EXPERIMENTAL EVALUATION OF Ca-K EXCHANGE SELECTIVITY REACTIONS IN ESTUARINE SEDIMENTS
نویسندگان
چکیده
~Clay materials separated from estuarine sediments in Louisiana exhibit a selectivity for exchangeable calcium in laboratory experiments. The studies were conducted at 25~ and 60~ in mixed calcium and potassium chloride solutions in which the equivalent fraction of potassium varied from 0.12 to 0.54. Calcium selectivity was observed to increase with an increase in the equivalent fraction of potassium in the external solution and the temperature. The results suggest the possible importance of ion exchange reactions in the regulation of calcium availability during early diagenetic reactions in sediments. I N T R O D U C T I O N The selective adsorption of cations by the exchange complex of soils and sediments has an important bearing on their physical and chemical properties. It is one of the processes normally operating in clay materials which is directly dependent on the mineralogy and distribution of charge deficiencies within the exchanger. A complete understanding of the variables influencing ion selectivity reactions might be useful in interpreting the chemical changes involved in the diagenesis and weathering of clay minerals. Selective cation adsorption has been related to solution concentration, pH, temperature, ion size, ion valence, ion hydration (Wiklander, 1964) and the structure and charge properties of the exchange substrate (Dolcaster et al., 1968). Rich and Black (1964) and Bolt et al., (1963)also related the selective adsorption of potassium by soil materials to the characteristics of the adsorbing surfaces. Kozak and Huang (1971) demonstrated the importance of hydroxy-aluminum interlayers in controlling potassium/ calcium selectivity of micaceous materials. Ion exchange selectivity also has been detected in laboratory studies of the changes in river sediments in contact with sea water. The equivalent fraction of exchangeable potassium (Q~/Qo) in Missouri River clays increased from 0.016 to 0.019 after 36 hr and reached 0.032 after 86 hr of exposure to sea water. (Data from Potts, in Keller, 1963). Values reported by Russell (1970) suggest a decrease in QK/Qo from 0.15 to 0.061 after two weeks of contact between sea water and Rio Ameca sediments. The experimental results yielded a value which was very close to the 0,065 ratio detected in the marine sediments of the area. Russell also detected an increase in the amount of fixed potassium and a corresponding decrease in total exchange capacity which helped to account for the changes in the potassium distribution. * Visiting Lecturer, 1975-76, University of Sheffield. 26 Changes in the ion exchange selectivity characteristics of the exchange substrate could produce changes of the type described above. Increasing solution concentration and decreasing exchange capacity (Wiklander, 1964) also would favour the adsorption of potassium. A number of other variables including temperature changes might modify the distribution of exchangeable ions and influence the early changes accompanying the diagenesis of clay mineral-rich sediments. This study reports the results of a laboratory investigation of the ion selectivity characteristics of the clay sized fraction of estuarine sediments from Louisiana. The most important variables studied were changes in the calcium to potassium content of the solutions in contact with the sediments and the temperature of reaction. The original clay materials are also slightly different in composition. M A T E R I A L S AND M E T H O D S Modem sediment samples were collected from five locations in Louisiana estuaries, whose approximate locations are indicated in Figure 1. The Big Island and Bay St. Denis sites are in areas where the average salinity of the overlying waters varies from 18 to 30 ppt. Lower salinities are usually present in Lake
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بررسی تعادل پتاسیم - کلسیم در برخی از خاک های آهکی خراسان
The presence of large amounts of calcium in the equilibrium system of calcareous soils of arid and semi-arid regions affects the chemical reactions of soluble and exchangeable phases. Exchange phenomenon is one of these chemical reactions in which calcium plays an important role. K-Ca equilibria are important equilibria in calcareous soils and may be an answer to many problems in soil chemistry...
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