COMPETITIVE ADSORPTION OF POTASSIUM AND CALCIUM ON MANGANESE OXIDE ASSTUDIED WITH 2 ION-SELECTIVE ELECTRODES

Competitive adsorption of K and Ca in a Mn oxide (birnessite) system w as investigated using a double indicator electrode technique. The data showed that Ca was more competitive than K for adsorption sites on th e Mn oxide. The preference of the oxide for Ca over K was partially at tributed to the specific adsorption of Ca on the oxide. The extent of the preference did not consistently increase as the amount of K and Ca was continuously added to the system, possibly due to a decrease in t he availability of the specific adsorption sites as these sites were i ncreasingly occupied by the added Ca. The pK-0.5pCa value for the syst em significantly increased when the solution to solid ratio was above 16 (mL g(-1)). Raising pH caused an increase in the solution K/Ca equi valent activity ratio and a linear decrease in the pK-0.5pCa value, th ough the influence of the pH on the K/Ca equivalent activity ratio was not substantial until the pH was above 5.23. These results demonstrat e an unique interaction of K and Ca on the surface of the Mn oxide, wh ich cannot be solely interpreted by the classical ratio law. This stud y is of significance in the chemistry of K in soils which have propert ies of variable charge and specific adsorption of the competitive cati ons.