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TECHNICAL REPORTS

Organic Compounds in the Environment

Nonlinear and Competitive Sorption of Apolar Compounds in Black Carbon-Free Natural Organic Materials

^a Department of Soil and Water, Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven, CT 06504-1106
^b Department of Civil and Environmental Engineering, Vanderbilt University, 400 24th Avenue South, VU Station B 351831, Nashville, TN 37235
^c Department of Environmental Sciences, Cook College, Rutgers University, New Brunswick, NJ 08901-8551
^d State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, The People's Republic of China
^e Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, MA 01003

^* Corresponding author (joseph.pignatello{at}po.state.ct.us)

Received for publication September 20, 2005. Numerous studies have reported a spectrum of sorption phenomena in soils, sediments, and organic matter isolates of those materials that are inconsistent with a partition model proposed in the late 1970s and early 1980s, a model predicated on a hypothesis that sorption is linear and noncompetitive. To explain these nonideal phenomena, prior studies have proposed a hard–soft (glassy–rubbery) model for SOM (soil and sediment organic matter), while others have attributed them singularly to BC (black carbon: soot and charcoal) particles present in topsoils and sediments. In this study, we demonstrated nonideal sorption behavior (isotherm nonlinearity, competitive effects) for a group of apolar compounds in a large set of natural and model organic materials, including a commercial lignin and humic acids from different sources. Complete oxidation of samples by an acidic dichromate method was taken to signify the absence of BC. (However, polymethylene units are stable even if functionalized on both ends, making the technique unreliable for quantifying BC.) Other samples were inferred free of BC by their source and method of preparation. Characterization by thermalanalytical methods indicated the glassy character of the organic materials. The origin of the nonideal behaviors appears to be the glassy character of these materials. Sorption nonlinearity increased or decreased by changing temperature, cosolvent content, or degree of cross-linking by metal ions as predicted for organic solids in a glassy state. We conclude that macromolecular humic substances in the environment may exhibit nonideal sorption behavior in soils and sediments, quite apart from any such behaviors attributable to BC.

Abbreviations: BC, black carbon • DCB, dichlorobenzene • HA, humic acid • HOC, hydrophobic organic contaminant • NAPH, naphthalene • OC, organic carbon • PHEN, phenanthrene • SOM, soil and sediment organic matter • TCB, trichlorobenzene • TMDSC, temperature-modulated differential scanning calorimetry • XYL, xylene