"利用氧化銅活化過二硫酸鹽降解2,4-二氯酚之研究：反應動力及反應機制"

Abstract

過二硫酸鹽具有高穩定性、不易在水中衰減的特性，近年來被廣泛應用於現地化學氧化程序處理地下水污染物，其注入地下井後，具有較遠、較廣的整治範圍。透過活化，過二硫酸鹽可轉變為具有高氧化還原電位的硫酸根自由基，可有效降解有機污染物。然而，在含有鹵素離子的水溶液中，硫酸根自由基會與其反應產生鹵素自由基，進而和有機物反應生成其他高毒性的含鹵有機污染物。 近來，有研究指出以氧化銅活化過二硫酸鹽可以有效降解地下水常見的氯酚類污染物，相異於一般的活化方法，此活化系統似乎不產生硫酸根自由基，便能有效降解污染物。然而，其反應動力卻沒有深入探討。本研究針利用氧化銅活化過二硫酸鹽去除2,4二氯酚之反應機制及反應動力進行探討，實驗結果指出，吸附於氧化銅表面上的過二硫酸鹽，為真正被活化、參與降解反應的氧化物，並非所有存在水相中的過二硫酸鹽皆參與反應，且自由基捕捉劑實驗證實此活化程序並無自由基產生。此外，本研究亦探討不同水質參數對污染物降解效率造成的影響，透過添加不同氧化銅劑量，發現2,4-二氯酚的降解與氧化銅之表面積成正比，以表面積標準化後，可得到趨於一致的反應速率常數。然而，在不同的過二硫酸鹽以及2,4-二氯酚添加量下，標準化後的反應速率並不一致，在過二硫酸鹽添加量越高、2,4-二氯酚添加量越低時，反應速率常數越高，此結果與理論相悖，推測可能原因為計算反應速率時，未以正確吸附至氧化銅表面之過二硫酸鹽量計算，造成誤差。另外，在不同酸鹼條件下，中性時具有最高的反應速率，當系統趨於酸性或鹼性條件時，由於過二硫酸鹽吸附於氧化銅表面之條件改變，使得反應速率降低。利用氧化銅活化過二硫酸鹽降解2,4-二氯酚是有效的處理程序，氧化銅在此程序中扮演電子傳遞者的角色，當其表面吸附之過二硫酸鹽接觸2,4-二氯酚時，電子即從2,4-二氯酚透過氧化銅傳遞至過二硫酸鹽，進而達成2,4-二氯酚的降解。

Peroxydisulfate (PDS) has been considered as a promising oxidant for in-situ chemical oxidation for groundwater remediation. PDS is stable in aqueous phase allowing it to travel a great distance to reach contaminants far from the injection well and can be activated to generate sulfate radical that is capable of degrading a variety of organic pollutants. In the presence of halide ions, however, sulfate radical can oxidize these ions to form halogen radicals that can react with ubiquitous organic matter to form toxic halogenated organic compounds. CuO has been recently used to activate PDS and superior efficiency was found for the removal of chlorophenols. Unlike other systems, no sulfate radical seems to be produced by this process. However, the kinetic for the PDS/CuO process is still unclear. In this research, the mechanisms and kinetics for the degradation of 2,4-dichlorophenol (2,4-DCP) by the PDS/CuO process were investigated. It was found that only the adsorbed PDS on the CuO surface instead of the PDS in the bulk solution was activated. Radical scavenging studies also demonstrated that the degradation of 2,4-DCP by the PDS/CuO process is a non-radical process. The effects of water chemistry on this process were also investigated. It was found that the rate of 2,4-DCP degradation was proportional to the CuO dosage employed and the surface area normalized initial rate constants converged to a single value. However, the normalized rate constants increased with the increasing PDS dosage and decreasing 2,4-DCP concentrations, which could possibly result from the use of bulk PDS concentration instead of adsorbed PDS concentration in the calculations. In terms of the influence of pH, the highest rate of degradation was found at the neutral condition. Overall, 2,4-DCP could be well-degraded by the PDS/CuO process. CuO acts like an electron shuttle transferring the electron from 2,4-DCP to adsorbed PDS.