Characterization of Charcoal Adsorption Sites for Aromatic Compounds:  Insights Drawn from Single-Solute and Bi-Solute Competitive Experiments

Charcoal, the residue of incomplete biomass burning that is found in many soils and sediments, is considered a high affinity sorbent for organic pollutants. However, little is known about the microscopic processes controlling sorption. The purpose of this study was to gain molecular-scale insight in...

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Bibliographic Details
Published in:Environmental science & technology 2005-03, Vol.39 (6), p.1606-1615
Main Authors: Sander, Michael, Pignatello, Joseph J
Format: Article
Language:English
Subjects:
Adsorbents
Adsorption
Applied sciences
Benzene - chemistry
Calibration
Carbon
Charcoal
Charcoal - chemistry
Chemistry
Electrons
Environmental Pollution - prevention & control
Exact sciences and technology
Experiments
General and physical chemistry
Global environmental pollution
Hydrocarbons
Models, Theoretical
Nitrobenzenes - chemistry
Pollution
Surface physical chemistry
Toluene - chemistry
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Summary:Charcoal, the residue of incomplete biomass burning that is found in many soils and sediments, is considered a high affinity sorbent for organic pollutants. However, little is known about the microscopic processes controlling sorption. The purpose of this study was to gain molecular-scale insight into the sorption on a charcoal of three weakly soluble aromatic compounds [benzene (BEN), toluene (TOL), and nitrobenzene (NBZ)] by conducting both single-solute and bi-solute experiments. The charcoal (420 m2 g-1) was produced from maple wood shavings by oxygen-limited pyrolysis at 673 K. Solute affinity for charcoal followed the order NBZ > TOL > BEN. Commonly employed sorption models did not adequately describe the single-solute isotherms. Competition in both TOL−BEN and the TOL−NBZ bi-solute systems was strong. Normalization of the isotherms for the hydrophobic driving force by using an existing free energy correlation between sorption and partitioning to an inert solvent (benzene or n-hexadecane) with a nonpolar aromatic compound calibration set resulted in a finding of enhanced sorption of NBZ relative to the coalesced BEN and TOL isotherms, indicating some specificity in the interaction of NBZ. The competitive data indicated 1:1 molar competition between BEN and TOL and between NBZ and TOL, showing conclusively that this specificity was not due to a subpopulation of sorption sites unique to NBZ. H-bonding was ruled out, as the relative affinity for the sorbent among the solutes did not change at all when increasing the solution pH from 6.5 to 11. 1H NMR experiments showed molecular complexation in chloroform between NBZ and model graphene polycyclic aromatic units (naphthalene, phenanthrene, and pyrene) which was absent for BEN and TOL. This result, in combination with the results of a companion study (Zhu and Pignatello, Environ. Sci. Technol. (in press)), is used to support the existence of π−π electron donor−acceptor interactions between NBZ (electron acceptor) and the polycyclic aromatic charcoal surface (electron donor) as the cause of enhanced NBZ sorption.
ISSN:0013-936X
1520-5851
DOI:10.1021/es049135l