Title: Adsorption of isoxaflutole degradates to aluminum and iron hydrous oxides Authors
|Wu, Si -|
|Goyne, Keith -|
|Lin, Chung -|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 9, 2010
Publication Date: March 1, 2011
Citation: Wu, S.H., Goyne, K.G., Lerch, R.N., Lin, C.H. 2011. Adsorption of isoxaflutole degradates to aluminum and iron hydrous oxides. Journal of Environmental Quality. 40(2):528-537. Interpretive Summary: The herbicide, Balance, is a relatively new product marketed as a substitute for the most commonly used corn herbicide, atrazine. Atrazine use has resulted in contamination of streams throughout the Corn Belt, and Balance has been promoted as a more environmentally benign herbicide for weed control in corn production. However, recent studies by the University of Missouri and ARS have shown that two of the Balance break down products, or degradates, were readily transported to shallow groundwater and based on their chemical properties are also likely to contaminant streams by surface runoff. One of the key mechanisms responsible for reducing the hydrologic transport of herbicides is their ability to bind to soil, a process referred to as sorption. Previous research has shown that the clay and organic matter content of the soil were not related to the sorption of Balance or it’s degradates. The key objective of this study was to determine if iron and aluminum oxides can sorb the Balance metabolites, diketonitrile (DKN) and benzoic acid (BA). The DKN degradate is the active herbicide ingredient in Balance and the BA degradate forms by break down of DKN. The parent compound in Balance is so unstable that it was not investigated since it would not persist in the soil environment. Our results showed that iron and aluminum oxides do sorb both metabolites, with slightly greater sorption to the iron oxides. The sorption intensity decreased with increasing pH and sorption intensity was very low for both degradates at pH values greater than 6. For both oxides, sorption intensity was readily reversible, showing that these degradates have an overall weak affinity for these oxides. This research indicates that metal oxides can serve as important adsorbents for DKN and BA only in acidic soils enriched with metal oxides. Such soils are highly weathered and are not typical of Corn Belt soils. While this research identified that iron and aluminum oxides can sorb the primary Balance degradates, their sorption intensity will be low in most Corn Belt soils and their risk for contaminating surface and ground waters will be high. This research benefits state and federal land management and regulatory agencies by providing needed information for better assessing the risk of water resource contamination by these two Balance degradates.
Technical Abstract: Isoxaflutole (IXF) is a pre-emergence 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, subsequently, DKN can be further degraded to a benzoic acid derivative (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 variable-charged soil solid phase constituents to further understand how these IXF degradates are retained by metal oxides. The DKN and BA degradates were adsorbed to hydrous aluminum and iron oxides, and the data were well described by the Freundlich model (R**2 > 0.91) with N values ranging from 0.89 – 1.2. Adsorption isotherms and Kd values demonstrate BA is adsorbed to HFO to a greater extent than previously observed for other IXF degradate–mineral reactions. The degree of hysteresis between adsorption/desorption isotherms was characterized as slight (Hysteresis Index values < 1.7), suggesting weak degradate retention to Fe and Al oxide surfaces. Degradate adsorption was observed to greatly diminish as suspension pH 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 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 can serve as important adsorbents for DKN and BA in acidic soils enriched with metal oxides.