Rate of Humic Substance Uptake During Activated Carbon Adsorption

The adsorption‐diffusion rate of a humic acid and three molecular size fractions thereof by two particle size fractions of activated carbon was investigated over an extended time period. Long‐term removal of humic substances was found, which deviated significantly from the equilibrium state predicted by single‐domain diffusion models. During the initial stages, i.e. up to 50% uptake, both pore and surface diffusion models adequately simulate the experimental data. Use of the pore diffusion model results in apparent tortuosities on the order of 10–20, indicating a hindered diffusion mechanism $(D_P=0.85\times {10}^{-12}\mathrm{to}3.5\times {10}^{-12}{m}^2/s)\text{.}$ Both rate and equilibrium experiments, as well as analysis of the adsorbent pore structure, support a two‐domain diffusion mechanism. No desorption of humic acid was observed in response to reducing the solution concentration or the ionic strength.