Theoretical analysis of kinetic effects on the quantitative comparison of K(d) values and contaminant retardation factors.

Distribution coefficients (K(d) values) describe contaminant partitioning between liquids and solids for linear sorption at equilibrium conditions. If experimentally-determined K(d) values do not represent sorption equilibria, errors are introduced in contaminant transport models. These errors may be further propagated when K(d) 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 K(d) values and retardation factors are always affected by sorption times under non-equilibrium conditions. The time-frames required for attaining constant K(d) values are not only dependent on kinetic sorption characteristics, but also the equilibrium K(d) values approached. The type of kinetic errors introduced is affected by the specific differences in sorption kinetics and equilibrium K(d) 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 K(d) value for systems with comparable kinetics, and the system with the slowest sorption kinetics for different kinetics.