| Summary: | 博士 === 淡江大學 === 水資源及環境工程學系博士班 === 95 === 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.
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