Synthesis of mesoporous MgO nanostructures using

Journal of Environmental Chemical Engineering 5 (2017) 3429–3438

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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

Synthesis of mesoporous MgO nanostructures using mixed surfactants template for enhanced adsorption and antimicrobial activity Jyoti Sharma, Manisha Sharma, Soumen Basu

MARK

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School of Chemistry and Biochemistry, Thapar University, Patiala 147004, India

A R T I C L E I N F O

A B S T R A C T

Keywords: MgO nanostructures Adsorption Dyes Mesoporous Antimicrobial activity

Mesoporous MgO nanostructures with the high specific surface area (180 m2 g−1) were synthesized via mixed surfactant-templating method. The synthesis of MgO with mixed surfactants system has rarely been discussed in the literature, which motivates us to synthesize MgO in this system. Alteration in catanionic surfactant molar ratio and chain length resulted in a change in surface area. Morphological and physical properties were analyzed using FESEM, HRTEM, FTIR and XRD. Synthesized MgO nanorods showed excellent adsorption properties for dye removal from aqueous solution. The obtained adsorption capacities were 333.33, 250 and 200 mg g−1 for Methylene Blue (MB), Alizarin red (AZ) and Rhodamine B (RD), respectively, which is higher or comparable with other reported methods in literature. Other than adsorption, MgO can act as a good bactericide. The effective antimicrobial activity of MgO was analyzed via minimal inhibitory concentration (MIC) method against E. coli and B. subtilis and obtained IC50 (half maximal inhibitory concentration) values after 24 h were 71.98 ± 0.03 and 94.01 ± 0.030 respectively.

1. Introduction Currently, research based on the nanomaterials are of more interest due to their unique physicochemical properties [1]. The combination of nanomaterials and porous structures represents the most interesting of the rapidly growing areas. Porous material includes microporous (< 2 nm), mesoporous (2–50 nm) and macroporous (> 50 nm) which have interconnected pore network of solid composites. Other than this, natural substances such as clays, biological tissues (e.g. bones), rocks, and synthetic materials together with metal oxides, ceramics, membranes and carbonaceous materials can be considered as porous material [2]. Porous solid catalysts with high surface area and different pore size control the adsorption of various toxic organic molecules [3–7]. Out of porous materials, mesoporous materials have been of great interest to the researchers. Metal oxide nanostructures are given more and more importance because of their high chemical activity and their specificity in interaction [8]. Many researchers have submitted their reports on the synthesis of mesoporous materials like ZnO [9], TiO2 [10–13], CaO [14], MgO etc [15]. MgO is a solid of highly ionic nature, crystalline structure and have simple stoichiometry which can be widely prepared in different shapes and sizes. Adsorption properties are affected by the morphology MgO crystals [16]. Earlier MgO was prepared by sol-gel method [17,18] and thermal decomposition of magnesium carbonate and hydroxides [19,20]. Many methods including the

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Corresponding author. E-mail addresses: [email protected], [email protected] (S. Basu).

http://dx.doi.org/10.1016/j.jece.2017.07.015 Received 17 April 2017; Received in revised form 3 July 2017; Accepted 5 July 2017 2213-3437/ © 2017 Elsevier Ltd. All rights reserved.

thermal oxidation [21], thermal reduction [22] and hydrophobic interactions (which helps in self-assembly of building blocks and template-assisted synthesis) have been used to prepare metal oxide nanostructures [23,24]. To overcome the problem of agglomeration, MgO is also obtained from the preferable ‘surfactant templating method’ over Sol-gel method [25]. Surfactants play a significant role in the synthesis of mesoporous structures or composites. Their behaviour can be determined by the nature of bonding between counter ions. The ionogenic Cetyltrimethylammonium bromide (CTAB) act as a soft template [26]. The long alkyl chains of surfactants help in forming the small voids in the composite material [27]. Other than the concentrations the arrangements of the templates depends upon the chemical nature, sizes and charges of the micelles formed from the surfactant.The arrangement of these templates depends on the concentrations, chemical nature of the used surfactants, their sizes and charges of the micelles formed [28]. Therefore, different surfactant templates may result in different structures and morphologies of inorganic materials. MgO with surfactant templating method results in formation of the rod-like structure [29]. Self-assembly and self-arrangement of surfactants helps in making the internal hollow sphere of MgO [30]. A cationic surfactant (CTAB) had been used in the sol-gel reaction, in which only spherical MgO was obtained, with less surface area [31]. MgO has been synthesized by using the mixtures of cationic and anionic surfactants in molar ratios from which 220 m2 g−1 surface area was recorded [32]. The


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literature based on the synthesis of high surface area MgO nanomaterials using mixed surfactant systems has been rarely investigated. The adsorption of pollutants on metal oxides is an interesting topic for both academic and industrial fields. Organic dyes are carcinogenic and acts as an obstacle in reusing the wastewater for various purposes. To maintain the human health, to preserve the aquatic life and to create an overall balance in the universe, a highly economical, efficient and a non-toxic method should be devised for removal of dyes [33]. Adsorption is a cost effective process which helps in removing synthetic dyes and hazardous pollutants from wastewater. Ahmed et al., in 2016 discussed the adsorption of fast orange and bromophenol blue dyes on MgO and reported adsorption capacities of 30 and 40 mg g−1 respectively [34]. Hu et al., 2010 discussed the adsorption of X3B and congo red on MgO nanosheets and calculated the adsorption capacities of 303.3 and 278.5 mg g1 respectively [35]. MgO also has an excellent bactericidal property against the different genre of bacterias. Inorganic antimicrobial agents proved to be more safe and stable under high temperatures. For a good bactericide, high surface area metal oxide is required. Powder metal oxides, ZnO [36], CaO [37], MgO [38] showing good antibacterial activity against S. aureus and E. coli has been evaluated by some researchers. Nanorods have much higher efficiency as bactericide due to their high surface area. The formation of reactive oxygen species such as superoxide, explained the antibacterial behaviour of MgO. It had also been reported that increase in the surface area of MgO with the increase in concentration of superoxide in solution, results into more effective destruction of the bacterial cell wall [39]. In this study, the formation mechanism of MgO nanorods synthesized via mix surfactants templating method has been discussed in detail. Also explored here are the effects of different molar concentration of catanionic surfactants and alteration in their chain length on the surface properties of synthesized MgO nanorods. MgO nanorods with high surface area are used as an efficient adsorbent for organic dyes like MB, AZ, RD from aqueous solutions. Kinetic and thermodynamic equilibrium studies were performed to measure the adsorption efficiency for different dyes. Other than adsorption, antimicrobial activity of MgO was analyzed via MIC method against E. coli and B. subtilis.

Table 1 Detailed composition of the materials used for the synthesis of mesoporous MgO. Sample Name

Concentration of MgCl2 (M)

Concentration of C16TAB (M)

Concentration of SDS (M)

Concentration of NaOH (M)

M-0.125 M-0.25 M-0.5 M-1 M-1.5

1 1 1 1 1

0.125 0.25 0.50 1.00 1.50

0.125 0.25 0.5 1.0 1.5

2 2 2 2 2

2.2. Characterization To verify the crystal structure of mesoporous MgO powder, X-ray diffraction measurements was done at room temperature using Pan Analytical (X’Pert-pro) diffractometer with Cu Kα1 radiation (λ = 1.5406 Å) with a scanning interval (2ϴ) from 15° to 75°. Brunauer Emmett Teller (BET) was used to determine the specific pore sizes and surface areas using BEL mini-II, Micro Trac Corp. Pvt. Ltd, Japan. Degassing of samples was done at 200 °C in vacuum for more than 2 h prior to the measurements. Fourier Transform Infrared Spectroscopy (FTIR) of the samples was obtained by using Agilent Technologies, Carry 660 FTIR spectrophotometer. Field Emission Scanning Electron Microscopic (FESEM) JEOL JSM-6510LV coupled with Energy dispersive spectroscopy (EDS) was used to detect the morphology and elemental components of MgO nanostructures. Transmission Electron Spectroscopy (TEM) images were obtained with FEI Technai G2 F20 operating at 200KV. 2.3. Adsorption study Adsorption of dyes on mesoporous MgO was performed in a batch system at ambient temperature using favorable conditions. Each dye underwent to the same procedure which is discussed below. Mesoporous MgO (0.01 g) was added into different dyes solution (100–400 mg L−1) and agitated for different time periods at a speed of 200 rpm. After the adsorption procedure, the solutions were centrifuged for 10 min at 5000 rpm. After centrifugation, the samples were analyzed by UV–visible spectrophotometer (Champion UV-500) by monitoring the absorbance changes at λmax 660 nm for MB, 600 nm for AZ and 510 nm for RD. The adsorption capacity (qe) was calculated by the uptake amount of dye adsorbed per unit mass of mesoporous MgO (mg/g) using the formula:

2. Experimental section 2.1. Material and methods

qe =

All reagents used for synthesis of MgO were of analytical grade and all solutions were prepared in Distilled water. Magnesium chloride, dodecyltrimethylammonium bromide (C12TAB), tetradecyltrimethylammonium bromide (C14TAB), cetyltrimethylammonium bromide(C16TAB) octadecyltrimethylammonium bromide (C18TAB), sodium hydroxide (NaOH), methylene blue (MB), sodiumdodecyl sulfate (SDS), alizarin red (AZ) and rhodamine B (RD) were purchased from Merck, India. Magnesium chloride was used as a magnesium source. An aqueous solution of MgCl2 (1 M) was added with different molar ratio of C16TAB + SDS mixture and the solution was stirred for 1 h after which 2 M of NaOH solution was added drop-wise and stirred continuously for another 30 min. Finally, the white precipitate of Mg(OH)2 was obtained by centrifugation (5000 rpm, 5 min), followed by washing (5 times) with distilled water and dried up at 80 °C for 12 h. Calcination was done at 400 °C with a heating ramp of 1 °C/min. The different ratio of C16TAB + SDS mixture used for the synthesis of MgO is given below (Table 1). For a variation of chain length of CnTAB (n = 12–18), we have followed the procedure mentioned above keeping the equimolar concentration of CnTAB:SDS = 0.125:0.125. For comparison purposes, we have also synthesized blank MgO by following the same procedure with the absence of surfactants and mentioned it as a blank sample.

(Co − Ce ) ×V m

(1)

Where Co is the initial and Ce is the equilibrium concentrations (mg/L), while V is the volume of the solution (L) and m is the weight of the adsorbent (g). 2.4. Antibacterial activity for mesoporous MgO The antimicrobial activity of mesoporous MgO nanomaterials against E. coli (MTCC-77) and B. subtilis (MTCC-441) was analyzed by using the MIC method. Bacterial cells were incubated with various concentrations (0–600 μg) of MgO. After incubation, optical density was analyzed at 600 nm with MgO for a period of 24 h at 35 °C and results for the mean value of 3 mutually independent experiments were plotted. 3. Results and discussion This work clearly shows the synthesis and characterization of MgO by using a mix surfactant system. Effect of different chain lengths and different ratios of surfactants alter the porosity as well as the surface area of synthesized nanomaterial, which was analyzed by BET surface area analyzer. 3430


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Fig. 1. (a) Nitrogen sorption plots and (b) Pore size distribution of synthesized blank MgO and by different ratios of surfactants, C16TAB and SDS (0.125, 0.25, 0.5, 1, 1.5 M).

3.1.1. Mechanism of formation of MgO nanorods The Critical micellar concentration (CMC) of cationic surfactant (CTAB) is 0.9 mM and for anionic surfactant (SDS) is 8.2 mM. At CMC, surfactant forms spherical micelles whereas, above this value, it results in the formation of rod shape lamellar micelle [41,42]. Organized micelle structures were obtained by electrostatic interaction between the cationic and anionic surfactants, as resulting in formation of gel-like suspension and decrease in critical aggregation concentrations (CAC) [43]. Mixtures of cat-anionic surfactants are generally more surface active than their pure individual form and results in the growth of lamellar structure. Sulfate and bromide groups from the cat-anionic surfactants were able to bind with the magnesium ions resulting in the formation of the zwitterions structures. Due to the micelle formation and passivation reactions, nanorods are formed [44]. Fig. 3 describes the mechanism of the nucleation and formation of MgO nanorods.

3.1. Effect of different molar ratios of surfactants on MgO The surfactant can decrease the surface energy and prevent agglomeration when adsorbed on the surface of the nanomaterials. Polymeric dispersants stabilize and control the size of the nucleating particles [40]. To check the effect of mixed surfactant on surface area, a mixture (1:1) of cationic (CTAB) and anionic (SDS) surfactants were used. Here, mesostructured MgO nanorods were synthesized using equimolar composition (1:1) of C16TAB and SDS (0.125, 0.25, 0.5, 1, 1.5 M) at ambient temperature. Fig. 1a represents the N2 adsorptiondesorption curves for the mesoporous MgO. According to BrunauerEmmett-Teller (BET) model and IUPAC classification, all the curves are of type IV with H3 hysteresis loop and the loop increases between 0.8 < P/P0 < 0.9, as the surfactant ratio decreases from 0.125 to 1.5. Fig. 1a clearly illustrated that with the increase in the surfactant ratio, there is a decrease in the sharpness of the hysteresis loop which shows a decrease in the average pore size and the broadening of the pore size distribution. The smaller decrease in pore size indicated the presence of agglomeration. Detailed physical parameters of nitrogen sorption for as-synthesized MgO are discussed in Table 2 and it is clearly seen that the surface area varied with the different ratios of cat-anionic surfactants. Fig. 1b shows the pore size distribution (30–12 nm) using differential BJH plot of the synthesized MgO nanorods with a different molar concentration of surfactants. As per the IUPAC classification, the synthesized MgO was a mesoporous material with high specific surface area. The mesostructures of the synthesized samples were investigated by FESEM, and the micrographs obtained are presented in Fig. 2(a–b). The FESEM images of MgO with different molar ratio of surfactant confirm the rod-shaped/lamellar-like structures of MgO (100–150 nm in length and 20 ± 5 nm in thickness). The rod shape morphology of MgO was also confirmed by TEM analysis. Fig. 2(c–d) shows the HRTEM images of MgO nanorods with different molar ratio of surfactants. The elemental composition of the MgO was confirmed by the EDS analysis as shown in Fig. 2e which confirms the purity of MgO nanorods.

3.2. Effect of chain length of surfactant To check the effect of chain length on the surface structure of MgO, we have varied the chain length of only cationic surfactant, CnTAB (n = 12–18) keeping the molar ratio of CTAB and SDS fixed to 0.125 M. The chain length is an important factor to change the pore size of the compound as it alters the size of the resulting micelle. The increment in the alkyl groups (n = 12–18) of cationic surfactant leads to the increase in the surface area. Fig. 4(a–b) shows the nitrogen adsorption-desorption and pore volume distribution curve using differential BJH plot for the synthesized MgO nanorods with a different chain length of CTAB. From the IUPAC classification, all are type IV and exhibit the characteristic hysteresis loop of mesoporous materials. Fig. 4(a–b) clearly shows the increase in surface area (50–180 cm2 g−1) and decrease in pore size (40–19 nm) with the increasing chain length of CnTAB (n = 12–18). Steric hindrance of the long alkyl chains of cationic surfactants results in the decrease in the pores size due to the agglomeration of the compound. Detailed N2 adsorption-desorption parameters for synthesized MgO nanorods with a different chain length of CTAB are shown in Table 3. The morphology of MgO nanorods with a different chain length of CTAB was analyzed by FESEM. Fig. 5(a–b) shows the appearance of long (150–200 nm) rod-shaped MgO with some irregular structures. HRTEM analysis Fig. 5(c–d) also confirms the rod shape structure of MgO. Although the surface properties of different MgO synthesized by varying the chain length of surfactant is different, but the morphology (size and shape) of the MgO is almost same (diameters of all nanorods are almost similar). The elemental composition of the MgO was confirmed by the EDS analysis as shown in Fig. 5e which confirms the purity of MgO. The crystalline nature of the mesoporous MgO nanorods with a different chain length of CTAB was analyzed through their XRD pattern.

Table 2 Nitrogen adsorption-desorption parameters for mesoporous MgO showing the effect of different molar concentration of catanionic surfactant. Sample Name

Ratio of C16TAB:SDS(M)

Surface Area (m2 g−1)

Pore Volume (cm3 g−1)

Pore Diameter (nm)

M-0.125 M-0.25 M-0.50 M-1.0 M-1.5

0.125:0.125 0.25:0.25 0.5:0.5 1.0:1.0 1.5:1.5

165 141 138 123 109

1.45 1.28 1.19 0.70 0.50

29.3 27.2 19.6 15.4 12.2

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Fig. 2. (a–b) FESEM images, (c–d) HRTEM images and (e) EDS spectrum of MgO with different surfactants ratio (0.125 M for (a)/(c) and 1.5 M for (b)/ (d)).

Fig. 6b shows the FTIR spectra of the synthesized mesoporous MgO nanorods and the blank MgO. The broad absorption band at 578 cm−1 confirms the symmetric stretching of Mg]O bond. The absorption bands at 3345 cm−1 (stretching) and 1431.7 cm−1 (bending) indicate the presence of hydroxyl groups (OH), which is due to the presence of Mg(OH)2. The bands at 1095 cm−1 are due to CeO stretching.

Fig. 6a presents the X-ray diffraction pattern of the MgO nanorods by using a mixture of Cn = 12–18TAB + SDS and blank MgO (without surfactant). The XRD pattern show the Bragg’s reflections at 2θ = 37.8°, 42.5° and 63.54° indexed to (111), (200), (220) planes respectively, confirming crystalline nature of MgO by comparison with the JCPDS card no. 45-0946.

Fig. 3. Mechanism showing the synthesis of mesoporous MgO nanorods.

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Fig. 4. (a) Nitrogen sorption plots and (b) Pore size distribution plots for MgO-0.125 with the different chain length of CnTAB (where n = 12–18).

and the adsorbent. Dye adsorption experiments on the MgO were performed in a batch system at room temperature using favorable conditions (pH-6).

Table 3 Nitrogen adsorption-desorption parameters for mesoporous MgO nanorods by varying the chain length of a cationic surfactant. Sample Name

C12TAB:SDS C14TAB:SDS C16TAB:SDS C18TAB:SDS

Ratio of CnTAB:SDS

Surface Area (m2 g−1)

Pore Volume (cm3 g−1)

Pore Diameter (nm)

0.125:0.125 0.125:0.125 0.125:0.125 0.125:0.125

50 110 165 180

0.73 0.97 1.45 1.48

40 32 29 19

3.3.4. Effect of temperature on adsorption To determine the effect of temperature on dyes (MB, AZ, RD), MgO was treated at 293, 313, 333 and 353 K respectively. The observed decrease in values of 1/T and ln(qe/Ce) with high temperature indicates the endothermic nature of the adsorption process [49]. The values of ln (qe/Ce) at different temperatures were analyzed according to Van’t Hoff equation:

q ΔH ΔS + ln ⎛ e ⎞ = − C RT R ⎝ e⎠

3.3. Adsorption study

⎜

3.3.1. Effect of pH pH is a significant parameter which affects both surface binding sites and aqueous chemistry of the adsorbents. To check the effect of pH on the adsorption process we have changed the pH of the solution from 3 to 10 by adding 0.1 N NaOH and 0.1 N HNO3 solution. Fig. 7a shows the results of the effect of pH on MB, AZ, RD removal efficiencies. The maximum adsorption for all the three dyes was noted at pH6 which clearly signifies the presence of maximum binding sites. The adsorption of MB, AZ, RD using MgO increases from 72.4 to 96.8%; 71.2 to 95.4% and 69.8 to 94.2% respectively, by adjusting the pH. At lower pH, adsorbent’s surface was surrounded by hydronium ions that compete with the dye, which prohibit the dye from approaching the binding sites on the adsorbents [45,46]. So with the increase in pH, there is less competition between the hydronium ions and the adsorbent’s surface, which enhances the adsorption.

⎟

(2)

Where R represents the universal gas constant (8.314 J/(mol K)) and T defines the absolute temperature (in Kelvin). A linear graph was obtained by plotting ln(qe/Ce) against 1/T Fig. 7d. Endothermic nature of adsorption process was confirmed by the positive value of ΔH and it states that the adsorption is favored at a higher temperature. Gibbs free energy of adsorption (ΔG) is calculated from the following relation: (3)

ΔG = ΔH − T ΔS

Results from Table 4 show the adsorption reaction were spontaneous and confirmed by the negative values of ΔG. 3.3.5. Kinetic study The kinetics of MB, AZ, RD adsorption on MgO were also evaluated using Lagergren’s first-order rate equation and pseudo-second-order kinetic model. Lagergren’s first-order rate equation is usually used to calculate adsorption kinetic and expressed as;

3.3.2. Effect of contact time Effect of contact time was analyzed between 10 and 120 min, which elaborated the efficiency of MgO on dyes. At 6 pH, the initial concentration of all the three dyes (MB, AZ, RD) was 100 mg/L. Fig. 7b shows that with increase in time, there is an increase in the adsorption capacity (qe). It also indicates that MB has the maximum absorbance with time in comparison to AZ and RD. The kinetics data explained that there is the presence of two phases: (i) fast phase shows the maximum adsorption and (ii) saturation phase explains the phase of equilibrium [47,48]. There is no significant increase in the adsorption after 90 min for all the three dyes.

log(qe − qt ) = log(qe ) −

K1 t 2.303

(4)

The pseudo-second-order rate equation is expressed as;

t 1 t = + qt qe k2 qe2

(5)

where qt and qe are the amount of metal oxide (mg/g) and a dye adsorbed at equilibrium and at any given time (min) respectively. While K1 and k2 represent the rate constant for the pseudo-first order reaction for adsorption (min−1) and pseudo-second order reaction (g/mg min) for adsorption respectively [50]. Fig. 8(a–b) shows the linear fit plot for pseudo-first and pseudo-second-order rate equation. Table 5 shows the kinetic parameters obtained from pseudo-first and pseudo-second-order kinetic model. In all cases, correlation coefficients were found to be closer to unity, but the proposed results suggest pseudo-second-order is best fitted in comparison to pseudo-first for adsorption due to the chemisorption process.

3.3.3. Effects of dye concentration As the concentration of dyes (MB, AZ, RD) affects the adsorption capacity of mesoporous MgO (0.01 g), the adsorption studies were done with different concentration (100–400 mg/L) of dyes. Fig. 7c shows the increase in adsorption capacity with the increase in the concentration of dyes which results to increase in the mass gradient between the solution 3433


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Fig. 5. (a–b) FESEM images, (c–d) HRTEM images and (e) EDS spectrum of MgO-0.125 with different chain length of CnTAB ((a)/(c) for C12TAB and (b)/ (d) for C18TAB).

Fig. 6. (a) X-ray diffraction pattern of MgO synthesized by different chain length of CnTAB (where n = 12–18) and (b) FTIR spectra of blank MgO and MgO-0.125 (C18TAB).

qt = K diff t 1/2 + C

3.3.6. Intraparticle diffusion Weber and Morris described that diffusion of the adsorbate from the surface to the internal pores of the adsorbent and referred it as intaparticle diffusion. The intraparticle diffusion model was analysed using kinetic study and mathematical relation is as follows:

where qt (mg g−1) is the amount of dye adsorbed at time t and Kdiff (mg g−1 min−1/2) is the rate constant for intraparticle diffusion. The thickness of the boundary layer can be calculated from the value of C, large intercept describes the great boundary layer effect. A plot of qt versus t1/2 can give a linear or multi linear and proves the involvement 3434


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Fig. 7. Plots for (a) effect of pH, (b) effect of time, (c) effect of concentration and (d) Van’t Hoff plots for the adsorption of MB, AZ, RD on MgO-0.125 (C18TAB).

3.3.7. Adsorption isotherms Data obtained from adsorption isotherm played a significant role in establishing the effectiveness of adsorption. The association between the concentrations of adsorbed and dissolved adsorbate at equilibrium along with the interactive behavior between the adsorbate and adsorbent can also be described by adsorption isotherm. In this study, Langmuir and Freundlich isotherm models were used to analyze the adsorption mechanisms, through which experimental results of dye can be explained in a wide variety of concentrations. The Langmuir equation is expressed as follows;

Table 4 Thermodynamic parameters at different temperatures for removal of MB, AZ and RD on MgO. Dyes

T(K)

ΔG (kJ mol−1)

ΔH (kJ mol−1)

ΔS (J mol K−1)

MB

293 313 333 353 293 313 333 353 293 313 333 353

−4.80 −4.67 −4.54 −4.41 −6.84 −6.71 −6.58 −6.43 −8.65 −8.49 −8.34 −8.19

−6.71

6.47

−8.84

6.80

−10.91

7.70

RD

AZ

Ce 1 C = + e qe Qo b Qo

(6)

where the equilibrium adsorption capacity is qe (mg/g) and the maximum amount of the MB, AZ and RD adsorbed per unit weight of the adsorbent is Q0 (mg/g). The linear plots of Ce/qe vs. Ce propose the validity of the Langmuir isotherms and the values of Q0 and b were obtained from slope and intercepts of the plots. The calculated maximum adsorption capacity of MgO nanorods for MB, AZ and RD were 333.3, 250 and 200 mg/g respectively. Also, when the surface is fully covered with dye, Q0 represents adsorption capacity, helping in the assessment of adsorption action of adsorbents. Langmuir equilibrium constant (b) which is related to the connecting spots and explains the bond energy for the adsorption reaction [52]. The Freundlich isotherm is a model of multilayer adsorption on the adsorbent which can be described as:

of intraparticle diffusion in the adsorption process. The linear graph from the origin suggests the intaparticle diffusion as a rate limiting step. The slowest step i.e., the rate limiting step control the overall adsorption process. The intraparticle diffusion can be explained by two parts, due to external diffusion the starting part of the adsorption is rapid and latter part said to be the rate limiting step known as intra-particle mass transfer rate [51]. Therefore, intraparticle diffusion is one of the factors which affect the rate of ease for the adsorption to reach equilibrium state. Fig. 8c illustrates Weber and Morris plot for all the three dyes MB, AZ, RD adsorption on MgO. Due to multi-linearity correlation it explained the significant role in adsorption mechanism. The graph indicates that it is not the solely rate limiting step.

log qe = log Kf +

log Ce n

(7)

Freundlich constants, Kf (mg1−1/n L1/n g−1) and n depict the adsorption capacity and intensity, respectively [53]. Fig. 9a and b showed the plots for Langmuir isotherm and Freundlich model for the 3435


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Fig. 8. Plots for (a) pseudo- first order, (b) pseudo-second order, (c) Weber Moris plot explaining mechanism of adsorption of MB, AZ, RD on MgO.

adsorption of dye on MgO. Table 6 shows the fitted parameters (constants and correlation coefficients) from experimental data for both Freundlich and Langmuir isotherms.

Where qe,exp and qe,cal are the experimental and model equilibrium capacity data, respectively. If the data from the model are similar to the experimental data, the value of χ2 will be small, otherwise it will be large.

3.3.8. Error analysis The non-linear regression was calculated using the software IBM SPSS Statistics 20 for the determination of isotherm models. Only R2 (coefficient of determination) is not only the non linear analysis which is important, there is one more non-linear analysis to evaluate the goodness of the models and describe the sorption phenomena. The correlation of the experimental data can further be estimated with the help of chi-square test and is given as:

χ2 =

∑

3.3.9. Desorption studies Synthesised MgO nanorods were found to be reusable for atleast 5 cycles after adsorption study for toxic dyes. After completion on each adsorption cycle, adsorbates were easily separated from aqueous solution of toxic pollutants by simple filtration or centrifugation process. No additional filtration techniques were required, so these adsorbates are considered as user-friendly and cost effective. Fig. 10 shows the plot of percentage removal vs. no. of cycles.

(qe ,exp − qe, cal )2 qe, cal

Table 5 Kinetic Parameter for adsorption of dyes on MgO. Dyes

MB AZ RD a b c

Pseudo-first-order

Pseudo-second-order

Intra-Particle diffusion

K1b

qea

R2

K2 c

qe a

R2

Kdiff

C

R2

0.075 0.019 0.021

388 57.6 140

0.91 0.89 0.92

1 × 10−5 1.8 × 10−5 6 × 10−5

400 132 285

0.96 0.98 0.97

4.18 3.54 5.25

25.54 16.73 5.95

0.87 0.90 0.87

mg g−1. min−1. g/mg min.

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Fig. 9. (a) Langmuir isotherm and (b) Freundlich isotherm for the adsorption of MB, AZ, RD on MgO-0.125 (C18TAB). Table 6 Isotherm constants and correlation coefficients for adsorption of MB, AZ, RD on MgO. Dyes

Blank MgO MB AZ RD a b

Freundlich 2

Kfa

N

R

20.0 160.78 120.78 97.72

3.00 1.99 2.40 1.41

0.40 0.96 0.95 0.93

Langmuir X

2

10.44 7.07 8.09 6.47

Q0

b

30.0 333.33 250 200

B

R2

X2

0.05 0.8 0.37 0.17

0.70 0.99 0.97 0.95

4.9 3.8 1.59 2.01

mg1-1/n L1/n g−1. mg g−1.

Fig. 11. Concentration vs percentage bacterial growth plot for antimicrobial activity of MgO-0.125 (C18TAB) using E. coli and B. subtilis.

4. Conclusion In summary, mesoporous MgO nanorods were synthesized by mixed surfactant templating method and analyzed were the effect of surfactants ratio and chain length on pore size distribution, surface area, and morphology. At particular concentration (0.125 M) of mixed cat-anionic surfactant, the surface area becomes high (180 m2 g−1) may be due to generation of the lamellar crystallinity for meso-structured MgO. Adsorption study showed that pseudo second order kinetic and Langmuir isotherm models were best fitted as values of correlation coefficients were more near to unity. The effects of adsorbate pH, temperature, contact time and adsorbent concentration were also explored. The antibacterial activity of mesoporous MgO nanorods was investigated against the gram-negative E. coli and gram-positive B. subtilis bacteria by MIC method.

Fig. 10. Plot showing percentage removal vs. no. of cycles.

Conflicts of interest

3.3.10. Antimicrobial activities The antibacterial activity of mesoporous MgO against E. coli and B. Subtilis is shown in Fig. 11. Being proportional to the concentration of the nanorods used, the antibacterial activity of MgO was analyzed via MIC method against E. coli and B. subtilis respectively. Obtained were the IC50 values i.e., the half maximal inhibitory concentration after 24 h were 71.98 ± 0.03 and 94.01 ± 0.030, respectively. As the pore size of the mesoporous MgO decreases with the increase in specific surface area, the potential number of reactive groups on the particle surface increases, due to which high antibacterial activity was expected [54].

The authors declare that there is no conflict of interests regarding the publication of this paper. Acknowledgements The authors are grateful to DAE-BRNS, Mumbai, India (Grant no 34/ 14/63/2014) for financial assistance and Thapar University (seed money grant) for infrastructure/instrumental facilities. DAE-BRNS, Mumbai, India 3437


J. Sharma et al.

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