Effect of pH and calcium on lead and cadmium uptake by earthworms in water

pH and calcium are important as factors determining heavy metal bioavailability to soil organisms. To study their intrinsic effect without the influence of soil adsorption, earthworms (Lumbricus rubellus) were exposed to lead and cadmium in reconstituted water adjusted to various levels of pH and calcium chloride. Concentration of cadmium in worms increased linearly with increasing concentration of exposure independent of pH. Concentration of Pb in worms, however, showed a pH-dependent increase with increasing concentration of exposure. The uptake of both metals was positively influenced by increasing pH. Calcium reduced the uptake of Cd, but had no significant effect on Pb uptake. It is suggested that Ca2+ and H+ compete with lead and cadmium ions for binding sites in the earthworm, and that the affinity for Pb is much greater than that for Cd. The results are discussed in relation to available data on Pb and Cd accumulation by earthworms in soil, showing the predominant importance of soil adsorption processes.