Removing hexazinone from groundwater with microbial bioreactors

Authors: Hunter, William; Shaner, Dale

Submitted to: Current Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2012
Publication Date: 2/2/2012
Citation: Hunter, W.J., Shaner, D.L. 2012. Removing hexazinone from groundwater with microbial bioreactors. Current Microbiology. 64:405-411.

Interpretive Summary: The herbicide hexazinone is used in agriculture to control weeds. Because of its use it is sometimes encountered as a contaminant in surface or groundwaters. For this study two types of microbial reactors were evaluated as techniques for removing hexazinone from water. One type of reactor that was evaluated was a nitrogen-limiting biobarrier similar to one that we previously used to remove atrazine from a simulated groundwater. The other biobarrier studied was a slow-sand-filter. Results showed that the nitrogen-limiting biobarriers were not as consistent at removing hexazinone from flowing water as were the slow-sand-filters and that a long acclimation period was required. Two nitrogen-limiting barriers were studies, one removed hexazinone with a efficiency of 95% but the other with an efficiency of only 50%. Quicker and consistent results were obtained with the slow-sand-filters. Four slow-sand-filters were evaluated and all behaved in a similar manner degrading hexazinone with removal efficiencies of ~97%. Also, the acclimation period, though still present, was shorter than that observed with the nitrogen–limiting barriers. The presence of an acclimation period in both types of reactor and the isolation of hexazinone degrading bacteria suggest that the degradation was a biological process. The results suggest that aerobic slow-sand-filters can be used to remove hexazinone from water.

Technical Abstract: Hexazinone, a triazine herbicide that is often detected as a ground and surface water contaminant, inhibits electron transport in photosynthetic organisms and is toxic to primary producers that serve as the base of the food chain. This laboratory study evaluated the ability of two types of microbial reactors, i.e., a vegetable-oil based nitrogen-limiting biobarrier and an aerobic slow sand filter, as methods for removing hexazinone from simulated groundwater. N-limiting biobarriers degraded hexazinone, but did so with a 52 week incubation period and a removal efficiency that varied greatly between replicates, with one biobarrier showing a removal efficiency of ~ 95% and the other an efficiency of ~ 50%. More consistent degradation was obtained with the aerobic sand biobarriers. Four aerobic biobarriers were evaluated and all behaved in a similar manner degrading hexazinone with removal efficiencies of ~97%; challenging two of the aerobic biobarriers with large amounts of influent hexazinone showed that these barriers are capable of remediating large amounts, > 100 mg L-1, of hexazinone at high efficiency. The remediation process was due to biological degradation rather than an abiotic processes. The long lag phase observed in both types of reactors suggests that an acclimation process, where microorganisms capable of degrading hexazinone increased in numbers, was required. Also, the isolation of bacteria that show a positive growth response to the presence of hexazinone in their growth media suggests biological degradation.