Sorption and related properties of the swine antibiotic carbadox and associated N-oxide reduced metabolites

Carbadox (CBX) (methyl 3-2-quinoxalinylmethylene]-carbazate N1, N4 dioxide) is a chemotherapeutic growth promoter and antibacterial drug added to feed for starter pigs. Toxicity of CBX and at least one of its metabolites (bis-desoxycarbadox; DCBX) has resulted in a number of studies regarding its stability and residence time in edible swine tissue; however, little is known on its environmental fate pertinent to the application of antibiotic-laden manure to agricultural fields. We measured sorption of CBX and DCBX by soils, sediment, and homoionic clays from 10 mM KCl and 5 mM CaCl2 solutions, sorption of two N-oxide reduced metabolites (N4 and N1) by a subset of soils from 5 mM CaCl2, octanol-water partition coefficients (Kow) of CBX and all three metabolites, and CBX solubility in water and mixed solvents. Sorption appeared well-correlated to organic carbon (OC) for the soils (e.g., log (Koc, L/kg OC) = 3.96 +/- 0.18 for CBX). However, sorption was enhanced in the presence of K+, competitive sorption from the metabolites was observed, and sorption by clay minerals was large (approximately 10(5) L/kg for SWy(-1)). Sorption by clays was inversely correlated to surface charge density (e.g., sorption decreased from 10(5) to 10 L/kg as charge density increased from 1 to 2 micromolc/m2), similar to what has been observed for nitroaromatic compounds. In the absence of a clay surface, hydrophobic-type forces dominated with Kow values and reverse-phase chromatographic retention times increasing with the loss of oxygen from the aromatic nitrogens. Therefore, it is likely that both OC and clay contribute significantly to sorption of carbadox and related metabolites by soils with relative contributions most dependent on clay type.