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標題: Effects of low molecular organic acids (LMWOAs) on Cr(III) photo-oxidation in the presence of Fe(III)
作者: Chung-Tse Chang
關鍵字: photocatalytic;Fe(III);Cr(III) photo-oxidation;Cr(VI);low molecular weight organic acid (LMWOA);hydroxyl radical;光催化;三價鐵;三價鉻光氧化;六價鉻;小分子量有機酸;氫氧自由基
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Chromium is a commonly found pollutant in the soils/sediments and water bodies because it can be discharged intentionally or unintentionally from various industrial activities. Chromium exists in two major forms of Cr(III) and Cr(VI). The physicochemical properties and toxicities of Cr(VI) and Cr(III) are quite different. Cr(III) is a cation, exhibiting a lower mobility and toxicity than that of Cr(VI), and thus, the possible oxidation of Cr(III) to harmful Cr(VI) in a natural system had received much scientific concerns in the past decades.
Photolysis of FeOH2+ is a well-known pathway occurred naturally which are capable of generating hydroxyl radicals (‧OH) for Cr(III) oxidation in an acidic solution. These reactions may be modified by the low molecular weight organic acid (LMWOA), because LMWOA distributes widely in the environment which may participate in the redox reactions. However, the role of LMWOA in regulating Fe species and subsequent Cr(III) photo-oxidation as influenced by pH remains unexplored. To evaluate precisely Cr transformation and fate, it is necessary to investigate the reactions of Fe(III) photolysis and Cr(III) photo-oxidation in detail when various LMWOAs are presented. Therefore, the objectives of this study are to evaluate systematically the effects of irradiation, pH and LMWOAs, including citric acid, oxalic acid and acetic acid, on the photo-oxidation of Cr(III) in the presence of Fe(III). Benzene was used as OH radical scavenger to investigate the reaction mechanisms of photo-oxidation of Cr(III) in the Fe(III)/organic system through the kinetic studies.
Result showed that Cr(III) could be oxidized in the presence of Fe(III) at pH 2-4 under UV irradiation. At pH 3, Cr(III) photo-oxidation exhibited the highest efficiency because the proportion of FeOH2+, a major Fe species of generating OH radicals upon its photolysis, was greatest according to the calculations by a chemical equilibrium model of Visual MINTEQ. Phenol, formed by interacting benzene with OH radicals, it was confirmed that OH radicals were the major oxidants for Cr(III) in the acidic solution containing Fe(III). Although both phenol and Cr(VI) formations were attributed to the OH radical productions, the ratio of phenol/Cr(VI) did not maintain a constant value. The results demonstrated that some side reactions, such as Cr(VI) reduction by Fe(II), were involved in the Cr(III) photo-oxidation reactions.
With the addition of 50-400 μM citrate and oxalate into a system with 400 μM Fe(III) at pH 3, 300 μM Cr(III) photo-oxidation was greatly inhibited, and thus, no discernible Cr(VI) were produced within 4 h reactions. Even if the OH radicals could still be produced through the photolysis of Fe(OH)2+ or Fe-organic complexes in the organic-containing systems, the rapid reduction of Cr(VI) by these two organic compounds may overwhelm Cr(III) photo-oxidation. Besides, a significant increase in Fe(II) productions, derived from the photolysis of Fe-organic complexes, were observed with the addition of the organic compounds. Rapid reduction of Cr(VI) by Fe(II) in an acidic solution may be the alternative reason inhibiting Cr(VI) productions. Dissolved oxygen (DO) was involved in photolysis of Fe-organic complexes, and thus, a decrease in DO concentrations was observed at the initial reaction time. However, with an accumulation of Fe(II), the interactions of O2●-, a photolytic product, with Fe(II) may release the O2 molecules and lead to an elevation of DO.
Unlike the system with citrate and oxalate, Cr(III) photo-oxidation reactions was not inhibited completely in the acetate system. Because acetate would not complex with Fe(III), photolysis of Fe-organic complexes did not occur. Therefore, the OH radical productions were mainly from the photolysis of Fe(OH)2+, and thus, acetate may serve as a reductant when it was added into the irradiated Fe(III)/Cr(III) system. However, with the addition of higher concentrations of acetate (i.e., > 50 μM), the oxidative product of acetate, i.e., ●CH2CO2H, may react with Fe(II) and promote Fe(OH)2+ productions. Thus, the inhibition of Cr(VI) production was comparable lower when higher concentrations of acetate were added.

環境中的鉻廣泛的存在於土壤以及自然水體中,其來源主要為工業廢棄物的排放。環境中的鉻通常以三價Cr(III)以及六價Cr(VI)兩種形態存在,其物理化學性質有明顯的差異。Cr(III)主要以陽離子型態存在,相對於以陰離子存在的Cr(VI)有較低的移動性以及毒性,因此近年來許多研究開始關注Cr(III)氧化成為Cr(VI)的潛勢探討。過去的研究發現三價鐵物種FeOH2+ 的光解可產生氫氧自由基(●OH),此一途徑被證實可以在酸性的環境下氧化Cr(III)。自然環境中廣泛存在的小分子量有機酸(LMWOA) 可與鐵離子錯合, 而此有機鐵錯合物在酸性的環境下, 可光解產生Fe(II)促進Cr(VI)的還原, 故三價鐵及有機鐵之光反應可控制環境中鉻物種的轉變, 但過去很少有研究系統性的探討此兩種光反應如何受到有機酸的影響而導致鉻物種的變化, 故本研究的目的為瞭解於Fe(III)的酸性系統中, UV光照、LMWOA(檸檬酸、草酸及醋酸)的添加, 以及初始LMWOA濃度對溶液中Cr(III)光氧化效率的影響,並透過氫氧自由基的定量以及溶液中氧氣的動力學變化,推測其反應的途徑。結果顯示,在暗反應的實驗中, 含鐵系統中不論有無LMWOA的添加皆不會產生●OH,因此也不會有Cr(VI)的生成。在無LMWOA系統中, 光照含400 μM Fe(III)與300 μM Cr(III)的酸性溶液後發現, Cr(III)的氧化效率與FeOH2+的物種分佈呈正相關,其中在pH 3的系統,Cr(VI)最高產量可達到4.3 μM;於pH 2和4下Cr(VI)產量約為 2.3 – 2.5 μM。Cr(VI)的高生成量伴隨著較低濃度Fe(II)的生成(<10 μM),相反地,於pH 2和4 系統內Fe(II)濃度在反應240分鐘內呈持續成長並符合零級反應(k=0.1029 μmol L-1 s-1), 且●OH的生成量沒有和Cr(VI)的生成量呈現固定的比值,此可能因Cr(VI)與Fe(II)於酸性環境中發生氧化還原反應所致。在有添加LMWOA的系統中,添加50–400 μM的檸檬酸和草酸於含鐵離子溶液中皆不會產生Cr(VI),但●OH的生成量會隨著此兩種有機酸濃度的提升而提高;醋酸的添加不會抑制Cr(III)的光氧化反應,且隨著醋酸濃度的提升, Cr(VI)及●OH的產量都會隨之提高, 由於草酸和檸檬酸對Fe(III)有較佳的錯和能力,故此二有機酸的存在除了直接抑制 ●OH直接由FeOH2+的光解產生外,大量由光解有機鐵還原生成的二價鐵亦會間接還原所生成的Cr(VI)。醋酸因與Fe(III)錯和能力低,故不會抑制FeOH2+的光解產生●OH,故有較高的Cr(VI)產生量, 此外, 以EPR檢測醋酸照光之產物發現, 其亦會產生●OH, 故可提升Cr(III)的氧化。
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