Equilibrium and nonequilibrium transport of boron in soil

Though B adsorption on soil is considered to be reversible and rapid, the use of models based on the assumption of local equilibrium often provide poor descriptions of B transport in soil columns. This study was conducted to reconcile inconsistencies between the findings of transport and batch-adsorption experiments. The B displacement experiments in the loamy sand soil were conducted at various pH values (6.9, 8.3, and 9.3) and pore-water velocities (3.6 and 0.16 cm h(-1)). The B transport in soil was strongly controlled by the pH-dependent and rate-limited adsorption (the soil heterogeneity was insignificant). The impact of rate-limited adsorption was dependent on pore-water velocity. The two-site (local equilibrium-nonequilibrium [LE-NE]) model accounting for the existence of equilibrium and nonequilibrium adsorption sites was used to describe nonideal transport of B in loamy sand soil. The Keren's phenomenological equation was used to simulate B adsorption on equilibrium sites and the Langmuir rate equation was applied for the rate-limited sites. The B adsorption parameters in the model were obtained from batch experiments. The fraction parameter f (representing the fraction of soil in which B adsorption is assumed to be rate-limited) and the dimensionless rate coefficients gamma0 (the Damkohler number) for B adsorption-desorption reactions were calculated by fitting the LE-NE model to the breakthrough curves (BTCs) for B measured from the fast-velocity experiments. The fraction parameter was >0.9, indicating that most of B adsorption sites on the loamy sand soil are rate-limited. The gamma0 values calculated from B adsorption BTCs were greater than that for desorption, indicating that hysteresis in B adsorption-desorption processes can be observed during nonequilibrium B transport in soil. The LE-NE model well reproduced the general B transport behavior in the soil over the observed pH and velocity ranges.