利用紫外光活化過硫酸鹽降解4-甲基亞芐基樟腦

Abstract

4-甲基亞芐基樟腦 (4-MBC) 為常添加於防曬乳的成分之一，並且已有研究指出4-MBC會影響雌激素的活性。由於傳統污水處理廠無法有效地去除4-MBC，因此近年來於許多環境水體中皆檢測出ng/L到µg/L範圍殘留濃度的4-MBC。而利用紫外光活化過硫酸鹽是目前認為具前景的高級氧化處理程序，亦有許多研究指出此處理程序能有效去除污水處理廠中難以被去除的有機污染物。因此本研究探討利用紫外光(254 nm)活化過硫酸鹽以降解水體中的4-MBC。並以暸解反應機制、主要參與反應之自由基、降解副產物及途徑、毒性變化及實際應用等面向為主要研究目標。 此處理程序在過硫酸鹽濃度範圍於4.2 µM 到 42 µM間皆符合擬一階動力學反應；過硫酸鹽濃度亦與反應速率常數呈正線性關係(R2=0.997)。4-MBC在單純只照射紫外光的情況下，只會於E型異構物與Z型異構物之間轉換並不會降解。然而，在4-MBC濃度0.39 µM、過硫酸鹽濃度42 µM、反應溶液pH值為7之條件下，本系統能有效地在6分鐘內達到90%的4-MBC去除率，此成效遠優於使4-MBC在單純只照射紫外光及在過硫酸鹽氧化反應下降解。此外，反應速率常數於酸性 (pH 5) 及中性情況下為11.8 × 10−2 min−1 到 11.0 × 10−2 min−1之範圍，然而於鹼性 (pH 9) 條件下則會大幅降低至6.8 × 10−2 min−1。 藉由自由基抑制與競爭動力學實驗結果發現，在此處理程序中硫酸根自由基 (SO4−•) 為主要與4-MBC反應之自由基，而SO4−•與4-MBC的二階反應常數經估算為 (2.95 ± 0.05) × 109 M−1 s−1。此外，所添加的過硫酸鹽於反應後之十分鐘內皆轉換為硫酸根離子。而4-MBC於降解過程將遵循兩種反應途徑：羥基化與去甲基化，並分別生成降解副產物P1 (C18H22O2, m/z = 271.1587) 與P2 (C17H22O, m/z = 242.2030)。另外經由費氏弧菌毒性試驗顯示，經由紫外光活化過硫酸鹽程序處理後溶液的毒性於反應的前20分鐘會不斷上升，且維持相同的毒性一段時間後才開始下降。經由毒性變化與副產物生成之數據結果得以推斷，4-MBC在反應結束前並沒有被完全礦化，且於反應的過程中生成尚未被發現且毒性比4-MBC還高同時亦更具光敏性之降解副產物。另一方面，此處理程序於實際泳池水中之4-MBC降解效率會從原先的93%去除率下降至48%，推測水中氯離子是造成此現象之主因。因此，未來若欲將此方法應用至實際污水處理，先行過濾無機離子的是重要的一個程序。

4-Methylbenzylidene camphor (4-MBC), a widely used UV filter, has been reported to show estrogenic activity. Owing to insufficient removal in conventional wastewater treatment plants, 4-MBC has been widely detected at the level of ng/L to µg/L in the aquatic environment, The UV-activated persulfate (UV/persulfate) process is a promising and efficient technology that has the potential to remove many recalcitrant organic contaminants. Thus, using the UV/persulfate process to degrade 4-MBC was first evaluated in the present study. The goals of this work were to determine the reaction mechanism, reactive species, transformation byproducts formation and pathways, and change in toxicity and to apply process in an actual water matrix. 4-MBC degradation can be well fitted by pseudo-first-order kinetics; the rate constant and the persulfate dosage have a linear relationship in the persulfate dosage range of 4.2 µM to 42 µM. Under the conditions of [4-MBC]0 = 0.39 µM, [persulfate]0 = 42 µM and initial pH = 7, up to 90% of 4-MBC was decomposed within 6 min by the UV/persulfate process, which is advantageous compared to the results obtained using UV irradiation alone and persulfate dark oxidation. Upon UV photolysis alone, 4-MBC experienced only photoisomerization between (E)- and (Z)-4-MBC. The rate constant remained similar, ranging from 11.8 × 10−2 min−1 to 11.0 × 10−2 min−1, in acidic (pH 5) and neutral pH, whereas it significantly decreased to 6.8 × 10−2 min−1 in basic conditions (pH 9). Radical scavenging and competition kinetics experiments indicated that SO4−• exhibited much higher reactivity toward 4-MBC than that of HO•, and the second-order rate constant of SO4−• with 4-MBC was estimated to be (2.95 ± 0.05) × 109 M−1 s−1. Moreover, after 10 min of reaction time, all the added persulfate was completely transformed into sulfate anion. 4-MBC followed transformation pathways including hydroxylation and demethylation, resulting in the generation of the transformation products P1 (C18H22O2, m/z = 271.1587) and P2 (C17H22O, m/z = 242.2030), respectively. Microtox® acute toxicity tests with Vibrio fischeri indicated that the inhibitory effect continuously increased in the first 20 mins and then remained at the same level for a certain time before starting to decrease. The rising toxicity indicated the formation of unknown transformation products that are more toxic and photolabile than 4-MBC itself, and 4-MBC was not completely mineralized at the end of the reaction. In contrast, the 4-MBC degradation rate was significantly attenuated in outdoor swimming pool water (the removal efficiency decreased from 93% to 48%), which resulted from the high concentration of Cl−. Consequently, removing inorganic anions would be an important pretreatment step if this UV/persulfate process were to be used in real wastewater environments.