Abstract
Anaerobic reductive dechlorination of trichloroethylene (TCE) and its daughter products, cis-1,2dichloroethylene (cDCE) and vinyl chloride (VC), is commonly inhibited by low pH and high TCE and
cDCE concentrations. Inhibition during bioremediation may cause a buildup of vinyl chloride which
can make the system more toxic than it was previously. One potential solution is to immobilize
dechlorinating microorganisms within a hydrogel to create a mass transfer barrier between microbes
and external toxins. Although, no studies to date have experimentally determined mass transfer
coefficients of TCE, cDCE, and VC through hydrogel materials or evaluated how immobilization may
impact microbial degradation. To address this knowledge gap, mass transfer coefficients of TCE,
cDCE, and VC were measured in 10% poly(vinyl alcohol) (PVA) hydrogels. Hydrogel crosslinking
method was varied to evaluate diffusion in different crosslinked networks, and ionic strength was varied
to determine the impact of near groundwater ionic strength on diffusion. Diffusion through cryogels
was significantly decreased by an increase in ionic strength from 0 to 0.01 M, but diffusion in
chemically crosslinked hydrogels was not. TCE and cDCE diffusion were significantly impacted by
the interaction between ionic strength and crosslinking method, but VC was not. These measured
diffusion coefficients were then utilized in a single spherical bead (biobead) reaction-diffusion
computational model to evaluate microbial degradation when mass transfer limitations were present.
Bead radii (0.1 - 1 cm) and cell concentration (106 - 1012 cells/mL) were varied to evaluate the impact
of external conditions such as initial TCE concentration and initial pH, as well as internal conditions
such pH inhibition and reaction rate increases, or dampening, on anaerobic reductive dechlorination.
pH inhibition and chlorinated hydrocarbon concentrations were found to still have significant impacts
on dechlorination, resulting in a buildup of cDCE in the biobeads tested. A large-scale computational
model of a multi-bead permeable reactive barrier (PRB) was also completed to evaluate if biobeads
could degrade TCE to ethylene. Multiple bead diameters, cell concentrations, inlet TCE concentrations,
and inlet pHs were tested using the Taguchi Method to evaluate PRB performance. Sensitivity analyses
were also completed to determine how changes in microbial reaction rate and inhibition due to
immobilization would impact PRB outlet concentrations. Results of each model were related back to
the Thiele Modulus to help inform on the balance between species transport and microbial reaction
rates for future use of immobilized microorganisms in the bioremediation of TCE. Degradation studies
were also conducted to determine the stability of hydrogels in anaerobic and aerobic environments for
evaluation of long-term use in bioremediation applications. The work herein presents measured mass
transport coefficients and evaluates biobead performance as a single bead and as multiple beads within
a PRB to determine how immobilization may impact microbial anaerobic dechlorination with and without changes to microbial kinetics. Degradation studies were utilized to evaluate PVA hydrogel
stability in anaerobic and aerobic environments in long-term studies.