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Journal of Chemistry
Volume 2015 (2015), Article ID 363405, 12 pages
http://dx.doi.org/10.1155/2015/363405
Research Article

Preparation of Na2Ti3O7/Titanium Peroxide Composites and Their Adsorption Property on Cationic Dyes

1Key Laboratory of Groundwater Resources and Environment, Ministry of Education, Jilin University, Changchun 130026, China
2Key Laboratory of Songliao Aquatic Environment, Ministry of Education, Jilin Jianzhu University, Changchun 130118, China
3School of Environment, Northeast Normal University, Changchun 130117, China

Received 4 February 2015; Accepted 16 March 2015

Academic Editor: José Morillo Aguado

Copyright © 2015 Meixia Zhao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Na2Ti3O7/titanium peroxide composites (TN-TP) were successfully prepared with the reaction of Ti foils, NaOH, and H2O2 at 60°C for 24 h in water bath. The Na2Ti3O7 appeared as nanorods in composites. Water bath temperature, water bath time, and the concentration of H2O2 and NaOH were crucial. The reaction mechanism was proposed. TN-TP was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and thermogravimetric and differential scanning calorimetry (TG-DSC). TN-TP was a mesoporous material and exhibited stronger adsorption capability for neutral red (NR), malachite green (MG), methylene blue (MB), and crystal violet (CV) than pure Na2Ti3O7 and pure titanium peroxide, and the saturated adsorption capacities were 490.21, 386.13, 322.81, and 292.74 mg/g at 25°C, respectively. It was found that the pseudo-second-order kinetic model and the Langmuir model could well describe the adsorption kinetic and isotherm of cationic dyes studied. The results of this work are of great significance for environmental applications of TN-TP as a promising adsorbent material for dyeing water purification.

1. Introduction

Cationic dyes are extensively used in industry, leading to the increasing discharge of dye to the water [1]. The dyeing wastewater reduces the solar light penetration and retards the photosynthetic activity of aquatic plant [2]. In addition, the colored effluence also triggers an increasing toxicity and carcinogenicity, which threatens the water security for human and animals [3]. This resulted in a demand to remove the dyes from effluents. Therefore, the treatment of cationic dyes raised much attention and adsorption has been found to be superior to other techniques for dyeing water purification in terms of initial cost and flexibility [4, 5]. For example, activated carbon has been regarded as an excellent adsorbent and was used widely. However, it was sometimes treated as one-off adsorbent due to the high regeneration cost [6, 7]. It is necessary to search for more efficient and cheaper alternate adsorbents. It had been found that titanate plays important roles of adsorbent in the removal of dyes [8, 9]. In addition, titanium peroxide also causes some concerns in recent researches. Titanium peroxide was first reported as a stable orange solution obtained by the coordination of Ti4+ and in 1891 [10]. Later, the reaction was applied to measure the concentration of Ti4+ and [11]. In 1970, the influence of pH on the structure of titanium peroxide was originally studied [12]. In recent years, some studies choose titanium peroxide as a precursor for preparing nanotitanium dioxide [1315]. It is reported [1618] that titanium superoxide could catalyze the selective oxidation of aromatic primary amines and phenols. Friese et al. [19] found peroxotitanium complexes can oxidize 2-propanol. Zhao [20] firstly prepared the titanium peroxide power with the reaction of titanium sulfate and H2O2, which showed good selective adsorption property on cationic dyes. There is little research about Na2Ti3O7/titanium peroxide composites at present.

In this work, we successfully prepared Na2Ti3O7/titanium peroxide composites (TN-TP) with the reaction of Ti foils and the mixed solution of NaOH and H2O2 (volume ration 1 : 1) at 60°C in water bath. The adsorption capabilities for cationic dyes of TN-TP were studied. The adsorption kinetic and isotherm were also studied. It certificated that TN-TP was a promising adsorbent material for dyeing water treatment. So, this work is of great significance.

2. Materials and Methods

2.1. Materials

Cold-rolled titanium foil, 99.5% in purity, was purchased from Baoji Fuxin Nonferrous Metal Products Co., Ltd., China. Hydrochloric acid (37%), sodium hydroxide (NaOH), and nitric acid (67%) were purchased from Beijing Chemical Works. Hydrofluoric acid (40%) was purchased from Tianjin Chemical Reagent Research Institute. Hydrogen peroxide (30%) was obtained from Xilong Chemical Co., Ltd. (China). Methylene blue (MB), neutral red (NR), and crystal violet (CV) were obtained from Tianjin Guangfu Fine Chemical Research Institute (China). Malachite green (MG) was obtained from Tianjin Bodi Chemical Co., Ltd. All reagents were of analytical grade. The water used was distilled.

2.2. Preparation

Ti foils (5 cm × 5 cm × 0.2 mm) were pickled in with a volume ratio of HF : HNO3 : H2O = 1 : 3 : 6 about 30 s at ambient temperature. After ultrasonical cleaning in distilled water, each two pieces of cleaned Ti foils were soaked in the mixed solution (sodium hydroxide solution (10 mol/L), hydrogen peroxide solution (30%), or both of them). Then, the reactants were kept at designated temperature in water bath for a certain time. After cooling to room temperature, precipitates were washed by distilled water for several times until the pH = 7 and dried by water bath at 40°C. The samples were obtained after grinding the dried precipitates. Samples were labeled in Table 1.

Table 1: Samples prepared on different conditions.
2.3. Characterization

XRD patterns were acquired on an X-ray diffraction spectrometer (BRUKER AXS D8 ADVANCE, Cu Kα,  Å). FT-IR curves were recorded on SHIMADZU 8400s Fourier transform infrared spectrometer. The SEM images were recorded with a model XL 30 ESEM FEG from Micro FEI Philips at room temperature. XPS measurements were carried out with a Thermo ESCALAB 250 spectrometer using an Al Kα (1486.6 eV) X-ray source. TG/DSC analyses were performed on a NETZSCH DSC 204 PC Instrument from 30 to 650°C at a heating rate of 10°C/min under (50 cm3/min at normal temperature and pressure). All of the measurements were carried out at room temperature (°C). The specific surface area was calculated from the adsorption isotherm using the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was determined using the Barrett-Joyner-Halenda (BJH) mathematical model. The sample was degassed at 50°C for 12 h before test.

2.4. Adsorption Test

All the adsorption experiments were conducted under stirring at room temperature (25°C) in the dark. The general experimental process was described as follows: 0.2 g of the sample was added to 200 mL of dye solution with certain initial concentration. At appropriate time intervals, the aliquots were withdrawn from the suspension and the adsorbents were separated from the suspension via centrifugation. SDPTOP UV 2600PC spectrophotometer was adopted to measure the concentration of residual dyes.

3. Results and Discussion

3.1. Preparation of TN-TP

As shown in Figure 1(a), the reaction product of Ti and H2O2 (30%) was buff gel with no precipitates after water bath in the study. The yellow gel was proved to be Ti(OH)2O2 according to the literature [21]. The reaction product of Ti and NaOH (10 mol/L) was colorless transparent liquid without precipitates as well (Figure 1(b)). This indicated that the insoluble TiO2 and titanate did not generate in alkaline condition. However, large amounts of light yellow precipitates appeared in the reaction products of Ti, H2O2, and NaOH (Figure 1(c)). As a result, the precipitates were the product of the reaction of Ti, H2O2, and NaOH. When the pH of the precipitates-containing solution is lower than 7 (hydrochloric acid added), the solution turned to be orange and precipitates in solution began to dissolve (Figure 1(e)); this phenomenon was in accordance with the characteristic of titanium peroxide in low pH solution [12]. After water washing and being dried, the precipitates turned to be yellow (Figure 1(d)), which might be ascribed to the absorbed on the surface of precipitates or the provided by the titanium peroxide which was one part of the precipitates [20]. As solid titanate was white and solid titanium peroxide was yellow, it could be hypothesized that the precipitate maybe titanate with large amounts of absorbed on its surface, a kind of titanate/titanium peroxide composites or pure titanium peroxide.

Figure 1: (a) Reaction product of Ti and H2O2 (30%). (b) Reaction product of Ti and NaOH (10 mol/L). (c) Reaction product of Ti, H2O2, and NaOH before washing. (d) Reaction product of Ti, H2O2, and NaOH after washing, being dried, and grinding. (e) Add hydrochloric acid in reaction product of Ti, H2O2, and NaOH (pH < 7).

As titanate was crystallizable, XRD was adopted to identify the reaction product of Ti, H2O2, and NaOH after washing, being dried, and grinding. As shown in Figure 2, all of the samples (solid) exhibited a strong peak around 10° and the other three weak broad peaks were around 24.5°, 28.34°, and 48.3°, respectively. Peaks of XRD could be approximately contributed to sodium titanate (Na2Ti3O7 JCPDS number 72-0148) with low crystallinity [22, 23]. In addition, peaks at 10° were concentrated with increasing water bath temperature (sample: (a)(b)) and the concentration of H2O2 (sample: (e)(a)(d)), which showed that the interlayered ions crystallinity of Na2Ti3O7 in samples was enhanced [8]. Na2Ti3O7 was generated in each preparation condition, but crystallinities of them were different, so the reaction product of Ti, H2O2, and NaOH after washing, being dried, and grinding (solid samples) was not the pure titanium peroxide.

Figure 2: XRD patterns of samples.

As shown in Figure 3, sample (b) prepared at relatively high temperature condition (70°C) and sample (d) prepared at relatively high concentration of H2O2 condition consisted of netlike structures with an average diameter of 50 nm and 40 nm, respectively. Titanium peroxide is amorphous in nature [20]; therefore, there was no titanium peroxide in sample (b) and sample (d). The netlike structure was identified to be Na2Ti3O7 [24] with low crystallinity according to XRD results. Compared to sample (b), sample (a) which was prepared at relatively low temperature (60°C) consisted of short nanorods and amorphous particles that were adhered on the surface of the short nanorods. Sample (c) was built up by layered sheets (terraces-like morphology) and little amorphous particles in the interlayers of large sheets. Sample (e) was just a large chunk and its surface was smooth. The titanate sheets could split into nanowires by prolonging the time of Ti foils treated in mixed solution of NaOH and H2O2 and then the nanowire layers formed with longer time, finally the netlike structure could be constructed [25]. The morphology change of sample (c) to that of sample (a) was in keeping with the formation of titanate nanowires from the sheets structure. Additionally, high temperature and high concentration of H2O2 were conductive to the generation of netlike Na2Ti3O7 [26]. So, both XRD and SEM observations presented the coincident results and showed that the short nanorods of sample (a) and the layered sheets of sample (c) were Na2Ti3O7.

Figure 3: SEM images of samples.

In order to identify the relationship of amorphous particles and , FT-IR was adopted. Figure 4 shows FT-IR spectra of samples. Differences of bands in the region of 400–4000 cm−1 observed were subtle except sample (e), which just had two obvious bands at 1630 cm−1 and 453 cm−1. The presence of Ti-OH and hydroxyl groups adsorbed on the surface of samples were confirmed by the appearance of broad intense bands at 3400 cm−1 and 3180 cm−1, respectively [9, 27]. There were almost no adsorbed hydroxyl groups on the surface of sample (e). The characteristic peaks around 1630 cm−1 and 1385 cm−1 could be assigned to H-O-H binding vibration mode and the Ti-O vibrations [9]. The wide band at 453 cm−1 in the sample spectra can be assigned to the crystal lattice vibration of TiO6 octahedra in Na2Ti3O7 [9, 28]. It confirmed the existence of Na2Ti3O7 in samples, which was consistent with XRD results. The peak at 895 cm−1 resulted in the peroxogroups provided by titanium peroxide [20] or excess absorbed on the surface of Na2Ti3O7. Peaks of sample (a) and sample (c) at 895 cm−1 were obviously stronger than others, which means that they possessed relatively larger amounts of . In addition, the peak height at 895 cm−1 in descending order was sample (a), sample (c), sample (d), sample (b), and sample (e). Combined with SEM results, the amount of was proportional to the amount of amorphous particles in sample (a) and sample (c). As a result, the in sample (a) and sample (c) maybe mainly provided by the amorphous particles. As sample (d) is prepared at a relatively high concentration of H2O2, the number of residual absorbed on its surface was not the largest. Obviously, the amount of absorbed on the surface of samples was limited. The titanium peroxide was the real provider. Sample (a) possessed the largest amount of . It could be hypothesized that titanium peroxide was present in sample (a).

Figure 4: FT-IR spectra of samples.

According to the SEM image of sample (e), the surface of sample (e) was smooth, and few hydroxyl groups and water were adsorbed on it, which could explain why the FR-IR curve of sample (e) had no obvious band at about 3180 cm−1 and 1630 cm−1. Combined with the XRD result, sample (e) was considered to be the Na2Ti3O7 chunk dropped from the Ti foils.

The XPS was adopted to identify the existing form of (absorbed on the surface of Na2Ti3O7 or covalently bound to Ti4+ to form the titanium peroxide) in sample (a). Before XPS test, sample (a) was dried at 100°C to remove the surface water and surface . Figure 5 shows the XPS spectra of sample (a). The peaks at 458.8 eV and 464.4 eV indicated the presence of oxidation state of Ti4+ [29]. The O1s spectra showed a main peak at 530.3 eV with two shoulders at 531.7 eV and 533.0 eV. The main peak at 530.3 eV was assigned to the Ti-O in Na2Ti3O7. The shoulder peak at 531.7 eV may be attributed to the Ti-OH in titanium peroxide [20]. The peak at 533.0 eV indicated the existence of structural in sample (a) [29]. The existence of Ti-OH and structural in sample (a) confirmed that sample (a) contains Na2Ti3O7 and titanium peroxide. The presence of Na1s spectra at 1071.9 eV indicated the existence of Na-O owing to Na2Ti3O7 [30]. The XPS results provided evidence on the existence of titanium peroxide in sample (a).

Figure 5: XPS spectra of sample (a).

From the above analysis, sample (a) was proved to be the Na2Ti3O7/titanium peroxide composites (TN-TP). The thermal analysis has been adopted to evaluate the thermal stability of TN-TP to be used as an adsorbent. Figure 6 shows the TG-DSC curves of TN-TP. It could be found that the curve of the DSC exhibited strong endothermic changes from room temperature to 200°C with about 20% weight losses, which should be attributed to residual water evaporation and dehydroxylation on the surface of TN-TP [31]. From 200°C to 400°C, there was no obvious peak in the curve of DSC with just about 4% weight losses due to the release of oxygen which was from the decomposition of peroxide root provided by titanium peroxide [20]. There was no obvious weight loss after 400°C, so water in TN-TP had almost released completely. The Na2Ti3O7 was thermally stable from 200°C to 600°C. In the following stage, there was an exothermic peak that appeared at 446.0°C. The titanium peroxide had decomposed to TiO2 and crystallized with the phase transformation at 446.0°C in this stage. It had been recognized that the temperature was about 450°C at which the transition of anatase to rutile starts [32]. TN-TP possessed good thermal stability from room temperature to 440°C.

Figure 6: TG-DSC curves of TN-TP.

The adsorption−desorption isotherm of TN-TP indicated a specific surface area of 32.26 m2/g by BET analysis. The corresponding BJH analysis (curve inserted) suggested a predominant pore diameter distribution of 17.4 nm and a total pore volume of 0.233 cm3/g. The BJH results indicated that TN-TP belonged to mesoporous material.

3.2. Reaction Mechanism

The reaction mechanism of Ti, H2O2, and NaOH was proposed to explain the generating process of Na2Ti3O7/titanium peroxide composites (TN-TP). In alkaline solution, dissociation of H2O2 formed the OOH ion in reaction (1). Then, the OOH ions reacted with Ti to form a metastable and highly soluble peroxide complex (TiO2). Reaction (4) took place immediately in the case of excess OOH ions, and reaction (5) followed to generate the Na2Ti3O7 [24]. Additionally, with the concentration of Na+ and OOH decreasing, TiO2 was going to condense to be stable , and then the further formed the titanium peroxide (reaction (6)) [12]. High temperature and high concentration of H2O2 were conducive to the generation of Na2Ti3O7 but not conducive to the generation of titanium peroxide, so sample (b) and sample (e) were pure Na2Ti3O7 without titanium peroxide. By prolonging water bath time, Na2Ti3O7 generated with the reaction of excess Ti and NaOH in solution: Ti + NaOH + H2ONa2Ti3O7 + H2 [33], which ensured the high concentration of Na2Ti3O7 to form the Na2Ti3O7 nanorods. Combined with the previous analyses, the best condition to prepare the Na2Ti3O7/titanium peroxide composites (TN-TP) was 60°C-24 h-1 : 1:

3.3. Adsorption Experiment

The adsorption activities of samples were demonstrated with MB (400 mg/L). As shown in Figure 7, all curves exhibited the same regularity. The concentration of MB decreased dramatically in the first 5 min. This was due to the strong electrostatic interaction between positively charged MB and negatively charged titanium peroxide and Na2Ti3O7 with hydroxyl groups absorbed on its surface [20, 34, 35]. Subsequently, the concentration of MB slowed down, and the adsorption rate was slower than that at the beginning stage. It could be explained that the decreasing adsorption points and vacant surface became more difficult to be occupied with reaction advanced, due to the repulsion between adsorbed MB molecules [8].

Figure 7: Removal efficiency of samples for MB (initial concentration 400 mg/L, pH = 7, and temperature 25°C).

It was obvious that the curve of sample (a) (TN-TP) decreased fastest in all curves. From SEM analysis, as the titanium peroxide adhered on the surface of Na2Ti3O7 nanorods, its molecular structure was not easily damaged and hydroxyl groups firmly bound to the to keep its negativity. In addition, as the titanium peroxide was condensed by the TiO2, which can help maintain the hydroxyl groups absorbed on the surface of Na2Ti3O7 nanorods, the negative charges of TN-TP can be stable which was constructive to the electrostatic adsorption. As a result, it possessed stronger adsorption ability than pure Na2Ti3O7 network structure (sample (c) and sample (e)). As sample (c) was terraces-like morphology, its specific surface area was smaller than that of TN-TP, and so was the adsorption ability.

Four different cationic dyes including MB, MG, CV, and NR were used to study the adsorption property of TN-TP. As can be seen from Figure 8, TN-TP showed great adsorption effect on them. In addition, the adsorption rates on NR, MG, MB, and CV were different (NR > MB > MG > CV). As the molecular structures were same to each other [20], the smaller the size of the molecular is, the easier the adsorption is. The result also showed that the experimental saturated adsorption capacities for NR, MG, MB, and CV were 490.21, 386.13, 322.81, and 292.74 mg/g at 25°C, respectively. Compared with the pure Na2Ti3O7 or pure titanium peroxide, the adsorption capacity of TN-TP increased [9, 20].

Figure 8: The adsorption curves of NR, MB, MG, and CV at different initial concentration (TN-TP dosage 1.0 g/L).

In order to investigate the mechanism and characteristics of TN-TP adsorption in dyes removal, the linear plots of pseudo-first-order and pseudo-second-order kinetic models were shown in Figures 9 and 10, and the adsorption kinetic parameters related to models were figured out in Table 2. It can be seen that the trend line of the pseudo-first-order model deviated obviously from the experimental data, but the trend line of the pseudo-second-order model passed through the whole experimental data. Correspondingly, the correlation coefficient values of pseudo-first-order model were lower than those of pseudo-second-order which were higher than 0.9994. The values of estimated from pseudo-second-order model were comparable with the experimentally determined values of , which indicated a better applicability of pseudo-second-order model to the adsorption of cationic dyes in this study. It also suggested that the rate of the adsorption process was controlled by the chemical adsorption, which involved valence forces through sharing or exchange of electrons between adsorbent and adsorbate [36].

Table 2: Equations and parameters of kinetic models and kinetic parameters of dyes onto TN-TP.
Figure 9: Pseudo-first-order kinetic plots for NR, MB, MG, and CV.
Figure 10: Pseudo-second-order kinetic plots for NR, MB, MG, and CV.

The adsorption process was further studied by two classical isotherm models, Langmuir and Freundlich, as shown in Figure 11. Their corresponding equations and parameters for adsorption of dyes onto the sample are listed in Table 3. It can be seen that the Langmuir model was quite suitable to the adsorption, and the correlation coefficients were higher than 0.9996. In addition, the of NR, MB, MG, and CV calculated through the Langmuir model were 497.51, 331.13, 395.26, and 304.88 mg/g, which was in accordance with the acquired from the experiment.

Table 3: Isotherm coefficients according to Freundlich and Langmuir.
Figure 11: Langmuir and Freundlich sorption isotherms of NR, MB, MG, and CV on TN-TP.

The FT-IR spectra of the TN-TP and dyes adsorbed on TN-TP were shown in Figure 12. Compared to TN-TP, the additional peaks at 1327, 1193 cm−1 (TN-TP-NR), 1392, 1334 cm−1 (TN-TP-MB), and 1170, 1367 cm−1 (TN-TP-MG, TN-TP-CV) were attributed to the characteristic peaks of NR, MB, MG, and CV, respectively [3740]. This confirmed the strong electrostatic interaction between the negatively charged TN-TP and positively charged NR, MB, MG, and CV.

Figure 12: FT-IR spectra of the TN-TP and dyes adsorbed on TN-TP.

4. Conclusion

In summary, the Na2Ti3O7/titanium peroxide composites (TN-TP) were successfully prepared through the reaction between Ti foils and the mixed solution of NaOH and H2O2 (volume ration 1 : 1) at 60°C for 24 h in water bath. High water bath temperature (70°C) and high concentration of H2O2 (volume ration 1 : 2) were conducive to the generation of Na2Ti3O7 without titanium peroxide. In the reactions, the was crucial. TN-TP exhibited stronger adsorption capability for NR, MB, MG, and CV than pure Na2Ti3O7 and pure titanium peroxide, and the adsorption capacities were 490.21, 322.81, 386.13, and 292.74 mg/g at 25°C, respectively. It was found that the pseudo-second-order kinetic model and the Langmuir model could well describe the adsorption kinetic and isotherm of the cationic dyes studied. Results of this work are of great significance for environmental applications of TN-TP as a promising adsorbent material used for dyeing water purification.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by the analysis and testing foundation of Jilin University and the National Natural Science Foundation of China (no. 51308252).

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