Magnetically recoverable nano-adsorbents for dual removal of organic dyes and heavy metals from aqueous media

Abstract

Freshwater contamination by heavy metals and synthetic dyes poses serious environmental and health risks, driving the need for efficient, cost-effective, and recoverable remediation technologies. Magnetic nanoparticles (MNPs) have emerged as a promising platform due to their high surface area and easy magnetic recovery. However, tailoring their surface chemistry is essential to maximize pollutant uptake and reusability. In this work, functionalized Fe₃O₄ MNPs were developed via chemical co-precipitation followed by surface functionalization with 3-aminopropyl trimethoxysilane (APTES), glutaraldehyde (GA), and citric acid (CA), yielding MNP-APTES, MNP-APTES-GA, and MNP-CA, respectively, for the efficient removal of heavy metal ions and organic pollutants from aqueous solutions. Physicochemical characterization—including FTIR spectroscopy, dynamic light scattering and zeta potential, electron microscopy, and thermogravimetric analysis—confirmed successful surface modification without compromising magnetic properties of the Fe₃O₄ core. Adsorption performance was assessed for copper(II) ions (Cu²⁺) in the presence of various competing heavy metal ions as well as for methyl orange (MO) dye. MNP-APTES-GA outperformed bare MNPs, MNP-CA, and MNP-APTES, achieving 69% Cu²⁺ removal at pH 6 and 83% MO decolorization at pH 3. Kinetic analysis demonstrated that Cu²⁺ uptake followed pseudo-first-order kinetics, whereas MO dye removal followed pseudo-second-order kinetics. The maximum Cu²⁺ adsorption capacity from the Langmuir model was 14.39 mg g⁻¹, and both Langmuir and Freundlich isotherms fit the experimental data well, suggesting heterogeneous, multilayer and competitive adsorption behavior. In multicomponent systems containing Co²⁺, Ni²⁺, Zn²⁺, and Al³⁺, Cu²⁺ removal remained 22–37%, confirming preferential Cu²⁺ uptake under competitive conditions. MNP regeneration experiments demonstrated robust reusability and good retention of adsorption capacity after five cycles using suitable eluents. Regeneration studies revealed strong reusability, with MO desorption remaining above 80% after five cycles, while Cu²⁺ desorption decreased from ∼76% to ∼30%. The enhanced dual-removal performance of MNP-APTES-GA is attributed to synergistic interactions from amine and aldehyde functionalities, enabling combined electrostatic attraction, coordination, and supramolecular interactions. These findings establish MNP-APTES-GA as a versatile, magnetically recoverable adsorbent with significant potential for scalable, multi-contaminant water remediation applications.