The Adsorption and Photocatalytic Transformation of Cr(VI) on Mn or Al Substituted Goethite
| Autor: | Wen-Ching Wu, 吳玟靜 |
|---|---|
| Rok vydání: | 2005 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 93 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. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
| Externí odkaz: |
|