Adsorption of Isoxaflutole Degradates to Aluminum and Iron Hydrous Oxides

Abstract

Isoxaflutole is a preemergence herbicide that has been marketed as a substitute for atrazine. It is rapidly transformed to a more stable and soluble diketonitrile degradate (DKN) after field application and can further degrade to a benzoic acid degradate (BA) within soil. 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 and BA with synthetic hydrous aluminum and iron oxides (HAO and HFO, respectively) to understand how variably charged minerals may influence adsorption of these compounds in soil. The herbicide degradates did adsorb to HAO and HFO, and the data were well described by the Freundlich model (R2 > 0.91), with N values ranging from 0.89 to 1.2. Adsorption isotherms and Kd values demonstrate that BA is adsorbed to HFO to a greater extent than other degradate–mineral combinations that were studied. The degree of hysteresis between adsorption/desorption isotherms was characterized as slight (hysteresis index values <1.7), suggesting weak DKN and BA retention to HFO and HAO oxide surfaces. Degradate adsorption was observed to greatly diminish as suspension pH increased. Attenuated total reflectance–Fourier transform infrared spectra show no evidence that DKN or BA adsorb to mineral surfaces as inner‐sphere complexes under hydrated conditions. Instead, DKN and BA adsorb to positively charged metal oxide surfaces as outer‐sphere or diffuse ion swarm complexes via electrostatic attraction. This research indicates that metal oxides may serve as important retardants for DKN and BA migration through acidic soils enriched with aluminum and iron oxides.

Soil Chemical Insights Provided through Vibrational Spectroscopy

“…The ATR-FTIR spectra of the diketonitrile (DKN) and benzoic acid (BA) degradates of isoxaflutole adsorbed to the hydrous metal oxides provided no evidence of inner-sphere complex formation between the ketone carbonyl group of DKN or the carboxylate group of BA with mineral surfaces. In contrast, DRIFTS spectra of the BA-mineral complex suggested the formation of an inner-sphere complex upon freeze-drying based on a shift in the asymmetric COO − stretch to a lower wavenumbers relative to the aqueous spectrum (Wu et al, 2011).…”

“…Therefore, evaluating the interactions of organic compounds with mineral surfaces is critical for understanding mineral dissolution, SOM stabilization, alteration of a solid's physicochemical properties, organic pollutant fate and transport in soil and sediment, and the formation of biomolecules on early Earth (Duckworth and Martin, 2001;Gu et al, 1995;Hazen and Sverjensky, 2010;Parikh et al, 2011;Wu et al, 2011;Yoon et al, 2004a). Vibrational spectroscopy provides a means to capture a spatially integrated response of the local bonding environment by measuring energies associated with the motion of bonded atoms, which can then be utilized to determine molecular structure (Amonette, 2002;Sposito, 2004).…”

AIMD Evidence of Inner Sphere Adsorption of Glycine on a Stepped (101) Boehmite AlOOH Surface

“…105,107−113 IR spectroscopic signatures of immersed particles have been used to discriminate between inner and outer sphere adsorption. Works on the adsorption of carboxylic acids on alumina polymorphs report outer sphere, 110,111,114 inner sphere adsorption, 108,109,114−117 or both. 105,107,112 In contrast, on γ-alumina, no glycine−surface well-defined adducts were identified by NMR spectroscopy.…”

“…Inner sphere adsorption through the COO – moiety is calculated here as much more exergonic than outer sphere adsorption. The question of inner versus outer sphere adsorption has mainly been addressed in the case of mineral surfaces that are in contact with natural water and contaminants, , particularly on Al polymorphs gibbsite, boehmite, or α-alumina. ,− IR spectroscopic signatures of immersed particles have been used to discriminate between inner and outer sphere adsorption. Works on the adsorption of carboxylic acids on alumina polymorphs report outer sphere, ,, inner sphere adsorption, ,,− or both. ,, In contrast, on γ-alumina, no glycine–surface well-defined adducts were identified by NMR spectroscopy …”