THE INFLUENCE OF PH ON THE DESORPTION AND SPECIATION OF COPPER IN A SANDY SOIL

The effect of pH on copper desorption from a sandy soil and on complexation by dissolved organic fractions was studied, using a soil, with pH ranging from 4.3 to 5.6, that had been polluted with copper a decade ago. For the desorption of exchangeable copper we used an unbuffered electrolyte solution (0.0033 M Ca(NO3)2; ionic strength 0.01 M). Free copper was determined using an ion exchange resin method. Copper bound by dissolved organic matter was divided into a humic and a fulvic fraction using ultrafiltration. The Cu desorption isotherms were nonlinear and dependent on pH. The (desorbed) exchangeable copper was less than 3% of the total copper in the soil. About 1% of exchangeable copper was complexed with nitrate, and free copper varied from 2% (pH 5.7, 0.3 mmol/kg total Cu) to 9% (pH 4.4, 3 mmol/kg total Cu). The remainder of exchangeable Cu was bound by dissolved organic matter. The amount of Cu bound by solid organic carbon (CuSOC) was almost equal to Cu bound by dissolved organic carbon (CuDOC) when both are expressed in mmol/g C, although there was a pH dependency. At low pH (4.4) Cu was predominantly bound by fulvic fraction, and at pH 5.7 most was bound by humic fraction. Desorption of copper from soil and copper binding by humic and fulvic fraction (all different pH dependent) could be described adequately with the Two Species Freundlich (TSF) equation. With this equation the competition between soil, fulvic fraction, and humic fraction for free copper (Cu2+) was shown.