| Carbon tetrachloride (CT) biodegradation in dual porosity (clay-sand) systems was investigated. Two laboratory-scale columns, the Halton column, packed with a till from the Halton region, Ontario, and the Rio column, packed with a clay from Rio de Janeiro, Brazil, were used in this study. The central core of each 10 cm diameter column was comprised of a 2.5cm diameter sand macropore. Biostimulation and bioaugmentation were done with methanol and Methanosarcina barkeri. A unique aspect of this study was the application of molecular tools, including DGGE (denaturing gradient gel electrophoresis), and qPCR (quantitative polymerase chain reaction) to determnie the spatial distribution of microorganisms in the dual porosity columns.; No initial methanol consumption was observed in the presence of 500mug/L of CT, but removal of CT, resulted in the onset of methanogenesis. Renewed CT addition resulted in conversion of CT to CF and DCM, with complete chloroemethane removal at inlet CT concentrations as high as 4.5mg/L. The clays contained indigenous eubacteria and archaea, despite dominant pore sizes that were smaller than the microbial cell sizes, including Methanosarcinae and Clostridia. Methanosarcina barkeri inoculated into the sand cores of the columns did not grow beyond the clay-sand interface. Growth of indigenous microorganisms within the columns was also predominantly in the sand cores. There was significant microbial growth of microorganisms in the clay near the sand-clay interface in the Halton column, but no significant growth was observed in this region in the Rio column. Calibrated first order degradation constants from modelling were consistent with biomass growth patterns, confirming that the majority of biodegradation occurred in the sand in the Rio column and in the sand and the sand-clay interface zones in the Halton column.; The Rio column had higher rates of CT biodegradation and less inhibition of methanol consumption. It also showed much more significant mineral precipitation. The Rio clay had higher porosity and higher iron content than the Halton column, which may have contributed to differences in column performance. The study demonstrates that clay physical structure, mineralogy, and microbiology must be considered in assessing contaminant transport and remediation in dual porosity sand-clay systems. |
