*3.1.2 Chromium*

Chromium is a trace element that is very important for human health. However, in large doses can lead to serious health problems. Prolonged exposure to this metal can lead to higher accumulation levels in human and animal tissues, causing toxicity and impacting human metabolism, reducing crop yields [47]. Ingesting Cr-contaminated foods can lead to liver damage, lung congestion, and skin irritation. Chromium and its compounds are widely used in many industrial applications such as plating, tanning, metal finishing, photography [48]. Wastewater from these industries can contain Cr with concentrations ranging from tens to hundreds of mg of L−1. Chromium exists in water mainly in two trivalent forms Cr(III) and hexavalent Cr(VI). Cr(III) is less mobile, non-toxic, and even a trace element for humans and animals to exist in this

### *Hybrid Magnetic-Semiconductor Oxides Nanomaterial: Green Synthesis and Environmental… DOI: http://dx.doi.org/10.5772/intechopen.107031*

form. In contrast, Cr(VI) is present in anionic forms as chromates (CrO4 2−), dichromates (Cr2O7 2−), and bichromates (HCrO4 − ). They are more mobile, highly soluble, and toxic to living organisms. In recent years, Cr pollution, especially Cr(VI) form, in both water and soil environments has increased due to human production activities. Therefore, the use of chromium in industrial production is being restricted gradually in some areas of the world. The treatment of Cr(VI) pollution in soil or water is an urgent issue, attracting much attention from scientists. Many different methods can be used to treat Cr-contaminated water. Among them, the common method is chemical redox combined with immobility. The disadvantages of this method are the prohibitive cost of the system, the consumption of chemicals, the generation of much sludge, the potential risk of re-pollution due to the leakage of sludge into the soil, and especially the recovery of metals to reuse after treatment is hardly feasible [49]. From many research results, the adsorption method has great potential for application in chromium removal, which overcomes the disadvantages of the traditional precipitation method. Shen et al. [50] synthesized Fe3O4 of different sizes using the co-precipitation technique (8 nm) and the polgol method using propylene glycol (35 nm). The Cr(VI) adsorption efficiency was tested, and the results showed that the adsorption capacity of Cr(VI) reached 35.46 and 7.45 mg g−1, respectively. The effect of pH on Cr(VI) removal efficiency was investigated. As a result, a low pH value (2–3) is optimal for adsorption. This is explained by the fact that under acidic conditions, H<sup>+</sup> ions adsorb to the adsorbent surface, making the surface positively charged, from which Cr(VI) exists in the form of oxyanions, which are readily adsorbed by interactions electrostatic action. This explanation agrees with the Cr(VI) adsorption mechanism previously proposed by Chen et al. [51]. A similar study has been done by Rajput et al. [52]. The ferromagnetic oxide nanoparticles are synthesized by the co-precipitation method. The obtained MNPs are spherical in shape, 15–30 nm in size, and the specific surface area is about 12.7 m<sup>2</sup> g−1, the point of zero charges (pHPZC)) 7.4. Adsorbent materials were regenerated in an alkaline environment. Unfortunately, their adsorption capacity decreases quite quickly after each regeneration. The results of these studies show that Fe3O4 can be used to adsorb Cr(VI), but the efficiency is not high, and the reusability is relatively poor. This can be explained because this adsorption process is physisorption, lacking the specific affinity of Cr(VI) for Fe3O4. In addition, the medium favors the adsorption of Cr(VI) under low pH, which easily leads to the dissolution of Fe3O4. Therefore, the study and enhancement of the adsorption capacity of Fe3O4 for Cr(VI) by denaturing, increasing the specific surface area, or introducing organic species with a particular affinity for this metal are being studied.

The research group of Shi et al. [53] attached Fe3O4@SiO2-NH2 to carboxylated biochar to form magnetic biochar to remove Cr(VI) and Cr(III) metals in the solution acid. The results show that Fe3O4@SiO2-NH2 not only enhances the adsorption capacity of Cr(VI) anions but also immobilizes Cr(III) cations. The proposed process mechanism consists of 3 steps: (1) adsorption of Cr(VI) anions on the surface by protonated functional groups; (2) reduction of the Cr(VI) anion to the Cr(III) cation by electron donor groups; (3) complexation and immobilization of Cr(III) by amine and carboxyl groups on magnetic biochar.

Polypyrrole (PPy) is an organic polymer synthesized by oxidative polymerization of pyrrole. PPy possesses many interesting properties such as high electrical conductivity, environmental stability, non-toxicity, and ease of preparation. These advantages make PPy very popular in many different applications [54, 55]. PPy is noted to be positively charged at the N atoms on the main chain, which is highly preferred in synthesizing adsorbents [56]. The PPy/Fe3O4 nanocomposite adsorbent was first synthesized by Bhaumik et al. to remove Cr(VI) [57]. The ferromagnetic nanoparticles were encapsulated by PPy through the in-situ polymerization of the pyrrole monomer. The removal efficiency of Cr(VI) was very high, reaching 100% with a 200 mg/L Cr(VI) solution at pH 2. The team also proposed that the main mechanism in Cr(VI) adsorption is the reduction and ion exchange on the PPy/ Fe3O4 nanocomposite surface. The adsorbent also showed the ability to reuse after two cycles of adsorption-desorption with almost no reduction in the adsorption capacity. The research group of Wang et al. [58] have synthesized a tertiary magnetic nanocomposite consisting of reduced graphene oxide (rGO), polypyrrole (PPy), and Fe3O4 nanoparticles (PPy-Fe3O4/rGO) for Cr(VI) removal application. The material's magnetism has been evaluated, showing a significant reduction in saturation magnetization value compared with Fe3O4/rGO. The Cr(VI) removal efficiency increased significantly after the Fe3O4/rGO nanocomposite was modified with PPy by an in situ polymerization. The results of surveying the influence of foreign ions showed that cations such as Na+ , K+ , Ca2+, and anions such as Cl− , and NO3− hardly affect the removal efficiency of Cr(VI). However, the presence of SO4 2− anion inhibited Cr(VI) adsorption.

## *3.1.3 Simultaneous removal of multiple other heavy metals*

In addition to chromium and arsenic, magnetic nanomaterials have been studied for the simultaneous removal of many other heavy metals. This aspect of research is interesting and has significant practical implications. Because, in practice, wastewater cannot exist as a single metal cation, they are always a complex mixture. The studies also tested the effects of impurities, including other metal ions commonly found in water sources such as Na+ , Ca2+, Mg2+ cations, or anions such as Cl− , SO4 2−, CO3 2−. Several studies have also conducted surveys to evaluate the effect of organic species on the removal efficiency of heavy metals.

Liu et al. [59] synthesized humic acid (HA) coated Fe3O4 nanoparticles by co-precipitation method to remove some heavy metal ions such as Hg(II), Cd(II), Cu (II). The results show that the Fe3O4/HA material system is nano-sized with a Fe3O4 core of approximately 10 nm. In solution, they form particles with hydrodynamic sizes up to 140 nm. Their magnetic saturation is relatively high with 79.6 emu g−1, which makes them easily recovered by an external magnetic field in a short time. The tests also show that Fe3O4/HA has high stability in tap water, natural water, and acidic environments from 0.1 M HCl to alkaline 2 M NaOH with low leaching (Fe ≤ 3.7%; HA ≤5.3%). The removal efficiency of heavy metals of Fe3O4/HA material system compared to Fe3O4 in other publications has been significantly enhanced. Specifically, the removal efficiency is up to over 99% for Hg(II) and Pb(II) and over 95% for Cu(II) and Cd(II) in tap water at optimum pH. The desorption of these metals to an aqueous medium is not significant. This shows the ideal potential of the Fe3O4/HA material system in treating polluted heavy metals in water sources. In another study, Ge et al. [60] synthesized Fe3O4 nanoparticles by co-precipitation method that followed functionalizing surface with a 3-aminopropyltriethoxysilane (APS) agent and then attaching a copolymer of acrylic acid and crotonic acid (AA-co-CA)tail. The material has a uniform size of 15–20 nm, saturation from 52 emu g−1−, and slightly lower with Fe3O4 of 79.67 emu g−1. Fe3O4@APS@AA-co-CA is used to remove heavy metal ions (Cd2+, Zn2+, Pb2+, and Cu2+) from an aqueous solution. Experimental results show that the synthesized adsorbent is highly effective with Pb2+ and Cu2+ ions. Their adsorption capacity reaches 166.1 mg g−1 and 126.9 mg g−1, respectively. For Cd2+ and

*Hybrid Magnetic-Semiconductor Oxides Nanomaterial: Green Synthesis and Environmental… DOI: http://dx.doi.org/10.5772/intechopen.107031*

Zn2+, the adsorption capacity was lower, 29.6 mg g−1 and 43.4 mg g−1, respectively. The limitation of this study is that the method and conditions for the adsorbent regeneration have not been specified. However, the authors have stated that a pH lower than two causes this material to be inactive.

The iron oxide nanomaterial facilitates quick and easy recovery of the adsorbent. However, they have the disadvantage that the chemical activity is quite sensitive. Iron oxides are easily dissolved in an acidic environment, leading to ineffective pollutant removal. Therefore, when using ferromagnetic as a pollutant treatment agent, it is necessary to take accompanying measures to enhance the durability and stability of this material. Especially in the heavy metal elution situation, regeneration of the adsorbent is usually carried out in an acidic medium. A core-shell structure magnetic nanomaterial Fe3O4@SiO2-NH2 was synthesized by Wang et al. [61]. The layer of SiO2 with acid resistance is used as a protective shell for Fe3O4 ferromagnetic cores. In addition, SiO2 rich in hydroxyl groups on the surface also helps to facilitate the functionalization of organic agents. The obtained Fe3O4@SiO2-NH2 had a specific surface area 216.2 m2 g−1, average size 18.4 nm. This material has high stability in an acidic environment. The solubility of Fe in 1 M HCl solution after 24 h is only 1.57%, much lower than 90.7% of bare Fe3O4. The adsorption capacity, affinity for heavy metal cations including Cu(II), Pb(II), and Cd(II), as well as the influence of pH, and foreign electrolytes, were evaluated. The results show that this material has good strength, high selectivity, and the ability to regenerate by acid agent (1 mol/L HCl acid solution) are all very effective. In particular, experiments show that after adsorption of energetic metals, Fe3O4@SiO2-NH2 is easily recovered quickly by an external magnetic field.

Some organic species that have the advantage of complexing with heavy metal ions are also used to modify the surface of nanoparticles with the expectation of high efficiency in heavy metal removal. Among them attracting much attention from researchers is ethylene diamine tetraacetic acid (EDTA). Liu et al. [62] synthesized core-shell magnetic nanomaterials based on Fe3O4, and the surface of the SiO2 shell was directly modified with EDTA with the ratio of Fe3O4:SiO2:EDTA components instead. in which the ratio of these components 2:5:1 gives the best adsorption capacity. Some characteristics of this material include a specific surface area of 24.07 m<sup>2</sup> g−1, an average pore size of 15.40 nm, a total pore volume of 0.09 cm3 g−1, and saturation magnetization from 34.49 emu g−1. Evaluating the effects of other metal cations such as K<sup>+</sup> , Na+ , Mg2+, and natural organic matter (NOM) such as humic acid and sodium alginate had proved that the Fe3O4@SiO2-EDTA material had high selectivity for heavy metals such as Pb(II) and Cu(II). In another study, Ren et al. used EDTA as an affinity enhancer for heavy metal ions [63]. Accordingly, the magnetic nanostructure with core-shell structure Fe3O4-SiO2 was surface modified with chitosan before being added with EDTA tail. The descriptive results show that the magnetic adsorbent has a size from 200 to 400 nm, and specific surface area, pore diameter, and pore volume are 1.04 m<sup>2</sup> g−1, 8.28 nm, and 2.2 × 10−3 cm3 g−1, respectively. The acid stability of the synthesized adsorbent was significantly enhanced after surface modification with chitosan and EDTA. Specifically, after soaking for 12 hours in 1 M HCl solution, the solubility of Fe3O4 of SiO2/Fe3O4, functionalized chitosan SiO2/Fe3O4 (CMS), and EDTA-modified CMS (EDCMS) were 1.28%, 1.02%, and 0.77%, respectively. The saturation magnetization of the materials after the modification steps of Fe3O4 microspheres, SiO2/Fe3O4 microspheres, CMS and EDCMS are 69.0, 56.3, 20.7, and 18.2 emu g−1, respectively. There is a decrease in magnetic saturation after each step of material modification. This phenomenon is also explained similarly to the previous

studies, as the mass content of magnetic Fe3O4 decreases with each addition of other agents. Although the magnetic saturation is reduced, they are still large enough to facilitate the separation of the nano adsorbents from the aqueous medium quickly and easily by the external magnetic field. The results of the heavy metal adsorption test showed that the nano adsorbents particles whose surfaces were modified by EDTA gave the adsorption capacity for Cu(II), Pb(II), and Cd(II) 0.699, 0.596, 0.563 mmol g−1, respectively. While for undenatured chitosan SiO2-Fe3O4 (CMS) nanoparticles, these values are only 0.495, 0.045, 0.040 mmol g−1. This enhancement is attributed to the presence of EDTA, which provides ease of complexation of metal ions. Zhang et al. [64] synthesized magnetic nanomaterials for mercury adsorption in the aqueous medium. Accordingly, the Fe3O4 magnetic cores are surrounded by SiO2 shells and attached to thiol (∙SH) bridges on their surface. This design is built based on Pearson's acid-base theory [65], in which mercury belongs to the group of soft acids, meaning that they quickly form strong bonds with soft Lewis base groups such as ∙CN, ∙RS, ∙SH. The results show that the Fe3O4@SiO2-SH material is nanoscale, with an average diameter of about 10 nm despite having a large size dispersion. Magnetic redundancy and reluctance are almost zero. Magnetic saturation is relatively high, 55.05, 25.45, and 20.47 emu g−1 correspond to Fe3O4, Fe3O4@SiO2, and Fe3O4@ SiO2-SH. The study on mercury adsorption capacity showed that this material has a large adsorption capacity with mercury, even under changing pH of the solution. Especially under low pH conditions, with competitive adsorption of H+ and other cations such as K<sup>+</sup> , Na+ , and Ca2+, the mercury adsorption capacity remained at a high level, 110 mg g−1.
