Bioavailability of the herbicide 2,4-D formulated with organoclays

Research on organoclays as sorbents of pesticides has shown the usefulness of these materials as pesticide supports to prolong the efficacy of soil-applied pesticides and to reduce the large transport losses that usually affect pesticides applied in an immediately available form. Nevertheless, little information exists on the availability of organoclay-formulated pesticides for bacterial degradation. In this work, laboratory experiments were conducted to determine the adsorption–desorption behavior of two hexadecyltrimethylammonium-treated Arizona montmorillonites (SA-HDTMA50 and SA-HDTMA100) for the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), and to evaluate the ability of these organoclays to slow the release of the herbicide and to reduce herbicide leaching losses as compared to the free (technical) compound. The kinetics of mineralization of free and formulated 2,4-D by adapted bacteria was also determined. Organoclay-based formulations of 2,4-D displayed slow release properties in water and reduced herbicide leaching through soil columns, while maintained a herbicidal efficacy similar to that of the free (technical) 2,4-D. The total amount of 14C-2,4-D mineralized at the end of the biodegradation experiment (t=130 h) ranged between 30% and 46% of the formulated herbicide, which represented 53–81% of the amount of free 2,4-D mineralized in the same conditions. The release, leaching, and mineralization patterns of the formulated herbicide were found to depend both on the affinity of the organoclay for the herbicide and on the degree of interaction promoted during the preparation of the herbicide–organoclay complex. This suggests the possibility to select diverse preparations to achieve the desired release, leaching and biodegradation behavior.