Sorption Hysteresis of Benzene in Charcoal Particles
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- 作者：Washington J. Braida ; Joseph J.Pignatello ; Yuefeng Lu ; Peter I.Ravikovitch ; Alexander V.Neimark ; and Baoshan Xing
- 刊名：Environmental Science & Technology
- 出版年：2003
- 出版时间：January 15, 2003
- 年：2003
- 卷：37
- 期：2
- 页码：409 - 417
- 全文大小：196K
- 年卷期：v.37,no.2(January 15, 2003)
- ISSN：1520-5851
文摘
Charcoal is found in water, soil, and sediment where itmay act as a sorbent of organic pollutants. The sorptionof organic compounds to natural solids often shows hysteresis.The purpose of this study was to determine the sourceof pronounced hysteresis that we found in the sorption ofa hydrophobic compound (benzene) in water to a maple-wood charcoal prepared by oxygen-limited pyrolysis at 673K. Gas adsorption (N2, Ar, CO2), 13C NMR, and FTIR showthe charcoal to be a microporous solid composed primarilyof elemental (aromatic) C and secondarily of carboxyland phenolic C. Nonlocal density functional theory (N2, Ar)and Monte Carlo (CO2) calculations reveal a porosity of0.15 cm3/g, specific surface area of 400 m2/g, and appreciableporosity in ultramicropores <10 Å. Benzene sorption-desorption conditions were chosen to eliminate artificialcauses of hysteresis (rate-limiting diffusion, degradation,colloids effect). Charcoal sorbed up to its own weight ofbenzene at ~69% of benzene water solubility. Sorptionwas highly irreversible over most of the range tested(10-4-103 
g/mL). A dimensionless irreversibility index(Ii) (0 
Ii 1) based on local slopes of adsorption anddesorption branches was evaluated at numerous placesalong the isotherm. Ii decreases as C increases, from0.9-1 at low concentration to ~0 (~fully reversible) atthe highest concentrations. Using sedimentation andvolumetric displacement measurements, benzene is observedto cause pronounced swelling (up to >2-fold) of thecharcoal particles. It is proposed that hysteresis is due topore deformation by the solute, which results in thepathway of sorption being different than the pathway ofdesorption and which leads to entrapment of some adsorbateas the polyaromatic scaffold collapses during desorption.It is suggested that intra-charcoal mass transport maybe influenced by structural rearrangement of the solid, inaddition to molecular diffusion.
g/mL). A dimensionless irreversibility index(Ii) (0 Ii 1) based on local slopes of adsorption anddesorption branches was evaluated at numerous placesalong the isotherm. Ii decreases as C increases, from0.9-1 at low concentration to ~0 (~fully reversible) atthe highest concentrations. Using sedimentation andvolumetric displacement measurements, benzene is observedto cause pronounced swelling (up to >2-fold) of thecharcoal particles. It is proposed that hysteresis is due topore deformation by the solute, which results in thepathway of sorption being different than the pathway ofdesorption and which leads to entrapment of some adsorbateas the polyaromatic scaffold collapses during desorption.It is suggested that intra-charcoal mass transport maybe influenced by structural rearrangement of the solid, inaddition to molecular diffusion.