The adsorption of paraquat (1,1-dimethyl 4,4'-dipyridylium dichloride) by a soil and two soil aggregate models containing tannic acid as the source of organic matter was studied. The isotherms obtained fit the Langmuir adsorption equation. The values of maximum adsorption found for the aggregates were higher than the cation exchange capacity indicating the existence of specific interactions as well as cation exchange. This is attributed, in agreement with the results of other studies, to the low Donnan potential of the water-paraquat-colloids of the soils system, owing to the sparingly ionized tannic acid. A study was also made of the adsorption of paraquat by the soil and the aggregates after removing the organic matter by oxidation. In all cases, a reduction in adsorption was observed. The values of the Langmuir K constants, of the sorptivity indices and of the mean equilibrium concentrations, indicate that the interaction mechanisms involved in the adsorption of the herbicide by the natural and oxidized samples are different.
