Effect of dissolved organic matter on copper-zinc competitive adsorption by a sandy soil at different pH values

The effect of dissolved organic carbon from sewage sludge on copper and zinc adsorption and interaction on samples from the surface layer (0 - 10 cm) of a sandy soil (Gleyic Podzol) were studied at two pH levels (4 and 7). This soil presented acidification hazards and sewage sludge with a high content of Cu and Zn was used as a fertilizer. 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 at a constant level (180 mg l(-1)) of dissolved organic carbon (DOC). Copper and zinc sorption capacity decreased in the presence of DOC. As the pH increased, the decrease in Cu adsorption due to interaction with DOC was more obvious. Conversely, without addition of DOC, Cu and Zn adsorption increased with pH. More Cu than Zn was adsorbed. Soil sorption of these cations was described by equilibrium isotherms that fitted both Langmuir and Freundlich type equations, presenting however a better fit to the Freundlich equation (R2>98%). Adsorption dependence on DOC was more noticeable at pH 7.