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標題: 鐵或錳氧化物於共存系統中對黑炭和六價鉻反應之影響
Reactions of hexavalent chromium with black carbon when co-existing with iron or manganese hydrous oxides
作者: 張雅菁
Chang, Ya- Jing
Contributors: 王尚禮
土壤環境科學系所
關鍵字: 六價鉻;黑炭;氧化物
Chromium(VI);Black carbon;Oxide
日期: 2012
Issue Date: 2013-11-07 11:39:33 (UTC+8)
Publisher: 土壤環境科學系所
摘要: Cr(VI)具有高毒性和致癌性,工業排放為Cr(VI)進入環境造成污染的主要來源。土壤中的有機質和黑炭為Cr(VI)主要的還原劑之一,已被證實能還原Cr(VI)成毒性和危害較低的Cr(III)。然而,卻仍有Cr(VI)被淋洗至較深土層甚至污染地下水的情況,推測是由於土壤組成複雜,其他組成與Cr(VI)的反應可能會延緩Cr(VI)的還原速率和減少Cr(VI)的還原量。因此本研究選擇黑炭為Cr(VI)還原劑的代表,探討氧化鐵和氧化錳與黑炭共存時,對黑炭與Cr(VI)的吸附和還原轉化之影響。結果顯示,針鐵礦與黑炭共存系統中,針鐵礦會與黑炭競爭吸附溶液中的Cr(VI),且因為遲滯效應導致吸附於針鐵礦上的Cr(VI)不易脫附釋出,而限制了黑炭對Cr(VI)之還原反應。但長時間反應後,Cr(VI)仍逐漸被黑炭所還原,所生成的Cr(III)則鍵結於黑炭表面。氧化錳與黑炭共存系統中,則分別探討Cr(III)和Cr(VI)的吸附和氧化還原反應。黑炭雖然可吸附Cr(VI)並將其還原為Cr(III),但氧化錳會氧化溶液中Cr(III)成Cr(VI),而抑制了Cr(VI)在黑炭表面的還原轉化。隨著反應時間的增加,氧化錳本身會逐漸被還原溶解,因此,反應產物最終仍以鍵結於黑炭上的Cr(III)物種為主。另外,黑炭與Cr(III)反應的系統中,在低pH時,黑炭表面具有淨正電荷而不利於吸附Cr(III),氧化錳將Cr(III)氧化成Cr(VI)的型態後,反而可藉由黑炭對Cr(VI)的還原反應,促進溶液中整體鉻的移除,將Cr(III)固定於黑炭上而降低其再被氧化成毒性較高的Cr(VI)之風險。總結本研究之結果,土壤中其他組成與Cr(VI)的反應,會對土壤中還原劑還原Cr(VI)之反應產生抑制的影響,因此可能會提高進入土壤中的Cr(VI)之移動性和有效性,進而增加其造成污染的風險。本研究結果亦顯示過去藉由了解單一土壤組成和Cr(VI)或其他污染物反應來推估其環境宿命有其不足之處,必須考慮不同組成共存之相互作用的影響,以及此相互作用如何決定鉻或其他污染物的物種轉化和傳輸過程,才有助於提供污染處理和評估上有效的資訊。
Cr(VI) is one of the pollutants of major concern in the environment due to its high toxicity and carcinogenicity. In oxic soils, organic matter (OM) and black carbon (BC) are the predominant reductants that can reduce toxic Cr(VI) to less toxic Cr(III). However, Cr(VI) still can be leached to deeper layers of soil or even into groundwater. The low reduction rate of Cr(VI) is presumably attributed to the reactions of other soil constituents with Cr(VI) that inhibit the reduction of Cr(VI) by OM and BC. Thus, in this study, the influences of hydrous oxides of Fe and Mn on the Cr(VI) reduction of BC were investigated. The results showed that the co-existence of goethite with BC inhibited the Cr(VI) reaction of BC due to the fast adsorption rate and strong desorption hysteresis of Cr(VI) on goethite. Nonetheless, the prolonged reaction of Cr(VI) with BC and goethite removed Cr(VI) from solution through the adsorption of Cr(VI) and the subsequent reduction of adsorbed Cr(VI) to Cr(III). The resulting Cr(III) is either released back into solution or bound on BC. When MnO2 coexisted with BC, the aqueous Cr(III) were oxidized to Cr(VI) by MnO2. The Cr(III) oxidation and releasing back into solution occur as coupled reactions. The oxidation of aqueous Cr(III) by MnO2 inhibited the Cr(VI) reduction by BC when MnO2 coexisted with BC. At lower pH, the net positive charge on the surface of BC resulted in electrostatic repulsion between Cr(III) and BC. On the other hand, the oxidation of Cr(III) by Mn oxide led to the formation of Cr(VI) in solution, which was subsequently reduced by BC and consequently enhanced the removal of total Cr by BC at lower pH when BC coexisted with MnO2. In summary, the presence of Fe and Mn oxides inhibited the reduction of Cr(VI) by BC, leading to an increasing risk of Cr(VI) contamination in soil. Thus, the interactions between different soil consitutents need to be considered in order to have better understanding of the environmental risks of Cr contamination in soil.
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