The distinct effects of Mn substitution on the reactivity of magnetite in heterogeneous Fenton reaction and Pb(II) adsorption
Graphical abstract
Introduction
The spread of a wide range of contaminants (e.g., organic pollutants and heavy metals) in surface water and groundwater has become a critical issue worldwide, due to the fast population growth and rapid development of industrialization [1]. Thus, it is imperative to develop novel, efficient and friendly materials and technologies to remove these contaminants. A number of iron oxides including goethite [2], [3], hematite [4], [5], magnetite [6], [7], ferrihydrite [8], [9] and lepidocrocite [10], [11] were applied to adsorb heavy metal species and initiate the advanced oxidation processes (AOPs) as heterogeneous catalysts for organic degradation.
Compared to other iron oxides, magnetite has several interesting structural features. Magnetite contains both Fe2+ and Fe3+ in spinel structure, with Fe2+ occupying the octahedral site and Fe3+ distributed between octahedral and tetrahedral sites. Fe2+ plays an important role as an electron donor to initiate the AOPs [12]. The accommodation of Fe2+ and Fe3+ on the octahedral sites allows the Fe species to be reversibly oxidized and reduced while keeping the structure unchanged [13]. In most natural magnetites, iron cations are isomorphously substituted by divalent (Co, Ni, Mn, etc.), trivalent (V, Cr, etc.) and tetravalent (Ti) cations [14]. These substitutions may vary the microstructure and physicochemical properties of magnetite and accordingly affect its surface reactivity in heavy metal adsorption and catalyzing AOPs. As a common constituent in soils and sediments, magnetite plays a vital role in the adsorption and redox behavior of heavy metals and organic contaminants [15].
Heterogeneous Fenton reaction, a typical advanced oxidation technology, is novel and promising for the destruction of organic contaminants in wastewaters [16]. In Fenton reaction, H2O2 is activated to generate radicals, especially OH, according to the classic Haber–Weiss mechanism [17]. OH has a quite high oxidation potential (2.8 eV) and can completely oxidize the organic pollutants in aqueous medium. Magnetite is especially efficient in catalyzing the heterogeneous Fenton reaction, ascribed to the presence of highly active Fe2+ species on magnetite surface and the fast electron transfer between Fe2+ and Fe3+ on the octahedral sites [13], [18]. The incorporation of transition metals in magnetite obviously affects the Fenton reactivity of magnetite. The introduction of Co [13], Ti [19], [20], V [18] and Cr [21] improves the Fenton catalytic activity of magnetite, while Ni shows an inhibitory effect [13]. Such distinct effects greatly depend on the oxidation states and occupancy of substituting cations [22], [23]. Therefore, the experimental determination of cation distribution in the spinel structure and their oxidation states is an interesting challenge. But the conventional methods, e.g., electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy are restricted to certain elements or not possible to perform under in situ condition [24]. X-ray absorption fine structure (XAFS) spectrum is a powerful tool for the characterization of chemical environment and is composed of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopes. XANES provides the information about overall oxidation states and site symmetry of the absorbing central atom. EXAFS reflects the crystal structure and local environment of the absorbing central atom [25]. The combination of XANES and EXAFS can present detailed description of atomic structure in substituted magnetite. From our previous publications [18], [20], [26], among the common substituting metals, the valence and distribution of Mn in magnetite structure are the most complicated. In this study, XAFS characterization was carried out to investigate the oxidation states and coordination environment of metal cations in Mn substituted magnetite (Fe3−xMnxO4). Moreover, through the batch experiments of acid orange II (AOII) decolorization and OH generation in heterogeneous Fenton system catalyzed by Fe3−xMnxO4, the relationship between local atomic structure of Fe3−xMnxO4 and its heterogeneous Fenton activity was discussed.
Besides being used as Fenton catalyst, magnetite is an important attenuator of heavy metals. Magnetite has been applied in the adsorption of various heavy metals, e.g., Cr(VI) [27], Hg(II) [28], As(V) [25], Sb(V) [29], Se(IV) [30] and U(VI) [31]. The adsorption mechanism includes surface site binding [32], electrostatic interaction [33], modified ligand combination [34] and oxidation–reduction interaction [35], [36]. Some substitutions have made obvious variations on the physicochemical properties of magnetite surface [26], [37]. For instance, some substituting cations (e.g., Cr3+ and Ti4+) increase the surface hydroxyl amount of magnetite. The surface hydroxyl groups are functional groups of iron oxides, due to its vital role of surface acidity in heavy metal adsorption. The specific surface area of magnetite is also varied by some substitutions, ascribed to the decrease in magnetism and thus particle aggregation. Such changes probably affect the adsorption properties of magnetite for heavy metals. But to the best of our knowledge, few studies have grasped this issue. In this study, Mn substituted magnetites were tested for the removal of Pb(II) in aqueous solution. Through the characterization of surface physicochemical properties, the effect of Mn substitution on the adsorption capacity of magnetite for Pb(II) was illustrated. The obtained results in this study will be of great significance for well understanding the interaction mechanism of magnetite-group minerals in the environmental self-purification process and their application in pollution protection.
Section snippets
Sample preparation
All the chemicals and reagents were of analytical grade. For the synthesis of Fe3O4, 0.90 mol L−1 of FeSO4·7H2O was dissolved in an HCl solution. 1.0 mL of hydrazine was added to prevent Fe2+ oxidation, and the pH was set below 1.0 to prevent Fe2+ oxidation, and hydroxide precipitation. This solution was heated to 90–100 °C. Equal volume of a solution containing 4.0 mol L−1 NaOH and 0.90 mol L−1 NaNO3 was added dropwise (10 mL min−1) into the heated iron solution and the reaction was maintained at 90 °C
Sample characterization
The chemical compositions for all the prepared magnetite samples are displayed in Table 1. The increase in Mn content is followed by a simultaneous decrease in Fe content, suggesting that Mn cations have replaced Fe cations.
The XRD characteristic of all the magnetite samples (Fig. 1) mostly corresponds to the standard card of magnetite (JCPDS: 19-0629), indicating that these samples mainly have spinel structure. But for samples with high Mn content (FM3 and FM4), a new phase of feitknechtite
Conclusion
In the present study, a series of Mn substituted magnetites was characterized by XAFS and tested in catalyzing the heterogeneous Fenton degradation of acid orange II and Pb(II) adsorption. The Mn substituted magnetites show better catalytic activity than magnetite without substitution. The best catalytic efficiency is reached by the sample with the minimum Mn content, while the other Mn substituted magnetites show no obvious variations. The Pb(II) adsorption capacity of magnetite is gradually
Acknowledgments
This is contribution No. IS-1858 from GIG CAS. We gratefully acknowledge Beijing Synchrotron Radiation Facility (BSRF) for providing us the beam time for the XAFS measurement. Financial supports were provided by National Natural Science Foundation of China (Grant Nos. 41172045, 41302026 and U1201233) and Shanghai Tongji Gao Tingyao Environmental Science & Technology Development Foundation (STGEF).
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