The equilibria as well as the rates of adsorption and desorption of the ions Pb2+, Cu2+, Cd2+, Zna+, and Caa+ by soil organic matter were determined in batch experiments as a function of the amount of metal ions added to an aqueous suspension of HC1-washed peat. Simultaneous determination of the metal ions and hydrogen ions in the solution by atomic absorption spectrophotometry and pH-measurements showed that the adsorption of one divalent metal ion by peat was coupled with the release of two hydrogen ions. Since this equivalent ionexchange process causes a corresponding increase of the electric conductivity of the solution, the rates of the adsorption and desorption processes were determined by an immersed conductivity electrode.The distribution coefficients show that the selective order for the metal adsorption by peat is Pb2+ > Cua+ > Cd2+ Zn2+ > Ca2+ in the pH range of 3.5 to 4.5.The slope of -2, as observed in a double logarithmic plot of the distribution coefficients versus the total solution concentration confirms the equivalence of the ion-exchange process of divalent metal ions for monovalent H,O+-ions in peat. The absolute rates of adsorption, as well as the rates for the fractional attainment of the equilibrium, increase with increasing amounts of metal ions added. This behaviour is also observed for the subsequent desorption of the metal ions by H,O+-ions. At a given amount of metal ions added, the absolute rates of adsorption decrease in the order Pba+ > Cu2+ > Cd2+ > Zna+ > Caa+, while the rates for the fractional attainment of the equilibrium decrease in the order Caa+ > Zn'f N -Cda+ > Pb2+ > Cua+. The half times for adsorption and desorption were in the range of 5 to 15 sec.

No abstract

The rate of ion exchange reactions of humic substances (humic acid and peat) with lead ions was investigated at 25 "C. All experiments were carried out under finite solution-volume conditions. To the well-stirred aqueous suspensions of the above samples lead ions were added in such small amounts that, after attainment of the equilibrium, the initial equivalent fraction of lead in the humic substance had increased only by a small amount. The kinetics of such differentially small ion exchange processes were studied as a function of the initial lead content of the samples. The experiments showed that the absolute rates for the uptake of small amounts of lead ions decrease, if the initial lead content of the samples increase. The relative rates of ion exchange, i.e. the rates with respect to the extent of each reaction, showed a more complicated behaviour, as they can exhibit a minimum at a certain initial lead content of the sample. The ratedetermining step of the above ion exchange experiments was shown, by interruption tests, to be diffusion of the ions through the Nernst-film surrounding the ion exchanger particles.Theoretical calculations of the rates of film-diffusion controlled ion exchange processes were carried out and the results revealed that the rates of any differential ion exchange reaction should decrease with increasing initial content of the ion adsorbed, their exact magnitude depending, among other quantities, strongly on the separation factor. The relative rates for the attainment of the equilibrium, on the other hand, depend much less on the initial ionic composition of the ion exchanger. A comparison of the experimental and the calculated results yields satisfactory agreement if the experimentally determined dependence of the separation factor on the lead content of the samples is taken into account.

No abstract

No abstract