Understanding Sorption Processes: A Focus on Distribution Coefficients and pH Influence, Summaries of Chemistry

Download Understanding Sorption Processes: A Focus on Distribution Coefficients and pH Influence and more Summaries Chemistry in PDF only on Docsity! 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). 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. 3. 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. 4. 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. Formation of mixed crystals due to substitution reactions, replacements of ions in the lattice of a crystalline compound or interstitial incorporation of the microcomponent into the lattice are related phenomena. 5. Differentiation of sorption mechanisms It is evident, that the sorption of a trace component would be the result of several different processes. It is rarely possible to unambiguously differentiate the various mechanisms outlined above since many mechanisms would be involved simultaneously. Generally, the probability of physical adsorption and chemisorption increases with increasing degree of hydrolysis of e.g. a trace metal, whereas the probability of ion exchange decrease. Still there is always an element of electrostatic interaction in almost any sorption process. The role of hydroxy groups, both on the solid and on the solute species, make a distinction between physical adsorption, electrostatic adsorption and chemisorption somewhat arbitrary or superficial. In some systems the hydrogen ion displacement concept appears to give a better explanation.of observed phenomena, while the electric double layer concept would be more suitable under other conditions. The increasing tendency for sorption with increasing degree of hydrolysis, e.g. on solids with very low ion exchange capacity, may possibly be due to lower hydration of the hydrolyzed ions, causing an enhanced ion uptake or exchange. An increased degree of hydro- lysis would, however, stimulate the action of van der Waals forces or possibly permit the formation of hydrogen bonds between the hydrolyzed solute and electronegative atoms on the surface of the solid. Ås a conclusion, it can be stated that the over-all sorption of a soiute on a sorbent can rarely be defined in terms of one single well-defined mechanism, to 3trictly differentiate between various mechanisms is generally difficult, or even impossible, a gradual change from one predominant sorption mechanism in a system to another one is feasible, e.g. by the change of pH, there are a large number of physical and chemical parameters that would have significant influence on che over-all sorption of e.g. a trace element. 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. 1. 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 ex /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. (4) Thus log D = npH + nlog C + log k or log D = npH + const. 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,OC A,B the ratio of the respective distribu- tion ratios (D) of two solutes measured under the same conditions. (DA)/(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. 1. If possible, put sample back after measurement. Repeat j. - 1. until equilibrium is considered to be reached, 2. Comments to "1. Experimental" a. Fractions used: 45-63 urn, 63-90 um, 90-125 um, 125-250 um. Clays have not been fractionated. b. 50 ml glass bottles and 25 ml polyetene or polypropene vials used. Amount of solid used 0.2, 0.5, 1.0 g. c. Amount of liquid used 45 ml or 20 ml. C» d.-e. This is done to remove fines and particles chat may form pseudo- colloids. A good wet-sieving could make this unnecessary. h. The species may be added in the last water change. If radionuclides are used, adding of a small amount to each vial may be practical. i. The use of reference samples corrects for sorption on vial walls and for radioactive decay of the species during measurement. Precipitation of the species with a component in the water will however give results chat are difficult to interprete. DISCUSSION OF MEASURED DATA. FACTORS THAT INFLUENCE K.. d 1. Time dependence For cesium and 3trontium the sorption on four different rocks and a large number of minerals has been investigated. For both nuclides a fast rise in "K " is observed during the first 24h, the value has increased a little after a week, while it is almost constant after three months. For most systems a fairly stable K,-value is obtained in one week. 2. Temperature dependence The temperature dependence of the 3orption has been studied fot clay and granite with a number of sorb ing species. The influence of a temperature change from 25 to 65 C was minor.