2.1. Synthesis of Magnetic Nanoparticles

The common methods for synthesizing MNPs are co-precipitation, thermal decomposition, hydrothermal synthesis, colloidal and high-energy ball mill. Depending on which method was used, a specific size and dispersion of size distribution of MNPs are obtained [48].

The modified co-precipitation method introduced by Massart includes the addition of FeCl₃·6H₂O and FeCl₂·4H₂O in deionized water, with the temperature rising to 60 °C in order to acquire a yellow and clear solution and stirred vigorously. Afterwards, aqueous ammonia is inserted drop by drop until the pH reaches 10. Then, it is left for the reaction to take place for about 30 min, under continuous and vigorous stirring. Throughout the whole experiment, N₂ is used as a protective gas. When the reaction finishes, the black precipitate is collected with the help of a strong magnet, it is washed many times with deionized water and ethanol. Lastly, the magnetite nanoparticles are lyophilized [49,50].

The thermal decomposition method carried out by Kluchova et al. produced maghemite nanoparticles by the solid-state isothermal decomposition in air at 400 °C of Fe(C₂H₃O₂)₂·2H₂O for 1 h. Before the synthesis, the solid was homogenized in an agate mortar, following in the precursor particles having a size distribution of 1–5 μm [51].

Zheng et al. [52] produced Fe₃O₄ nanoparticles by hydrothermal synthesis, where 1 mmol of Fe(NO₃)₃·9H₂O and 0.5 mmol of sodium bis(2-ethylhexyl)sulfosuccinate were added in 10 mL deionized water, following the addition of 2 mL of 50% hydrazine hydrate while stirring. This solution was put into a 20 mL Teflon-lined stainless-steel autoclave of and heated to 160 °C at a pace of 5 °C/min. For 10 h, the temperature was kept at 160 °C and then the mixture brought to room temperature naturally. The nanoparticles were filtered, washed several times with ethanol and deionized water and air-dried. The morphology and the structure of the MNPs were characterized by TEM, HRTEM and XRD.

Another procedure for synthesizing Fe₃O₄ magnetic nanoparticles is by the microwave-solvothermal method suggested by Li et al. [53], where 5 mmol of FeCl₃·6H₂O, 400 mg of Na₃C₆H₅O₇ and 50 mmol of NH₄CH₃CO₂ were added in a solution of 70 mL of ethylene glycol. The black solution was stirred and heated and then transferred into a microwave transparent PTFE-TFM-lined alumina ceramic vessel, with the capacity of 100 mL. Then, it was heated for 15 min to 260 °C for 2 h in a microwave reaction system. Finally, the black powder of Fe₃O₄ was gathered with the help of a strong magnet and washed several times with ethanol.

2.2. Synthesis of Fe₃O₄-GO Nanocomposites

Kyzas et al. [50] presented two ways of preparing magnetic GO (mGO): by co-precipitation (mGOp) or by impregnation (mGOi). In the former method, graphite oxide is scattered in deionized water with the help of a sonicator for 30 min. Then, specific amounts of FeCl₃·6H₂O and FeCl₂·4H₂O are dissolved in deionized water at room temperature and the mixture is added drop by drop to the GO solution, under vigorous stirring and under a N₂ atmosphere. After the completion of ion exchange, 28% v/v ammonia solution is added dropwise so the pH reaches the value of 10 for the synthesis of MNPs. Next, the solution is heated to 80 °C and after agitating for 45 min, the black solid is collected via centrifugation, is washed many times with CH₃OH and is lyophilized. In the latter method, graphite oxide is again added in deionized water and put into a sonicator for 30 min to obtain GO and then an amount of Fe₃O₄ nanoparticles are added to the mixture. After another 30 min, a homogenous suspension is obtained and the nanocomposites are collected by centrifugation and freeze-dried.

The aforementioned material was studied for dye adsorption experiments, with Reactive Black 5 (C₂₆H₂₁N₅Na₄O₁₉S₆) being the target compound. From the X-ray diffraction measurements (XRD) presented in the paper, the average crystallite size of the MNPs were found to be around 18.4 nm. Scanning electron microscopy images were taken to identify the morphology of the mGOi nanoparticles and the iron distribution map of the material. Also, the magnetization of the two materials were studied and the mGOi adsorbent had 65 emu/g, which was a bit higher than the same value of mGOp. Lastly, from Fourier transform infrared (FT-IR)spectra, the identification of functional groups, such as carboxylic groups, epoxy groups and ammonium groups, was carried out.

2.3. Synthesis of GO Membranes Composites

Deng et al. [54] firstly prepared hydroxylated graphene (GOH), by using ferrous chloride and hydrogen peroxide at certain volume ratios, while adjusting the pH with diluted HCl, and GO with a modified Hummers method. Then, 1 mg of GO and 1 mg of GOH were mixed into a beaker, under ultrasonication for 30 min to form a uniform dispersion and the solution was put onto the polyethersulfone membranes of pore sizes 0.22 μm and 1 μm under vacuum.

The morphology and validation of the deposition of the material on the membrane was done via scanning electron microscopy (SEM) and X-ray diffraction (XRD). The performance of the adsorption properties was evaluated with dye compounds, such as rhodamine B and methyl orange, and with the protein albumin from bovine serum.

2.4. Synthesis of Graphene Aerogels

Zhi et al. [55] showcased in a review seven different ways of preparing graphene aerogels, either with no templates or with a template. The methods that require no template are the hydrothermal method, where a GO colloidal dispersion is prepared in ethanol or water, heated at a certain temperature and freeze-dried, dried in atmospheric and in supercritical conditions, the chemical reduction method, by using a reducing agent, the chemical cross-linking method, to enhance the interaction between the graphene oxide sheets with the usage of glutaraldehyde or poly(vinyl alcohol), and a 3D printing method, with the preparation of a high viscosity ink of SiO₂ and GO suspension. The latter methods thar require a template are the chemical vapor deposition method, where 3D graphene is prepared by the chemical vapor deposition of carbon onto a metal foam, like copper or nickel, the ice template method, by freezing a GO suspension and its gradual melting to form a porous aerogel with GO, and the bubble template method, with the utilization of a surfactant under vigorous stirring so that the GO is trapped in the bubbles and begins to form.

2.5. Synthesis of GO Alginate Beads

Arshad et al. [56] prepared alginate beads of GO by adding 0.2 g sodium alginate in a 5 wt.% GO suspension in deionized water and then deposited dropwise into a 5 wt.% calcium chloride solution. The alginate beads that were formed were kept in the solution for 3 h. These beads were used for the removal of metals and organic compounds, with maximum adsorption capacities reaching 588.2 mg/g bead.