A microcosm study of the biodegradability of adsorbed toluene by acclimated bacteria in soils
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Groundwater contamination by man-made chemicals is increasingly being reported in the United States. The potential for detrimental health effects is substantial and has been addressed by the environmental engineering profession. Typically, contaminated groundwater is pumped to the surface and treated in a variety of methods including air stripping, carbon adsorption, and biodegradation. In situ biodegradation is increasingly being considered as an alternative to pump-and-treat technology.
The primary goal of this research was to determine the fate of an organic chemical adsorbed to a subsurface soil when exposed to acclimated bacteria. Toluene was chosen as a representative compound because it is a major constituent of groundwater contaminated by gasoline. In addition, toluene is known to be both biodegradable and adsorbable. Sybron Biochemical, Inc. supplied the aerobic bacteria Psgudomonas gutjga known to readily transform toluene.
Soil microcosms were established in test-tubes and conditions simulated those of a saturated, aerobic aquifer. Gas chromatography was used to quantify changes in toluene concentration due to adsorption and biodegradation. The addition of an aqueous toluene solution to sterile microcosms resulted in the rapid and extensive adsorption of toluene to the soil. Subsequent analysis revealed the slow adsorption of an additional small fraction of toluene.
Biodegradation studies entailed the addition of acclimated bacteria to sterile soil microcosms in which substantial toluene adsorption had occurred. Addition of small doses of hydrogen peroxide effectively maintained aerobic conditions for biodegradation. As a result, E, putjda was able to transform all measurable toluene in the microcosms.
Additional desorption studies revealed that a "resistant" component of toluene remained adsorbed to the soil during biodegradation. This component was neither acted upon by bacteria nor readily extractable by methylene chloride. However, slow desorption of toluene was shown to occur at a rate comparable to slow adsorption. To achieve complete removal, groundwater treatment methods must address the rate-controlled desorption of the resistant toluene component.
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