Environ. Sci. Technol. 1990, 2 4 , 736-741
Effect of Naturally Occurring Soluble Organic Matter on the Adsorption and Movement of Simazine [2-Chloro-4,6-bis(ethylamino)-s-triazine] in Hanford Sandy Loam Frank C. Spurlock” and James W. Biggar Department of Land, Air and Water Resources, University of California, Davis, California 95616
A series of batch, miscible displacement, and solubility experiments were conducted to examine the effect of soluble soil organic matter (SOM) on the adsorption of simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] in Hanford sandy loam. Batch equilibrium data conformed to the linear adsorption model and indicated no effect on adsorption from the presence of SOM up to 45 ppm organic carbon over a simazine concentration range of 30-1900 mg/mL aqueous. Breakthrough curve (BTC) data from six steady-state soil column experiments were evaluated by use of two transport models, a one-dimensional convective dispersive equilibrium linear adsorption model, and a two-site/two-region nonequilibrium formulation of the transport equation. The latter model was more consistent with batch results than the former. For this soilherbicide combination, no substantial effect on BTC-fitted linear distribution coefficients or simazine water solubility was observed for SOM concentrations up to approximately 80 and 100 ppm organic carbon, respectively. Introduction
The positive correlation between soil organic matter and adsorption of nonionic organic compounds (NOCs) such as herbicides has long been recognized. Attention has recently been turned to the fact that the soil solution contains naturally occurring dissolved organic constituents that may have some influence on the environmental behavior of NOCs. Investigations on the influence of soluble soil organic matter (SOM) have demonstrated significant solubility enhancement for highly nonpolar solutes such as DDT, PCBs, and cholesterol (1-4). The work of Chiou et al. (2)suggests that substantial solubility increases would be restricted to those compounds which themselves are highly insoluble (hydrophobic). A partition-like mechanism between hydrophobic solutes and intramolecular nonpolar regions of large molecular weight organic matter molecules was postulated to account for the increased solute solubility. Under typical environmental conditions, herbicides and pesticides are generally found at levels much lower than their aqueous solubilities. I t is therefore prudent to consider the possibility of other SOM effects that may be observed beyond solubility enhancement, such as a decrease in adsorption of moderately nonpolar compounds to soil particles by a competitive mechanism, or by the formation of specific soluble organic matter-solute complexes, effectively sequestering adsorbate species. Other postulated effects include an increase in adsorption resulting from the coating of exposed mineral surfaces by SOM. SOM has been suggested by Ballard (5) to increase mobility of DDT in lysimeter experiments by acting as a carrier for the pesticide. Khan (6) using 2,4-D, suggested that fulvic acid molecules adsorbed to clay particles, with subsequent physical (reversible) adsorption of 2,4-D to the resulting organo-clay complex. Bowman (7) observed mixed effects with methyl parathion and parathion in montmorillinite suspensions in the presence of fulvic acids. 736
Environ. Sci. Technol., Vol. 24, No. 5, 1990
Other laboratory studies have examined NOC binding with soluble soil organic matter preparations using gel exclusion chromatography. Hayes et al. (8) postulated a London-van der Waals (physical adsorption) interaction between atrazine and dissolved organic matter. Atrazine, an s-triazine similar to simazine, eluted from gel filtration columns earlier in the presence of a dissolved fulvic acid preparation than those experiments conducted without it. Soil organic matter preparations are typically subject to substantial chemical and physical manipulation in the extraction and purification process. As a result, SOM preparations obtained may be far removed from SOM as it exists in the field. More recent investigations have examined pesticide interactions with “naturally occurringn (water extractable) soluble soil organic matter. In gel filtration experiments similar to those of Hayes et al., Wijayaratne and Means (9) provided qualitative evidence indicating the ability of soluble estuarine organic matter (nominal molecular weight,
