Magnetic nanoadsorbents are particularly attractive as they can be easily retained and separated from treated water. Iron and iron oxides NPs are extensively described in the literature for the removal of different heavy metals, such as Ni2+ (Hooshyar et al. 2013; Poguberovi? et al. 2016), Cu2+ (Poguberovi? et al. 2016), Co2+ (Hooshyar et al. 2013), Cd2+ (Ebrahim et al. 2016), as well as for the remediation of chlorinated organic solvents (Han and Yan 2016; Guo et al. 2017). Nonetheless, there are some challenges when using NPs for the remediation of environmental contaminants. Aggregation is one of the major concerns as it can significantly affect the reactivity of the material, and consequently reduce the advantage of using nano scale materials as means of improving efficiency. Another challenge when working with metal and metal oxide NPs is due to the possible toxicity of the materials involved. In addition, costs and fate of the remediation technology are also important factors to consider when opting for the use of NPs as a remediation material. Some examples of strategies to overcome some of these challenges are presented in this section.Iron NPs typically exhibit a core-shell structure, with elemental iron (i.e. Fe0, also termed “zerovalent”) comprising the core and mixed valent (Fe(II) and Fe(III)) oxides forming the shell (Kharisov et al. 2012). Figure 2 illustrates the mechanisms through which iron NPs can be used for the remediation of environmental contaminants. Both chlorinated compounds and heavy metals can be reduced via electron donation from the zerovalent iron core.  Additionally, the shell portion of the NPs can also facilitate the remediation of contaminants, such as those heavy metals that present a higher standard potential (E0) than of the Fe2+/Fe couple (Li and Zhang 2006). Figure 2. Degradation mechanism of chlorinated contaminants and heavy metals from aqueous systems using iron NPs In an effort to investigate how to overcome aggregation of the NPs, Hooshyar (2013) explored the use of sonication of the iron NP solution to enhance Ni(II) and Co(II) removal. It has been demonstrated that nanoclusters can be dispersed when exposed to sonication, but the freed NPs are subject to reaggregation if submitted to longer sonication times. The iron NPs used in the experiments were spherical and 12 nm in diameter. Hooshyar and co-workers (2013) demonstrated that sonication of the iron nanoparticle solution can enhance the removal of Ni(II) and Co(II) with an optimal sonication time of approximately 20 min for nickel removal and 30 min for cobalt removal. The maximum removal efficiencies obtained for each case were 38% for nickel and 59% for cobalt.Several studies have investigated the use of bimetallic NPs as means of overcoming some of the limitations associated with monometallic NPs including their propensity toward aggregation and their low stability. Often, different stabilizers and surfactants are employed to increase the stability of nanoparticle solutions, however, the addition of a second metal to the formulation can enhance the solution stability of the material and obviate the need for stabilizers and surfactants (Khin et al. 2012). Improved stability can contribute to increased efficiency and capacity, and can accelerate the degradation rate of contaminants (Tao 2012). The incorporation of a second metal, such as Pd (Lien and Zhang 2005; Chen et al. 2008; Nagpal et al. 2010; Z. Zhang et al. 2010; X. Wang et al. 2009), Ni (Tee et al. 2005; Wu and Ritchie 2006; Fang et al. 2011; Xie et al. 2014) and Cu (Zheng et al. 2009; Zhu et al. 2010), has been reported in the literature as a strategy for enhancing the stability of zero-valent iron NPs (nZVI). Some noble metals (i.e. resistant to corrosion and oxidation in moist air) can be combined with nZVI to catalyze dechlorination and hydrogenation reactions with contaminants, therefore resulting in a more efficient remediation method (Karn et al. 2009). For instance, in their study of the effects of the addition of Pd to nZVI particles, Wang et al. (2009) described an enhanced rate of dehalogenation of chlorinated organic compounds, albeit at a higher cost due to the expense of elemental palladium. Other alternatives to enhance the stability of NPs include the use of supporting materials, such as several examples that follow in our discussion of polymer-based materials (vide infra).Furthermore, another concern regarding the use of metal-based NPs is due to the possible toxicity of the chemicals used to synthesize the material and of the byproducts generated from the contaminant degradation. Poguberovic (2016) successfully demonstrated the use of nZVI for the removal of Ni(II) and Cu(II) from aqueous solutions. The nZVI used in the experiments were synthesized using oak and mulberry leaf extracts. The compounds present in these high antioxidant extracts react with iron(III) to form nZVI (Machado et al. 2013). Using natural products for the fabrication of environmental remediation materials is important to overcome concerns regarding the possible toxicity of chemicals and byproducts when using chemical synthesis approaches. In addition, the “green” synthesis of nZVI demonstrated by Poguberovi and co-workers (2016) have the advantage of adding value to natural resources, such as the leaf extracts, that are otherwise considered waste and providing a low-cost adsorbent for the remediation of heavy metals from water. The study demonstrates fast kinetics and rate of adsorption of the nZVI, with adsorption isotherm results showing a higher capacity for Ni(II), 777.3 mg Ni/g when using oak leaf extracts, and higher capacity for Cu(II), 1,047 mgCu/g when using mulberry leaf extracts.  The maximum removal of Ni(II) was achieved at pH=8.0, while the maximum removal of Cu(II) was achieved at pH=7.0. While this study is promising, further investigation is necessary for a full-scale application of this material in wastewater treatment. Furthermore, a pH sensitive remediation technology may impose limitations to its applicability to in situremediation, as some of the environment may not present adequate conditions for the remediation to occur successfully and efficiently.Another binary mixed magnetic nanoparticle system has been developed by Ding et al. XMFA12 , they synthesized lecithin-loaded Ni/Fe nanoparticles by microemulsion method and was tested through employing 3,3?,4,4?-tetrachlorobiphenyl (PCB77) as target pollutant. Lecithin was used as a biocompatible surfactant for forming monodispersed and stable (lecithin-Ni/Fe NPs) and to capture the targeted organic contaminant. The results revealed high efficiency and rapid removal of PCB77 by lecithin-nano Ni/Fe than that by blank carrier (nanolecithin). They succeeded to develop relatively low toxic and inexpensive organic-inorganic combined material for targeting polychlorinated biphenyls contaminants.

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