|其他題名: ||Nitrate removal by zero-valent iron (ZVI)|
|作者: ||陳一銘;Chen, Yi-ming|
|上傳時間: ||2010-01-11 07:22:05 (UTC+8)|
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.