Adsorption of Dissolved Organic Compounds by Black Carbon

Black carbon is the carbonaceous product of pyrolysis or incomplete combustion of biomass or fossil fuels. Due to its widespread occurrence in atmospheric aerosols, soils and sediments, and its intrinsic strength as an adsorbent, black carbon potentially plays an important role in the partitioning of organic pollutants from water and air to natural solids, especially at low pollutant concentration. The adsorptive strength of black carbon depends greatly on pyrolysis time, temperature and other formation conditions, as well as subsequent weathering in the environment. The predominant property of black carbon governing its adsorptive strength is its small-pore porosity. The filling of micropores and mesopores of molecular dimensions eliminates the need for the cavity penalty that otherwise accompanies partitioning of molecules into bulk nonporous phases such as organic liquids, polymers and natural organic matter to accommodate incoming molecules. However, the filling of these small pores exhibits a steric effect due to size exclusion at pore throats. The polyaromatic (graphite-like) surface of black carbon serves as a strong pi-electron donor in the formation of pi-pi-electron donor-acceptor complexes with strong pi-deficient aromatic systems, for example, polynitroaromatics and charged aromatic amines. Polar functional groups on the rims of polyaromatic sheets attract water clusters that crowd out adsorbates, regardless of polarity. Nevertheless, compounds such as phenols, carboxylic acids and others that are capable of forming especially strong hydrogen bonds with carboxyl or phenoxyl groups on the surface may interact strongly. In the environment, the adsorptive strength of black carbon becomes quickly attenuated by fouling with humic substances, which block pores and compete for adsorption sites. Quantifying the contribution of native black carbon to retention of a contaminant in a given natural sample is a challenge due to the lack of reliable methods for determining black carbon content in geosolids, the difficulty of separating black carbon particles from the sample, the absence in most cases of a basis for choosing an appropriate reference standard, and a poor quantitative understanding of the weathering process. Adding to the challenge is the strong hysteresis that is often seen to accompany adsorption to black carbon materials, but that is unpredictable and poorly understood mechanistically.