| 摘要: | 本研究利用批次實驗，探討二價鐵離子、鈣離子與硫酸根離子等無機離子存在時，對零價鐵降解TCE之影響，並且利用SEM-EDS與XPS等精密儀器，進行鐵粉表面微觀觀察與表面氧化物物種與含量的分析，以瞭解鐵粉表面氧化物種類的變化對TCE降解的影響。 研究結果顯示，未經酸洗前處理之鐵粉，對TCE不具有降解能力，由XPS分析結果發現，係因其表面存在Fe2O3鈍化膜，阻礙零價鐵釋放電子，而與水中TCE發生氧化還原反應的機會。然而，經過酸洗前處理後之鐵粉，TCE的降解速率大幅增加，係因表面氧化物物種的組成，Fe2O3含量大幅降低，而轉變為易水解之α-FeOOH，因而增加鐵粉可參與反應的表面積及氧化能力，提高還原脫氯TCE的能力，因此在反應時間37小時，TCE去除率即可達100 ％。此外，二價鐵離子存在於TCE水溶液時，實驗結果發現，在本研究條件下，對TCE並不具有降解能力，而經由Nernst equation計算發現，必須在低pH及高二價鐵離子濃度下，二價鐵離子對TCE才具有氧化還原的能力。當二價鐵離子存在於未經酸洗前處理之鐵粉時，在反應初期10分鐘內，TCE幾乎沒有降解的情形發生，然而隨著反應時間的增加，TCE的降解速率也隨之增加，且降解速率隨初始二價鐵離子濃度的增加而增加，當二價鐵離子的濃度由0 mg/L增加至300 mg/L時，反應77小時後，水中TCE的濃度由90 mg/L降低至20 mg/L，未經酸洗前處理之鐵粉，二價鐵離子對TCE降解速率的增加，係因為二價鐵離子會吸附至鐵粉表面的Fe2O3晶格，將Fe2O3還原轉化成具半導體特性Fe3O4，允許由零價鐵表面釋放之電子通過。而當二價鐵離子存在於酸洗前處理之鐵粉時，實驗結果發現，當初始二價鐵離子濃度由0 mg/L增加至 300 mg/L，TCE的降解速率由0.084 h-1下降至0.056 h-1，利用SEM-EDS與XPS觀察鐵粉表面發現，反應初期，溶液中之二價鐵離子主要形成氫氧化亞鐵沈澱物，覆蓋於鐵粉表面，隨者反應時間增加，進一步氧化為Fe3O4，雖然Fe3O4具半導體特性，可使零價鐵釋出之電子通過，與TCE接觸發生氧化還原反應，但相較之下，其電子傳遞速度，仍不如具導體性質之零價鐵，故二價鐵離子存於酸洗前處理之鐵粉時，會使鐵粉表面電子傳遞能力下降，同時降低TCE去除速率。 此外，在本實驗條件下，反應時間77小時內，水溶液中鈣離子與TCE的濃度幾乎沒有變化，主要係因為溶液中之氫氧根離子與碳酸根離子濃度，尚不足與鈣離子形成氫氧化鈣或碳酸鈣沈澱，降低鐵粉表面活性。 硫酸根離子存在於Fe0-TCE系統時，硫酸根離子與零價鐵氧化還原反應後之氫氧化亞鐵，形成可溶性硫酸亞鐵，避免氫氧化亞鐵氧化繼續轉化為Fe3O4，降低鐵粉可反應表面積與電子傳遞能力，因而提升TCE還原脫氯速率。 The purpose of this study was aimed to investigate the effects of inorganic ions, such as ferrous ion, calcium ion, and sulfate ion on the reductive dechlorination of trichloroethylene (TCE) with batch tests in aqueous solution by zero-valent iron (ZVI). Also, the observation of the surface of metal iron by scanning electron microscopy with energy dispersive X-ray spectrum (SEM-EDS) and X-ray photoelectron spectrometer (XPS) analysis were carried out to identify the formation of hydroxide precipitations on the surface of metal iron and that of effects of TCE degradation. Experimental results indicated that the metal iron without acid washing pretreatment could not reduce TCE effectively because the passive film of maghemite (Fe2O3) might coat on the surface of metal iron by the observation of XPS analysis. The precipitations of maghemite on the surface of metal iron could hinder the electrons released from the oxidation of metal iron to the bulk solution and affect the reductive dechlorination of TCE. However, the degradation rate of TCE with ZVI would increase after pretreatment of metal iron with acid washing. At the 37 h of reaction time, the removal percentage of TCE would reach 100 %. The observation of the surface of metal iron by XPS analysis showed that the content of maghemite would decrease and the conformation of oxides on the surface of metal iron with acid washing changed from maghemite to more hydrated α-FeOOH, therefore, the reactive specific area and the ability of oxidation of metal iron might increase. Additionally, the ferrous ion could not directly reduce TCE in this study. According to the Nernst equation, the reductive dechlorination of TCE by the ferrous ion should be at the low pH and high concentration of ferrous ion. Experimental results indicated that in the presence of ferrous ion with unacid washing metal iron in the Fe0-TCE system, the degradation of TCE would not occur during the initial 10 min of the reaction time. However, the degradation rate of TCE increased with the increase of reaction time. Also, the degradation rate increased with the initial concentration of ferrous ion. When the concentration of ferrous ion increased from 0 mg/L to 300 mg/L, the residual concentration of TCE in the solution decreased from 90 mg/L to 20 mg/L after the 77 h of reaction time. The reasons of the increase rate of TCE degradation with unacid washing metal iron was that ferrous ion absorbed to the lattice of maghemite which coating on the surface of metal iron and converse maghimite to the semiconductive materials of magnetite(Fe3O4) that was permitted electrons to pass those precipitations from the metal iron surface and reduce TCE in the solution, therefore, the degradation rates of TCE was enhanced. On the other hand, when the ferrous ion coexisted with acid washing metal iron, the rate constant of TCE decreased from 0.084 h-1 to 0.056 h-1 as the concentration of ferrous ion increase from 0 mg/L to 300 mg/L. The observation of the surface of metal iron by SEM-EDS and XPS analysis showed that the ferrous ion would form the ferrous hydroxide coating on the surface of ZVI and decreased the degradation rate of TCE during the initial reaction time. With the increase of reaction time, the ferrous hydroxide precipitations would be oxidized to form magnetite and had not hindered the electrons to contact with TCE for reductive dechlorination. However, the transfer rate of electrons from magnetite on the surface of metal iron was still less than that of ZVI. Consequently, it revealed that the removal rate of TCE would decrease when the ferrous ion coexisted with acid washing acid. Additionally, the concentrations of calcium ion and TCE would not be different in the solution during the 77 h of reaction time when calcium ion coexisted in the Fe0-TCE system. This was because the both concentration of OH- and carbonate was not enough to form the precipitations of calcium hydroxide and calcium carbonate on the surface of ZVI in this study. Also, the calcium ion have no an effect on the reductive dechlorination of TCE. When the sulfate ion coexist in the Fe0-TCE system, the sulfate ion would react with the ferrous hydroxide that be oxidized by the ZVI, and form the soluble ferrous sulfate (FeSO4) in the solution. Further, it avoided that the ferrous hydroxide was oxidized to form the magnetite continually, thus, the reactive specific area and the ability of electrons releasing of ZVI could decrease. Consequently, the sulfate ion would enhance the degradation rate of the TCE by ZVI in the Fe0-TCE system |
