利用金屬氧化物活化過硫酸鹽降解有機污染物: 反應動力、反應機制及反應牆模組應用之研究

Abstract

過硫酸鹽包括過單硫酸鹽(PMS)和過二硫酸鹽(PDS)，為水處理程序及現地化學氧化法用於整治受污染地下水體時常用的氧化劑。過硫酸鹽因在水中穩定，需加以活化產生自由基或其他反應活性物質以用於降解有機污染物。近年來，人們致力於開發由金屬氧化物活化過硫酸鹽的異相過硫酸鹽活化方法，此活化方法具有低能耗且高降解效率的特性。然而，此類型之活化方法其反應機制仍不明確，且未建立完整的反應動力式。本研究首先使用氧化銅(CuO)、氧化鎳(NiO)、氧化鈷(Co3O4)、氧化鐵(Fe2O3)和二氧化錳(MnO2)做為活化劑活化過硫酸鹽用以去除水中之2,4-二氯酚污染物。研究結果發現金屬氧化物之表面羥基氧、氧空缺和零電荷點pH(pHpzc)之協同效應可同時影響金屬氧化物與過硫酸鹽間的表面相互作用關係，從而影響硫酸鹽被活化能力。由控制實驗、自由基捕捉試驗、電子自旋共振分析和離子強度效應的結果顯示，被吸附的過二硫酸鹽生成表面結合氫氧自由基(surface-bound OH•)是氧化銅活化過二硫酸鹽系統中，造成2,4-二氯酚降解的主要反應物質。經由反應機制，本研究考慮了幾項重要反應步驟建立反應動力學模型，包含(1)過二硫酸鹽吸附到氧化銅表面形成內層表面錯合物，(2)過二硫酸鹽錯合物經活化後形成表面結合硫酸根自由基(surface-bound SO4•−)與表面結合氫氧自由基，(3)2,4-二氯酚再與表面結合氫氧自由基反應從而被降解。開發的動力學模型可做為其他以表面結合自由基為主之過硫酸鹽氧化系統的主要反應框架，以更好地解釋過硫酸鹽活化系統之反應動力學。 本研究接續以過二硫酸鹽為氧化劑、氧化銅為材料，整合現地化學氧化法和透水性反應概念，建立氧化銅透水性反應牆模組，以去除地下水中之2,4-二氯酚、2,4,6-三氯酚及五氯酚。使用合成水樣評估在不同氯酚濃度(10-150 µM)和流速(1.8-14.4 mL/min)下對系統性能的影響，可發現當[PDS]/[氯酚]>1與足夠停留時間下，氯酚類有機污染物的去除率皆大於90%。使用實際地下水做為實驗水樣時，2,4-二氯酚及2,4,6-三氯酚的去除率些微下降，而五氯酚之去除效果僅剩20%。研究發現，地下水中之碳酸氫根濃度為影響氧化銅透水性反應牆模組去除地下水中五氯酚污染物之關鍵參數，五氯酚之去除率降低可能是由於主要參與反應之自由基與碳酸氫根反應生成反應性較弱的自由基、碳酸氫根離子會競爭氧化銅表面上過二硫酸鹽的活性點位或由五氯酚本身的空間位阻所造成。

Persulfates including peroxymonosulfate (PMS) and peroxydisulfate (PDS) are promising oxidants for water treatment and for in-situ chemical oxidation to remediate contaminated groundwater. They require activation to generate radicals or other reactive species to degrade organic pollutants. Recently, efforts were devoted to developing heterogeneous persulfate activation systems by metal oxides due to the lower energy consumption and high degradation efficiency. However, the mechanism was still ambiguous and the kinetics was either not quantitative or empirical in nature. In this research, the removal of 2,4-dichlorophenol (2,4-DCP) using persulfate activated by CuO, NiO Co3O4, Fe2O3 and MnO2 were first examined. The results showed that the synergistic effects of hydroxyl oxygen, oxygen vacancy and pHpzc collectively influence the interaction between metal oxides and persulfates, thereby determining the driving force for persulfate activation. Controlled experiments, radical scavenging experiments, electron paramagnetic resonance (EPR) studies and ionic strength effects showed that surface-bound OH• generated from the adsorbed PDS was the main reactive species responsible for the degradation of 2,4-DCP in the 2,4-DCP/CuO/PDS system. A kinetic model considering the important reaction steps, which includes the adsorption of PDS onto CuO to form inner-sphere surface complex, activation of adsorbed PDS complex to form surface-bound SO4•− and then surface-bound OH•, and degradation of 2,4-DCP by surface-bound OH•, was developed to better elucidate the reaction kinetics. The developed kinetic model could serve as a framework to characterize other persulfate oxidation systems relying on surface-bound radicals. A novel system integrating ISCO and permeable reactive barrier (PRB) using PDS as the oxidant and CuO as the reactive barrier material was developed for the removal of chlorophenols (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and pentachlorophenol (PCP)). The influences of chlorophenol concentration (10-150 M) and flow rate (1.8-14.4 mL/min) on the system performance were evaluated using synthetic solutions. The removal efficiencies of target chlorophenols were greater than 90% when sufficient PDS was supplied ([PDS]/[chlorophenol]>1). When groundwaters were employed, the removal efficiencies of 2,4-DCP and 2,4,6-TCP reduced to 90% and 85%, while that of PCP dropped to 20%. The level of [HCO3−] in the groundwater played a key role in manipulating the performance of Cu-PRB module for PCP removal. The reduced removal efficiency could be due to the formation of weaker radicals, the stronger competition between bicarbonate ions and PDS for the activation sites on the CuO surfaces and the steric hindrance of PCP.