Alkaline earth ions distribution coefficients

Because distribution coefficients are often dependent on pH, buffers were used. The pH of 5 was selected.for most of the experiments because montmorillonite is stable at this acidity, interference of observations from hydrolysis and from precipitation of alkaline earth carbonates is precluded, and adsorption on possible hydrous oxide impurities is minimized. An acetate buffer was usually used to maintain pH because acetate does not have a strong tendency to complex many ions. 

These three-layer silicates are characterized by permanent surface charges (due to isomorphic substitutions). Therefore the binding of cations is assumed to be caused by stoichiometric ion exchange of interlayer ions. These concepts hold well for alkaline and earth-alkaline cations their adsorption and their ionic strength dependence can be characterized by distribution coefficients derived from ion exchange theory. It has been known for some time, however, that... 

What is the simplest adsorption model that can adequately define sorption in the system of interest for purposes of our study The simpler the model, the less information is needed to parameterize it. The distribution coefficient model requires only entry of the mass of sorbent in contact with a volume of water and a value for K,. Pesticide adsorption can often be modeled adequately using a simple K ) approach (cf. Lyman et al. 1982). For smectite and ver-miculite clays and zeolites that have dominantly pH-independent surface charge, ion-exchange or power-exchange models may accurately reproduce adsorption of the alkaline earths and alkali metals. If the system of interest experiences a wide range of pH and solution concentrations, and adsorption is of multivalent species by metal oxyhydroxides, then an electrostatic model may be most appropriate. 

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