Effect of pH on Boron Adsorption-Desorption Hysteresis of Soils

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Abstract

Abstract:

Lime is regularly used to increase soil pH, which will increase boron (B) fixation in soils. The effects of raising pH on B adsorption and the reversibility of adsorbed B in acid soils are essential for soil B management. In this study, B adsorption and desorption in limited concentration as a function of pH were measured in five fine-textured red soils. Immediately after adsorption of B, four consecutive desorption steps were carried out by successive dilution. The sorption results were described well by the Freundlich equation. Boron adsorption markedly increased as pH increased. This increase was more pronounced when Ca(OH)2, as opposed to NaOH, was used for pH adjustment. The presentation of desorption data was based on the traditional isotherm approach and on time-dependent isotherms. Apparent hysteresis was observed in that both derivational families of desorption isotherms deviated from the adsorption isotherm for the five soils at their original pH. Moreover, hysteresis coefficients, ndes/nads and λtrad for the traditional approach, had a positive correlation with organic carbon content, but they had a negative correlation with free Fe2O3 and free Al2O3 content. As pH increased, the hysteresis diminished, and adsorbed B became more likely to be desorbed. This finding may be because B adsorption and raising pH increase the negative charge on the surface, which creates electrostatic repulsion and results in an increase in desorption as pH increases.

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