Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/28/2010
Publication Date: 10/22/2010
Citation: Trigo, C., Koskinen, W.C., Celis, R., Sadowsky, M.J., Hermosin, M.C., Cornejo, J. 2010. Bioavailability of organoclay formulations of atrazine in soil. Journal of Agricultural and Food Chemistry. 58(22):11857-11863. Interpretive Summary: Pesticides that are highly soluble in water, minimally sorbed by soil particles, and not readily degradable can move rapidly with water and hence, have a high potential to be found in surface and ground waters. Many widely-used pesticides have these characteristics, and thus if accidentally reach high concentrations in soil as in an accidental spill, they have to be immobilized as soon as possible to avoid further potential water contamination. The potential off-site movement of these agrochemicals can be decreased by creation of sorptive or immobilizing zones in the soils by incorporating an appropriate adsorbent, such as a clays modified with an organic cation, in the affected area. However, the nature of the organic cation on the clay influences the adsorption capacity and selectivity of organobentonites for selected pesticides. Suitable selection of the chemical characteristics of the organic cation, can optimize the adsorbent properties of the organoclays for pesticides. In general, organoclays with high affinity for selected pesticides showed a higher resistance to desorption than organoclays with less affinity for them. While, these results reveal that organoclays might be potentially useful as agents to selectively immobilize pesticides in soils or to delay their movement towards underground waters, the question arises as to whether pesticides bound to organocalys are bioavailable to degrading microorganisms. The results of incubation experiments show that the bioavailability of atrazine was similar in the case of the organoclay formulations and as free atrazine. This indicated that while more atrazine was sorbed and less likely to be transported in soil, when formulated as organoclay complexes, it was ultimately accessible to degrading bacteria results. This suggests that the amount of atrazine applied as organoclay formulations can retard leaching and runoff, but is still bioavailable to degrading microorganisms and is likely to be naturally attenuated by soil microorganisms. These results will aid scientists in their development of inexpensive and efficient organoclays to be used to decontaminate spill sites. The viability of the use of natural organic cations to prepare organoclays is particularly interesting for minimizing the impact of the adsorbent once added to soil or aquifers.
Technical Abstract: Pesticide formulations based on organoclays have been proposed to prolong the efficacy and reduce the environmental impact of pesticides in soil. This research addressed the question of whether organoclay-based formulations of atrazine are irreversibly sorbed or are bioavailable for bacterial degradation in soil. Different cations were incorporated into Na-rich Wyoming montmorillonite (SWy-2) and Ca-rich Arizona montmorillonite (SAz-1) at 100% of the cation exchange capacity of the clays as a strategy to enhance the affinity of the clay minerals for atrazine. A Buse loam soil from Becker, Minnesota was treated with three organoclay-based formulations of 14C-atrazine or free herbicide and incubated for two weeks. To determine the bioavailability of 14C-atrazine, the soil was inoculated with Pseudomonas sp. strain ADP, which rapidly mineralizes atrazine. At Day 0, and after a 2-wk incubation, mineralization and the amount of 14C-atrazine residues distributed between the aqueous-extractable, methanol-extractable, and bound fractions in the soil were determined to characterize the availability of non-aged and aged atrazine residues. By the end of the 2-wk incubation, the microorganisms mineralized more than >80 % of the initial readily-available (water-extractable) and > 70 % of the less readily-available (methanol-extractable) 14C-atrazine in the soil. Bound residues increased from <4 % at Day 0 to ~17 % after the 2-wk incubation for both the formulated and free forms of atrazine. The results of our incubation experiments show that the bioavailability of atrazine was similar in the case of the organoclay formulations and as free atrazine. This indicated that while more atrazine was sorbed and less likely to be transported in soil, when formulated as organoclay complexes, it was ultimately accessible to degrading bacteria, so that the herbicide is likely to be naturally attenuated by soil microorganisms.