Electrolyte sorption

To be useful in modeling electrolyte sorption, a theory needs to describe hydrolysis and the mineral surface, account for electrical charge there, and provide for mass balance on the sorbing sites. In addition, an internally consistent and sufficiently broad database of sorption reactions should accompany the theory. Of the approaches available, a class known as surface complexation models (e.g., Adamson, 1976 Stumm, 1992) reflect such an ideal most closely. This class includes the double layer model (also known as the diffuse layer model) and the triple layer model (e.g., Westall and Hohl, 1980 Sverjensky, 1993). 

The application of ion-exchange dynamics for the determination of HETP or HTU values should be studied in ion-exchange systems characterized by invariable static and kinetic parameters such as ion-exchanger swelling, electrolyte sorption, separation coefficient, and interdiffusion coefficients in solution and resin phases over experimentally investigated ranges of component variation. 

Lichens are adapted to accumulate all the elements necessary for their life from the atmosphere. They have no root system and absorb very little from the substrate on which they grow. Atmospheric materials, including trace metals and radionuclides, can be concentrated by particulate entrapment, ion exchange, electrolytic sorption and processes mediated by metabolic energy (Crete et al., 1992). Passive particulate trapping is, however, thought to be the dominant uptake mechanism. This is also true for mosses which absorb nutrients directly through leaf and stem surfaces. 

Electrolyte Sorption. In fact an ion exchanger does not totally exclude the sorption of co-ions from an external electrolyte solution, and therefore any ideal model has to account for all other permeant species over and above the exchange of counter-ions. 

Fig[ure 5.4 Non-exchange electrolyte sorption of NaCl and HCl by styrenk anion and cation exchange resins respectively against external concentration [m = internal molality A = macroporous SB A (low capacity) B = macroporous SBA (conventional) C = isoporous SBA (gel) D = macroporous SAC E = gel SAC]... 

Current surface complexation models were developed with a focus on minor and trace ions and hence do not consider sorption in the diffuse layer. Even the triple-layer model (34), which can include electrolyte sorption as outer-sphere complexes, does not consider sorption in the diffuse layer. To... 

For the application of these membranes to the electrolytic production of chlorine-caustic, other performance characteristics in addition to membrane conductivity are of interest. The sodium ion transport number, in moles Na+ per Faraday of passed current, establishes the cathode current efficiency of the membrane cell. Also the water transport number, expressed as moles of water transported to the NaOH catholyte per Faraday, affects the concentration of caustic produced in the cell. Sodium ion and water transport numbers have been simultaneously determined for several Nafion membranes in concentrated NaCl and NaOH solution environments and elevated temperatures (30-32). Experiments were conducted at high membrane current densities (2-4 kA m 2) to duplicate industrial conditions. Results of some of these experiments are shown in Figure 8, in which sodium ion transport number is plotted vs NaOH catholyte concentration for 1100 EW, 1150 EW, and Nafion 295 membranes (30,31). For the first two membranes, tjja+ decreases with increasing NaOH concentration, as would be expected due to increasing electrolyte sorption into the polymer, it has been found that uptake of NaOH into these membranes does occur, but the relative amount of sorption remains relatively constant as solution concentration increases (23,33) Membrane water sorption decreases significantly over the same concentration range however, and so the ratio of sodium ion to water steadily increases. Mauritz and co-workers propose that a tunneling process of the form... 

Figure 7. The electrolyte sorption of Nafion during the charge and discharge cycles of HBr cell under various initial conditions (50). Key +, presoaked in H20, operation started with charge Q, presoaked in 45% HBr, started with charge A, presoaked in H20, started with discharge X, presoaked in 7 % HBr, started with discharge —, steady-state operation.

One of the reasons for local corrosion at the metal-polymer interface is sorption of electrolytes by polymers and permeability of the polymer barrier towards electrolytes. Sorption of electrolytes (acid solutions, bases and salts) leads to essential variation in the service characteristics of the protecting polymer coatings and anticorrosion packaging films under mechanical loads. These variations under mechanical loads, especially in seals and friction joints, are much deeper and can affect mechanisms of contact interactions. 

Sorption characteristics for these films in concentrated NaOH solution at 80 C are listed in Table IV. The concentration of Na" " is seen to rise abruptly in these environments, due to the sorption of electrolyte solution. This electrolyte sorption is also reflected in the mole ratio of water to sodium ion in the polymers. This ratio is quite generally found to be smaller in the polymer phase than in solution, even in cases where a large fraction of sodium ion content is due to electrolyte sorption (14). This can be ascribed to the lower dielectric constant and less aqueous character of the polymer phase. However, in Table IV it is seen that the ratios closely approach those of the solution phase for these materials. This also suggests that microporosity is a prominent feature of these aromatic carboxylate films, as reflected in the highly solution-like environment which these ratios indicate. 

Following electrolyte sorption, CO2 typically undergoes competitive reactions that advance simultaneously on the electrode surface, yielding different products, with a product distribution primarily determined by (i) the identity of the electrode and (ii) the composition of the electrolyte (aqueous or organic, with various possible salts and additives). 

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