A distributed reactivity model for sorption by sails and sediments. 14. Characterization and modeling of phenanthrene desorption rates

Rates and extents of phenanthrene desorption were studied for more than 250 days as functions of sorbent type, initial loading level, and aging. Appar ent first-order desorption rate constants for the slowly desorbing fraction were found to ii) range from 0.00086 to 0.148 days(-1) for geosorbents tha t contain geologically mature kerogen and less rigid humic-type soil organi c matter, respectively, (ii) decrease by as much as an order of magnitude w ith decreasing initial sorbed solid-phase phenanthrene concentration, (iii) decrease by a factor of 2 with increasing aging time far a humic topsoil b ut remain unaffected by aging time beyond 3 months for a shale, and (iv) be 1-2 ord e rs of magnitude lower than rate constants for th e rapidly desor bing phenanthrene fractions for any given contaminated sample. Six models w ere used to fit the desorption rate data. Biphasic diffusion and biphasic f irst-order models with three fitting parameters possess broad utility and a re potentially useful in a variety of environmental applications. Disadvant ages of a five-parameter triphasic first-order desorption model, a two-para meter gamma-function model, and a one- or two-parameter pore diffusion mode l are also discussed.