Does an Unconventional Source with a High Silica Content (Obsidian) or a High Alumina Content (Bauxite) Affect the Type of Potassium Zeolite Formation? Preliminary Data on Synthetic Products’ Efficiency in Ammonium Removal

Abstract

In this paper, obsidian (OS) and bauxite (BX) are used to synthesize zeolites via KOH prefusion, followed by a conventional hydrothermal process (HY) or an unconventional vapor-phase crystallization method (VPC) at 90 °C. The results indicate that, due to their chemical and mineralogical composition, both natural sources prove to be suitable raw materials for potassium zeolite synthesis. However, the higher silica content in obsidian is decisive for the formation of merlinoite (MER topology) and, subordinately, merlinoite-chabazite intergrowths (MER-CHA), whereas the higher alumina content in bauxite controls chabazite (CHA topology) synthesis. Moreover, our hypothesis is that the lower water content in the VPC method enhances the Al concentration during the dissolution of pre-fused raw materials, thus favouring the formation of zeolite N (EDI topology) in BX, the raw material with the highest Al content. The synthetic products were further investigated for their magnetic properties and potential application in ammonium removal from polluted water. The predominant magnetic phases (i.e., ferrimagnetic or antiferromagnetic) demonstrate that the magnetic properties are mainly governed by the transformation pathways of iron-bearing phases, which are controlled by water availability during synthesis. In turn, this gives the possibility of tailoring both structural and magnetic order through careful selection of the raw material and synthesis conditions, while simultaneously enabling efficient ammonium removal from contaminated water.