Hydrogel beads with the consistency of gummy candy are made with an ingredient used in processed foods to clean up groundwater contaminated with dangerous and widely used volatile organic compounds (VOCs). Among the contaminants addressed by the method are 1,1,1-trichloroethane, cis-1,2-dichloroethene, and 1,4-dioxane — degreasers commonly used by industry and the military. The chemicals can infiltrate groundwater through leaky underground storage tanks or runoff, or by simply being dumped on the ground.
The new decontamination method works because the microbes produce an enzyme that oxidizes the toxins when groundwater contaminants diffuse into the beads. The result is a transformation of the contaminants into harmless compounds. The process, called long-term aerobic cometabolism, is an enclosed, passive, self-sustaining system for groundwater remediation.
Current practices call for gaseous growth substrates such as propane and methane to be added directly to the subsurface. The substrates nourish indigenous microbes, which in turn produce enzymes that transform the contaminants to non-toxic byproducts. Often, however, the growth substrates chemically compete for those crucial enzymes, which significantly inhibits the transformation process. The new system eliminates that competition, freeing all of the enzyme to oxidize contaminants.
The team co-encapsulated the bacteria culture Rhodococcus rhodochrous and a slow-release growth substrate within hydrogel beads produced in the lab. The cylindrical beads — made of gellan gum, a common ingredient in processed foods — are 2 millimeters long. As groundwater flows by the beads, the contaminants diffuse into the beads, where the slow-release substrate reacts with groundwater to produce alcohol that sustains the Rhodococcus bacteria. The bacteria contain a monooxygenase enzyme that transforms the contaminants into harmless compounds including carbon dioxide, water, and chloride ions. The purified water and the byproducts then diffuse out of the beads and rejoin the groundwater plume.
In bead-filled test columns supplied with a continuous flow of contaminated water, the system functioned continuously for more than 300 days on the original growth substrate. The beads removed more than 99% of the contaminants and their concentrations declined from several hundred parts per billion to less than 1 part per billion.
The system’s longevity will depend mainly on how long the bacteria live, which is a factor of how long the growth substrate lasts. Current cometabolic remediation methods require regular additions of growth substrates to ensure that key microorganisms flourish and that necessitates regular site monitoring, biochemical adjustments, and related costs.
Several possibilities exist for deploying the beads: mixing beads directly into contaminated subsurface material; digging a trench in the path of groundwater flow and filling it with beads, creating a permeable reactive barrier; and packing beads into reactors, a simple form being mesh bags, that can be placed in wells.