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標題: 錳或鋁取代針鐵礦對六價鉻之吸附及光催化反應
The Adsorption and Photocatalytic Transformation of Cr(VI) on Mn or Al Substituted Goethite
作者: 吳玟靜
Wu, Wen-Ching
關鍵字: Chromium;鉻;Giethite;Photocatalytic;針鐵礦;光催化作用
出版社: 土壤環境科學系
鉻在自然界中主要以三價[Cr(III)]及六價[Cr(VI)]兩種形式存在,其中Cr(VI)為致癌性物質,屬於陰離子,其吸附極易受環境中pH值的影響,由於移動性高,易造成環境污染,而Cr(III)在環境中主要以氫氧化物或氧化物形式存在,移動性低,且對作物的毒性較低,因此藉由礦物將Cr(VI)固定或是將其還原成Cr(III),可降低Cr(VI)對環境的危害。土壤中的針鐵礦可增進光催化Cr(VI)還原為Cr(III)的能力,而錳氧化物卻可氧化Cr(III)成Cr(VI),但自然界中錳取代型針鐵礦對鉻的吸持甚至氧化還原反應的影響,仍屬未知。因此,本實驗使用錳取代針鐵礦,並以鋁取代型針鐵礦作為佐證,研究其在光照下對鉻物種轉變的影響。結果顯示,由於錳取代型或鋁取代針型鐵礦的比表面積均比純針鐵礦高,因此錳取代或鋁取代型針鐵礦對於Cr(VI)的吸附能力比純針鐵礦高,最高可達0.1 mmol g-1,而純的針鐵礦對Cr(VI)僅有0.04 mmol g-1的吸附量;在光催化還原Cr(VI)方面,純的針鐵礦具有良好的光催化還原Cr(VI)的效果,在實驗經過12小時後純針鐵礦依舊對六價鉻具有光還原的能力,在實驗進行12小時後,已將0.07 mmol g-1的Cr(VI)還原,而取代型的針鐵礦,在實驗經過12小時後光還原的效果逐漸減弱,高取代量的錳取代型針鐵礦甚至沒有還原的現象。經萃取實驗及氧化實驗證明,將Cr(III)加入錳取代型針鐵礦懸浮液後,以磷酸二氫鉀將吸附在錳取代型針鐵礦表面上的吸附物萃取出後,發現有Cr(VI)吸附在錳取代型針鐵礦的表面,由此可知,錳取代型針鐵礦具有將Cr(III)氧化為Cr(VI)的能力,且每克的10 %錳取代型針鐵礦在12小時內,能氧化出0.02 mmol的Cr(VI),因此會造成抑制光還原的現象,但是鋁取代型針鐵礦並沒有氧化Cr(III)的能力,故不會隨反應時間,降低光還原Cr(VI)之能力。因此抑制光還原Cr(VI)的現象並非只因錳將Cr(III)氧化為Cr(VI)的關係,由於光催化還原反應僅發生在吸附在針鐵礦表面的Cr(VI),所以當針鐵礦中的鐵被其他離子取代後,會減少活化鐵的光還原位置,且被光還原的Cr(III)沈澱在吸附劑表面時,亦會減少吸附及光活化位置,由此可知,取代型的針鐵礦對六價鉻的吸附量雖然較純針鐵礦多,但是其光催化Cr(VI)的能力卻會因為離子的取代反應造成針鐵礦表面性質的改變(例如:表面位的變化與表面光活化位置的降低等),而造成光還原能力的抑制。

Chromium exists in the environment as two major forms, Cr(VI) and Cr(III). Chromium(VI) is a Class A human carcinogen by inhalation and an anion with a strong oxidizing ability. It can be adsorbed on minerals; however, the adsorption is greatly influenced by the pH. The characteristic makes Cr(VI) mobile and may cause environmental problems. In contrast, Cr(III) is non-toxic and an essential element for human health. It is found predominantly in insoluble forms in soils, such as Cr(OH)3 or Cr2O3. Thus, the fixation or the reduction of Cr(VI) using specific minerals may lead to an elimination in ecological hazards of such element. Goethite has been confirmed capable of enhancing Cr(VI) reduction in the presence of light. On the other hand, Mn oxides are well known oxidizers for Cr(III). Nevertheless, it is unclear for the redox/adsorption reaction of Cr on Mn-substituted goethite. Therefore, the objective of this study is to investigate the transformations of Cr on Mn-substituted goethite under illumination. Al-substituted goethite was also used in the study for comparisons. The results showed that metal-substitution in the goethite structures could increase Cr(VI) adsorption probably due to an increase in the surface area. Up to 0.1 mmol Cr(VI) was adsorbed on 1 g metal-substituted goethite but only was 0.04 mmol Cr(VI) adsorbed on the same amount of pure goethite. Cromium(VI) could be photo-reduced on goethite and the photo-catalytic reaction continued for at least 12 h. Around 0.07 mmol Cr(VI) was reduced after 12 h reaction. Metal-substituted goethite showed different behavior in the photo-redox reaction. The rate of photo-reduction of Cr(VI) on Mn-goethite gradually decreased as a function of time. There was even no photo-reduction of Cr(VI) observed on high Mn-substituted goethite. The occurrence and the inhibition of photo-reduction of Cr(VI) were further examined by extraction and oxidation techniques on the goethite/metal substituted goethite. A significant decrease in the amount of Cr(VI) extracted by phosphate was observed for light treated samples experienced light which evidenced that Cr(VI) can be reduced on goethite. Through oxidation experiments, we observed that Mn-goethite could oxidize Cr(III). For instance 0.02 mmol Cr(VI) was produced within 12 h when Cr(III) was added to 1 g Mn-substituted goethite. The oxidation of Cr(III) explain the decrease in the amount of Cr(VI) being photo-reduced because a portion of reduced Cr were re-oxidized. We believe that Mn in the goethite proceeded this oxidation reaction because Cr(III) oxidation did not occur in Al-goethite system. Moreover, metal-substitution may decrease the “active site” on goethite, and these sites contribute largely to Cr(VI) reduction. In general, metal-substitution can enhance Cr(VI) adsorption on goethite. In contrast, the substitution may decrease the efficiency for the photo-reduction of Cr(VI), particularly in the presence of Mn.
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