Mutual influence of Cu and a cationic herbicide on their adsorption-desorption processes on two selected soils

Adsorption of Cu and the cationic herbicide chlordimeform was carried out on two acidic soils (S-48 and LM). Chlordimeform adsorption occurred by cationic exchange and was higher on LM soil due to its higher CEC, partly associated to the presence of vermiculite. Cu adsorption was also higher on LM soil and was related to its higher iron and manganese oxides content. The presence of the herbicide in solution always decreased Cu adsorption on S-48 soil, but on LM soil the amount of Cu adsorbed only decreased at the highest concentrations used. This together with the lower amounts of Cu desorbed from LM soil after five successive desorptions in comparison to S-48 soil, indicates that Cu was being mostly adsorbed on oxides which exhibit a very high affinity for the heavy metal. The maximal amount of Cu desorbed on both soils only reached up to 13% of the amount adsorbed. Chlordimeform adsorption decreased in the presence of Cu as a result of its competition for the adsorption sites on both soils. Cu competition on LM soil was not so strong as on S-48 soil due to the higher adsorption of the heavy metal on oxides, for which the herbicide's affinity is not so high. The lower the amounts of chlordimeform adsorbed on both soils the higher its desorption percentage. This was attributed to steric impediment of chlordimeform molecule itself for the desorption from lamellar silicates.

The authors acknowledge the Comisión Interministerial de Ciencia y Tecnología (CICYT) for financial support (Project N. AMB97-0692). Dr. Undabeytia acknow- ledges the Spanish Government for a research contract (Project N. AMB98-0888).

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