There have been numerous pieces of evidence in history that repeatedly proved that the presence of contaminants affects aquatic organisms and can spread quickly to large areas, making the contamination of fresh and marine sediments and water environments by oil spills, urban runoff, and industrial and domestic effluents a serious concern. Heavy metals like mercury, lead, and manganese, as well as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and dichlorodiphenyltrichloroethane (DDT), are common trace contaminants found in freshwater and marine environments. The existence of these pollutants is concerning, as trace amounts of the organic compounds have been shown to cause cancer in mammals and to be harmful to human health as well as the aquatic biota. Technologies for toxicityToxicity reduction are frequently used in innovative processes for treating wastewater that contains heavy metals. Environmental safetyEnvironmental safety is just one of the many scientific and technological domains that have been impacted by nanotechnology. Water remediation and treatment is one of the most promising and advanced environmental uses of nanotechnology. Various nanomaterialsNanomaterials can help purify water through a variety of mechanisms, such as the adsorptionAdsorption of heavy metals and other pollutants, the removal and inactivation of pathogens, and the conversion of toxic materials into less toxic compounds. Functionalized magnetic materials are gaining significant attention in wastewater purification due to their efficiency, reusability, and unique properties. These materials leverage the magnetic core for easy separation and functionalized surfaces for targeted adsorptionAdsorption or catalytic activities. These days, metals (such as cadmium, copper, lead, mercury, nickel, and zinc), nutrients (such as phosphate, ammonia, nitrate, and nitrite), organics, algae (such as cyanobacterial toxins), viruses, bacteria, parasites, and antibiotics are all revealed and removed by nanoparticles, nanomembranes, and nanopowder. Recent developments in nanotechnology for wastewater treatmentWastewater treatment are described in this chapter by linking these technological advancements to the physicochemical characteristics of magnetic nanomaterialsMagnetic nanomaterials. The features, processes, and potential production of magnetic nanoparticlesMagnetic nanoparticles (MNPs) in water purificationWater purification are discussed.