Sorption-desorption of 2,4-dichlorophenoxyacetic acid by wetland sediments

Abstract

The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is among the most frequently detected pesticides in the water-column of surface waters in Canada. Bottom sediments (0 to 15 cm) were collected in 41 wetlands across the prairie region of Canada and analyzed for organic carbon content (OC), pH, and texture. Using batch equilibrium experiments at 5 and 25°C, the herbicide sorption coefficient, Kd, was determined for 41 sediments, along with more comprehensive herbicide sorption and desorption isotherms for 7 of these 41 sediments. The 2,4-D Kd was positively correlated with OC and negatively correlated with sediment pH. A small (3%) significant increase in the 2,4-D Kd occurred when the temperature was at 25°C rather than 5°C. Desorption rates were faster for sediments with ≤ 2.4% OC and exhibited little or no hysteresis, compared to sediments with ≥ 5.9% OC that consistently exhibited hysteresis. We conclude that bottom sediments could serve as a source of 2,4-D to the water-column regardless of water temperature (5 to 25°C). However, the potential for accumulation of 2,4-D in wetland sediments would be small because between 62 and 100% of the 2,4-D sorbed by sediments was released after 8 hours.