ANAEROBIC DECHLORINATION OF CARBON-TETRACHLORIDE BY FREE-LIVING AND ATTACHED BACTERIA UNDER VARIOUS ELECTRON-DONOR CONDITIONS

The dechlorination of carbon tetrachloride (CCl4) by free-living and a ttached bacteria under anaerobic conditions was studied to examine the relationship between porous media and electron donor. Two batch-type experiments, the free-living and attached bacterial systems, were cond ucted with and without addition of 0.5-mm glass beads. Glucose and ace tate were selected as the primary electron donors because they are eas ily biodegradable. Direct epifluorescence technology, the DAPI (4' 6-d iamidino -2-phenylindole) method, was used for counting the microbial activities. Adding glass beads could accelerate the dechlorination rat e of CCl4. Removals of 44%-57% were observed in free-living bacterial system. Whereas a two- to fivefold increase in the CCl4 dechlorination rate was observed in the attached system. Experimental results and th ermodynamic calculations indicated that glucose is a better supplement ary substrate than acetate for stimulating the dechlorinating capabili ty of microorganisms because of its relatively high available free ene rgy. A higher concentration of substrate provided more reducing power for attached bacteria to initiate the dechlorination reaction. The pse udo-first-order rate constants of CCl4 dechlorination ranged from 0.00 7 day(-1) to 0.017 day(-1) and from 0.011 day(-1) to 0.0625 day(-1) fo r free-living and attached bacterial systems respectively. Microscopic observation revealed a three- to eightfold difference of microbial nu mber between the free-living and attached bacterial systems. On the ba sis of the results in this study, we can conclude that the presence of porous media and an electron donor can change the dechlorination capa bilities of the microorganisms. This work will be valuable in the desi gn of in situ bioremediation as it discusses the specific area of the medium and supplementation with an electron donor to stimulate the ind igenous microflora.