The potential impact of the addition of sewage sludge from domestic waste water treatment plants on agricultural soils is a worldwide concern. A proportion of heavy metals in sewage sludge will be present in the soil solution; their equilibria can be modified by the competitive interactions between them, changing not only their mobility but also their adsorption pattern. The competitive adsorption of heavy metals by soils has been studied by several authors in simple model substrates and synthetic minerals, and also in soils but restricted to binary adsorption. In the present paper the modification of the Cu, Pb, Ni, and Zn interactions with soils as a consequence of a competitive adsorption on the available adsorption sites on Andisols were determined.

The single- and multi-component adsorption of Cu, Pb, Ni, and Zn were carried out at constant ionic strength (0.1 m KNO₃). The adsorption was monitored by ICP-OES and the Langmuir model was applied to fit the adsorption isotherms.

Competitive adsorption isotherms indicate a reduction of the adsorption in the control soils when compared to the single adsorption isotherms. The maximum adsorption constants for single- and multi-component isotherms for Diguillín soil were 38.2 and 33.3 mmol/kg for Cu, 48.1 and 30.0 mmol/kg for Pb, 5.4 and 2.1 mmol/kg for Ni, and 18.0 and 8.1 mmol/kg for Zinc, respectively. With Ralun soil, lower values were obtained, but the same tendency was observed.

The maximum adsorption and the affinity constant increased for all cations, after the addition of sewage sludge to Diguillin soil.