Clay-Supported Fe3O4 Magnetic Nanocomposites as Adsorbents for Heavy Metal Removal from Water and Wastewater: A Mini Review on Trends and Future Perspectives

This mini-review presents the major research trends in the synthesis, performance, and mechanisms of clay-supported magnetic iron oxide nanocomposites for the adsorption of heavy metals in water and wastewater treatment applications. The immobilization of iron oxide nanoparticles onto the hydrophilic natural or synthetic nanoclay matrices not only minimized the magnetic nanoparticles’ tendency to aggregate in aquatic solutions but also facilitated their recovery from the solutions via magnetic separation after adsorption. For these reasons, research on such materials emerged in the early 2010s, leading to the development of highly efficient nanocomposite adsorbents. At optimum conditions, including solution pH values between 5 and 7, rapid equilibrium times ranging from 30 to 180 min, and ambient or moderately elevated temperatures (up to 60 °C), maximum adsorption values of up to 225 mg/g were reported for certain heavy metals. Moreover, the nanocomposites demonstrated reusability, maintaining adsorption performance towards heavy metals for up to five adsorption–desorption cycles when common acids (such as HNO3 and HCl) were used as regenerating agents. However, the current findings are all based on batch-scale laboratory experiments. To move toward industrial-scale applications, further research is necessary to address scale-up challenges and evaluate the performance of the clay-supported magnetic iron oxide nanocomposites under real-world conditions. All the critical limitations are highlighted in the context of this mini review to support future efforts toward achieving their economic and environmentally sustainable application for the adsorption of heavy metals from water/wastewater streams.