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 Kd 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 Kd 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 Kd was invariant with input pulse for the 10−7 detection limit. Conversely, the measured Kd values were significantly smaller at low input pulses for the 10−3 detection limit.