In recent years, the application of nanomaterials as sorbents for the extraction and concentration of different compounds has attracted the attention of researchers. [1]. Magnetic materials stand out among others [2]. Magnetic nanoparticles (MNPs) are inorganic material particles 5–100 nm in size with a certain level of magnetic properties. MNPs are most often used in biomedicine [3], magnetic resonance tomography [4, 5], data storage, environmental restoration [6], magnetic liquids [7], immunoanalysis [8], and detectors [9].
Iron, nickel and cobalt, as well as their oxides, are predominantly used as materials to produce MNPs. However, the magnetization of pure metals decreases with the oxidation of the surface, which is in contact with atmospheric oxygen, which is their drawback, although there are materials devoid of these disadvantages, namely, ferrites. Their general formula is MO-Fe2O3, where М is a bivalent metal ion. The best known representative of this group is magnetite (Fe3+[Fe2+Fe3+]O4). The main advantages of magnetite are its low oxidation susceptibility, high magnetic properties and low cost. When the magnetic material particles are reduced to the level of one domain (less than several tens of nanometers), the magnetite acquires superparamagnetic properties; i.e., at removal of the magnetic field, the particles completely lose their magnetization, return to their initial state and can be easily resuspended in the solution.
The main requirements to the procedure of MHP synthesis are simplicity, low cost and reproducibility. When producing magnetite, the method of chemical coprecipitation [7] is most often used, when MNPs are obtained from water-salt solutions at addition of the base in inert atmosphere at room or elevated temperature. This method was proposed as far back as in 1938 by W.Z. Elmore, so that it is sometimes called “classical” [10]. Despite the popularity of this method, the question of optimization of the synthesis conditions to produce the nanoparticles (NPs) with a narrow size distribution has been very little researched.
Magnetic inorganic materials have several significant disadvantages: 1 – they are oxidized in air and can change into other forms; 2 – they can aggregate, which lowers their sorption capacity and kinetics as a result of a reduction in their surface area. All this necessitates modification or functionalization of the magnetic material surface. Modification not only changes the surface properties and regulates the sorption selectivity but also increases the particle resistance in different environments, which is important for their further application. The magnetic sorbent surface may be covered by inorganic (silicon oxide or aluminum oxide) or organic components (polymers in the form of surfactants). Therefore, for biomedicine and ecology, the most important consideration is the investigation of the processes of iron oxidation over time during different changes in the environment (acidity, ultraviolet radiation, etc.), as iron oxidation may have an essential influence both on the mechanism of material influence in vivo and on the safety of iron oxidation.
The mechanisms of NP synthesis are often combined with their stabilization processes, which involve the application of various protective shells. Surface modification can not only ensure NP stability in different biological environments with high-strength ions but also control the nature of their interaction with the object, which determines NP biocompatibility. The surface characteristics are among the most important determining characteristics of the bioactive properties of NPs. Therefore, a tangible understanding of the problem emerges after obtaining additional structural characteristics of modified NPs and demonstrating their different functional properties. Having determined, for instance, the high potential of the sorption properties of functionalized NPs to different toxicants, it is necessary to analyze and predict the characteristic properties of the new preparations for their further application in different environments. It is important for the magnetic properties to be preserved during particle modification. The priority tasks as before include the following: (1) the development of new synthesis methods; (2) material functionalization; and (3) the enhancement of physicochemical stability and selectivity.
For ultradispersed systems, the most important characteristic is the combination of two general features (heterogeneity and dispersity): the specific surface area (Ss), which is determined by the ratio of the interface area (S) to the volume (V) of the body, S/V. The heterogeneity, dispersity and Ss are related to each other: the dispersity and Ss increase with decreasing characteristic particle size (D). A decrease in D results in an increase in the material Ss, leading to a greater contribution of excess surface energy to the total free energy of the system. By changing the shape of the particles at the same V, it is possible to adjust the S/V ratio, i.e., change the surface properties. With D decreasing below 1 nm, the majority of the atoms are located in such structures on the particle surface. The structures formed by such particles are called ultradispersed or cluster structures. The range of particle dimensions of 1 – 100 nm is called the nanosize, and the materials formed by clusters are called nanomaterials. In this region, the substance has special surface excess properties and volume phase properties, leading to qualitatively new physicochemical properties.
The role of NP dimensions and the interface has been discussed in many works. Therefore, for instance, in [11], it was shown that TiO2 NP size influences the size of their specific surface (and, thus, their sorption capacity). With increasing TiO2 crystallinity and increasing temperature, the specific surface area decreased from 409 (25 °С) to 39 m2/g (600 °С), and the crystallite size increased. To determine the influence of size and crystallinity on the arsenic sorption capacity, As(III) oxidizability and nature of surface complexes, the authors of [12] studied the sorption of As(III) and As(V) isolated solutions on nanosized amorphous and crystalline TiO2. Nanosized sorbents such as elementary iron, titanium oxide and iron oxide are more effective than their macrosized analogs, mainly because of (1) their high surface area relative to weight, (2) their high surface reactivity, and (3) their unique catalytic activities. Macroscopic investigations of metal sorption have shown that amorphous metal oxides have a high sorption capacity compared with their crystalline polymorphs due to their extremely large surface area [13, 14]. Amorphous TiO2 has a small coherent particle size, unordered surface structure and high specific surface area, making it an efficient sorbent. In [15], bimodal nanocrystalline mesoporous TiO2 powders with high photocatalytic activity were produced by a hydrothermal method. With increasing temperature and hydrothermal treatment time, the mean crystallite size and mean pore size increased, and the specific surface area, pore volume and porosity decreased.
One review [16] considered a new promising method for improving the properties of solid materials for hydrogen storage: the nanoconfiguring of metal particles into scaffolds in two ways: the reduction of particle dimensions to several tens of nanometers and the steric stabilization and treatment of such particles by building them into frames (nanoporous solids, polymers, stabilizers, etc.). Metal NPs based on noble metals and on Mg were considered. Here, the influence of particle size on the adsorption and desorption properties of porous oxide NPs at different temperatures was studied by using water molecules as a model compound. At all temperatures, fine NPs have a greater concentration of saturated surface, as they have a greater curvature, greater deviation from the flat surface, greater number of active regions, and greater amount of surface adsorbate. With decreasing temperature, NPs have a higher concentration of saturated moisture, and they are more prone to moisture adsorption and similar contamination.
Particle size essentially influences the physical properties of substances: at the nanoscale, the ratio of surface and volume atoms is very high, which leads to the formation of a large atom population on the particle surface with changed electronic properties. Owing to the reduction in the size of the particles in the metal-hydrate system, the limits of hydrogen solubility can be changed, and the enthalpy of hydrate formation can be reduced [17]. Moreover, NP surface atoms are in contact with the environment, leading to an extremely developed surface area and hence to enhancement of the kinetics of surface reactions, particularly solid‒gas reactions. Finally, owing to their small size, the paths of the reaction, related to atom diffusion inside the particles, are shortened, ensuring a very high reaction rate. Therefore, size enables the regulation of both thermodynamic and kinetic properties [18].
The authors [19] measured the macroscopic edges of Cu2+ sorption by hematite NPs with average diameters of 7 nm, 25 nm and 88 nm for 0.1 М NaNO3. The surface areas were 188 m2/g and 62 m2/g for the 7 nm and 25 nm hematite samples, respectively; the surface area of the 88 nm particles was 9.1 m2. These results emphasize the uniqueness of the reactive surface capacity of crystalline iron oxide particles with decreasing NP diameter.
The sorption capacity increases with decreasing mean size of the chitosan NPs [20]. NPs with a mean size of 40 nm have a sorption capacity of 98 mg/g when in contact with a 100 g/ml solution of lead ions. An increase in the NP size increased the time needed to reach equilibrium. Particle size can have a considerable influence on chitosan sorption characteristics [21]. The influence of particle size on sorption effectiveness is shown using sorption isotherms at рН 4. The absorption is essentially influenced by the sorbent particle size distribution. The maximal absorption depends on the specific area or external treatment of the sorbent.
The number of studies devoted to investigating biomedical possibilities for MNP application has increased continuously. This fact is indicative of the considerable potential of this direction for medical programs. This work addresses the dynamics of the sorption process of iron NPs produced by highly productive physical synthesis via the molecular beam method.