THE DISTRIBUTION COEFFICIENT CONCEPT AND
ASPECTS ON EXPERIMENTAL DISTRIBUTION STUDIES
B.
Allard, K. Andersson and B. Torstenfelt
Department of Nuclear Chemistry
Chalmers University of Technology
S—412
96 Göteborg, Sweden
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Understanding Sorption Processes: A Focus on Distribution Coefficients and pH Influence, Summaries of Chemistry
Insights into the distribution coefficient concept, sorption mechanisms, and factors influencing trace element sorption, with a particular focus on pH effects. It discusses various sorption processes such as physical adsorption, electrostatic adsorption (ion exchange), and chemisorption, and their interplay with pH. The document also touches upon the terminology used in sorption studies.
What you will learn
- How does pH affect trace element sorption?
- How does the distribution coefficient concept apply to sorption studies?
- What is the role of electrostatic adsorption (ion exchange) in sorption processes?
- What are the different sorption mechanisms and how do they differ?
- What are the key factors influencing trace element sorption?
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THE DISTRIBUTION COEFFICIENT CONCEPT AND
ASPECTS ON EXPERIMENTAL DISTRIBUTION STUDIES
B. Allard, K. Andersson and B. Torstenfelt
Department of Nuclear Chemistry
Chalmers University of Technology
S—412 96 Göteborg, Sweden
SUMMARY
Aspects on the distribution coefficient concept, sorption mechanisms and
measurements of sorption phenomena are given.
(This is an introductory summary; a full revised Technical Report with the
same title will be issued in late 1983).
SORPTION PROCESSES
Various kinds of processes can be recognized which would remove a dissolved substance from an aqueous phase, either by adsorption or by other mecha- nisms. Some basic processes, somewhat arbitrarily defined, are summarized below.
- Physical adsorption
Physical adsorption will occur due co non-specific forces of attraction involving the entire electron shells of the solute and the adsorbent. These forces, which are denoted as van der Waals forces, have a very short range. The process is rapid and reversible and does not require any activation energy. It is fairly independent of the chemical nature of the adsorbent. There is usually no specific adsorption site. Several consecutive layers of adsorbed particles can be developed, and che selectivity is poor. Thus, the process is fairly independent of ionic strength and even solute concentra- tions at low total concentrations. However, the composition of the aqueous phase in terms of pH and concentrations of complexing agents would be decisive for the sorption process, since these parameters would determine the chemical state of Che solute in che aqueous phase.
- Electrostatic adsorption
Electrostatic adsorption (ion exchange) will occur due to the action of attractive coulombic forces between electrically charged solute species and oppositely charged sorbent surfaces. The range is much greater than for physical adsorption processes. The process is usually rapid and largely reversible and may require some activation energy. It is strongly dependent of the chemical nature of the sorbent, and specific adsorption sites may sometimes be recognized.
Charge and size restrictions can result in a certain selectivity in some systems. The process is generally highly dependent on the ionic strength of the aqueous phase as well as of the chemical state and concentration of the solute.
Ion exchange reactions can cake place becween non-complexed cacions (e.g. M , M^ ) in che solute and ions having che same charge forming part of che surface crystal layer of che sorbent, or which are presenc in the outer
part of the electric double layer existing on most sorbent surfaces. Thus, ion exchange reactions can be considered as isomorphous ion replacements, specific of the solid sorbent or as non-specific adsorption processes that can occur on most solid sorbents, depending on magnitude and polarity of the total charge of the electric double layer.
The electrostatic adsorption processes may frequently be considered as replacement reactions involving displacement of hydrogen ions from non- dissociated surface groups of the sorbent. Thus, ion exchange reactions may be observed on solids which have a very low or negligible charge or even a charge of the same sign as the adsorbed ion, according to this concept.
- Chemisorption
Chemisorption will occur due to the actions of specific chemical forces, and can be considered as a chemical bonding involving a sharing or possibly transfer of electrons. Energies associated with chemisorption are large. The process may be slow and partly irreversible, and highly selective. A large activation energy may be required, leading to a pronounced tempera- ture dependence. The reaction is insensitive to the ion strength but highly dependent of the solute concentration, often with a characteristic satura- bility, related to the formation of a. single adsorption layer.
- Precipitation, coprecipitation and substitution
The removal of dissolved material due to precipitation, because the solu- bility product is reached should be recognized as one mechanism that would reduce the total solute concentration in solution. Many of the long-lived elements in spent nuclear fuel would form sparingly soluble complexes with 2- 3- OH , CCL , F and PO, , e.g. the actinides in their lower oxidation states. Thus, due to changes in pH or the redox potential the solubility product may be exceeded, even at total solute concentrations as low as 10 -LO M in some cases. Coprecipitation is generally defined as the precipitation of a solute, at concentrations below the solubility product of any sparingly soluble compound, in conjunction with the precipitation of some other macro component. The microcomponent is incorporated or attached to the solid precipitate either by the formation of isomorphous mixed crystals or by the adsorption on the precipitate or occlusion.
FACTORS INFLUENCING TRACE ELEMENT SORPTION
Some major parameters that would significantly affect the sorption of trace element, e.g. in conjunction with geologic storage of spent nuclear fuel are summarized below.
- Effects of pH
The pH of the aqueous phase is one of the principal parameters affecting sorption, both due to the effect on the properties of the sorbent (surface charge, surface alterations) and the solute (chemical state, hydrolysis).
The adsorption of cations at trace concentrations is generally small at low pH, but increases with increasing pH above a certain level. If the adsors- tion increase were due only to the decreasing H -concentration, the follow- ing reaction would be valid:
M2 +^ Mz+^ nH
where species adsorbed on the solid are denoted by bars. This exchange reaction can be defined bv
k e x e x /f^ (2)
where f is the activity coefficient ratio and K the corresponding chermo- dynamic constant.
Assuming high exchange capacity (C) of the solid and a negligible change of the composition due to the exchange with M the following relations would be valid
ZCMZ+J Ä • 3 )
ex Ö F J where D is the distribution ratio» CMz+J/fMz+J.
Thus log D = npH + nlog C + log k or log D = npH + const.
assuming constant C and f. At low pH the following pH-dependence would be expected:
o log D vs pH would have a slope of n, which is equal to or proportional to Z o the increase in sorpcion would shift towards lower pH if n or k increases, i.e. with the charge and/or the affinity for the adsorbent.
However, eqn. (6) is rarely fulfilled other than qualitatively because
o C is not high enough, o C is not pH-independent (variation of the surface charge), o f is not pH-independent, o competition with other cations, o the trace element is hydrolyzed above a certain pH.
A steep increase in the over-all sorption of cations is usually observed when pH increases up to and above the level where a significant hydrolysis starts. The state of the trace element changes with pH, and the exchange reaction can not be expressed in any simple form similar to eqn. (3). Moreover, pH in the immediate proximity of a charged surface can differ substantially from pH in the bulk of the solution, and different solute species may exist in the solution layer adjacent to the surface and in the bulk. This phenomenon could be expressed as an enhanced degree of hydrolysis on the surface and the net result would be equal to a precipitation process in the presence of the sorbent surface. The adsorption of hydrolyzed species usually reaches a maximum in the pH-range where neutral hydroxides would dominate, which indicate that the sorption can hardly be considered an ion exchange process. The formation of anionic hydroxy species at still higher pH would usually lead to a reduced sorption.
The theories concerning the sorption of hydrolyzed species on solid sor- bents and the various models describing reactions in the double layer, surface reactions etc. are numerous, and it is not within the scope of this report to discuss these in detail. It can be concluded, that ic is possible to roughly predict the pH where sorption starts, reaches a maximum and decreases. Calculations of the absolute magnitude of the distribution of a hydrolyzed species between a solid and an aqueous phase can rarely be accomplished without empirical data. Some limits are, however, set by solubility constants.
VA
Partition constant, fC the ratio of the activity of a given species in phase I to its activity in phase II with which it is in equili- brium.
I -»II
Comment: The distribution ratio (D) is an experimental parameter whose value varies with experimental conditions, and its value does not necessarily imply that partition equilibrium between the phases has been achieved. The ratio should normally be expressed as concentration in phase I divided by that in phase II.
The distribution constant (IC) is constant for one particular species under specified conditions only.
If the pure phasss are taken as standard states, 1C — total concentrations of dissolved materials decreases.
as the
Distribution isotherm (Synonym: sorption isotherm)
the relationship between the concen- trations of a solute in phase I and the corresponding concentration of the same solute in phase II at equilibrium with it at some specified temperature.
Reparation factor,OCA,B the ratio of the respective distribu- tion ratios (D) of two solutes measured under the same conditions.
( D (^) A) / ( (^) V
Loading capacity (Synonymus: saturation capacity, maximum loading).
the maximum concentration of a solute in phase I under certain specified conditions.
3ArCH MEASUREMENT TECHNIQUE
The technique discribed has been developed since K, measurements started at the department in 1976. It has been used for rock (granite, gneiss, diabase etc.)i pure minerals (over 30 different), clays, artificially prepared in- organic solids and concrete. In most cases with artificial groundwater, but also with brine, artificial seawater and concrete pore water solutions. A number of species have been studied.
I. Experimental
a. Crush and sieve the solid into desired fractions. Wet sieving is recom- mended for very small fractions (<150 um).
b. Weigh desired amount of solid in clean vial (a quality chat can be centrifuged).
c. Add liquid phase, note weight of vial and solid and liquid.
d. Shake vial to mix the phases, let stand to separate. If phase separation is slow, centrifuge.
e. Remove as much as possible of water, with solid intact. Add new water.
f. Shake to equilibrate solid-liquid (Id - Iw).
g. Centrifuge and repeat e.
h. Add species to be studied in small (but exact) volume of water.
i. Add species to "reference" vials with water but no solid.
j. Shake until sampling time.
k. Centrifuge - take sample. Compare with reference.
- If possible, put sample back after measurement.
Repeat j. - 1. until equilibrium is considered to be reached,
- Liquid/solid ratio
The influence of the liquid/solid ratio has been studied for Cs and Sr with a change in this ratio of a factor 100. This nas only given minor changes in K,. d
- Concentration of sorbing species
The concentration dependence has been studied for Cs over a range from 10
. to 10 M for granite, orthoclase and hornblende. In all cases a concentra- ™ tion dependence that can be fitted into a Freundlich-type equation, i.e. g » A'c , where g - concentration in solid phase at equilibrium, A and n are constants. The value of n ranges between 1.1 - 1.8. The values of A and n changes with time, probably due to the change in sorption mechanism from surface to volume reaction and the alteration of the solid phase. 5. pH-dependence
For all systems- studied, K, has shown to be pH-dependent. This is due to the influence the pH has on ion exchange processes and on hydrolysis.
CONCLUSIONS
,^^ To obtain reproducible "K." values the most important variables to keep controlled (and to specify when reporting data) are pH, concentration of the studied species and for redox-sensitive elements the redox potential.