Two-region flow and rate-limited sorption of uranium (VI) during transport in an unsaturated silt loam

Uranium (VI) sorption during unsaturated transport was evaluated using a centrifuge method; conventional saturated columns and batch incubation procedures were used to isolate flow effects on sorption processes. Uranium (VI) sorption was dependent on flow velocity, and modeling indicated that 60% of sorption was rate‐limited. Breakthrough curves were described and independently predicted with a two‐site model; the mass transfer coefficient for sorption was assumed to be linearly related to velocity. While two‐region flow contributed to disparity in apparent Kd values (linear distribution coefficient) at different water contents, the effect was explained by increased velocity in the mobile flow domain and not a decrease in availability of sorption sites. Hydrodynamic parameters for two‐region transport, determined for a nonsorptive tracer at faster velocity (16 cm h−1), were fixed and described U(VI) transport at slower velocity (2 cm h−1). The rate‐limited parameter for mass transfer between mobile and immobile water domains was linearly related to the average pore water velocity. Effective retardation factors varied with moisture saturation and the apparent Kd value and ranged from 7 for faster velocity experiments in a saturated column to 44 at slow velocity and 40% moisture saturation.