本研究探討電化學還原(ECR)和化學還原(CR)處理含有Cr(VI)的廢水。在ECR過程中，Cr(VI)還原在反應過程中隨著pH的降低而增加，要達到完全還原Cr(VI)的時間與初始pH無關。在初始pH 7和9的條件下要完全去除Cr（VI），其所需的Fe(II)分別比化學計量值高10％和13％。對於pH 3和5，該值比化學計量值小約32.0％，XPS結果證明在反應期間生成的顆粒物造成吸附是主要的原因。相反，間接還原是控制pH值為3,7和9時去除Cr(VI)的主要機制。而維持pH 5的條件下，間接還原和吸附都是Cr(VI)去除的原因。為了探討溶氧飽和度對於Cr(VI)還原的影響，本研究針對曝氣、氮氣和磁石攪拌之三種方法進行更深入的探討。其中，在氮氣吹掃下，還原1莫耳的Cr(VI)需要的Fe(II)比化學計量值高3.67％。對於磁石攪拌和曝氣，還原1莫耳的Cr(VI)需要的Fe(II)比化學計量值分別高12.79％和15.82％。因此，溶氧的含量確實影響Cr(VI)的還原。
在CR過程中，依據ECR過程中產生的鐵劑量滴加Fe(II)。在控制pH 3,7和9的條件下，還原1莫耳Cr(VI)所需的Fe(II)與化學計量值相同，表明間接還原是主要的去除原理。控制溶液pH為5時，扣除其化學計量值後得出吸附率約為54％。在電流為0.75和1 A時，電化學還原所需的成本高於化學還原。而在低電流（0.25和0.5 A）下的成本則低於化學還原。其中能源的消耗是電化學還原過程中的主要的成本。

Electrochemical reduction (ECR) and chemical reduction (CR) processes were employed to treat Cr(VI)-containing wastewater. The Cr(VI) reduction was extremely fast with reaction kinetics limited by electro-generation of Fe(II) and chemical dosage of Fe in ECR and CR processes, respectively. 
In ECR process, the Cr(VI) reduction increased with decreasing pH at the initial stage of reaction, but the time to reach complete Cr(VI) reduction is pH independent. The amount of Fe(II) required was 10% and 13% higher than the stoichiometric value to remove Cr(VI) completely for initial pH of 7 and 9, respectively. For pH 3 and 5, the values were around 32.0 % less than the stoichiometric value. XPS results proved that the adsorption of Cr(VI) onto the produced flocs was the reason. In contrast, the time to reach 100% removal was pH dependent if the system pH was controlled throughout the reaction. Indirect reduction was the main mechanism for Cr(VI) removal under controlling pH of 3, 7, and 9. At controlled pH of 5, both indirect reduction and adsorption were responsible for the removal of Cr(VI). Various mixing mechanism including aeration, nitrogen purging, and mechanic mixing were investigated to study the effects of DO. Fe(II) needed for one mole of Cr(VI) reduced was 3.67% higher the stoichiometric value of 3 under nitrogen purging. The values were 12.79% and 15.82% higher the stoichiometric value of 3 moles of Fe(II) needed for one mole of Cr(VI) for mechanical mixing and aeration conditions, respectively. Therefore, the content of DO did affect the reduction of Cr(VI) using ECR. 
In CR process, iron is dosed dropwise every 4 min according to the dosage of Fe(II) generated from electrochemical reduction process. The reaction was time and pH-independent. Under controlled solution pH, the values were the same as the stoichiometric value of 3 moles of Fe(II) needed for one mole of Cr(VI) being reduced for pH 3, 7 and 9, indicating that indirect reduction was the main removal mechanism. The value of 0.73 was two times higher than the stoichiometry value of 0.33 under controlled pH of 5, indicating that adsorption was around 54% after subtracting the stoichiometric value of 3. 
The operation cost of ECR process was higher than CR process at current intensities of 0.75 and 1 A. Meanwhile, the cost was lower than that of CR process at low current intensities. Energy consumption was the main cost for ECR process. Thus, the operation cost of ECR process depends the current intensity applied.

Table of content
List of Figure	VIII
List of Table	XII
Chapter 1 Introduction	1
Chapter 2 Background information	3
2.1 Chromium speciation in the environment	3
2.2 Chromium removal processes	4
2.2.1	Adsorption	5
2.2.2	Chemical reduction (CR) process	6
2.2.3	Electrochemical reduction (ECR) process	9
2.3 The removal mechanisms responsible for Cr(VI) removal in an ECR process	11
2.3.1	Direct reduction and direct electroreduction	11
2.3.2	Indirect reduction	12
2.3.3	Adsorption	13
2.3.4	Co-precipitation	14
Chapter 3 Materials and methods	15
3.1 Wastewater characteristics	15
3.2 Experimental setup	15
3.2.1	Electrochemical reduction	15
3.2.2	Chemical reduction	17
3.3 Experimental method	18
3.3.1	Electrochemical reduction	18
3.3.2	Chemical reduction	20
3.4 Analytical methods	21
3.4.1	Colorimetric method	21
3.4.2	Flame atomic absorption spectrophotometer	23
3.4.3	X-ray photoelectron spectroscopy (XPS)	23
Chapter 4 Results and discussion	24
4.1 Electrochemical reduction process	24
4.1.1	Effect of current intensity	24
4.1.2	Effect of initial pH 	29
4.1.3	Effect of fixed pH 	35
4.1.4	Effect of dissolved oxygen (DO) 	37
4.2 Chemical reduction (CR) process	42
4.2.1	Effect of initial pH 	42
4.2.2	Effect of control pH 	44
4.3 Operation cost	47
Chapter 5 Conclusions and suggestions	50
5.1 Conclusions	50
5.2 Suggestion	52
Reference	53


List of Figure
Figure 1. Redox potential (Eh)-pH diagram for Cr-O-H system..	4
Figure 2. Removal of Cr(VI) by reduction and precipitation process using bisulfite reductant.	7
Figure 3. Percentage of Fe ions species precipitation as a function of pH modeled by Mineql+.	9
Figure 4. Schematic diagram of bench-scale electrocoagulation process.	10
Figure 5. The schematic of electrochemical reduction system.	16
Figure 6. The schematic of chemical reduction system.	17
Figure 7. Standard curve for Cr(VI) analysis.	21
Figure 8. Standard curve for Fe(II) analysis.	22
Figure 9. The efficiency of Cr(VI) reduction vs. reaction times under different current. Experimental condition: Cr(VI) concentration = 0.86 mM; pH = 7.0; mechanic mixing = 100 rpm; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	25
Figure 10. (A) The pH changed vs. reaction under different current (B) The final pH vs. electron. Experimental condition: Cr(VI) concentration = 0.86 mM; pH = 7.0; mechanic mixing = 100 rpm; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	26
Figure 11. (A) experimental Fe generated vs. theoretical Fe generated. (B) Cr(VI) removed vs. theoretical Fe generated. Experimental condition: Cr(VI) concentration = 0.86 mM; pH = 7.0; mechanic mixing = 100 rpm; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	28
Figure 12. Total Cr removal efficiency vs. reaction time. Experimental condition: Cr(VI) concentration = 0.86 mM; pH = 7.0; mechanic mixing = 100 rpm; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	29
Figure 13. The efficiency of Cr(VI) reduction as function of reaction times with different initial pH. Experimental condition: Cr(VI) concentration = 0.86 mM; mechanic mixing = 100 rpm; current intensity = 0.75 A; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	30
Figure 14. The pH changed vs. reaction time for various initial pH. Experimental condition: Cr(VI) concentration = 0.86 mM; mechanic mixing = 100 rpm; current intensity = 0.75 A; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	31
Figure 15. Cr(VI) removed vs. theoretical Fe generated. Experimental condition: Cr(VI) concentration = 0.84 mM; mechanism mixing = 100 rpm; current intensity = 0.75 A; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	32
Figure 16. XPS analysis at different initial pH. Experimental condition: Cr(VI) concentration = 0.86 mM; mechanic mixing = 100 rpm; current intensity = 0.75 A; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	33
Figure 17. Total Cr removal vs. reaction time. Experimental condition: Cr(VI) concentration = 0.86 mM; mechanic mixing = 100 rpm; current intensity = 0.75 A; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	34
Figure 18. (A) Cr(VI) removed vs reaction time. (B) Cr(VI) removed vs. theoretical Fe generated. Experimental condition: Cr(VI) concentration = 0.86 mM; mechanic mixing = 100 rpm; current intensity = 0.5 A; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	36
Figure 19. Total Cr removal efficiency as a function of reaction time. Experimental condition: Cr(VI) concentration = 0.86 mM; mechanic mixing = 100 rpm; current intensity = 0.5 A; current density = 24.15 mA/cm2; conductivity = 4 mS/cm.	37
Figure 20. (A) Cr(VI) removed vs reaction time. (B) Cr(VI) removed vs. theoretical Fe generated. Experimental condition: Cr(VI) concentration = 0.84 mM; pH = 7.0; mechanism mixing = 100 rpm; current intensity = 0.75 A; current density = 24.15 mA/cm2; conductivity = 4 mS/cm; aeration flow rate = 5 L/min; nitrogen gas flow rate = 5 L/min.	39
Figure 21. pH changed vs. reaction time. Experimental condition: Cr(VI) concentration = 0.86 mM; pH = 7.0; mechanism mixing = 100 rpm; current intensity = 0.75 A; current density = 24.15 mA/cm2; conductivity = 4 mS/cm; aeration flow rate = 5 L/min; nitrogen gas flow rate = 5 L/min.	40
Figure 22. Total Cr removal vs. reaction time. Experimental condition: Cr(VI) concentration = 0.86 mM; pH = 7.0; mechanism mixing = 100 rpm; current intensity = 0.75 A; current density = 24.15 mA/cm2; conductivity = 4 mS/cm; aeration flow rate = 5 L/min; nitrogen gas flow rate = 5 L/min.	41
Figure 23. The pH changed as a function of reaction time. Experiment condition: Cr(VI) concentration = 0.86 mM; Fe concentration = 2.58 mM; mechanism mixing = 100 rpm.	42
Figure 24. Total Cr removal as a function of reaction time under different solution pH. Experiment condition: Cr(VI) concentration = 0.86 mM; Fe concentration = 2.58 mM; mechanism mixing = 100 rpm.	43
Figure 25. Cr(VI) reduction efficiency as a function of Fe(II) added.  Experiment condition: Cr(VI) concentration = 0.86 mM; Fe concentration = 3.53 mM; mechanism mixing = 100 rpm.	44
Figure 26. Cr(VI) reduction as a function of Fe(II) added. Experiment condition: Cr(VI) concentration = 0.86 mM; Fe concentration = 3.53 mM; mechanism mixing = 100 rpm;	45
Figure 27. Total Cr removal efficiency as a function of Fe(II) added. Experiment condition: Cr(VI) concentration = 0.86 mM; Fe concentration = 3.53 mM; mechanism mixing = 100 rpm;	46
Figure 28. Operation cost for electrochemical reduction and chemical reduction processes	49
 
List of Table
Table 1. Cr(VI) removal by adsorption with different adsorbents.	6
Table 2. Chemicals and reagents	15
Table 3. Effect of different current intensity.	19
Table 4. The time and Fe dosage consumed to removal 100% Cr(VI) by ECR and CR.	47
Table 5. Operation costs (USD/mole Cr(VI) for ECR and CR processes. ECR condition: Cr(VI) concentration = 0.86 mM; current density = 24.15 mA/cm2; conductivity = 4 mS/cm; initial pH = 7. CR condition: Cr(VI) concentration = 0.86 mM; Fe concentration = 3.53 mM; fixed pH of 5.	49

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