美沙酮於氯化消毒下之太陽光解 – 降解動力學及其對N-亞硝基二甲胺(NDMA)形成影響

Sunlight photolysis of methadone in the chlorine-based disinfected water: degradation kinetics and effect on N-nitrosodimethylamine (NDMA) formation

謝明季(Ming-Chi Hsieh)

指導教授 ： 林郁真

國立臺灣大學/工學院/環境工程學研究所/博士(2020年)

https://doi.org/10.6342/NTU202000699

摘要

N-亞硝基二甲胺 (N-nitrosodimethylamine, NDMA) 是一種具有致癌性的新興消毒副產物。根據先前文獻指出，飲用水中NDMA與含有胺基之化合物經由氯胺的氧化有關。然而在後續的文獻指出，於游泳池水中 (水來源為飲用水) 發現高濃度的NDMA。游泳池水中的NDMA主要是藉由游泳池中的自由餘氯與有機質物質(泳客游泳時釋出的化合物)之間的反應而形成。考量到泳客於游泳的過程中可能會無意間暴露到NDMA，因此NDMA於游泳池中的生成為本研究所熱衷探討的議題。本篇研究目的是探討自由餘氯太陽光解或氯胺太陽光解過程中對於藥物的去除以及隨後NDMA形成的情況。此外，我們全面研究了自由餘氯太陽光解或氯胺太陽光解過程中的降解動力學、反應物種的相對貢獻、副產物的形成、毒性的變化以及水質參數的影響。
本研究使用潛在的NDMA前驅物，美沙酮作為目標污染物。其結果發現與直接光解相比，自由餘氯光解能夠提高美沙酮的降解 ([Methadone] = 0.2 μM, [Free chlorine] = 4 μM, and pH = 4)，並且美沙酮的自由餘氯光解速率可透過擬一階動力方程式加以說明。結果指出美沙酮於自由餘氯光解以及直接光解下之擬一階反應速率常數分別為7.0±1.1 × 10–2 min−1 and 1.4±0.2 × 10–2 min−1。美沙酮的降解速率在自由餘氯太陽光解過程中能夠顯著提升在於氫氧自由基 (HO•) 以及活性氯物種 (RCS) 的產生。自由餘氯太陽光解在酸性以及中性條件下的主要物種為 HO• 以及RCS，而直接光解是在鹼性條件下造成美沙酮降解的主要途徑。此外，初始pH (pH 4−11) 以及自由餘氯濃度 (1−6 μM) 顯著影響了美沙酮的降解效率。碳酸氫根 (HCO3–)、氯離子 (Cl–) 和溶解性有機物的存在可能與HO•以及RCS發生競爭性的反應，從而減緩了美沙酮的降解。因此當使用飲用水進行自由餘氯太陽光光解時，將會發現美沙酮的降解效率降低了50%。
在氯胺作為氧化劑的部分，相較於直接光解以及單純氯胺氧化，氯胺太陽光解能提升美沙酮的降解並且降低NDMA的生成。在氯胺太陽光光解過程中，美沙酮的降解遵循擬一階反應動力，並且其降解速率常數隨著二氯胺劑量增加而上升。美沙酮氯胺光解的過程中在Cl2/N為1.5的情況時發現有最高的NDMA生成，並且其NDMA產量隨著pH值的升高而增加。最後為硝酸鹽 (NO3–) 的存在能進一步減少NDMA的生成，原因在亞硝酸鹽 (NO2–) 會於NO3–在太陽光的照射下形成，進而消耗一氯胺的量，因此降低NDMA的生成。以上結果考慮太陽光的存在下，不同水基質進行了自由餘氯氧化或氯胺氧化來評估美沙酮降解以及NDMA的生成，這些都會可能發生在污水處理的後加氯程序或是在戶外游泳池的場景中。

關鍵字

N-亞硝基二甲胺 ； 美沙酮 ； 自由餘氯太陽光光解 ； 反應動力學 ； 氯胺太陽光光解 ； 活性氯物種

並列摘要

N-nitrosodimethylamine (NDMA) is a potent carcinogenic emerging disinfection byproduct (DBP) for humans at ng/L level. The presence of NDMA in drinking water is most commonly associated with chloramination of amine-based precursors. However, previous literature indicated that the high concentration of NDMA formed during the chlorination of swimming pool water (considered pools filled with drinking water). The formation of NDMA in swimming pool water mainly occurs through the reaction between free chlorine and natural organic matter, which can be derived from the pool water and swimmers. Swimmer activities in swimming pools release various substances (such pharmaceutical), the unintentional exposure to NDMA in swimming pool water is a particularly important area of concern. The aim of this study was the investigation and evaluation of the removal efficiency of pharmaceutical and associated NDMA formation during sunlight photolysis of free chlorine (sunlight/FC) and sunlight photolysis of chloramine (sunlight/chloramine) process. Additionally, the degradation kinetics, relative contributions of the reactive species, byproducts formation, change in toxicity, and effect of water quality parameters during the sunlight/FC or sunlight/chloramine process were comprehensively explored.
This study selected methadone, a potential NDMA precursor, as a model compound and found that sunlight/FC markedly enhances the degradation rate of methadone, over that obtained using sunlight alone. The pseudo-first-order rate constants of methadone degradation under acidic conditions ([Methadone] = 0.2 μM, [Free chlorine] = 4 μM, and pH = 4) for sunlight/FC and sunlight photolysis are 7.0±1.1 × 10–2 min−1 and 1.4±0.2 × 10–2 min−1, respectively. The improved methadone degradation can be attributed to the production of HO• and reactive chlorine species (RCS) during sunlight/FC photolysis. HO• and RCS predominantly accounted for degradation during sunlight/FC photolysis under acidic and neutral conditions, while direct photolysis was the major contributor towards methadone degradation in alkaline conditions. The initial pH (pH 4−11) and free chlorine concentration (1−6 μM) significantly influenced the overall degradation efficiency of methadone. The presence of HCO3−, Cl− and dissolved organic matters, which may competitively react with HO• and RCS, retard the degradation of methadone in synthetic wastewater. Consequently, a 50% lower methadone degradation rate was observed when deionized (DI) water was replaced with tap water.
Sunlight/chloramine enhances the degradation of methadone and reduces NDMA formation compared to those under sunlight or chloramination processes alone. The degradation of methadone under all investigated sunlight/chloramine conditions followed pseudo-first-order kinetics, and the degradation rate constant increased with the dosage of dichloramine (NHCl2). The highest conversion of NDMA from methadone occurred at a chlorine-to-ammonia (Cl2/N) of 1.5, and the NDMA formation increased with increasing pH (pH = 4−9.5). The presence of nitrate further decreased NDMA formation since the production of nitrite consumes monochloramine (NH2Cl) under sunlight irradiation. These results emphasize the need to consider different water matrices influencing methadone degradation and the formation of NDMA in chlorinated or chloraminated waters under natural sunlight, as is found in oxidation ponds post-chlorination or in swimming pools.

並列關鍵字

N-nitrosodimethylamine (NDMA) ； Methadone ； Free chlorine sunlight photolysis ； Reaction kinetics ； Chloramine sunlight photolysis ； Reactive chlorine species

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