improve the performance of UV-AOPs treating TrOCs. This critical review provides a comprehensive framework outlining the multifaceted impact of NO3-/NO2- in UV-AOPs, contributing insights for basic research and practical applications of UV-AOPs containing NO3-/NO2-.