Characterization of Sorbent Properties of Soil Organic Matter and Carbonaceous Geosorbents Using n-Alkanes and Cycloalkanes as Molecular Probes
Nonspecific interactions and modes (i.e., adsorption vs absorption) of sorption by noncondensed, amorphous organic phases (here termed organic matter; OM) in soils and by and condensed phases (termed carbonaceous rigid, aromatic, and condensed phases (termed carbonaceous geosorbents; CGS) were investigated using n-alkanes and cycloalkanes as molecular probes. Sorption isotherms of n- and cyclooctane from water for seven CGs (charcoal, lignite coke, activated carbon, graphite, partially oxidized graphite, diesel soot, bituminous coal), four sorbents with a predominance of OM (lignite, peat, two sapric soils), and two soils containing OM and high amounts of CGs were measured in batch systems. The peat and the sapric soils showed extensively linear sorption, while the CGs exhibited highly nonlinear (Freundlich exponents 0.2-0.7) and strong (K-oc values being up to 10(5) times those for the OM-rich materials at low concentrations) sorption for the alkanes studied, showing that enhanced sorption by CGs can occur to completely apolar sorbates that do not undergo any specific interaction. Sorption by CGS was generally stronger and more nonlinear for n-octane than for cyclooctane, which suggests a strong dependence of sorption on the 3-D structure of sorbate molecules. The n-octane-to-cyclooctane sorption coefficient ratios (K-n/K-c) for adsorption to CGs were >= 1, being distinctly different from those for absorption to the OM-rich materials (K-n/K-c < 1). The measured sorption isotherms and the CG compositions in the soils determined by quantitative petrography analysis suggest however that CGs occurring in,, soils may be far less effective sorbents than the reference CGs used in the sorption experiments at least for nonspecifically interacting sorbates, probably because of competitive sorption and/or pore blocking by natural OM. The presented approaches and results offer a basis for interpreting sorption data for other organic compounds, as nonspecific interactions and sorption modes are relevant for any compound.
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