Advanced TiO2âSiO2âSulfur (TiâSiâS) Nanohybrid Materials

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Advanced TiO2-SiO2-Sulfur (Ti-Si-S) Nanohybrid Materials: Potential Adsorbent for the Remediation of Contaminated Wastewater Kishore K. Jena, Vijay Kumar Shankarayya Wadi, Hemant Mittal, Ganesh Kumar Mani, and Saeed M. Alhassan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09140 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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ACS Applied Materials & Interfaces

Advanced TiO2-SiO2-Sulfur (Ti-Si-S) Nanohybrid Materials: Potential Adsorbent for the Remediation of Contaminated Wastewater Kishore K. Jena a, Hemant Mittal a, Vijay S Wadi a, Ganesh Kumar Mani b and Saeed M. Alhassan a* a Department

of Chemical Engineering, Khalifa University , SAN Campus ,PO Box - 127788, Abu Dhabi, United Arab Emirates (UAE)

bMicro/Nano

Technology Center, Tokai University (Shonan Campus), 4-1-1 Kitakanamae, Kanagawa, Japan 259-1292

ABSTRACT In this present work, TiO2-SiO2-sulfur (Ti-Si-S) nanohybrid material was successfully prepared by using TiO2 nano powder, TEOS sol-gel precursor and elemental sulfur as raw material by sol-gel process and hydrothermal method at 120 °C temperature. Raman spectroscopy, XRD, SEM, TEM and N2 absorption‐desorption characterized the synthesized nanohybrid material. The characterization results confirmed the homogeneous distribution of sulfur in the nanohybrid material. The size of the Ti-Si-S nanohybrid material is vary between 20-40 nm and the surface areas of the nanohybrid material was measured using N2 absorption‐desorption, which showed value of 57.2 m2g−1. The potential of Ti-Si-S nanohybrid material as an adsorbent was further tested to adsorb methylene blue (MB) from aqueous solution. Adsorption performance of hybrid material was highly influenced by the solution pH and mass of adsorbent. Adsorption of MB using Ti-Si-S nanohybrid material was homogeneous monolayer adsorption which followed Langmuir adsorption isotherm with qe,max value of 804.80 mg.g-1 and pseudo-second-order rate equation. Dye diffusion mechanism followed partially both intra-particle and liquid film diffusion mechanism. Thermodynamics studies predicted spontaneous and endothermic nature of the whole adsorption process. Furthermore, Ti-Si-S nanohybrid material was repeatedly for six cycles of adsorptiondesorption of MB dye. KEYWORDS: Dye adsorption; adsorption isotherm; adsorption kinetics; elemental sulfur; XPS * Supporting Information (S)

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INTRODUCTION Easy availability and low cost of synthetic dyes have increased their use tremendously in different industries especially textile, paper and pharmaceutical. Approximately, 10,000 tons of synthetic dyes are produced per annum out of which 1-2% during their production and 1-10% during application is discharged in the environment 1. Most of the synthetic dyes are hazardous and cause environmental pollution and have serious health-risk factors. Despite of their extremely hazardous nature, their use in industries is extensively increasing day by day because of their easy availability and comparatively much lower cost. Use of synthetic dyes is very important for industrial revolution but at the same time protection of environment is of equal importance. Therefore, the treatment of dyes effluent before its discharge into environment is very crucial and with the increasing production of dye stuff it is a big challenge for environmental scientists. For water purification, there is always an efficient and cost-effective technology is needed. Different technologies have been developed and used previously to remove due stuff from contaminated water but out of all the water purification technologies, adsorption is the most effective, reliable and efficient technology used so far because of the advantageous such as low operational cost, minimum use of energy and lack of interaction with toxic substances . Different adsorbents mainly activated charcoal, clay, zeolites, polysaccharides based hydrogels were used previously to adsorb synthetic dyes from aqueous solution2-5. Some of these adsorbents have shown very good adsorption capacities but they are associated with limitations such as high operational cost and production of secondary waste after adsorption. Polymer-nanocomposites has been reported in literature as potential adsorbents to remove both organic

6, 7

and inorganic

8, 9

pollutants. Some various nano-structured materials such as metal oxide nanoparticles, CNTs and graphene based materials and magnetic nanoparticles were also used successfully to adsorb synthetic dyes from wastewater 10-12. The protection of environment and the improvement of water quality by the removal of dyes and metal ions from industrial waste using different hybrid /nano materials have been studied more than a decade

13-16.

Introduction of nanotechnology in water

remediation applications have shown very promising results, furthermore, the performance of these nano-materials can be further enhanced by either by modifying their surface with some suitable functional groups or through composite formation by embedding them into polymer matrix. Activated carbon is also highly demandable adsorbent because of its unique properties such as high adsorption capacity and large surface area but its high production cost liming its use 2 ACS Paragon Plus Environment

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and forcing researchers to think of other cost effective and highly efficient adsorbents which can be used in large scale for the adsorption of both organic and inorganic contaminants. Sulfur based hybrid materials also possess very good surface properties such as high porosity and surface area, very good cation exchange capacity and easy availability, therefore, they can be used as an alternative for activated carbon in water remediation applications. However, the application of sulfur-based hybrid materials is limited and broadly used in Li-S battery, supercapacitor and mercury capture application 17-19. In this work, we developed hybrid materials by using TiO2, SiO2, and elemental sulfur as an adsorbent for cationic dye i.e. methylene blue (MB) and in respect to our knowledge and previous literature study, this work is reported first time. The aim of this work was to synthesize a nanohybrid material by using simple synthesis process and by-product within reasonable costs. Therefore, we adopted the sol-gel synthesis and hydrothermal method to develop the hybrid material in a single step synthesis procedure and TiO2, SiO2, and elemental sulfur as the starting materials. The novelty of this work consists in obtaining TiO2-SiO2-sulfur (Ti-Si-S-25) nanohybrid material with tailored phase arrangement by hydrothermal and sol-gel synthesis. MB was used as a cationic dye to test the feasibility of Ti-SiS-25 nanohybrid material to be used as a potential adsorbent in water remediation applications. Ti-Si-S-25 nanohybrid material showed pH dependent adsorption behavior with high value of qe,max 804.80 mg.g-1, which was tried to explain on the basis of different characterization techniques, different two and three-parameter adsorption isotherm models and various adsorption kinetics models. Main advantageous of using Ti-Si-S-25 nanohybrid material in water remediation applications especially dyes adsorption are its exceptionally high adsorption capacity, reusability, consumption of low energy during synthesis and the use of easily available and cost effective raw materials. EXPERIMENTAL METHODS Materials. Titanium dioxide (TiO2, < 100 nm (BET)), tetraethyl orthosilicate (TEOS, solgel precursor, Mw= 208.33 gmol-1), glacial acetic acid (CH3COOH, Mw = 60.05 gmol-1) and cetyltrimethylammonium bromide (CTAB, Mw = 364.45 gmol-1) were obtained from Sigma Aldrich, UAE. Granulated elemental sulfur was supplied by Abu Dhabi Gas Industries Limited (GASCO, Abu Dhabi, UAE). All chemicals were used as received without further purification. Synthesis of SiO2 nanoparticles. SiO2 nanoparticles were synthesized from hydrolysis and condensation reaction of TEOS by sol-gel method. Initially, TEOS was added in absolute ethanol 3 ACS Paragon Plus Environment


followed by the addition of distilled water. Molar ratio of TEOS: H2O was fixed at 1:4. pH of the reaction mixture as well as the condensation and hydrolysis reactions were controlled using acetic acid (0.05wt. %). The reaction mixture was stirred continuously to get a clear solution. Thereafter, 0.05 wt. % of CTAB was added and after stirring for 6 h, the sol was aged for 24 h, it was altered into gels. Nanoparticles were obtain after evaporating water molecules and organic materials by drying the gels at 150°C for 2h. Finally, dry gels were powdered and sintered at 500°C in tube furnace to obtain desired SiO2 nano powders. Synthesis of titania-silica-sulfur (Ti-Si-S) nanohybrid material by hydrothermal method. Titania-Silica-Sulfur (Ti-Si-S) nanohybrid material was synthesized through a single step hydrothermal method. In synthesis procedure, TiO2 nano-powder (1.875g) and SiO2 (1.875g) nano-powder were dispersed in 45 ml deionized water and 5 ml toluene under vigorous magnetic stirring. After stirring for 30 min, sulfur powder (1.25g) was added to the above solution and stirred for 1h. The dispersed solution was placed into an autoclave to treat hydrothermally under autogenous pressure. Hydrothermal temperatures of 120oC and heating durations 24h were employed. After cooling to room temperature, product was recovered by filtration, washed with distilled water and finally dried at 80°C for about 24h. The preparation of nanohybrid material with 25 % sulfur concentration was denoted as Ti-Si-S-25.The details of the characterization techniques and MB adsorption studies are explained in Supporting Information. The chemical synthesis route of ordered Ti-Si-S-25 nanohybrid material prepared by sol-gel and hydrothermal process is shown in Scheme S1. RESULTS AND DISCUSSION Structural analysis. The phase composition of TiO2, SiO2, sulfur and Ti-Si-S-25 nanohybrid material have been identified from XRD patterns. XRD diffraction patterns of TiO2, SiO2, sulfur and Ti-Si-S-25 (Figure S1 (a), Supporting Information) showed a broad peak located approximately at 2θ = 22.6° suggesting an amorphous characteristic of SiO2 which agreed with the reported JCPDS data (card No. 01-086-1561). The main diffraction peaks observed for TiO2 were centered at 25.5° (A, 101), 27.7°(R, 110), 37.8° (A, 004), 48.2° (A, 200), and 54.7°(R, 211) respectively. All the diffraction peaks were absolutely indexed to the anatase (tetragonal structure) and rutile phase of TiO2. All values of diffraction peaks were in accordance with Powder Diffraction Standards database (JCPDS 21-1272 and JCPDS 21-1276) 20. The diffraction patterns of sulfur powder are shown in Figure S1, Supporting Information. Diffraction peaks at 23.1°, 4 ACS Paragon Plus Environment

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25.9°, 27.8°, and 28.6° are assigned to the presence of crystallized orthorhombic sulfur (S8) structure

21, 22.

XRD pattern for Ti-Si-S-25 nanohybrid material is shown in Figure S1(a) (top)

,which showed peak at 2θ = 23.1° that can be attributed to sulfur and other peaks assigned to crystalline TiO2. No diffraction peaks from amorphous silica at 22.6° are detectable. The crystalline diffraction peaks of TiO2 at 25.5°, 27.7°, 37.8°, 48.2° and 54.7° and noticeable low intensity diffraction peaks of crystallized orthorhombic sulfur (S8) at 2θ = 23.1° are observed in Ti-Si-S-25 nanohybrid material, indicates the possible exfoliation of the sulfur in the hybrid without significant aggregation. Compared with the pure TiO2, a systematical peak shift towards low diffraction angles is recognizably detected in the Ti-Si-S-25 nanohybrid material, suggests a lattice expansion when introducing sulfur into the TiO2 crystal lattice. The morphology and exfoliation of the sulfur are explained in more detail in SEM and TEM analysis. In addition, the crystallite size based on the major diffraction peak at 25.5° (A, 101) is 25.8 nm for the hybrid composite. Figure S1 (b), Supporting Information shows the Raman spectra of TiO2, SiO2, sulfur and Ti-Si-S-25 nanohybrid material in the range of 100-4000 cm−1. Raman spectra analysis elucidates the predominant anatase form of TiO2. Six Raman active modes (A1g +2B1g+ 3Eg) for anatase TiO2 were observed in the spectrum of TiO2. The intense peak 143 cm−1 is assigned as Eg peak along with 199 and 637 cm−1 are as low intense Eg peaks. One B1g peak appears at 393 cm−1 and the (A1g+ B1g) peak appear at about 515 cm−1 23. SiO2 Raman spectrum shows peaks at 1113 cm-1, 3074 cm-1 and 3586 cm-1 correspond to Si-O-Sistr, C-Hstr and O-Hstr, respectively. Raman spectrum of Ti-Si-S-25 nanohybrid material is not showing any clear peaks at 1113 cm-1 (Si-O-Si) , 3074 cm-1 (C-Hstr) and 3586 cm-1 (O-Hstr) except 143 , 217, 464 and 641 cm-1 peaks, which indicate the presence of Eg , Ti-O-Si , S-S and Ti-O-Ti in nanohybrid material. The Raman spectra of TiO2 and Ti-Si-S-25 nanohybrid are very similar, except a slight shifting in the absorption peaks, could be due to changes in the electron density. XPS analysis. The structural combination between SiO2, TiO2 and sulfur in the Ti-Si-S-25 nanohybrid was analyzed by X-ray photoelectron spectroscopy (XPS) and the individual element spectrum was shown in Figure 1. Figure 1(a) shows the survey scan spectrum of the Ti–Si-S-25 nanohybrid material. The survey scan spectrum shows that Ti, Si, O, C and S elements are present at different binding energy in the Ti-Si-S-25 nanohybrid material. The presence of C in the Ti– Si-S nanohybrid material may be attributed to contamination by adventitious carbon and Si-O5 ACS Paragon Plus Environment


CH2-CH3 groups. There may be some evidence that CO or CO2 species may play a role in the gradual appearance of carbon on pristine surfaces within the vacuum of the XPS chamber 24. In addition to the survey scan, the binding energies of high-resolution XPS spectra of Ti2p, O1s, Si2p, C1s and S2p near 460 eV, 530 eV, 105eV, 285eV and 165 eV were individually analyzed by deconvolution. Ti (2p) XPS spectra of Ti–S-S-25 nanohybrid material as a Ti (2p3/2) and Ti (2p1/2) doublet are shown in Figure 1(b). The peaks at 459.1 and 464.3 eV represent Ti (2p3/2) and Ti (2p1/2), respectively. The peak at 459.1 eV for Ti (2p3/2) was shifted to higher binding energy in the deconvolution spectrum could be due to the contribution of lower valence 25. The Ti (2p3/2) peak for pure TiO2 is observed at 458.8 eV 26, which is lower binding energy than that of Ti-Si-S-25 nanohybrid. Therefore, it is believed that the TiO2 nanoparticles dispersed in SiO2 sol are chemically bonded to the SiO2 nanoparticles to form Ti-O-Si linkages in the Ti–Si-S-25 nanohybrid material. Different chemical state of oxygen (O1s) element according to the binding energy are analyzed by deconvolution study and shown in Figure 1 (C). We deconvoluted O1s spectrum peak to six kinds of chemical states at different binding energy level. Deconvoluted peaks observe at 529.4 eV, 529.6 eV, 530.8 eV, 532.7 eV, 532.9 eV and 533.5 eV binding energy may be attributed to Ti–O–Ti, O-CH2, Si–O–Ti, Si–O–Si ,O-H and H2O (adsorbed water) , respectively with increasing binding energy . We can assume that TiO2 and SiO2 nanoparticles formed network structure by Si–O–Ti covalent bond, which can form hybrid material with high crosslinking density. The deconvoluted Si2p (Figure 1(d)) zone shows four major peaks at 102.2 eV, 103.1 eV, 103.8eV and 104.5eV, which correspond to Si-O-Ti, Si–O–Si, Si-O-C and Si–OH bonds, respectively 27. The presence of Si-O-Ti peak at 102.2 eV in the hybrid samples can be ascribed to the crosslinking structure between SiO2 and TiO2. In the S2p (Figure 1 (e)), three peaks are observed in the sulfur region. Peaks at 162.7eV, 163.8 eV and 168.7eV correspond to S2p1/2, S2p3/2 and S2p (oxidized sulfur), respectively. The S2p1/2 peak at 162.7 eV and S2p3/2 peak at 163.8 eV with an intensity ratio of 1:2 are the characteristic of solid sulfur in composite 28. These peaks indicate the presence of S2p in the Ti-Si-S-25 nanohybrid material. A weak broad peak observed at 168.7eV could be attributed to the surface oxidation of sulfur 29. These deconvolution studies of individual elements of Ti-Si-S-25 nanohybrid material gives the direct evidence of formation of ordered hybrid material prepared by sol-gel and hydrothermal Process.


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Figure 1. XPS spectra of Ti-Si-S-25 nanohybrid material (a) survey spectrum of Ti-Si-S-25 nanohybrid material; (b)-(e) Ti2p, O1s, Si2p and S2p high-resolution deconvoluted spectra, respectively; (f) survey spectra of the TiO2 and Ti-Si-S-25 nanohybrid material. In the survey spectra (f), the elements Ti, Si, O, S and C are clearly identified. Morphology analysis. Figure 2 (a, b) shows the TEM images of the SiO2 and TiO2 nanoparticles. The TiO2 nano particles possess uniform tetragonal anatase structure. Figure 2(c-g) presents the TEM, EDS and SAED results of the TiO2-SiO2-sulfur nanohybrid material (Ti-Si-S25). The morphology of TiO2 nanoparticles changed dramatically with the addition of SiO2 and sulfur. First, irregular size of aggregated TiO2 nanoparticles with size ranging from 10 nm to 30 nm was observed in Figure 2(b). The HRTEM (100 nm) image in Figure 2 (d) shows the particle 7 ACS Paragon Plus Environment


size of TiO2 anatase phase in the range 70-90 nm with exfoliated structure and sulfur nanoparticles are uniformly distributed in the TiO2–SiO2 nanohybrid. The corresponding SAED pattern (Figure 2(f)) indicates that the TiO2 nanoparticles in Ti-Si-S-25 nanohybrid material are polycrystalline anatase structure, in good agreement with the XRD results. TEM analysis reveals that the TiO2 nanocrystals without sulfur loading present irregular morphologies with a size ranging from 10 nm to 30 nm, while the TiO2 nanocrystals in Ti-Si-S-25 nanohybrid are nearly monodisperse in an average size of 80 nm (Figure 2(d)). This could be observed due to molecular sulfur chains, which may act as a growth controlling agent to produce well shaped nanocrystals with high surface area ( clearly analyzed in BET analysis) in the present hybrid system. The EDS spectrum of Ti-Si-S-25 nanohybrid material was recorded to analyze the changes in elemental composition (Figure 2(g)). Strong peak for sulfur, Ti, Si and O elements were observed in the EDS spectra, indicates the presence of sulfur at high concentration in the hybrid material. Figures 3 (a, d, e, h and i) are the SEM micrographs of TiO2, SiO2 and TiO2-SiO2-sulfur nanohybrid material (Ti-Si-S-25). Figure 3 (d and h) are the magnified (400nm) TiO2 and SiO2 images and Figure 3 (m) is the higher magnified (500nm) SEM micrograph of Ti-Si-S-25, revealing that these material are smooth and uniform nanostructures. Figures 3 (d and h) are the evidence of several nanometers tetragonal and globular nanostructures of TiO2 and SiO2. These tetragonal nanostructures are of anatase TiO2 and high crystalline in nature as previously observed in XRD and TEM analysis. In order to confirm the existence of sulfur in the Ti-Si-S-25 hybrid material, energy dispersive X-ray spectroscopy (EDXA) (Figure 3 (p, r and q) and elemental mapping ( Figure 3(b,c,f,g,j,k,l and n)) were analyzed from the SEM image of the TiO2 and SiO2 and Ti-Si-S-25 hybrid material. As shown in EDXA and mapping figures, strong signals of Ti, O, Si, and S can be detected from the hybrid sample, indicating the homogeneous distribution of sulfur in the hybrids, which was in accordance with the TEM results.


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Figure 2. TEM morphology of hierarchically organized TiO2-SiO2-sulfur (Ti-Si-S-25) nanohybrid materials. (a) Pure SiO2 nanoparticles (20nm); (b) TiO2 nanoparticles (0.2µm); (c) TiO2-SiO2sulfur nanohybrid material (Ti-Si-S-25) ((0.2µm); (d) TiO2-SiO2-sulfur nanohybrid material (TiSi-S-25) ((100nm); (e) HRTEM image of selected area of Ti-Si-S-25; (f) selected-area electron diffraction pattern of Ti-Si-S-25; and (g) EDS spectrum of TiO2-SiO2-sulfur nanohybrid material (Ti-Si-S-25).



Figure 3. SEM morphology of hierarchically organized TiO2-SiO2-sulfur (Ti-Si-S-25) nanohybrid materials. (a) Pure TiO2 nanoparticles (1 μm); (b) and (c) O and Ti elemental mapping of TiO2 nanoparticles ; (d) TiO2 nanoparticles (400nm at different area) ; (e) SiO2 nanoparticles (1 μm) ; (f) and (g) O and Si elemental mapping of SiO2 nanoparticles; (h) SiO2 nanoparticles ( 1 μm at 10 ACS Paragon Plus Environment

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different area); (i) TiO2-SiO2-sulfur nano hybrid material (Ti-Si-S-25) ( 1 μm); (j), (k), (l) and (n) O, Si, Ti and S elemental mapping of Ti-Si-S-25 nanohybrid material; (m) expanded zone of TiSi-S-25 nanohybrid material (500nm) and; (o), (p), (q) and (r) EDAX spectra of TiO2 , SiO2 and TiO2-SiO2-sulfur nanohybrid material (Ti-Si-S-25). Elemental mapping of sulfur evidence that sulfur nanoparticles were uniformly distributed in the TiO2-SiO2-sulfur nanohybrid material. BET analysis. The pores size and the surface area of the TiO2-SiO2 and Ti-Si-S-25 nanohybrid materials have been studied and confirmed by BET size distribution analyses (Figure S2). Both the TiO2-SiO2 and Ti-Si-S-25 nanohybrid materials show the H3 hysteresis loop and Type IV isotherm in this study. Figure S2 (a), (b) and (c), Supporting Information shows 14.3 m2/g, 57.2m2/g and 3.91nm, 3.94nm BET surface area and pore diameter of TiO2-SiO2 and Ti-SiS-25 nanohybrid materials. The increased surface area and constant pore size were observed after addition of elemental sulfur in the TiO2-SiO2 nanohybrid material. The increased surface area of the Ti-Si-S-25 nanohybrid material might be due to the lack of big crystal growth and condensation reaction between SiO2 and TiO2 functional groups in the presence of elemental sulfur. The elemental sulfur effectively barred coalescence of the TiO2-SiO2 nanoparticles in this work. The detailed explanation of the particles size and uniformity is provided in the above TEM and SEM analysis sections. Moreover, the observed H3 hysteresis loop and Type IV isotherm trend of the BET surface area has already been reported in the literature 30, 31. In addition, Figure S2(c) displays the comparative pore size distribution for the TiO2-SiO2 and Ti-Si-S-25 nanohybrid materials, in which the both hybrids show pore size in the range of 3-4 nm, which is mesoporous materials and satisfied with the TEM morphology. MB dye adsorption studies Variation in Ti-Si-S-25 nanohybrid material mass. Effect of adsorbent mass on percentage adsorption was studied by taking a varied mass of Ti-Si-S-25 nanohybrid material. Initially a gradual increase in percentage adsorption was observed with increasing mass of Ti-SiS-25 nanohybrid material. Percentage adsorption increased from 24 to 83 % while changing mass of adsorbent from 0.1-0.4 g.L-1 (Figure 4(a)). This increased performance of Ti-Si-S-25 nanohybrid material can be attributed to the comparatively larger number of adsorption sites with increased mass of adsorbent

32.

However, after attaining the equilibrium adsorption percentage

adsorption did not changed significantly with further increase in the mass of Ti-Si-S-25 nanohybrid material. This saturation in the adsorption performance at higher mass of adsorbent was due to the 11 ACS Paragon Plus Environment


agglomeration of Ti-Si-S-25 nanohybrid particles, which aggregated adsorption sites, reduced the total available surface area, and increased diffusion path 33, 34. Variation in solution pH. Firstly, point of zero charge (pzc) of Ti-Si-S-25 nanohybrid material was determined to examine the influence of variation in pH of dye solution. The pzc of Ti-Si-S-25 nanohybrid material was found to be 4.73 (Figure 4(b)). It is reported that for the promising adsorption of cationic pollutants solution pH should be higher than pzc of adsorbent, whereas, for anionic solution pH should be less than pzc 35. MB dye is a cationic dye, therefore, a comparatively higher adsorption was expected in the solutions having pH > pzc of adsorbent i.e. Ti-Si-S-25 nanohybrid. In the strongly acidic solutions i.e. dye solutions having pH values lesser than pzc, adsorption efficiency was less, whereas, with increased solution pH the adsorption efficiency also increased (Figure 4(c)). Furthermore, in strongly acidic solutions, a higher concentration of H+ ions was present which also competed with cationic dye molecules for the adsorption sites of Ti-Si-S-25 nanohybrid and reduced overall adsorption efficiency, whereas, with increasing solution pH from acidic to alkaline, the concentration of H+ ions in the solution decreased progressively, therefore, more adsorption sites were available for the dye molecules to adsorb and value of adsorption efficiency increased. Maximum performance was observed in MB dye solution having neutral pH, therefore, rest of the adsorption studies were conducted at this pH. Influence of the presence other cations on performance. As MB is a cationic dye and if the adsorption involves electrostatic interaction between dye molecules and adsorption sites having opposite charges then the presence of other cations will definitely influence the adsorption efficiency or overall performance of the adsorbent i.e. nanohybrid material. Therefore, to study the effect of various cations i.e. Na+ and Ca+2 ions, dye solutions were prepared in NaCl and CaCl2 salt solutions of various ionic strength of cations (0.1 to 0.6 M) (Figure 4d). Adsorption efficiency decreased progressively as the ionic strength of cation increased in respective salt solutions, which confirmed the presence of electrostatic interactions between dye molecules and Ti-Si-S-25 nanohybrid material particles. Moreover, the adsorption efficiency decreased to a larger extent in the solutions of Ca+2 ions as compared to the solutions of Na+ ions of same ionic strength and this might be due to the higher cationic charge of Ca+2 ions which occupied more adsorption sites as compared to Na+ ions.


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Figure 4(a-d): (a) Influence of variation in mass of on the adsorption efficiency having initial concentration of dye = 100 mg.L-1, dye solution volume = 50 mL; (b) plot for the calculation of pzc of Ti-Si-S-25 nanohybrid material; (c) effect of solution pH and (d) ionic strength of cations where adsorbent dose = 0.4 g.L-1 and dye volume = 50 mL. Adsorption isotherm. MB dye solutions having concentration 100-1000 mg.L-1 were used to study the adsorption isotherm at different temperatures. Initially, it was observed that with increasing temperature the performance of hybrid composite improved and a higher value of adsorption capacity was obtained. The adsorption isotherm data was fitted to non-linear equations of six two-parameter and eight three-parameter isotherm models (Figures 5(a-c)). Different twoparameter isotherm models applied to experimental isotherm data were Jovanovich, DubininKaganer-Radushkevich (DKR), Temkin, Freundlich, Hasley and Langmuir isotherm models 36, 37 at 298.15 K (Figure 5(a)), 308.15 K (Figure 5(b)) and 318.15 K (Figure 5(c)). The mathematical equations of all the isotherm models applied in this study are tabulated in Table S1, Supporting Information , whereas, values of various adsorption isotherm parameters obtained by different 13 ACS Paragon Plus Environment


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isotherm models are compiled under Table 1. Applicability of most suitable isotherm model to the experimental data was decided by maximum value of R2 i.e. correlation coefficient and least values of error factors such as akaike information criterion (AIC) and bayesian information criterion (BIC). Based on the values of these adsorption parameters, adsorption isotherm for the adsorption of MB onto Ti-Si-S-25 nanohybrid material can be most correctly defined by homogeneous monolayer Langmuir adsorption isotherm according to which all the adsorption sites of Ti-Si-S25 nanohybrid material are energetically identical and dye molecules have equal affinity for all the adsorption sites. The adsorption capacities at three temperatures were found to be 804.80 (298.15K), 830.01 (308.15K) and 864.58 mg.g-1 (318.15 K). Furthermore, value of RL i.e. separation factor at all the three temperatures between 0 and 1 also confirmed applicability of Langmuir isotherm 38. In Temkin model, higher values of β at increased temperature also supported an increase in adsorption capacities with increasing temperature and also suggested the evenly distribution of binding energies on the surface of Ti-Si-S-25 nanohybrid material

39.

In case of

Freundlich isotherm, the calculated value of 1/n was less than unity which predicted that the synthesized adsorbent i.e. Ti-Si-S-25 nanohybrid material was applicable over the studied dye concentration range

32.

Moreover, the value of equilibrium capacity calculated using Langmuir

isotherm was different from the value of adsorption capacities calculated using the DKR model, which was due to the difference in the assumptions considered while formulating different isotherm models 39. According the apparent energy (E) value determined from DKR model, the process of the adsorption of MB onto Ti-Si-S-25 nanohybrid material is chemisorption process (Table 1) 39. Experimental adsorption data was further fitted to the non-linear forms of eight threeparameter isotherm models namely Brouers-Sotolongo, Vieth-Sladek, Koble-Corrigen, Hill, Toth, Sips, Radke-Prausnitz and Redlich-Peterson isotherms models at 298.15 K (Figure 6(a), 308.15 K (Figure 6(b)) and 318.15 K (Figure 6(c)). Values of all the adsorption parameters and error factors calculated using these three-parameter isotherm models are given in Table S2, Supporting Information. Hill and Koble-Corrigen isotherm models were fitted best to experimental data among all three-parameter isotherm models studied for the adsorption of MB onto the Ti-Si-S-25 nanohybrid material, which also predicted the applicability of monolayer Langmuir adsorption isotherm.


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Figure 5(a-c): Plots for the fitting of experimental adsorption isotherm data to two-parameter isotherm models for the adsorption of MB onto Ti-Si-S-25 nanohybrid material at (a) 298.15 K; (b) 308.15 K and (c) 318.15 K where solution pH =7.0, adsorbent dose = 0.4 g.L-1 dye solution volume = 50 mL. Table 1: Different isotherm parameters determined for the adsorption MB onto Ti-Si-S-25 nanohybrid material. Isotherm models

Parameters (mg.g-1)

Langmuir

qm KL (L.mg-1) RL Reduced χ2 RSS R2

Temperature (K) 298.15 308.15 804.80 830.01 0.046 0.052 0.03-0.178 0.026-0.161 36.99 12.13 184.96 60.695 0.999 0.999 15

ACS Paragon Plus Environment

318.15 864.58 0.056 0.024-0.151 30.021 150.106 0.999


Freundlich

Temkin

DKR

Jovanovic

Halsey

AIC BIC KF (L.mg-1)1/n(mg.g-1) 1/n Reduced χ2 RSS R2 AIC BIC β (mg.g-1) KT (L.mg-1) Reduced χ2 RSS R2 AIC BIC qm (mg.g-1) KDR (mol2.J-2) Es (kJ.mol-1) Reduced χ2 RSS R2 AIC BIC qm (mg.g-1) KJ (L.mg-1) Reduced χ2 RSS R2 AIC BIC KH (mg.g-1) n Reduced χ2 RSS R2 AIC BIC

36.91 26.75 198.99 0.204 4941.11 2.47x104 0.875 71.18 63.01 17.72 0.816 1870.05 9.3x103 0.952 64.38 56.22 689.19 1.9x10-5 12.25 8531.91 4.2x104 0.785 75.00 66.84 730.02 0.033 1274.81 6374.07 0.967 61.69 53.53 5.1x109 4.22 4941.11 2.4x104 0.875 71.18 63.01


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29.11 20.95 213.73 0.232 5577.92 2.78x104 0.872 72.63 63.86 18.41 0.948 2162.18 10.8x103 0.950 75.09 66.93 716.95 1.6x10-5 13.62 8641.18 4.3x104 0.802 65.39 57.23 755.72 0.038 1346.43 6732.17 0.969 62.08 53.91 1.06x1010 4.30 5577.92 2.7x104 0.872 72.037 63.86

35.45 27.29 227.02 0.228 6582.61 3.29x104 0.866 73.19 65.02 18.98 1.030 2624.66 13.1x103 0.946 66.75 58.59 749.54 1.4x10-5 14.85 9185.96 4.5x104 0.813 75.52 67.36 788.46 0.041 1383.34 6916.72 0.971 62.27 54.10 1.57x1010 4.32 6582.61 3.2x104 0.866 73.19 65.02

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Figure 6(a-c): Plots for the fitting of experimental adsorption isotherm data to three-parameter isotherm models for the adsorption of MB onto Ti-Si-S-25 nanohybrid material at (a) 298.15 K; (b) 308.15 K and (c) 318.15 K where solution pH =7.0, adsorbent dose = 0.4 g.L-1 dye solution volume = 50 mL. As compared to some other reported adsorbents, the adsorption capacity of Ti-Si-S-25 nanohybrid material was very good and it can be successfully used in large-scale industrial applications for cationic dyes removal (Table 2) 40-48.



Page 18 of 31

Table 2: Comparison of the maximum adsorption capacity of Ti-Si-S-25 nanohybrid material with some other reported adsorbents for the adsorption of MB. Adsorbent Montmorillonite clay h-XG/SiO2 nanocomposite CMT-g-PAM/silica nanocomposite Coir pith Polymer containing -cyclodextrin and carboxyl Activated carbon Bamboo based activated carbon Gg-cl-P(AAm-co-MAA) hydrogel polymer Amphoteric straw-based adsorbent Ti-Si-S-25 nanohybrid adsorbent

Adsorption capacity 289.12 497.5 43.86 120.43 64 315 454.20 694.44 138 804.40

Reference 40 41 42 43 44 45 46 47 48

This study

Adsorption thermodynamics. The increased adsorption capacity of Ti-Si-S-25 nanohybrid material with increasing temperature (Table 1) suggested that the adsorption of MB onto Ti-Si-S25 nanohybrid material was endothermic in nature. To validate this further, the adsorption thermodynamics of the adsorption of MB onto Ti-Si-S-25 nanohybrid material was studied and different parameters related to adsorption thermodynamics were calculated as per the following expressions: ∆𝑆° ―∆𝐻° 𝑙𝑛𝐾𝐿 = + 𝑅 𝑅𝑇

(5)

∆𝐺° = ― 𝑅𝑇𝑙𝑛𝐾𝐿 𝐾𝐿 = 𝑚

(6)

𝑞𝑒

(7)

𝐶𝑒

where, R and t are gas constant and temperature, respectively, whereas KL is distribution coefficient. Values of ΔG°, ΔS° and ΔH° were calculated using plot of lnKL vs 1/t (Figure S3, Supporting Information). The spontaneity of adsorption process was predicted by negative value of ΔG°, whereas positive values of ΔH° and ΔS° also supported the increase in qe,max at elevated temperatures along with the increased disorder and randomness at solid-liquid interface, respectively (Table 3).


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Table 3: Different thermodynamic parameters determined to adsorb MB using Ti-Si-S-25 nanohybrid material. ΔG° (kJ.mol-1) 298.15K

308.15K

318.15K

-1.558

-1.642

-1.723

ΔH° (kJ.mol-1)

ΔS° (kJ.mol-1.K-1)

8.911

0.0821

The maximum adsorption capacity of Ti-Si-S-25 nanohybrid material for MB removal was further compared with the elemental sulfur and TiO2-SiO2 using the Langmuir adsorption isotherm at 298.15K and the results are compiled in Figure S4, Supporting Information. It was observed that the maximum adsorption capacity in case of elemental sulfur was found to be only 11.09 mg.g-1, however for TiO2-SiO2 it was found to be 595.16 mg.g-1 (Table S3, Supporting Information ), which is much lesser than the Ti-Si-S-25 nanohybrid material (804.80 mg.g-1). These results proved that the dyes adsorption capacity of the material improved considerably after incorporation of sulfur. This could be observed due to the homogeneous distribution of sulfur, which forms different type of strong physical interaction with cationic dye. Different type of interactions (Hbond, coordinate bond, ionic bond) between advanced Ti-Si-S-25 nanohybrid materials and cationic dye (Methylene Blue) are presented in Figure S5, Supporting Information. Adsorption kinetics. MB dye solutions of different concentration i.e. 50, 100 and 150 mg.L1

were used for kinetics studies. The adsorption kinetics for all the three concentrations studied

followed almost the similar trend (Figure 7(a)). In the starting, the adsorption rate was quite high which decreased progressively while reaching towards equilibrium. This comparatively higher adsorption rate in the starting of adsorption was because of the availability of a higher concentration of adsorption sites which continuously reduced while progressing towards equilibrium, therefore, the adsorption rate kept on decreasing. With increasing concentration of dye solution the time to reach adsorption equilibrium also increased i.e. in 50 mg.L-1 concentration dye solution adsorption equilibrium reached fastest followed by the solution of 100 and 150 mg.L1,

respectively. This adsorption behavior was because of the presence of stronger adsorption

driving forces in lower concentration dye solutions as compared to higher concentration solutions 33.



The experimental adsorption kinetics data was fitted to non-linear forms of different adsorption kinetics models namely pseudo-first-order (Figure 7(b)), pseudo-second-order (Figure 7(c)) and Elovich (Figure 7(d)) 39, 49. Mathematical expressions of these models are compiled in Table S4, Supporting Information. On the basis of the values various adsorption kinetics parameters and error factors, pseudo-second-order rate equation fitted most suitably to the adsorption kinetics for the adsorption of MB using Ti-Si-S-25 nanohybrid material (Table 4). Applicability of pseudo-second-order rate equation also predicted that the adsorption of MB in this case was chemisorption 50.

Figure 7(a-d): The plots of (a) adsorption kinetics; (b) pseudo-first-order; (c) pseudo-second-order and (d) Elovich models for the adsorption of MB onto the Ti-Si-S-25 nanohybrid material where solution pH =7.0, adsorbent dose = 0.4 g.L-1 dye solution volume = 50 mL.


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Table 4: Different adsorption kinetics parameters derived using kinetics models for the adsorption of MB onto Ti-Si-S-25 nanohybrid material. Models

Parameters (min-1)

Pseudo-first-order

Pseudo-second-order

Elovich

Intraparticle diffusion

Liquid film diffusion

k1 qe,cal (mg.g-1) Reduced χ2 RSS R2 AIC BIC K2 (g.mg-1.min-1) qe,cal (mg.g-1) Reduced χ2 RSS R2 AIC BIC B (g.mg-1) kE (mg.g-1.min) Reduced χ2 RSS R2 AIC BIC kID (S1) (mg.g-1.min1/2) C(S1) (mg.g-1) RSS R2 (S1) kID (S2) (mg.g-1.min1/2) C(S2) (mg.g-1) RSS R2 (S2) kFD (min-1) C1 RSS R2

Dye concentration (mg.L-1) 50 100 0.170 0.06 120.53 243.65 14.20 46.27 113.66 647.91 0.985 0.982 34.30 67.21 31.21 67.53 1.57x10-3 2.74x10-4 138.41 282.65 1.427 6.40 11.41 89.64 0.999 0.999 11.32 35.57 8.23 35.89 3.71x10-2 1.77x10-2 76.92 54.61 27.98 42.11 223.87 1009.54 0.984 0.988 41.08 74.31 37.99 74.63 28.38 39.95

150 0.037 367.51 65.69 1051.09 0.985 80.92 81.88 9.41x10-5 443.58 12.55 200.90 0.999 51.13 52.09 1.129x10-2 48.12 187.28 2996.62 0.987 99.78 100.73 47.37

2.09 100.76 0.982 4.14

-8.26 332.55 0.986 17.13

-17.12 745.08 0.988 13.85

96.55 6.37 0.894 0.124

11.81 359.59 0.843 0.053

214.79 214.79 0.911 0.040

6.6x10-2 0.543 0.973

8.14x10-3 0.493 0.988

-0.154 2.29 0.951

Diffusion of dye molecules to the internal structure of adsorbent passes via different stages i.e. initial passage of dye molecules to outer surface of adsorbent followed by transport to internal surface and finally transportation to the adsorption sites

39.

Therefore, to determine the actual



mechanism controlling the overall dye diffusion, the adsorption kinetics data was further fitted to the linear forms of intraparticle diffusion (Figure 8(a)) and liquid film diffusion models (Figure 8(b)). If intraparticle diffusion mechanism controls the overall dye diffusion mechanism then its plot should pass through the origin and it should not possess any slope 51. But in this case, both the conditions were not fulfilled and the plot of intraparticle diffusion model was found to have two regions S1 and S2 (Figure 8(a)) suggesting the possibility of the contribution of some other diffusion mechanisms. Furthermore, it was also observed that region S1 had higher value of slope as compared to region S2 (Table 4) which also suggested much higher transportation rate for relocating dye molecules from bulk solution to boundary layer as compared to relocating dye molecules from boundary layer to internal surface. To check the possible contribution of other diffusion mechanisms, the adsorption kinetics data was further fitted to the liquid film diffusion model (Figure 8(b)). Plot of this model also did not passed through the origin as well as it was also found to have slope. Therefore, the dye diffusion mechanism was also not solely controlled by film diffusion mechanism but it was a combination of both intra-particle diffusion as well as liquid film diffusion mechanism 38. Desorption and reusability. Desorption of already adsorbed MB dye particles onto Ti-SiS-25 nanohybrid material and the reusability of adsorbent was studied for consecutive six cycles of adsorption-desorption (Figure S6, Supporting Information). The adsorption capacity remained almost same for all the six cycles whereas, the desorption capacity decreased a little bit which was even less than 5 % after six cycles. However, in first four cycles the desorption capacity remained almost constant but it reduced little bit in the last two cycles. Therefore, from these studies it can be concluded that Ti-Si-S-25 nanohybrid material can be repeatedly used again and again to adsorb cationic dyes from aqueous solution. This analysis was again supported by TGA experiment. TGA data showed that 21.1 % sulfur still present after adsorption study. (Reported in Figure S7, Supporting Information).


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Figure 8(a-b): Plots of (a) intraparticle and (b) liquid film diffusion models where solution pH =7.0, adsorbent dose = 0.4 g.L-1 dye solution volume = 50 mL. CONCLUSIONS The hydrothermal and sol-gel methods were used to obtain ordered Ti-Si-S-25 nanohybrid material of high purity in nanometer scale. Nanostructured Ti-Si-S-25 nanohybrid material was structurally characterized by XRD, XPS and Raman spectroscopy techniques. SEM, TEM and BET nitrogen adsorption analysis studied the morphological characteristics and specific surface area of hybrid materials. Furthermore, Ti-Si-S-25 nanohybrid material was successfully utilized to adsorb MB from dye-contaminated wastewater. The adsorption of MB using Ti-Si-S-25 nanohybrid material followed Langmuir isotherm predicting that the adsorption of MB was homogeneous monolayer adsorption with qe,max value of 804.80 mg.g-1, whereas, among three-parameter isotherm models it followed Hill and Koble-Corrigen isotherm models. The experimental adsorption kinetics data most suitably fitted to pseudo-second-order rate equation. Furthermore, results of thermodynamics studies confirmed spontaneity and endothermic nature of adsorption process. Adsorptiondesorption studied suggested that Ti-Si-S-25 nanohybrid material can be repetitively used six cycles without any considerable decrease in adsorption capacity. Therefore, from these studies it can be suggested that Ti-Si-S-25 nanohybrid composite can be used as regenerable, cost-effective and potential adsorbent for the remediation of synthetic dyes contaminated wastewater.



Supporting Information. Sol-Gel and Hydrothermal synthesis steps of hierarchically organized and ordered TiO2-SiO2-sulfur (Ti-Si-S) nanohybrid material, XRD diffraction patterns and Raman spectra of TiO2 , SiO2 , sulfur and Ti-Si-S-25 nanohybrid material , BET surface area analysis and pore size distribution of TiO2-SiO2 and TiO2-SiO2-S (Ti-Si-S-25) nanohybrid materials, Van’t Hoff Plot for the adsorption of MB onto the Ti-Si-S-25 nanohybrid material, Langmuir plots for the MB adsorption using elemental sulfur and TiO2-SiO2, different type of interactions mechanism, Adsorption-desorption cycles and TGA curves of sulfur, TiO2-SiO2, Ti-Si-S-25 and Ti-Si-S25(after adsorption). Conflicts of interests. There are no conflicts to declare. AUTHOR INFORMATION Corresponding author: Dr. Saeed M. Alhassan, Associate Professor Department of Chemical Engineering, Director of Gas Research Center (GRC) Telephone: +97126075944, Fax: +97126075200, Email: [email protected] ACKNOWLEDGMENTS This work is funded by the Gas Subcommittee of Abu Dhabi National Oil Company (ADNOC) Research & Development. We would like to thank Mr. Samuel Stephen, Tharalekshmy Anjana, and Abeer Ali Nasser Al Yafeai for assistance with material characterization. REFERENCES 1. Forgacs, E.; Cserháti, T.; Oros, G., Removal of Synthetic Dyes from Wastewaters: a review. Environment International 2004, 30, (7), 953-971. 2. Iqbal, M. J.; Ashiq, M. N., Adsorption of Dyes from Aqueous Solutions on Activated Charcoal. Journal of Hazardous Materials 2007, 139, (1), 57-66. 3. Kausar, A.; Iqbal, M.; Javed, A.; Aftab, K.; Nazli, Z.-i.-H.; Bhatti, H. N.; Nouren, S., Dyes Adsorption using Clay and Modified Clay: A review. Journal of Molecular Liquids 2018, 256, 395-407. 4. Aguiar, J. E.; Cecilia, J. A.; Tavares, P. A. S.; Azevedo, D. C. S.; Castellón, E. R.; Lucena, S. M. P.; Silva, I. J., Adsorption Study of Reactive Dyes onto Porous Clay Heterostructures. Applied Clay Science 2017, 135, 35-44.


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