Sorption and desorption of cadmium from some New Zealand soils: effect of pH and contact time

The effects of soil pH on the desorption of native soil cadmium (Cd), and on the sorption and desorption of added Cd at low concentrations, have been examined for 6 New Zealand soils ranging from pH 4.9 to 6.2. The effect of contact time with added Cd on subsequent desorption from soil has also been studied. Cadmium desorption was determined by repeated equilibrations in 0.01 M Ca(NO3)(2) solution. Cadmium sorption ranged between 38% and 96% from an initial addition of 2 mu g Cd/g soil. The effect of increasing soil pH was to increase substantially the amount of Cd sorbed. Sorption isotherms were all linear, with a negative intercept on the y-axis. Sorption data also fitted a linearised Freundlich sorption equation. Cadmium desorption was also very sensitive to pH, with a dramatic reduction in the amount of native Cd desorbed from the soil as pH increased, as was observed for samples where Cd was added. The cumulative amounts of native Cd desorbed represented only a relatively small proportion (0-22%) of total soil Cd concentrations. Added Cd desorption ranged between 22% and 99% of the Cd initially sorbed on the soil at varying pH. Organic matter appeared to be the most important soil component controlling both sorption and desorption in the soils studied. In the contact period experiment, the proportion of Cd desorbed was decreased by increasing initial contact time to 70 days before desorption for all 4 soils studied. Contact time had the greatest effect on Cd desorption in soils with the highest amounts of soil oxide components. Implications of the study are that, for the soils studied, soil pH, Cd contact time, and soil organic matter content are controlling factors on Cd desorption into soil solution, and are therefore likely to play an important role in Cd phytoavailability.