Arsenite removal with Fe3O4, MnFe2O4 and CoFe2O4 Nanoparticles

Using fixed bed column and magnetic filtration

Morales-Amaya C.G.

Alarcón-Herrera M.T.

Departamento de Ingeniería Sustentable, CIMAV, CIMAV 110, Ejido Arroyo Seco, C.P. 34147, Durango, Dgo. México. 

Astudillo-Sánchez P.D.

Departamento de Ciencias Basicas y Aplicadas, Centro Universitario de Tonalá. Av. Nuevo periférico Ote. 45425, Tonalá, Jalisco.

Lozano-Morales S.A.

Cátedras-CONACYT-Centro de Investigación en Química Aplicada , Blvd. Enrique Reyna 140, Saltillo, C. P. 25294.Coahuila, México.

Licea-Jiménez L.

CIMAV, S.C. Alianza Norte 202, Parque de Investigación e Innovación Tecnológica PIIT C.P. 66628 Apodaca, NL., México.

Teynoso-Cuevas L.

Cátedras-CONACYT. Departamento de Ingeniería Sustentable, CIMAV, Calle CIMAV 110, Ejido Arroyo Seco, C.P. 34147, Durango, Dgo. México.

This work evaluates the ability of nanoparticles (NPs) to remove arsenic (As⁺³) from water. Metallic NPs of Fe3O4, as well as bimetallic NPs of CoFe2O4 and MnFe2O4, were synthesized by chemical coprecipitation. The three types of NPs consist of nanocrystals with high monodispersity and excellent stoichiometry, and show superparamagnetic properties at room temperature. X-ray diffraction (DXR) shows that the NPs have the characteristic intense peak of inverse spinel-like crystalline phase in the 311 plane, which is typical of a ferrite. Scanning Electron Microscopy (SEM) shows nanocrystals with uniform quasi-spherical morphology; surface area for the NPs was 168.8 m²/g (MnFe2O4), 198.6 m2/g (CoFe2O4), and 158.8 m²/g (Fe3O4). The size of the NPs, determined by Dynamic Light Scattering (DLS), averaged 12-50 nm. The removal of As⁺³ from water was carried out in fixed-bed columns with continuous flow and magnetic filtration. Magnetic separation favored the filtration and separation of the NPs. The adsorption capacity results show that it is possible to reduce the concentration of As⁺³ below the maximum permissible limit (25 µg/L) established by the official Mexican standard for water for human consumption (NOM-127-SSA1-1994), and that 0.1 g/L of the NPs can achieve nearly complete removal. Adsorption results were adjusted with an R²= 0.99 to the Freundlich and Langmuir isotherms. The adsorption capacity of the NPs was 115 (MnFe2O4), 130 (CoFe2O4), and 43,450 mg/g (Fe3O4).

Arsenic (As) contamination in groundwater is a global threat. It is estimated that more than 300 million people worldwide drink water with As levels that exceed the permissible limit (10 µg/L) [1]. Many parts of Latin America are among the most severely polluted regions of the world [2].

The last decade brought new findings of As in water for human consumption in Argentina, Brazil, Chile, Colombia, Ecuador, El Salvador, Guatemala, Mexico, Nicaragua, and Peru [2]. More than 4.5 million people in Latin America are chronically exposed to concentrations greater than 50 µg/L of As. McClintock et al., 2012, and the World Health Organization (WHO), 2011, outlined the health effects of people who are chronically exposed to As in drinking water. Vascular diseases (premature heart attack) stand out in their analysis, as well as respiratory diseases, skin lesions and lung and bladder cancer [3,4].

The application of nano-adsorbents for water treatment has become increasingly common, due to a high removal efficiency for As [5]. The aim of this study was to synthesize low-cost metal nanoparticles that can be used in water treatment, specifically in the removal of As. In this work, the technical and economic viability of the three metal nanoparticles was synthesized and evaluated, MxFe3-xO4 (where M=Co and/or Mn and x=2).

Methodology

NPs Synthesis

The NPs were prepared by chemical coprecipitation [6]. For the synthesis of MnFe2O4 NPs, Fe(NO3)3•9H2O and MnSO4•H2O were dissolved, with a stoichiometry ratio of 2:1, in 5 mL of deionized water and 1 mL of HCl at 1 M. The solution was mixed and vigorously stirred. Then, 100 mL of NaOH was added dropwise to 3 M. Once the addition was complete, it was brought to 90 °C for 60 min. This same procedure was applied for CoFe2O4 NPs, using CoSO4•7H2O and Fe(NO3)3•9H2O as precursors. Fe(NO3)3•9H2O and FeCl2•4H2O were used to produce Fe3O4. NPs were then allowed to cool until reaching thermal equilibrium with the environment. The NPs obtained were recovered from the solution with the help of a magnet and repeatedly rinsed. Afterward, the values of pH were adjusted to 6, 7, 8, and 8.5. Finally, the products were baked at 50 °C for 48 h and subsequently ground.

NPs characterization

The morphology and dimensions were ef the NPs were observed through scanning electron microscopy (SEM), using a FEI Nova NanoSem200 with a low vacuum detector. A PANalytical X-ray diffractometer model Empyream of Malvern with a K-Alpha Cu anode of 1.54 nm, at an amperage of 40 mA and a voltage of 45 kV, with a scanning step of 0.02 in 2θ degrees, was used to know the crystal structure of the NPs. The values of the specific surface area were determined by the Brunauer, Emmett, and Teller (BET) method, using the Quantachrome Nova Corporation 1000 series equipment. The samples were degassed in vacuum at 150 °C for 10 h. Also, X-ray photoelectron spectrometry (XPS) analyses were carried out with a Thermo Scientific Escalab 250Xi instrument. The base pressure during analysis was ~10⁻¹⁰ mbar and the photoelectrons were generated with the Al Kα (1486.68 eV) X-ray source with monochromator and a spot size of 650 µm. The X-ray voltage and power were 14 kV and 350 W, respectively. The acquisition conditions for the high-resolution spectra were 20 eV pass energy, 45º take-off angle and 0.1 eV/step. The recorded photoelectrons peaks were analyzed with the Avantage software V 5.41. The magnetic properties of the NPs were analyzed at room temperature with an AGM MICROMAG magnetometer.

Absorption propierties

To evaluate the adsorption of As+3, a stock solution of NaAsO2 was prepared at a concentration of 40 μg/L, and divided into three containers for testing, each with 1 L of solution. After adding 0.1 g of the adsorbent NPs, the pH of the solutions were adjusted to 6, 7, and 8 by aqueous solutions of NaOH and HNO3. The solutions were stirred at room temperature for 10 min, then transferred to the continuous flow column of a magnetic filtration column with high gradient magnetic separation (HGMS). The filtered solutions were stored in jars for further analysis. An HGMS device comprises a bed of magnetically susceptible cables placed inside an electromagnet or external magnetic fields. When a magnetic field is applied across the column, the wires dehomogenize the magnetic field in the column, producing large field gradients around the wires that attract magnetic particles to their surfaces and trap them there [7]. For the successful collection o

f magnetic particles by HGMS, the magnetic forces that attract the particles to the wires must dominate the entrainment of the fluid, the gravitational, inertial and fusion forces as the suspension of the particles flow through the separator. The particles were tested in continuous flow columns with magnetic transmission. In Figure 1, the diagram of the column of the magnetic transmission is shown.

Figure 1. a) Scheme and b) photography of magnetic separator and magnetic filtration column.

The total concentrations of As were determined by a GBS atomic absorption spectrophotometer (Avanta Sigma model) coupled to a hydride generator (HG-AAS) with flame (air-acetylene). Samples were prepared with 3 mL of concentrated HCl and 3 mL of KL, allowed to stand for 3 h for later determination.

Results and Discussion

Brunauer-Emmett-Teller (BET) surface analysis determined that the specific surface area values of MnFe2O4, CoFe2O4, and Fe3O4 were 198.6, 188.8, and 158.8 m2/g, respectively.

Figure 2 shows the SEM images of a) MnFe2O4,  b) CoFe2O4   and c) Fe3O4, where it is observed that the samples consist of nanoparticles with relatively uniform size and quasi-spherical morphology. Average sizes of 38, 22 and 57 nm were obtained for the NPs of MnFe2O4, CoFe2O4 and Fe3O4, respectively.

Figure 2.  Scanning electron microscopy (SEM) of NPs:  a) MnFe2O4,  b) CoFe2O4 and c) Fe3O4.

The XRD patterns of MnFe2O4, CoFe2O4, and Fe3O4 are shown in Figure 3a-c. Figure 3a corresponds to the XRD pattern of the sample of MnFe2O4, where the peaks 2θ of 30.31°, 36.60°, 44.57°, 58.68°, 57.12°, and 65.78° are indexed to planes (220), (311), (400), (511) and (440), respectively; corresponding to the cubic structure centered on the face of MnFe2O4, according to the  card JCPDS- 742403 of the International center diffraction data.

Figure 3b shows the diffraction peaks for the sample of CoFe2O4 located at the values of 2θ at 18°, 30°, 36°, 43°, 57°, and 62° with the respective crystal planes (111), (220), (311), (400), (511) and (400), respectively; corresponding with the card JCPDS-22-1086 of the International center diffraction data [8].

The XRD pattern of Figure 3c corresponds to the sample of Fe3O4, which contains a high coincidence with the values and intensities of the JCPDS-01-084 3854 sheets of the International center diffraction data. The sample presented peaks in 2θ corresponding to 20.5°, 30.31°, 36.60°, 44.57°, 54°, 57.12°, and 65.78°, which are indexed at (111), (220), (311), ( 400), (422), (511) and (440) planes, respectively, correspond to a cubic unitary cell, characteristic of a cubic spinel structure [9].

Figure 3.  XRD image of: MnFe2O4, b) CoFe2O4 and c) Fe3O4.

Figure 4 shows the peaks of the binding energies of each element, as determined by the XPS analysis, where the corresponding to the surface molar ratio of Fe/Mn and Fe/Co was according to MnFe2O4, and CoFe2O4 was 2:1, which coincides with the expected, since, is the ratio of metal ions in the solution.

Figure 4. XPS for Fe3O4, CoFe2O4, and MnFe2O4.

The hysteresis cycle of NPs was studied to verify paramagnetic behavior. The hysteresis curve in these NPs is shown in Figure 5, in which the hysteresis cycle is narrow, typical of soft magnetic materials, the magnetization of MnFe2O4, CoFe2O4 and Fe3O4 was 48.39, 58.74 and 28.03 emu/g, correspondingly. These properties make adsorbents easily separate from solution when an external magnetic field is applied.Figure 5.  Hysteresis cycle of MnFe2O4, CoFe2O4 and Fe3O4.

The zeta potential, as a function of the pH in contact with the synthesized NPs, is shown in Figure 6. The difference is negligible between the pH values over the zeta potential in CoFe2O4 and MnFe2O4, in contrast with Fe3O4. However, the charge of the NPs at pH 2 value is positive, as shown in Figure 6. For the case of pH 4, 6, 8, 8.5, and 10, in the NPs the surface of these is negative, so the isoelectric point of these materials would be at pH 2.Figure 6.  Zeta potential as a function of pH of  MnFe2O4,  CoFe2O4 and  Fe3O4.

Effect of the removal of As⁺³ in continuous flow with MnFe2O4, CoFe2O4 and Fe3O4

As described in the methodology section above, the experiments were carried out in a liter of solution with an initial concentration of 40 µg/L of As⁺³, and with an adsorbent dose of  0.1 g/L. The solutions and nanoparticles were stirred for 10 minutes at 450 rpm, after which the continuous flow in the HGMS device was 100 mL/min. Magnetic separation favored the filtration and separation of the NPs.

Figure 7 shows the comparison chart of the 3 NPs in the removal of As+3 in continuous flow, in the Table 1 shows the results of the removal of As+3 with each of the NPs. The results of the adsorption capacity of As+3 as a function of pH (6, 7 and 8) are reported in Table 2.

Figure 7.  Removal  As⁺³ in continuous flow.

It is shown both in the graph and in the table, that the removal of As+3 with the use of NPs as adsorbents in continuous flow occurs in the first 10 minutes, the removal efficiencies for MnFe2O4 and CoFe2O4 are greater than 90%, on the other hand, the Fe3O4 removal percentage is closer to 85%. Nevertheless, the final concentrations of the three experiments are able to enter the NOM-1994 standards for water for human consumption [10], which makes this type of NPs a technology with high technical feasibility in the removal of As+3 in continuous flow.

The effect of pH on the adsorption of ions in the samples with Fe3O4, MnFe2O4, and CoFe2O4 is shown in table 2. The solution reaches As+3 equilibrium after only 10 min, probably because the NPs have a negative surface charge (approximately -98 mV for MnFe2O4, -90 mV for CoFe2O4, and -75 mV for Fe3O4). H3AsO3 is positively charged in the pH range of 6-8 used here, and electrostatic attraction te the NPs accounts for the high adsorption efficiency of As⁺³.

The adsorption capacity of As⁺³ remained similar in the range of pH studied; this is possible because the lack of competition from the hydroxyl groups (OH^–) that were generated in the adsorption processes, which kept the adsorption sites active, and as well as the non-deprotonation of the NPs [11]. This effect involves a two-step ligand exchange reaction: first, the hydroxyl group of the metal hydroxide is protonated; then, the H2O ligand is replaced with the oxyanion, so the adsorption is affected by protonation of the pH-dependent metal hydroxide surfaces. The affinity differences of the adsorption between the oxyanion species are generally small; this is usually attributed to the deprotonation of the surface of the metal hydroxides with the increase of pH [12]. The surfaces of our NPs play an essential role in the electrostatic interaction towards As⁺³, for the exchange of ligands.

Conclusions

This work presents nanoparticles (NPs) with the potential for removing As+3 ions from water for human consumption. Adsorption capacity measurements show that water carrying 40 μg/L of As⁺³ can be brought not only below the official Mexican standard (25 μg/L), but also below of the WHO standard (10 μg/L). The adsorbents MnFe₂O4 and CoFe2O4 showed better performance than Fe3O4. Within the tested range, pH has no significant effect on the adsorption of As⁺³, because As⁺³ was found as H3AsO3⁰ in this range.

ACKNOWLEDGEMENTS

This work was supported with Project No. 267666 of the FONCICYT CONACYT-INNOVATE UK 2015. Likewise, thanks to FORDECYT project No. 297116: “Water Consortium.” To CONACYT for its support with the scholarship 486760.

The acknowledgments also To M.S.A Luis Arturo Torres-Castañón, for all the analytical facilities and collaboration in analytical determinations.

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