Effect of Sorbate Planarity on Environmental Black Carbon Sorption

Soot and charcoal, collectively termed "black carbon" or BC, can exhibit extremely strong sorption of many hydrophobic organic compounds. In order to include BC sorption in fate models, it is important to know BC nanopore surface areas. In addition, it is useful to know for which compounds BC sorption can be expected to be important. By nitrogen adsorption measurements at ultralow pressures on sediment that was strongly enriched in BC by HF treatment and/or chemothermal oxidation at 375 degrees C, we found that environmental BC has nanoporosity in the <4-10 A size range. The nanopore surface area (<50 A) of BC in Lake Ketelmeer (The Netherlands) sediment was approximately 58 m2/g. We measured sorption isotherms over a wide concentration interval for four compounds with the same Kow (10(46+/-0.1): planar anthracene (ANT), phenanthrene (PHE), and 4-chlorobiphenyl (4-PCB) along with nonplanar 2,2'-dichlorobiphenyl (2,2'-PCB). The environmental BC sorption coefficients of these iso-Kow compounds decreased in the order ANT > PHE approximately 4-PCB >> 2,2'-PCB and spanned a factor of 50-200, depending on concentration. Nonplanar 2,2'-PCB showed much more linear BC sorption (nF = 0.92) than the planar compounds (nF = 0.54-0.70). This shows that steric hindrance strongly attenuates BC-sorbate interactions for a nonplanar PCB. Thus, BC is more important for environmental sorption of planar compounds (>50% sorbed to BC in the nanogram per liter range) than for nonplanar ones (<10-20%). Using the measured BC nanopore surface area, a close agreement between modeled and measured BC sorption data could be found.