Adsorption of isoxaflutole degradates to hydrous aluminum and iron oxides

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Isoxaflutole (IXF) is a pre-emergence herbicide that is rapidly transformed to a more stable and soluble diketonitrile degradate (DKN) after field application and, subsequently, DKN can be further degraded to a degradate (BA) within soil. Due to the short half-life (t₁/₂) of IXF and the significantly longer t₁/₂ of its degradation products, DKN and BA are more prone to transport from soil to water resources than IXF. However, no previous research has been conducted to investigate DKN and BA sorption to metal oxide minerals. The primary objective of this research was to elucidate the interactions of DKN or BA with variable-charged soil solid phase constituents to further understand how these IXF degradates are retained by soil. The DKN and BA degradates were adsorbed to hydrous aluminum and iron oxides, and the data were well-fitted by the Freundlich model (R² [greater than] 0.91) with [nu] values ranging from 0.89 - 1.2. Adsorption isotherms and [kappa][subscript-b] values demonstrate BA is adsorbed to HFO to a greater extent than observed in other IXF-mineral reactions. The degree of hysteresis between adsorption/desorption isotherms was characterized as slight (HI [lesser than] 1.7). Degradate adsorption was observed to decrease rapidly from pH 4 to 5.5 and continued decreasing to a lesser extent as pH was further increased. Attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectra show no evidence that DKN or BA adsorb to mineral surfaces as inner-sphere complexes. Instead, the compounds adsorb to positively-charged metal oxide surfaces as outer-sphere or diffuse ion swarm complexes via electrostatic attraction. This research indicates that metal oxides can serve as adsorbents for DKN and BA in acidic soils enriched in metal oxides.