Influence of pore-water velocity on transport behavior of cadmium: Equilibrium versus nonequilibrium

The effect of pore-water velocity (6-82cm/h) on transport behavior of cadmium was investigated in three soils using column experiments. The symmetrical bromide breakthrough curves indicated the absence of immobile water region at the applied pore-water velocities. The equilibrium model could not describe the breakthrough curves of cadmium, particularly at high pore-water velocities. The shorter residence times at higher pore-water velocities result in the more significant early breakthrough and long tailing, reflecting a higher degree of nonequilibrium. While both the two-site nonequilibrium model and the effective dispersion equilibrium model could describe the transport behavior of cadmium, the latter appeared to be inappropriate because it overestimated the dispersion coefficients as high as 6.3-fold compared with the coefficients obtained from the bromine breakthrough curves. Thus, the nonequilibrium model was used to analyze the results. With increasing pore-water velocity, the sorption rate coefficient increased suggesting that diffusion was responsible for the rate-limited sorption, and the fraction of instantaneous sorption decreased inferring a range of diffusion times to the sorption sites. The retardation also increased, probably because the soils were exposed to a greater mass of cadmium at higher pore-water velocities over the same timespan.