Nonequilibrium Sorption during Displacement of Hydrophobic Organic Chemicals and ⁴⁵Ca through Soil Columns with Aqueous and Mixed Solvents

A series of miscible displacement experiments was conducted to investigate the significance of intraorganic matter diffusion (IOMD) as the rate-limiting step in sorption of organic and inorganic solutes during steady water flow in soil columns. Displacement studies were performed using Eustis surface soil and the same soil treated with hydrogen peroxide to reduce soil organic carbon content from 0.2% to <0.01%. Two herbicides (diuron and atrazine) were also displaced with binary solvent mixtures containing varying fractions of methanol and water (0-50% methanol). A bicontinuum nonequilibrium sorption model was used to simulate the measured effluent breakthrough curves (BTCs) obtained by simultaneous displacement of a nonadsorbed tracer (tritiated water), an organic solute (herbicide), and a metal cation (⁴⁵Ca). Support for IOMD as the rate-limiting step in sorption was provided by the following: (i) lack of asymmetry for the BTCs of tritiated water and ⁴⁵Ca in contrast to the asymmetrical BTCs of the two herbicides for aqueous displacements; (ii) symmetrical BTCs for all solutes displaced through the soil that was treated to remove organic matter; (iii) decreasing asymmetry with increasing methanol content in the displacing solution. An analysis of the bicontinuum sorption model parameters also supported this conclusion.