Acid Tolerant CAH Bioremediation: Combined Experimentation and Modeling of Hydrogel Encapsulated Biobeads

Trichloroethylene (TCE) and other chlorinated aliphatic hydrocarbons (CAHs) are a widespread issue in groundwater due to their acute toxicity, carcinogenicity, and volatility. Long term TCE exposure at low levels has been linked to kidney cancer, non-Hodgkin's lymphoma, cardiac defects, and death. Anaerobic reductive dechlorination by microorganisms (bioremediation) is a cost-effective solution to treat TCE; however, during remediation acid (hydronium ions) can accumulate to levels that inhibit or are lethal to degrading microorganisms and halt degradation in a more hazardous state.

Biodegradable, spherical hydrogel encapsulated microorganisms, also known as “biobeads”, allow for tuned control of mass transport and the ability to couple non-biological processes to reduce the acid generated during bioremediation. Our research group has concentrated on characterizing mass transport behavior of CAHs and hydronium ions in different polymer hydrogel systems, creating methods to characterize alteration of microorganism viability during encapsulation, and developing computer simulations of coupled reaction-diffusion processes to optimize biobead design.

Mass transport of CAHs, including TCE, cis-1,2-dichloroethylene (cDCE), and vinyl chloride (VC) in hydrogels were measured spectrophotometrically from temporal measurements using a custom diaphragm cell. Computer simulations were developed in Matlab from a central finite difference method using published kinetic parameters for CAH degradation and pH inhibition. Measured diffusion rates of CAHs were used in the model. A sensitivity analysis was performed using cell concentration, CAH diffusivity, and biobead radius.

Effective diffusivity of TCE, cDCE, and VC was enhanced in PVA/Alg membranes over strictly aqueous diffusivity by -10±25%, +39±10%, and +69±17%, respectively. The difference in diffusivities between CAHs may be explained by dipole interactions in correlation to the symmetry of the molecules. Measured diffusivity rates were used in the reaction/diffusion model. Even at fully saturated external TCE concentrations, simulated microbes degraded contaminants while encapsulated within the hydrogel. For a single biobead, without accounting for microbial pH inhibition, as cell concentration increased, generation and efflux of VC to the environment increased to a maximum then decreased and was dependent on hydrogel radius.

Measured diffusivities of CAHs represent the first characterization of mass transfer coefficients for CAHs in a hydrogel matrix and these parameters along with the reaction/diffusion model represent a critical component for engineering and optimizing a encapsulated bioremediation solution. Because the geometry of biobeads allow for simultaneous degradation of TCE, cDCE and VC and can be modified to respond to stimuli and environmental conditions, these hybrid materials represent a promising tool for remediation of CAHs.