Activated carbon adsorption of humic substances

The activated carbon isotherms of heterodisperse humic substances are analyzed with a widely accepted model developed for well-characterized polymer adsorption. The four humic substances examined included both fulvic and humic acids isolated from aquatic and terrestrial sources. The adsorption of the humic substances exhibits a dependence on the ratio of adsorbent mass to solution volume, resulting in a set of isotherms. Modifying the adsorption isotherm to explicitly account for the adsorbent dose results in a unique isotherm that relates the adsorbed mass to the unadsorbed mass expressed on a per unit adsorbent basis. A modified Freundlich equation was found to represent the adsorption behavior over a wide range of initial concentrations and adsorbent doses and for different adsorbents, ionic strengths, and temperatures. Reduction in the solution concentration did not result in measurable desorption of humic substances from the adsorbent surface. This was attributed to multiple-site adsorption and to the heterogeneous nature of both humic substances and activated carbon. Desorption was found to occur only at elevated pH values.