Competitive adsorption is almost universally investigated in systems with multiple adsorbates competing for space on a single adsorbent. If the adsorption equilibrium constant for each adsorbate is independent of the adsorption density, this scenario leads to the classical “competitive Langmuir isotherm”. Frequently, however, experimental data suggest that the average binding strength of an adsorbent for an adsorbate declines as the adsorption density increases. Analysis of this scenario is far more complex and, historically, has been accomplished most effectively using the “ideal adsorbed solution theory (IAST)”. This model envisions that adsorption occurs not by binding of adsorbate molecules to individual surface sites, but rather by generation of a new fluid-like phase located between the solid and solution.<br/><br/>While the IAST model is useful, analyzing competitive adsorption in such cases within the site-binding paradigm has both conceptual and practical value. We analyzed adsorption of natural organic matter (NOM) onto two different adsorbents – heated aluminum oxide particles (HAOPs) and powdered activated carbon (PAC) – in systems with either one or both adsorbents present. NOM comprises a diverse group of molecules with a range of affinities for each adsorbent. A subset of those molecules contributes significantly to fouling of membranes in water treatment processes. The experiments had the surprising outcomes that, although progressive replacement of PAC with HAOPs in the adsorbent mixture led to progressively better removal of the NOM responsible for membrane fouling, the overall NOM removal efficiency peaked at intermediate mixtures of the two adsorbents and then declined as the mixture shifted toward either pure HAOPs or pure PAC (Figure 1). A conceptual and mathematical model that is based on the site-binding paradigm can explain these observations remarkably well and serves as an example of how such a model can yield insights into competitive adsorption in mixed adsorbate – mixed adsorbent systems.

Development and verification of a new model for competitive adsorption on multiple adsorbents

255th National Meeting (2018)

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