Investigations of Reactivity of Nanoscale Iron Particles for Degradation of Chlorinated Organic Contaminants in Seawater

摘要

The use of nZVI, as a cost-effective and environmentally friendly remediation technique has been extensively investigated over the last decade for the degradation of a range of different contaminants; however, few studies have focused on its application in seawater environments. In this thesis, the degradations of trichloroethylene (TCE) and polychlorinated biphenyl (PCB) are examined in seawater environments with further investigations into a range of strategies to improve the reactivity of the nZVI particles in seawater.With regard to the synthesis of nZVI particles, the possibility of using sodium dithionite (Na2S2O4) as an alternative to sodium borohydride (NaBH4) is examined in view of its lower cost and increased safety. The observed differences in the physico-chemical properties of particles formed using the different reducing agents, together with differing palladium speciation on the particle surface following the addition of Pd in catalytic quantities, lead to a reduced dechlorination efficiency of either monometallic or bimetallic dithionite-synthesized particles for TCE reduction. In relation to the degradation of PCB, the experimental data indicated that only palladized nZVI(BH4) particles were able to effectively dechlorinate PCB.Experiments comparing the reactivity of both particle types in deionized water or seawater demonstrated only a slightly slower dechlorination of PCB and TCE and less complete transformation to de-chlorinated products in seawater compared to deionized water. The expected reduced reactivity in seawater appears to be largely counteracted by the positive effect of pitting corrosion of the passivation layer due to the high salt concentration. To improve the reactivity of the particles in seawater, three possible strategies to counter the adverse effects of seawater were examined, coupled with investigations to understand the underlying mechanism of the strategies employed. The first strategy involved the addition of the ligand (ethylenediaminetetraacetic acid (EDTA)) with marked enhancement in PCB degradation via removal of the passivating layer observed following the exposure of particles to EDTA. The second strategy investigated was the use of stabilizer, and results indicated that the steric stabilizer (polyvinylpyrrolidone (PVP)) proved to be more effective for the degradation of PCB in seawater compared to the electrostatic stabilizer carboxymethyl cellulose (CMC). Moreover, the addition of EDTA to the CMC-coated particles demonstrated no improvement in PCB removal. Finally, the use of wet particles relative to those which had been oven-dried were found to be superior at decomposing PCB, as the dried particles underwent aggregation and possessed a less reactive oxide shell.