Many organic substances in natural waters are amphipathic; i.e., they contain both a hydrophobic and a hydrophilic moiety. To assess the adsorptive behavior of such substances and to appreciate the factors that influence the distribution between particles and water, we need to understand how these substances interact with polar or nonpolar interfaces. To gain such an understanding, the adsorption of fatty acids of various chain lengths has been investigated on two model surfaces: (1) on a Hg electrode and (2) on 6-A1203 particles. While hydrophobic expulsion dominates the adsorption on the nonpolar Hg surface, the adsorption of short-chain fatty acids (<8 carbon atoms) on the surface of hydrous A1203 is characterized by specific coordinative interaction. Phase-selective polarography is a convenient tool to measure the extent and kinetics of adsorption of those amphipathic substances that become adsorbed primarily because of hydrophobic expulsion.
Actinide sorption on colloids may be described as a competition between the formation of complexes in solution and the build up of surface complexes. The role of particle and of carbonate concentrations on the sorption/desorption of 241 Am on montmorillonite, illite and hematite colloids is investigated. Since the partition coefficient (K p ) values are virtually independent of the colloid concentrations, within the range 1 to 300 ppm, no significant aggregation takes place in the sorption/desorption experiment. At pH 8, a slight decrease of K p is observed if the concentration of total carbonate exceeds 10~2 M. The formation of the carbonato-(and hydroxo-carbonato-) complexes in the solution competes with the formation of surface complexes on the colloids. A relationship between the sorption coefficient and the complexation of 241 Am in the solution has been found. This leads to the conclusion that, besides free americium cation, the hydroxo-, and carbonato-as well as the mixed hydroxo-carbonatocomplexes are sorbed. Only when the tricarbonatocomplex [Am(C0 3 ) 3 ] 3~ prevails (total carbonate concentration >10~2 M), a significant decrease of the distribution coefficient is observed. At pH 10 this decrease disappears because under these conditions the strong hydroxo-complexes dominate. A pragmatic and relatively simple application of surface complexation model describes the observed features.
SummaryThe influence of the ionic strength and of pH on the adsorption/ desorption processes of Pb, Bi and Po on montmorillonite has been investigated. For Pb, a strong dependence of the adsorption and desorption processes on the ionic strength was observed at pH < 7, whereas, at higher pH values, this dependence totally disappears. The ionic medium had no influence on the sorption/ desorption of both Bi and Po. For all of these nuclides, large distribution ratios (Rj) are measured. They range from 10 3 to 10 5 ml · g" 1 for Pb and from 10 4 to 10 7 ml · g" 1 for 210 Bi and 210 Po. When adsorption and desorption coefficients are compared, an agreement is found for both adsorption/desorption Rj s of Pb, whereas, for Bi and Po adsorption R/s were several orders of magnitude lower than those obtained for desorption. The chemical activities of free Bi and Po in the liquid phase are limited by the formation of Bi and Po-colloids prior to the sorption step. This fact could explain the differences in the R d values. While the adsorption of Pb was reversible, only very small amounts of Bi and Po could be desorbed from the montmorillonite (quasi-irreversible adsorption).The radioactive decay of adsorbed 210 Pb to 210 Bi, which in turn decays to 210 Po, can lead to significant changes in the desorption behaviour of the daughter nuclides. Whereas the sorption is nearly irreversible if Bi and Po adsorb on montmorillonite from the aqueous phase, they desorb more easily if they are generated by the radioactive decay of adsorbed 210 Pb. The difference in the distribution coefficients R d is approximately one order of magnitude in the case of Po, and more than 2 orders of magnitude in the case of Bi.
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