Rate-limited boron transport in soils: The effect of soil texture and solution pH

Although boron (B) adsorption significantly depends on soil texture and pH, few attempts have been made to estimate their influence on B transport behavior. Adsorption and transport of B in three soils of different textures were investigated in batch and column experiments. The B adsorption on the soils in batch experiments was rapid and an apparent adsorption equilibrium was reached over the first several hours. The extent of adsorption on each of the soils was strongly dependent on pH, increasing sharply as the pH increased from 7.0 to 9.2-9.4. The Langmuir equation with the pH-dependent adsorption coefficient simulated well the B adsorption at equilibrium. Miscible-displacement experiments were conducted at two water fluxes of 2.9 and 0.19 can h(-1) and at two pH values. The transport of B through the soil columns was retarded. The retardation increased with the increase of clay content and solution pH. The impact of the rate-limited adsorption on B transport was dependent on water flux and was controlled by mass-transfer processes. The B breakthrough curves (BTCs) for the loamy sand and sandy loam soils (where Br- transport was ideal) were simulated using the ideal-transport-based (local equilibrium-nonequilibrium [LE-NE]) model. This model successfully described B transport at different water fluxes and pH values when using the adsorption parameters derived from batch equilibrium experiments. The adsorption rate coefficient values obtained from BTCs were smaller than those obtained from the batch kinetic data. The non-ideal transport behavior of B in the clay soil was associated with intra-aggregate and rate-limited adsorption in the mobile domain. A successful simulation of B BTCs for this soil for the two aggregate sizes (< 2 and 4-4.75 mm) was obtained when used the two-domain, two-rate (TD-TR) model. The results indicate that the influence of rate-limited adsorption on NE B behavior was more pronounced at fast than at slow water flux. However, at slow water flux, the rate-limited adsorption had a secondary importance in comparison with the rate-limited mass-transfer between mobile and immobile water domains.