臺灣博碩士論文加值系統

鉻的用途隨著工業發展及人類生活水準的提高而日益增加，相對地，鉻污染環境的可能性也隨其用途的增加而加劇。鉻在水體中通常以三價鉻和六價鉻兩種形態存在，兩者的物化特性和毒性相差甚大。Cr(Ⅲ)與固體粒子具高反應性，可直接沈澱或吸附在懸浮粒子上，形成難溶之氫氧化物，移動性低故溶液中Cr(Ⅲ)濃度較低，一般而言對環境危害不大；反之，Cr(Ⅵ)溶解度大，移動性高，且毒性較Cr(Ⅲ)強(約高300倍)。因此，根據化學行為和毒性間的差異，多半將環境中之Cr(Ⅵ)還原成Cr(Ⅲ)來降低其危害風險。
許多研究已證實，有機配位基與半導體於含Fe(Ⅲ)溶液，經紫外光照射後可將Cr(Ⅵ)還原生成Cr(Ⅲ)。然而於同相系統中，無機鹽類對Cr(Ⅵ)之光反應研究並不多，此反應之重要性在於電鍍廢液中常含有不同型式高濃度的無機陰陽離子，一旦這些含鉻廢液排放至水體，在光的照射下，可能對鉻的氧化還原反應有所影響。因此，本研究主要目的為探討在不同pH值(pH 1.0、1.5和2.0)條件下，Fe(Ⅲ)與陰離子(Cl-、NO3-、SO42-、ClO4-和PO43-)對於光還原Cr(Ⅵ)之影響，並進一步從動力學角度探討其反應行為與機制。
研究結果顯示，於Cl-和NO3-系統中，經中壓汞燈(100 W)的照射下，Fe(Ⅲ)的存在與否均能使Cr(Ⅵ)還原，溶液pH值降低和Fe(Ⅲ)濃度提升，有利於Cr(Ⅵ)光還原反應的進行，而改變Cr(Ⅵ)初始濃度對Cr(Ⅵ)光還原速率並無明顯之影響。此外，於含氯離子情況下，Cr(Ⅵ)的還原主要來自FeCl2+光解生成之Fe2+所引起，而於硝酸根系統，則是NO3-光解生成的NO2-所貢獻，且對照兩種環境Fe2+之生成量，更可證實此一論點。至於反應動力部分，在Cl-系統中，Fe(Ⅲ)對Cr(Ⅵ)之光還原反應行為屬零級反應(R2＞0.95)，此表示其速率決定步驟與Cr(Ⅵ)濃度無關；在NO3-系統中，反應級數會因濃度及pH而改變，且無任何較為符合的動力模式，此說明Cr(Ⅵ)光還原反應在NO3-存在下非常複雜，詳細反應機制仍有待釐清。而水體中所共存的陰離子，因其與Fe(Ⅲ)形成錯合物時，會造成氧化還原電位的改變，導致對光還原Cr(Ⅵ)之影響有所不同，在單一體系中，影響大小依序為Cl- > SO42- > ClO4- > NO3-，含10 mM Cl-復合體系則是PO43- > SO42- > ClO4- NO3-。因此，根據本研究結果得知，應可利用電鍍廢液之基本特性，經UV光照射來還原處理水中Cr(Ⅵ)，以降低Cr(Ⅵ)的污染與危害。

The detrimental impact of chromium to the environment gradually increased accompanied with the extensive utilization of chromium due to its requirement to the development of industries and to improve the living standard for human. Chromium exists in water in two major oxidation forms: Cr(III) and Cr(VI). There are distinct differences in physical/chemical properties and toxicities between these two Cr species. Chromium(III) has a high affinity to particles. Through the precipitation or adsorption of Cr(III) onto suspended particles, the concentration and mobility of Cr(III) is usually low in solutions. Contrary to Cr(III), Cr(VI) exhibits high solubility and mobility, and its toxicity is much higher than that of Cr(III). Therefore, the reduction of Cr(VI) to less toxic Cr(III) is one of the feasible strategies to decrease harmful effect s of Cr.
Many previous studies had approved that organic ligands and semiconductors could promote the reduction of Cr(VI) upon the exposure to UV/VIS light in the presence of Fe(III). Nonetheless, the photo-reduction of Cr(VI) as catalyzed by inorganic electrolytes is less mentioned in the literature. Since many electroplating wastewaters contain various inorganic anions and cations, the irradiation of the acidic solutions with these anions and cations may lead to the formation of activated radical, influencing indirectly the abiotic transformation of Cr. Therefore, the objective of this study is to evaluate the effects of pH (1~2) and various anions (i.e, Cl-, NO3-, SO42-, ClO4-, and PO42-) on the photo-reduction of Cr(VI) with or without Fe(III). The possible reaction mechanisms are also investigated through the kinetic studies.
The results showed that Cr(VI) could be rapidly reduced in the presence of Cl- and NO3- under illumination. A decrease in solution pH accompanied with the increase of Fe(III) would enhance Cr(VI) photo-reduction; however, the photo-reduction rate of Cr(VI) was less affected with an increase of Cr(VI) concentrations. The formation of Fe2+ resulting from the photolysis of Fe-Cl complexes is responsible for Cr(VI) reduction in a solution with Cl-. In the presence of NO3-, the photolysis of NO3-, leading to the formation of NO2- may contribute to Cr(VI) reduction. The photo-reduction of Cr(VI) could be described using a zero-order kinetic model with R2 > 0.95 in Cl- system; however, the order of kinetic reactions would be changed with Cr(VI) concentrations and pH in NO3- system. The results demonstrate that the photo-reduction of Cr(VI) involving NO3- is very complicate, and the detail reaction mechanism merits further study.The association of anions with Fe(III) would change not only the potential of the Fe(III)/Fe(II) redox couple but also the photolytic reaction of Fe(III)-anion complexes, resulting in a drastic effect of Cr(VI) photo-reduction. In a system with single anion, the influence of Cr(VI) photo-reduction follows the order of Cl- > SO42- > ClO4- > NO3-. In a multi-anion system with 10 mM Cl-, the order is PO42- > SO42- > ClO4- ≈ NO3-, which seems to obey their complexation constants with Fe(III). That is, the anion forming a more stable complex with Fe(III) would have the greatest effect in Cr(VI) photo-reduction. Accordingly, this study provide a new pathway to convert the toxic Cr(VI) to Cr(III) without the addition of any reductants. Based on the current results, a cost-effective treatment technology may be developed by irradiating directly Cr(VI)-containing wastewaters to reduce and immobilize Cr(VI). Thus, the potential harms of Cr(VI) to ecosystem can be greatly decreased.

目 錄

摘要（中文）. . . . . . . . . . . . . . . . . . . . . . .Ⅰ
摘要（英文. . . . . . . . . . . . . . . . . . . . . . . .Ⅲ
目錄. . . . . . . . . . . . . . . . . . . . . . . . . . .Ⅴ
圖次. . . . . . . . . . . . . . . . . . . . . . . . . . .Ⅶ
表次. . . . . . . . . . . . . . . . . . . . . . . . . . .Ⅹ
第一章 緒論. . . . . . . . . . . . . . . . . . . . . . . .1
1-1研究緣起與目的. . . . . . . . . . . . . . . . . . . . .1
1-2研究內容. . . . . . . . . . . . . . . . . . . . . . .2
第二章 文獻回顧. . . . . . . . . . . . . . . . . . . . . .3
2-1鉻的特性與污染 . . . . . . . . . . . . . . . . . . . .3
2-1-1鉻之性質 . . . . . . . . . . . . . . . . . . . . . 3
2-1-2鉻於環境中的遷移變化 . . . . . . . . . . . . . . . 6
2-1-3環境鉻污染來源 . . . . . . . . . . . . . . . . . . 7
2-1-4鉻的危害 . . . . . . . . . . . . . . . . . . . . . 8
2-1-5法令規定.. . . . . . . . . . . . . . . . . . . . . 9
2-1-6工業電鍍廢水成分性質. . . . . . . . . . . . . . . 10
2-1-7含鉻廢水處理技術. . . . . . . . . . . . . . . . . 11
2-2光催化還原六價鉻. . . . . . . . . . . . . . . . . . .13
2-2-1光催化反應之種類. . . . . . . . . . . . . . . . . 13
2-2-2鐵對六價鉻光催化反應的影響. . . . . . . . . . . . 15
第三章 研究方法與步驟 . . . . . . . . . . . . . . . . . .18
3-1實驗試劑與器材. . . . . . . . . . . . . . . . . . . .19
3-2批次實驗步驟. . . . . . . . . . . . . . . . . . . . .20
第四章 結果與討論 . . . . . . . . . . . . . . . . . . . .25
4-1硝酸根離子系統下光還原六價鉻影響因子探討. . . . . . .25
4-1-1光照的影響. . . . . . . . . . . . . . . . . . . . 25
4-1-2初始pH對光還原Cr(Ⅵ)的影響. . . . . . . . . . . . 29
4-1-2-1溶液中不含Fe(Ⅲ)時，pH對光還原Cr(Ⅵ)的影響 . . 29
4-1-2-2溶液中含Fe(Ⅲ)時，pH對光還原Cr(Ⅵ)的影響 . . . 29
4-1-3三價鐵對光還原Cr(Ⅵ)的影響. . . . . . . . . . . . 33
4-1-4六價鉻初始濃度對光還原Cr(Ⅵ)的影響. . . . . . . . 39
4-2氯離子系統下光還原六價鉻影響因子探討. . . . . . . . .41
4-2-1光照的影響. . . . . . . . . . . . . . . . . . . . 41
4-2-2初始pH對光還原Cr(Ⅵ)的影響. . . . . . . . . . . . 44
4-2-2-1溶液中不含Fe(Ⅲ)時，pH對光還原Cr(Ⅵ)的影響 . . 44
4-2-2-2溶液中含Fe(Ⅲ)時，pH對光還原Cr(Ⅵ)的影響 . . . 44
4-2-3三價鐵對光還原Cr(Ⅵ)的影響. . . . . . . . . . . . 49
4-2-4六價鉻初始濃度對光還原Cr(Ⅵ)的影響. . . . . . . . 56
4-3無機陰離子對光還原Cr(Ⅵ)的影響. . . . . . . . . . . .58
第五章 結論 . . . . . . . . . . . . . . . . . . . . . . .65
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