C Taiwei | PubMed Health

PubMed2000

On the Application of Surface Complexation Models to Ionic Adsorption.

Surface complexation models (SCMs) have been used extensively to describe the acid-base properties and adsorption behaviors of inorganic and organic cations and anions on many single-phase adsorbents. The success of SCMs results from the conformation of adsorption to mass-action and material-balance laws and the flexibility in adjustable parameters. However, the assumptions made in the derivation of the SCMs are really not justified and the SCMs are not so thoroughly grounded in fundamental chemistry. The SCMs are really the data-fitting model, rather than the chemical model. The premises of application of SCMs are the accurate identification of surface species and the low surface coverage. Though the application of the SCMs comes into vogue, the SCMs should be applied with caution. Copyright 2000 Academic Press.

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PubMed2000

On the Applicability of the Langmuir Equation to Estimation of Adsorption Equilibrium Constants on a Powdered Solid from Aqueous Solution.

The Langmuir adsorption equation has become popular for summarizing solution-solid adsorption data and estimating adsorption equilibrium constants. The applicability of the Langmuir equation to estimation of adsorption equilibrium constants at solid-aqueous solution interfaces was theoretically examined in terms of (1) comparisons of solid-solution adsorption with solid-gas adsorption, (2) deductions of the Langmuir mathematical expression for the nonelectrolyte/solvent exchange, the phase exchange, the ion exchange, and the simple association reactions, and (3) the fitting procedure. The restrictive assumptions underlying the Langmuir equation for solid-gas adsorption may not be valid for adsorption of solute of small molecules or ions by a powdered solid surface from the aqueous solution phase. The fit of adsorption data to the Langmuir mathematical expression cannot be ascertained solely by the linearity of the commonly used equilibrium concentration/amount adsorbed versus the equilibrium concentration plot and does not provide the actual mechanism of the loss of small molecules or ions from aqueous solution. Therefore, the parameters of fitting to the Langmuir mathematical expression may not provide an unambiguous chemical meaning. In addition, we attempt to bring attention to the importance of the standard states of surface phase and solution phase. Copyright 2000 Academic Press.

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