Reaction of the herbicide ethofumesate with activated sludge

Abstract

Biochemical decomposition of the water solution of the herbicide Ethofumesate (EFS) with the actived sludge (AS) adapted on EFS was studied. The EFS initial concentration was in the range S (0) = 0.05 - 0.2 mmol dm -3, the AS initial concentration (expressed as the weight of dry biomass in the volume unit of the reaction mixture) was X(0) = 0.4 - 2.2 g dm -3. Experiments were carried out in a batch isothermal stirred reactor at temperatures of 15 - 40 °C. It was found out that EFS is decomposed by means of AS very slowly. At given conditions, the initial velocity of the EFS docomposition was v = 3 to 30 mg EFS h -1 g -1 AS. Concentrations of AS and dissolved oxygen in the reaction mixture were practically constant during the whole reaction time in all the experiments. The value of v decreased at 20 - 25 °C with increasing ratio of initial AS and EFS concentrations p = X0/S0. At temperatures of 30 and 35 °C, an inductive period of ca two hours took place before EFS decomposition. In the liquid phase of the reaction mixture an unknown substance was found beside EFS, which is not a decomposition product of EFS bur originates probably from the microorganisms of AS. All the analyses carried out indicate a complicated reaction mechanism (probably dictated by physical rather then chemical steps) and the fact that EFS is a species hard to decompose biologically.