# Silica-Coated Magnetic Nanocomposites for Pb\({}^{2+}\) Removal from Aqueous Solution
## 1 Introduction  
The modernization of human life is directly linked with soil, air and water pollution, one of the main concerns being related to the development of industries that release into the environment untreated chemicals and toxic wastes. The removal of pollutants represents nowadays a global challenge and, in particular, heavy metals dispersed in the ambient are highly detrimental for human health being one of the major cause of cancer, birth defects or neurological problems to name a few, not to mention damage to flora and fauna [1]. Materials scientists have developed several strategies for the removal of heavy metals from water, ranging from chemical to physical, magnetic or electric techniques and the best performing in terms of cost, efficiency, simplicity, separation and regeneration seem to rely on chemical or physical adsorption [2]. Therefore different engineered nanomaterials were developed recently, including silica or carbon-based, metal or graphene oxide nanoparticles. Among these, magnetic iron oxide nanoparticles emerged not only for their good adsorption properties, but also for the possibility of easy removal from aqueous systems [3]. Iron oxide magnetic nanoparticles (MNPs), indeed, were shown to have large surface areas with many active sites for adsorption of metal ions, good selectivity and low toxicity [4; 5; 6; 7]. However, due to their hydrophobic surfaces, aggregation occurs in aqueous media, which lowers the efficiency of adsorption [8]. Therefore,coating with organic or inorganic shells with porous, amorphous or ordered structures was carried out by several groups [9; 10]. In this area, silica-based materials are hydrophilic and also considered as excellent adsorbents, being biocompatible, chemically and physically stable and cheap [11], so the combination of the two materials is considered a winning strategy for obtaining the best adsorbents. Moreover, silica coating due to the abundant presence of hydroxyl groups facilitates the functionalization of the shell [7]. A thorough bibliographic search showed that great efforts have already been carried out to obtain MNPs@SiO2/R nanocomposites, where R is usually an organic or hybrid organic-silica coating offering N or O-donor sites to coordinate the heavy metal center. Thus absorbents for heavy metals were obtained with Fe3O4@SiO2/TSD-TEOS (TSD--(trimethoxysil)propyl]ethylenediamine, TEOS--tetraethyl orthosilicate) [12], Fe3O4@SiO2/TETA (TETA--triethylenetramine) [13], Fe3O4@SiO2/Chitosan [14], Fe3O4@SiO2/NH2 [15], Fe3O4@SiO2/Vaniline [16], Fe3O4@SiO2/DMSA (DMSA--meso-2,3-dimercaptosuccin acid) [17], MNPs@SiO2/Bismuthid II [18]. The synthesis usually involves a three-step procedure: (i) the obtaining of magnetic MNPs; (ii) the synthesis of MNPs@SiO2 by the coating of MNPs with silica and (iii) a subsequent covering of the silica coating of MNPs@SiO2 by co-condensation of another functional silica shell [19; 20], covalent binding a functional derivative [12] or adsorption through electrostatic attraction between opposite charges [14]. Surprisingly, no systematic studies have been reported on the adsorption properties of non-functionalized MNPs and MNPs@SiO2 nanocomposites, although some groups investigated their abilities to adsorb heavy metals mostly comparative to the functionalized materials in order to highlight the improvement of their properties post-functionalization [13; 15; 16; 17] and there is one report on core-shell Fe3O4@SiO2 microspheres showing Pb2+ removal efficiencies of 97% and good recyclability [21]. Both Fe3O4 nanoparticles and Fe3O4@SiO2 nanocomposites are good adsorbents especially for Pb2+ ions due to the quite strong non-covalent interactions between the metal ion stereochemically active lone pair of electrons and the octahedral voids of Fe3O4 in the former and electrostatic interactions between the positively charged metal and negatively charged SiO2 coating in the latter [14].