The synthesis of gmelinite microspheres and their post-synthetic modification for improved defluoridation

ABSTRACT A method for synthesizing microspheres of the zeolite gmelinite (GME) with mean diameter 8 µm, and narrow dispersion, has been developed and establishes the key role of aging the gel prior to heating to produce a narrow size distribution. Fluoride adsorption from acidic media has been evaluated for GME microspheres following post-synthetic modifications by calcium ion exchange, iron(III) surface modification, and dealumination. In each instance, the post-synthetic modification leads to appreciable equilibrium fluoride loadings, with the highest loadings observed for dealuminated GME microspheres, capable of 97.4% fluoride removal from acidified 22 ppm fluoride solutions at equilibrium. Accordingly, dealuminated GME microspheres show promise as a potential adsorbent for reducing the aqueous fluoride content in dangerously high natural sources (up to 30 ppm) to safe drinking levels (≤1.5 ppm) in one treatment.