Kinetics, isotherm, and thermodynamic studies of methylene blue selective adsorption and photocatalysis of malachite green from aqueous solution using layered Na-intercalated Cu-doped Titania

Applied Clay Science 183 (2019) 105323

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Research paper

Kinetics, isotherm, and thermodynamic studies of methylene blue selective adsorption and photocatalysis of malachite green from aqueous solution using layered Na-intercalated Cu-doped Titania

T

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Amar Kundu , Aparna Mondal Department of Chemistry, National Institute of Technology Rourkela, Odisha 769008, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Cu-doping Selective adsorption Semiconductor Na-intercalation

In this study, layered Na-intercalated copper-doped titania (Cu-NaTi) have been synthesized using controlled precipitation method where the sodium ions are self-incorporated. Physio-chemical, microstructural, and optical properties of the prepared materials have been studied using different analytical techniques. Structural data shows that pure and 0.5 wt% Cu-doped samples exhibit pure anatase phase of titania while higher Cu-doped (1 and 2 wt%) titania nanostructures show mainly anatase phase of titania along with a spectrum of monoclinic sodium tritatanate. HRTEM micrographs demonstrate the presence of interlayered architecture. The inner-space of the titania layers may create capillaries with specific size since the adsorption results showed adsorption selectivity towards Methylene blue (MB) from a mixed aqueous solution containing methylene blue and rhodamine B (RhB). The MB adsorption selectivity has been also confirmed from other adsorption experiments mixing with Rose bengal (RB) and bismark brown Y (BB-Y). It is believed that the planner molecular structure and small size of MB makes it unique to enter into the specific capillary to participate in the adsorption process through cation exchange. The 1 wt% Cu-doped sample shows maximum MB removal in just 10 min. of adsorption experiment. Rate kinetics and isotherms of the selective MB adsorption are studied in detail. It has been found that pseudo-second-order kinetic model well describes the adsorption kinetics. Moreover, photodegradation experiment has also been carried out using aqueous solution of malachite green (MG) under direct sunlight. The results indicate approx. 95% photodegradation of MG by 0Cu-NaTi sample with kinetic rate of 0.040 min−1. Results of this study are of great implication for environmental applications, where the layered Naintercalated copper-doped titania can act as a capable adsorbent and photocatalyst for the purification of water.

1. Introduction The worldwide environmental problem has been challenging the human's survival and development since over a decade (Carneiro et al. 2010). As a solution, numerous physiochemical approaches have been used to eliminate pollutants from waste water (Varghese et al. 2003; He et al. 2008; Osugi et al. 2009; Khayet et al. 2011; Labanda et al. 2011; Giri et al. 2013). Now a days, nanostructured materials have been discovered to have the potential to serve as promising adsorbents as well as photocatalysts for the environmental treatment with great applications (Babel and Kurniawan 2003; Ahmaruzzaman 2011; Ali 2012; Unuabonah and Taubert 2014; Hussain 2015). Among all kinds of water pollutants, dyes have become one of the main components due to their extensive applications in numerous fields, e.g. textile, paper, rubber, plastic, leather, cosmetic, pharmaceutical and food industries (Gong

⁎

et al. 2009). After every industrial usage, about 5–15% dyes are lost and released into the open water with a simple pretreatment (Al-Ghouti et al. 2003). This results in a considerable waste of resources and catastrophe to the environment (Rai et al. 2005). Most dye pollutants are toxic and even carcinogenic and teratogenic due to their high solubility in water and poor biodegradability (Carvalho et al. 2008; Rafatullah et al. 2010). Dye industries like textile and printing extensively use Methylene blue (MB) (Wang et al. 2008; Sajab et al. 2011; Kundu and Mondal, 2019). MB is a toxic organic molecule and it can lead to numerous health problems like cancer, vomiting, ad nauseam, mental disorders, permanent injury on the eyes if it is contacted for long time (Ghosh and Bhattacharyya 2002; Salleh et al. 2011; Alver and Metin 2012; Cui et al. 2015). Hence, removal of MB from wastewater has gained a specific consideration in recent years. Different technologies like adsorption, biological methods, chemical oxidation, and

Corresponding author. E-mail addresses: [email protected] (A. Kundu), [email protected] (A. Mondal).

https://doi.org/10.1016/j.clay.2019.105323 Received 25 January 2019; Received in revised form 24 September 2019; Accepted 3 October 2019 0169-1317/ © 2019 Published by Elsevier B.V.

Applied Clay Science 183 (2019) 105323

A. Kundu and A. Mondal

followed by maintaining the corresponding temperatures for another 2 h. Na-intercalated titania is prepared by following same synthetic procedure without using the dopant. The sample is identified as 0CuNaTi in the text.

photocatalytic degradation have been developed to remove dye pollutants from wastewater. Among these, the most effective and promising methods are adsorption and photocatalyst because these processes have extraordinary removal efficiency, low processing cost, and easy operational process. Different nanostructures have been developed over time which act as high-efficient adsorbents and photocatalysts (Ahmaruzzaman 2008, 2011; Wu and Zhao 2011; Valle-Vigón et al. 2013; Singh et al. 2014; Kundu and Mondal 2019a). However, the challenge remains while developing nanostructure materials with enhanced specific adsorption and photocatalytic performance. Potential applications of fabricated titania in the treatment of environmental pollutants can be found in many literature reports (Yang et al. 2008; Rafatullah et al. 2010). In addition, several types of low-dimensional titania-related materials with high morphological specificity like; nanowires, nanotubes, nanoribbons, and nanofibers have been fabricated using various physicochemical methods (Kasuga et al. 1999; Humar et al. 2006; He et al. 2010; Zhang et al. 2010; Cavaliere et al. 2011; Swami et al. 2011; Luo et al. 2014; Zampetti et al. 2015, 2015; Kundu and Mondal, 2019a, b). Typically those nanostructures are found attractive as a catalyst in advanced applications (Adachi et al. 2000; Amano et al. 2009; Vijayan et al. 2012). But, synthetic method for fabricating the above nanostructures is expensive. Therefore, fabrication of cost-effective titania nanostructures is still a challenging task. So, as a selective adsorbent and photocatalyst, titania nanostructures have rarely been studied. In this study, we report a facile environmentally friendly co-precipitation procedure for the preparation of Na-intercalated titania and Na-intercalated copper-doped titania nanostructures. The copper-doped nanostructures have layered architectures. It is evident from the experimental results that the prepared nanostructures act as potential selective adsorbents and photocatalysts.

2.3. Characterization Powder X-ray diffraction patterns (XRD) of the samples are recorded on Rigaku Ultima IV X-ray diffractometer using 0.154056 nm Cu-Kα radiation. Crystallite size of the samples are estimated using Scherrer's equation. Raman spectra are recorded in Witec alpha-300 confocal Raman microscope with solid-state laser source using excitation line of 532 nm. Nitrogen adsorption-desorption isotherms are acquired from Quantachrome Autosorb-1 apparatus afterwards degassing the samples at 200 °C for 2 h at relative pressure (p/p0) range 0.03–1.00. Microstructures and elemental mapping of the samples are recorded on Nova NanoSEM 450/ FEI equipped with QANTEX EDS detector. HighResolution micrographs are acquired from FEI Tecnai F30 S-TWIN. UV2450 Shimadzu spectrometer with liquid and solid assembly setup used to acquire the absorbance of the dye solutions and UV−vis DRS (BaSO4 as reflectance standard), respectively. The band gap energies of the synthesized samples are calculated from the Tauc's plot using KubelkaMunk equation. FluoroMax-4 spectrophotometer is used to record the photoluminescence (PL) spectra. The 230 nm wavelength line is used for the excitation. The absorbance of the dye solutions is recorded using the same UV–Vis spectrophotometer with liquid assembly setup. 2.4. Selective adsorption experiments Analytical reagent grade Methylene Blue (C16H18ClN3S) (MB) and Rhodamine B (C28H31ClN2O3) (RhB) are used as the model pollutants (or adsorbate) and are purchased from S.D. Fine chemical and Merck, respectively. The adsorption efficiency of the samples is evaluated by the extent of adsorption of MB in aqueous solution under dark environment. The selective adsorption experiment has been carried out using a mixed solution of MB (100 mL) and RhB (100 mL) dye solution with initial concentration of 5×10−5 (M) and 2×10−5 (M), respectively at room temperature. The mixed solution of MB and RhB has been identified as “M-R” solution. For each reaction set, 50 mg of adsorbent is added into 200 mL of M-R solution and kept under dark at room temperature with constant stirring (500 r.p.m). No steps are taken to maintain the pH during the process. The experimental process has been carried out for a time range 0–40 min. Thereafter, from the reaction mixture at a different time interval, about 7 mL of suspension has been withdrawn and centrifuged. The changes in concentration of MB and RhB with respect to time is then analyzed by UV–Vis spectrophotometer. Absorbance of MB and RhB in the solutions are observed from their characteristic absorption band maxima 663 nm and 552 nm, respectively. The percentage of dye removal and the adsorption capacity has been estimated by the following equations,

2. Experimental section 2.1. Materials used Titanium (IV) oxysulphate (TiOSO4) (Aldrich), copper (II) sulphate pentahydrate (CuSO4, 5H2O) (Merck) are used as TiO2 and Cu sources respectively. Brij C10 (Aldrich) (average Mn ~ 683), and sodium hydroxide (NaOH) (Merck) are used as template and precipitating agents. All the procured chemicals are of A.R. grade and are used without any further purification. Deionized water has been used during the experimental work. 2.2. Materials synthetic procedure Na-intercalated Cu doped titania has been synthesized by co-precipitation method using a non-ionic surfactant template with 0.5, 1, and 2 wt% Cu-content. The samples are respectively identified as 0.5CuNaTi, 1Cu-NaTi, and 2Cu-NaTi. For each synthesis, a clear 1/2 (M) aqueous solution of Ti4+ is prepared by dissolving TiOSO4 in distilled water with constant stirring. Then 15 wt% isopropanol solution of Brij C10 is prepared separately in a beaker and mixed with the precursor solution with constant stirring (600 r.p.m) for 1 h at room temperature. Afterwards, an aqueous solution of Cu-salt is added to the subsequent solution and left it for another 1 h. 5(M) NaOH solution is then introduced into the resulting solution at a rate of 5 drops/min to obtain the precipitate. After complete precipitation, pH of the precipitated solution has been maintained at 9 ± 0.2. The precipitated solution is then aged for two days and then, is filtered followed by washing with distilled water (50 mL) and ethanol (50 mL) for a total of four times (each 2 times) to remove the surfactant and other salts. The subsequent precipitates are dried in an oven at 60 °C for two days and after being cooled, the resulting solid has been crushed using mortar pastel to make fine powder. The powders are then annealed using RECIO Muffle Furnace at 500 °C in air atmosphere with a heating rate of 5 °C/min

%Removal = (1 − ct / c0) × 100

(1)

qe = (c0 − ct ) V / m

(2)

where c0 (mg/L), ct (mg/L), V(L),and m(g) represent the initial dye concentrations, equilibrium dye concentrations, volume of the solution and mass of the adsorbent, respectively. The adsorption experiment is further carried out with two other different dyes along with MB to confirm the selectivity. Aqueous solution of Rose bengal (RB) and bismark brown Y (BB-Y) with initial concentration of 5×10−5 (M) and 4×10−5 (M) are combined with MB (conc. = 5×10−5 (M)) to peform the experiments. In addition, all other parameters are reserved same as M-R adsorption experiment. The mixed dye solution containing RB and MB marked as “R-M” and “M-BB” for MB and BB-Y. 2

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Fig. 1. (a) XRD patterns of the respective samples and (b) lower shift pattern in Bragg's angle in (101) peak of anatase phase.

easily doped into the anatase lattice (López et al. 2010). The resemblance in the Cu (0.72 Å) and Ti (0.68 Å) ionic radii allows the interstitial incorporation of the dopant into the anatase lattice network. Cu could be placed in the interstitial sites owing to rTi < rCu producing the strain of the titania lattice, and hence a displacement of the (101) signal in the XRD pattern (López et al. 2010). Umek et al. synthesized 3 wt% Cu-doped titania which have the XRD patterns of anatase TiO2 (Umek et al. 2008). Additionally, the literature contains much debate about the exact structure of sodium-titanate. The closest match to the XRD spectra of the 2Cu-NaTi nanostucture appears to be a spectrum of layerd Na2Ti3O7 (monoclinic sodium tritatanate) (JCPDS 72-0148). However, there is a mismatch between some of the peak positions, and some additional peaks are present in the collected spectra. These observations suggest that the crystal structure of 2Cu-NaTi deviates from the structure of Na2Ti3O7, which can be considered by ridged layers of three edge-sharing TiO6 octahedra linked via corners and interleaved with Na+ ions (Feist and Davies 1992). The cystallite size of the samples has been calculated from the Scherrer's equation using the XRD peak of anatase (101) plane. Lower intensty XRD peaks indicate small amorphous nature in the samples.The crystallite size for the samples are calculated and found to be 9.52, 9.85, 9.81, and 10.21 nm for 0Cu-NaTi, 0.5Cu-NaTi, 1Cu-NaTi, and 2Cu-NaTi, respectively.

2.5. Photocatalytic activity test The photocatalytic performance of the samples is evaluated by the extent of degradation of malachite green (MG) dye in an aqueous solution under solar light irradiation at ambient temperature. In a typical experiment, 100 mL of 2×10−5 (M) MG solution and 25 mg photocatalysts is taken in a beaker. Then the MG/catalyst suspension has been exposed to direct sunlight with vigorous stirring. At a distinct time interval, the photoreacted solution is extracted from the reaction mixture and then centrifuged. The MG concentration in the residual solutions is evaluated using UV–Vis spectrophotometer by recording its characteristic absorption band maximum (λmax = 616 nm). The degradation efficiency is then calculated using Eq. (1). 3. Result and discussion 3.1. XRD analysis XRD pattern of the samples are shown in Fig. 1a. The XRD peaks at 25.3°, 37.9°, 48°, 54.1°, 55°, 62.8°, 68.8°, 70.2° and 76.1° are corresponding to the crystal planes (101), (004), (200), (105), (211), (204), (116), (220) and (215) respectively, suggesting anatase phase (JCPDS 21-1272) of TiO2. There are no other characteristic peaks present in 0Cu-NaTi and 0.5Cu-NaTi. The XRD patterns of the mentioned nanostructure contain sharp peaks indicating well-formed titania crystals. The XRD patterns of 1Cu-NaTi sample shows anatase phase and three broad peaks of low intensity positioned at 29.6°, 32.4° and 44.3° which are difficult to index and are originated due to the formation of a secondary phase. However, XRD peaks observed for 2Cu-NaTi sample indicate anatase phase and some major peaks approxmately at 24.5°, 28.6° and 48.0° assinged to sodium titanates in accordance with the previous reports (Chen et al. 2002). The nanostructures haven't any major peaks which is related to copper oxide lattice. The major peaks arised for copper oxide are aprroximately at 32.8°, 35.9°, 39.1°, and 49.1° (Abdel-Monem et al. 2017). The copper has been doped into the anatase lattice as we can see from Fig. 1b, since there is a lower shift in bragg's angles. We have seen from the previous repot that Cu can be

3.2. Raman spectra Raman spectroscopy is performed to gain the insight into the structural rearrangement. The Raman shift in the nanostructures has been shown in Fig. 2a. Raman peak at about 147 cm−1 is observed for all samples, which is attributed to the main Eg anatase vibration mode. Likewise, vibration peaks at 200 cm−1 (Eg, weak), 399 cm−1 (B1g), 511 cm−1 (A1g), and at 642 cm−1 (Eg) are present in the spectra for all samples, which indicates the predominant species anatase TiO2 (tetragonal structure). The observed Raman shift position of our synthesized samples has nearly the same value as the reported values with small variation (Cong et al. 2007). A lower intense raman peak is observed nearly at 287 cm−1 for 2Cu-NaTi, which might be arising due to the NaeOeTi stretching vibration (Meng et al. 2004). Vibration mode at 3

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Fig. 2. (a) Raman spectra of 0Cu-NaTi, 0.5Cu-NaTi, 1Cu-NaTi, and 2Cu-NaTi samples and (b) shifting pattern of the Eg bands.

147 cm−1 (Eg) illustratrates a minor blue shift (Fig. 2b) as the dopant concentration increased, excluding 2Cu-NaTi. The difference might have occurred due to the disordered lattice structure of anatase, where the Cu-atoms are substitutionally incorporated into the anatase frameworks. The Raman outcomes are also supporting the XRD results.

Table 1 Surface area, pore volume, and average pore size of the respective nanostructures.

3.3. Specific surface area and pore structure The BET isotherm and pore size distribution of the samples are shown in Fig. 3a and b. Type IV isotherm has been observed, suggesting the presence of mesopores. These isotherms display hysteresis loops of type H3, and can be attributed to the adsorbate condensation in capillary spaces between parallel plates or open slit-shaped capillaries, indicating the formation of slit-like pores arising from the aggregation of sheet-like particles (Zhang et al. 2017). The micrograph of 1Cu-NaTi (Fig. 3a. inset) in the nanometer scale confirmed the above assumption by showing oriented nano-layers which are overlapped. The observed

Sample name

Surface area (m2/ g)

Total pore volume (cc/g)

Average pore size (nm)

0Cu-NaTi 0.5Cu-NaTi 1Cu-NaTi 2Cu-NaTi

84.6 102.5 77.2 32.7

0.97 1.01 0.54 0.28

22.8 39.4 49.6 70.1

BJH pore-size distribution curves indicate formation of mesopores. The surface area, pore volume, and average pore size of the samples are listed in Table 1. The data indicates increase in pore size while surface area is decreasing except for 0.5Cu-NaTi and it's because the pores are not truly pores, but are the interspace between the grains. Moreover, these nanolayers may be the adsorptive sites, which promote the

Fig. 3. (a) BET isotherm and (b) pore size distribution profile (inset) of the respective nanostructures. 4

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A. Kundu and A. Mondal

Fig. 4. FESEM micrographs and respective particle size distribution profile (inset) of (a) 0Cu-NaTi, (b) 0.5Cu-NaTi, (c) 1Cu-NaTi, and (d) 2Cu-NaTi samples and TEM, HRTEM micrographs (inset), SEAD patterns (inset) of the (e) 0Cu-NaTi, and (f) 1Cu-NaTi nanostructures.

particle size in the range of 10–60 nm. But, only the 0.5Cu-NaTi nanostructure holds the maximum number of particles in the lower diameter range and therefore displays high surface area as evident from the BET isotherm. The TEM-HRTEM micrographs have been obtained for selected samples. Fig. 4e and f are the TEM micrographs of 0Cu-NaTi and 1CuNaTi nanoparticles, respectively. The 0Cu-NaTi nanostructures have a particle size in the range of 10–18 nm, and are irregular in shape. The HRTEM micrographs (Fig. 4e.1) of it show formation of clear lattice

adsorption of organic pollutant from wastewater. A high surface area, about 102.5 m2/g is observed for the 0.5Cu-NaTi sample. 3.4. Microstructural property The microstructures of the distinct nanostructures are shown in the Fig. 4(a–d). The samples are agglomerated and have geometrical shape. The particle size distribution profile of the samples is illustrated in the inset of their individual micrographs. All the nanoparticles have the 5

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A. Kundu and A. Mondal

Fig. 5. Elemental mapping, EDS plot (inset) of the (a) 0Cu-NaTi, (b) 0.5Cu-NaTi, (c) 1Cu-NaTi, and (d) 2Cu-NaTi samples and a table represent the at.% of the elements in the respective samples.

3.5. Elemental mapping and EDS analysis

fringes with an interplanar distance 0.342 nm, indicating the (101) plane of the anatase phase. The SAED pattern in Fig. 4e.2 shows spotty ring patterns without any additional diffraction spots and rings of second phases, revealing a well crystalline nature of the anatase phase. The TEM micrographs of the 1Cu-NaTi nanoparticles (Fig. 4f) shows an interlayer nanostructure, as the layers have different contrast. The HRTEM micrographs (Fig. 4f.1) display the lattice fringes of the anatase phase with interplanar distance (d (101)) of 0.347 nm. Moreover, the SAED patterns of the nanostructure (Fig. 4f.2) shows that the brightness and intensity of polymorphic ring is not so strong, and therefore it is partly crystallized and partly amorphous.

Elemental mapping and EDAX of the nanoparticles indicate the presence and the atomic content of elements Ti, O, Na, and Cu. Elemental mapping has been carried out to understand the distribution of Na and Cu throughout the lattice framework. Fig. 5 represents the elemental mapping of the respective samples and the atomic percentage of the elements present tabulated in it. The different colour dots symbolize the Ti, O, Na, and Cu atoms in the samples. Elemental mapping gives a clear view that the Na and Cu atoms are present and distributed homogeneously. We have also performed the XPS analysis (Fig. S2: full

6

Applied Clay Science 183 (2019) 105323

A. Kundu and A. Mondal

Fig. 6. (a) UV−vis DRS spectra (b) indirect band gap energies and (c) photoluminescence profile of the nanostructures.

quenching the photoluminescence (Kundu and Mondal 2019b). So, from the study we can say that the dopant modified nanostuctures may act as a good photocatalysts.

survey spectrum) only for 0Cu-NaTi samples which also confirmed the presence of Na+ ion in the nanostructures.

3.6. Band gap energies and optical properties 4. Selective adsorption study Fig. 6a shows the absorption spectra of the nanostructures. The absorption edge displays a steep rising at wavelength about 392 nm (3.16 eV), which may signify the intrinsic band gap absorption of undoped sample. On the other hand, the addition of dopant to the lattice shows a red shift of absorption edge and a significant improvement of light absorption at 400–650 nm. The above changes become more significant with increasing Cu-content. Earlier, some literature reported that metal ions incorporated into the TiO2 lattice does not change its valence band edge's position but it actually creates new energy levels to the band gap of TiO2 (Aguilar et al. 2013). Therefore, shifting of the absorption edges towards the longer wavelengths may originate from the electronic transition from the Cu2+ (valance band) energy level to the conduction band of TiO2. Tauc's plots have been drawn to estimate the band gap energies (Fig. 6b). A plot of (αhν) 1/2 versus photon energy (hν) is drawn to estimate the indirect band gap energies, where “α” is the Kubelka–Munk function. The indirect band gap energies are identified in the Fig. 6b. The photoluminescence (PL) study is carried out to acquire better insights on the recombination of the free charge carriers i.e. electrons (e−) and holes (h+). The PL spectra of the samples are shown in Fig. 6c. As shown from the figure, the PL intensity decreases with dopant concentration. PL emission spectra is generated from the recombination of charge carriers, but the lower intense PL spectra indicates a lower recombination rate or inhibition of photogenerated (e−) and (h+) carriers. Therefore in the nanostructures, Cu-ion acts as trapping site and subsequently enhances the separation of the charge carriers by

The synthesized materials only adsorbed MB from M-R solution. The UV–Vis absorption spectra has been taken at different time intervals from the M-R solution. Fig. 7a represents the absorption spectra of the M-R solution at different time intervals by the respective nanostuctures. The corresponding λmax of MB and RhB are 663 and 552 nm. As we can see, selectively MB has been adsorbed in maximum percentage in the first 2 min. of the adsorption process. Just after 2 min. the adsorption are 52%, 66%, 68%, and 64% for 0Cu-NaTi, 0.5Cu-NaTi, 1Cu-NaTi, and 2Cu-NaTi, respectively. Afterwards, the adsorption rate becomes very slower. The 0Cu-NaTi adsorbed 62% of MB and a maximum of 93% by 1Cu-NaTi during the adsorption experiment. Visual images of selective adsorption from M-R solution before and after are shown in Fig. 7b (inset. set 1). The images give a clear view about the selective adsorption of MB dye, where the intensity of the blue colour of MB is reduced and the violet colour of RhB becomes prominent. On the other hand, the individual adsorption experiment presented in Fig. 7b (inset. set 2), exhibits a colour change on the adsorbent surface. After MB adsorption, the surface becomes blue and no colour change is observed for RhB. The selectivity towards MB dye is similarly tested using additional dyes. We have carried out two other adsorption experiments with R-M and M-BB mixed solutions. Individual experiments with RB and BB-Y dye solutions are also carried out to get a clear understanding. The experiment has been carried out with 1Cu-NaTi sample because of its highest adsorption capacity. The time dependent UV–Vis spectra of MB, 7

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A. Kundu and A. Mondal

Fig. 7. (a) Time dependent UV–Vis absorption spectra of M-R solutions, and (b) removal efficiency of MB from the mixed M-R solution by the respective samples.

kinetic models are expressed as follows

RB and BB-Y adsorption from R-M and M-BB solution and the individual adsorption of RB and BB-Y are illustrated in Fig. 8. In Fig. 8a, the absorption peak at 559 nm (λmax of RB) shows no prominent changes in concentration of RB, but the absorption peak at 664 nm (λmax of MB) shows lower absorbance with time. Therefore, from the R-M mixed solution only the MB dye has been adsorbed on the nanostucture. Individual adsorption experiment of RB dye is also indicating no prominent changes in RB concentration as shown in Fig. 8b. Similarly, the MBB adsorption setup displays only adsorption of MB as shown in Fig. 8c. Therefore, from these results we can conclude that synthesized nanostuctures are adequately efficient for selective adsorption of MB from aqueous solution. Kinetics, isotherms and thermodynamic studies of selective MB adsorption from the M-R mixed solution have been carried out in detail.

ln(qe − qt ) = ln(qe ) − kI t

(3)

t 1 t = + qt qe kII qe2

(4) −1

−1

−1

where, qe/t (mg/g), kI (min ), and kII (g mg min ) denote the quantity of MB (adsorbed at equilibrium), the pseudo first order rate constant, and pseudo second-order rate constant, respectively. By using the linear equations, we have evaluated the values of the kinetic parameters (kI, kII, correlation coefficients (R2), and qe, cal) (Maruthapandi et al. 2018). The qe values calculated from the pseudo second order kinetic plots are in better agreement with the experimental values of qe with slight deviations. In addition, the higher R2 (≥ 0.99) values indicate that the MB sorption system follows secondorder kinetics. The kinetic parameters of adsorption for pseudo first and second order model are presented in Table 2. The rate of adsorption is higher in 0Cu-NaTi but 1Cu-NaTi shows the maximum adsorption capacity, which is about 30.3 mg. g−1. The intraparticle diffusion model also investigates the adsorption process. The model implies that the uptake of the adsorbate changes with the square root of adsorption time if the rate-controlling factor is intraparticle diffusion (Maruthapandi et al. 2018). The linear

4.1. Kinetics study of selective adsorption Kinetics studies on the selective MB adsorption have been carried out at room temperature (27 °C). Kinetics data can deliver information about the mechanism and rate of adsorption. Pseudo first and pseudo second-order kinetics models of the selective MB adsorption process on the nanostructures are showed at Fig. 9a and b. The linear forms of the 8

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A. Kundu and A. Mondal

Fig. 8. Time dependent UV–Vis absorption spectra of (a) R-M, (b) RB, (c) M-BB, and (d) BB-Y dye solutions.

where, Ce (mg. L−1), qe(mg. g−1), KL(L. mg−1), and Qm(mg. g−1) are equilibrium concentration of adsorbate, adsorption capacity, Langmuir constant (maximum adsorption capacity), and Langmuir constant, respectively. The Freundlich isotherm model commonly refers to heterogeneous systems. It can be applied to multilayer adsorption, with non-uniform distributions (Sarkar et al. 2014). The linear form of the Freundlich isotherm is expressed by the following equation,

intraparticle diffusion model is expressed in Eq. (5).

qt = ki t 1/2 + C

(5)

where, C (mg/g) represents the intercept which is significantly related to the adsorption steps and ki (mg/ g min−1/2) signifies the diffusion rate constant. Fig. 9c demonstrates the multilinear plots of the intraparticle diffusion process of the selective MB adsorption on the respective nanostuctures. The adsorption goes on with two steps process as presented in Fig. 8c. The first phase points towards the film diffusion model, which is the diffusion of MB molecules from the solution to the interlayered surface of the respective nanostuctures. The second stage can be ascribed to the intraparticle diffusion phase, due to the slow adsorption on surface of the samples. Fig. 8c displays the slope (ki) of the intraparticle diffusion phase in the respective nanostuctures, which is smaller than the film diffusion stage. Therefore, the results implied that the intraparticle diffusion process is gradual one. The higher values of C indicates the additional mass transfer of MB molecules onto the nanostuctures and it occurs at initial stage of adsorption.

lnqe = (1/n) lnCe + +lnKF

where the Freundlich isotherm parameters KF(L. mg−1) and n are adsorption capacity and constant related to adsorption intensity respectively. Value of n > 1 signifies a favourable adsorption condition (Haque et al. 2010; Santos et al. 2017). The Langmuir and Freundlich model parameters presented in Table no 5 are obtained from the plots of (1/qevs. 1/Ce) and (lnqe vs. lnCe) respectively. The adsorption feasibility is evaluated by the Langmuir isotherm separation factor (RL). It is calculated using following equation,

RL =

4.2. Isotherm models of selective adsorption

1 1 + KL C0

(8) −1

−1

where, KL(L. mg ), C0 (mg. L ) are the Langmuir constant and the initial MB concentration, respectively. The RL values signify isotherm types; favourable (0 < RL < 1), irreversible (RL = 0), unfavourable (RL > 1), and linear (RL = 1). The calculated RL values of MB adsorption on synthesized samples are between 0 and 1, indicating a favourable adsorption isotherm. The Langmuir and Freundlich isotherms profile of the respective samples with linear fitting are presented in Fig. S2.a and b, respectively. The isotherm models revealed that both supported the selective MB adsorption data because of high R2 values. This can be considered by

Langmuir and Freundlich models in the linearized form have been evaluated to optimize the maximum adsorbent usage and to understand the distribution of the adsorbate molecules between solid and liquid phases for MB adsorption (Huo and Yan 2012). The Langmuir adsorption isotherm has been immensely used to appreciate the performance of different adsorbents. The model supports monolayer adsorption, and indicates homogeneous adsorption (Haque et al. 2011). The linear form of the Langmuir isotherm can be expressed as.

1/ qe = (1/ Qm KL )(1/ Ce ) + 1/ Qm

(7)

(6) 9

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Fig. 9. (a) Pseudo-first-order kinetic model, (b) pseudo-second-order kinetic model and (c) intraparticle diffusion plot of selective MB adsorption from M-R solution on the respective nanostructures.

the monolayer coverage (19.08–45.66 mg. g−1) and sorption intensity on heterogeneous energies (26.3–63.1 mg. g−1), which had a corresponding R2 value between 0.80 and 0.99 and 0.94–0.99 for Langmuir and Freundlich isotherm models, respectively (Table 3). The selective MB uptake capacity of the synthesized adsorbent form aqueous medium are comparable with previously reported other adsorbents (Table 4).

Table 3 Langmuir and Frendulich isotherm parameters of selective MB adsorption from R-M mixed solution by the samples. Sample name

4.3. Thermodynamic analysis

0Cu-NaTi 0.5Cu-NaTi 1Cu-NaTi 2Cu-NaTi

Fig. 10a represents the temperature study of the selective MB adsorption process on the respective nanostuctures. All the parameters are fixed as used for M-R adsorption experiment setup. We have carried out the experiment with a fixed contact time of 40 min. at three different temperatures 25 °C, 35 °C, and 45 °C. The results indicate that removal

Langmuir isotherm

Frendulich isotherm

Qm (mg. g−1)

KL (L. mg−1)

R2

RL

KF (L. mg−1)

1/n

R2

45.66 19.08 21.97 36.90

2.63 2.71 6.70 2.07

0.99 0.88 0.80 0.87

0.04 0.04 0.01 0.05

63.10 28.84 26.30 29.51

0.18 0.29 0.18 0.34

0.99 0.97 0.94 0.96

Table 2 Model kinetic parameters of selective MB adsorption from R-M mixed solution by the respective nano- adsorbents. Sample name

Pseudo first order rate kinetics parameters

Pseudo second order rate kinetics parameters

Intraparticle diffusion

kI (min−1)

qe,Cal (mg. g−1)

qe,Exp (mg. g−1)

R2

kII (g. mg−1. min−1)

qe,Cal (mg. g−1)

qe,Exp (mg. g−1)

R2

ki (mg. g−1. min1/2)

C (mg. g−1)

R2

0Cu-NaTi

0.0017

44.16

41.97

0.50

0.125

19.72

19.92

0.99

0.5Cu-NaTi

0.0019

61.31

57.68

0.46

0.057

27.38

27.62

0.99

1Cu-NaTi

0.0023

67.35

63.24

0.33

0.065

29.84

30.30

0.99

2Cu-NaTi

0.0020

26.26

55.42

0.76

0.035

26.26

26.34

0.99

ki1 ki2 ki1 ki2 ki1 ki2 ki1 ki2

C1 C2 C1 C2 C1 C2 C1 C2

0.86 0.96 0.76 0.83 0.92 0.70 0.78 0.99

10

= = = = = = = =

1.41 0.17 2.34 0.37 4.11 0.11 1.58 1.01

= = = = = = = =

14.90 18.66 19.04 25.12 17.05 29.13 19.11 9.90

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A. Kundu and A. Mondal

Table 4 Comparison of adsorption capacities of methylene blue onto different adsorbents. Adsorbent

Maximum adsorption capacity (mg. g−1)

Ref.

Eucalyptus, Palm bark, Anaerobic digestion residue Amorphous silica, and Zeolite NaOH-treated pure kaolin Fe(III)/Cr(III) hydroxide PANI, and PPY (C-dot initiated polymerization) Carbon nanotube Na-incorporated titania

2.06, 2.66, and 9.5 22.66, and 10.82 20.49 22.8 19.67, 19.96 35 45.66

[49] [50] [20] [51] [52] [53] This work

Table 5 Thermodynamic parameters of selective MB adsorption from M-R solution onto different samples. Sample name

T (K)

0Cu-NaTi 0.5Cu-NaTi 1Cu-NaTi 2Cu-NaTi

298, 298, 298, 298,

308, 308, 308, 308,

ΔGo (kJ/mol) 318 318 318 318

−2.39, −2.47, −6.70, −2.07,

−2.65, −2.72, −7.68, −2.53,

−2.99 −3.19 −7.89 −2.89

ΔHo (kJ/mol)

ΔSo (J/mol K)

6.46 8.31 6.48 13.16

29.68 36.08 37.71 50.31

(L. mol−1). Fig. 10b represents the van't Hoff plots (ln(KL) vs. 1/T) of the respective nanostuctures. From the plot thermodynamic parameters, enthalpy and entropy changes are determined and tabulated in Table 5. At different temperatures, the spontaneous nature and feasibility of the selective adsorption process is confirmed since the ΔGo values are negative (Fu et al. 2015). The positive enthalpy values suggest that the adsorption process is endothermic in nature. In addition, the positive ΔSo values indicate that there may be some structural changes in the samples and MB which leads to the increased randomness at solid−liquid junction throughout the adsorption period. 4.4. Reusability of the adsorbent After the first adsorption experiment, the adsorbent (1Cu-NaTi) is washed with small amount of 30% methanol solution, until the surface of the adsorbent becomes colourless. The adsorbent is then heated in an oven at 60 °C for four hours after the 2nd cycle is performed. Similarly, we have run a total of six cycles and calculated the removal percentage of the MB from the M-R solution. From the Fig. 11, it is clear that the nano-adsorbent losses its activity by approximately 50% after sixth cycle of experiment. In 2nd and 3rd cycle of adsorption experiment the decrease in removal percentage may arise due to the loss of Na+ ions from the interlayered surface. After that, the adsorption took place only to the specific sites of the nanostructures due to electrostatic attraction between the negatively charged interlayered surface and the cationic MB dye. The zeta potential value of 1Cu-NaTi is found to be −22.9 mV,

Fig. 10. (a) Effect of temperature on the selective MB adsorption from M-R solution onto the respective samples, and (b) van't Hoff plots for thermodynamic parameters.

efficiency of MB from M-R solution rises to some extent when the temperature is raised to 45 °C. Therefore, we can say that the selective adsorption of MB on the nanostuctures is favored at higher temperatures within 25–45 °C temperature range. Therefore, the synthesized materials can be judged as a very effective adsorbent to eradicate MB from the aqueous solutions. We have evaluated the basic thermodynamic parameters like, Gibbs free energy (ΔGo), enthalpy (ΔHo), and entropy (ΔSo) for the selective MB adsorption process. Langmuir isotherm is used to evaluate the parameters using the following equations (Fu et al. 2016).

ΔGo = −RTln (KL) ln (KL) =

(9)

ΔS o ΔH o − R RT

(10) −1 −1

Fig. 11. Reusability efficiency of the 1Cu-NaTi nano-adsorbent after several adsorption cycles.

where, R is the universal gas constant (8.314 J. mol K ), T is the system temperature (K), and KL is the Langmuir equilibrium constant 11

Applied Clay Science 183 (2019) 105323

A. Kundu and A. Mondal

which indicates that the nanostuctures have negative surface charge. Therefore, the adsorbent can be reused, as its activity remained to some extent even after the several cycle of experiments. 4.5. Mechanistic pathway The adsorption selectivity appears because of the molecular shape and size of MB molecules. Planar structure and small molecular size make MB unique. The MB molecules are embedded into the interspace between the interlayered stuctures (BET isotherm and microstructural study confirmed presence of interlayered structure) through cation exchange (by replacing Na+ ion). The electronic attraction is not part of the mechanism because both MB and RhB are cationic dyes. A plausible mechanistic pathway is represented in Fig. S3. The cation exchange mechanism has been reported previously for the adsorption of cationic dyes on titanate nanotubes (Te Hsieh et al., 2008; Lee et al. 2008a; LIN et al. 2009; Lin et al., 2010). The adsorption study shows that 1Cu-NaTi nanoparticles have the highest adsorption capacity, whereas the 0CuNaTi has the lowest. The high adsorption capacity by the aforementioned nanostuctures may be due to the containing of maximum number of capillaries created from the titania layers. Moreover, the Cu doped samples displayed approximately same removal percentage of MB after 40 min. As we can see from the HRTEM micrographs 0Cu-NaTi nanostructure is more particle like compared to 1Cu-NaTi (which is mostly layered). Moreover, if we compare among the Cu-doped samples, maximum adsorption capacity is achieved by 1Cu-NaTi because of maximum Na-content in the nanostructure (Fig. 5). When the number of Na+ ions are more, large number of MB+ ions can be replaced through cation exchange. Previously reported data reveals that high sodium content enhances the adsorption capacity in titanates. e.g. Lee et al. studied the effect of the Na-content in titanate nanotubes on the adsorption of Basic Violet 3, and they reported that the specific microstructure with high sodium content (> 7.23 wt%) have the best adsorption capacity (Lee et al. 2008b). We have found that the MB+ ions are actually intercalated between the titania layers. XRD analysis (Fig. S4: after intercalation) indicates formation of a new broad peak approximately at 6° arising due to the MB intercalation (Klika et al. 2007). The peak is not sharp due to the low content and amorphous nature of MB.

Fig. 12. (a) Photodegradation profile at different time interval and (b) the pseudo-first order rate kinetics plot of MG photodegradation by the respective samples. Table 6 Pseudo-first order kinetics parameters of photocatalysis of MG dye under direct sunlight using the different nanostructured samples.

5. Photodegradation study

Sample name

The photocatalytic activity of all the nanostuctures in terms of degradation efficiency is evaluated. A maximum of ~94% degradation is observed within 70 min. using 0Cu-NaTi catalyst under sunlight. Fig. 12a displays the percentage of degradation of MG dye at different time intervals by the respective samples. As we can see, in the first 15 min. the degradation rate is much faster compared to the other. The percentage of degradation by the 0.5Cu-NaTi, 1Cu-NaTi, and 2Cu-NaTi catalyst are ~88%, ~91%, and ~75% respectively. The photocatalytic reactions follow the pseudo first order rate kinetics (Fig. 12b). The rate constant (k) can be calculated using the equation, ln (Co/Ct) = kt where, Ct and C0 are the concentrations at “t” time and initial concentration, respectively. The pseudo-first order kinetic parameters have been calculated and summarized in Table 6. The nanostuctures have approximately same photodegradation rate except 2Cu-NaTi. Low band gap energy and photoluminescence intensity of Na-intercalated Cudoped titania indicates that those should contribute to the better photodegradation efficiency but here anatase crystallinity plays its role. XRD data demonstrates the formation of secondary phase and deprived anatase crystallinity with increased dopant concentration. The amorphous nature in the surface of the samples may has strong effects leading to a decrease in the photodegradation process. We have compared the rate of MG photodegradation with some reported literature. Saikia et al. reported MG photodegradation rate of 0.014 and 0.017 min−1 for synthesized ZnO nanoparticles (hydrothermal) and

Rate (min R2

−1

)

0Cu-NaTi

0.5Cu-NaTi

1Cu-NaTi

2Cu-NaTi

0.040 0.99

0.035 0.94

0.038 0.94

0.021 0.94

ZnO nanoflowers, respectively (Saikia et al. 2015). Asiltürk et al. reported MG photodegradation (under visible light) rate of 0.0136 and 0.0131 min−1 for 0.3 mol% Fe-doped TiO2 sample when the MG concentrations were 2.5 mg/L and 5.0 mg/L, respectively (Asiltürk et al. 2009). Kant et al. synthesized Fe0.01Ni0.01Zn0.98O/polyacrylamide nanocomposite which shows MG photodegradation rate of 0.018 and 0.022 min−1 for dye concentrations of 10−5 (M) and 10−6 (M), respectively (Kant et al. 2014). Rajabi et al. reported MG decolorization (15 ppm) using ZnS and Zn0.955Fe0.05S nanoparticles, which have kinetic rates 0.017 and 0.022 min−1 respectively (Rajabi et al. 2013). As we can see from data the pseudo-first order kinetics are faster for our systems. 6. Conclusion Na-intercalated titania and Na-intercalated Cu-doped titania are successfully synthesized via precipitation method. The nanostructures exhibit excellent adsorption of MB from aqueous solution selectively. MB adsorption followed pseudo second-order kinetics and well fitted to both Langmuir and Freundlich isotherms. XRD and EDX analysis reveal 12

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A. Kundu and A. Mondal

that the polymorphs have the important anatase phase and also the presence of Na and Cu–content in their respective nanostructures. BET isotherms, FESEM, and HRTEM micrographs illustrate interlayered structures of the samples which allow MB molecules to pass and then embed into the inner space of the layers to show selective adsorption. The thermodynamic studies showed the selective adsorption of MB onto the nanostructures to be spontaneous and endothermic. Moreover, the synthesized nanostructures are reusable and the adsorption capacities are comparable to the literature data. The materials also show good photocatalytic behaviour with malachite green in aqueous solution under direct sunlight irradiation. Therefore, Na-intercalated titania and Na-intercalated Cu-doped titania are excellent selective adsorbent and photocatalytic materials for the removal of MB and MG from aqueous solution.

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Declaration of Competing Interest We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. Acknowledgment The authors are thankful to Department of Chemistry, National Institute of Technology Rourkela for providing experimental facilities. Amar Kundu acknowledges National Institute of Technology Rourkela, India for his research fellowships. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2019.105323. References Abdel-Monem, Y.K., Emam, S.M., Okda, H.M.Y., 2017. Solid state thermal decomposition synthesis of CuO nanoparticles from coordinated pyrazolopyridine as novel precursors. J. Mater. Sci. Mater. Electron. 28, 2923–2934. https://doi.org/10.1007/ s10854-016-5877-3. Adachi, M., Murata, Y., Harada, M., Yoshikawa, S., 2000. Formation of titania nanotubes with high photo-catalytic activity. Chem. Lett. 29, 942–943. https://doi.org/10. 1246/cl.2000.942. Aguilar, T., Navas, J., Alcántara, R., Fernández-Lorenzo, C., Gallardo, J.J., Blanco, G., Martín-Calleja, J., 2013. A route for the synthesis of Cu-doped TiO2 nanoparticles with a very low band gap. Chem. Phys. Lett. 571, 49–53. https://doi.org/10.1016/j. cplett.2013.04.007. Ahmaruzzaman, M., 2008. Adsorption of phenolic compounds on low-cost adsorbents: a review. Adv. Colloid Interf. Sci. https://doi.org/10.1016/j.cis.2008.07.002. Ahmaruzzaman, M., 2011. Industrial wastes as low-cost potential adsorbents for the treatment of wastewater laden with heavy metals. Adv. Colloid Interf. Sci. https:// doi.org/10.1016/j.cis.2011.04.005. Al-Ghouti, M.A., Khraisheh, M.A.M., Allen, S.J., Ahmad, M.N., 2003. The removal of dyes from textile wastewater: a study of the physical characteristics and adsorption mechanisms of diatomaceous earth. J. Environ. Manag. 69, 229–238. https://doi.org/ 10.1016/j.jenvman.2003.09.005. Ali, I., 2012. New generation adsorbents for water treatment. Chem. Rev. https://doi.org/ 10.1021/cr300133d. Alver, E., Metin, A. ü, 2012. Anionic dye removal from aqueous solutions using modified zeolite: adsorption kinetics and isotherm studies. Chem. Eng. J. 200–202, 59–67. https://doi.org/10.1016/j.cej.2012.06.038. Amano, F., Yasumoto, T., Shibayama, T., Uchida, S., Ohtani, B., 2009. Nanowire-structured titanate with anatase titania: characterization and photocatalytic activity. Appl. Catal. B Environ. 89, 583–589. https://doi.org/10.1016/j.apcatb.2009.01.013. Asiltürk, M., Sayilkan, F., Arpaç, E., 2009. Effect of Fe3+ ion doping to TiO2 on the photocatalytic degradation of malachite green dye under UV and vis-irradiation. J. Photochem. Photobiol. A Chem. 203, 64–71. https://doi.org/10.1016/j.jphotochem. 2008.12.021. Babel, S., Kurniawan, T.A., 2003. Low-cost adsorbents for heavy metals uptake from contaminated water: a review. J. Hazard. Mater. 97, 219–243. https://doi.org/10. 1016/S0304-3894(02)00263-7. Carneiro, P.A., Umbuzeiro, G.A., Oliveira, D.P., Zanoni, M.V.B., 2010. Assessment of water contamination caused by a mutagenic textile effluent/dyehouse effluent bearing disperse dyes. J. Hazard. Mater. 174, 694–699. https://doi.org/10.1016/j. jhazmat.2009.09.106. Carvalho, M.C., Pereira, C., Gonçalves, I.C., Pinheiro, H.M., Santos, A.R., Lopes, A., Ferra,

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