Thursday, 13 July 2006

Competition Between Glyphosate and Phosphate for Adsorption Sites on Soil Minerals.

Anne Louise Gimsing and Ole K. Borggaard. Royal Veterinary & Agricultural University, Thorvaldsensvej 40, Frederiksberg C, DK-1871, Denmark

Glyphosate (N-phosphonomethylglycine) is among the most widely used herbicides worldwide. It is the active ingredient in a number of well-known products such as Round-Up. Glyphosate, which contains a phosphonic acid moiety, has been suggested to adsorb to soils and minerals by ligand exchange through the phosphonic acid moiety in a way similar to the adsorption of phosphate. Accordingly, it has been suggested that phosphate and glyphosate compete for adsorption sites, that the phosphate concentration is the most important factor in determining the amount of glyphosate adsorbed, and that phosphate in some cases is able to completely desorb glyphosate . Phosphate may thus play an important role in determining the bioavailability and leaching potential of glyphosate. Competitive adsorption of glyphosate and phosphate on the oxides goethite, ferrihydrite, hematite and gibbsite and on the clay minerals illite, montmorillonite and two kaolinites differing in surface area was evaluated. The results show that glyphosate and phosphate are competing for the adsorption sites, but the degree of competition depends on the adsorbent. On the three iron oxides the competition is in favour of phosphate, especially on goethite where phosphate completely desorbs glyphosate. On gibbsite the competition is more equal, but still phosphate is favoured, while on illite, montmorillonite and kaolinite the competition is almost equal. The amounts of glyphosate and phosphate, which can be adsorbed also depends on the adsorbent: the oxides adsorb more than the clay silicates, and the ferrihydrite which has the largest surface area adsorbs most. The amount adsorbed on kaolinite was dependent on the specific surface area. Changes in the surface area did not affect the competition between glyphosate and phosphate for adsorption sites. The results indicate that differences among soils of different mineralogical composition regarding the adsorption of glyphosate and phosphate can be expected.

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