High-selectivity and regenerable iron oxide-coated resins for enhanced iron and manganese removal

Ion exchange resins used for water softening are susceptible to irreversible fouling in the presence of iron and manganese. This study introduces regenerable iron oxide-coated resins (IOCRs), synthesized by binding Fe3+ ions to sulfonate-based cation exchange resins, followed by hydrolysis and aging, to enhance the selective removal of Fe and Mn. Comparative experiments were conducted to evaluate the performance of IOCRs in relation to pristine resins, focusing on selectivity, removal capacity, and regeneration stability. IOCRs exhibited a selectivity ratio greater than 1.95 for Fe and Mn over Ca2+ and Mg2+, approximately 5 times higher than the 0.4 ratio observed for uncoated resins, due to surface complexation and oxidation. IOCRs maintained stable Fe and Mn removal, meeting irrigation water quality standards and outperforming uncoated resins. Their volumetric treatment capacity increased by 49% compared to pristine resins, with stable operation sustained up to 820 bed volumes. This corresponds to a molar metal removal that is approximately 3–7 times greater than previously reported values for granular media. Periodic backwashing, performed every 50 bed volumes, effectively sustained performance, keeping filter pressure below 2 kPa. Optimal regeneration was achieved with a single 7-hour immersion in 20% NaCl, ensuring long-term stability and reusability of the material. IOCRs combine the filtration consistency of polymeric resins with the strong affinity of iron oxide for transition metals, enabling the efficient removal of Fe and Mn. These findings position IOCRs as a promising sorbent for mitigating metal-related water quality issues and highlight the potential for upcycling spent resins into high-performance filtration materials.