Chlorite exchange capacity

The data are "normalized" with regard to the ion exchange capacity C of the sorbents. The sorption curves of the illite and of the < 40-pm chlorite are strongly non-linear, whereas that of the montmorillonite approaches linearity. 

Quite often Al, Fe, and Mg hydroxides partially fill the interlayer position of the derived vermiculites and decrease their exchange capacity and their ability to contract completely to 10 A when heated or when treated with a potassium solution. This material can usually be removed by treating the clay with a solution of sodium citrate (Tamura,1958). As the content of hydroxy interlayer material increases, the expandable clay tends to assume the character of a chlorite. Thus, in the weathering of a mica or illite it is not uncommon to form discrete vermiculite-like, beidellite-like, monf-morillonite-like and chlorite-like layers. These various layers can occur as discrete packets or interstratified in a wide variety of proportions. 

Initially in this study, it was planned to critically evaluate AG data for complex clays, including chlorite, illite, and the smectites. However, there is much evidence that these clays dissolve Incongruently so that the apparent equilibria in solution are determined by secondary phases, such as gibbsite, boehmite, amorphous silica, and ferric oxyhydroxldes. The smectites are frequently the dominant clays in the colloidal size fraction in natural sediments. They have very large exchange capacities, and exhibit wide chemical variations. Usually, one or more of these factors have not been considered in the experimental solubility work. Even if appropriate corrections could be made, it is uncertain whether a AG value so obtained would have applicability to natural systems. 

There are inconsistencies in the model for the calculation of activity products for the "clays. Exchangeable cations are disregarded for the low exchange capacity kaolinite, halloysite, chlorite, and moderate capacity illite. For certain expansible layer silicates and two zeolites, the logjo of the activity of selected cations is added into the sum of the activity products. 

Of special significance with respect to their properties as sorbents are the clay minerals (e.g. kaolinite, montmorillonite, vermiculite, illite, chlorite), mainly due to their high exchange capacity. 

Chlorites have rather low specific surface areas and cation exchange capacities (see Chapter 3, section 3.2), a result of the blockage of the interlayer regions by hydroxy sheets, and do not expand at all in water. Exchangeable cations are likely to be found only on external surfaces and edges of chlorite particles. 

Chemical Properties. An important chemical property of clays, which directly affects fines migration is the cation exchange capacity (CEC) (6-9). CEC is a measure of the capacity of a clay to exchange cations. It is usually reported in units of milliequivalents per 100 g of clay (meq/100 g). The CEC depends on the concentration of exchangeable cations in the diffuse Gouy-Chapman layer (see later). This concentration depends on the total particle charge, which may vary with pH. Unless stated otherwise, the reported values of CEC are measured at neutral pH. CEC values (meq/lOOg) of common clay minerals are as follows smectites, 80-150 vermiculites, 120-200 illites, 10-40 kaolinite, 1-10 and chlorite, <10 (10). 

With these reservations in mind, the approximate range of cation exchange capacities of the vermiculite group can be written as 120 to 200 meq/100 g air-dry Mg-vermiculite. A more satisfactory basis would be to record the exchange capacity as meq/100 g interlayer-water-free and interlayer-cation-free mineral (Walker [1965]), and on this basis, the vermiculites range approximately from 140 to 240 meq. Values of cation exchange capacity below those quoted above have been reported, but their validity is in considerable doubt. These low values have invariably been obtained, not from pure vermicuUtes, but from mixed-layer minerals such as hydrobiotite or chlorite-vermiculite, a correction being applied for the proportion of non-vermiculite layers estimated to be present. The error involved in such corrections is considerable. 

Over the past 5 years, Metrohm (Herisau, Switzerland) also developed a number of surface-aminated PS/DVB-based resins. Metrosep A Supp 1 is a medium-capacity universal anion exchanger with a special selectivity in that bromide elutes behind nitrate. It is predominantly used both for anion analysis in samples with large concentration differences and for separating oxyhalide ions such as bromate, chlorite, and chlorate. 

In 1999, Wagner et al. [61] developed another derivatization technique for the analysis of bromate, which can be directly combined with the US EPA Method 300.1 [62], In Part A, Method 300.1 describes the separation of the seven standard anions bromide, chloride, fluoride, nitrite, nitrate, orthophosphate, and sulfate, and in Part B the separation of bromate, bromide, chloride, and chlorite on an lonPac AS9-HC high-capacity anion exchanger. The technique developed by Wagner et al. is based on the reaction of bromate with o-dianisidine (ODA) in an acidic solution [63,64] with subsequent photometry of the reaction product at 540 nm. 

Figure 10.28 Separation of chlorite, bromate, bromide, and chlorate at trace levels in reagent water and spiked tap water using ERA Method 317.0 using a high-capacity carbon-ate-seiective anion exchanger. Separator coiumn lonPac AS9-HC with guard coiumn dimensions 250mm x4 mm i.d. eiuent 9mmoi/L Na2C03 flow rate 1.3ml7min detection suppressed conductivity and...

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