Cation-exchange isotherms

By this method, the ion-exchange isotherms and selectivity coefficients can precisely be determined in a wide surface concentration range, which allows the construction of the ion-exchange isotherm and selectivity function, and the integration of the selectivity function (Chapter 1, Section 1.3.4.2.1, Equation 1.81). An example of a cation-exchange isotherm and isotherm parameters is shown in Figure 2.2 for the cation exchange of cobalt ions and calcium-montmorillonite. 

Schweich, D., Sardin, M., and Gaudet, J. P. (1983) Measurement of a Cation Exchange Isotherm from Elution Curves Obtained in a Soil Column Preliminary Results, Soil Sci. Soc. Am. J. 47, 32-37. 

In absence of any selectivity, the amount of each cation adsorbed in the cell wall must be exactly proportional to the amount of each cation present in the treatment solution. For example, the proportions of two bivalent cations adsorbed in the cell wall must be the same as in the equilibrium solution. In other words, the ion exchange isotherm must be diagonal. 

Chang, C. and Lenhoff, A.M., Comparison of protein adsorption isotherms and uptake rates in preparative cation-exchange materials, ]. Chromatogr. A, 827, 281, 1998. 

Sorption and desorption are usually modeled as one fully reversible process, although hystersis is sometimes observed. Four types of equations are commonly used to describe sorption/desorption processes Langmuir, Freundlich, overall and ion or cation exchange. The Langmuir isotherm model was developed for single layer adsorption and is based on the assumption that maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface, that the energy of adsorption is constant, and that there is no transmigration of adsorbate on the surface phase. 

A 0.5-gram mass of either the organo-treated or inorganic cation exchanged zeolite and 50 mL of 10 mM/L arsenate or chromate aqueous solutions were placed into Erlenmeyer flasks and mechanically shaken in reciprocating mode to attain equilibrium. Different equilibrium periods for individual zeolite modifications and both aqueous oxyanions species have been established. The adsorption isotherm experiments were conducted using above mass/ volume ratio of samples with an initial metal concentrations ranged from 0.5 to 100 mM/L at laboratory temperature. The... 

Competitive adsorption on sepiolite clay of a monovalent dye (e.g., methyl green or methyl blue) and of the divalent organo-cationic herbicides diquat and paraquat was studied by Rytwo et al. (2002). To evaluate a possible competitive adsorption between the two organic compounds, separate aqueous solutions of each cation were used and adsorption isotherms were obtained. Fig. 8.27 shows the amount of diquat, paraquat, and methyl green adsorbed on sepiolite as a function of total added divalent cation. It may be observed that, when the added amounts were lower than the cation exchange capacity of the sepiolite (O.Mmol kg ), aU cations were completely adsorbed. 

Na -loess clay, where batch experiments were analyzed by X-ray diffraction and infrared and far-infrared measurements. The adsorption isotherm (Fig. 8.36) shows that loess clay is selective for cesium cations. The raw material contained a large amount of quartz, and the clay material was a mixture of kaolinite and an interstrati-fied iUite-smectite mineral as a result, equilibrium Cs" adsorption data are not consistent with a single site Langmuir model. Cesium adsorption on this particular soil clay occurs by cation exchange on sites with various cesium affinities. At low concentration, far-infrared spechoscopy shows the presence of very selective adsorption sites that correspond to internal collapsed layers. At high concentration, Cs MAS-NMR shows that cesium essentially is adsorbed to external sites that are not very selective. 

The i29Xe chemical shift and the adsorption isotherm of xenon adsorbed on Y zeolites are dependent on the size, location and nature of cations in the zeolite intraframvork space. The variation of cation location in a partially cation-exchanged Na—Y can also be monitored by Na NMR. 

Figure 11.9 Sorption isotherms for some charged organic compounds interacting with natural solids (a) quinolinium cation on a subsoil of /oc = 0.024 and cation exchange capacity of 84 mmol/kg (Zachara et al., 1986), ( >) anilinium cation on a surface soil with /oc = 0.013 and cation exchange capacity of 112 mmol/kg (Lee et al., 1997), and (c) sorption of 4-(2,4-dichloro-phe-noxy)-butyrate anion on a sediment with/oc = 0.015 and unknown anion exchange capacity (Jafvert, 1990).

Box 11.2 General Derivation of Ion Exchange Isotherms for Cationic Organic Compound (/ = BH+). 

Figure 11.13 Adsorption isotherms for a series of alkyl ammonium compounds on sodium montmoril-lonite (adapted from Cowan and White. 1958). The horizontal dashed line indicates the cation exchange capacity of the clay.

Figure 11.15 Observed sorption of dodecylpyridinium on a soil (EPA-12) exhibiting an overall cation exchange capacity of 0.135 mol-kg"1. Two Langmuir isotherms (defined with particular values of C,s max and K/l, recall Eq. 9-5) are placed on the data to illustrate how different portions of the observed isotherm may reflect the influence of different materials in the complex soil sorbent or possibly different mechanisms (data from Brownawell et al., 1990).

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