Heavy metal accumulation (lead and cadmium) and ion exchange in three species of Sphagnaceae : I. Main principles of heavy metal accumulation in Sphagnaceae.

Three different species of Sphagnum mosses originating from a bog ("Hochmoor") were examined with respect to their behaviour of accumulation of heavy metal ions lead and cadmium. A strong bond of double charged Pb could be found independent of the Sphagnum species. When competitive reactions were performed in test solutions, with two ions (Pb2+ and one alkali or one earthalkali ion) or three (Pb2+ and one earthalkali and one alkali ion) Pb2+ was always accumulated too. This leads to a suppression of exchange of the other ions. Cadmium is by far less strongly bound than lead. Cadmium was tested in combination with two or three ions as was done for lead. The behaviour of cadmium adsorption is similar to that of calcium. For a number of ions typical tendencies were observed, which give insight in the quantity and strength of their binding to the exchanging material. The following series of accumulation was found: Pb2+>Cd2+≥Ca2+>Mg2+>K+>Na+≥NH inf4sup+ . (Accumulation was determined in test solutions containing each ion in the concentration 0.01N.) Differences in this series selectivity could not be found within the three Sphagnum species. But there exist typical differences in the total amount of adsorbed ions per unit dry weight. This species specific value corresponds with total capacities for ion exchange of the three species. Furthermore acid strength as well as pH-dependence of the ion exchanging Sphagnum material were investigated. Its physico-chemical behaviour may be described as that of a weak acid polyelectrolyte with variable charge, depending on ionic strength.