Tuesday, November 14, 2006 - 2:00 PM

Effect of natural organic matter on the surface and adsorptive properties of char particles in soil.

Joseph Pignatello, Seokjoon Kwon, and Yuefeng Lu. P.O. Box 1106, Connecticut Agricultural Experiment, Connecticut Agricultural Experiment Station, 123 Huntington St., New Haven, CT 06504-1106

            Black carbon (BC) is present in soil as a result of fires and soot deposition. Since BC particles are highly microporous, they are expected to strongly influence the adsorption and bioavailability of organic compounds. However, over time natural organic substances in soil may bind to BC and attenuate its surface activity relative to raw BC. Such attenuation has been claimed, but, because BC is not readily separated from soil, the claims are based on assumptions that soil BC is accurately quantified and that adsorption parameters obtained with raw BC standards are applicable. We report major effects on surface area and moderate effects on benzene sorption after weathering wood char particles in soil suspension. Subsequently, we examined specific natural substances to determine their influence on adsorption of hydrophobic compounds (benzene, 1,2,4-trichlorobenzene, naphthalene, phenanthrene) and on char surface and pore characteristics measured by N2 and CO2 porisimetry. These studies included char suspended in solutions of humic or fulvic acids, char co-precipitated with humic acid by evaporation of the water, char co-flocculated with humic acid by addition of Al, and char adsorbed with soy oil to model the lipid (polymethylene fraction) of humic substances. The following conclusions were drawn. 1) BC surface activity in soil is attenuated with aging by coverage of external surfaces with humics. 2)  Nitrogen is not a useful probe for hydrophobic microporosity due to blockage of internal pore networks by humic substances at the cryogenic temperature of the analysis (77 K).  3)  Adsorption of organic compounds by BC can be suppressed by competition with humic substances. Competition increases with adsorbate size because adsorbate is more and more restricted to external surfaces where humics interact. Competition is facilitated by coagulation, presumably by increasing the local surface humic concentration.  4) Competition by humic substances increases the linearity of the adsorbate isotherm.