materials Review Heavy Metal Adsorption Using Magnetic Nanoparticles for Water Purification: A Critical Review Christos Liosis 1 , Athina Papadopoulou 2 , Evangelos Karvelas 3,4 , Theodoros E. Karakasidis 4, * and Ioannis E. Sarris 3   Citation: Liosis, C.; Papadopoulou, A.; Karvelas, E.; Karakasidis, T.E.; Sarris, I.E. Heavy Metal Adsorption Using Magnetic Nanoparticles for Water Purification: A Critical Review. Materials 2021, 14, 7500. https:// doi.org/10.3390/ma14247500 Academic Editors: Fernando Gomes de Souza Junior and Francisco Javier Guzmán Bernardo Received: 17 September 2021 Accepted: 1 December 2021 Published: 7 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Civil Engineering, University of Thessaly, 38334 Volos, Greece; cliosis@uth.gr 2 Inorganic Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, 15771 Athens, Greece; athinapapad@chem.uoa.gr 3 Department of Mechanical Engineering, University of West Attica, 12243 Athens, Greece; karvelas@uth.gr (E.K.); sarris@uniwa.gr (I.E.S.) 4 Condensed Matter Physics Lab, Department of Physics, University of Thessaly, 35100 Lamia, Greece * Correspondence: thkarak@uth.gr; Tel.: +30-22310-60280 Abstract: Research on contamination of groundwater and drinking water is of major importance. Due to the rapid and significant progress in the last decade in nanotechnology and its potential applications to water purification, such as adsorption of heavy metal ion from contaminated water, a wide number of articles have been published. An evaluating frame of the main findings of recent research on heavy metal removal using magnetic nanoparticles, with emphasis on water quality and method applicability, is presented. A large number of articles have been studied with a focus on the synthesis and characterization procedures for bare and modified magnetic nanoparticles as well as on their adsorption capacity and the corresponding desorption process of the methods are presented. The present review analysis shows that the experimental procedures demonstrate high adsorption capacity for pollutants from aquatic solutions. Moreover, reuse of the employed nanoparticles up to five times leads to an efficiency up to 90%. We must mention also that in some rare occasions, nanoparticles have been reused up to 22 times. Keywords: adsorption; contamination; magnetic nanoparticles; heavy metals 1. Introduction Nowadays, water issues, such as exhaustion of resources and quality of drinking water, have attracted the interest not only of researchers but also of national and international organizations and governments [1]. Terms such as water stress and water scarcity are now subjects of research on a daily basis. The first term refers to situations where the quantity of available water is not sufficient for agricultural, industrial, or domestic uses. It takes into account several physical aspects related to water resources, such as water quality, environmental flows, and water accessibility [2–4]. On the other hand, the second term refers to the volumetric abundance of water supply [5–7], which is the ratio of human water consumption to available water supply for a specific area [8,9]. The main factors that cause water crisis in the long run are population growth, expansion of industrial activities, urbanization, climate change, depletion of aquifers, and water contamination [10–12]. It is obvious that the coverage of global demands for safe drinking water in the near future is utopian, if we consider that water quantity is nearly constant due to the hydrologic cycle in various forms, such as seawater, groundwater, surface water, and rainwater [13,14]. However, climate change will affect the quality and quantity of potentially available drinking water due to increased flooding, more severe droughts, and enhanced toxicity of chemical contaminants in the environment [15,16]. Of more immediate concern is the efficiency of existing water treatment methods due to increasing pollution resulting from the anthropogenic activities [17]. Thus, purification of water from polluted sources is essential to enable the utilization of sustainable global water [18,19]. Materials 2021, 14, 7500. https://doi.org/10.3390/ma14247500 https://www.mdpi.com/journal/materials