Influence of pH on copper-zinc competitive adsorption by a sandy soil

Copper and zinc adsorption and interaction were studied at different pH levels in a sandy soil (Gleyic Podzol) to understand heavy metal behaviour in soils with acidification hazards, polluted by the use of sewage sludge with high content of Cu and Zn. Soil samples were treated by solutions of Cu and Zn as nitrate salts with concentration levels up to 30 mg l(-1) in a Ca(NO3)2 background. More Cu than Zn was adsorbed, mainly specifically adsorbed. Copper and Zn in the exchange complex varied with soil solution pH. Adsorbed Cu and Zn increased with pH and as pH increased, more Cu than Zn was specifically adsorbed. Soil sorption of these cations was described by equilibrium isotherms that fitted either Freundlich or Langmuir type equations. The competitive Langmuir and extended Freundlich equations predicted for Cu and Zn values are in good agreement with the experimental data. Adsorption dependence on pH was more marked above pH 5 with a sharp increase of the distribution ratio D. Values for pH50 offering a relative measure of the selectivity of the adsorbent for the metal cations, were higher for Zn than for Cu increasing with the amount of heavy metal added to the soil sample.