||Nitrate removal by Zero-valent iron (ZVI)
||Department of Water Resources and Environmental Engineering
本實驗採用零價鐵流體化反應器還原硝酸鹽，本系統可有效控制適合硝酸鹽反應之pH值，在水力停留時間15 min，其硝酸鹽去除率隨著初始ZVI劑量的增加而增加，在ZVI為33.3 g l-1時，在pH值未控制時，去除率為13 %；而pH值4.0時去除率則上升到92 %，而當pH值控制在4.0時，水力停留時間縮短至3 min時去除率還有87 %，在氮回收率方面，在pH4.0時約只有50 %，而pH值未控制則有100%，而實驗中以証明排除硝酸鹽或氨吸附於氧化鐵上所造成的可能性，因此推測在pH值4.0之下反應是有可能有含氮氣體產生。
本實驗採用零價鐵流體化反應器以CO2加壓來控制硝酸鹽反應時之pH值，且此系統可比曝氣系統大量減少CO2之用量且快速平衡pH值，實驗中發現，pH值會隨著反應時間增加而增加，而其硝酸鹽去除率效率將隨著減少，而整體去除率將隨著起始ZVI劑量的增加而增加，但在8.25 g l-1以上便趨於穩定，而初始硝酸鹽濃度增加至100 mg l-1時，對去除率只有些微的影響，而氮回收率方面實驗結果不若先前強酸控制pH值系統之結果，在各條件下均有100%之回收率，有可能是ZVI與硝酸鹽在不同pH值控制方式下有不同之反應路徑。
||Zero-valent iron (ZVI) and fluidize technique was successfully integrated in this study for nitrate removal, with three related processes developed, namely fluidized zero valent iron reactor, two fluidized ZVI reactors system, and pressurized CO2/zero valent iron system. Each of these is elaborated in the following three sections.
Fluidized zero valent iron bed reactor
With fluidized zero valent iron reactor, the pH of solution can be maintained at optimal conditions for rapid nitrate reduction. For hydraulic retention times of 15 min, the nitrate reduction efficiency increases with increasing ZVI dosage. At ZVI loadings of 33 g l-1, results indicate that the nitrate removal efficiency increases from less than 13% for systems without pH control to more than 92% for systems operated at pH of 4.0. By maintaining pH at 4.0, we are able to decrease the hydraulic retention time to 3 min and still achieve more than 87% nitrate reduction. The recovery of total nitrogen which is defined as the total of nitrate, ammonium, and nitrite was less than 50% for the system operated at pH 4.0, and was close to 100% for a system without pH control. The possibility of nitrate and ammonium adsorption onto iron corrosion products was ruled out by studying the behavior of their adsorption onto freshly hydrous ferric oxide at various pHs, suggesting the probable formation of nitrogen gas species during reaction in pH 4.0.
Two fluidized ZVI reactors system
A two fluidized ZVI reactors system was proposed to treat nitrate-contaminated water. The first column was employed to achieve an efficient nitrate reduction, while the second column was installed as the post-treatment process for neutralizing the effluent pH. The results of experiment show the pH increases and total nitrate removal decreases with increasing by pass ratio (BPR). Results from XRD analyses of the used ZVI taken from conditions at pH4.0 and 8.5 indicate only metallic iron was identified under pH4.0 condition and Fe2O3 and Fe3O4 along with metallic iron were identified for pH 8.5. Regarding to N-recovery deficiency problem, gaseous product was collected and nitrogen gas produced was confirmed.
Pressurized CO2/zero valent iron system
A fluidized zero valent iron reactor pressurized by CO2 gas for pH control was employed for nitrate reduction. The proposed CO2 pressurized system has advantages of using less CO2 gas and reaching equilibrium pH faster than CO2-bubbled system. However, due to weak acid nature of carbonic acid, system pH gradually increased with increasing oxidation of ZVI and reduction of nitrate. As pH increased with progress of reaction, nitrate removal rate decreased continuously. The results indicate that nitrate removal efficiency increases with increasing initial ZVI dosage but reaches plateau at ZVI doses of higher than 8.25 g l-1, and initial nitrate concentration up to 100 mg l-1 as N has minimal impact on the removal efficiency. Unlike the fluidized system with pH control by strong acid reported in our pervious study, near 100% of nitrogen recovery was observed in the current process, indicating that nitrate reduction by ZVI with different pH controlled mechanisms will have different reaction routes.
第一章、 序論 1
1-1 前言 1
1-2 研究目的 3
第二章、 文獻回顧 4
2-2 硝酸鹽去除技術之簡介 5
2-4 零價鐵還原機制之探討 9
2-5 氮回收率之相關研究 15
2-7 實驗流程之設計及其原由 22
第三章、 實驗設備與方法 23
3-1 研究流程 23
3-2 實驗材料 24
3-2 實驗設備 25
3-3 分析方法 27
3-4 研究主題之分述 33
第四章、 結果與討論 43
第五章、 結論 95
第六章、 參考文獻 98
圖 1、The illustration of nitrate reduction by ZVI. 9
圖 2、The process of this investigation. 23
圖 3、The peak spectrum of ion chromatographic. 27
圖 4、The calibration curve of NO3-N concentration measured by ion chromatographic method. 29
圖 5、The calibration curve of NO2-N concentration measured by ion chromatographic method. 29
圖 6、The calibration curve of NH3-N concentration measured by Phenate method. 30
圖 7、The calibration curve of Fe concentration measured by a atomic absorption spectrometer. 31
圖8、The calibration curve of Fe2+ concentration measured by Phenanthroline Method. 32
圖 9、Schematic experimental setup of fluidized ZVI reactor. 35
圖 10、Schematic experimental setup of two fluidized ZVI reactors system. 38
圖 11、Schematic experimental setup of pressurized CO2-ZVI system. 41
圖 12、The micrograph of ZVI by SEM(a)before the reaction(b) after the reaction(pH=4.0，NO3-Initial = 25 mg-N l-1，HRT = 15 min，ZVI = 33.3 g l-1) 44
圖 13、The nitrate removal efficiency as a function of time for various pH conditions. ZVI=33 g l-1, HRT= 15 min. Initial nitrate concentration of 25 mg-N l-1. 46
圖 14、The apparent pseudo-first order kinetic constant as a function of pH. ZVI=33 g l-1, HRT= 15 min. Initial nitrate concentration of 25 mg-N l-1. 50
圖 15 、The recovery of N-containing species and ammonium concentration as function of time for various pH conditions. Error bars represent one standard deviation (n≧13).ZVI=33 g l-1, HRT= 15 min. Initial nitrate concentration of 25 mg-N l-1. 52
圖 16、The experimental process of ammonium and nitrate adsorption onto iron oxides. 53
圖 17、The effect of pH on the adsorption of nitrate or ammonium onto freshly precipitated ferric hydroxide (5.6 g/L). Error bars represent one standard deviation (n=3) 54
圖 18、The effect of pH on the speciation of dissolved iron at HRT 30 min, ZVI 33 g l-1 and initial nitrate concentration of 25 mg N l-1. Error bars represent one standard deviation (n = 10). 55
圖 19、The effect of pH on the stoichiometric ratio between nitrate reduction and iron dissolution at HRT 30 min, ZVI 33 g l-1 and initial nitrate concentration of 25 mg N l-1. 58
圖 20、The N recovery as function of the stoichiometric ratio between nitrate reduction and iron dissolution 59
圖 21、The nitrate removal efficiency, recovery of nitrogen-containing species (including nitrate, nitrite, and ammonium), and ammonium concentration as function of the hydraulic retention times at pH 4.0. ZVI = 33 g l-1. Initial nitrate concentration of 25 mg N l−1. Error bars represent one standard deviation (n = 20). 60
圖 22、The recovery of nitrogen-containing species vs. ammonium concentration. 61
圖 23、The apparent pseudo-first order kinetic constant as a function of HRT. Error bars represent one standard deviation (n=20) 62
圖 24、The average nitrate removal efficiency and the apparent pseudo-first order kinetic constant as a function of ZVI dosages at pH 4.0. Error bars represent one standard deviation (n=20) 64
圖 25、(a) The removal efficiency and the residual amount of ZVI as the function of time and (b) the apparent pseudo-first order kinetic constant as a function of the amount of ZVI remained for the exhaustion test. ZVI = 33.3 g l-1. HRT = 30 min, pH 4.0, and initial nitrate concentration of 25 mg N l-1. 67
圖 26、The typical profiles for the effluent pH of the second column and nitrate concentrations of the reactors 1 and 2 as a function of time. The pH of reactor 1 was fixed at 4.0. BPR=1. HRT and ZVI dosages for both reactors are 30 min and 33.3 g L-1, respectively. 69
圖 27、The schematic experimental setup for ZVI dry in anaerobic condition. 73
圖 28、Iron speciation of used ZVI surface. (a) ZVI taken from the reactor 1 of fluidized systems (pH=4.0). (b) ZVI taken from the reactor 2 of fluidized systems (pH=8.5, BPR=3.0). 75
圖 29、Iron speciation of used ZVI surface for long term operation. (pH=8.5, BPR=3.0). 77
圖 30、The illustration of potential - pH for iron speciation. 77
圖 31、The illustration of pE - pH for nitrogen speciation. 78
圖 32、Schematic experimental setup for nitrogen collection. 79
圖 33、The spectrum of GC/MS for nitrogen qualitative analysis.(a)air, (b)sample. 80
圖 34、pH and total amount of CO2 dissolved in DI water system as a function of CO2 pressure. 82
圖 35、pH variation as a function of time in DI water system for various CO2 pressures. (a) Pressurized system. (b) Bubbled system. 83
圖 36、The pH variation in DI water of theoritical calculation and experiment value as function of CO2 pressure. 84
圖 37、pH variation as a function of time in DI water systems containing ZVI only and ZVI/Nitrate. CO2 pressure fixed at 3 bars. ZVI = 33 g l-1. Nitrate = 25 mg N l-1. 85
圖 38、Concentrations of oxidized iron species as a function of time in DI water systems containing ZVI only and ZVI+Nitrate. CO2 pressure = 3bars. ZVI = 33 g l-1. Nitrate = 25 mg N l-1. Error bar represents a standard deviation from the mean for triplicate experiments. 86
圖 39、pH variation as a function of nitrate removal percentage in DI water systems containing ZVI (33 g l-1) and nitrate (25 mg N l-1) with CO2 pressure fixed at 3 bars. 87
圖 40、Final pH and Nitrate removed as a function of initial ZVI dosages at reaction time of 30 min. CO2 pressure fixed at 3 bars, Nitrate = 25 mg N l-1. Error bar represents a standard deviation from the mean for triplicate experiments. 89
圖 41、Final pH and Nitrate removal efficiency as a function of initial nitrate concentration at reaction time of 30 min. CO2 pressure = 3 bars, ZVI = 33 g l-1. Error bar represents a standard deviation from the mean for triplicate experiments. 90
圖 42、Pseudo-first order plot and final pH under various reaction time for nitrate removal in system containing ZVI (33 g l-1) and nitrate (25 mg N l-1) with CO2 pressure fixed at 3 bars. 92
圖 43、Nitrate and ammonium concentration as a function of initial ZVI dosages at reaction time of 30 min. CO2 pressure fixed at 3 bars, Nitrate = 25 mg N l-1. Error bar represents a standard deviation from the mean for triplicate experiments. 94
表 1、Research paper about N-recovery. 17
表 2、The research about Pseudo-first order constant. 20
表 3、The material of this investigation. 24
表 4、The peak information of ion chromatographic. 28
表 5、The results of element analysis of ZVI surface by EDS 45
表 6、Normalized nitrate removal rate under various pH conditions 48
表 7、The speciation of dissolved iron for various pH conditions. ZVI dosage=33 g l-1. Initial Nitrate concentration= 25 mg-N l-1. HRT=30 min. Error bars represent one standard deviation (n = 10). 56
表 8、Effect of BPR on the fate of iron species, ammonium concentration, pH of the second column, and total N recovery ratio. 72
表 9 、Qualitative comparison of nitrate removal efficiency. 93
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