Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Raman and IR spectroscopic investigation of As adsorbed on Mn3O4 magnetic composites
Graphical abstract
Highlights
► Arsenic adsorption onto Mn3O4 magnetic composite. ► XANES shows that As(III) is oxidized to As(V). ► Raman and IR spectra of As loaded samples show peaks which indicate As(V) monodentate and bidentate complexes. ► Precipitation of manganese arsenate is observed for high As loading.
Introduction
Arsenite and arsenate adsorption in metal oxides is one of the processes that control arsenic distribution in the environment. Manganese oxides are effective oxidants in the transformation of As(III) to As(V) and allow a stronger fixation and removal of greater amounts of arsenic at pH below 7. In particular, manganese dioxides are the most reported manganese oxides in literature used in water treatment [1], [2], [3]. Some researchers suggest that the oxidation of As(III) to As(V) by manganese dioxides involves the reduction of Mn(IV) to Mn(III) and, further, of Mn(III) to Mn(II) [4], [5], [6], [7], [8]. Many works on the complexation of arsenite and arsenate by manganese dioxides using XAFS show that As(III) is oxidized to As(V) and only arsenate is found as the adsorbed species on MnO2 [7], [9], [10]. The majority of these works shows that arsenate is adsorbed in the edge of MnO2, binding to the Mn(IV) octahedra in bidentate binuclear form [7], [9]. A more recent work [10] has shown that arsenate can bind to the Mn(IV) octahedra in a monodentate mononuclear form and to the Mn(III) octahedra in a bidentate mononuclear form. Dias et al. [8] have shown that during As(III) sorption in a manganese dioxide (Na-birnessita, Na0.55Mn2O4·1.5H2O), hausmannite (Mn3O4), a manganese oxide very common in soils, is the intermediate product of reductive dissolution of manganese dioxide and arsenate is adsorbed in Mn3O4 surface. Moreover, it has been demonstrated that precipitation of a Mn(II) arsenate, Mn3(AsO4)2, occurs for high arsenic concentrations (above 160 mg/L). Hausmannite [Mn3O4 or (Mn2+)(Mn3+)2O4] is a spinel manganese oxide with structural distortions caused by Jahn–Teller effect, having Mn(II) ions in tetrahedral coordination and Mn(III) ions in distorced octahedral coordination. Mn3O4 magnetic composites combine the oxidative property of Mn3O4 with the magnetic property of ferromagnetic iron oxides, such as magnetite (Fe3O4) and maghemite (γ-Fe2O3), which will help the solid-liquid separation process that follows arsenic sorption. In a previous work [11], we have synthesized Mn3O4 magnetic composites to remove As(III) from solutions and have shown that the magnetic composite presents high affinity for arsenic. However, no detailed study has been carried out to show how arsenic is complexed on the surface of hausmannite. Raman and IR spectroscopy are techniques that measure the vibrational modes of molecules and present the benefits of the identification of less crystalline and amorphous phases, and small amounts of material. Many works have used both vibrational spectroscopic techniques to elucidate the molecular structure of many systems, such as natural and synthetic organic and inorganic materials to be applied in biotechnology, catalysis, magnetism, electronics and optics [12], [13], [14], [15]. Our group has applied vibrational spectroscopy in environmental area for the investigation of surface-bound complexes on a variety of materials, such as oxi-hidroxides, soil constituents, modified clays and biomass. The techniques contributed to define the speciation of the chemical element, the identification of molecules and the types of bonds involved, which allow inferring about the mobility of the contaminants in environmental systems [8], [16], [17], [18], [19], [20]. The present work aims to understand the arsenic complexation by a magnetic Mn3O4 composite using Raman and IR spectroscopy.
Section snippets
Experimental
All chemicals were of analytical grade and used without further purification. All solutions were prepared with deionized water with a conductivity of 18.2 μS/cm obtained with a Milli-Q water purification system (Millipore).
Results and discussion
The Raman spectrum of Mn3O4 magnetic composite shows the main bands reported in literature for Mn3O4: a very sharp peak at 661 cm−1; two smaller peaks located at 293, 323 and 378 cm−1; and a weak signal at 485 cm−1 (Fig. 1) [21].
The arsenic sorption isotherm (initial As concentrations from 1 mg/L to 50 mg/L) for the Mn3O4 magnetic composite is shown in Fig. 2. The maximum sorption capacity and affinity of arsenic ions were evaluated from the isotherms by combined Langmuir and Freundlich model (Fig. 2
Conclusions
The sorption of As on the synthesized Mn3O4 magnetic composite samples was investigated using Raman and IR spectroscopy. XANES results have shown that the As adsorbed in the samples is at the oxidized arsenic form, As(V). Raman spectra of adsorbed As(V) species onto Mn3O4 occur at about 830 cm−1 and 900 cm−1. IR spectral data suggests the presence of three bands at ∼737, 774 and 835 cm−1 and two bands at ∼899 and 915 cm−1. The results are in agreement with monodentate and bidentate mononuclear
Acknowledgments
We would like to thank CNPq, Fapemig, CAPES and the National Institute of Science and Technology on Mineral Resources, Water and Biodiversity (INCT-Acqua) for their financial support. We would also like to thank Dr. Eduardo Henrique Martins Nunes and Prof. Wander Luiz Vasconcelos (DEMET-UFMG) for IR data and Laboratorio Nacional de Luz Sincrotron-LNLS for XANES measurements.
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