Abstract:
Anaerobic biodegradation processes have been extensively used to remediate tetrachloroethene (PCE) and hexachloroethane (HCA) and are often combined with chemical processes such as Fenton's reactions to achieve more economic and flexible treatment. However, Fenton's reactions could have adverse effects on anaerobic cultures. Thus, this research set out to investigate the degradation of PCE and HCA by Fenton's reactions using different iron catalysts, H202 concentrations and pH conditions to study the oxidative and reductive mechanisms involved in the degradation of PCE and HCA. The effect of Fenton's reactions on the anaerobic microbial community structures and the subsequent effect on the anaerobic biodegradation of PCE and HCA were also explored. The Fenton's reactions with different iron catalysts showed that the reactive moieties produced dependea' largely on the types of the iron catalysts used to initiate the reactions. The variety of interactions between reactive moieties that occur in each system, the cyclic nature of the Fe(II)Fe(III) redox interactions and the interference of PCE and HCA with the cycling of reductant and oxidant moieties in the system make for a complex system. Fe(III) was more effective than Fe(II) as a catalyst in degrading PCE and HCA. The PCE and HCA degradation was enhanced when they were in a mixture in the Fe(II)-catalysed reactions, but decreased in the Fe(III)-catalysed reactions. The PCE and HCA degradation in the iron-citrate-catalysed reactions was not as effective - the PCE degradation was slower and the HCA degradation was negligible - as in the iron-catalysed reactions. The anaerobic cultures prepared in the laboratory lost its biodegradability of PCE and HCA after 1 h exposure to Fenton's reactions, but retained the biodegradability after 2 d exposure. The biodegradation of PCE and HCA was enhanced with increasing recovery times. Overall, the exposure to Fenton's reactions changed the microbial community structures but had beneficial effects in developing a more diverse culture. The work presented in this thesis makes a valuable and significant contribution to the increasing pool-of-knowledge required for the enhancement of chlorinated contaminant remediation by the combined Fenton's reactions and anaerobic biodegradation.