Negative adsorption of co-ions

In Figure 9.11, the expulsion of co-ions is depicted for a Gouy-Chapman-Stern double layer. Let c+ be the concentration of co-ions at a positively charged surface. In a Gouy-Chapman-Stem double layer, the co-ion expulsion per unit surface area, that is, the negative adsorption of co-ions F+ is given by... 

V.1. The negative adsorption of co ions > determines the excluded-volume per grain... 

The area is an important surface parameter for catalytic studies. It is needed to evaluate the rate constant of the surface reaction from the kinetics as well as to allow a fair comparison to be made of the effectiveness of different catalysts. Areas are commonly determined by nitrogen or krypton gas adsorption interpreted by the Brunauer-Emmett Teller (BET) isotherm [30, 32], A number of other methods has been proposed and utilised including microscopy, isotopic exchange, chromatography, gas permeability, adsorption from solution, and negative adsorption (desorption) of co-ions [30, 33]. 

Thermodynamically it is the total charge balance of the adsorbed ion and co-adsorption of counter ions. The value can be obtained by independent measurements of adsorbed charge and adsorbed mass. The charge is usually determined by chronoamperometry and integration of potential scans either in the negative direction (adsorption) or the positive one (desorption and anodic stripping method). The latter method is usually preferred because the adsorption film reaches a stable condition. The mass can be determined by several methods. The experimental techniques and examples will be described in Section 4.2. 

The Berea sandstone had been split into clay and quartz fractions, but the Berea whole rock was still more negative relative to the other core listed for this study. Even though the trend was the same for both brines, the divalent cations in the 2.1% TDS brine produced less negatively charged surfaces than did the NaCl brine. This behavior was attributed to adsorption of these ions into the Stem layer or, in the case of earbonates, to preferential dissolution of CO over Ca or Mg in the presenee of excess divalent cations in the aqueous phase. It was also noted that adsorption of metal hydroxide ions or mineral transformation reaetions at the solid surface may play a role. 

There are several variations to this technique. The negative adsorption method is based on the exclusion of co-ions from the electrical double layer surrounding charged particles [76]. 

The bicarbonate ion, HC03, is a prevalent species in natural waters, ranging in concentrations up to 0.8 X 10 3. As was indicated previously, carbonate ions have the ability to form complexes with plutonium. Starik (39) mentions that, in an investigation of the adsorption of uranium, there was a decrease in the adsorption after reaching a maximum, which was explained by the formation of negative carbonate complexes. Kurbatov and co-workers (20) found that increasing the bicarbonate ion concentration in a UXi (thorium) solution decreased the amount of thorium which formed a colloid and became filterable. This again was believed to be caused by the formation of a soluble complex with the bicarbonate. 

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