Groundwater contamination with volatile organic compounds (VOCs) is a widespread issue throughout the United States. Over 50% of more than 3500 groundwater samples collected from 98 major drinking water supply aquifers from 1985-2001 contained at least one anthropogenic contaminant, with VOCs detected most frequently. Among the top 15 VOCs detected, eight were chlorinated aliphatic hydrocarbons (CAHs): chloroform (CF), perchloroethylene (PCE), trichloroethylene (TCE), 1,1,1-trichloroethane (1,1,1-TCA), cis-1,2-dichloroethylene (cis-DCE), trans-1,2-dichloroethylene (trans-DCE), dichloromethane (DCM), and 1,1-dichloroethane (1,1-DCA). All of these CAHs are listed by the Centers for Disease Control and Prevention as being likely human carcinogens. Of increasing concern are emerging co-contaminants, such as 1,4-dioxane (1,4-D), which is also a likely human carcinogen. Common remediation techniques, such as pump-and-treat, are not sustainable for treating contaminant mixtures that slowly diffuse from low permeability zones in the subsurface. These issues highlight the need for long-term, passive, and economical remediation techniques. Passive and sustainable systems are proposed for the aerobic cometabolism of emerging contaminants, such as 1,4-D, that are mixed with CAHs. These passive systems will be created by co-encapsulating axenic bacterial cultures with a slow release compound (SRC) in hydrogel beads. The SRC will slowly hydrolyze in the beads to produce an alcohol, which will serve as a microbial growth substrate and as an inducer for non-specific contaminant-degrading monooxygenases. Groundwater contaminants will diffuse into the hydrogels where they will be cometabolically transformed to non-toxic products. In preliminary studies, the alkane-oxidizing bacterium Rhodococcus rhodochrous ATCC 21198 was co-encapsulated with an orthosilicate SRC in a gellan gum hydrogel. Continuous degradation of 1,1,1-TCA, cis-DCE, and 1,4-D was maintained for over 300 days. In the proposed work, proteomic analyses of this model bacterium will be performed to identify the active monooxygenases and to characterize the enzymatic and physiological changes that ultimately limit the long-term activity of this bacterium in the encapsulated systems. Genome- enabled approaches will also be used to rationally identify other microorganisms with different monooxygenase compliments that can also be co-encapsulated with SRCs to achieve the degradation of a broad range of emerging contaminants. Material science research will determine how to produce hydrogels beads that maintain mechanical integrity for extended periods, focusing on processes that can be easily scaled-up for producing large quantities of beads needed for in-situ treatment. The beads will then be used in different platforms at the laboratory scale to create passive permeable reactive barriers. The hydrogel beads might also be used for treating contaminated soils and sediments and other emerging contaminants.
The proposed research is relevant to public health, because a sustainable technology will be developed for the passive bioremediation of complex mixtures of groundwater contaminants, including chlorinated solvents and 1,4-dioxane via aerobic cometabolism. The co-encapsulation of pure cultures of bacteria along with slow release compounds in hydrogels beads has the potential to lower costs and achieve high treatment efficiencies. The beads can be used to treat a broad range of contaminants and in many different platforms, including permeable reactive barriers.
