Cation exchange process

Monovalent cations are good deflocculants for clay—water sHps and produce deflocculation by a cation exchange process, eg, Na" for Ca ". Low molecular weight polymer electrolytes and polyelectrolytes such as ammonium salts (see Ammonium compounds) are also good deflocculants for polar Hquids. Acids and bases can be used to control pH, surface charge, and the interparticle forces in most oxide ceramic—water suspensions. 

The Multi-Purpose Processing Facility was installed in F Canyon (reprocessing plant) at SRP for separation of Californium and trans-californium elements using newly developed, high-pressure, chromatographic cation exchange processes. 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

For crown ethers in which the H-nmr probes Ha and Hb are diastereotopic in the free crown ether too, the cation exchange process (50) becomes more complicated. For 1 1 complexes the measured free energy of activation also involves conformational inversion components (AG = AG + AG ), whereas exchange between free and complexed crown ether (2 1 ratio of crown ether to salt) only involves the free energy of activation for decomplexation (AG ). Bradshaw et al. (1979b) have recently reported AG -values for cation exchange in RNH3X complexes of crown ethers [257]. The AGf term was estimated to be 0.7, 0.5, and 1.2 kcal mol-1 for compounds [257 n = 1] with... 

Sutherland and co-workers have studied the cation-exchange processes in alkylammonium complexes of crown ethers [258]—[262], Depending on the position of the exchanging H-nmr sites, the measured zlG -values may refer to three different types of cation exchange (0 cation exchange between free and... 

Campbell has studied the separation of technetium by extraction with tributyl phosphate from a mixture of fission products cooled for 200 days. Nearly complete separation of pertechnetate is achieved by extraction from 2 N sulfuric acid using a 45 % solution of tributyl phosphate in kerosene. Ruthenium interferes with the separation and is difficult to remove without loss of technetium other radioisotopes can be removed by a cation-exchange process. However, this separation procedure has not been widely applied because of the adverse influence of nitrate. 

Contaminant adsorption includes retention on the porous medium solid phase, as a result of cation exchange processes, and surface retention of neutral molecules, due to van der Waals forces. 

The large mass difference relative to the mass of the element between Li and Li of abont 16% is a favorable condition for their fractionation in nature. Taylor and Urey (1938) fonnd a change of 25% in the Li-isotope ratio when Li-solutions percolate throngh a zeolite colnmn. Thns, fractionation of Li-isotopes might be expected in geochemical settings in which cation exchange processes are involved. 

X and y represent the number of charges of competing B and A ions (sign omitted since the equation represents both anion and cation exchange process), respectively... 

Step 7. H-saturate fulvic acid fraction by immediately passing 0.1 N NaOH eluate through cation-exchange resin in H-form. Continue cation-exchange process until final concentration of Na+ is less than 0.1 ppm. 

The separation of rare earths by anion exchange resins depends on somewhat different factors for a given chelating agent than to the cation exchange resins. For EDTA in an anion exchange process the order of elution is not the inverse order of the stability constants as in cation exchange process, rather [79] 4... 

In the first case we speak of anion-exchange, since the exchanged ions U and X are negatively charged. The second reaction illustrates a cation-exchange process. It is clear that different stationary phases will typically be used for the two types of IEC. Thus, there... 

A continuing problem with the cation exchange process as used in production operations is that it has not been sufficiently selective and therefore allows considerable carryover of the MSE salt constituents and impurities with the plutonium and americium. This isn t serious with plutonium since plutonium can be subsequently purified by anion exchange. For americium, however, the subsequent recovery process is oxalate precipitation which is less selective and carries some of the impurities into the final product. 

This work demonstrates that considerable improvement in the cation exchange process can be made by incorporating the following changes into the process procedures ... 

Cation exchange work now in progress comprises evaluation of the optimized cation exchange process with the BioRad AG MP-50 macroporous resin that is now a replacement candidate for the Dowex 50W-X8 gel-type resin. Other work being considered includes use of HC1 wash solutions prior to conversion to the nitrate system and possible use of chelating agents. 

Recovery of actinides at the RFP with an organic phosphorous bidentate extractant has been proposed. A conceptual production flow sheet is shown in Figure 3. The bidentate reagent, dihexyl-N, N-diethylcarbamoylmethylenephosphonate (DHDECMP), is especially attractive since it can recover actinides from MSE residues containing aluminum. The cation exchange process is unable to effect actinide purification when aluminum is present. (DHDECMP extracts actinides and lanthanides, but does not extract common RFP contaminants, e.g., aluminum. No lanthanides are used in process streams at RFP.)... 

The cation exchange process, however, could not be piloted with nonradioactive chemicals. Instead, each separate batch was... 

As seen previously, the acidity depends on the interlayer cation, and so catalytic activity can be affected by cation-exchange processes. In addition, when the interlayer cations, or even the cations of the octahedral sheet (e.g., Fe2+ or Fe3+), have different oxidation states, clay minerals can catalyze redox reactions, too. 

When studying the surface acid-base properties of montmorillonite, it is essential to understand that hydrogen ions and cations of the support electrolyte can also participate in cation-exchange processes. The processes on the internal and external surfaces have to be taken into consideration simultaneously, and they both have to be included into the equilibrium thermodynamical models. 

Hydrogen ions participate in the cation-exchange processes of the interlayer space. As will be seen later (Section 2.7.1), they have a very large affinity for the layer charge. Hydrogen and hydroxide ions are potential-determining ions of the external surfaces via the protonation and deprotonation processes of aluminol and silanol sites. In acidic media, the degradation of aluminosilicates can be observed. 

Since the cation-exchange processes of the main cations (sodium, potassium, magnesium, calcium) have a significant role in the nutrient cycle of soils, the classical literature has discussed their cation-exchange processes in detail (e.g., Boyd et al. 1947 Gaines and Thomas 1953 Howery and Thomas 1965 Sposito 1981 Filep 1999). As described earlier, cation exchange is characterized by the selectivity coefficients and... 

---