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Existing field data indicate that soot may significantly affect the environmental speciation of polycyclic aromatic hydrocarbons (PAHs). To expand hydrophobic partition models to include soot partitioning, we need to quantify f sc , the soot fraction of the solid matrix, and K sc , the sootcarbon-normalized partition coefficient. To this end, we have developed a method that allows quantification of soot carbon in dilute and complex sedimentary matrices. Nonsoot organic carbon is removed by thermal oxidation, and inorganic carbonates are removed by acidification, followed by CHN elemental analysis of the residual soot carbon. The selectivity of the soot carbon method was confirmed in tests with matrices of known compostion. The soot quantification technique was applied to two sets of natural sediments, both previously analyzed for PAHs. The input histories of PAHs and soot recorded in a lacustrine sediment core followed the same general trends, and we thus infer a coupling between the two. Our measures of f sc and calculations of K sc , approximated from studies of PAH sorption onto activated carbon, were applied to rationalize previously generated in situ K oc values. Intriguingly, we find that the elevated PAH K d values of two marine sediment-porewater systems are now quantitatively explainable through the extended, soot-partioning inclusive, distribution model. The importance of the soot phase for PAHs in the environment has implications for how we perceive (and should test) in situ bioavailability and, consequently, also for the development of sediment quality criteria.

residue particulate carbon, and spilled organic liquids. Certain manipulations of sorbates or sorbent media may help reveal sorption mechanisms, but mixed sorption phenomena complicate the interpretation of macroscopic data regarding diffusion of HOCs into and out of different matrices and the hysteretic sorption and aging effects commonly observed for geosorbents. Analytical characterizations at the microscale, and mechanistic models derived therefrom, are needed to advance scientific knowledge of HOC sequestration, release, and environmental risk.

[1] Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified

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We hypothesized that two mechanisms, absorption into natural organic matter and adsorption onto combustion-derived black carbon (BC), act in parallel to bind polycyclic aromatic hydrocarbons (PAHs) to Boston Harbor sediments. To focus on BC-adsorption, we removed the non-BC fraction by combusting near shore sediments at 375 degrees C for 24 h under air, leaving ca. 16% of the reduced carbon. The isotherm for pyrene sorption onto our combusted sediment was nonlinear with a Freundlich exponent of 0.62+/-0.12 and a BC-normalized distribution coefficient (microg/ kgBC)/(microg/L)n of 10(6.25)+/-0.14. Pyrene sorption to untreated sediment was reasonably estimated using Kd = f(oc) 10(4.7) + fBC 10(6.25) Cw(0.62-1) where f(oc) was the non-BC organic carbon content, 10(4.7) was the organic carbon-normalized absorption coefficient for pyrene (L/kg(oc)), fBC was the BC content, and Cw was the dissolved pyrene concentration (microg/ L). C/H/N ratios indicated that our environmental BC differed substantially from NIST diesel soot, possibly due to inclusion of larger BC particles from near source atmospheric fallout and urban runoff. The impact of BC on total PAH sorption may explain reports of nonlinear isotherms, Koc values for PAHs that exceed their respective Kow values, and discrepancies in bioavailability between planar and nonplanar sorbates.

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