Sorbate size-dependent maximum capacities for adsorption of organic compounds in the slowly and very slowly desorbing domains of a sediment

Maximum capacities were determined for adsorption of six polycyclic aromatic hydrocarbons, seven polychlorinated biphenyls, and five chlorobenzenes in both the slowly desorbing domain and the very slowly desorbing domain of a sediment. For separate compound classes in the two desorption domains, log-transformed maximum adsorption capacities were linearly related to the relative magnitude of the shadow of the sorbate on an imaginary planar surface. For planar compounds, the ratio of maximum adsorption capacities for the slowly desorbing domain and the very slowly desorbing domain was approximately two. This ratio was eightfold higher for strongly nonplanar polychlorinated biphenyls, which suggests that adsorption depends on sorbate thickness in the very slow desorption domain but not in the slow desorption domain. The rates of decrease of the log-transformed sum of maximum capacities for adsorption in the slow desorption domain and the very slow desorption domain with increasing relative magnitude of the shadow of the sorbate on a planar surface were similar to those derived from literature data regarding maximum capacities for adsorption onto a soil and a sediment. It is proposed that the rates of decrease for both the slow desorption domain and the very slow desorption domain may have general applicability to soils and sediments.