A thermodynamics-based estimation model for adsorption of organic compounds by carbonaceous materials in...

A model was developed to estimate Langmuir affinities for adsorption of low-polarity organic compounds from either water or air by carbonaceous sorbents. Sorption enthalpies and entropies provided the basic information for the description of sorption affinities in terms of the entropy of melting and either solubility in water or vapor pressure. For m-xylene, polycyclic aromatic hydrocarbons (PAHs) and chlorobenzenes on 10 different sorbents, 80% of the measured sorption affinities fall within a factor of four of the model estimates. Equations for the limiting distribution coefficients in terms of either octanol-air (K(OA)) or octanol-water partition (K(ow)) coefficients were derived from regressions of calculated affinities combined with an estimated relation between experimental Langmuir sorption capacities and K(ow). Estimated soot-water distribution coefficients were within a factor of three of measured data for polychlorobiphenyls (PCBs) and lower molecular weight PAHs on automotive soot samples and captured the dependence of PCB distribution coefficients on the extent of ortho substitution. For higher molecular weight PAHs, sorption was underestimated. For soot in sediment-water distribution coefficients of PAHs and PCBs, estimated values captured both the trend of measured data with K(ow) and the dependence on sorbate planarity. Tentative application to aerosol-air distribution explained the observed independence of distribution coefficient-K(OA) relations for PCBs on the extent of ortho substitution and suggested nonequilibrium conditions for PAHs in comparison with recent measurements.