(752f) Biodegradation of Carbon Tetrachloride in Laboratory Flow Channels

Carbon tetrachloride
(CT) is a solvent used in several applications including degreasing and
fumigation. CT is a common groundwater pollutant and a suspected carcinogen.
Remediation feasibility studies were conducted by mesocosm experiments; a
chamber was divided into six channels and filled with soil, and plants were
grown on the soil surface. Each channel was fed with CT contaminated water near
the bottom and collected at the outlet, simulating groundwater flow conditions.
CT was introduced at a concentration of about 2 mg/L (~13 μmoles/L) in three
channels, two of them with alfalfa plants and the other with grass. The other
three channels were used for a different study. After the system had attained
steady state, the concentrations of CT at inlet and outlet were monitored and
the amount of CT disappearing in the saturated zone was studied. Since no
degradation products were found at the outlet after about 100 days, anaerobic
conditions were created in one channel (with alfalfa) by adding one liter of 0.2%
glucose solution. The glucose solution was fed once every month starting from
day 111 and continued until day 328. From day 203 to day 445, one liter of 0.1%
emulsified soy oil methyl esters (SOME) was fed once a month to another channel
(with alfalfa). The monthly SOME addition dates were the same as for glucose. Degradation
compounds of CT were detected in the outlet liquid of the channels fed with
SOME and glucose; however, the extent of degradation in the SOME fed channel was
not as great as that of the glucose fed channel. No degradation compounds were
observed in the outlet solution of the channel (grass grown on top) in which no
carbon and energy supplements were added. The total chlorinated methanes (CMes)
decreased to less than 1 μmoles/L in channels 5 and channel 6; however,
the CMes remained low for a longer time in the glucose fed channel. In addition
to glucose, corn starch and cheese whey were also added to channel 5, as growth
substrates. Supplements such as glucose, corn starch, cheese whey and SOME
stimulated the indigenous microbes and helped in the degradation of CT. In both
glucose and SOME fed channels, the degradation continued several weeks after
stopping the feeding of supplements. The soil matrix and the microorganisms
were able to store the supplements/degradation products of supplements and
provide a long-term source of carbon and hydrogen. This finding is very valuable
in the design of remediation systems in field sites since it determines the
frequency of supplement addition.