Payne Emma M, Langelan Emma G, Linden Karl G
Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, 4001 Discovery Dr., Boulder, CO, 80303, United States.
Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, 4001 Discovery Dr., Boulder, CO, 80303, United States.
Water Res. 2025 Sep 1;283:123754. doi: 10.1016/j.watres.2025.123754. Epub 2025 Apr 30.
Interest in krypton chloride excimer lamps, which emit primarily at 222 nm (UV222), for applications to water treatment has been growing rapidly in the last few years, due to the greater contaminant degradation and pathogen inactivation enabled at this low wavelength. Nitrate absorbs UV very strongly at 222 nm (ε=2747 M cm) and is thus of particular interest in KrCl* water treatment. While the ability of nitrate to promote hydroxyl radical formation under UV irradiation from other UV sources has been well-demonstrated, the effect of nitrate on UV222/HO has not been previously investigated. When nitrate is present at 5 mg-N/L or greater, addition of hydrogen peroxide, a common radical promoter in UV advanced oxidation processes, leads to a 7.3 to 20.8 % decrease in degradation of para-chlorobenzoic acid (pCBA), a probe compound for hydroxyl radical formation. This effect is attributed to 1) HO acting solely as a scavenger, rather than source, of hydroxyl radicals due to light screening by nitrate during 222 nm UV and 2) increased formation of nitrite from nitrate photolysis when peroxide is present, leading to more hydroxyl radical scavenging. Nitrite was found to exceed the maximum contaminant level of 1 mg-N/L when nitrate was present at 7.5 mg-N/L, presenting a possible challenge for applications of UV222. However, it was also found that nitrite may act as a source of hydroxyl radicals due to its high absorbance and quantum yield at 222 nm, which can compensate for the increased hydroxyl scavenging by photo-produced nitrite. Lastly, the impact of irradiation path length, an important experimental design parameter, was investigated for the UV/nitrate process and found to significantly influence chemical degradation results (k varied by 1.4-1.9 times as a function of path length), due to the high light absorption of nitrate violating several key assumptions in the standard methods for calculating UV fluence. In particular, this work challenges the inclusion of the water factor in calculating UV fluence in 222 nm studies, as the water factor corrects for photon attenuation by the background water matrix but leads to erroneous results when nitrate is present due to both nitrate's impact on the water factor by photon absorption and its role as the primary source of hydroxyl radicals during 222 nm irradiation. This work demonstrates the significant, and unexpected, impact of nitrate on UV222 advanced oxidation, and identifies key issues that researchers of this technology should consider.
在过去几年中，由于在这个低波长下能够实现更大程度的污染物降解和病原体灭活，人们对主要发射222纳米（UV222）光的氪氯准分子灯在水处理中的应用兴趣迅速增长。硝酸盐在222纳米处对紫外线有很强的吸收（ε=2747 M cm），因此在氪氯准分子灯水处理中特别受关注。虽然硝酸盐在其他紫外线源的照射下促进羟基自由基形成的能力已得到充分证明，但硝酸盐对UV222/HO的影响此前尚未被研究。当硝酸盐浓度为5毫克氮/升或更高时，添加过氧化氢（紫外线高级氧化过程中一种常见的自由基促进剂）会导致对羟基苯甲酸（pCBA，一种用于检测羟基自由基形成的探针化合物）的降解率下降7.3%至20.8%。这种影响归因于：1）由于硝酸盐在222纳米紫外线照射期间的光屏蔽作用，HO仅作为羟基自由基的清除剂而非来源；2）当过氧化物存在时，硝酸盐光解产生亚硝酸盐的量增加，导致更多的羟基自由基被清除。当硝酸盐浓度为7.5毫克氮/升时，发现亚硝酸盐超过了1毫克氮/升的最大污染物水平，这对UV222的应用可能构成挑战。然而，还发现亚硝酸盐由于其在222纳米处的高吸光度和量子产率，可能作为羟基自由基的来源，这可以弥补光产生的亚硝酸盐增加的羟基清除作用。最后，研究了照射路径长度（一个重要的实验设计参数）对紫外线/硝酸盐过程的影响，发现它对化学降解结果有显著影响（k随路径长度变化1.4 - 1.9倍），这是因为硝酸盐的高吸光率违反了计算紫外线通量的标准方法中的几个关键假设。特别是，这项工作对在222纳米研究中计算紫外线通量时纳入水因子提出了挑战，因为水因子校正了背景水基质对光子的衰减，但当硝酸盐存在时会导致错误结果，这是由于硝酸盐通过光子吸收对水因子的影响以及它在222纳米照射期间作为羟基自由基主要来源的作用。这项工作证明了硝酸盐对UV222高级氧化有显著且意想不到的影响，并确定了该技术研究人员应考虑的关键问题。
