Adsorption and desorption of boron by goethite

Abstract

The effects of varying pH, concentration of boron, and competing anions on the adsorption of boron were examined. Boron adsorption initially increased with pH, peaked at approximately pH 8, and then declined. The presence of phosphate reduced adsorption over the pH range 5.2-10.6; the effect of sulfate was less than that of phosphate and ceased at about pH 7. The presence of mannitol caused virtually no difference in boron adsorption up to about pH 6, but reduced adsorption at higher values. The model by Bowden and coworkers was successful in describing both the charge characteristics of goethite in the absence of boron adsorption and the effects of pH and boron concentration on adsorption of the element. The best fit of the model was achieved when B(OH)4-, B3O3(OH)4-, B4O5(OH):- and B5O6(OH)4- were considered as the adsorbing species, although B(OH)4- and B4O5(OH)24- were predicted to be the major adsorbing ions. Boron adsorption was found to be reversible with respect to both concentration and pH. Elevated temperature increased the amount of desorption at both pH 6 and 8. The effect of mannitol on boron desorption mirrored the effect of the compound on adsorption of the element; there was no effect up to pH 6, but at higher pH values desorption was increased. The significance of the desorption data for soil testing for boron is discussed.