Removal of toxic hexavalent chromium ions from water using magnetic protic poly (ionic liquids) nanocomposites

Novel magnetic nanocomposites (MNCs) based on Fe₃O₄ and NiFe₂O₄ nanoparticles were synthesized at low temperature via an in-situ approach using linear and crosslinked quaternized protic poly (ionic liquid) (PIL) matrices. Linear PILs were prepared from quaternized triethanolammonium acrylate and quaternized triethanolammonium 2-acrylamido-2-methylpropane sulfonate to form LQAA, while crosslinked copolymerization with N,N'-methylenebisacrylamide yielded CQAA hydrogels. For comparison, acrylic acid-co-2-acrylamido-2-methylpropane sulfonic acid (CAA) hydrogels were also fabricated. The resulting MNCs were comprehensively characterized to elucidate their chemical structure, thermal stability, magnetic behavior, surface morphology, and crystallographic features. The nanocomposites exhibit superparamagnetic behavior, high structural stability, and uniform dispersion of ferrite nanoparticles within the ionic and hydrogel matrices. Their adsorption performance toward Cr ions removal from aqueous solutions was systematically investigated, including the effects of pH, contact time, and adsorbent dosage. The fate of Cr⁶⁺ in the aqueous solution, as well as the surface of the material, was determined; the results demonstrated not only the successful adsorption of Cr⁶⁺ from solution, but also confirmed the presence and in situ reduction of Cr⁶⁺ to Cr³⁺ on the surface. Kinetic and isotherm analyses reveal rapid adsorption governed by surface chemisorption and diffusion processes. Remarkably, the MNCs demonstrate exceptionally high equilibrium Cr ions adsorption capacities (800-950 mg g^-1), exceeding those of most reported magnetic adsorbents. This superior performance arises from the synergistic combination of functionalized Fe₃O₄ and NiFe₂O₄ surfaces with highly charged PIL networks, which provide a high density of accessible binding sites. Competitive adsorption studies using simulated industrial wastewater containing Cr⁶⁺, Cu²⁺, and Ni²⁺ (up to 1000 mg L^-1 each) further confirm the strong affinity, selectivity, and stability of the developed materials, particularly toward Cr ions. These findings highlight the potential of PIL-based magnetic nanocomposites as efficient, magnetically recoverable, and reusable adsorbents for advanced treatment of heavy metal contaminate.