Synthesis of nanoadsorbent entailed mesoporous organosilica for decontamination of methylene blue and methyl orange from water

ABSTRACT Water, being a universal solvent, is contaminated by highly toxic pollutants, which is alarming for life on this planet. However, eco-friendly human efforts are constantly trying to overcome this problem. In continuation of these efforts, the present study reported the synthesis of template-supported nanoadsorbents consisting of mesoporous organosilica (MPOS) through poly-condensation of organic (3-methacryloxypropyltrimethoxysilane) and inorganic (sodium silicate) silicon precursors. The nanoadsorbent with suitable textural characteristics, heat-resistant competency and highly porous nature provided a good platform for achieving an excellent performance to remove methylene blue (MB) and methyl orange (MO) from water. Due to easy handling and efficient adsorption capacity, the MPOS nanoadsorbent shows 93.1% and 66.7% adsorptions of MB and MO, respectively. The intra-particle diffusion kinetic model of synthesised materials explained high surface interactions of adsorbate on active sites of adsorbent, which also support possible chemisorption. The MPOS exhibited adsorption capacities of 57.58 mg/g for MB and 56.62 mg/g for MO by using the multilayered sorption mechanism. Reusability of MPOS is also checked up to file cycles and the results indicate the minute decline in the adsorption proficiency of regenerated MPOS. An eco-friendly synthetic approach, efficient exclusion of MB and MO and better regain results make these MPOS a competent nanoadsorbent for water decontamination.


Introduction
Industrial and domestic needs for different chemicals increased the environmental water pollution, which spoilt the water ecosystem [1,2].Organic dyes, which are the main source of water pollution, are widely used in different industries [3,4].Azo organic dyes are largely utilised in the textile industry for dyeing silk, cotton, nylon and wool [5,6] and their excess amounts are directly released in water reservoirs and cause water pollution.The use of such contaminated water triggered numerous diseases in humans and other living organisms [7][8][9].Moreover, azo dyes show more stability in environment, which made their exclusion necessary, but it becomes hard by using the orthodox adsorption method, separation by oxidation and microbial degradation and photodegradation [10][11][12].However, the economical and simple operational approach for elimination of dyes made batch adsorption more superior [13].
Although, various kinds of adsorbent have been synthesised for the removal of azo dyes [14,15], mesoporous silicas are found to be more effective adsorbents [16].After discovery of M-41S1 and FSM-16, new prospects have been unlocked for preparation of proficient mesoporous materials [17,18].
Subsequently, synthesis of well-ordered mesoporous organosilicates (MPOSs) having organic-inorganic linkages [19] developed new features in catalyst and adsorbent surfaces [18] in which organic moieties positioned between two silicon atoms (Si-R-Si).Furthermore, multi-functional MPOS can be prepared by utilising more than one organosilane precursors on templates by hydrolysis and polycondensation reactions, which causes further improvements in the surface properties and modifications in mechanical and optical characteristics [20] of the materials.The MPOS structure, pore size, pore volume and wall thickness can be precisely organised by controlling the nature of surface-directing agents (SDA), molar ratio of silicon precursors, choice of solvents and process temperature [21].The silica precursor has many advantages over other silicon materials for the preparation of MPOS like its high transparency with low optical loss than zirconia or titanium [22], higher thermal resistance [23], easy attachment of organic inorganic hybrids [24] and more stability of inorganic moieties with an organic linkage network [25].
The present research comprises preparation of low-cost nanoadsorbent having mesoporous organosilicates (MPOSs) using an efficient available natural precursors such as sodium silicate, for their adsorption study.The organic moiety (R) present between two silicon atoms may gain uniform repetition in the whole structure by using SDA, which enhances the reactive nature of synthesised MPOS.A large number of surfactants such as cetyltrimethylammonium bromide (CTAB), triblock copolymers and non-ionic surfactants can be used as SDAs to prepare highly porous materials by the sol-gel strategy.A proposed interaction between SDA and precursor molecules is shown in Scheme 1, which demonstrated that hydrophobic and hydrophilic ends of SDA regulate the size of mesoporosity, whilst the hydrophilic chains of organosilane precursors may cause the development of microporosity in the synthesised MPOS [21].
The periodicity in the pore size and tuneable wall thickness can be achieved by controlling the temperature, stirring rate, pH and precursor's concentration and by using appropriate SDA, which enables us to formulate a 2D hexagonal or cubic framework in synthesised MPOS materials [26].The resulting MPOS can be used as an effective adsorbent for water purification.Azo pollutants such as MB and MO causing environmental problems force to research for environmental protection [27].The prepared MPOSs due to their economical synthesis, structural tunability and large surface area made them as efficient analytical tools for the decontamination of MB (93.1%) and MO (66.7%) from water within a short contact time of 48 minutes.

Synthesis of MPOS
The dynamic and cost-effective approach provided more importance to the current work for synthesis of MPOS with small discrepancies in reported methods (Scheme 1) [19,28].In a typical experiment, aqueous solution (120 g) of HCl (0.65 g) and Tween-80 (0.7 g) templates was prepared with constant stirring at room temperature shown in Scheme 1.Soon after, sodium silicate (0.9 g) and 3-methacryloxypropyltrimethoxysilane (1.7 g) were introduced in mixture solution.Stirring time of 2 hours at 45°C progressed hydrolysis and co-condensation.Later on, the temperature was increased up to 85°C for 24 hours for ageing to prepare methyacryl-based MPOS.Ethanol (380 g) was added at 56°C with constant stirring of 5 hours, for the removal of surfactant from the assynthesised product.Solid MPOSs were collected through filtration and washed with ethanol and deionised water.the obtained solid product was desiccated at 100°C for 24 hours.Scheme 1. Schematic illustration of mixed bridged MPOS synthesis.

Characterisation
A Bruker D8 Advanced X-ray diffractometer was used to study the amorphous nature of MPOS.An Alpha Bruker Fourier-transformed infrared spectrometer revealed the presence of functional groups in the MPOS framework.A Quantachrome NovaWin -Data Acquisition and Reduction version 11.04 instrument monitored the N 2 adsorptiondesorption isotherm, Whilst a PerkinElmer thermal analyser was used to examine thermal behaviour of synthesised materials.Cary Eclipse (MY 18060003) Photo-luminescence and CeCil CE-7400 ultraviolet visible spectrophotometers were used for absorption studies and a Lab-companion SK-300 Shaker was used for well interaction between adsorbent and adsorbate.Moreover, a JSM-IT-100 scanning electron microscope and a Hitachi transmission electron microscope (TEM) system enlightened the external and internal surface studies of MPOS.

Adsorption experiment
MB and MO adsorptions on MPOS nanoadsorbent were observed and repeated five times with continuous shaking and scanning of all the mixtures under a UV-vis spectrophotometer under constant experimental conditions as shown in Table S2.The adsorption competences of MPOS were monitored at an 8-minute time interval, which showed removal efficiencies (% R) and adsorption capacities of MPOS by considering Equations ( 1) and ( 2), where R is the removal efficiency, C e and C i are the equilibrium and initial concentration (mg/L) of azo dyes, V is the volume of dyes solution and m is the adsorbent amount (g).
For the adsorption kinetic study, specific amounts of MB and MO samples were acquired at a regular 8-minute time interval and examined using a UV-visible spectrophotometer (Figure 3(a,b)).Various adsorption kinetic plots were used for the kinetic study as shown in Figure 4 and calculated linear correlation coefficient constant (R 2 ) values are used to find the best fitted kinetic model for explaining the adsorption of MB and MO on MPOS adsorbent.The mathematical forms of these models are given in equation ( 3) and (4) to represent the pseudo-first and second order reaction [29,30], whilst equation ( 5) and ( 6) are used to obtain liquid-film diffusion and intra-particle diffusion [31,32], where T is the adsorption time in minutes and Q t and Q e (mg/g) are the adsorption capacities at equilibrium.K 1 (h −1 ) and K 2 (g mg −1 h −1 ) are the rate constants for pseudofirst order and pseudo-second order adsorption, respectively, whilst K f (h −1 ) is the adsorption rate constant for liquid-film diffusion.

Characterisation
Sol-gel synthesis converted sodium silicate into silica, which provided silica linkages, whilst alkoxy groups (-OR) of organic Si-precursor were converted into silanol groups by hydrolysis.Moreover, the double bond in the methacrylate moiety provided positions for organosilica linkages to produce mixed bridging MPOS by co-condensation [21], as shown in Figure S5.XRD spectra enlightened the structural study of synthesised organosilica-based material.The wide angle x-ray diffractometer showed the developing pattern, signifying 2D hexagonal nature of the MPOS [33] (Figure 1(a)) and the incorporation of the organic moiety in synthesised material [22].Moreover, the MPOS average particle size was measured by using Scherer's equation, D = Kλ /βCosϴ, and was found to be 2.31 nm.In this equation, D is the average size of crystallite, K = 0.9, λ = 1.54056 °A, β is the full width at half maximum (FWHM) and ϴ corresponds to the diffraction angle of the peaks [34].The significant FTIR absorption peaks at 690, 780, 1103, 1295, 1460, 1721 and 2940 cm −1 further confirmed the successful integration of the organic moiety in MPOS (Figure 1(b).The interactions -Si -C and -Si -OCH 3 showed absorption peaks at 690 cm −1 and 780 cm −1 , respectively [35].However, C-H revealed stretching and bending at 2940 cm −1 and 1460 cm −1 , respectively [22,36].Furthermore, Si-CH 2 bending [16] displayed the positive presence at 1295 cm −1 , whilst C -O -C and Si -O -Si linkages showed stretching vibrations at 1721 cm −1 and 1103 cm −1 , respectively [37], indicating efficacious integration of mixed bridging in MPOS, which highlighted the effectiveness of the co-condensation method [38] and made the present work more successful.
The thermal response of MPOS was observed by TGA (Figure 1(c) and the thermogram was obtained at 30 °C to 1000 °C by keeping the heating rate and flow rate of N 2 gas at 20 ° C/min and 20 ml/min, respectively.At 200 °C, MPOS showed 7% mass reduction due to evaporation of water and physisorbed solvents.Afterwards, chemisorbed water and organic solvents were removed at 350 °C [36,39] and showed 18% weight loss in the thermogram, which revealed the presence of varying ratios of organic moieties in and onto the surfaces of synthesised MPOS.
Optical behaviour of synthesised MPOS was monitored by photoluminescence (Pl) and UV-visible spectroscopy.MPOSs were found to be active for fluorescence at 756 nm (Figure 1(d)) and showed very small absorbance at 660 nm with the band gap at ~3.76 eV (Figure 3(c)) showing charge transfer [40] and chemisorbed points for adsorbents.Nitrogen adsorption-desorption results are depicted in Fig. S1 and S2.
The pore size of MPOS was calculated using the BJH adsorption method.The surface characteristics such as mesopore diameter, pore volume and BET-surface area of assynthesised MPOS (Table S1) were found to be 32.616Å, 0.027 cm 3 /g and 241.287 m 2 / g, respectively.
SEM images (Figure 2(a,b)) demonstrated 2D-hexagonal shapes of particles [21], which largely depend on the synthesis approach, types of precursors used and proceeding reaction conditions.Additionally, sponge-shaped particles were also observed, indicating the presence of unreacted monodispersed particles [22].Moreover, TEM images (Figure 2(c,d)) showed organised stacking of spheres, leading to ladder-shaped hexagonal symmetry comprising well-ordered porosity (inset to Figure 2(c)).

Inspecting chemisorption point for adsorbates
The possible availability of electrons during chemisorption of adsorbates was investigated by photoluminescence.The experiment was performed by using 6 mL of 0.01 M NaOH in which 3 mM of terephthalic acid (TA) and 0.1 M KCl (3 mL) were also added.
Another comparable mixture of the same composition was prepared and 3 mg of adsorbent was added in the mixture.Both samples were kept in a 2 × 2 x 2.5 ft 3 box fitted with a visible light source (Tungsten bulb (200 W)) and irradiated for 20 minutes with constant stirring.Photoluminescence spectra were recorded (Figure 3(d)) and studied.Methyacryl adsorbent produced hydroxyl free radicals, which chemically converted terephthalic acid (TA) into hydroxyl terephthalic acid (TAOH), which showed more fluorescence [41] than TA.These effective results enlightened the possibility of chemisorbed adsorbates due to the presence of pi-electrons in methyacryl-adsorbent during the adsorption study.

Effect of pH on adsorbent efficiency
Different pH dye solutions were used to find the best conditions for MPOS adsorbent to remove MB and MO.The adsorption capacity of MB on the adsorbent surface was increased as the pH increased from 3 to 9 as shown in Fig. S7.The results favoured to basic solution for the removal of MB by using MPOS adsorbent as below pH 3, with MPOS approaching zero charge state, which reduced the chemisorption removal of MB.But, in the case of MO removal, the adsorption capacity of MPOS was decreased upon increasing pH from 3 to 9 as shown in Fig. S8 as above pH 9 and MPOS may lose their surface charge, which diminished the binding of adsorbate on the adsorbent surface and thus reduced the adsorptive removal of MO, which indicates that MPOS adsorbent will show higher adsorption competency for MO under acidic conditions.

Adsorption kinetic study
Initially, the change in the contact time was observed, which encouraged significant adsorption of MB and MO on synthesised MPOS.The literature study showed the decrease in adsorption efficiency by adding a surface directing surfactant (SDS), but the improved adsorption competency in the current research work may be enlightened on the use of non-ionic Tween-80 instead of ionic SDS [22,42].In Table 1, the comparative analysis of different adsorbents demonstrated that functionalised MPOS showed better adsorptions of MB and MO by providing sufficient binding sites.
Kinetic study models (Figure 4(b-d) and Fig. S4)) précised in Table 2 explained pseudofirst order, pseudo-second order, intra-particle diffusion and liquid film diffusion kinetics for adsorption of MB and MO on MPOS adsorbent.R 2 ≈ 1 values indicate the homogenous surface of MPOS for multilayer adsorptions [22,42,43] of MB and MO.
The linear correlation coefficient (R 2 ) values for these models indicated the best-fitted intraparticle diffusion model to explain adsorption of both dyes on adsorbent due to a greater value of R 2 (for MB R 2 = 0.99, for MO R 2 = 0.943) than other models (Table 2).The adsorption rate was explained by K d and controlled by intra-particle diffusion, whereas macropore and mesopore diffusions were also demonstrated by the straight line of Figure 4(d).Furthermore, the intra-particle diffusion kinetic model of synthesised materials explained high surface interactions of adsorbate on active sites of adsorbent, which also support possible chemisorption (Figure 3(d)).The adsorption process was largely controlled by the surface reaction due to excited molecules and radicals and less dependent on temperature [44].

Effect of the contact time
The increase in the contact time increased the adsorption of MB and MO on adsorbent (Figures 3(a,b)), 4(a) and Fig. S3)).At the start of contact time, the adsorbent has more active sites and showed high adsorption capacity and diffusion rates.Although the size of cationic MB (1.7 × 0.76 × 0.33 nm) is larger than that of anionic MO (1.31 × 0.55 × 0.18 nm), the adsorption competency of MB is higher than that of MO.This is possible due to the presence of a large number of negative active sites on the MPOS surfaces as expected in Figure 5 [52].Results (Fig. S3) claimed 93.1% and 66.7% diffusion rates of MB and MO, respectively.The MPOS adsorbent showed enhanced efficiencies than reported adsorbents as shown in Table 1 and revealed a higher adsorption rate for MB due to strong interactions.

Proposed mechanism of adsorption
The synthesised MPOS showed a BET-surface area of 241.287 m 2 /g (Table S1) and thus provides a large number of active sites for the sorption of low-molecular weight azo dyes [53].The linear correlation coefficient (R 2 ) values for MB R 2 = 0.99 and for MO R 2 = 0.943 indicated the best fitted intra-particle diffusion model to explain adsorption of both dyes on adsorbent.The adsorption of MB and MO was also controlled by hydrogen bonding, π-π interaction and electrostatic connections [54].Moreover, the N-and O-containing functional groups present in MB and MO established hydrogen bonding with hydroxyl groups of MPOS adsorbent, whilst possible pi-electrons of the methyacryl moiety in MPOS may interact with the conjugated system of dyes and increases the adsorption capacity of adsorbent.Besides, weak electrostatic interaction may be established between completely positive and negative sites of dyes with opposite active sites of MPOS and may cause enhancement of dyes molecules trapping in adsorbent.

Reusability of MPOS nanoadsorbent
The reusability of the MPOSs makes them ideal adsorbent in addition to their greater adsorption capacity and high diffusion rates.Adsorbent was collected by centrifuging the sample solutions after each adsorption and washed with ethanol and distilled water at room temperature.Regenerated MPOS was reused according to the adsorption protocol and percentage removal of dyes was recorded by repeating five cycles.Even after the fifth cycle, the adsorption capacity of MPOS decreased to a small extent (Figure 6) as shown in Table S3.

Conclusion
In the present work, MPOS nanoadsorbent was successfully synthesised by the wetchemical method for the adsorption exclusion of MB and MO.This method directed cocondensation of silicon precursors to form well-organised MPOS, which showed high adsorption competency for MB and MO.The synthesised MPOS exhibited 93.1% and 66.7% adsorptions of MB and MO, respectively, within a contact time of 48 minutes, which proved it proficient adsorbent for the decontamination of MB and MO from water.Moreover, synthesised nanoadsorbent entailed MPOS proficiently regenerated and was used in five adsorption cycles of MB and MO.A small decrease in adsorption competency indicated durability of the adsorbent for a long-time study.These promising results make synthesised nanoadsorbent an impressive source for the removal of dye pollutants from water.

Figure 2 .
Figure 2. (a and b) SEM images of synthesised MPOS and (c and d) TEM images of synthesised MPOS.

Figure 3 .
Figure 3. (a) Effect of time on absorption for removal of MB, (b) effect of time on absorption for removal of MO, (c) UV absorption with the Tauc plot of synthesised MPOS and (d) photoluminescence (Pl) of MPOS.

Figure 4 .
Figure 4. (a) Adsorption performance of MPOS for MB and MO and fitting results of (b) pseudo-first order, (c) pseudo-second order and (d) intra-particle diffusion kinetic model.

K 1 =
Pseudo-first order constant, Q e = Adsorption capacity near equilibrium, R 2 = Linear correlation coefficient, K 2 = Pseudo-second order constant, K d and C = Liquid-film diffusion constants, K f and A = Intra-particle diffusion constant.

Figure 5 .
Figure 5. Expected adsorption mechanism for MB and MO removal by MPOS adsorbent.

Figure 6 .
Figure 6.Percentage removal of MB and MO by regenerated MPOS.

Table 1 .
Comparison of adsorption efficiencies of various adsorbents for the removal of MB and MO from water.

Table 2 .
Adsorption kinetic study for the removal of methylene blue (MB) and methyl orange.