Sustainable ammonium recovery from wastewater: Improved synthesis and performance of zeolite N made from kaolin

Although zeolite N (K12Al10Si10O40Cl2 center dot 8H(2)O) is used commercially for the removal and recovery of ammoniacal nitrogen from wastewater, it has received limited attention in academic literature. The hypothesis was that if an improved understanding of zeolite N synthesis and resultant physical parameters and performance can be acquired, then the application of this material may be accelerated. This study successfully addressed existing research challenges including: maximizing the cation exchange capacity (CEC); selection of preferred synthesis conditions; control of the size, shape, and dispersion of zeolite N crystals; and improvement of ion exchange properties. Zeolite N was hydrothermally synthesised from kaolin clay added to solutions of potassium hydroxide and potassium chloride in a "one pot" process. CEC values increased to 563 meq/100 g (c.f. 528 meq/100 g in previous literature). Reaction between 120 and 140 degrees C for 2-4 h under static conditions was preferred as higher temperatures (180 degrees C) produced kalsilite. The KOH molarity (1.2-4.3 M) controlled the average zeolite crystal size, dispersion, and crystal shape; with highest molarity values optimal. Whereas, added KCl acted as a template which promoted zeolite N formation. Removal of KCl resulted in creation of zeolite F (K-13(OH)(3)Al10Si10O40 center dot 13H(2)O) which also belonged to the EDI structural type. Ammonium ion exchange from aqueous solution was improved when diffusion restrictions were eased; as evidenced by using smaller zeolite crystals which were highly dispersed and not agglomerated. Not only was a greater cation capacity recorded but also the time to reach equilibrium was decreased.