鋁取代型針鐵礦對六價鉻之吸附及光催化反應

Adsorption and Photocatalytic Transformation of Cr (VI) on A1 Substituted Goethite

10.30058/SE.200606.0006

吳玟靜(Wen-Ching Wu)；鄒裕民(Yu-Min Tzou)

鉻 ； 鋁取代型針鐵礦 ； 光催化作用 ； chromium ； A1-goethite ； photoreduction

土壤與環境

9卷1&2期（2006 / 06 / 01）

59 - 68

繁體中文

本研究使用鋁取代型針鐵礦，並以純針鐵礦作為對照組，研究其在UV光照下對鉻物種轉變的影響。結果顯示，1%的鋁取代型針鐵礦對Cr（VI）的吸附量達0.06 mmol／g，而純的針鐵礦則有0.04 mmol／g的吸附量，由於鋁取代型針鐵礦的比表面積較純針鐵礦大，因此鋁取代型針鐵礦對於Cr（VI）的吸附能力比純針鐵礦高；然而，由於鋁離子對於鉻陰離子的親和力不如鐵離子強，因此隨著鋁離子的取代量增加，其對Cr（VI）的吸附能力逐漸減弱。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，純針鐵礦可將0.07 mmol／g的Cr（VI）還原，且光還原六價鉻的能力仍然存在，而鋁取代型的針鐵礦，對Cr（VI）的光還原卻隨時間逐漸減弱。經萃取試驗證明， 鋁取代型針鐵礦在施加光照下，確實具有光還原六價鉻的能力，然而其光催化能力卻較純針鐵礦的弱。由於光催化還原反應的發生，Cr（VI）必須先吸附有針鐵礦的表面，所以當針鐵礦中的鐵被其他離子取代後，可能會影響及減少表面鐵的光活化位置，此外，光還原產物，如Cr（III），亦可能沉澱在吸附劑表面，減少針鐵礦表面吸附及光活化位置。由此實驗結果可知，取代型的針鐵礦對六價鉻的吸附量雖然較純針鐵礦高，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：表面光活化位置的降低等），而造成光還原能力的抑制。

In the current study, the adsorption and photo-reduction of Cr (VI) on A1-substituted and goethite were evaluated using a batch technique. The results showed that Cr (VI) adsorption on goethite was rapid. About 0.06 mmol/g Cr (VI) was adsorbed on 1% A1-goethite as compared with 0.04 mmol/g Cr (VI) adsorption on pure goethite after 12 h. The increase in Cr (VI) adsorption on low A1-substituted goethite (i,e., 1 %) was attributed to increase in the surface area. However, Cr (VI) adsorption would decrease when the percentage of A1 substitution in goethite was further increased. The lower complexation constant of Cr (VI) with structural A1 as compared with that of Fe may result in the decrease in Cr (VI) adsorption when A1 substitution increased. Pure goethite exhibited an excellent ability of photo-reduction of Cr (VI) under illumination. For instance, as high as 0.07 mmol/g of Cr (VI) was reduced after 12 h, and the photo-reduction of Cr (VI) continued over prolonged time. The occurrence of Cr (VI) reduction instead of adsorption was confirmed using an extraction technique. Since phosphate was an efficient exchanger for adsorbed Cr (VI), a low extraction efficiency of Cr (VI) by phosphate indicated Cr (VI) reduction upon exposure to the UV light. On the contrary, the amount of Cr (VI) reduction was greatly inhibited on A1-substituted goethite, and the reaction rate decreased over the irradiating time. The elimination of ”activated sites” on A1-substituted goethite was proposed a possible reason for the decrease in Cr (VI) adsorption and subsequent reduction. In summary, low A1-substitution facilitates Cr (VJ) adsorption on goethite; however, the A1-substitution will result in a decrease in the efficiency for the photo-reduction of Cr (VI).