Comparative performance of iron-modified natural zeolites for chromium(VI) removal from aqueous solutions

Natural zeolites modified with iron(II) ions or iron(III) phases represent two promising approachesfor treating hexavalent chromium contamination in water due to their reduction and adsorptioncapacities respectively. However, comprehensive comparative analysis of their efficiency remainslimited. Fe(II)-modified zeolites can reduce Cr(VI) to the less toxic and insoluble Cr(III) form, whileFe(III)-modified zeolites primarily remove Cr(VI) through surface adsorption.This study evaluates the effectiveness of Fe(II)- and Fe(III)-modified zeolites using synthetic Crsolutions and real contaminated surface water. The materials were characterized via X-raydiffraction, scanning electron microscopy, transmission electron microscopy and x-ray fluorescencespectroscopy to assess their chemical, structural and morphological changes before and afterchromium sorption. Batch and column tests determined Cr(VI) removal capacity, while Cr-isotopeswere used to track reduction reactions.The Fe(II) enrichment via cation exchange was highly effective. The Fe(II)-modified zeolitedemonstrated a higher capacity for Cr(VI) removal from contaminated solutions, primarily through areduction mechanism that converted Cr(VI) to Cr(III), as was confirmed by isotopic analysis. Itsperformance improves under higher salinity conditions. However, it requires prolonged contact timewith the contaminant to achieve optimal efficiency.The Fe(III)-modified zeolite exhibited heterogeneous distribution of iron oxide/hydroxide phases onits surface. This material showed some favorable characteristics in batch and column tests, includinga notably faster Cr removal rate due to direct Cr(VI) sorption. On the other hand, it can only performwell when no other competitive ions are present in the solution, severely impacting its application incomplex, real matrices.