KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER. IV. ADSORPTION AND DESORPTION OF Pb²⁺, Cu²⁺, Cd²⁺, Zn²⁺ AND Ca²⁺ BY PEAT

Abstract

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.

Batch metal removal by peat. Kinetics and thermodynamics

“…Except for nickel and copper respective positions, these results are in good agreement with metal-humic substances previous results (Bunzl et al, 1976;Giesy, 1983). Although these experiments enable the comparison of the respective metal-peat binding energetics, they also show clearly that the maximum binding capacities in 10 mM metal cation solutions are very similar for the different metals and peats: all values range between 180 and 200 mmol kg -~ dry wt (Table 2).…”

“…Kinetics of metal ion binding by peat has been previously studied by Bunzl (1974a,b) and Bunzl et al (1976) using either continuous or discrete metal cation addition to preacidified sphagnum peat in batch experiments. Reaction kinetics were much more rapid, reaction half-times ranging between 5 and 15 s (instead of 15-30 min in this paper).…”

“…proton release when metal cations bind to peat (Bunzl et al, 1976;Bloom and McBride, 1979;Meisel et al, 1979;Aho and Tummavuori, 1984); the pH buffering capacity of peat weak acid groups, limiting possible pH variations related to previously mentioned proton release (Attal et al, 1985).…”

Effects of Long-Term Wastewater Application on Chemical Properties and Phosphorus Adsorption Capacity in Soils of a Wastewater Land Treatment System

“…The mechanical composition, soil water characteristics, bulk density, and hydraulic conductivity are remarkably similar among different samples taken from different fields of the same type of soil (12, 13). We tested variations of the concentrations of exchangeable Na, K, Mg, and Ca, Bray-1 P, and total organic carbon (TOC) in the top 60 cm soils sampled from fields 5,6,8,9,17,18,28,29, and 34. We found that each type of soil also has very similar chemical properties, even though they may be sampled from different fields, which allows use of only a few samples to obtain the information with negligible error.…”

Geochemistry of three contrasting British peatlands: Complex patterns of cation availability and implications for microbial metabolism

“…Andersen et al (1987) demonstrated with Sphagnum species that, when exchange sites are less saturated with Mg, more heavy metals are adsorbed. Bunzl et al (1976) Thus, unsaturation of the Ellergower Moss peat explains why quotients were lower in the bog than in the fen sites and why Ellergower Moss was the poorest site in soluble Mg (despite high exchangeable concentrations of Mg).…”