Cadmium soil sorption at low concentrations: I. Effect of time,cadmium load,pH, and calcium

Sorption of Cd at low concentrations onto two Danish soils (loamy sand, sandy loam) was examined in terms of kinetics and governing factors. From an environmental point of view soil sorption of Cd is a fast process: More than 95% of the sorption takes place within 10 min, equilibrium is reached in 1 hr, and exposures up to 67 wk did not reveal any long term changes in Cd sorption capacities. The soils have very high affinity for Cd at pH = 6.00 (10^?3 M CaCl₂) exhibiting distribution coefficients in the order of 200 to 250 (soil Cd concentration/solute Cd concentration). However, the sorption isotherms describing the distribution of Cd between soil and solute are slightly curvelinear. In the pH-interval 4 to 7.7, the sorption capacity of the soil approximately increases 3 times for a pH increase of one unit. Increasing the Ca concentration from 10^?3 to 10^?2 M reduces the sorption capacity of the sandy loam to one third.