Results
Sorption experiments
Adsorption isotherms from the soils and clay minerals were plotted using the 48-hour equilibrium concentrations. Previous studies have shown sorption past 48 hours is inconsequential to the total amount sorbed.1 Initial phosphate concentrations were 5.3, 50.5, and 101.1 mmol/l in order to represent concentrations normally added to agricultural soils and glyphosate concentrations ranged from 7.75E-3 to 2.96 mmol/l.
Isotherms depicting glyphosate sorption with varyinglevels of phosphate are shown in Fig. 1. As the initial level of phosphate increases, sorption of glyphosate decreases. For all systems, addition of as little as 5.3 mmol/l of phosphate causes a significant decrease in sorption of glyphosate. Also, as initial levels of glyphosate increased, the effect of the phosphate tended to have less of an impact on glyphosate sorption.
Isotherms were characterized as being of the Langmuir type, indicating a decrease in the slope of the sorption isotherm, consistent with sorption on material with a limited number of sorption sites. It is also consistent with the hypothesis that glyphosate sorption will decrease in the presence of ortho-phosphate assuming that they compete for the same surface sites.
Table 2 contains the Langmuir curves and regression analysis for the phosphate and glyphosate sorption isotherms. Langmuir curves are used to estimate the total sorption due to surface site saturation [Eq. (5)]. Results indicate that glyphosate sorption should be much greater than phosphate sorption; however, only three points were used to generate the phosphate curve and underestimation is possible. In order to make a more accurate prediction of total phosphate capacity it would be necessary to determine sorbed concentrations much
higher than those studied in this experiment: