Measuring equilibrium sorption coefficients with the miscible-displacement method

Abstract

The miscible-displacement method is one commonly used approach for measuring equilibrium sorption coefficients. The objective of this research was to examine the impact of experiment conditions (detection limit, input-pulse size, input concentration) on the measurement of retardation factors and sorption coefficients for a system that exhibits significant nonideal sorption behavior. A series of miscible-displacement experiments was conducted wherein effluent solute concentrations were monitored over a range of approximately seven orders of magnitude, allowing characterization of asymptotic tailing phenomenon, which was significant. The magnitude of K_d increased asymptotically with the increase in the extent of the elution tail measured. The results also showed that the fraction of the tail required to obtain close-to-maximum measures of K_d is greater for smaller input pulses. Investigating the impact of analytical detection limit (equivalent to relative concentrations of 10⁻³ and 10⁻⁷) revealed that the magnitude of K_d was invariant with input pulse for the 10⁻⁷ detection limit. Conversely, the measured K_d values were significantly smaller at low input pulses for the 10⁻³ detection limit.

Keywords:

• Distribution coefficient
• nonlinear sorption
• column experiment

Acknowledgments

This research was supported by the NIEHS Superfund Research Program (Grant #ES 4940).