Environmental Engineering Reference
In-Depth Information
and impracticality of fully removing all trapped contaminant sources [3],
researches have explored in situ bioremediation and biological stabilization as
safe and efficient alternatives to physicochemical remediation strategies [4, 5].
Bioremediation refers to mineralization and/or transformation of pollutants into
less harmful compounds via microbial degradation [6]. Quantification of advec-
tive and dispersive transport of bacteria and contaminants in the subsurface, and
characterization of biochemical reactions involved in pollutant biodegradation,
have been major areas of research in the past few decades. The roles of enzymes
and genes in biodegradation are now relatively well understood. Current research
efforts are focused on identifying additional features of bacterial behavior that
may help enhance the rate of biodegradation [7]. Bacterial chemotaxis, a directed
movement toward or away from chemicals (energy sources), is one such beha-
vioral response that has the potential to enhance in situ bioremediation [7].
Chemotaxis may be applied to current technologies in bioaugmentation,
monitored natural attenuation (MNA) and contaminant containment to
improve overall remediation efficiency. Bioremediation is often limited by
inadequate distribution of bacteria in regions favorable for pollutant degrada-
tion. For example, contaminants trapped in low-permeability pockets within
the subsurface are persistent sources of long-term contamination in the envir-
onment; chemotaxis enables bacteria to locate, penetrate, and remediate these
pollutant sources. While high concentrations of many environmental pollutants
can be toxic, bacteria are able to degrade them at lower concentrations. Direc-
ted migration away from toxic hot spots toward lower peripheral chemical
concentrations maximizes biodegradation efficiency by removing bacteria
from high contaminant concentrations that inhibit growth and degradation
and allowing them to accumulate in niches optimal for growth and metabolism.
Chemotaxis has the potential to significantly improve in situ bioremediation
processes; however, in order to apply this emerging technology to site remedia-
tion plans it is important that we optimize the unique transport processes that
facilitate bacterial chemotaxis. This chapter presents a review of current litera-
ture on bacterial chemotaxis toward environmental pollutants, describes the
specific assays available to quantify chemotactic transport parameters, and
provides recommendations on how chemotaxis may be exploited to improve
field-scale applications of in situ ground-water bioremediation.
7.2 Bacterial Chemotaxis
The ability of bacteria to sense and respond to chemical gradients in their
surroundings and to direct their migration either toward or away from increas-
ing concentrations of chemicals is known as chemotaxis [5, 8]. Bacterial migra-
tion toward and away from chemicals helps bacteria navigate to niches that are
optimal for their growth and survival [5] and is termed positive and negative
chemotaxis, respectively. Generally, chemicals that attract bacteria as sources
