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][3][4][2,3,4]. On the other hand, the second term refers to the volumetric abundance of water supply ^[5][6][7][5,6,7], which is the ratio of human water consumption to available water supply for a specific area ^[8][9][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][11][12][10,11,12]. 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][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][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][18,19].

Water pollutants are categorized into point source, where the pollution originates from a single and identifiable source, and nonpoint source, where pollutants originate from a variety of sources [20]. Based on their main physicochemical characteristics, they can also be classified into radioactive, thermal, microorganism, nutrient, suspended solid and sediment, and organic and inorganic pollutants [21]. Inorganic pollutants consist of heavy metals, fertilizers, sulphides, ammonia, oxides of nitrogen, acids, and bases ^[22][23][24][22,23,24]. Since water quality is improving with increasing advances in technologies [17] and water purification is highly required to prevent toxic effects and disruption of ecological balance, this particular research review focuses on water contamination by heavy metals and ways of heavy metal removal.

Pollutants can contaminate water through both natural processes and anthropogenic activities ^[25][26][25,26]. Their concentrations in water depend on the local geological, hydrogeological, and geochemical characteristics of the aquifer. A significant burden on water quality and, consequently, public health is the increasing concentrations of heavy metals due to their toxicity, persistence, and bioaccumulative nature ^[25][27][25,27]. Unlike organic contaminants, heavy metals, which are toxic and mostly carcinogenetic, are not biodegradable and tend to accumulate in organisms [28]. Hence, exposure to toxic heavy metal through drinking water has long been a critical public health concern across the world [29]. Elements whose density exceeds 5 g/cm³ fall into the category of heavy metals [30] and are listed in Table 1 based on their toxicity, which is related to the maximum contaminant level (MCL) [31]. Specific differences in metal ion toxicities arise from differences in solubility, absorbability, transport, chemical reactivity, and the complexes that are formed within the body [32].

Heavy metals are classified into essential (Zn, Cu, Fe, and Co) and nonessential (Cd, Hg, As, and Cr) based on their toxicity. At low concentrations, essential heavy metals are harmless, unlike nonessential metals, which are highly toxic ^[34][35][34,35]. It is worth noting that for nonessential heavy metals, toxicity is very high, even at low concentrations ^[36][37][36,37]. Water contamination by heavy metals occurs in anthropogenic activities or in natural processes. Sources of contamination include metal corrosion, atmospheric deposition, soil erosion of metal ions, leaching of heavy metals, sediment resuspension, and metal evaporation from water resources to soil and groundwater. Phenomena such as weathering and volcanic eruptions fall into the category natural contamination [38]. In contrast, the major sources resulting from anthropogenic activities are mining wastes, landfill leaches, municipal wastewater, urban runoff, industrial wastewater, electroplating, and electronic and metal finishing industries ^[39][40][39,40]. Moreover, due to an increasing production of metals from technological activities, the problem of waste disposal has become of paramount importance; hence many aquatic environments face metal concentrations that do not meet water quality criteria designed to protect the environment, animals, and humans [41]. Metals such as Cu, Cd, Cr, Hg, and Ni have the ability to produce reactive radicals, resulting in DNA damage, lipid peroxidation, depletion of protein sulfhydryls, and other effects [42]. In this context, the availability and release of pollutants from anthropogenic sources needs to be considered [43]. The source of pollution and the effect of each heavy metal on human health are presented in Table 2.

However, the removal of heavy metal ions from water has been a vital and challenging issue for several decades ^[47][48][47,48], without reaching the heart of the problem. On the other hand, in recent years nanotechnology has been integrated with several novel techniques for the removal of heavy metals from water systems to improve removal efficiency ^[49][50][51][49,50,51].

Several water purification methods for heavy metals exist, such as chemical precipitation and coagulation, flocculation, electrochemical methods, photocatalytic degradation, membrane filtration, ion exchange, bioremediation, and adsorption, just to mention few ^[16][52][53][16,52,53]. The present work focuses on the adsorption methods developed during the last decade, where nanotechnology contributed quite a lot to great progress. In this context, adsorption capacity and reuse efficiency of magnetic nanoparticles for capturing heavy metals in water environments are investigated. In addition, the findings are analysed based on certain characteristics, such as viability method, time of purification, water quality after purification (adsorption capacity), and nanoparticle reusability.

The main interest in the adsorption method is the interaction between nanoparticles and adsorbents, which depends on their physicochemical properties [54]. The term physicochemical covers the particle size, surface area, surface charge, agglomeration, morphology, surface coating, and so forth. Particles with sizes below 100 nm are defined as nanoparticles [55], and their applications vary according to their size ^[54][56][57][54,56,57]. Magnetic, mechanical, optical, and electrical properties affect the formation and aggregation of nanoparticles [44]. Aggregation of nanoparticles is affected by surface charge, particle size, and composition. Nanoparticles without magnetic properties have limited applicability in water purification due to the difficulty of separation from the aqueous solution [58]. Magnetic nanoparticles present advantages due to their large surface area, size, and shape-dependent catalytic properties and can be separated from the aqueous solution with the use of a magnet field ^[48][59][48,59]; thus many methods have been investigated for their potential application in both environmental and biological fields ^[60][61][62][60,61,62].

Our focus was on studies that have been published during the last decade. The selection of relevant research articles was performed in several stages. In the first stage, the search was based on article title, abstract, and keywords. The terms for the search were (a) water purification, (b) magnetic nanoparticles, (c) heavy metals, and (d) reusability. In the second stage, we selected articles according to purification method (adsorption process). The final stage included the expansion of the bibliography using the remaining articles’ reference lists.

2. Main Findings during the Last Decade

Restrictions on the use of bare hematite, magnetite, and maghemite nanoparticles force the majority of researchers to synthesize modified nanoparticles from iron oxide. A large number of parameters apart from those of the magnetic nanoparticle synthesis, which have been analysed above, affect the adsorption efficiency of heavy metals, among them being pH, contact time, temperature, adsorbent dose, and initial ion concentration ^[58][63][58,166]. pH is a factor that is involved not only at this stage of the method but also during the synthesis of nanoparticles; almost in all cases, the pH values are different between these two stages. pH is directly related to the competition ability of hydrogen and metal ions to the adsorbent surface active sites, for by increasing the pH value due to the formation of soluble hydroxylated metal complexes, the metal uptake capacity decreases ^[64][161], the maximum adsorption capacity that is observed varies from pH 2 to 9, and most researchers achieve optimum adsorption capacity at pH 5–7. The adsorbent dosage is directly related to the adsorbent capacity since it determines the contact areas between the adsorbent and the adsorbate ^[65][167]. Moreover, when the adsorbent dose is increasing, the number of available binding sites is also increasing; but as the equilibrium point of adsorption is reached, the efficiency does not reflect the available sites and remains steady ^[64][161]. A critical point also exists for the contact time parameter, since initially, removal rates increase rapidly with time; then they gradually decrease due to the availability of the binding sites until the equilibrium is reached ^[64][161]. Experiments show that even for the same adsorbent but for different pollutants, the ideal contact time may vary significantly, as in some cases it is between 30 min to 12 h ^[66][157]. The factor that has been investigated less is the effect of coexisting cations/anions, as in previous parameters, the initial ion concentration does not differ; thus removal rates increase with increasing initial concentrations until a point where the rates remain unaffected ^[64][161]. Research shows that these cations/anions that are contained in water have no significant influence on adsorption capacity ^[67][68][168,169], since the adsorbent surfaces suggest multisurface adsorption active sites. Moreover, differences in the radius of heavy metal ions have significant influence on adsorption efficiency ^[69][170], ions with a smaller radius have higher mobility in aqueous solutions, and therefore, they have a lower tendency to adsorb on magnetic nanoparticles. Additionally, the temperature that is used during the adsorption process varies between 15 and 45 °C, although in some rare occasions, it reaches 70 °C. Every research method must be evaluated according to the criteria that have been set from the beginning. At this phase of the literature review, the results of heavy metal adsorption in aquatic solutions using bare or modified magnetic iron oxide nanoparticles are summarized. In order to be able to evaluate the methods through the experimental results since 2010, many factors must be taken into consideration. Initially, many researchers used different nanoparticle compounds for various pollutants, but also observed a large selectivity of the initial parameters (i.e., adsorbent dosage, contact time, pH, temperature), which creates difficulties in the categorization of the experimental results. Despite that, the main purpose of every work is the removal of heavy metals; thus adsorption capacity is the main factor that has been focused on in each research. Hence, Table 35 provides useful information, such as the time of equilibrium/contact time, pH, adsorption capacity or removal efficiency, and temperature during the adsorption process, which could lead to safe extractions for the applicability of the method, which is based on the findings during the last decade. Additionally, the findings are listed in chronological order. Due to several nanoparticle compounds, the equilibrium time is not constant but has large dispersion. [171], so the nanoparticles could be used again. Among the selected articles, all have employed desorbing agents during the desorption process. The performance of the desorption process for magnetic nanoparticles is directly related to the size, coating, magnetic behaviour ^[266][159] and pH of the solution ^[267][357], while other critical factors are the kind of regenerative solutions (i.e., NaOH, HCl) but also their concentration. For example, 2 M of HNO₃ has a desorption efficiency of Cr (VI) equal to 73%, but the efficiency drops to 20% when 0.1 M of HNO₃ is employed ^[136][231]. Additional benefits of the desorption process are the limited cost of desorbing agents and the time of the process, which could be achieved in less than 1 h. The usage of the desorption process is enhanced by the ease of collection, which comes from the selectivity of the paramagnetic nanoparticles assisting the technique, because they could be readily separated from the solution when a magnetic field is applied; thus iron oxide nanoparticles are more preferable to other nanoparticles with no magnetic cores. Additionally, computational fluid dynamics could be employed at this stage, reducing the cost of the materials. The major advantage of computational water treatment methods compared with an experimental method is that the steps of synthesis and characterization of magnetic nanoparticles are not time-consuming since they do not exist. The aims of microfluidic mixing and driving simulations for water purification from heavy metal ions are to achieve rapid mixing and desired guidance of nanoparticles ^{[48][51][59][268]}[48,51,59,358]. Nonetheless, the effectiveness of the adsorption–desorption process is evaluated by the efficiency of heavy metal adsorption after each recycling. An ideal adsorbent is considered to be one that simultaneously possesses high adsorption capability and high desorption efficiency ^[269][359]. Of critical importance is the effectiveness of the process in several studies, in which up to five cycles of adsorption without a significant decrease in efficiency have been achieved ^[270][360]. In addition, high adsorption and desorption efficiencies equal to 98.4% after seven cycles ^[214][306] and 98% (constant) for over 15 cycles have been reported ^[70][171]. The deterioration of active binding sites on the surface absorbent during recycling results in a decrease in efficiency. It should be noted, however, that in some cases after the desorption process, the adsorption efficiency did not decrease, but instead, it increased in the next cycle ^[271][361]; this phenomenon is based on the increase in the positively charged surface of nanoparticles, which leads to increased electrostatic attraction forces between the iron oxide nanoparticles and the pollutant. Recycling of the adsorbent is important to obtain the process that enhances the viability of the adsorption method. The recycling efficiency and adsorption capacity for each cycle are presented in Table 46 and Table 57, respectively. We must mention here that there are very few articles that have investigated the adsorption capacity. This fact has a negative impact on the applicability of the method. During the literature review were recorded cases where the recycling effectiveness was measured with adsorption capacity instead of efficiency. Additionally, both adsorption and desorption efficiency decreased through the regeneration cycles and due to the difficulty in reversing adsorption ^[272][362]. A crucial stage in the evaluation of the applicability of the experimental methods is the possibility of reducing the production cost and time. This reduction could be achieved by reusing the existing nanoparticles; thus the stages of synthesis and characterization could be avoided. Desorption processes may occur either by thermal treatment or through suitable desorbing agents and are necessary for recycling ^[70]