Cadmium soil sorption at low concentrations. VII: Effect of stable solid waste leachate complexes

Soil sorption of Cd in solid waste leachates of landfill, sewage sludge, and incinerator residue was studied in the laboratory with emphasis on the behavior of Cd present as stable complexes. A previously developed speciation method was employed to determine free divalent Cd and complexed Cd of various stability. The experiments involved 13 soils covering a wide range of clay contents. Speciation of Cd present in solution after exposure to increasing amounts of soil showed that stable Cd complexes did not sorb onto the soil. This was supported by equilibrium isotherms exhibiting similar solute concentration intercepts and by a leaching study of soil columns which resulted in the same concentration of stable Cd complexes. The stable complexes were of the order of 1 to 10 μg Cd L^?1, the higher values found in landfill and sewage sludge leachate. Very little Cd present in the leachates was free divalent Cd (a few percent), which may explain the low Cd distribution coefficients (3 to 70 L kg^?1).