Boron Removal from Aqueous Solutions by Synthetic MgAlFe Mixed

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Boron Removal from Aqueous Solutions by Synthetic MgAlFe Mixed Oxides Angeĺ ica C. Heredia,*,† M. M. de la Fuente García-Soto,‡ Adolfo Narros Sierra,‡ Sandra M. Mendoza,§ ́ ez Avila,† and Moń ica E. Crivello† Jenny Gom †

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CITeQ-CONICET, Universidad Tecnológica Nacional, Facultad Regional Córdoba, Maestro Marcelo López esq. Cruz Roja Argentina, Ciudad Universitaria, Córdoba X5016ZAA, Argentina ‡ Departamento de Ingeniería Química Industrial y del Medio Ambiente. E. T. S. de Ingenieros Industriales, Universidad Politécnica de Madrid, Madrid 28006, España § Universidad Tecnológica Nacional, CONICET, Facultad Regional Reconquista, Reconquista 3560, Argentina S Supporting Information *

ABSTRACT: The boron removal capacity from an aqueous solution using MgAlFe mixed oxides from layered double hydroxides (LDH) was studied. They were synthesized by the coprecipitation method at 70 °C and were characterized as potential filter materials. The Fe3+ analyzed by X-ray photoelectron spectroscopy and UV−visible diffuse reflectance showed their tetrahedral and octahedral coordination. Scanning electron microscopy micrographs and thermogravimetric and differential scanning calorimetry analysis evidenced the presence of clusters and particles aggregates and decreased dehydroxylation temperature when the iron content increased. Mixed oxides and boron solution in a ratio of 20:1 Mg/B were put in a batch reactor at different contact times. The borate removal process was due to the memory effect of the mixed oxides and superficial adsorption by electrostatic attraction. This fact is directly related to the specific surface area, Fe content, and surface charge. The maximum boron removals were achieved with the CS25 and CS50 samples with values higher than 85%.

1. INTRODUCTION

content can be increased as a result of wastewater discharges, but this practice has decreased significantly. There is no universal method to eliminate boron from water. Different separation methods may be applied on the basis of the boron concentration in the medium. Many separation technologies have been applied in its removal from aqueous solutions, including adsorption, reverse osmosis, ion exchange, electrocoagulation, chemical coagulation, and a hybrid process.6−12 At very low boron concentrations, adsorption is an extremely effective way to remove it from aqueous solutions. Different sorbents are utilized in adsorption processes, including activated carbon, fly ash, clays, natural minerals, biological materials, oxides, mesoporous silica, nanoparticles, complexing membranes, and selective resin.13−18 A family of anionic clays named layered double hydroxides (LDHs) or hydrotalcite-like compounds has become of interest for industrial development as well as for scientific research.19 LDHs are also naturally abundant; the materials synthesized by replacement of Mg(II) by trivalent metal cation M(III) in the brucite layers represent a simple and economical way. In the interlayer region between the two brucite-like sheets, the anions

Boron is an essential micronutrient for human beings; however, it sometimes pollutes drinking water sources, leading to various environmental and health problems.1,2 This element is generally found in nature as borates (B(OH)4−) and/or boric acid (H3BO3). The range in which it is converted from a nutrient to a contaminant is quite narrow. The boron excess in water can affect the blood plasma and the endocrine system of a human being. The boron compounds are within the second group of toxic substances,3 and the World Health Organization (WHO) has defined a boron dose of 1−13 mg/day to be safe and adequate for a healthy individual. Therefore, the WHO has set a guideline value of 0.5 mg L−1 for boron in drinking water,4 while the recommended concentration for irrigation water is 0.75 mg L−1. Boron compounds are used in the manufacture of glass, soap, and detergent and as flame retardant.5 Naturally occurring boron is present in groundwater primarily because of leaching from rocks and soils containing borates and borosilicates. In nature, boron has never been found in its elemental state. Conversely, it mainly occurs as boric acid (B(OH)3) and its salts or as borosilicate. Boric acid can be easily dissolved in water, where it performs as a very weak Lewis acid (Ka = 6 × 10−10); this value may increase due to the presence of OH−1 in surface solid structures such as HDLs. In surface water, the borate © XXXX American Chemical Society

Received: April 25, 2019 Accepted: May 17, 2019

A

DOI: 10.1021/acs.iecr.9b02259 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Table 1. Experimental and Theoretical Chemical Composition of Analyzed Samples Including the Respective Values of SSA and Pore Volume (Vp) molar chemical composition

Mg 2 + sample S0 S25 S50 S75 S100

3+

Al

+ Fe 3 3 3 3 3

SSAb (m2 g−1)

MP-AESa

theoretical

Mg 2 + 3+

Mg

Fe

Al

75 75 75 75 75

0 6.25 12.5 18.75 25

25 18.75 12.5 6.25 0

3+

Al

+ Fe3 −

3.89 3.57 3.35 2.67 2.51

Mg

Fe

Al

PSx

CSx

79.55 78.12 76.99 72.71 71.48

0 6.36 13.41 21.70 28.52

20.45 15,51 9.60 5.58 0

100 90 77 64 57

248 190 176 87 82

Vpc (cm3 g̵−1) 0.29 0.27 0.20 0.21 0.20

a

Microwave plasma-atomic emission spectroscopy. bSpecific surface area. cAnalyzed by CSx samples.

(CSx), respectively, and the calcined sample names that were evaluated in the boron removal are preceded by letter B (BCSx). 2.3. Characterization of Materials. The elemental analysis was carried out by microwave plasma-atomic emission spectroscopy (MP-AES) in an Agilent 4200 instrument (Agilent, USA). Prior to elemental analysis, samples were dissolved by acid digestion in a microwave oven (SCP Science, Canada). The XRD analysis was carried out using an X’pert diffractometer (PANanalytical, Netherlands) with Cu Kα radiation at room temperature. The PCPDFWIN software has been used for matching the XRD pattern. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a SPECS multitechnique analysis instrument equipped with a dual X-ray source Mg/Al and a hemispherical analyzer PHOIBOS 150 in fixed analyzer transmission mode (FAT). High-resolution spectra were recorded in the constant pass energy mode at 30 eV with a Mg anode operated at 100 W. The pressure in the analysis chamber was maintained at lower than 10−9 mbar. UV−vis diffuse reflectance spectra (DRUV−vis) were recorded using a Jasco V-650 spectrometer with an integrating sphere in the wavelength range of 200−1000 nm. Also, a Spectrolon was used as the reflectance standard. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out in a TA Instruments SDT Q600 (0−1500 °C) instrumentation apparatus in a flowing air atmosphere. The sample was heated at a rate of 10 °C/min up to 550 °C. The SSA analysis was carried out in an ASAP 2000 instrument (Micromeritics, USA) and was calculated by the Brunauer− Emmett−Teller (BET) method. Prior to the determination of the adsorption isotherms, the precursors were treated at 200 °C and the calcined samples, at 390 °C during 60 min under 1.0 × 10−3 mbar vacuum. The micrographs of the mixed oxides were obtained by SEM model JSM-6380 LV (JEOL, Japan) equipped with a Supra 40 (Carl Zeiss, Germany). 2.4. Boron Removal Study. The mixed oxides (0.075 g) were kept in contact with a boron solution (15 mL of 50 mg L−1) in a 20:1 Mg/B ratio according to previous research.21 The test was carried out at 25 °C, and for the first 30 min, the mixture was kept at a constant stirring speed of 200 rpm; then, the sample was allowed to stand at different contact times. After finishing the adsorption test, the mixture was filtered and the obtained solid materials (BCSx) were dried in a nitrogen atmosphere at 90 °C. The materials were analyzed by XRD after adsorption. In order to study the memory effect and corroborate the borate anion incorporation in the laminar structure, the experiment was carried out in a N2 atmosphere. The boron measurement in

were stored to compensate the difference of charge. When the LDHs are calcined, mixed oxides are obtained, which present a high surface area and thermal stability. The memory effect is a property in which the mixed oxides allow the reconstruction, under mild conditions, of the layered structure when they are in contact with an aqueous solution or in humid atmosphere containing anions. This property, together with the good exchange capacity, makes them very appropriate materials for a household filtering mechanism.20 In this work, MgAlFe mixed oxides are synthesized from LDHs. The materials obtained were characterized by atomic emission spectroscopy (MP-AES), X-ray diffraction (XRD), Xray photoelectron spectroscopy (XPS), UV−vis diffuse reflectance spectroscopy (DRUV−vis), specific surface area (SSA), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) and then used for boron removal.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All solutions were prepared with deionized (DI) water (18 MΩ cm). Analytical grade reagents included Azomethine H (Merck, Ukraine) and NaOH (Baker, Argentina); the others, disodium ethylenediaminetetraacetate (Na2EDTA), acetic acid (HAc), ammonium acetate (NH4Ac), Mg(NO3)2, Al(NO3)3, and Fe(NO3)3, were from SigmaAldrich. A 500 mg L−1 boron stock solution was prepared by dissolving an appropriate quantity of H3BO3. 2.2. Synthesis of Mixed Oxides. The precursor materials were obtained by coprecipitation. These consist of a low supersaturation method at constant pH = (10.0 ± 0.5). A molar ratio of [Mg2+]/[Al3+ + Fe3+] = 3 was maintained. Experimental and theoretical chemical compositions are shown in Table 1. Two solutions were prepared: one of them (solution A) contained Mg(NO3)2·6H2O, Al(NO3)3·9H2O, and Fe(NO3)3· 9H2O while the other solution (B) was 0.085 mol L−1 of Na2CO3 as a source of carbonate anions. The solutions A and B were dropped at 60 mL h−1. The pH was controlled by adding a NaOH solution with a concentration of 2.0 mol L−1. The coprecipitation was carried out at 70 °C, and the gel was continuously magnetically stirred. The mixture was kept under magnetic stirring for 4 h. The precipitate was aged in the mother liquor overnight for 18 h, washed with DI water, and centrifuged at 2000 rpm until pH ∼ 7. The resulting material was dried at 90 °C and calcined in open air at 450 °C for 8 h. The aluminum content is gradually replaced by the iron content in a molar ratio from 0 to 100. The samples have been denominated as Sx where x is the percentage of iron incorporated. The precursors and calcined names are preceded with the letter P (PSx) and C B


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Figure 1. XRD patterns. (A) LDHs precursors and (B) mixed oxides. (□) periclase MgO and (+) hematite Fe2O3.

observed; this is because the crystalline structure appears when the calcined temperature is higher than 800 °C.25 3.2. XPS Spectroscopy. The XPS spectra of calcined samples are shown in Figure 2A. The analysis provides important data related to the surface composition of the calcined materials. The presence of Fe3+ cations can be assigned by the principal Fe 2p3/2 signal and their satellite signal around binding energy (BE) of 719.3 ± 0.2 eV,26−31 while the corresponding signals to Fe 2p1/2 are attributed at 723.9 eV.attributed at 723.9 eV.29 The Fe3+ species can exist in a tetrahedral (A sites) at 711.5 ± 0.2 eV and octahedral (B sites) at 714.0 ± 0.3 eV (peak II).30 This fact is correlated to the two contributions observed in the main Fe 2p3/2 signal. The tetrahedral environment is associated with Fe3+ in the spinels phase26 (not observed by XRD, likely by their small size). The O1s spectra for the calcined samples are shown in Figure 2B. The signals at 530.4 ± 0.1 and 532.5 ± 0.1 eV are attributed to O2− distributed on two different kinds of surface oxygen species. The oxygen atoms can be found in different locations depending on the coordination and bond length of metal attached to the O2−.31 The OI at low BE is the characteristic of lattice oxygen bound to the metal cation of the structure, while the signal at higher BE OII is related to surface oxygen species in the hydroxyl groups.32 3.3. DRUV−vis Analysis. The DRUV−vis spectra of LDH precursors and calcined samples are shown in Figure 3. The spectra have been deconvoluted into a set of bands that can be assigned to the different Fe species.33−36 All LDH precursors exhibit a band at ∼260 nm (Figure 3A) that can indicate the Fe3+

soluble fraction was evaluated by molecular absorption spectroscopy in the UV−visible range employing Azomethine H as a colorimetric reagent. This compound is not considered a standardized reagent for boron analysis in waters.22 However, it has been selected owing to its simplicity and competitiveness over other spectrophotometric reagents.23

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction. The powder X-ray diffraction patterns of the precursor samples are presented in Figure 1A. In all samples, the characteristic peaks relative to the hydrotalcite phase are observed, agreeing with the International Centre for Diffraction Data (PCPDFWIN 70-2151).24 The distance between the brucite-like sheets indicates the size of the charge compensating anions (carbonate or possibly boron species). This distance is calculated using the signal 2θ corresponding to the planes (003) and (006). The sample PS0 exhibits intense and sharp signals, while as the presence of iron in the samples increased, the signals become less intense and wider. This effect can be associated with the decreased crystallinity of the samples. Figure 1B shows powder XRD patterns of calcined materials. The layered structure collapsed, and the signals corresponding to the characteristic hydrotalcite phase (2θ of 11.62° and 23.38°) disappear. The MgO in the periclase phase (PCPDFWIN 78-0430)24 is observed in all samples. It can also be observed that the ion Fe3+ crystallizes like hematite Fe2O3 (PCPDFWIN 79-1741).24 In the diffraction patterns of calcined samples at 450 °C, the alumina phase Al2O3 was not C


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Figure 2. XPS spectra of the calcined samples: (A) Fe 2p1/2 and Fe 2p3/2; (B) O1s.

ions in octahedral coordination in the laminar structure33 The band observed at ∼350 nm has been assigned to an octahedral coordination of Fe3+ in a small cluster of oxy-hydroxide structures located out of the brucite-like sheets.34 Finally, the band at ∼476 nm can be related to large iron oxides outside of the brucite layer.35,36 Therefore, the intensity of the bands at ∼350 and ∼476 nm, related to the Fe3+ in oxides and hydroxides outside of the hydrotalcite-like sheet, increased with the higher iron content in the samples. Figure 3B shows DRUV−vis spectra of mixed oxides; the band observed at ∼260 nm is related to the Fe3+ in octahedral coordination. The band at ∼350 nm can be associated with the presence of isolated Fe3+ in periclase phase Mg(Fe, Al)O.33 As previously mentioned, the band at ∼476 nm is associated with large iron oxide clusters and aggregated iron oxide nanoparticles.35−37 In this respect and as can be seen in Figure 3B, the presence of the iron nanoparticles as well as their size grow by the iron increment in the samples. 3.4. Elemental and Texture Analysis. Table 1 summarizes the theoretical and experimental molar chemical compositions of the set of synthesized samples, as well as the values of SSA and pore volume (Vp) for the set of precursors and mixed oxides. The MP-AES analysis indicates that the experimental molar percentage values of the Mg, Fe, and Al have a deviation of its theoretical values of 4.7−6%, 1.76−15.73%, and 10.72−23.2%, respectively. According to this effect, the Mg2+/(Al3+ + Fe3+) molar ratio also has a deviation of its theoretical value between 11% and 30%.

N2 adsorption−desorption isotherms determined at 77 K for CSx are shown in Figure 4. Additional data associated with the adsorption−desorption isotherms has been included as Supporting Information. The physisorption isotherms behavior of the CSx samples can be assigned to reversible type II isotherms according to the IUPAC classification. The values of C obtained in all the samples were higher than 80 (94−144); a little knee is observed in the isotherms, and point B is slightly defined.38 From the adsorption data analysis, it has been observed that these materials are slightly porous or macroporous. This fact is in agreement with that reported by Ramos Guivar et al. in γ-Fe2O3−TiO2.39 It has also been observed that the SSA values decrease with the increase of the Al replacement for the iron in the synthesized samples; both in precursors and in the calcined samples, this effect can be assigned to the decrease of amorphous alumina. In all cases, the area of the calcined sample is greater than that of its precursors. In this regard, after calcination, the samples S0, S25, and S50 duplicate their values of SSA, while the samples S75 and S100 only increase 36% and 44%. According to the report by Triantafyllidis et al., the gradual isomorphic substitution of Al3+ for Fe3+ prevents the Mg2+ from being incorporated in the brucite layer and consequently distorts the precursor structures.40 3.5. Thermal Analysis (TGA and DSC). Thermal properties of the samples have been assessed by TGA and DSC. These studies were carried out in air. The total weight losses and the temperatures of the maximum endothermic peak in the DSC D


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Figure 3. DRUV−vis spectra. (A) LDHs precursors: (a) octahedral Fe3+, (b) octahedral Fe3+ in small oxy-hydroxides, and (c) Fe3+ nanoparticles. (B) Mixed oxides: (a) octahedral Fe3+, (b) Fe3+ in periclase phase, and (c) large Fe3+ oxides.

and simultaneously, the Al content (Al2O3 amorphous) decreases. Also, elemental mapping verified the presence and dispersion of metals. Figure 6E,F shows images corresponding to the good distribution metals above the surface in the samples CS50 and CS100. The molar percent values related to the Mg, Al, and Fe are shown in Table 3. The expected data from the synthesis protocol and the results of the elemental mapping have a good approximation. 3.7. Boron Removal by Mixed Oxides. The boron removal from the aqua solution was evaluated through the different calcined samples. The removal test was carried out by keeping an aliquot of 15 mL with 50 mg L−1 H3BO3 in contact with a sample of 0.073 g of mixed oxides (Mg/B ratio of 20:1); the process time was 285 min at room temperature. The pH was not adjusted, but its value was monitored through all of the boron removal experiments and remained constant around 10 ± 0.5. It is well-known that, in aqueous solutions, a distribution of borates is established between mono- and polyborate on the basis of concentration, temperature, and pH.41 At low concentrations, the dissociation of H3 BO 3 yields only monomolecular species [B(OH)4]−. A good capacity of the boron removal is observed in all analyzed calcined samples; the processed data are shown in Figure 7.

profiles, corresponding to the obtained mixed oxides, are reported in Table 2. Two weight loss stages were observed on the TGA, corresponding to the two endothermic peaks on the DSC profiles. This fact demonstrates that the decomposition of LDH proceeded in two steps. The first weight loss below ∼100 °C is assigned to the water physically adsorbed, which corresponds to a shoulder in the DSC profiles; see Figure 5. The loss of interlayer water corresponds to a weight loss between 100 and ∼300 °C and an endothermic peak in the DSC profile. The brucite layer dehydroxylation and the interlayered carbonate loss generate the mixed oxides; this fact takes place between 300 and 410 °C. A decrease in the total weight loss next to an increased iron content in the samples indicates a less crystalline and distorted LDH structure. This is correlated with the shift to a lower temperature of endothermic peak assigned to brucite layer dehydroxylation and mixed oxides generation. 3.6. SEM Analysis. The SEM images of mixed oxides are shown in Figure 6. The samples CS25 and CS50 show porous structures with small aggregate crystals (Figure 6A,B) between ∼15 and ∼20 μm. The images of the samples CS75 and CS100 exhibit large particles or domains (Figure 6C,D) between ∼30 and ∼40 μm. This effect is in concordance with the decreased SSA when the Fe content was gradually increased in the samples, E


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Figure 4. Nitrogen adsorption−desorption isotherms for calcined samples.

Table 2. TG-DSC: Total Weight Lost and Dehydroxylated Temperature sample

total weight loss (%)

DSC maximum (°C)

S0 S25 S50 S75 S100

40.3 39.2 35.7 30.1 21.4

395 379 375 364 346

The mixed oxides, under mild humidity conditions, can be reconstructed to their original precursor structures. The XRD pattern analysis (Figure 8) indicates that the samples BCS0, BCS25, and BCS50 reconstituted their precursor structures after 45 min of exposure to the boron solution. The laminar reconstruction by the memory effect is slower when the iron content in the samples is increased; due to this, two contact times (45 and 285 min) were analyzed by XRD for the samples BCS75 and BCS100. The presence of periclase and hematite phases, besides small peaks related to the LDH structure, indicated the partial reconstitution of the laminar structure by the memory effect when the mixed oxides are exposed to the boron solution at different contact times. It is well-known that mixed oxides interact strongly with anionic species due to the existence of positive charges over their external surface (adsorption process) or regenerating the structure by the memory effect. Therefore, the interaction of samples CS0, CS25, and CS50 with soluble species incorporates borate anions within their interlaminar spaces. Thus, borate anion removals in the C75 and C100 samples would take place by electrostatic attraction, between mixed oxides and borate solution, and by means of the memory effect by incorporating

Figure 5. DSC profile of the precursor samples.

F


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Figure 6. SEM images of mixed oxides: (A) CS25, (B) CS50, (C) CS75, and (D) CS100. SEM elemental mapping: (E) CS50; (F) CS100.

Table 3. Molar Ratio Values Acquired by Elemental Mapping in a SEM Analysis (Mg2+ × 100%)/(Mg2+ + Al3+ + Fe3+)

(Al3+ × 100%)/(Mg2+ + Al3+ + Fe3+)

(Fe3+ × 100%)/(Mg2+ + Al3+ + Fe3+)

sample

theoretical

mapping

theoretical

mapping

theoretical

mapping

CS0 CS25 CS50 CS75 CS100

75 75 75 75 75

77.5 73.5 76.7 74.5 74.0

25 18.8 12.5 6.2 0.0

22.5 17.7 11.9 6.2 0.0

0.0 6.2 12.5 18.2 25

0.0 8.8 11.4 19.3 26.0

coprecipitation method; the resulting gel containing the corresponding metallic cations was maintained at 70 °C. The boron removal capacity of synthesized mixed oxides was carried out in a batch reactor during a 285 min contact time. The maximum boron removal values were achieved with CS25 and CS50 when 0.07 g of mixed oxide kept in contact with 15 mL of 50 mg L−1 boron solution. With respect to the aluminum content in the samples, the amorphous Al2O3 in the calcined materials enhances the surface area and increases the iron species dispersion.

borate anions in the laminar structure. The XRD patterns obtained from the samples analyzed in an inert atmosphere, without the presence of carbonate ion, corroborated the borate anion incorporation in the laminar structure.

4. CONCLUSION A set of mixed oxides from synthetic layered double hydroxides, feasible of being used in a filtering system, has been characterized by spectroscopic, microscopic, and elemental analysis. The mixed oxides were obtained by calcination of layered double hydroxides. The precursors were prepared by the G


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content by SEM analysis. The TG and DSC analysis showed a change to lower temperatures for mixed oxides generation besides a lower weight loss according to the increased iron content in the sample. The presence of aluminum increased the SSA, promoting the incorporation of the borate anion through the pores and the recovery of the hydrotalcite-like structure, while the Fe3+ presents an advantage in the interactions of the borate anions with the mixed oxides surface. Thus, borate anion removals would take place by electrostatic attraction, between mixed oxides and borate solution, and by means of the memory effect by incorporating borate anions in the laminar structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02259. Additional data about N2 adsorption−desorption isotherms of the precursors samples (PDF)

Figure 7. Capacity of calcined samples for boron removal when aliquots of 15 mL are exposed for 285 min to 0.07 g of mixed oxides.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Angélica C. Heredia: 0000-0002-2917-005X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was from the Consejo Nacional de Investigaciones Cientı ́ficas y Tecnológicas (CONICET), Universidad Tecnológica Nacional−Facultad Regional Córdoba (UTN-FRC), CIN-CONICET PDTS 517. Thanks are given to ANPCyT for the purchase of the SPECS multitechnique analysis instrument (PME8-2003).

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REFERENCES

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Figure 8. X-ray diffraction patterns of mixed oxides after boron adsorption. Diffraction planes of laminar phase are indicated. (□) periclase MgO and (+) hematite Fe2O3.

The trivalent iron ions have been observed in two different tetrahedral and octahedral coordinations by X-ray photoelectron spectroscopy and UV−vis diffuse reflectance spectroscopy, while the small aggregate crystals and large particles or domains have been observed according to the rise in iron H


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Boron Removal from Aqueous Solutions by Synthetic MgAlFe Mixed

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