| dc.description.abstract | This study presented and discussed the results from batch and continuous flow experiments of nickel, cyanide, and Ethylene Diamine Tetraacetic Acid (EDTA) treatment using iron-based media. The removal mechanisms of all three contaminants have been investigated. The activated iron media (AIM) system can be further applied to the electroplating wastewater treatment for the removal of nickel, cyanide, and EDTA.
Nickel removal is possible for both the Zero-Valent Iron (ZVI) and AIM systems. The removal mechanism in the ZVI system is mostly contributed by the reductive reaction. Fe0 on the iron particle surface reduces the dissolved Ni2+ ions to insoluble Ni0, while oxidizes itself and releases as Fe2+. In the AIM system, it is the replacement reaction that dominates the removal mechanism. The labile Fe(II) from surface CPs are substituted by dissolved Ni2+ ions resulting in the change of the FeOx structure. The X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Diffraction (XRD) spectra of the sediments indicated the detection of Ni0 in the ZVI system and the formation of NiFe2O4 (trevorite) in the AIM system. These evidences support the reductive and replacement mechanisms, which also agreed with the difference in final dissolved Fe2+ concentration in the two systems. The results of the continuous batch and Continuous Stirred-Tank Reactor (CSTR) experiments show that the AIM system was sustainable and more efficient than the ZVI system. Therefore, the AIM system would be a robust and time-tested treatment procedure for nickel removal.
The removal of cyanide was not preferable in anoxic condition. In the iron-precipitation process, cyanide forms ferrocyanide with dissolved Fe2+ and further to insoluble ferrous ferrocyanide. However, the process is reversible and the sediments are unstable. The anion exchange procedure allows cyanide ions to exchange with ions in iron media, which reduced the aqueous cyanide concentration. The cyanide ions, whereas can be exchanged back to the solution by ions with higher binding affinity. Therefore, this process is hard to operate and maintained. Cyanide treatment can be feasible in the oxic environment. The oxic AIM system can produce a notable amount of hydroxyl radicals, which oxidized cyanide. Cyanide and iron-chelated cyanide can be converted to nitrogen gas step by step. With the quantification of the intermediate products (ferricyanide, cyanate, and ammonia) and comparison of the inhibited system, the AIM system was proven to treat cyanide irreversibly with no other reagent needed. The continuous batch test further implied the reliability, efficiency, and sustainability of the system. Therefore, the oxic AIM system would be a trustworthy treatment procedure for cyanide-containing wastewater.
EDTA can be removed by the adsorption process in both ZVI and AIM systems. In the anoxic environment, only a certain amount of EDTA can be adsorbed due to the limited surface loading of both media. It is not sustainable and reliable. EDTA treatment is feasible in the oxic AIM system by the oxidative process. Hydroxyl radicals, produced in the oxic AIM system, are able to break down the long-chain EDTA molecule to the smaller molecule products. The detection of iminodiacetic acid (IMDA), nitrilotriacetic acid (NTA), glycine, and ammonia has proved that EDTA was degraded through the ‘glycine’s pathway’ during the process. Additional H2O2 injection leads to the efficiency improvement of the EDTA treatment in the oxic AIM system. H2O2 performs Fenton reactions with dissolved Fe2+ in the system, which expedites the hydroxyl radical’s production. In the H2O2-AIM system, the final EDTA removal was increased. The continuous batch experiment proves the H2O2-AIM has higher removal efficiency than AIM itself. Therefore, the H2O2-AIM system would be a possible treatment procedure for wastewater containing EDTA.
The reduction of cyanide and EDTA were nearly unaffected when they combined with nickel. However, due to the strong chelating effect with EDTA, the nickel treatment was limited. The addition of H2O2 accelerates the decomposition of cyanide and EDTA, which consequently improves nickel treatment. In the H2O2-AIM system, with the increased amount of additional H2O2, the dissolved nickel concentration dropped faster than it in the AIM-only system. The continuous batch and CSTR experiment proved the H2O2-AIM system can simultaneously treatment of nickel, cyanide, and EDTA. Therefore, the H2O2-AIM system can be a reliable and sustainable process for simultaneous treatment of nickel, cyanide, and EDTA in industrial wastewater. | |
