Effects of residence time and degree of water saturation on sorption nonequilibrium parameters

This study reports the impact of the degree of water saturation on sorption nonequilibrium parameters. Two nonionic organic compounds (benzene anti di methylphthalate) and three nonaggregated sandy soils were utilized. Local e quilibrium assumptions were found to be invalid for describing the transpor t of these compounds even at pore-water velocities as low as 0.7 cm/h. Sorp tion nonequilibrium appeared to be of a diffusive nature rather than due to a slow chemical reaction. Sorption mass-transfer coefficients varied propo rtionally with pore-water velocity. A strong correlation between the mass-t ransfer coefficient and residence time was found utilizing present and prev iously reported laboratory data. A similar relationship was also found for the mass-transfer coefficient between mobile and immobile water regions. Fi eld data indicate that the sorption mass-transfer coefficient may continue to decrease in a consistent way even at residence times as large as 5 x 10( 3) h. Variations in the degree of water saturation had no impact on the val ue of the sorption mass-transfer coefficient other than what would be expec ted due to changes in the residence time. This suggested that movement into the solid grains of the large emptied pores through diffusion from the wat er-filled pores into stagnant water covering these grains was relatively fa st compared to the sorption rate. (C) 1999 Elsevier Science B.V. All rights reserved.