Relationship between the highest occupied molecular orbital (HOMO) electron density of adsorption sites on carbon and the Freundlich exponent Original Research Article

This paper discusses the relationship between the electron density for various groups of phenolic compounds and their adsorption capacity on activated carbon. Chloro- and nitrophenols were used as test adsorbates, and as-received or HNO3-modified granular activated carbon (GAC) samples were used as adsorbents. The isotherms for these systems were determined by the batch bottle technique. The highest occupied molecular orbital (HOMO) electron density at the adsorption site was estimated using the CAChE program, and a relationship between the Freundlich exponent 1/n and the HOMO density of the adsorbent was found by combining the experimental and computational results for the modified GAC. The Freundlich equation is described as: nads=k(C)1/n, where nads, k, C, 1/n are the weight of adsorbate per weight of adsorbent (g g−1), the Freundlich coefficient, the concentration of adsorbate in bulk solution (g l−1), and the Freundlich exponent, respectively. The value of the Freundlich exponent 1/n for the systems investigated decreased linearly with an increase in ‘adsorbent’ HOMO density. The HOMO electron density of both the adsorbate and the adsorbent were the major factors determining the value of the Freundlich exponent, 1/n, for phenolic adsorbate–carbonaceous adsorbent systems.