Assessing the Combined Roles of Natural Organic Matter and Black Carbon as Sorbents in Sediments

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 °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 (μg/kgBC)/(μg/L)n of 106.25±0.14. Pyrene sorption to untreated sediment was reasonably estimated using K d = f oc 104.7 + f BC 106.25 C w (0.62-1) where f oc was the non-BC organic carbon content, 104.7 was the organic carbon-normalized absorption coefficient for pyrene (L/kgoc), f BC was the BC content, and C w was the dissolved pyrene concentration (μg/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, K oc values for PAHs that exceed their respective K ow values, and discrepancies in bioavailability between planar and nonplanar sorbates.