Rob N.J. Comans

Sorption of cesium on illite: Non-equilibrium behaviour and reversibility

Abstract Adsorption, desorption, and isotopic exchange experiments have been used to investigate the kinetics and reversibility of cesium sorption on illite. Trace concentrations of cesium were used and experimental conditions were kept close to those of many freshwater environments. Initial adsorption of cesium on illite was rapid but was followed by slower uptake processes with time scales of weeks to months. The uptake proceeded faster and reached much higher Kd values in a Ca2+ environment than in a K+ environment. The apparent reversibility of sorption was affected by both the slow sorption processes and the nature of the competing cation. The desorption equilibration patterns indicate that cesium is released from rapidly accessible sorption sites but that the slow forward reaction continues. On time scales relevant for modelling its transport and retention in aquatic systems, cesium shows partially irreversible behaviour. A mechanistic interpretation for the observed sorption and isotopic exchange behaviour is proposed whereby cesium migrates slowly to energetically favourable interlayer sites from which it is not easily released.

Kinetics of cesium sorption on illite

Abstract The behaviour of anthropogenic cesium isotopes in the aquatic environment is controlled by sorption on solid particles, especially illitic clays. We review data previously published and present new results which show that cesium sorption on potassium- and calcium-saturated illite is kinetically controlled. Two kinetic models incorporating Freundlich isotherms and an irreversible process are used to describe sorption data spanning a range of cesium concentrations. Although empirical in nature and origin, the models are consistent with mechanistic hypotheses proposed by earlier workers. The models predict that effectively instantaneous and reversible kinetic processes control cesium sorption over time scales of a few days and less. Irreversible or apparently irreversible sorption is more significant over longer times. The implications of our findings for radiocesium remobilization in anoxic sediments and transport in the water column are discussed.

Sorption of trace metals on calcite: applicability of the surface precipitation model

Published Sorption isotherm data of Cd2+, Mn2+, Zn2+, and Co2+ on calcite are adequately described by the surface precipitation model which was originally developed by FArley et al. (1985) for the sorption of cations on metal oxides. In addition to monolayer adsorption, the model accounts for the formation of a surface phase with a composition that is described by a solid solution having as end members the sorbent calcium carbonate mineral and a pure carbonate precipitate of the sorbing trace metal. The model thus specifies a continuum between adsorption and precipitation. This feature is supported in the literature by observations on the reaction kinetics and the amount of surface coverage during trace metal sorption on calcite. The apparent adsorption constants of these trace metals, as derived from the model, can be ranked according to the degree to which their ionic radii match the ionic radius of Ca2+.

Adsorption, desorption and isotopic exchange of cadmium on illite: evidence for complete reversibility

Adsorption, desorption and isotopic exchange of Cd on illite clay have been studied at low Cd concentrations and low ionic strength. The results indicate that under the conditions of the experiments Cd sorption on illite is completely reversible. Long equilibration times (7–8 weeks) were shown to be essential because of slow desorption kinetics.

Sorption of cadmium on hydroxyapatite

Equilibrium and kinetic data for Cd sorption on hydroxyapatite are presented for a wide range of Cd2+ concentrations. Experimental conditions were kept close to those found in freshwater environments. Dissolved Cd2+ concentrations after equilibration ranged from environmental levels (< 0.1 μg l−1) up to values 6 orders of magnitude higher. At low Cd2+ concentration (<500 μg l−1) the equilibrium data follow a Langmuir isotherm, but at higher concentrations solutions are supersaturated with respect to CdCO3. Precipitation of this phase further controlled dissolved Cd2+ concentrations. For low concentrations of Cd2+, an initial rapid uptake of Cd is followed by a period of relatively slow removal from solution. Because of slow sorption processes Cd continues to be taken up even after 28 days. The high affinity of the hydroxyapatite surface for Cd, when compared to other natural solids studied under similar experimental conditions, is consistent with the enrichment of Cd in phosphorite deposits.

Geochemical studies in the drainage basin of the Rio Vouga, Portugal. IV. Impact of land use on the hydrogeochemistry of natural waters in the Vouzela region

A hydrogeochemical study of the impact of land use on the composition of natural waters in the Vouzela region in northern Portugal was carried out during the summer of 1983. Water samples were collected from spring and major streams in the area and analyzed for major constituents and some trace elements. Analysis of variance and subsequent pairwise contrast tests demonstrated that waters from agricultural areas are significantly enriched in constiluents such as Na, K, Ca, Mg, Sr, Cl, SO4, and NO3. High concentrations of these ions in agricultural areas are the result of both the application of fertilizers and enhanced evaporation by the intensive irrigation of agricultural lands. Constitutents such as H4SiO4, HCO3, F, and probably Li, which are typically related to mineral weathering, were not affected by land use. In waters from forested areas the concentration of most constituents was about 20% higher than in waters from uncultivated areas with mainly a grass cover. This reflects the difference in the evapotranspiration of these two vegetation types. The chemistry of the Rio Zela clearly reflects differences in land use in the Rio Zela valley.

Intermittent stirring: A method for achieving equilibrium in adsorption systems with a solid phase sensitive to the breakdown of particles

The method of agitation has a great influence on the kinetics of adsorption of cobalt by synthetic geothite /α-FeOOH/ suspended in 10−2 mol l−1 NaCl at pH 7. Three methods of agitation were compared, reciprocating shaking, continuous stirring and intermittent stirring, and only with the latter was equilibrium achieved. This suggests that in sorption systems with a solid phase subject to aggregation of particles the breakdown of aggregates can be prevented by intermittent stirring.

ISOTOPICALLY EXCHANGEABLE PHOSPHATE IN FRESHWATER SEDIMENTS: EFFECTS OF U.V.-IRRADIATION, FORMALDEHYDE, SOLID/SOLUTION RATIO, AND pH ON ITS EXPERIMENTAL DETERMINATION

Abstract The influence of bioactivities, solid/solution ratio and the pH on the isotopic exchangeability of phosphate in a freshwater sediment was investigated. From the comparison of the results obtained for the same sample in the presence or absence of formaldehyde, it is concluded that microorganisms can effect the analysis for isotopically exchangeable phosphate. Irradiation with u.v.-light caused a sharp rise in isotopic exchangeability. In the pH-range 6.6–8.4 isotopic exchangeability of phosphate increases with decreasing pH-value which is attributed to an easier exchange of H2PO4− than of HPO42−. The influence of the solid/solution ratio on the isotopic exchangeability of phosphate in the solid phase is small or nil. However, because of the relatively large amount of phosphate that goes into solution, the total isotopic exchangeability Ei of phosphate in the solid and liquid phase together is strongly increased at a low solid/solution ratio. From these results it is concluded that in order to make a meaningful comparison of isotopically exchangeable phosphate in different soils or sediments it is essential to work at a nearly constant pH and solid/solution ratio. Such a comparison was made for 26 freshwater sediments from the Rhine/Meuse delta, in the presence and absence of 0.17 mol l−1 formaldehyde as a biological inhibitor and it is concluded that addition of the latter is essential. The lowest total isotopic exchangeabilities of phosphate, (15–25) were measured in the sediments collected from the Haringvliet, whereas higher values (40–80) were found in the sediments from the Brielse Meer and the Grote Rug. This could well be indicative of a similar variance in the biological availability of the phosphate in the investigated sediments.