The soil sorption coefficient Kd and the soil organic carbon sorption coefficient KOC of pesticides are basic parameters used by environmental scientists and regulatory agencies worldwide in describing the environmental fate and behavior of pesticides. They are a measure of the strength of sorption of pesticides to soils and other geosorbent surfaces at the water/solid interface, and are thus directly related to both environmental mobility and persistence. KOC is regarded as a 'universal' parameter related to the hydrophobicity of the pesticide molecule, which applies to a given pesticide in all soils. This assumption is known to be inexact, but it is used in this way in modeling and estimating risk for pesticide leaching and runoff. In this report we examine the theory, uses, measurement or estimation, limitations and reliability of these parameters and provide some 'rules of thumb' for the use of these parameters in describing the behavior and fate of pesticides in the environment, especially in analysis by modeling.

The use of commercially available formulations of alachlor has resulted in a serious environmental problem due to its leaching and migration to water sources. The objective of this study was to develop organo-clay based formulations that can significantly reduce leaching and improve weed control efficacy. We adsorbed the herbicide to montmorillonite clay particles whose surfaces were modified from hydrophilic to hydrophobic by preadsorbing to the clay an organic cation, such as benzyltrimethylammonium (BTMA) to reduce release of the herbicide into the soil. Alachlor adsorption was determined by gas chromatography. Organo-clay complexes of 0.5 mmol BTMA/g of clay gave larger adsorbed amounts and better formulations of alachlor as compared to BTMA preadsorbed up to the cation exchange capacity (0.8 mmol/g), whereas formulations without organic cations were not effective and gave little adsorption of the herbicide. The use of Fourier transform infrared spectroscopy showed that the optimal formulation also yielded the largest shifts of the stretching vibrations of the phenyl ring. The organo-clay formulations yielded slow release of the herbicide to the environment, maintaining the herbicidal activity in the top soil as measured by a bioassay using green foxtail and wheat as test plants. Laboratory and field experiments showed improved weed control at significantly reduced applied rate when alachlor was applied as organoclay formulations.

This study aimed to optimize organo-clay formulations for reduction of leaching of the herbicides alachlor, metolachlor, and norflurazon, which include a phenyl ring in the structure. The adsorbed amounts of herbicides increased severalfold when montmorillonite was preadsorbed by an organic cation; benzyltrimethylammonium (BTMA) was more effective than benzyltriethylammonium (BTEA). Fourier transform infrared studies indicated interactions between alachlor molecules and adsorbed BTMA. The adsorption affinity of the herbicides increased with BTEA loading up to the cation exchange capacity (CEC) of montmorillonite but reached a maximum at a BTMA loading of 5/8 of the CEC. The enhanced adsorbed amounts of herbicides are mainly due to interactions between the phenyl rings of herbicide molecules and organic cations, which are favored with the smaller cation, BTMA. BTMA preadsorbed on the clay up to the CEC forms a fraction (14-18%) of charged dimers so that less phenyl rings are available for interacting with herbicide molecules. This effect is small for preloading by BTEA, so that the amounts adsorbed increase with the degree of preloading. Thus, optimization of claybased herbicide formulations requires a selection of structurally compatible organic cations preadsorbed on the clay at optimal coverage.

Removal of anionic pollutants (imazaquin, sulfentrazone, sulfosulfuron) and neutral pollutants (alachlor, acetochlor, chlorotoluron, bromacil) from water by micelles preadsorbed on montmorillonite was studied. Micelles of octadecyltrimethylammonium and benzyldimethylhexadecylammonium (BDMHDA) were used. The micelle-clay systems (1% w/w) removed 87-99% of the pollutants from their water solutions containing 1-33 mg/L of herbicide. The nature of the headgroup of the organic cation, which forms the micelles, is critical. Desorption of imazaquin and acetochlor from 0.3% (w/w) suspension of BDMHDA-clay complex after 24 h was around 7% in the range of adsorbed amounts from 0.6 to 15.3 mg/g. These results indicate rather slow rates and small extents of release of pollutants from micelle-clay complexes. Column filters (25 cm) made of a mixture of quartz sand and BDMHDA micelle-clay complex at 100:1 w/w ratio removed at least 99% of above pollutants from initial solutions containing 10 mg/L; 99.5 and 97% of sulfosulfuron and alachlor were removed from their initial solutions containing 200 and 5 microg/L, respectively. These data indicate that micelle-clay complexes are very efficient for water purification from organic contaminants.