A distributed reactivity model for sorption by soils and sediments. 11. Slow concentration dependent sorption rates

Long-term temporal phase distribution relationships (PDRs) were measured for sorption of a hydrophobic organic contaminant probe by seven EPA reference soils and sediments and six shale and kerogen samples. The times required for attainment of apparent sorption equilibrium by the phenanthrene probe were found to be highly dependent upon the aqueous phase-solute concentration, C(t), for a given sorbent, and the type of soil organic matter (SOM) associated with a particular sorbent. Organic-carbon-normalized single-point temporal distribution ratios corresponding to low residual solution phase concentrations were found to approach their respective apparent equilibrium values after times ranging from several days to 90 days for the EPA soils and sediments and from 90 days to greater than or equal to 368 days for the shales and kerogens. Conversely, at residual solution phase concentrations 2 orders of magnitude larger, apparent equilibrium conditions were attained within a few hours for the EPA soils and sediments and within a year for the share and kerogen samples. The observed dependencies of sorption rate on C(t) and on the type of SOM appear to result from differences in solute diffusion behavior within chemically reduced and structurally condensed SOM domains and that in highly amorphous SOM domains. In the former case the very slow and concentration-dependent non-Fickian behavior observed is likely attributable to the slow and energetically driven reconfiguration of local condensed SOM structures to accommodate solute migration into the matrixes. The results of the study extend the applicability of the Dual Reactive Domain Model introduced earlier in this series of papers to the interpretation and description of sorption rate behavior.