Adsorption and desorption processes of boron in calcareous soils.

Boron (B) availability is regulated by its equilibrium concentration that in turn is buffered by adsorption and desorption reactions. Ionic strength, pH, OM content, and the type and amount of minerals are the major factors affecting B sorption reactions. To evaluate the influence of calcium carbonate equivalent (CCE) and ionic strength on B chemical behavior, its adsorption and desorption isotherms were measured in eight calcareous soils differed in CCE (0-85%). Adsorption and desorption data were described by the Langmuir and the Linear adsorption equations, respectively. No statistically significant relation was found between model parameters and soil properties. However, in comparison, soils with higher reactive particles (clay and OM) and higher pH adsorbed more boron. Removing CCE from a soil sample (CCE=18%) lowered B adsorption maximum by 35%. In contrast, increasing electrolyte concentration (0.01 M NaCl) to 0.1 and 0.5M caused to increase B adsorption maximum by 30% and 75%, respectively. At the equi-molar concentration, CaCl(2) increased B adsorption stronger than NaCl. The positive effect of ionic strength was attributed to a better screening of surface charges and compaction of double layer thickness. Desorption data were deviated from adsorption isotherms only at equilibrium concentrations smaller than 2 mM. Analysis of boron solution speciation and adsorption-desorption data revealed that B is mainly adsorbed as spectroscopically proved outer-sphere complex in the studied soil samples. The experimental data and model prediction could be used to manage B bio-availability and to optimize remediation processes in calcareous soils.