U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Sorption of Polycyclic Aromatic Hydrocarbons to Aquifer Materials Containing a Low-Organic Carbon Content from Bemidji, Minnesota

Abstract

Three polycyclic aromatic hydrocarbons, naphthalene, phenanthrene, and pyrene -- were sorbed to a single sample of aquifer material containing a low-organic carbon content at two different temperatures, 5°C and 25°C, using batch and column experimental techniques. Controls on batch and column techniques were incorporated to ensure that the experimental vessels were organic-carbon free, biological and photolytic activity was absent, and solution supernates were colloid-free. The sorptive uptake curves indicated a two-step approach to equilibrium. The data were modelled using a two-site sorption kinetic model. Times to equilibrium were at least 40 hours. Small increases in equilibrium distribution coefficients were observed with decreased temperature which resulted in enthalpy of sorption values which were consistent with a partitioning mechanism. Small decreases in rate constants were occurred as temperature decreased. The values of the solids/water distribution coefficients for the batch experiments were consistently 4 to 60 times larger than those obtained from the column experiments, probably because of differences in experimental boundary conditions or to creation of fine particles in the batch experiments that would increase the surface area available for sorption. Although the relative effects of organic-carbon phase sorption versus mineral phase sorption were not isolated directly, the magnitude of the observed distribution coefficients and rate constants are consistent with sorption controlled by the organic-carbon content of this aquifer material.