Environmental Engineering Reference
In-Depth Information
studies. Paul et al. [37] evaluated chemotaxis of Ralstonia sp. SJ98 toward
p-nitrophenol in soil microcosms using qualitative and quantitative plate and
tray assays. However, no effort was made to evaluate the effect of chemotaxis
on biodegradation.
One concern for in situ bioremediation is the bioavailability of contami-
nants, or the proximity of target pollutants to degrading microorganisms.
Many subsurface pollutants are hydrophobic, sparingly soluble in ground-
water, and form a non-aqueous-phase liquid (NAPL) [6]. These NAPLs remain
trapped in low-permeability regions in heterogeneous subsurface environments,
making them difficult to remove by conventional pump-and-treat treatment
methods. Chemotaxis is a mechanism for bringing cells in close proximity to
contaminants [7, 8, 25] thereby reducing limitations in bioavailability due to
mass transfer limitations or low contaminant solubility [7]. It also enables
bacteria to adjust their proximity to toxic repellents, thereby increasing their
odds for survival and optimizing their distribution in conditions favorable for
bioremediation. A vigorous chemotactic response can enhance the availability
of carbon and/or energy resources significantly and hence chemotactic bacteria
can grow faster than their non-chemotactic counterparts. Faster chemoattrac-
tant consumption causes localized depletion of the contaminant which creates
even steeper chemical gradients that trigger higher driving forces for dissolution
of the contaminant. In light of this, groundwater treatment technologies can
take advantage of bacterial chemotaxis for enhancement of contaminant
removal. Scenarios of groundwater remediation in which chemotaxis can
potentially be exploited to enhance biodegradation are described below.
7.7.1 Enhanced Remediation Due to Chemotaxis in Heterogeneous
Porous Media
Contaminants often remain trapped in pockets of low permeability within the
subsurface. Figure 7.2 depicts a contaminated aquifer scenario where contami-
nant is trapped in a low-permeability clay lens. In Fig. 7.2a, non-chemotactic
degradative bacteria flow with advective ground water through the surrounding
high-permeability region, limiting remediation to the slow diffusion of con-
taminants into the high-permeability region. Lanning et al. [53] recently
reported that chemotactic bacteria can swim transverse to the flow direction
at fluid velocities greater than typical ground-water flow velocities. Thus che-
motactic bacteria flowing with ground water in the high-permeability regions
can sense chemical gradients induced by diffusing contaminants and migrate
toward the source (Fig. 7.2b). Chemotactic bacteria are able to swim upgradient
in typical ground-water velocities [47] to penetrate low-permeability regions
from the bulk flow in high-permeability areas [49]. Accumulation of bacterial
bands surrounding contaminated low-permeability regions can significantly
enhance contaminant removal.
