Far-UVC Photolysis of Peroxydisulfate for Micropollutant Degradation in Water

Abstract

Increasing radical yields to reduce UV fluence requirement for achieving targeted removal of micropollutants in water would make UV-based advanced oxidation processes (AOPs) less energy demanding in the context of United Nations' Sustainable Development Goals and carbon neutrality. We herein demonstrate that, by switching the UV radiation source from conventional low-pressure UV at 254 nm (UV 254 ) to emerging Far-UVC at 222 nm (UV 222 ), the fluence-based concentration of HO • in the UV/peroxydisulfate (UV/PDS) AOP increases by 6.40, 2.89, and 6.00 times in deionized water, tap water, and surface water, respectively, with increases in the fluence-based concentration of SO 4•− also by 5.06, 5.81, and 55.47 times, respectively. The enhancement to radical generation is confirmed using a kinetic model. The pseudo-first-order degradation rate constants of 16 micropollutants by the UV 222 /PDS AOP in surface water are predicted to be 1.94−13.71 times higher than those by the UV 254 /PDS AOP. Among the tested water matrix components, chloride and nitrate decrease SO 4•− but increase HO • concentration in the UV 222 /PDS AOP. Compared to the UV 254 /PDS AOP, the UV 222 / PDS AOP decreases the formation potentials of carbonaceous disinfection byproducts (DBPs) but increases the formation potentials of nitrogenous DBPs.

Far-UVC Radiation Activating Percarbonate to Accelerate the Removal of Methotrexate: Degradation Efficiency, Mechanisms, and Moiety-Dependence Transformation

“…In addition, the presence of certain water matrix components could promote radical generation in UV 222 /SPC. For example, NO 3 – had a promoting effect on free radical production in the UV 222 /SPC system, , whereas its low molar absorption coefficient leads to radical scavenging and reduced MTX degradation in the UV 254 /SPC system. , This difference in radical behavior between two systems may contribute to their varying effectiveness in degrading MTX in a real secondary effluent.…”

“…This reduction is likely due to the fact that the excess HCO 3 – consumes •OH (eq 7), lowering MTX degradation efficiency. Conversely, the presence of NO 3 – exhibited a promotional effect on MTX degradation, aligning with previous research. , This effect may be attributed to the fact that NO 3 – can promote the generation of •OH and the generation of reactive nitrogen species (RNS), , thus increasing the fluence-based degradation rate of MTX.…”

“…In addition, DOM in the real secondary effluent samples may further reduce MTX degradation rates through quenching of excited states or radicals . Notably, the fluence-based degradation rate of MTX decreased by 45% in the UV 254 /SPC system, whereas it decreased by 38% in the UV 222 /SPC system, indicating that UV 222 /SPC exhibited better stability for MTX degradation, which is similar to previous studies . This improved performance in the UV 222 /SPC system may be attributed to its more effective free radical generation, which mitigated the impact of radical scavenging by components in the wastewater, thus maintaining a higher MTX degradation rate.…”

“…Sodium percarbonate (SPC), as a low-cost, stable, adaptable, and eco-friendly solid replacement for liquid-phase hydrogen peroxide (H 2 O 2 ), has shown promise in removing various contaminants. − UV/SPC, particularly with low-pressure mercury UV lamps (LPUV) at 254 nm, has demonstrated effectiveness in contaminant removal. , However, LPUV has some drawbacks during operation, such as the possibility of harm to human health, the risks of mercury pollution, and requiring a long stabilization time. , In contrast, KrCl* excimer lamps at 222 nm have received increasing attention within water treatment realms due to their environmental friendliness, minimal risk to human skin and eyes, and rapid stabilization periods. , UV 222 has demonstrated superiority over UV 254 in various wastewater treatment technologies, − positioning it as a potential alternative to LPUV lamps in future water treatments. One notable advantage of UV 222 is its higher photon energy (539 kJ einstein –1 ) compared to UV 254 (471 kJ einstein –1 ), which may enhance the direct photolysis rates of contaminants. ,, In addition, many oxidizing agents exhibit higher molar absorption coefficients and/or quantum yields at 222 nm, ,− suggesting that UV 222 -AOPs could induce higher concentrations of free radicals, facilitating more effective contaminant degradation. , However, carbonate (CO 3 2– ) released by SPC shows a higher molar absorption coefficient at 222 nm (42.3 M –1 cm –1 ) but exhibits minimal light absorption at 254 nm, suggesting that the reactivity and reaction mechanism of UV 222 /SPC could be different from UV 254 /SPC. In addition, the generation of the radicals and their relative contributions to the degradation efficiency of UV/SPC are not well understood when the wavelength shifts from 254 to 222 nm.…”

Control of Micropollutants in Water by Far-UVC Photolysis of Peracetic Acid

“…While the UV 254 /PAA process can generate radicals for micropollutant degradation, the yield of HO • is relatively low (e.g., compared to the UV 254 /H 2 O 2 process with the same H 2 O 2 initial concentration), − due to the small molar absorption coefficient of PAA at 254 nm (5.0–8.0 M –1 cm –1 ), despite that the innate quantum yield for PAA photolysis at 254 nm is 0.88 mol einstein –1 . KrCl* excimer lamps emitting far-UVC radiation at around 222 nm (UV 222 ) has emerged as a promising UV radiation source during the COVID-19 pandemic. , Several previous research has reported the enhanced generation of reactive species from the photolysis of oxidants including H 2 O 2 , chlorine, and persulfate by replacing the conventional low-pressure mercury lamps with the KrCl* excimer lamps. − For example, the UV fluence-based concentrations of HO • generated in the UV 222 /H 2 O 2 process is 9.4-fold and 3.7-fold higher than that in the UV 254 /H 2 O 2 process in deionized water and groundwater, respectively; the UV fluence-based concentrations of HO • and reactive chlorine species (e.g., Cl • ) is 5.0-fold and 12.0-fold higher in the UV 222 /chlorine process than the UV 254 /chlorine process; switching the UV wavelength from 254 to 222 nm also increases the UV fluence-based concentrations of HO • and sulfate radicals (SO 4 •– ) generated in the UV/peroxydisulfate process by 6.0-fold and 55.5-fold, respectively, in real surface water …”