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Division S-1—Soil Physics

Equilibrium and Nonequilibrium Transport of Boron in Soil

Institute of Soil, Water and Environmental Sciences, the Volcani Center, Agricultural Research Organization (ARO), P.O. Box 6, Bet Dagan 50250, Israel

^* Corresponding author (rkeren{at}agri.gov.il)

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 ₀ (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 ₀ 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.

Abbreviations: BTC, breakthrough curve • LE, local equilibrium • LE–NE, local equilibrium-nonequilibrium • NE, nonequilibrium • SAR, sodium adsorption ratio

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