Cadmium and nickel distribution coefficients for sandy aquifer materials

Distribution coefficients (K(d)) were measured for cadmium (Cd) and nickel (Ni) in laboratory batch experiments for 18 sandy aquifer materials at environmentally relevant solute concentrations (Cd: 0.1-15 μg/l; Ni: 4-200 μg/l). The K(d)-values ranged three orders of magnitude: Cd: 2-1770 l/kg and Ni: 3-7250 l/kg. Correlating observed K(d)-values to characteristics of the aquifer materials (particle size fractions, organic-C content, surface area, pH) revealed very good correlation with pH in the pH range 4.9 to 8.9 (Cd: r² = 0.942; Ni: r² = 0.929). Including any other of the measured aquifer material characteristics did not improve the correlation significantly. The results indicate that at the same pH-values, the K(d)-values for aquifer materials are 4-10 times lower than K(d)-values for soils reported in the literature. However, the mobility of Cd and Ni, as reflected in the K(d)-values, is still very limited in sandy aquifers, unless the pH is very low. The results indicate that at pH 6 the relative mobility of both Cd and Ni is of the order of 1%.