Hydrogel Bead Production for the Immobilization of Bacteria and Slow Release Compound for Bioremediation of Chlorinated Aliphatic Hydrocarbons.

Groundwater contamination by chlorinated aliphatic hydrocarbons (CAHs) and co-contaminant 1,4-dioxane (1,4-D) is widespread throughout the United States. The long-term contamination of groundwater by CAHs and 1,4-D has allowed contaminants to diffuse into low-permeability zones (LPZs). Common remediation methods in LPZs, including pump-and-treat, are unsustainable due to diffusion limitations.

One potential solution to treat LPZs is bioremediation, an in situ remediation technique where microorganisms are used to transform contaminants into less harmful byproducts. Hydrogel beads containing immobilized bacteria and a slow release compound (SRC) can treat contaminants over long periods of time, while reducing cost of treatment, when packed into well reactors to form passive permeable reactive barriers (PPRBs). The SRC hydrolyzes into an alcohol to maintain the long-term activity of the microorganism. We have demonstrated that the bacterium Rhodococcus rhodochrous ATCC 21198 can transform cis-1,2-dichloroethene, a CAH, when immobilized in hydrogel beads with the SRC tetrabutyl orthosilicate. High volumes of hydrogel beads with uniform size distribution are required to fill PPRBs.

The goal of this project is to optimize a production method for hydrogel beads using a coaxial air bead generator and to conduct experiments informed by a Design of Experiments (DOE) algorithm. Hydrogel beads are formed from a polymer solution which is dropped into a crosslinking bath. During crosslinking the polymer strands are locked in place, causing the polymer solution to form a hydrogel bead. The bead generator can produce hydrogel beads with a controlled size distribution at high polymer flow rates to produce large amounts of beads. In the coaxial dual nozzle system, polymer solution flows through the inside nozzle and air flows through the outside nozzle to direct airflow onto polymer droplets as they fall into the crosslinking bath. The polymer used in this study was concentrated Sodium Alginate (NaAlg) solution with Calcium Chloride (CaCl2) crosslinking solution.

The initial DOE factors were polymer flow rates (10-70 mL/hour), airflow rates (0.75-1.5 dial setting), and distance to crosslinking bath (20-200 mm). Hydrogel bead diameter was the response. DOE analysis revealed that polymer flow rate was insignificant with respect to bead diameter. Distance to crosslinking bath and airflow rate both were both significant, as well as the interaction of these two factors. Next steps include a DOE with higher polymer flow rates and production rate as a response.