Interaction of selected herbicides with soil constituents Public Deposited

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  • The behavior of six herbicides in soil was studied under controlled laboratory conditions. Sorption of bromacil, chiorotoluron and diuron was assessed by the batch equilibration procedure over a broad range of aqueous phase sorbate concentration. Linear equilibrium sorption constants (K[subscript p]) were slightly higher at 4° C than at 25° C and many-fold higher in the organic soil than in loamy sand soil. By referencing sorption to soil organic carbon content (OC), this vast variation was reduced to less than 3-fold variation in K[subscript oc]. The isosteric heats of sorption were exothermic and low suggesting an entropy driven mechanism. Water solubilities (S) and octanol-water partition coefficients (K[subscript ow]) of the compounds were determined and used to derive equations for predicting K[subscript oc]. These equations are useful in estimating sorption of related pesticides and pollutants from knowing their K[subscript ow] and S. Degradation of the three herbicides was followed by monitoring [superscript 14]C0₂ evolution from soil. Temperature affected degradation more than did soil type or initial herbicide concentration. The degradation was tripled with a 10° rise in temperature from 25 to 35° C. The presence of herbicides did not affect the activity of soil micro-organisms indicating that degradation was a co-metabolism process. Degradation fit first-order kinetics with no lag phase and with minimum half-lives of about 200 days for diuron and chiorotoluron and 2600 days for bromacil. The binding of these three herbicides plus diquat, glyphosate and simazine with water-soluble soil organic materials (WSSOM) and soil humic acids (HA) was studied by gel filtration chromatography. The chromatographic method adapted by Hummel and Dreyer (1962) was very successful in proving the existence of binding. Noted differences in the elution behavior of herbicides were related to their physical and chemical properties and their Interaction complexes. Infrared (IR) spectroscopy showed the involvement of ion exchange reactions in diquat and H-bonding in diuron, chiorotoluron and bromacil binding with WSSOM and HA. Shifts in some characteristic IR absorption bands suggested the formation of charge transfer complexes in WSSOM and HA interactions with bromacil, chiorotoluron and diuron but not with diquat. The results appear significant in evaluating pesticides and pollutants mobility and transport in soil and water environments.
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