Reductive debromination of hexabromocyclododecane by nanosized zerovalent iron
| Autor: | Chih-Ping Tso, 左致平 |
|---|---|
| Rok vydání: | 2010 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 98 Hexabromocyclododecane (HBCD) belongs to the large family of brominated flame retardants (BFRs) and is widely used as additives in many household and commercial products to reduce their flammability. Because of its environmental persistence and high production volume in the past, HBCD has been widely detected in the environment. In this study we conducted to investigate the removal of HBCD by using nanoscale zero-valent iron (NZVI) particles and two stabilized NZVI suspensions, sodium carboxymethyl cellulose (CMC) stabilized NZVI (CMC-Fe) and CMC-Ni/Fe bimetallic nanoparticles. We also evaluate the effects of environmental factors such as temperature, pH, anions, and humic acid on the degradation kinetics. HBCD was almost removed from aqueous solutions by NZVI or CMC-Ni/Fe with 5 g/L iron loading in 30 min. The pseudo-first-order the rate constants were 0.208 min-1 and 0.144 min-1 for NZVI and CMC-Ni/Fe, respectively. However, only 25.5 % removal efficiency of HCBD with CMC-Fe nanoparticles was observed within 1 hr. The CMC could occupy the reactive sites on the iron surface then hinder the mass transfer of HBCD to the reactive sites of Fe. With the increase of temperature, the degradation kinetics of both NZVI and CMC-Ni/Fe increased. The activated energies of NZVI and CMC-Ni/Fe nanoparticles were 18.0 and 7.10 kJ/mol, respectively. The effect of pH was not obvious for these two particles. The removal rates in the presence of these three anions were observed in the order of: pure water＞Cl- "≅" NO3-＞HCO3-, which may result from the quick aggregation of bare NZVI particles in the presence of these electrolytes and the possible complexation of anions with oxidized iron surface. In the contrary, for CMC-Ni/Fe nanoparticles, Cl- and HCO3- slightly affect degradation rates, whereas the high concentration of NO3- could inhibit the reactivity of CMC-Ni/Fe0. The transformation efficiency of NO3- to NH4+ decreased with increasing NO3- concentration, and less NO3- transformation in CMC-Ni/Fe system was observed as compared to bare NZVI. Degradation rates of NZVI or CMC-Ni/Fe nanoparticles slightly increased with humic acid (HA) concentration increased up to 5 mg/L. High concentration of HA (＞5 mg/L) could inhibit the degradation kinetics of NZVI and CMC-Ni/Fe, possibly because the complexes of humic acid and dissolved iron species may compete for the reactive sites on the iron surface with HBCD and forming surface passivating layers; subsequently inhibited iron corrosion and reduce the HBCD reduction rate. The removal rate of NZVI particles declined in soil solution; however, for CMC-Ni/Fe nanoparticles, the fast removal efficiency can still be achieved in 5 minutes, and the particle size sustained in about 65 nm through the reaction. The less brominated byproducts of HBCD were identified by gas chromatography-mass spectroscopy to suggest the reductive debromination process. Because the stabilized CMC-Ni/Fe can maintain nanoscale size to transport through subsurface effectively to remove halogenated compounds and to be unaffected by the above discussed common environmental factors, we suggest CMC-Ni/Fe nanoparticles having a high application potential in the environmental treatments. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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