**3.2 Degradation of organic pollutants**

Organic pollutants are toxic organic compounds that cause human health problems and diseases when residues exceed permissible limits. Industrial products such as detergents, organic solvents, dyes, pesticides, or some organic pollutants, which are biochemical products of bacteria, fungi, and mainly algae secreted, etc., are toxic and carcinogenic. They can exist in different forms in environments where typical physicochemical or biological techniques have become ineffective in removing them [66, 67].

Magnetic adsorbents are usually designed in a core-shell structure. Magnetite nanoparticles will have a core of magnetic material and a shell that adsorbs and treats pollutants outside. Zhang and Kong [68] have synthesized Fe3O4/C magnetic adsorbent in which the carbon (C) adsorbent layer is covered on the surface of the magnetic nanoparticle. The synthesized nanoparticles, which have an average diameter of about 250 nm, are good dispersion in aqueous media and are quickly and easily separated by external magnetic fields. The adsorption efficiency was tested with MB and CR pollutants. The adsorption capacity of Fe3O4/C for these pollutants was 44.38 mg g−1 and 11.22 mg g−1, respectively.

Microcystins (MCs) are a class of toxins produced by certain species of freshwater cyanobacteria, commonly known as blue-green algae. There are more than 50 different compounds belonging to this group. They are disocyclic heptapeptides with a molecular weight of about 1000 Da, such as MC-RR, MC-YR, and MC-LR. Among them, MC-LR is the most popular. Microcystin belongs to a group of toxins that are very dangerous to humans, livestock, and pets if ingested [69]. In order to remove them from water, methods such as coagulation and mechanical filtration can be used, but only the insoluble particles are removed. For dissolved poisons, activated carbon

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

can be used, but effective removal of MC requires a large amount of activated carbon adsorbent. With chemicals for chlorination or ozonation, there are similar limitations. In addition, using these methods carries the risk of creating secondary toxic products [70]. Deng et al. [71] synthesized mesoporous microspheres, sandwich structures of approximately 500 nm in size, to treat this type of organic poison. In this study, the core-shell structure Fe3O4@SiO2 was fabricated by the sol-gel method. The porous structure of the shell is formed by a composite layer of cetyltrimethylammonium bromide (CTAB) and silica after the removal of CTAB by acetone extraction. As a result, this porous shell has a uniform diameter, about 70 nm thick. The authors also proved that these porous capillaries have a direction perpendicular to the material's surface. The pore size is about 2.3 nm, the specific surface area is 365 m<sup>2</sup> g−1, and the total pore volume is 0.29 cm3 . g−1, magnetic saturation is 53.3 emu g−1. MC treatment results show that with a relatively low dose, about 0.05 mg μg−1, the adsorbent can remove MCs in solution, and the removal efficiency is more than 95%. After extraction with acetonitrile/water mixture regenerates, the adsorbent can be reused with MC removal efficiency above 90% after eight cycles of use. This result shows that the magnetic material synthesized by the research team has very high efficiency in removing toxic MCs, convenient recovery and regeneration, can be reused many times, and is effective in economic and technical aspects.

Recently, photocatalysis technology has received significant attention for treating polluted organic compounds. Photocatalysis uses the excitation energy from sunlight to decomposing organic compounds into harmless products of CO2 and water or other particles along the water flow. Their advantages are that they do not use additional treatment chemicals, do not generate secondary pollution products, and the toxic organic compounds are always decomposed on the catalyst surface without having to be recovered for treatment at another step [72, 73]. However, like in heavy metal treatment, the catalyst material must also be recovered to avoid environmental emissions. Therefore, synthesizing materials with both photocatalytic activity and magnetism to facilitate recovery and treatment has attracted significant attention from researchers.

In a publication by Chi et al. [74], the research team synthesized magnetic nanomaterials dopped by active silver metal centers, which are catalysts for 4-nitrophenol (4-AP) treatment. With the help of polyvinylpyrrolidone (PVP) as a reducing agent and stabilizing agent, the synthesized Fe3O4@SiO2-Ag composite material has a spherical shape, a core size of about 200 nm, and a SiO2 shell. The average thickness is about 35 nm, and the size of Ag NPs is controlled at about 3.65 nm and is evenly distributed on the Fe3O4@SiO2 background. The magnetism of the materials is not significantly reduced compared to the magnetic core, whereby the magnetic saturation of Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2-Ag is 78.5, 66.4, and 63.8 emu g−1, respectively. The treatment efficiency of 4-AP is excellent. The rate constant of this compound degradation is higher than that of some Ag catalysts carried on other substrates published previously. The catalytic activity of the material was also maintained stably after eight treatment cycles. The 4-AP conversion still reached over 99%. After that, a decrease in activity was initiated, but this was negligible.

Titanium oxide is considered one of the materials that was attracting much attention in its use as photocatalysts in treating organic pollutants. It has high chemical and biological inertness and solid oxidizing force. The main disadvantage of these metal oxides is that they have a reasonably wide band gap, about 3.2 eV, which means that their excitation energy must be ultraviolet radiation, not radiation in the visible region. In addition, poor recovery efficiency is also one of the limitations of this

material. Many studies used magnetic metals/metal oxides integrated with TiO2 to facilitate the recovery of this material after treatment. Mortazavi-Derazkola et al. [75] synthesized a core-shell magnetic photocatalyst Fe3O4@SiO2@TiO2@Ho to treat rhodamine B and methyl orange dyes under UV radiation conditions. This catalyst material is synthesized from the magnetic core Fe3O4 covered by the SiO2 shell. This SiO2 shell, which protects the Fe3O4 core from acid attacks, is also used to add silane coupling agents to facilitate the deposition of TiO2 on the surface. Holmium (Ho) is doped onto the surface of the adsorbent, which acts as electron traps to separate the electron-hole by creating local electric fields. The descriptive results show that the particle size of Fe3O4@SiO2@TiO2@Ho is about 52 nm, the outermost Ho shell is about 3 nm thick, and the TiO2 NPs layer is 3.5 nm thick. Their saturation magnetism decreased quite deeply after coating many outer shells of the Fe3O4 magnetic core. Specifically, the saturation magnetization decreased from 57.42 to 24.5 emu g−1 when coated with SiO2. This value decreased to 15.9 and 6.2 emu g−1 when TiO2 and Ho were coated, respectively. Although the saturation magnetization is relatively low, they are still sufficient to separate the adsorbent from the aqueous solution by an external magnetic field. Testing the ability to handle pollutant compounds rhodamine B and methyl orange showed that under dark conditions, without UV irradiation, this material only adsorbs up to 4% of organic pollutants in the water solution. Meanwhile, with an irradiation time of more than 2 h, the decomposition efficiency of Rhodamine B and Methyl Orange was 92.1% and 78.4%. In particular, this type of catalyst is readily regenerated by washing with clean water and ethanol and used again after being recovered by an external magnetic field. After each cycle of use, the catalytic activity and the degree of conversion decreased very little. This stable shows a great potential application of this material in treating organic dyes that pollute water sources. Zinc oxide (ZnO) is a semiconductor with unique properties such as a stable hexagonal wurtzite structure and a wide band gap of 3.37 eV, which is the large binding energy of 60 meV at room temperature. Hence, ZnO possesses some unique abilities, such as bactericidal properties and photocatalytic activity [76, 77]. Studies on the ability in water treatment, photocatalytic activity, the influence of factors such as dopped element, synthesis method, particle size, etc. were published [78–81]. However, in order to lead to a possible practical application, magnetic nanomaterials based on ZnO have been thought of as a means of increasing the ability to recover and reuse this potential material. The Fe-Zn binary oxide material was synthesized by Kumar et al. [82]. The optical, magnetic, and photocatalytic properties and the influence of Fe-doped content in methylene blue (MB) dye treatment were evaluated. The survey results show that Fe-Zn binary oxide has enhanced photocatalytic performance compared to simple ZnO under UV radiation and sunlight. The magnetism of this material was found to be dependent on the Fe dose. Similar material was studied by Falak et al. but with a specific and more Fe content [83]. A magnetic ZnO-ZnFe2O4 binary composite was created by the research team. The catalytic activity of this composite was also studied on MB. The results show more than 40% of their catalytic activity compared to ZnO nanoparticles. In addition, the magnetization saturation value of ZnO-ZnF2O4 was about 5.8 emu/g, which is high enough that they can be collected by applying an external magnetic field. In another study, Boutra et al. [84] synthesized a nanocomposite photocatalyst from ZnO, manganese ferrite (MnFe2O4), and tannic acid (TA) by hydrothermal method. The photocatalytic activity was evaluated through its ability to decompose Cong Red (CR) under visible light irradiation. The results showed that CR decomposition efficiency reached 84.2%, higher than simple ZnO. The catalyst is easily separated and reused without even washing, drying,

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

or any other technique to remove CR. The performance after the fifth reuse remains high, up to 77.5%. Dlugosz et al. [85] synthesized Fe3O4/ZnO magnetite nanoparticles and tested their photocatalytic activity on series of organic dyes, including MB, MO, Quinoline Yellow, Eriochromic Black T(EBT), and Trypani Blue (TB). The synthetic magnetic nanomaterial is 30% Fe3O4 by mass, with saturation magnetization of about 9.5 emu/g. The recovery of Fe3O4/ZnO reached 83.91%, slightly lower than Fe3O4 (94.80%). Notably, the catalytic activity of this material was found to increase with the molecular weight of the dye. Specifically, the photodegradation efficiency of Fe3O4/ZnO reached 76.90% for TB (872.9 g/mol), 63.02% for EBT (461.4 g/mol) and 13.23% for MB (319.9 g/mol). We have studies the synthesis of (polyethylene glycol)–Fe3O4/ZnO material [86]. The main objective of this study is the preparation of a PEG (polyethylene glycol)–Fe3O4/ZnO magnetic nanocomposite using a green sonochemical synthesis method with rambutan peel extract as a stabilizing agent for photocatalytic methylene blue degradation. The result showed the size of nanocomposite was 20–30 nm and had the band gap ennergy of 2.58 eV. Measurements of the degradation efficiency of the photocatalyst showed that the photocatalytic degradation of methylene blue follows pseudo-first order kinetics with good correlation and linear regression coefficient. This study found that the maximum degradation of the methylene blue dye was approximately 96%, with pH = 4.0, a PEG–Fe3O4/ZnO concentration of 1.0 g L−1, a methylene blue concentration of 200 mg L−1, and a time of 90 min. In the dark, the Langmuir adsorption constant and the maximum adsorbable methylene blue quantity were calculated as *K*L = 0.0451 L mg−1 (and *K*L = 11.275*K*LH) and *Q*max = 21.05 mg g−1. This study concludes that for the Fe3O4/ZnO magnetic nanocomposite, the adsorption process supports catalytic methylene blue degradation, reducing the decomposition time, increasing the efficiency of the catalytic process, and increasing the sample recovery due to the magnetic properties of the material.

From these research results, integrating ferromagnetic oxide nanoparticles into ZnO will yield a new magnetic material. The degree of magnetism will usually depend on the Fe3O4 content in the material. Therefore, in practice, if the material must achieve certain levels of magnetism in order to facilitate a particular separation requirement, consideration should be given to adjusting the mass ratio of this component in the material. In addition, in the presence of Fe3O4, the photocatalytic activity of the material increases quite significantly.
