Solid-solution equilibria of cadmium in soils

Seven cadmium (Cd)-treated soils (10 mg Cd kg(-1) soil) with contrasting pH (4.9 - 8.4), organic matter content (0.1 - 1.8%) and mineralogy were used to investigate the role of solid-phase fractions on Cd solubility and equilibrium. The concentration of total dissolved Cd in soil solution (69.95-512 mug Cd l(-1)), dissolved organic C (58-750 mg Cl-1), and estimated concentrations of free Cd2+ (0.1-16%) and Cd-organic complexes (82.1-99.9%) varied considerably amongst the soils. Organically bound Cd is a major component of soluble Cd in the soil solutions and this emphasizes the need to consider the complexes when speciating soluble Cd. Solid-phase fractionation of Cd together with correlation studies revealed that the dissolved concentrations of Cd can be predicted significantly (R-2 = 0.98, P<0.001) using a multivariate model including pH and the amounts of Cd associated with organic binding sites. The data presented demonstrated the usefulness of the solidphase fractionation in understanding the solid-solution equilibria of Cd in contaminated soils. (C) 2002 Elsevier Science B.V. All rights reserved.