Effects of pH and Ionic Strength on Boron Adsorption by Kaolinite

Plants uptake boron (B) from the soil solution, in which boron concentration is controlled by adsorption and desorption reactions on soil organic and inorganic colloids. Considering the importance of aluminosilicate minerals in ion adsorption and desorption reactions in soil, in this research, B adsorption behavior on kaolinite was studied as a function of equilibrium concentration, pH, kaolinite concentration, and ionic strength. Adsorption isotherms on kaolinite at pH 5 and 8, and at concentration range of 0 to 1319 μmol/L, indicated that B adsorption increased with increase in its equilibrium concentration and its maximum adsorption at pH 8 reached 20 μmol/L. Adsorption edges also indicated that B adsorption proportionally increased with pH until pH 9, where it reached a maximum level. It was then reversed by further increase in pH.  Experimental data at constant B concentration exhibited an adverse effect of kaolinite concentration on B adsorption. Boron adsorption at three levels of ionic strength showed a negative effect between salt concentration and B adsorption. This adverse effect is likely due to the effect of ionic strength on kaolinite charging behavior, and on the boron solution speciation, and the competitive adsorption of nitrate ion with B. Due to the high amount of boron adsorption on kaolinite and similarity in anion adsorption by different aluminosilicate clay minerals, it can be concluded that these minerals play an important role in boron adsorption in soil and controlling its availability for plants.