Encapsulation of Microbes and Slow Release Substrate for Bioremediation of Chlorinated Solvents

Encapsulation of microorganisms in a gel matrix offers several advantages in groundwater remediation, such as high biomass and resistance to environmental stressors. Other compounds, such as primary substrates, can also be captured in the gel matrix, providing more favorable conditions for the microbes. The present research considers the use of tetra-n-butyl orthosilicate (TBOS) as a slow releasing primary substrate for microorganisms encapsulated in a variety of gels. TBOS can be hydrolyzed to create four butanol molecules, which can be used as a primary substrate by Rhodococcus rhodochrous.

Calcium alginate, agar, agarose, and polyvinyl alcohol (PVA) gels were each used to encapsulated R. rhodochrous and determine the rate of substrate transformation. Each gel was also tested for feasibility of encapsulation with TBOS. Batch tests with R. rhodochrous and TBOS encapusulated in agarose beads were performed to test the ability of the encapsulated microbe to metabolize repeated injections of a mixture of 1,4-dioxane, 1,1-dichloroethene, and cis-dichloroethene.

Cells encapsulated in calcium alginate, agar, and agarose gels had similar rates of substrate transformation to suspended cells. Cells encapsulated in PVA had a lower rate of substrate transformation due to reduced diffusion of oxygen and substrates into the gel. It was possible to encapsulate TBOS into agar, agarose, and PVA gels. Encapsulation of TBOS into calcium alginate gels was possible with use of PVA to reduce interfacial tension between the alginate solution and TBOS. TBOS was also shown to be a viable slow release substrate in the long-term batch tests, as multiple injections of the chlorinated solvent mixture were metabolized without further addition of substrate, although it was less successful than the control reactors with suspended cells. Future work will aim to reduce the size of the gel beads from 5 mm to less than 100 μm to allow for transport of beads within an aquifer. Tests of chlorinated solvent transformation by encapsulated cells and TBOS will be continued in continuous-flow column reactors.

Keywords: Chlorinated solvent, bioremediation, Rhodococcus rhodochrous, microbial transformation, immobilization, encapusluation