Environmental Engineering Reference
In-Depth Information
chemotaxis can potentially be exploited to enhance in situ bioremediation in the
subsurface, particularly in heterogeneous and low-permeability regions, where
low solubility contaminants such as NAPLs remain trapped.
7.6.3 Impact of Chemotaxis on Contaminant Degradation
A quantitative evaluation of enhanced chemoeffector degradation due to che-
motaxis is presented by Law and Aitken [42]. Naphthalene desorption and
degradation from a model NAPL was faster for a chemotactic wild-type
P. putida G7 strain compared with non-chemotactic strains, which is attributed
to the steep concentration gradient created by chemotactic bacteria near the
NAPL surface. Thus, chemotaxis can be useful in increasing the rate of mass
transfer and biodegradation of NAPL-associated hydrophobic pollutants.
The role of chemotaxis in naphthalene degradation by P. putida G7 in a
heterogeneous aqueous system was evaluated experimentally by Marx and
Aitken [46]. They demonstrated that mass transfer was a rate-limiting step in
naphthalene biodegradation by non-chemotactic strains. In contrast, the
removal rate clearly exceeded the mass transfer rate in the case of the chemo-
tactic wild-type strain of P. putida G7 and was approximately five times faster
than the non-chemotactic strains. These results clearly indicate the possibility of
enhancing bioremediation in aqueous systems by chemotaxis.
7.7 Field-Scale Application of Chemotaxis to Ground-Water
Bioremediation
Bacterial movement in soil as a result of chemotaxis and randommotility is well
documented by soil microbiologists [50, 51]. However, studies demonstrating
direct evidence of enhanced bioremediation at the field scale due to chemotaxis
are very rare. Witt et al. [52] demonstrated faster migration of Pseudomonas
stutzeri KC, a denitrifying strain chemotactic toward nitrate, in comparison to
groundwater flow velocities in a bench scale study of carbon tetrachloride (CT)
degradation. CT and nitrate were injected with groundwater into a model
aquifer column containing CT-saturated sediments. Bacteria and tracers were
inoculated at the top of a column and it was observed that bacteria migrated
through the column faster than the traces, removing both adsorbed and aqu-
eous CT. This enhanced migration of strain KC was attributed to the chemo-
tactic response as a result of nitrate depletion in the vicinity. Olson et al. [21]
also reported chemotaxis of P. putida F1 toward TCE in small-scale laboratory
columns packed with glass-coated polystyrene. Two field-scale bacterial trans-
port studies recently reported faster transport and greater recovery of motile
Pseudomonas compared with non-motile bacterial strains [12, 43]. However, no
distinction between chemotaxis and motility was made in these two field
