Wetland systems provide both anaerobic (reducing) and aerobic (oxidizing) zones for the biodegradation of chlorinated aliphatic hydrocarbons (CAH). In particular, wetland plant roots provide micro-oxidizing environments for methanotrophic bacteria from the presence of methane, which is produced in anaerobic zones, and oxygen, which is brought to the subsurface by roots of wetland plants; this shows the potential for cometabolic degradation of common organic pollutants.
This study explored the natural attenuation of CAHs by methanotrophic bacteria naturally associated with roots of the common wetland plant, Carex comosa. Root microcosms were amended with varying concentrations of methane; trichloroethene; cis 1,2-dichloroethene; 1,2-dichloroethane; or dichloromethane. Transformation Yield (Ty) increased with increasing CAH concentration. Ty and pseudo first-order degradation rate constants were often at least one order of magnitude lower than published values.
A suite of halogenated aliphatic hydrocarbons (HAH) were studied in bench scale root microcosms for their potential to be cometabolically degraded by methanotrophic bacteria naturally associated with the roots of Carex comosa. Among the HAHs
investigated, 1,1,2-trichloroethane, 1,2-dichloropropane, and 1,3-dichloropropene did not cometabolically degrade in the aerobic microcosm systems. However, four trihalomethanes (chloroform, bromodichloromethane, dibromochloromethane, and bromoform) as well as 1,1-dichloroethene; 1,2-dichloroethane, and 1,2-dibromoethane were found to cometabolically degrade in the systems. Ty as well as pseudo first-order degradation rate constants normalized to biomass (k1-CAH) were calculated for compounds that did degrade.
Past studies have explored the possibility of plant-microbe interactions for the bioremediation of harmful pollutants in soil. In this study, numerous chlorinated aliphatic hydrocarbons were examined for their potential to cometabolically degrade by aerobic bacteria naturally associated with Carex comosa. The bacteria present along with the roots were able to degrade trichloroethene and cis 1,2-dichloroethene as well as low amounts of 1,1-dichloroethene; trans 1,2-dichloroethene; and 1,2-dichloroethane without the need for added growth substrate or an incubation period. Carbon tetrachloride and chloroform did not degrade in the systems. Ty was determined for compounds that did degrade but were often one to two orders of magnitude lower than published values. The growth substrate naturally present in the microcosm systems was unclear. Immediate degradation of chlorinated ethenes and removal of low levels of other types of CAHs indicates that cometabolic microorganisms are naturally present in and around the roots of wetland plants-even if the wetland system has not been previously exposed to chemical pollutants.
Results of this study help to fill in key data for cometabolism of emerging pollutants by methanotrophic bacteria naturally associated with wetland plant roots. Results provide support for the use of wetland systems as means of natural attenuation of contaminated groundwater and provide more realistic degradation rate constants for natural attenuation.