The solidsolution equilibria of lead and cadmium in polluted soils

Summary A method for the measurement of Pb and Cd in equilibrium soil solutions involving soil equilibration with a dilute Ca electrolyte, centrifugation and filtration to <0.2 μm was evaluated. The procedure was subsequently used for the analysis of 100 Pb‐ and 30 Cd‐contaminated soils. Solutions were analysed for Pb‐ and Cd using graphite‐furnace AAS and the concentrations of Pb 2+ and Cd 2+ were estimated using standard speciation calculations. The concentrations of Pb and Cd found in the soil solutions were in the range 3.5–3600 μg dmp −3 and 2.7–1278 μg dm −3 respectively; both ranges represented less than 0.1% of the total metal concentration in the soils. Depending on solution pH, Pb +2 accounted for between 42–78% of Pb in solution while about 65% of Cd in solution was present as Cd +2 . The concentrations of Pb 2+ and Cd 2+ in solution suggested that the soil solutions were undersaturated with respect to the solid phases PbC0 3 and CdC0 3 but supersaturated with respect to Pb 5 (P0 4 ) 3 Cl and, for some samples, Cd 3 (P0 4 ) 2 respectively. However, for both metals, a good empirical relationship was obtained between the total metal concentration in soil (mol kg −1 ), free metal concentration in solution (mol dm −3 ) and solution pH. The relationships took the general form of a pH‐dependent Freundlich adsorption equation: image For both lead and cadmium relationships, the values of n and K 1 were close to unity, so that the distribution coefficient could be estimated from pH and a single metal‐dependent constant, K 2 . The algorithms appeared to be valid over a metal concentration range of four logarithmic units and pH range of 3.5–7.5.