Amino-Functionalized Titanate Nanotubes: pH and Kinetic Study of a Promising Adsorbent for Acid Dye in Aqueous Solution

This work reports the functionalization of sodium titanate nanotubes with amine groups obtained from the reaction of titanate nanotubes with [3-(2-Aminoethylamino)propyl]trimethoxysilane, NaTiNT−2NH, and 3-[2-(2-Aminoethylamino)ethylamino]propyltrimethoxysilane, NaTiNT−3NH. It was verified that the crystalline and morphological structures of NaTiNT were preserved after the functionalization, spectroscopies showed that aminosilane interacted covalently with the surface of NaTiNT, and the incorporation of the aminosilane groups on the surface of NaTiNT can be confirmed. The adsorbent matrices NaTiNT−2NH and NaTiNT−3NH were used to remove the anionic dye from remazol blue R (RB) in aqueous medium, and the highest adsorption capacity was around 365.84 mg g−1 (NaTiNT−2NH) and 440.70 mg g−1 (NaTiNT−3NH) in the range of pH 5.0 to 10.0 and the equilibrium time was reached in 210 min (NaTiNT−2NH) and 270 min (NaTiNT−3NH). Furthermore, the Elovich model, which reports the adsorption in heterogeneous sites and with different activation energies in the chemisorption process, was the most appropriate to describe the adsorption kinetics. Thus, these adsorbent matrices can be used as an alternative potential for dye removal RB in aqueous solution.


Introduction
The reduction of environmental impacts that derive from industrial development [1,2], in particular water pollution from contaminants such as dyes [3][4][5], metals [6][7][8], and pharmaceuticals [9,10], has been the subject of several studies in recent years. Among the contaminants, basic [11,12] and anionic [13][14][15] dyes can cause several problems to living organisms due to their carcinogenic and mutagenic characteristics, being acid dyes (for example, remazol blue dye (RB) ) with high water solubility and toxicity, making it difficult to remove from wastewater [16]. Furthermore, the pollution resulting from this class of compounds harms several biochemical processes in living beings [1]. In this context, adsorption is a promising method against the challenge of environmental pollution [16][17][18][19]. The adsorption process uses various materials as adsorbents, such as clays, activated carbon, biopolymers, nanomaterials, among others [20][21][22]. Thus, the development of new materials that can be used as adsorbents with high adsorption capacity represents an important action to mitigate the impacts of environmental pollution [23]. In this scenario, titanate nanotubes are very efficient adsorbents for basic (cationic) dyes due to their high surface area and cation exchange behaviors [11][12][13]. However, sodium titanate nanotubes (NaTiNTs) have practically no attraction for acid (anionic) dyes, and their functionalization with amine groups is of fundamental importance to be used as adsorbents for anionic dyes in aqueous media, because adsorption can be favored by electrostatic interactions due these groups being easily protonated under acidic conditions and the acid dye (for example, RB) is negatively charged [15]. Because of this, amino-functionalized titanate nanotubes are versatile adsorbents that have been used to remove acid dyes in aqueous solution [13][14][15], due to the combined action of high surface area functional groups decoration [15,24]. Thus, due to the positive characteristics of amino-functionalized titanate nanotubes in relation to the adsorption of acid dyes from an aqueous medium, more studies are needed regarding the production of new routes of synthesis of functionalized titanate nanotubes with amino groups and their applications in the adsorption of acidic dyes from the environment. Therefore, the production of trimethoxysilane and 3-[2-(2-Aminoethylamino)ethylamino] propyltrimethoxysilane functionalized titanate nanotubes and their application in the adsorption of acid dyes is necessary, because there are no studies in the literature related to this route of synthesis of this adsorbent for application in the adsorption of acid dyes from the aqueous medium. Thus, this work describes the feasibility of using sodium titanate nanotubes (NaTiNTs) with amine groups (NaTiNT−2NH and NaTiNT−3NH), obtained through the reaction of NaTiNTs with [3-(2-Aminoethylamino)propyl] trimethoxysilane and 3-[2-(2-Aminoethylamino)ethylamino] propyltrimethoxysilane, characterizes them by XRD, BET, SEM, EDS, Raman, FTIR, and XPS, and applies them in the adsorption of the acid dye remazol blue R (RB) with the variation of pH and contact time. In addition, the experimental results of kinetics were fitted to pseudo-first-order, pseudo-second-order and Elovich nonlinear kinetic models.

Synthesis of Sodium Titanate Nanotubes
For the production of sodium titanate nanotubes (NaTiNT), 3.00 g of TiO 2 (anatase) were placed in 90 mL of 10.0 mol L −1 NaOH. Subsequently, this solution was placed in a Teflon reactor for 96 h at 140°C. Finally, after cooling the system, a white solid was formed, which was washed several times with deionized water until the pH was between 10.0 and 12.0, and, subsequently, this material was dried in vacuo for 24 h [15].

Point of Zero Charge (pH pzc )
The point of zero charge (pH pzc ) was carried out using the method described in the literature [25,26]. Twenty mg of the solid (NaTiNT, NaTiNT−2NH or NaTiNT−3NH) were placed in Erlenmeyers containing 20 mL of NaCl solution (0.1 mol L −1 ) with different pHs (1.0 to 12.0). These pHs were regulated with the addition of HCl (0.1 mol L −1 ) and/or NaOH (0.1 mol L −1 ) solutions. Soon after, the Erlenmeyers were left for 24 h under agitation (140 rpm). Subsequently, the suspensions were centrifuged (3500 rpm/5 min), and the final pH (pH f ) was measured. e ∆pH versus pH i graph was plotted, where ∆pH is the difference between the initial and final pH (∆pH = pH i − pH f ).

Remazol Blue R Adsorption
The RB dye adsorption assays with the adsorbents NaTiNT−2NH and NaTiNT−3NH were carried out using 20 mg of the modified nanotubes with 20 mL of the dye solution (room temperature). The dye concentration (initial and final) was determined by UV-Vis, as shown in the literature [27,28]. The adsorption process varying the pH occurred with: the dye concentration-1000 mg L −1 , the time-24 h, the temperature-25°C, and the pH-1.0 to 12.0. The adsorption process varying the time occurred with: concentration-1000 mg L −1 , time-0 to 360 min, temperature-25°C and pH-6.5 (natural pH of the dye solution). The equilibrium adsorption amount (Q e ) of RB was calculated according to Equation (1): where Q e is the equilibrium adsorption amount (mg g −1 ), C 0 and C e are the initial and equilibrium concentrations of remazol blue (mg L −1 ), respectively, M is the mass of adsorbents (g), and V is the volume of dye solution (L).

Kinetic Models
Equation (2) shows the nonlinear Equation used to fit the experimental data to the pseudo-first order model (Lagergren): where q e,cal (mg g −1 ) is the adsorbed amount per gram of adsorbent, q t (mg g −1 ) is the adsorbed amount per gram of adsorbent at time t (mim), and k 1 (min −1 ) is the pseudo-firstorder velocity constant [29]. Equation (3) presents the non-linear Equation used to perform the experimental adjustments to the pseudo-second order model ( [30]): where q t (mg g −1 ) is the adsorbed amount per gram of adsorbent at time t (mim) and k 2 (g mg −1 min −1 ) is the pseudo-second-order rate constant [30]. The Elovich model is represented by the following equation in nonlinear form (Equation (4)): where qt (mg g −1 ) is the adsorbed amount per gram of adsorbent at time t (mim), β (g mg −1 ) is the adsorption constant and α (mg g −1 min −1 ) is the initial adsorption rate constant [31].

Characterizations
X-ray diffraction (XRD) patterns were obtained with a Bruker D8 diffractometer, using CuKα radiation (λ = 1.5406 Å) operating at 30 mA and 40 kV. A scan rate of 2°min −1 was used and the 2°range was 5-70 degrees. Surface area measurements were performed using a BELSORP-mini II, BEL JAPAN equipment. Initially, samples were treated at 105°C for 14 h under N2 flow. The nitrogen adsorption/desorption (BET) study was carried out at 77 K with a maximum pressure of 1 atm and a relative pressure of 0.99. Raman spectroscopy was performed in a confocal Raman spectrometer, Bruker Senterra, with objective lenses with 50× and 785 nm laser excitation source. Low laser power density was used to prevent the sample from overheating. A spectral resolution of 3 cm −1 was used and the range used was 50-3700 cm −1 . Fourier transform infrared (FTIR) spectroscopy was recorded using a KBr configuration on a Vertex 70 spectrometer (Bruker company). A total of 64 scans and a resolution of 4 cm −1 were used to obtain spectra with good signal-tonoise ratios. The range used was 400-4000 cm −1 . The morphology of the adsorbent was investigated by scanning electron microscopy (SEM) using a FIB-SEM TESCAN MIRA3 microscope operating at 200 keV, coupled with an energy dispersive X-ray spectrometer (EDS). The samples were prepared by placing an aqueous suspension of the material powder on a grid of the copper coated with carbon and allowing water to evaporate at room temperature. X-ray photoemission (XPS) spectra were obtained with a Scienta Omicron ESCA + spectrometer system equipped with an EA 125 hemispherical analyzer and a monochrome Xm 1000 X-ray source in Al Kα (1486.7 eV). For corrections in peak shifts due to the remaining charge effect, the binding energy of all spectra was scaled using the main peak of C1s at 284.5 eV as reference [15]. Dye concentration was quantified in a UV-VIS spectrophotometer (Cary 60, Agilent Technologies, Santa Clara, CA, USA) at a wavelength (λ) of 593 nm, from a calibration curve.

Material Characterizations
The X-ray diffraction data of the materials (NaTiNT, NaTiNT−2NH, and NaTiNT−3NH) are shown in Figure 1a. The diffractogram profile of the NaTiNT sample highlights four peaks, which can be related to the crystalline planes of Na 2 Ti 3 O 7 · nH 2 O indexed as (200), (110), (211), and (020), respectively [32,33]. These peaks are in agreement with data published in the literature [34][35][36][37]. Figure 1 shows the XRD of the nanotubes (NaTiNT, NaTiNT−2NH, and NaTiNT−3NH). From the figure, it can be seen that the crystalline structure of the titanate nanotubes was preserved after the incorporation reaction of the amino groups due to the similarity of the diffractograms of the NaTiNT, NaTiNT−2NH, and NaTiNT−3NH samples [15,38]. XRD shows that there is no excess of aminosilanols in the functionalized nanotubes due to the absence of aminosilanol reflections in the diffractograms [24]. From the nitrogen adsorption-desorption studies of the materials (NaTiNT, NaTiNT−2NH, and NaTiNT−3NH), shown in Figure 1b, it can be seen that the isotherms are of type IV with hysteresis of type H3, which indicates the presence of mesopores [15]. The BET surface area and total pore volume of NaTiNT−2NH and NaTiNT−3NH were lower than that of NaTiNT (Table 1), which indicates that the silane molecules efficiently coat the surface of the adsorbents, thus promoting a reduction in available surface area and pore volume. Some similar results are also shown in the literature [6]. The micrographs shown in Figure 2 illustrate the morphological characterization of titanate nanotubes as synthesized and after chemical modification. In common, all products had predominantly agglomerated materials and a compact and very porous fibrous structure, with fibers randomly intertwined or arranged in parallel bundles. As illustrated in Figure 2a,c,e, it was not possible to make a morphological distinction between the samples. The chemical compositions present in the samples were investigated by dispersion analysis by X-ray spectroscopy (EDS). For NaTiNT, signs of Ti, O, and Na were observed [37]. The EDS spectra of the nanotubes after modification (Figure 2d,f) showed additional signals corresponding to Si and N when compared to NaTiNT (Figure 2b), indicating the presence of aminosilanols in the samples-NaTiNT−2NH and NaTiNT−3NH.  The Raman spectra, shown in Figure 3a, exhibit vibrational characteristics typical of NaTiNT. From this Figure, it can be seen that the functionalization did not change the characteristic vibrational modes of the titanate nanotubes due to the preservation of the vibrational modes of NaTiNT in the spectra of NaTiNT−2NH and NaTiNT−3NH [39]. Furthermore, the spectra of the functionalized samples show bands (1000-3500 cm −1 ) with characteristic vibrations of organic groups, indicating that there are vibrational modes of the aminosilanol groups present on the surface of the titanate nanotubes [15]. Figure 3b shows the FTIR spectra of the materials (NaTiNT, NaTiNT−2NH, and NaTiNT−3NH). All samples have a broad band in the range 2600 to 3700 cm −1 , related to stretching vibrations of the OH groups present in the nanotubes and a strong band at 1637 cm −1 , which can be related to the flexural H−O−H mode of vibration of water molecules. In addition, a band at approximately 900 cm −1 can be observed, which is attributed to Ti−O bonds (nonbridging oxygen bonds) [38,40]. After the functionalization reaction of NaTiNT with aminosilanol molecules, significant changes can be seen that show the proposed functionalization to obtain NaTiNT−2NH and NaTiNT−2NH. The functionalized nanotubes showed a band at 1478 cm −1 , referring to the symmetric bending deformation of NH + 3 [39]. Another band at 1570 cm −1 was related to NH 2 deformation vibration. This confirms the presence of the amino-terminal groups of the aminosilane molecules [6]. The N−H stretch vibration (band at 3500 cm −1 ) cannot be observed due to the superposition of the O-H stretch band. The CH 2 vibrations of the aminopropyl segment can be observed in the range of 1450-1300 cm −1 [37]. The bands at 1122 cm −1 and 1036 cm −1 are related to the vibrations of the Si-O-Si groups [38,39]. These results prove the incorporation of aminosilane groups on the surface of titanate nanotubes of TiNT−2NH and TiNT−3NH materials [15]. The results of the X-ray photoelectron spectroscopy (XPS) of the materials (NaTiNT− 2NH and NaTiNT−3NH) are shown in Figure 4. From XPS, the presence of the peaks attributed to the electrons Ti, O, and Na, referring to the starting material (NaTiNT), can be observed. In addition, the presence of the peaks attributed to the N and Si electrons can be observed. The presence of these peaks confirms the incorporation of aminosilane groups on the surface of the nanotubes after the modification reaction in the materials (NaTiNT−2NH and NaTiNT−3NH) [15]. The peak related to the Cl electrons, present in the spectra of materials, is related to a small amount of CCl 2 H 2 residue used as a solvent in the modification reaction.
Thus, the characterization results indicated that the grafting reactions of aminosilane groups on the surface of NaTiNT was performed by the condensation of silanol units and hydroxyl units present on the surface of the nanotubes, as shown in Figure 5.    Figure 6 shows the curve of the point of zero charge, pHpzc, of the NaTiNT, NaTiNT− 2NH, and NaTiNT−3NH matrices, which shows that the main difference is in the pH pzc : 7.6 (NaTiNT), 9.2 (NaTiNT−2NH), and 8.4 (NaTiNT−3NH). This result shows that the alteration in the pH pzc of the modified materials is related to the incorporation of aminosi-lanes groups on the surface of NaTiNT that promote the increase of the positive charge density due to the protonation of the nitrogen present in these groups [15,41]. In general, at pH < pH pcz , the adsorbent matrix can be used in the anionic dye adsorption process (for example, RB dye, because it is negatively charged). At pH > pH pcz , the matrix can be used in the adsorption of cationic dye (for example, Methylene Blue dye), as it is a negatively charged surface [42].

Adsorption Studies
The study of pH in the RB dye adsorption process using adsorbent matrices (NaTiNT− 2NH and NaTiNT−3NH) is shown in Figure 7. From the graphs, it can be seen that as the pH increases, there is an increase in the adsorption of the RB dye by the two materials until pH 5, and the adsorption capacity was around 365.84 mg g −1 for NaTiNT−2NH and 440.70 mg g −1 for NaTiNT−3NH. From pH 5 onwards, the amount of dye adsorbed by the two materials remains practically constant until pH 10. This result is due to the interactions that occur between the adsorbent matrices and the dye. At pH < pH pzc , the adsorbent matrices (NaTiNT−2NH and NaTiNT−3NH) retain protons from the medium, and present positive charges on their surface due to the protonation of amine groups. Thus, as the anionic dye RB has a negative charge in an aqueous medium at these pHs, the adsorption process, in the two adsorbent matrices, occurs mainly through electrostatic interactions, as shown in Figure 8. The presence of hydrogen bonds and Van der Waals force during adsorption is also possible, as shown in some works [3,15,43]. The decrease in the amount of dye adsorbed by materials at a pH below 5 is due to the presence of excess H+ ions. The excess of H+ ions prevent the interaction between the adsorbent materials and the RB molecules, because these ions interact electrostatically with the RB molecules (anionic molecules) making interactions with the adsorbent materials impossible [1,18,20]. This result is important because the pH range mentioned above covers the pH that was used to perform the adsorption kinetics tests (pH 6.5).   Figure 9 shows the study of the influence of time on the adsorption of remazol blue dye (RB) using the surface of NaTiNT−2NH and NaTiNT−3NH. It can be observed that for the functionalized nanotubes, after 210 min (NaTiNT−2NH) and 270 min (NaTiNT−3NH) of contact with the RB dye, the amount adsorbed is almost constant, that is, this indicates that this time is sufficient for the adsorption equilibrium to occur. With these results, it is observed that after the proposed modifications in the nanotubes, the equilibrium between adsorbent/adsorbate occurs longer in NaTiNT−3NH, most likely due to the increase in adsorption sites with a greater incorporation of amine groups, which favors the greater capacity of adsorption of the dye in this matrix [15].  Table 2 shows the kinetic parameters obtained for the adsorption of RB dye in NaTiNT−2NH or NaTiNT−3NH adsorbent matrices. From the results obtained, it can be observed that the Elovich model presented the best fit, as it presented the highest value of the correlation coefficient (R 2 = 0.9757 (NaTiNT−2NH) and 0.9982 (NaTiNT−3NH)) compared to the other models. Thus, this model, which reports the adsorption in heterogeneous sites and with different activation energies in the chemisorption process, is the most appropriate to describe the kinetic behavior of the adsorption systems discussed, as already observed in the adsorption of the remazol red dye in the surface of pure cellulose [15,42].
The two adsorbent materials of amino-functionalized titanate nanotubes showed promise for the removal of acid dyes, such as acid dye remazol blue R (RB), from aqueous media. The results presented in this work were better than some results in the literature, as shown in Table 3.

Conclusions
The reactions to obtain the adsorbent matrices of amino-functionalized sodium titanate nanotubes (NaTiNT−2NH and NaTiNT−3NH) were confirmed with the proposed characterizations. Through XRD and SEM, it was verified that the crystal and morphological structures of NaTiNT were preserved after the functionalizations. Raman and FTIR verified that the aminosilane interacted covalently with the surface of NaTiNT. Through XPS the incorporation of the amino silane groups on the NaTiNT surface was confirmed in the materials (NaTiNT−2NH and NaTiNT−3NH). The RB dye adsorption experiments using adsorbents (NaTiNT-2NH and NaTiNT-3NH) in aqueous solution showed that the highest adsorption capacity was around 365.84 mg g-1 (NaTiNT−2NH) and 440.70 mg g −1 (NaTiNT−3NH) in the range of pH 5.0 to 10.0 and the equilibrium time was reached in 210 min (NaTiNT−2NH) and 270 min (NaTiNT−3NH). Furthermore, the Elovich model, which reports the adsorption in heterogeneous sites and with different activation energies in the chemisorption process, was the most appropriate to describe the adsorption kinetics. The adsorbents NaTiNT−2NH and NaTiNT−3NH can be used as alternative materials for the removal of acid dyes (for example, RB) in aqueous solution, since the matrices have good adsorption capacity.
Author Contributions: D.A.S.: acquisition of data, formal analysis, data curation. P.N.S.L.: acquisition of data. L.S.S.: acquisition of data, analysis and/or interpretation of data. T.M.F.M.: formal analysis, writing-original draft, analysis and/or interpretation of data, revising the manuscript critically for important intellectual content. S.B.S.G.: formal analysis, acquisition of data, analysis and/or interpretation of data, drafting the manuscript. O.P.F.: writing-original draft, analysis and/or interpretation of data, drafting the manuscript, revising the manuscript critically for important intellectual content. A.G.: writing-original draft, analysis and/or interpretation of data, drafting the manuscript, revising the manuscript critically for important intellectual content. Y.G.: conceptualization, writing-original draft, data curation, formal analysis, conception and design of study, analysis and/or interpretation of data, revising the manuscript critically for important intellectual content. A.Í.S.M.: acquisition of data. R.D.S.B.: conceptualization, drafting the manuscript, conception and design of study, revising the manuscript critically for important intellectual content. E.C.S.-F.: writing-original draft, analysis and/or interpretation of data, drafting the manuscript, revising the manuscript critically for important intellectual content. B.C.V.: conceptualization, funding acquisition, conception and design of study, acquisition of data, analysis and/or interpretation of data, drafting the manuscript, revising the manuscript critically for important intellectual content. All authors have read and agreed to the published version of the manuscript.