Insights and Model for Understanding Natural Organic Matter Adsorption onto Mixed Adsorbents

Adsorption-based processes are commonly used to remove natural organic matter (NOM) from drinking water sources and thereby mitigate its impacts on other water treatment processes and the quality of the finished water. These processes are complicated by the fact that NOM comprises multiple fractions that can exhibit disparate adsorption behaviors. Prior modeling of NOM adsorption has invariably focused on systems with a single adsorbent, but results presented here demonstrate surprising and counterintuitive behavior in systems containing two or more adsorbents. Specifically, if the sequence of affinities of the different NOM fractions are the same for two adsorbents, then overall adsorption changes monotonically as one adsorbent is gradually replaced by the other. However, if the sequence differs for the two adsorbents, overall adsorption can increase even when the nominally stronger adsorbent is gradually replaced by the weaker one. This work demonstrates and explains such behavior for a particular mixture of adsorbents and introduces a mathematical model that illustrates how other mixtures might behave.