Theoretical analysis of kinetic effects on the quantitative comparison of Kd values and contaminant retardation factors

Distribution coefficients (Kd values) describe contaminant partitioning between liquids and solids for linear sorption at equilibrium conditions. If experimentally-determined Kd values do not represent sorption equilibria, errors are introduced in contaminant transport models. These errors may be further propagated when Kd values are used to compare contaminant mobility under different chemical solution conditions. Our theoretical analysis based on pseudo-first order sorption kinetics shows that, independent if two systems have the same or different sorption kinetics, relative comparisons of Kd values and retardation factors are always affected by sorption times under non-equilibrium conditions. The time-frames required for attaining constant Kd values are not only dependent on kinetic sorption characteristics, but also the equilibrium Kd values approached. The type of kinetic errors introduced is affected by the specific differences in sorption kinetics and equilibrium Kd values between the two systems. For systems with the same sorption kinetics, relative increases or decreases in contaminant velocities are always underestimated. In case of different kinetics, either an under- or overestimation of relative differences seems possible. Experimental sorption times should aim to equilibrate the system with the highest Kd value for systems with comparable kinetics, and the system with the slowest sorption kinetics for different kinetics.