Enhancing Anion Selectivity in Natural Clinoptilolite through Iron Modification for Water Remediation

1. INTRODUCTION Natural zeolites are widely investigated for water purification due to their ion-exchange capacity, low cost and environmental sustainability. Clinoptilolite, one of the most abundant natural zeolites, shows high affinity toward monovalent and divalent cations, making it effective for the removal of toxic metals such as thallium from contaminated waters. However, its efficiency toward anionic species, such as arsenate and selenite, is limited. Since arsenic and selenium contamination represent a serious environmental and health issue in several areas, including Tuscany (Italy) [1], the development of a green and cost-effective material capable of selectively removing both cationic and anionic pollutants is highly desirable. In this study, natural clinoptilolite was modified with iron to enhance its adsorption performance toward anionic species. 2. RESULTS AND DISCUSSION Natural clinoptilolite exhibited excellent removal efficiency for thallium, confirming its strong cation-exchange behaviour, while arsenic and selenium removal was negligible. Iron modification resulted in a dramatic improvement in arsenic and selenium removal: natural clinoptilolite exhibited very low efficiencies for arsenic (7%) and selenium (6%), while the Fe-modified material achieved removal efficiencies of up to 98% for both elements after 24 h. XRD analysis revealed peak shifts associated with iron incorporation into the zeolite channels and the formation of secondary iron oxide/hydroxide phases (Fig. 1). TG–DTA analyses showed distinct thermal behaviour between natural and Fe-modified clinoptilolite, both before and after adsorption, indicating structural and surface changes induced by iron loading and pollutant uptake. SEM–EDS observations demonstrated that arsenic and selenium were preferentially located at particle edges, where iron-rich domains were detected, confirming the role of surface-bound iron in anion adsorption (Fig. 2). Iron modification also induced changes in particle morphology and size, occasionally leading to the formation of spherical aggregates. Fig. 1 - Powder X-ray diffraction patterns of natural clinoptilolite (black) and iron-modified clinoptilolite (red). Clinoptilolite is the dominant phase, with minor accessory phases such as albite and K-feldspar (orthoclase) due to the natural origin of the material. Iron modification induces slight peak shifts and the appearance of additional reflections related to iron oxide/hydroxide phases. 3. EXPERIMENTAL Natural clinoptilolite was modified by contact with an oversaturated FeCl₂ solution, promoting iron incorporation within zeolite channels and pores as well as the formation of iron-rich aggregates on the external surface [2], [3]. Batch adsorption experiments were carried out using 0.2 g of zeolite in contact with 1 mL of aqueous solutions containing arsenate or selenite at an initial concentration of approximately 500 ppm. Adsorption performance was evaluated after 24 h of contact time. Thermal analyses (TG–DTA) were performed on natural and Fe-modified clinoptilolite, both before and after adsorption, to investigate changes in thermal behaviour related to iron loading and anion uptake. Structural modifications and the presence of secondary iron oxide or hydroxide phases were assessed by powder X-ray diffraction. Particle morphology and elemental distribution were examined by SEM–EDS, while ICP analyses were used to quantify arsenic and selenium removal and to evaluate potential iron release into solution. Fig. 2 - SEM image in secondary electron mode (left) of iron-modified clinoptilolite after arsenic adsorption, showing particle morphology and surface features, and corresponding EDS spectrum (right) confirming the presence of arsenic associated with iron-rich areas on the zeolite surface. 4. CONCLUSIONS Iron-modified natural clinoptilolite proved to be an efficient and selective material for the removal of both cationic and anionic toxic metals from water. The presence of iron significantly enhanced arsenic and selenium adsorption without substantial iron leaching, ensuring material stability. This study highlights the potential of iron-modified natural zeolites as sustainable, low-cost and environmentally friendly solutions for water remediation in arsenic- and selenium-contaminated areas. References [1] G. Tamasi and R. Cini (2004), Science of the Total Environment, vol. 327, no. 1–3, pp. 41–51. [2] A. Badeenezhad, A. Azhdarpoor, S. Bahrami, and S. Yousefinejad (2019, Mol Simul, vol. 45, no. 7, pp. 564–571. [3] M. K. Doula (2009), Water Res, vol. 43, no. 15, pp. 3659–3672.