Ion-exchange behavior of zeolite NaA and maximum aluminum zeolite NaP

The framework structures of zeolite NaA and maximum aluminum zeolite NaP (also known as MAP) significantly effect the rate at which these two zeolites remove hardness ions, calcium and magnesium, from solution at similar particle sizes and ion-exchange conditions. Both the rate of hardness ion removal and the overall hardness ion capacity of a zeolite are important factors for determining if a zeolite can be used as a detergent builder. Although maximum aluminum zeolite NaP's framework structure allows for a higher selectivity for calcium ions as compared to zeolite NaA, the rate of calcium uptake during the 12-15 minutes of the wash cycle is faster in zeolite NaA than in maximum aluminum zeolite NaP. Since the rate of ion removal by a zeolite is controlled by the limiting pore dimension, the faster calcium ion removal by zeolite NaA can be explained by considering that this zeolite has a three-dimensional pore system with a pore opening of 4.2 Angstrom. Maximum aluminum zeolite NaP, on the other hand, has a two dimensional channel system with elliptical pore openings of 3.5 Angstrom, x 4.5 Angstrom and 2.8 Angstrom x 3.5 Angstrom. In addition to the ion depletion data collected on both zeolites, isotherms and the derived Kielland plots, log K versus Z(Ca) were determined. As predicted, the calcium selectivity coefficient, log K, for zeolite A decreases as calcium loading, Z(Ca), increases. However, for maximum aluminum zeolite P, the calcium selectivity coefficient increases as calcium loading increases until Z(Ca) reaches 0.6, at which point, the calcium selectivity coefficient begins to decrease with higher calcium loadings. X-ray diffraction (XRD) results of the various calcium loaded maximum aluminum zeolite P's revealed that the maximum aluminum zeolite P structure undergoes a phase change as calcium ions replace the sodium ions in the framework. These XRD results explain the maximum in the Kielland plot.