# An Overview of Nanofiltration and Nanoadsorption Technologies to Emerging Pollutants Treatment
## 4 Synthesis and Fabrication of Nanomaterials for the Remediation of Emerging Pollutants
### Metal-Based Nanoadsorbents  
Ferromagnetic or ferrimagnetic nanoparticles are also widely studied and applied to remove contaminants. These nanomaterials can be produced by some synthetic routes, which involve chemical, physical, and biological processes [118]. Among the chemical processes applied to the production of these nanomaterials, coprecipitation, thermal decomposition, microwaves, sol-gel, hydrothermal and solvothermal synthesis, microemulsion, sonochemistry, and polyol stand out. Chemical methods are widely used because they present high yield of nanoparticle production with low economic investment [119, 120, 121, 122, 123]. Another method used is the wet chemical reduction, which allows the morphological control of the developed nanomaterials and incorporates organic or inorganic ligand [124]. Regarding the production of nanoparticles by physical methods, mechanical milling, electron beam lithography, gas-phase deposition, vapor deposition/standardization, and the electrical explosion of wires are applied [125]. The milling process stands out for its simplicity, in addition to being one of the most used methods for obtaining nanomaterials. In the electron beam lithography method, an electron beam is applied to convert iron particles into iron oxides [126]. Concerning gas-phase deposition, there is the development of a one-dimensional structure of iron oxide through catalyst-assisted chemical vapor deposition on alumina and gold-coated substrates [127]. Physical vapor deposition (evaporation, sputtering and laser ablation) develops thin films by transferring magnetic particles onto a substrate. The material for the development of nanoparticles can also be deposited by electrodeposition processes [128, 129]. The electric wire explosion method applies an electric current sufficient to evaporate a metal wire [130]. The biological synthesis of nanoparticles takes place in the bottom-up approach, where metal ion redox processes and stabilization processes control the size of the nanoparticles. Coatings such as proteins, tannins, and peptides as covering agents are incorporated into the nanoparticles, improving the stability and dispersion of the nanomaterials [131]. The biosynthesis of these nanomaterials can be carried out through biological routes by different organisms, including viruses, algae, microalgae, bacteria, and plants. Furthermore, nanoparticles can be synthesized from macromolecules [132]. Regarding biosynthesis by microorganisms, the formation of nanoparticles can occur both intracellularly and extracellularly [133]. In the intracellular mechanism, there is the diffusion of metal ions in the cells that are reduced by enzymes to produce the nanoparticles. The extracellular process is based on the electrostatic interaction of metal ions, which are attracted to the cell wall of the microorganism for enzymatic reduction to occur [134]. Microalgae are commonly applied to produce metallic nanoparticles. These nanomaterials are formed by the heavy metal biosorption process and are potentially suitable for the treatment of effluents [135]. The biosorption process occurs with the binding of heavy metals at specific sites in the microalgal cell structure. The cell walls of these microorganisms are composed of functional groups of carbohydrates and negatively charged polysaccharides (such as hydroxyl, amino, carboxyl, or sulfhydryl) that represent the binding sites for heavy metals in aqueous solution [136]. The contribution of microalgae in the removal of contaminants does not occur only with the production of metallic nanoparticles. Microalgae reduce the contaminants through mechanisms of bioaccumulation, biodegradation, and biosorption. Thus, these microorganisms are applied to mitigate emerging pollutants, including pesticides, hormones,  
containing the contaminant, in landfills requires that the pollutant remain adsorbed for a long period to avoid its release into the environment.