Decrease in Cation Exchange Capacity of Zeolites at Neutral pH: Examples and Proposals of a Determination Method

Cation exchange capacity (CEC) is an important characteristic of zeolites, especially when they are used as adsorbents in the aqueous system. However, no international standard method exists for the determination of CEC of zeolites. We determined CEC of Linde-type A and Na-P1 type zeolites at various pH (4 to 10) with a simple method, where Na+-saturated zeolites were prepared, and then various amounts of HCl were added. CEC was simply calculated by subtracting the amount of Na+ in the final supernatant from the content of Na+ of the Na+-saturated zeolites. CEC of the zeolites decreased with decreasing pH and with decreasing Na+ concentration of the final supernatant. The concentration of Na+ of the supernatant, CEC of the zeolites began to decrease at weakly alkaline or neutral pH, and that of the Linde-type A zeolite became about half at pH around 6. When CEC was plotted against pH-pNa; where pNa is negative logarithm of the activity of Na+; CEC of each zeolite was expressed by a curve. It indicates that the CEC or the amount of Na+ retention is univocally determined by the ratio of activities of Na+ and proton.