Evaluation of ferromagnetic hybrid polymers obtained using cation exchangers

Highlights

• Iron oxides were deposited within the matrix of different cation exchangers.

• Hybrid polymers exhibited significantly different Fe(III) to Fe(II) ratios.

• Products contained mixtures of iron oxide forms.

• There were identified magnetite, maghemite, lepidocrocite, green rust and Fe(OH)₂.

• Ferromagnetic hybrid polymers were obtained.

Abstract

Ferromagnetic hybrid polymers containing iron oxide deposit were obtained using sulfonic and carboxylic cation exchangers. The influence of ion exchange capacity, chemical constitution and porous structure of the polymeric matrix on the form and the quantity of iron oxide deposit was studied. Obtained materials contained the iron oxide load amounting to 9.0–23.0% Fe. The XRD, and IR and Mössbauer spectroscopic studies showed that hybrid polymers obtained using sulfonic cation exchangers contained mainly maghemite and magnetite within the polymeric matrix, while the inorganic deposit incorporated within the matrix of caboxylic cation exchangers was the mixture of iron oxide polymorphs (magnetite, maghemite, lepidocrocite, green rust and also Fe(OH)₂). Due to high content of magnetite and maghemite deposited within the polymer matrix of the macroporous sulfonic cation exchanger this product showed highest magnetization saturation amounting 26 emu/g.

Introduction

Iron oxide-based materials are widely studied for their potential environmental applications such as contaminated groundwater, drinking water and wastewater treatment technologies. Different forms of iron oxides exhibit very good adsorption characteristics toward a wide variety of inorganic and organic species. Iron oxide nanomaterials doped with noble metals, iron oxide-oxalate and iron oxide-TiO₂ systems are promising photocatalysts in decomposition of many organic pollutants. Simultaneously, their magnetic properties allow simple separation of the spent sorbent or catalyst and make them useful in remediation technologies [1], [2], [3], [4], [5].

Among various iron oxide polymorphs, magnetite exhibits high saturation magnetization which enables its rapid separation from solutions or suspensions in an external magnetic field [6], [7], [8]. Due to this property, many composite materials containing magnetite have been synthesized based on resins, functionalized silica, organic functionalized surfactants, natural polymers such as chitosan and alginate, or other biosorbents [9], [10], [11], [12]. These types of magnetic sorbents present high sorption capacity toward a wide spectrum of many organic and inorganic contaminants of water, such as natural organic matter, humic acids, organic dyes and oil, as well as heavy metal ions (Co, Ni, Pb, Hg, Cr) and radionuclides (²³⁸Pu, Eu, Am, Cs), since their adsorptive properties depend on the functionality of the matrix or shell which covers the magnetic component, ensuring only the magnetic behavior of the material [7], [9], [11], [13], [14]. Simultaneously, the functionalized shell prevents the loss of magnetic properties which may be a result of chemisorption processes and changes of the chemical structure of the magnetic component [3], [7], [8].

Iron oxides due to the presence of surface hydroxyl groups in the aqueous environment are also efficient and specific adsorbents. The Fe atoms act as Lewis acids and exchange the hydroxyl groups for other ligands such as phosphate, arsenate and silicate, forming inner sphere complexes. In turn, the specific adsorption of cations Pb(II), Cd(II), Cu(II), Ni(II) involves interaction with deprotonated surface hydroxyl groups leading to mono- and binuclear inner sphere complexes [1], [15], [16], [17], [18], [19]. A remarkable property of magnetite, due to the presence of structural Fe(II), is the potential to reduce some water contaminants: U(VI), Tc(VII), Cr(VI), CCl₄. In consequence of the redox reaction, the passivation of the magnetite surface occurs as a result of a decrease of reactive surface sites. However, in the case of Cr(VI) the reduction reaction leads to formation of non-toxic Cr(III) immobilized in the form of Fe(III)–Cr(III) (oxy)hydroxide and in the case of U(VI) reduction leads to precipitation of highly insoluble UO₂. As a result of the reductive adsorption, both toxic contaminants may be permanently immobilized on the surface of magnetite [20], [21], [22], [23], [24].

To improve the prospects of iron oxide applications, many attempts have been made to develop iron oxide-based nanomaterials. Due to the high surface free energy and high surface area to volume ratio, nanoparticles exhibit high adsorption capacities in comparison to their bulk counterparts. Simultaneously, due to the smaller size of sorbent particles, the lower resistance of mass transfer within its porous structure accelerates the adsorption processes. However, to date the aforementioned benefits resulting from the features of magnetic nanoparticles have not found many practical applications in water or wastewater purification processes due to the tendency of aggregation, which results in the reduction of adsorption capacity and regeneration efficiency. Another concern is the still limited data available on the toxicity of iron oxide nanoparticles, which due to their small sizes and high reactivity have a potential to migrate in environmental and biological systems [8], [25], [26]. To overcome these difficulties, many studies have been undertaken to stabilize the iron oxide nanoparticles. These methods are based on electrostatic or steric stabilization using surfactants, polyelectrolytes such as humic acids, block copolymers and other macromolecules such as poly(acrylic acid), carboxymethyl cellulose or chitosan [8], [25], [27], [28], [29], [30]. These coatings may enhance the sorption properties of the material, due to the presence of additional functional groups, but also may constrict the contact of contaminants with the surface of iron oxide. Another disadvantage is the reversibility of the coating deposition, since under certain conditions in treated wastewaters the adsorbed stabilizers may be desorbed from the nanoparticle surface [8]. These problems can be avoided by immobilization of thin layers of iron oxides on the surface of porous supporting materials insoluble in aqueous solutions such as natural and synthetic zeolites, activated carbon, carbon nanotubes, as well as natural and synthetic ion exchangers. These types of materials combine the adsorptive and magnetic properties of iron oxides with physical and chemical stability of the support [3], [10], [31], [32], [33], [34].

Ion exchangers offer an attractive option for encapsulation of iron oxides within their polymer structure due to their physical and chemical strength, high surface area, geometric porosity and presence of ionic functional groups. Their preparation consists of the following two steps: the ion exchange reaction and the redox reaction, or only the redox reaction if the polymeric support is the macroporous oxidant [35], [36], [37], [38], [39]. However, these products usually contain paramagnetic amorphous iron oxides and only a few sorbents containing ferromagnetic crystalline forms such as magnetite or maghemite were obtained on the basis of sulfonic cation exchangers. In the first step the sulfonic groups were converted into Fe(II) form and in the second step NaNO₃ under a N₂ atmosphere or controlled doses of O₂ in a mixture with N₂ were used as the oxidant leading to magnetite deposition. Employing the stronger oxidizing agent H₂O₂ enabled incorporation of maghemite into the polymeric matrix [40], [41], [42], [43], [44], [45]. The aim of this study was to obtain hybrid polymers with a high magnetic response, containing different functional groups (sulfonic and carboxylic). The incorporation of ferromagnetic iron oxide was conducted using alkaline NaNO₃ solution under a N₂ atmosphere as the oxidant, which is the standard method used for micrometric magnetite synthesis [46]. The influence of ion exchange capacity, chemical constitution and structure of the polymeric matrix (gel-type or macroporous) on the form and the quantity of iron oxide deposit was studied.