Insights about Adsorption Mechanism - American Chemical Society

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Influence of pH and of the Interactions Involved in Etheramine Removal in Kaolinite: Insights about Adsorption Mechanism Zuy Maria Magriotis,* Priscila Ferreira de Sales,‡ Teodorico Castro Ramalho,§ Marcus Vinícius Juliaci Rocha,∥ and Paulo Vitor Brandaõ Leal†,⊥ Departamento de Química, Universidade Federal de Lavras, C.P. 3037, 37200-000 Lavras-MG, Brazil S Supporting Information *

ABSTRACT: As the adsorption process involves interactions between molecules of the adsorbate and the surface of the adsorbent material, theoretical studies are developed with the aim of elucidating the mechanism. In this work, the influence of pH on adsorption of etheramine in kaolinite was investigated. The results showed that adsorption was most efficient at pH 10, which was attributed to the equilibrium of the etheramine molecules between ionic and molecular forms creating an ionic−molecular complex that promotes adsorption. Theoretical calculations confirmed these results and showed that the ionic−molecular dimer is the most stable structure among the possible dimers (ionic−ionic, molecular−molecular, and ionic−molecular).

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INTRODUCTION Increase in population and industrialization worldwide have had harmful consequences for the environment. The mineral working and processing industry has been a significant contributor to this harm, as considerable volumes of water are needed throughout the process. Flotation, the most commonly used mineral concentration process, consumes a huge amount of water, on average 5 m3 h−1 per tonne of mineral processed.1 Apart from the significant amounts of water used, it is worth emphasizing that the flotation process involves the use of a variety of chemical products intended to improve the efficiency of the process. Etheramines were the focus of this study, because they are used in large quantity in the flotation of various minerals, chiefly iron ore.2 Etheramines show a high chemical oxygen demand (COD) value, are toxic to aquatic organisms, and degrade over approximately 28 days.1,3 However, with the silting of ore tailings dams and the possible outflow of residue from them via the spillway, the residence-time of etheramines in the tailings dams may become insufficient for their degradation, bringing risks to the environment.1 Adsorption justifiably stands out from among a variety of economically viable effluent treatment methods. In effluent treatment, the search for low-cost materials to be used as adsorbents has increased in recent years.4 Prominent among the low-cost adsorbents are clay minerals, due mainly to their great capacity for cationic transfer and chemical and mechanical stability.4 Kaolinite is a clay mineral with phyllosilicate structure, formed by regular alternate stacking of a sheet of silica tetrahedra and one of aluminum octahedra, which gives rise to the basic 1:1 units, maintained by hydrogen bonds between the basal oxygen of the tetrahedra and hydroxyls of the octahedral layer.5 © 2013 American Chemical Society

As the adsorption process involves interactions between molecules of the adsorbate and the surface of the adsorbent material, theoretical studies have been developed with the aim of elucidating the mechanism of contaminant removal.6−9 The objective of this work was, therefore, to study the influence of pH on the adsorption of an organic contaminant (etheramine) by a natural adsorbent, kaolinite, seeking to appraise the adsorption mechanism through the use of computational methods.

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METHODOLOGY

Characterization of the Adsorbent. The kaolinite, ́ provided by Mineradora Quimica e Minérios de Ijaci, Minas Gerais State, Brazil, was first crushed and sieved (0.425 mm sieveTyler series). The textural properties of the kaolinite were determined by measurements of adsorption and desorption of N2 at −196 °C in an ASAP 2020 Analyzer (Micromeritics). The ζ potential of the kaolinite was measured with a Zeta Meter 3.0+, model ZM3-D-G (Zeta Meter Inc.). The suspensions of adsorbent, ground beforehand to below 37 μm, were sedimented/conditioned at a temperature of 22 °C, for 2 h, at the selected pH, in 250 mL cylinders with the addition of 2.0 × 10−3 mol L−1 of NaNO3 solution, used as an indifferent electrolyte. The applied tension was varied in the range 75−200 mV. Twenty measurements were carried out to obtain the average potential measured. X-ray diffraction analyses (XRD) were carried out in a Phillips model Received: June 3, 2013 Revised: September 20, 2013 Published: September 24, 2013 21788

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Figure 1. Structural representation of the etheramine molecule.

PW 1710 spectrometer using Cu Kα1 radiation with scanning from 4° to 90° and sweep speed of 0.6° min−1. Adsorbate. The tests of adsorption were carried out using, as adsorbate, the etheramine Flotigan EDA (Clariant)/etheramine acetate, with dodecyl radical, neutralized at 30% (Figure 1). The etheramine solutions used in the experiments were diluted from a 4 g L−1 solution. Adsorption Experiments. The adsorption experiments were carried out in batches, in orbital shakers with a speed of 100 rpm, at temperature 25° ± 1 °C, using 10 mL of etheramine solution with an initial concentration of 200 mg L−1, adsorbent mass (g)/adsorbate volume (mL) ratio 1:100 (corresponding to 0.1 g of kaolinite) and pH 2, 2.85, 4, 7, 10, 12, and 13. The solutions were adjusted to the respective pH values using acetic acid P.A. and sodium hydroxide solution 0.1 mol L−1. The samples were withdrawn at regular time intervals and filtered, and the remaining concentration of etheramine was monitored by UV−vis spectroscopy with wavelength 410 nm, using the bromocresol green method.1 All experiments were carried out in triplicate to guarantee reproducibility. The adsorption was also studied by infrared spectroscopy. For this, adsorption at a temperature of 25 ± 1 °C was evaluated using the pH optimized in the process of adsorption. After the adsorption time, the samples were filtered and dried in an oven at 30 °C for 24 h. Kaolinite samples, before and after adsorption, were analyzed in the middle infrared region, in the range from 500 to 4000 cm−1 (resolution of 4 cm−1). The analyses were carried out using KBr pellets (30 mg of sample to 300 mg of KBr) in a Digilab Excalibur spectrophotometer, series FTS 3000. Study of the Interaction between Etheramine and Kaolinite by Computational Methods. All calculations were carried out with the Gaussian 03 package.10 For each structure studied, calculations were made for optimization, using the DFT with the B3LYP functional11,12 employing the 6-31G basis set. A polarization function was defined for the protonated nitrogen atom. No symmetry constraint was imposed during the optimization process. Those optimized geometries were used in all subsequent calculations. Furthermore, a force constant calculation was made to verify if the optimized structures were indeed local minima (no imaginary frequencies) or transition states (one imaginary frequency).13,14 This theoretical level was also used for the frequency calculations to obtain the infrared spectrum. All discussions concerning the energy differences and the energy barriers refer to the thermodynamic parameters corrected for the zero point energy at 298.15 K. For all the studied species we have checked S2 values to evaluate whether spin contamination can influence the quality of the results. In all cases we have found that the calculated values differ from S (S + 1) by less than 10%.

Table 1. Textural Properties of Kaolinite specific area (BET) pore area (BJH) pore volume (BJH) avg pore size (BJH)

34.3 m2 g−1 39.8 m2 g−1 0.2187 cm3 g−1 219.9 Å

kaolinite showed a specific surface area value of (34.3 m2 g−1) (Table 1), higher than the range found in the literature (13.7− 28.1 m2 g−1).15,16 The difference observed can be attributed to the difference in the chemical composition or in the crystallinity of kaolinites from different regions of origin.15,16 The surface charge of the kaolinite diminished as pH increased (Figure 2). The observed phenomenon may be related to the fact

Figure 2. Influence of pH on the ζ potential of kaolinite.

that in acid pH the aluminol (Al−OH) group is protonated by H+ ions present in the solution, forming a positive species, Al−OH2+. Meanwhile, in a basic environment the Al−OH species gives up a proton to the hydroxide ion present in the solution, creating an anion (Al−O−) and water. In this way, the edge charges are positive in the acid pH range and negative in the basic pH range, as was observed in reactions 1 and 2:17 Al−OH + H+ → Al−OH 2+

(1)

Al−OH + OH− → Al−O− + H 2O

(2) +

So, in an acid medium there is adsorption of H ions at centers of negative charge, while basic mediums promote the adsorption of OH− ions at centers of positive charge in kaolinite.17 The XRD results (Figure 3) show that the kaolinite contained some impurities (a quartz fraction), but in insufficient quantities to hinder the adsorption process. Adsorption Studies. The kaolinite showed minimum and maximum adsorptions at pH 2.85 (3%) and 10 (80%), respectively (Figure 4). The adsorption of etheramine at the kaolinite surface occurs particularly due to interactive forces of electrostatic nature

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RESULTS AND DISCUSSION Characterization of the Adsorbent. The results of the textural properties of kaolinite are shown in Table 1. The 21789

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Figure 3. XRD of kaolinite.

sites exposed at the edge-plane of the adsorbent material.20 Thus, the greater adsorption of etheramine at pH 10 can be explained by the fact that at this pH the adsorbent has a negative surface charge and the etheramine molecules are evenly balanced between the ionic and molecular forms, which can result in the formation of an ionic− molecular complex that promotes adsorption.1,21 At pH 12and 13, despite the kaolinite surface having an even stronger negative charge, there was less removal of etheramine, which can be explained by the fact that, at these pH values, etheramine is in 100% molecular form. In these cases only the mechanism of capillary condensation between the carbonic chains of the etheramine would be responsible for the adsorption at the kaolinite surface. The process of etheramine adsorption in kaolinite was also studied using FTIR. The spectra of the etheramine solution, of kaolinite before and after adsorption, are shown in Figure 5.

Figure 4. Influence of pH on the adsorption of etheramine onto kaolinite.

between the protonated groups of the surface of the adsorbent and the etheramine, followed by a chemical reaction between them.18 Etheramine is predominantly found in ionic form at pH values below 9 and predominantly in molecular form at pH values above 11.5. At pH 10, etheramine is 50% in ionic form and 50% in molecular form.19 The lower adsorption value at pH 2.85 may be explained by the fact that this is the isoelectric point (IEP) of kaolinite. Thus, although the etheramine is in ionic form, the surface of the kaolinite presents zero charge, preventing the operation of electrostatic attraction with the molecules of adsorbate in ionic form. The low removals obtained at pH 2 and 4 can be attributed to the positive charge of the kaolinite (pH 2) and to the reduced negative surface charge (pH 4). When the etheramine molecule is close to the surface of the kaolinite, the oxygen atoms of the Si−O− and Al−O− groups attract the hydrogen of the NH2 group, forming hydrogen bonds between the O−Si−O and O−Al−O groups and the etheramine. In an acid medium, these bonds are less likely to occur, since the H+ ions end up competing with molecules of adsorbate for the adsorption sites and, thus, reducing the electrostatic interactions which envelop the surface of the adsorbent material with the etheramine molecules, in ionic form, NH3+. At basic pH, adsorption takes place through the electrostatic mechanism and hydrogen bonding involves Si−OH and Al−OH

Figure 5. FTIR spectra of etheramine (A), kaolinite pure (B), and kaolinite after adsorption (C).

Table 2. Relative Values of Energy

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species

ΔE (kcal mol−1)

ionic−molecular ionic−ionic molecular−molecular

0.00 31.35 18.81

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Figure 6. Etheramine dimer structures at the B3LYP/6-31G level. Ionic−ionic (A); ionic−molecular (B); molecular−molecular (C).

The etheramines used in mineral processing, at operational conditions of pH 10, have a molecular structure corresponding to R−O−(CH2)3−NH2 and, in ionic form, R−O−(CH2)3−NH3+. Therefore, in the infrared spectrum of etheramine, the 2960 and 2920 cm−1 bands identify axial deformation vibrations of the hydrogen atoms bonded to carbon (methyl groups). The absorption in the region between 2300 and 1900 cm−1 is associated with angular deformations of NH2 and N−H: axial deformation of double bonds (nonaccumulated). Between 1590 and 1550 cm−1, there is angular deformation of NH2. The region between 1500 and 600 cm−1 is associated with various types of vibration: axial and angular deformations of C−O, C−N, and C−C bonds. The band in the 1462 cm−1 region concerns the angular deformation of (CH2)3. In the region between 1150 and 1070 cm−1, there is the axial deformation C−O of aliphatic ethers. As there is an ether bond in the structure of the etheramine studied, the 1100 cm−1 band refers to this type of vibration.22 Upon analyzing the spectra of kaolinite before and after adsorption, it can be seen that there are two bands that differ in those spectra, identified at 2856 and 2948 cm−1. In the infrared analysis, these bands come from the asymmetric and symmetric axial deformations of the NH3+ group. These bands effect the adsorption process, as at the surface of the adsorbent there would be an interaction (chemical adsorption) with the adsorbate in ionic form. Further, it is possible to identify, in the spectrum of pure kaolinite, bands characteristic of the vibration of the atoms that make up its structure. The band at 3680 cm−1 corresponds to the stretching of OH groups. The 1630 cm−1 band corresponds to the deformation of the water molecule (absorption of water, accompanied by rotational transitions).23 Study of the Interaction between Etheramine and Kaolinite by Computational Methods. As observed in Figure 4, at pH 10 adsorption was shown to be most efficient, which can be explained by the interaction between the molecular and ionic forms in equilibrium of the adsorbate molecules forming an

Figure 7. Potential energy curves to represent the interaction between kaolinite and dimer.

ionic−molecular complex that helps in the adsorption process. Given these results, and with the aim of clarifying the mechanism 21791

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Figure 8. Theoretical infrared spectrum of etheramine in the ionic form (A) and the etheramine in molecular form (B), and the dimer (C).

protonated nitrogen atom and hydroxyl group from the kaolinite surface as related in a previous study,23 where the amine groups are close to the surface and the oxygen atoms of the Al−O groups attract hydrogen from the NH2 group, forming hydrogen interactions between those groups. The results are shown in Figure 7. In accordance with our theoretical results, the protonated nitrogen atoms interact strongly with the hydroxyls of the kaolinite surface through electrostatic interactions. In this situation, even with the steric repulsion between the dimer and the kaolinite, the adsorption energy is favorable Theoretical calculations of vibrational frequencies (Figure 8) were carried out to corroborate the experimental values obtained by infrared spectroscopy. Those calculations were based on the adsorption of etheramine in the molecular and ionic forms, as well as that occurring between two etheramine molecules and the kaolinite surface. The region around 2800−2900 cm−1 (Figure 8C) indicates bands originating from asymmetrical and symmetrical axial deformations of the NH3+ group near the kaolinite surface, as demonstrated in the experimental infrared spectrum when an ionic−molecular dimer of etheramine is adsorbed at the surface of the kaolinite. In fact, the results for theoretical IR frequencies reported in Figure 8C show much better agreement with the experimental infrared spectrum (Figure 5). Thus, the theoretical results reinforce the experimental findings and point to the adsorption of an ionic−molecular dimer of etheramine at the surface of the kaolinite, as suggested by Figure 9.

of etheramine adsorption on kaolinite, theoretical calculations were carried out to verify the interaction between the molecular forms of the adsorbate, as well as that occurring between the etheramine molecules and the surface of the adsorbent material. First, analysis was carried out of the possible ways of interaction when two etheramine molecules form a complex. In this case, it is possible to analyze three types: those that involve adsorbate molecules of the same form (ionic−ionic and molecular−molecular) and in distinct forms (ionic−molecular). The relative values of energy interaction are shown in Table 2. The results presented in Table 2 show that the dimer in ionic− molecular form is the most stable structure. The relative energy was calculated by potential energy surface analysis (PES), the most stable structure found in each dimer was reoptimized at the B3LYP/6-31G level. Figure 6 shows all the etheramine dimer structures used in this work. The PES profiles were also introduced in the Supporting Information (Figures S1, S2, and S3). These results corroborate the experimental results in which the best adsorption occurs at pH 10, when there is equilibrium between the ionic and molecular forms. With the aim of evaluating how the most stable dimer structure interacts with kaolinite, calculations were made of the potential energy curve of the dimer with the kaolinite surface. For this, three types of interactions were tested: the first with the complex in dimer form interacting with the surface; the second with one molecule already adsorbed and the other approaching to form the complex on the kaolinite surface. The distance between the etheramine complex and the kaolinite surface was based on the 21792

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Figure 9. Adsorption of etheramine ionic−molecular dimer at the surface of kaolinite.

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CONCLUSION The study of etheramine adsorption at different pH values showed that this parameter has considerable influence on the removal of the contaminant, since it acts both to modify the surface charge of the adsorbent material and upon the balance of ionic and molecular forms of the etheramine molecules. The greater etheramine removal at pH 10 was attributed to the equilibrium of the etheramine molecules between the ionic and molecular forms, creating an ionic−molecular complex that enhances adsorption. Theoretical calculations confirm these results and show that the dimer in ionic−molecular form is the most stable structure among the possible dimers.

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Rio de Janeiro, Rio de Janeiro, Brazil) for XRF and adsorption/ desorption analyses, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

Potential energy surface profiles, and complete author list of ref 10. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: +55 35 38291889. Fax: +55 35 38291812. E-mail addresses: [email protected]fla.br. Present Address †

Departamento de Ciências Agrárias e Tecnológicas, Universidade Federal do Tocantins, C.P. 66, 77402-970 Gurupi-TO, Brazil.

Notes

The authors declare no competing financial interest. ‡ E-mail: [email protected] (P.F. de Sales). § E-mail: [email protected]fla.br (T.C. Ramalho). ∥ E-mail: [email protected] (M.V.J. Rocha), ⊥ E-mail: pvitorufl[email protected] (P.V.B. Leal).

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ACKNOWLEDGMENTS We express our appreciation to CAPES and FAPEMIG for financial support, to Intercement Brasil S.A (Ijaci, Minas Gerais, Brazil), and to F.B. Noronha (Instituto Nacional de Tecnologia, 21793

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