In recent years, there has been an increasing use of engineered magnetic nanoparticles for remediation and water treatments, leading to elevated public concerns. To this end, it is necessary to enhance the understanding of how these magnetic nanoparticles react with contaminants and interact with the surrounding environment during applications. This review aims to provide a holistic overview of current knowledge of magnetic nanoparticles in environmental applications, emphasizing studies of zero-valent iron (nZVI), magnetite (Fe3O4) and maghemite (γ-Fe2O3) nanoparticles. Contaminant removal mechanisms by magnetic nanoparticles are presented, along with factors affecting the ability of contaminant desorption. Factors influencing the recovery of magnetic nanoparticles are outlined, describing the challenges of magnetic particle collection. The aggregation of magnetic nanoparticles is described, and methods for enhancing stability are summarized. Moreover, the toxicological effects owing to magnetic nanoparticles are discussed. It is possible that magnetic nanoparticles can be applied sustainably after detailed consideration of these discussed factors.

A novel magnetic polymeric adsorbent, namely, magnetic hydrogel, was used to investigate its reusability and applicability in Cr(VI)-bearing wastewater treatment using magnetic separation. Different concentrations and amounts of NaCl solution and a stepwise approach were used for the regeneration experiment. A stepwise adsorption process followed by stepwise 3.0 M NaCl regeneration with a 40:1 wastewater-to-recovery volume ratio was found to be the most applicable working condition. The Cr concentration in the recovery solution was increased 25−30 times to 500−600 mg/L. The Cr(VI) removal and recovery performance of magnetic hydrogel was maintained for 20 cycles. An industrial wastewater treatment prototype, including a magnetic separation unit, was developed. The magnetic separation unit was designed to provide a magnetic field at the bottom with a zigzag pathway feature for maximizing the chance of capturing magnetic hydrogel. The separation efficiency for the magnetic hydrogel was above 97% throughout the 20 cycles of treatment.

Many magnetic adsorbents reported in the literature, such as iron oxides, for Cr(VI) removal have been found effective only in low pH environments. Moreover, the application of polymeric hydrogels on heavy metal removal has been hindered by difficulties in separation by filtration. In this study, a magnetic cationic hydrogel was synthesized for Cr(VI) removal from contaminated water, making use of the advantages of magnetic adsorbents and polymeric hydrogels. The magnetic hydrogel was produced by imbedding 10-nm g-Fe 2 O 3 nanoparticles into the polymeric matrix via radical polymerization. Characterization of the hydrogel was undertaken with Fourier transform infrared and vibrating sample magnetometer; swelling properties were tested and anionic adsorption capacity was evaluated. The magnetic hydrogel showed a superior Cr(VI) removal capacity compared to commercial products such as MIEX Ò . Cr(VI) removal was independent of solution pH. Results show that Cr(VI) removal kinetics was improved drastically by grinding the bulk hydrogel into powder form. At relevant concentrations, common water anions (e.g., Cl À , SO 4 2À , PO 4 3À ) and natural organic matter did not exhibit significant inhibition of Cr(VI) adsorption onto the hydrogel. Results of vibrating sample magnetometer indicate that the magnetic hydrogel can be easily separated from treatment systems. Regeneration of the magnetic hydrogel can be easily achieved by washing the Cr(VI)-loaded hydrogel with 0.5 M NaCl solution, with a recovery rate of about 90% of Cr(VI).