The effects of dispersion methods and organic acids on the reactivity of iron nanoparticles to remove halogenated organic contaminants
| Autor: | Chih-ping Tso, 左致平 |
|---|---|
| Rok vydání: | 2016 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 104 Nanoscale zerovalent iron (NZVI) and bimetallic Fe nanoparticles (NPs) have significant potential for the remediation of a wide array of priority pollutants. Their properties of a large surface area and high reduction potential generated significant interest in their application for in-situ remediation. However, Fe NPs aggregate immediately that significantly reduce their mobility and reactivity. Furthermore, corrosion processes form precipitates on the Fe surface, whose passive layers resulted in a rapid decrease in activity and longevity. Therefore, methods to enhance/extend the colloidal stability and reactivity of Fe NPs are needed. On the other hand, it is important to understand the reactivity of Fe ions and Fe oxides after the application of Fe NPs. Fe NPs were dispersed successfully via physical (ultrasonication (US)) and chemical (CMC stabilizer) dispersion methods under different environmental conditions. Carboxylic acids including formic acid (FA), oxalic acid (OA) and citric acid (CA), were applied to prolong the Fe particles reactivity by removing passive layers on the Fe surface. Furthermore, the activity of Fe (Cu) complexes/precipitates with humic acid (HA) to mimic natural environments was assessed for subsequent treatments. Well-dispersed bare Fe NPs enhanced the adsorption of contaminants such as pentachlorophenol onto the Fe surface, as compared to Fe aggregates. In the presence of common anions (Cl−, NO3− and HCO3−), NO3− was reduced by Fe NPs and generated more Fe (hydro)oxides that responded to increase the adsorption/coagulation of the target compound. The inhibition of Fe reactivity by HCO3− may be due to the formation of precipitates on the Fe surface while Cl− only caused a small increasing in Fe reactivity. The presence of CMC suspended Fe NPs very well and dispersed them into individual particles instead of nanoparticle aggregates. Due to the properties of CMC, the reactivity of CMC-Fe NPs toward decabromodiphenyl ether was enhanced under alkaline conditions. The reaction rate was considered as a surface chemical reaction because the CMC layers induced diffusion for the target compound to the Fe surface. But CMC layers may also hinder the electron transfer. Anions did not influence the reactivity of CMC-Fe NPs compared with bare Fe NPs, indicating that the CMC layers may inhibit surface corrosions and thus prolong the reactivity of Fe NPs in the environment. Common carboxylic ligands (FA, OA and CA) induced the reactivity of Fe particles toward trichloroethylene following an order of FA > OA > pure water ≅ CA by dissolving Fe oxides from the Fe surface. FA provided protons to promote the dissolution of passive layers and to convert iron oxides to form magnetite which increased the adsorption of the target compound onto the Fe surface. With the strong complexing ability, OA and CA could form dissoluble complexes to remove passive layers. But a high concentration of OA resulted in reprecipitated of Fe oxalate back onto the Fe surface which then inhibited its reactivity. Moreover, these Fe-ligand complexes could further degrade the target compound depending on their redox properties. The activity of Fe2+ and HA-Fe complexes depended on the pH which had strong interactions toward the target compound at pH 9. RHA-Fe complexes had higher reactivity than UHA-Fe, but these complex forms potentially reduced the Fe2+ reactivity because of electron competition with the target compound. Moreover, RHA prevented Fe2+ from precipitating but also caused higher Fe2+ oxidation. Furthermore, HA co-precipitated Fe and Cu colloids showed ability to remove the organic contaminants such as Reactive black 5. HA-Cu colloids had higher reactivity than HA-Fe colloids, which could result from the nature of metal, the shape and the morphology of particles. RHA-Cu colloids stably dispersed in aqueous solutions. Among them, a small amount of Cu0 was generatead by RHA. RHA-Cu colloids also had a stronger reactivity than UHA-Cu colloids for breaking azo bonds in the target compound. To conclude, this study presents the reaction characteristics and removal mechanisms of bare Fe NPs that were well suspended under different conditions and the potential for CMC-Fe NPs for in-situ treatments. The characteristics and reactivity of metal complexes/colloids with ligands and HA in reducing environments have pointed out the potential of these Fe (Cu) complexes/colloids for sustainable/green remediation. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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