Joint work with Peter C. D'Adamo.
Bioremediation involves complex interactions of biological, chemical, and physical processes, and requires integration of phenomena operating at scales ranging from that of the microbial cell (1 Ám) to that of the field site (10-1000 m). Field and laboratory studies have been conducted at a former manufactured gas plant in Baltimore to examine some of the scaling issues pertinent to bioremediation and bioavailability. The site has extensive coal tar contamination.
Sediments obtained from the site and incubated in the laboratory indicate that the indigenous bacteria are capable of mineralizing the principal aromatic compounds in the groundwater plume (benzene, naphthalene, and phenanthrene). Aerobic conditions were most favorable for the biotransformations. Only two sediment microcosms exhibited mineralization of naphthalene (<15%) under anaerobic conditions with nitrate. Supplementing the natural sediment microcosms with oxygen and nutrients (N & P) enhanced the extent of mineralization for benzene, naphthalene, and phenanthrene under aerobic conditions. Batch studies conducted under mixed electron acceptor (nitrate with microaerophilic oxygen levels) conditions, resulted in the degradation of BTEX, naphthalene, and phenanthrene with toluene degraded using nitrate as a terminal electron acceptor in the presence of oxygen. Sequential removal of contaminants was observed in these studies. Biodegradation rates in the field are significantly slower than in the laboratory because of reduced bioavailability. Bioavailability is affected by sorption/desorption in two ways. First, rates of contaminant biotransformation are directly proportional to aqueous concentration, which is directly reduced by sorption. Second, when processes of mass transfer from the immobilized phase are sufficiently slow, the aqueous concentrations may be diminished to an even greater extent than equilibrium partitioning models would predict, thereby enhancing the first effect. In this case, the rates of both biotransformation and bacterial growth can be ultimately controlled by the rate of desorptive mass transfer. Batch mineralization studies conducted in the presence of natural geosorbents revealed that the rates of naphthalene and phenanthrene mineralization were diminished (50 to 89% depending on the "aging" period) in the presence of these solids as compared to mineralization studies without solids. For two geosorbents, Borden sand and Bozeman sediments, mineralization rates were further reduced as a function of contaminant exposure time, or "aging." Concurrent desorption rate experiments with these solids demonstrated that phenanthrene and naphthalene desorption mass transfer rates were a function of aging with slower rates observed with increased contaminant exposure time. First-order phenanthrene desorption mass transfer rates decreased from 0.003 hr-1 to 0.008 hr-1 and from 0.011 hr-1 to 0.002 hr-1 for Bozeman sediments and Borden sand, respectively, as the aging time was increased from 7 to 270 days. These rates were significantly less than pseudo first-order biodegradation rates estimated independently in batch biodegradation studies conducted without solids (0.45 hr-1). The influence of sorption on biodegradation is quantified by defining a Bioavailabilty Factor, Bf. A Thiele Modulus which indicates the ratio of characteristic times for desorption and biodegradation is helpful for determining the extent of mass transfer control during biodegradation of the aromatic compounds. Some fruitful areas of further research involving modeling and experimentation will be discussed.
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