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TECHNICAL REPORTS

Organic Compounds in the Environment

Sorption and Mobility of Ivermectin in Different Soils

^a Copenhagen Univ., Faculty of Pharmaceutical Science, Dep. of Pharmaceutics and Analytical Chemistry, Section of Toxicology and Environmental Chemistry, Universitetsparken 2, DK-2100 Copenhagen, Denmark
^b present address, Danish Medicines Agency, Axel Heides Gade 1, DK-2300 Copenhagen, Denmark

^* Corresponding author (kak{at}farma.ku.dk).

Received for publication November 7, 2007. Avermectins are widely used to treat livestock for parasite infections. Ivermectin, which belongs to the group of avermectins, is particularly hazardous to the environment, especially to crustaceans and to soil-dwelling organisms. Sorption is one of the key factors controlling transport and bioavailability. Therefore, batch studies have been conducted to characterize the sorption and desorption behavior of ivermectin in three European soils (Madrid, York, and artificial soil). The solid-water distribution coefficient (K_d) for ivermectin sorption to the tested soils were between 57 and 396 L kg^–1 (determined at 0.1 µg g^–1), while the organic carbon-normalized sorption coefficients (K_oc) ranged from 4.00 x 10³ to 2.58 x 10⁴ L kg^–1. The Freundlich sorption coefficient (K_F) was 396 (after 48 h) for the artificial soil over a concentration range of 0.1 to 50 µg g^–1, with regression constants indicating a concentration-dependent sorption. The obtained data and data in the literature are inconclusive with regard to whether hydrophobic partitioning or more specific interactions are involved in sorption of avermectins. For abamectin, hydrophobic partitioning seems to be one of the dominant types of binding, while hydrophobicity is less important for ivermectin, which is probably due to the lower lipophilicity of the molecule. Furthermore, the presence of cations such as Ca²⁺ leads to decreasing sorption. Thus, it is presumed that ivermectin binds to soil by formation of complexes with immobile, inorganic soil matter. In contrast to abamectin, hysteresis could be excluded for ivermectin in the studied soils for the evaluation of sorption and desorption. The sorption mechanism is highly dependent on physicochemical properties of the avermectin.

Abbreviations: CPM/DPM, counts per minute/disintegrations per minute • d.w., dry weight • K_d, solid-water distribution coefficient • K_F, Freundlich sorption coefficient • K_f ^des, Freundlich desorption coefficients • K_oc, organic carbon-normalized sorption coefficient • K_ow, octanol/water coefficient • LC-MS, liquid chromatography-mass spectrometry • LC-MS/MS, liquid chromatography-tandem mass spectrometry • LSA, liquid scintillation analyzer • OM, organic matter • SPE, solid phase extraction