Sonication Effects on Atrazine Dissipation in Vadose Zone Sediment Slurries

Abstract

Herbicide atrazine easily leaches to groundwater, where it is persistent. We studied whether sonication accelerates atrazine dissipation (100 mg·L −1 ) in vadose zone sediment slurries. Sediments were from 11.3 to 14.6 m depths in an atrazine-contaminated groundwater area. The slurries and autoclave-sterilized slurries were sonicated (bath, 43 kHz, 320 W) for 0, 5, 10, 20, or 30 min once/twice a day, and atrazine concentrations were followed. Atrazine concentrations raised in the sterilized slurries sonicated twice a day for 10 min (86.0 ± 7.7 mg·L −1 ), while they remained low in the slurries (56.6 ± 10.9 mg·L −1 ) due to microbial degradation. Twice a day sonications for 20-30 min did not enhance microbial atrazine degradation. Chemical dissipation may have occurred in the sterilized slurries sonicated twice a day for 30 min. However, sonication did not decrease atrazine concentrations below those in the non-sonicated slurries (55.1 ± 7.8 mg·L −1 ) and sterilized slurries (67.1 ± 7.9 mg·L −1 ). Atrazine concentrations in the sterilized slurries were higher than in the slurries, indicating changes in sediment structure and adsorption due to autoclaving. Sonication parameters needed for releasing atrazine from interactions with particles may be close to those damaging microbial cells. This suggests difficulties in enhancing microbial atrazine degradation by sonication, though chemical degradation can be enhanced.

Improved Short-Term Microbial Degradation in Circulating Water Reducing High Stagnant Atrazine Concentrations in Subsurface Sediments

“…In long-term (180 days) degradation experiments, using low atrazine concentrations of 20 µg L −1 in water circulating columns at a flow rate of 2 mL min −1 , atrazine dissipation rates have been estimated to be two to five times faster than in stagnant flasks [5], while pharmaceuticals dissipation was not enhanced at flow rates of 1.7-3.4 mL min −1 [65]. The short-term results of this study showed that at quite a high atrazine concentration below 10-18 mg L −1 , high flow rate (16.7 mL min −1 )-induced changes in the sediment columns did not improve atrazine dissipation, apparently due to the good colloid formation ability [21][22][23], while the amount adsorbed to the sediments was low (about 50.1 µg). The atrazine distribution coefficient K d , which is the ratio of atrazine adsorbed to that dissolved into the liquid phase, increases when the atrazine concentration decreases [66].…”

“…The rapid increase in microbial numbers in the liquid phase of the atrazine-added sediment slurries, but not in the treatments without atrazine, indicates that some of the atrazine was degraded as a microbial growth substrate (Table 4). The subsurface sediments contaminated with about 13 µg kg −1 of atrazine [32] appeared to inherently contain microbes that are able to rapidly degrade atrazine, which is often disputed due to the absence of microbial pesticide degradation at concentrations below the water solubility limit of 33 mg L −1 [6,21,54].…”