Modeling the dynamic adsorption desorption of a NOM mixture: Effects of physical and chemical heterogeneity

Because natural organic matter (NOM) can act as a carrier for contaminants, it is of great importance to understand its dynamic adsorption/desorption behavior. NOM is a mixture of organic molecules that vary both in chemical and physical properties. The adsorption/desorption behavior of a NOM mixture to the solid matrix cannot be adequately described using simple equilibrium sorption isotherms, like the Langmuir isotherm. Often adsorption/ desorption hysteresis is found or "adsorption maxima" keep increasing slowly. In this paper a relatively simple model is developed to describe the kinetic adsorption/desorption of NOM. The model is calibrated using experimental data. Model simulations of experimental data show that adsorption/ desorption hysteresis is an inherent property of a heterogeneous mixture of molecules. The composition and thus the properties of the mixture vary with the total amount of NOM added and with surface/volume ratios. Therefore the relationship between the overall adsorption and the overall solution concentration is nonunique and thus a nonthermodynamic isotherm. Although adsorption in terms of mass of carbon seems to reach equilibrium relatively fast, the distribution of individual components can still be far from equilibrium. This indicates that the composition of the NOM mixture may vary with time as well.