Alkaline-mediated hydrothermal treatment is optimized to obtain hierarchical zeolite from naturally obtained chabazite (8 6 4 membered ring sizes, framework density, and use this material for selective removal of NH4+ from competing cations (primarily Na+, Ca2+, and Mg2+). 6 or less membered ring sized cage in the parent chabazite offers lower inter-crystalline movement dimension to exchangeable cationic species, which results in lower ion-exchange capacity. We report proof of the parent chabazite undergoing crystalline reconstruction in an alkaline environment at elevated temperature. The choice and concentration of alkali, time of treatment, and temperature are critical parameters in the remodeling of the zeolite framework. Through XRD studies, we show the appearance of distinct sharp peaks in modified chabazite; these 2 angles are markers for 10 6 4 and 8 4 membered ring sizes with 18.0T/1000 Å3 and 16.4T/1000 Å3 respectively. The treated chabazite is denser in number of building blocks with greater number of 8 or more membered rings. Moreover, it is observed through SEM/EDS that the controlled environment develops mesoporosity in the treated sample due to desilication of parent chabazite which facilitates the movement of species toward active sites. We were able to increase the cation exchange capacity of the zeolite from 2.5 meq./dry g (parent material) to 5.25 meq./dry g. Results of cation selectivity vis-à-vis the order of NH4+ &gt; Na+ &gt; Ca2+ for the parent zeolite will be discussed. Results of ion-exchange isotherm, kinetic tests, and regeneration efficiency of the modified chabazite, as well as the ability of the process to produce high-purity NH4+ salt as a fertilizer will be presented. The modified chabazite has promise to be used in a fixed-bed configuration to selectively remove NH4+ from wastewater and recover NH4+ as a high-purity fertilizer in a passive, scalable, and sustainable process.

Alkaline hydrothermal treatment to modify chabazite and use it for selective NH4+ removal from wastewater and recover NH4+ as a high-purity fertilizer

ACS Spring 2022

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