Heavy metal accumulation (lead and cadmium) and ion exchange in three species of Sphagnaceae : II. Chemical equilibrium of ion exchange and the selectivity of single ions.

Chemical equilibrium of ion exchange reactions of Sphagnum mosses was investigated for four different combinations of ions. These combinations were: Pb/Ca; Cd/Ca; Pb/K; Cd/K. Based on the law of mass action one can find mathematical descriptions of ion exchange reactions which permit one to calculate a state of equilibrium. As a measure of ion selectivity a rational coefficient of selectivity can be used. For culture experiments three different species of Sphagnum originating from a bog ("Hochmoor") in southern Bavaria were used. With these species coefficients of selectivity of the mentioned heavy metals were determined in solutions of different total ion content. Furthermore for each test solution of fixed total concentration the portions of the ions were altered. The Sphagnum material shows the behaviour of a relative ideal ion exchanger, whose states of equilibrium can be well described by the mathematical equations. The coefficients of selectivity are independent of Sphagnum species. In all cases they reflect the preferred binding of lead to the exchanger. Cadmium and calcium are adsorbed in approximately equivalent amounts. Representative limit isotherms permit an estimation of the errors caused by variation of the rational coefficients of selectivity. The applicability of the determined interactions and regularities on heavy metal monitoring programs is discussed.