Changes in partitioning of cadmium-109 and zinc-65 in soil as affected by organic matter addition and temperature

Soil collected from the A horizon of a soil developed from alum shale was added organic matter art the rate of 0 and 4% (w/w) and placed in temperature-controlled climate chambers at 9 and 21 degreesC. After spiking the soil with Cd-109 and Zn-65, sequential extraction of the tracers was performed on soil subsamples collected at time intervals ranging from 0.5 to 8760 h (1 yr). The activity concentrations of Cd-109 and Zn-65 determined in the seven extracts were used in a three-component model to calculate the kinetics of Cd-109 and Zn-65 transfer between the water-soluble, reversibly sorbed and irreversibly sorbed model components. The rates and the time-dependent distribution coefficient constants for Cd-109 and Zn-65 distribution among the three components were determined, and the impacts of organic matter addition and increasing temperature on these constants were assessed. The reversible sorption of the tracers occurred rapidly, within 0.5 h, while the transfer of reversibly sorbed Cd-109 and Zn-65 into the irreversibly sorbed fractions was a significantly slower process. The addition of organic matter reduced the rate of Cd-109 and Zn-65 transfer into the irreversibly sorbed fractions, whereas the transfer rate Increased with increasing temperature. The interaction between organic matter and temperature affected both the rates and the pseudoequilibrium constants, and the temperature regimes and rate of organic matter addition may thus influence the potential mobility of the investigated metals.