Sorption hysteresis of benzene in charcoal particles

Charcoal is found in water, soil, and sediment where it may act as a sorbent of organic pollutants. The sorption of organic compounds to natural solids often shows hysteresis. The purpose of this study was to determine the source of pronounced hysteresis that we found in the sorption of a hydrophobic compound (benzene) in water to a maplewood charcoal prepared by oxygen-limited pyrolysis at 673 K. Gas adsorption (N₂, Ar, CO₂), ¹³C NMR, and FTIR show the charcoal to be a microporous solid composed primarily of elemental (aromatic) C and secondarily of carboxyl and phenolic C. Nonlocal density functional theory (N₂, Ar) and Monte Carlo (CO₂) calculations reveal a porosity of 0.15 cm³/g, specific surface area of 400 m²/g, and appreciable porosity in ultramicropores < 10 Å. Benzene sorption-desorption conditions were chosen to eliminate artificial causes of hysteresis (rate-limiting diffusion, degradation, colloids effect). Charcoal sorbed up to its own weight of benzene at ∼69% of benzene water solubility. Sorption was highly irreversible over most of the range tested (10^-4-10³ μg/mL). A dimensionless irreversibility index (I_i) (0 ≤ I_i ≤ 1) based on local slopes of adsorption and desorption branches was evaluated at numerous places along the isotherm. I_i decreases as C increases, from 0.9-1 at low concentration to ∼0 (∼fully reversible) at the highest concentrations. Using sedimentation and volumetric displacement measurements, benzene is observed to cause pronounced swelling (up to > 2-fold) of the charcoal particles. It is proposed that hysteresis is due to pore deformation by the solute, which results in the pathway of sorption being different than the pathway of desorption and which leads to entrapment of some adsorbate as the polyaromatic scaffold collapses during desorption. It is suggested that intra-charcoal mass transport may be influenced by structural rearrangement of the solid, in addition to molecular diffusion.