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標題: Oxidative polymerization reactions of hydroquinone and chromium(VI) in an aqueous solution
作者: Ching-Yun Cheng
關鍵字: 氧化聚合;對苯二酚;對苯醌;六價鉻;過氧化氫;類腐植物質;oxidative polymerization;hydroquinone;1,4-benzoquinone;chromium(VI);hydrogen peroxide;humic-like substances
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對苯二酚與重金屬鉻皆被廣泛地應用在半導體製造、皮革製造、顯影、油漆等化學工業上,是工業廢水中常見的污染物。此外,在降解受苯酚汙染的廢水處理過程中,苯酚也會轉變成有毒性且不易降解的對苯二酚和醌類等的中間產物。六價鉻具有高毒性、致癌性以及會對生物造成突變性,因此,將廢水中的六價鉻還原成三價鉻以降低六價鉻在自然界中的毒性和移動性為最主要減低鉻危害的策略。六價鉻為一強氧化劑,當其與水溶液中之有機物/膠體反應時,可將有機物氧化,且此反應可因光的介入而增強。然而,反應中有機物亦可能因部分分解造成可溶性有機碳(DOC)的增加和/或生成有毒性的中間產物,影響後續廢水處理的成效。根據前人研究指出,錳氧化物能促進有機分子(如對苯二酚)氧化聚合生成類腐植物質,故當六價鉻與酚類化合物反應時,有機化合物被六價鉻氧化成DOC或二氧化碳可能並非唯一的反應途徑,因此,本研究的目的為探討在酸性條件下,六價鉻與多酚化合物,即對苯二酚,的氧化還原反應,以及在施予光照 (100瓦中壓汞燈,最大輻射強度於波長366 nm) 時對所生成反應產物之影響,同時,反應所形成的氧化聚合產物之結構將以光譜分析,並與天然的腐植酸以及在高pH值下由多酚類/醌類所合成的聚合物相互比較。此外,過氧化氫亦為一氧化劑,在本研究中,其將被用來取代六價鉻來氧化對苯二酚,以評估三價鉻金屬離子在氧化聚合過程中所扮演的角色。結果顯示,當67.3 µM六價鉻與181.6 µM對苯二酚反應時,對苯醌為唯一的有機產物;而在光系統下,則有產物1,2,4-苯三酚和2-羥基-對苯醌的生成。隨著對苯二酚和六價鉻濃度的增加,有機分子會發生氧化聚合反應並生成含三價鉻的有機聚合物,且研究發現,對苯醌可能為促使後續氧化聚合的重要中間產物。根據X光吸收近邊緣結構的線性疊加擬合分析結果,此有機聚合物主要有78.0-90.4%是以三價鉻鍵結至較大的有機物聚合物的型態存在,顯示六價鉻不僅會氧化對苯二酚,也能進一步藉由生成的三價鉻將部分分解的中間產物聚合,生成含有三價鉻鍵結的類腐植物質。不同於天然腐植酸有脂肪族碳和芳香族碳的結構,該有機聚合物是以芳香族碳結構為主。當提高溶液pH值至10.0時,對苯二酚或對苯醌亦會發生聚合反應,而不同於六價鉻系統,其聚合物有較明顯的羧酸(COOH)結構。對苯二酚與對苯醌在高pH下的聚合產物其化學結構仍以芳香族碳的結構為主,但對苯醌系統存在有較明顯的脂肪族結構。另一方面,不論光照與否,在沒有金屬離子的存在下,即過氧化氫系統,則無法有效地促進對苯二酚的聚合反應,而生成的對苯醌會再轉變成2-羥基-對苯醌和其它有機酸。故在酸性條件下,六價鉻能促進對苯二酚的氧化聚合反應。然而,相較於對苯二酚及對苯醌在高pH下的系統,則金屬扮演著連接聚合之芳香性有機分子的一個重要之角色。

Hydroquinone (H2Q) and chromium (Cr) are widely used in semiconductor manufacturing processes, tanning, photography, paint, etc., and they are the major environmental pollutants in industrial wastewaters. It was also found that the formation of toxic and less degradable intermediate products, such as H2Q and quinones, at early stages of phenol degradation in phenol-containing wastewaters. Chromium(VI) is a toxic and carcinogenic element, and it will cause mutagenic effects to organisms. Therefore, reduction of Cr(VI) to Cr(III) in the wastewater is an important strategy for eliminating the toxicity and mobility of Cr(VI) in nature. Chromium(VI) is a strong oxidant, and upon its interaction with organic matter/colloids in solutions, the organic compounds can be mineralized and the reaction is enhanced under illumination. However, an increase in dissolved organic carbon (DOC) and/or the formation of toxic intermediate products may occur if the organic materials are partially decomposed which may affect the efficiency of wastewater treatment systems. It is well-known that manganese (Mn) oxides can promote the oxidative polymerization of H2Q to humic-like polymers. Based on the findings, we hypothesized that the oxidative decompositions of organic compounds, forming DOC or CO2, may not be the only pathway of the redox reaction involving Cr(VI) and phenolic compounds. Thus, this study aims to investigate the redox reactions of Cr(VI) and H2Q, a model organic compound of the polyphenols, in an acidic condition, and their reactive products as influenced by ultraviolet (UV) irradiation (a medium-pressure mercury lamp with the maximum intensity of illumination at 366 nm). The structures of oxidative polymerization products are spectroscopically analyzed and compared to the natural humic acids and the reactive products of polyphenols/quinones at a high pH value. In addition, the hydrogen peroxide (H2O2), a substitute oxidant of Cr(VI), will be used to oxidize H2Q for evaluating the roles of metal ions of Cr(III) in the oxidative polymerization process. The results showed that when 67.3 µM Cr(VI) reacted with 181.6 µM H2Q, 1,4-benzoquinone (BQ) was the only organic product, but 1,2,4-benzenetriol (Ph(OH)3) and 2-hydroxy-1,4-benzoquinone (BQ(OH)) were produced when UV irradiation was provided. With an increase in the H2Q and Cr(VI) concentrations, the oxidative polymerization of organic molecules occurred, and the derived organic polymers contained Cr(III). The BQ was probably the primary intermediate of H2Q that induced the subsequent oxidative polymerization reaction in the presence of Cr(VI). The linear combination fitting of the XANES spectra demonstrated that the organic polymers consisted of 78.0-90.4% Cr(III)-humic-like polymers, suggesting that Cr(III) may act as a linkage of organic molecules during the polymerization processes of H2Q. This study indicates that polymerization of oxidized organic molecules occurs even in the presence of a strong oxidant of Cr(VI). The aromatic domains dominated the chemical structures of the organic polymers, different from natural humic acids with both aliphatic and aromatic carbons. The polymerization of H2Q or BQ also occurred when the solution pH value was adjusted to 10.0. Unlike the products of Cr(VI)/H2Q, the polymerized products of H2Q/BQ bore mainly the aromatic domains with more characteristic peaks of COOH exhibited in the polymers. It was found that the obviously aliphatic structures existed in the BQ system. The H2O2 could not promote the polymerization of H2Q either in the presence or absence of UV irradiation. Because the BQ, the important precursor of organic polymer, would be converted to BQ(OH) and other organic acids upon its formation in the presence of H2O2, we presumed this associated with the lack of binding metals may lead to a change in the reactive pathways of H2Q. In summary, the oxidative polymerization of H2Q could be promoted by Cr(VI) in an acidic solution; however, metals played an important role of linkage of these aromatic enriched polymers as compared to the reactive products of H2Q and BQ at a high pH.
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