ARS | Publication request: Removal of Hexazinone from Water with Bioreactors. Remediation of Chlorinated and Recalcitrant Compounds.

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: December 8, 2011
Publication Date: May 20, 2012
Citation: Hunter, W.J., Shaner, D.L. 2012. Removal of Hexazinone from Water with Bioreactors. Remediation of Chlorinated and Recalcitrant Compounds. Meeting Abstract. 1.

Technical Abstract: Background/Objectives. Hexazinone is a broad-spectrum triazine herbicide that inhibits electron transport in photosynthetic organisms. The presence of hexazinone in surface and groundwater is a concern because it is toxic to primary producers that serve as the base of the food chain. Long term laboratory studies evaluated two types of reactors as methods for removing hexazinone from simulated groundwater. Approach/Activities. Two types of laboratory scale reactor were evaluated. One was a vegetable-oil based nitrogen-limiting biobarrier and the other an aerobic slow sand filter. The N-limiting biobarrier was 25 cm long and was formed by coating 195 g of silica sand with 2.5 g of soybean oil. This barrier was placed in a 2.6-cm by 30-cm glass chromatography column covered with 5 centimeters (40 g) of oil-free sand. The sand filter reactors were made from a 2-L glass graduate cylinder filled with 43 cm of sand (~ 2 L). Water flowed downward through the sand filter to an uptake tube placed 28 cm below the surface of the sand, thus the upper 28 cm of the sand filter was unsaturated and the lower 15 cm was saturated. Reactors were fed a moderately hard simulated groundwater containing ~ 6 mg L-1 hexazinone pumped at a rate of ~11 ml day-1. Reactors were maintained in the dark. Results/Lessons Learned. Hexazinone was degraded by the N-limiting biobarriers, but two problems were observed. First, a 52 week incubation period was required before the maximum removal efficiency was reached, and second the removal efficiencies differed greatly between replicates with one biobarrier showing a ~95% removal efficiency and the other a ~50% efficiency. We were not able to demonstrate that the microbial population within the N-limiting biobarriers was degrading hexazinone as a source of nitrogen. A shorter initial incubation period, ~22 weeks, and higher, more consistent removal efficiencies were obtained with the slow sand filters. Four sand filter based reactors were appraised and all degraded hexazinone with removal efficiencies of ~97%. Challenging two of the sand filter reactors with high concentrations of influent hexazinone showed that these barriers were capable of remediating large amounts, > 100 mg L-1, of hexazinone at high efficiency. With both types of reactors the degradation was due to biological processes rather than abiotic processes. Evidence of this comes from the long lag phase observed in both types of reactors which indicates that an acclimation process, where microorganisms that are able to degrade hexazinone increased in numbers, was required. Also, with the sand filters, bacteria were isolated that show a positive growth response that correlated with the amount of hexazinone that was present in their growth media. In this study the aerobic sand filters were more consistent and had higher removal efficiencies than did the N-limiting biobarriers.