CADMIUM SORPTION IN SOILS 25 YEARS AFTER AMENDMENT WITH SEWAGE SLUDGE.

Long-term changes in the solubility and bioavailability of heavy metals in soils, accumulated as the result of waste application, cannot be predicted without knowledge of the nature of metal retention in these soils. To test the theory that Fe- and Al-rich mineral phases in sewage sludge-amended soils can act as long-term sinks for heavy metals, soils were sampled from two field sites, each with a similar history of multiyear application of either high-Fe, high-Al, or high-Ca sludge (~25 years earlier) but with different textural characteristics. These soils were amended with Cd in the form of CdCl2 in the laboratory to determine Cd solubility as a function of total added Cd over the range of 0 to 20 mg/kg. The slopes of these linear solubility functions, used to determine the strength of Cd sorption, revealed that the high-Fe and high-Al sludge amendments did not improve the soilsʹ affinity for Cd at either site. The high-Al sludge treatment decreased the affinity for Cd in the more coarse-textured soil. The high-Ca sludge amendment, conversely, increased the affinity for Cd, probably a result of the higher soil pH that has persisted since the sludge application. The results suggest that sludge Fe and Al may not be effective in binding Cd in all soils of humid-temperate climates.