Silica-polymer hybrid materials as methylene blue adsorbents

Silica-polymer hybrid materials as methylene blue adsorbents

Accepted Manuscript Title: Silica-polymer hybrid materials as methylene blue adsorbents Author: Hem Suman Jamwal Sapana Kumari Ghanshyam S. Chauhan N...

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Accepted Manuscript Title: Silica-polymer hybrid materials as methylene blue adsorbents Author: Hem Suman Jamwal Sapana Kumari Ghanshyam S. Chauhan N.S. Reddy Jou-Hyeon Ahn PII: DOI: Reference:

S2213-3437(16)30421-3 http://dx.doi.org/doi:10.1016/j.jece.2016.11.029 JECE 1342

To appear in: Received date: Revised date: Accepted date:

16-7-2016 19-11-2016 22-11-2016

Please cite this article as: Hem Suman Jamwal, Sapana Kumari, Ghanshyam S.Chauhan, N.S.Reddy, Jou-Hyeon Ahn, Silica-polymer hybrid materials as methylene blue adsorbents, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.11.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Silica-polymer hybrid materials as methylene blue adsorbents Hem Suman Jamwal1, Sapana Kumari1, Ghanshyam S. Chauhan1*, N.S Reddy2, Jou-Hyeon Ahn3 1

Himachal Pradesh University, Department of Chemistry, Shimla–171005, India School of Materials Science and Engineering, Engineering Research Institute, Gyeongsang National University, Jinju, 660-701, Korea. 3 Department of Materials Engineering and Convergence Technology and RIGET, Gyeongsang National University, 501 Jinju-daero, Jinju 52828, Republic of Korea email: [email protected], [email protected] Ph: +911772830944, +919418003399 (M), FAX: +91177283077 2

Corresponding Author *E–mail: [email protected], [email protected], Ph: +911772830944, +919418003399 (M), FAX: +911772830775.

Graphical abstarct

Highlights

 New poly(methacrylate)/silica nanohybrid were synthesized by emulsifier-free emulsion polymerization.  Synthesized materials were used as methylene blue (MB) adsorbents following a parametric framework.  Materials exhibited high maximum adsorption capacity between 56.625-91.324 mg g −1 under optimum conditions.  Materials are reusable

1

Abstract: Nanohybrid materials have emerged as effective adsorbents for the removal of contaminants from the polluted water bodies. In this study we report two new hybrid materials as adsorbents for methylene blue from its aqueous solutions. Nanohybrid materials were prepared from methacrylic acid and methyl methacrylate or 2-hydroxyproypl methacrylate by emulsifier–free emulsion polymerization using 3– aminopropyltriethoxysilane (APTES) as silane coupling agent, and tetraethoxysilane (TEOS) and polyvinyl alcohol (PVA) as silica component precursor and polymeric colloid stabilizer, respectively. Adsorption was studied as a function of various factors, including contact time using two cationic dyes methylene blue (MB) and malachite green and one anionic dye Congo red. Since the results obtained suggest more affinity of the adsorbents for MB than other two dyes, hence the former was selected to assess the effect of variation of temperature, pH and concentration, which control the dye adsorption process. The hybrid materials exhibited high adsorption capacity both in the cumulative as well as in the reusability studies. Experimental data was subjected to different kinetic models and adsorption isotherms to understand the adsorption mechanism. Keywords: Adsorption; Hybrid materials; Poly(methacrylates); Methylene blue, Adsorption capacity; Reusability

1.

Introduction Discharge of dyes from different industries causes severe damage to the living

beings with hazardous health disorders including carcinogenic and mutagenic changes [1]. The dyes have complex chemical structures and are stable to light, heat and oxidation. Dyes are not easily biodegradable though their degradation with fungi has been reported [2–4]. Consequently, different physico–chemical methods like coagulation [5], ultra– 2

filtration [6], electro–chemical [7], adsorption and photo–oxidation [8] have been extensively reported for the removal of various dyes from wastewater [9]. However, adsorption is more cost-effective and a number of low cost adsorbents for dye removal, including activated carbon [10–12], activated sludge [13–15], biowaste [16–24], and clays [25,26], have been reported in literature. Out of these the polymeric hybrid materials, comprising of inorganic and organic components, where the latter is a polymer, are highly promising materials as adsorbents for metal ions [27-32] and dyes [31–35]. High efficacy of the hybrid materials, than the mono or individual components used separately, results from the synergetic effect of the two components into the secondary matrix. Among these the hybrids materials synthesized from silica as one component and polymer as the other component have been reported to be efficient dye adsorbents [34,35]. In the present work, new poly(methacrylate)/silica hybrid materials were synthesized and used for the removal of methylene blue (MB), a cationic dye, from its aqueous solutions. Its removal by different adsorbents has been reported elsewhere [10, 23, 36–40]. There is scanty information on its removal by silica–polymer nanohybrid materials and the hybrid materials reported in this work are new and not reported elsewhere. The polyacrylate–based silica hybrid materials have been reported elsewhere using monomers styrene, butyl acrylate and methacrylic acid with tetraethoxysilane (TEOS) as the Si precursor [41]. However, in the present study hybrid materials were synthesized from a binary monomer system containing methacrylic acid (MAA) and methyl methacrylate (MMA) or 2-hydroxypropyl methacrylate (HPMA). Synthesis was carried out by emulsifier–free emulsion polymerization using 3– aminopropyltriethoxysilane (APTES) as silane coupling agent, and TEOS and polyvinyl alcohol (PVA) as the silica component precursor and polymeric colloid stabilizer, respectively. The removal of MB from its aqueous solutions was studied as a function of 3

time, temperature, pH and concentration. The reusability of the synthesized hybrid materials was also investigated. 2. Materials and methods 2.1. Materials Methacrylic acid (MAA), methylmethacrylate (MMA), 2-hydroxypropyl methacrylate (HPMA) (Merck, Schuchardt, Germany), 3-aminopropyltriethoxysilane (APTES), tetraethoxysilane (TEOS) (Himedia, Mumbai, India), polyvinyl alcohol (PVA) (CDH Lab reagents, New Delhi, India), potassium persulfate (KPS), sodium hydroxide (SD fine Chem. Ltd., India), methylene blue (MB), malachite green (MG), Congo red (CR) (Alpha Chemika, Mumbai, India), all of analytical grade, were used as received. The concentration of MB in the sorption experiments were determined with Photo lab 6600 UV–Vis series and pH values were measured with pH meter (Eutech 20). 2.2. Synthesis of hybrid polymers Silica–based nanohybrid materials were synthesized via emulsifier-free emulsion polymerization method by modifying an earlier reported protocol [41]. Firstly, PVA (10 % w/v in water) was taken in a reaction flask that was fitted with a reflux condenser, mechanical stirrer and a digital thermometer. After stirring at 350 rpm for 30 min, in the next step, monomers (MAA and MMA or MAA and HPMA), APTES and TEOS were added into the vessel. Solution of initiator (KPS) was prepared in water and added slowly, within 1 h, to the reaction vessel maintained at 80 °C. After the addition, the polymerization system was kept undisturbed at 85 °C for 2 h. The nanohybrid materials thus obtained were designated as HMMA and HHPMA, respectively. The details of the materials used and the proposed mechanism for the synthesis of hybrid materials are presented in Table 1 and Scheme 1, respectively. 2.3. Characterization of hybrid polymers 4

The synthesized materials were characterized by Fourier transform infrared (FTIR) spectroscopy, Brunauer−Emmett−Teller (BET) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and scanning electron microscopy–energy dispersive X–ray spectrometry (SEM–EDX). FTIR spectra were recorded on a Nicolete 5700 instrument in transmittance mode in KBr. The FTIR spectra were recorded to investigate the presence of various functional groups present in the hybrid materials. The %weight of the elements present was determined by EDS recorded on SEM QUANTA 250 D9393. Surface morphology of the samples was observed by scanning electron microscopy. TEM studies were carried out after sonification of the sample in EtOH for 1 h. XRD studies were taken to analyze whether the synthesized samples have crystalline or amorphous nature. Surface area and pore size of the hybrid materials were analyzed using a BET surface area analyzer (SMART SORB 92/93). The samples were degassed at 100 °C before the measurement. 2.4. Batch adsorption experiments and optimization of adsorption parameters Adsorption study was carried out with 0.05 g of the hybrid materials each was added to 50.0 mL (10.0 ppm solution) of cationic dyes MB, MG and anionic dye CR at pH 7.0. The effect of the contact time on adsorption was studied at 30 °C. MB was selected for further studies as the hybrids were showing best adsorption results for MB under the optimum conditions of time. Temperature variation was studied at 300 min and 10.0 ppm of MB concentration. Effect of pH variation (2.0-11.0) on the dyes adsorption was studied under the optimum conditions of time (300 min) and temperature (45 °C). Effect of different concentration (5.0-40.0 ppm) of MB was studied at the optimum conditions of time, temperature and pH (9.0). For measuring the time dependent dye loading, aliquots were removed at specified time intervals, and the absorbance was measured at 664 nm on Photo lab 6600 UV–Vis series. The readings were taken in 5

triplicate and the average of the values was used for further studies. The results of adsorption behavior were calculated in terms percent uptake (Pu) and adsorption capacity (q) as follows [38]:

𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑢𝑝𝑡𝑎𝑘𝑒 (𝑃𝑢 ) =

𝐶𝑜 − 𝐶𝑡 . 100 𝐶𝑜

𝐴𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 (𝑞) (𝑚𝑔/𝑔) =

𝐶𝑜 − 𝐶𝑡 .𝑉 𝑀

(1) (2)

where, q is the amount of the dye adsorbed onto unit dry mass of the adsorbent (mg/g), Co and Ct is the concentrations of dye in the feed solution and in the aqueous phase after treatment for a certain period of time t, respectively (mg/L), 𝑉 is the volume of the MB solution and 𝑀 is the weight of dry hybrid materials (g).

2.5. Desorption and reusability experiments The adsorption−desorption cycles were repeated six times to determine the reusability of the hybrid materials. After adsorption experiments (volume, 50 mL; hybrid materials (HMMA or HHPMA), 0.05 g; initial concentration, 30 ppm; pH value, 9.0; contact time, 300 min; temperature, 45 °C; agitation speed, 200 rpm), the hybrid materials adsorbed with MB were separated from the solution by filtration, then added to 20 mL of the stripping solution (0.1N NaOH), and stirred at 200 rpm for 60 min at 30 °C, and the final MB concentration was determined. After each adsorption−desorption cycle, hybrids were washed with the double distilled deionized water and used in the subsequent cycle. 2.6. Determination of point of zero charge (PZC) The PZC was measured to study the effect of pH on the surface charge of the hybrid materials. It represents the pH in relation to an electrolyte at which the surface charge of the material is zero or the pH at which there is change in the polarity of the 6

adsorbent surface. For the purpose, 0.1 g of the hybrid material was put in 20 ml of 0.1 M NaNO3 solution in different beakers to make a suspension. The initial pH (pHi) of the suspensions was adjusted using standard HCl or NaOH solution from 2.0 ̶ 10.0 using a pH meter (Cyberscan, Eutech, India). The solution was kept for equilibration under stirring for 24 h. After 24 h, the final pH (pHf) of the solution was observed using a pH meter. A graph between (pHf - pHi) and pHi was plotted, wherein pH corresponding to zero value of (pHf - pHi) gives the PZC of the hybrid material. 2.7. Adsorption kinetic and isotherm modeling Three different models viz. pseudo–first–order [42], pseudo-second order equation [43], Elovich equation [44] were used to evaluate the adsorption kinetics. The experimental kinetics was fitted to the pseudo first-order, pseudo second-order kinetics and Elovich equation by linear method. The non-linear equations were linearized to give the following linear forms. The pseudo–first order equation was used: log(𝑞𝑒 − 𝑞𝑡 ) = log(𝑞𝑒 ) −

𝑘1 2.303

𝑡

(3)

The linear form of the pseudo–second order equation is given as: 𝑡 𝑞𝑡

=

1 𝑘2 𝑞𝑒

2

+

1 𝑞𝑒

𝑡

(4)

The linear form of the Elovich equation used is given as: 𝑞𝑡 =

1 𝛽

ln(𝛼𝛽) +

1 𝛽

ln(𝑡)

(5)

The adsorption mechanism was evaluated using Langmuir and Freundlich isotherms as a function of MB concentration [45,46] as also reported elsewhere [47]. The Langmuir adsorption isotherm assumes that surface is uniform with all of the adsorption

7

sites having equal adsorbate affinity. The linear form of Langmuir isotherm may be represented as: 1 𝑞𝑒

=

1 𝐾𝐿 𝑞𝑚

.

1 𝐶𝑒

+

1

(6)

𝑞𝑚

The Freundlich adsorption isotherm illustrates adsorption as a heterogeneous or multilayer phenomenon. In linear form it can be represented as: log 𝑞𝑒 = log 𝐾𝐹 +

1 𝑛

𝑙𝑜𝑔𝐶𝑒

(7)

2.8. Thermodynamic studies The thermodynamics of the hybrid materials were evaluated by studying the effect of temperature on adsorption in the temperature range 298-323 °K. The values of ΔG° were calculated using the equation: ΔG° = -RT ln Kc

(8)

However ΔH° and ΔS° values were evaluated from the slope and intercept of the lnKc vs.1/T. 3. Results and discussion The synthesized hybrid polymers were characterized by different techniques and further investigated for uptake of MB. 3.1. Characterization of hybrid polymers Characterization of the hybrid materials was carried out by SEM, TEM, EDAX and FTIR spectroscopy. The evidence of synthesis of hybrid materials was obtained by analyzing the characteristic absorption bands of functional groups present in the respective polymers. The FTIR spectra of the candidate materials are presented in Figure 1. From the spectrum of HMMA important absorption bands can be observed at 3436 cm–1, 2999.2 cm–1, 1716.2 cm–1, 1390.2 cm–1, 1264.4 cm–1 and 1166.6 cm–1and for HHPMA 8

absorption bands were observed at 3475.2 cm–1, 2988.3 cm–1, 1718.6 cm–1, 1390.2 cm–1, 1270.6 cm–1 and 1175.1 cm–1 due to the N–H stretching, C–H stretching, C–O double bond stretching, C–N stretching, C–O stretching and Si–O–Si stretching, respectively. The absorption bands at 1716.2 cm–1 or 1718.6 cm–1 is a collapsed band due to the ester and carboxylic group C=O stretching vibrations. SEM images of the hybrid materials reveal that the surface is porous with rough morphology (Figure 2). However the SEM of the MB–loaded samples shows that MB is also adsorbed on the surface of the hybrid materials thus bringing about a change in its surface morphology (Figure 2). The hybrid materials synthesized composed of C, N, O and Si elements whose presence was confirmed from the EDAX data (Figure 3). TEM images reveal that these materials are spherical in nature with nearly uniform size < 50 nm (Figure 4). XRD spectra of the hybrid materials contain broad and diffused peaks revealing their amorphous nature (Figure 5). The plot of pHf –pHi vs. pH is presented in Figure 6. The values of PZC for HMMA and HHPMA were observed to pH 2.8 and 2.7, respectively. Thus these materials have positive charge below pH 2.8 and 2.7 for HMMA and HHPMA, respectively, and obtain negative surface charge above these values. It has also been reported in literature that the point of zero charge of hybrid materials having Si-OH group lies in between 1.5 and 4.5 pH [47,48]. 3.2. Adsorption studies of dyes Dye adsorption process depends on various factors such as size and number of sites on the adsorbent, the accessibility of the sites, the chemical state of the site (i.e. availability) and affinity between the adsorption site and the dye. 3.2.1. Effect of time on adsorption The adsorption of dyes increases with an increase in contact time before assuming equilibrium [38,49]. Of the three dyes, both HMMA and HHPMA showed the maximum 9

uptake for MB with q values of 4.12 and 4.17 mg g–1, respectively, after 300 min at pH 7.0. The effect of contact time on the dye adsorption is represented in Figure 7a. The adsorption is very fast especially as the dye uptake for more than 3.0 mg/g was adsorbed within 5 min. Both the materials reached equilibrium within 300 min. These adsorption capacity values are significant as the initial dyes concentration (10.0 ppm) as well as the temperature (30 °C) was low. The obtained q values are higher than reported in literature [23,50]. As the hybrid materials showed the best adsorption of MB, it was selected for further studies. 3.2.2. Effect of pH on adsorption The variation of MB adsorption was studied with pH variation as shown in Figure 7b. The adsorption increased with pH from 2.0 to 9.0, reaching the maximum value at pH 9.0 and decreasing sharply from pH 9.0 to 11.0, indicating the pH dependence of adsorption. This may be explained on the basis that at low pH values, the surface of the hybrids obtains high positive charge and thus the cationic dye is repelled by the positively charged surface. On the other hand, an increase in pH resulted in the increase of negatively charged sites resulting in more adsorption of the dye [38]. At high pH, most of adsorption sites get occupied by the small size hydroxide ions and thus a decrease in the number of the unoccupied adsorption site, and so decreases in the amount of the dye adsorbed. Furthermore, the PZC value also justifies the uptake behavior of the material with low adsorption under the highly acidic pH owing to positive surface charge and optimum adsorption of the hybrid material at basic pH due to the negative surface charge. 3.2.3. Effect of temperature

10

The effect of temperature (25–50 °C) was studied at pH 7.0 with 10 ppm MB for 300 min. The adsorption capacity (mg g–1) increased linearly with rise in temperature from 25 to 45 °C and became almost constant afterwards (Figure 7c). 3.2.4. Effect of dye concentration The effect of variation of concentration on the MB dye adsorption is represented in Figure 7d. The adsorption capacity (mg g–1) increased linearly initially with the increase in concentration and reached an equilibrium value of 27.546 and 25.34 mg/g for HMMA and HHPMA, respectively, at 30 ppm. From the obtained results, it can be stated that the effect of using a more hydrophobic monomer poly(HPMA) in HHPMA does not significantly affect adsorption capacity as adsorption sites apart from the Si–framework in both the samples remain the same (–COO– groups). The slight difference in adsorption capacity, that is lower in the case of HHPMA, is attributed to the low partitioning of the dye from the aqueous solution to the hybrid phase due to somewhat hydrophobic character of the poly(HPMA). The optimum value and q relationship is also supported for contact time and concentration [36] or pH variation [37] from the reported literature for the adsorption of MB on different silica–based adsorbents. 3.3. Reusability studies In the present study hybrid materials were subjected to seven feeds of MB solution of 30 ppm, at 45 °C with initial pH 9.0. Desorption experiments were carried out with 0.1M NaOH, the regenerated hybrid materials were reused for six adsorptiondesorption cycles effectively, and the results are illustrated in Figure 8. The cumulative adsorption capacity of 85.33 and 87.37 mg g–1, was observed for the hybrid materials containing MMA and HPMA, respectively, from their reusability studies. Such maximum adsorption capacity of the hybrid adsorbents has not been reported in literature [37,51-53] (Table 2). 11

3.4. Kinetic and isotherm studies Three kinetic models, pseudo–first order, pseudo–second order, and Elovich equation, were applied to examine the adsorption kinetics (Figure 9). Values of qe and k calculated from the slope and intercept of the plot, respectively, are presented in Table 3. The pseudo–second order model best fits the experimental data, compared to the other models, with coefficient of determination (R2) close to 1.0. Since R2 is not sufficient to describe the applicability of the applied models to the experimental data, better deductions can always be obtained when the theoretical values of the calculated equilibrium adsorption capacities (qt) from the applied models are compared and found closer to the experimental values as revealed in the present case (Figure 9d). Thus, it is suggested that the adsorption kinetics of MB can be best described by the pseudo–second order model. Since we applied linear forms of the different equations which generally are reported to be less applicable than the non-linear forms due to the more probability of error, hence to see the their applicability in the present case we used chi square (χ2) error analysis [54-55]. (𝑞𝑒𝑥𝑝 − 𝑞𝑐𝑎𝑙 ) 𝜒 =∑ 𝑞𝑐𝑎𝑙

2

2

(9)

where qexp is the equilibrium capacity (mg/g) from the experimental data and qcal (mg/g) is the equilibrium capacity calculated from model applied. Low value of χ2 for pseudosecond order model further validates the best fit for adsorption process than the pseudofirst order. The adsorption isotherms Langmuir and Freundlich were evaluated, and the equations were used to model the adsorption capacities obtained as a function of the concentration of the adsorbed MB and residual MB in solution. The values of the 12

isotherm constants and maximum adsorption capacities (qmax) were calculated from the slope and intercept of Figure 10 and are presented in Table 4. The high qm values of 91.324 mg/g and 56.625 mg/g, respectively, for the HMMA and HHPMA manifest importance of designing an effective adsorbent by choosing the proper polymeric component and also the high potential of the HMMA for MB removal. The higher value of R2 for the Langmuir isotherm indicated that the Langmuir model best fits the adsorption process via the monolayer formation with active sites predominantly having uniform structure such as –COO_ and adsorption capacity is dependent on the adsorbate concentration at equilibrium. A high KL value in Langmuir model indicates a low residual dye concentration in liquid phase in equilibrium with a high dye concentration in the hybrid material. Thus, the Langmuir isotherm provides a more rational description of the adsorption by the hybrid materials because it accounts for similar binding sites and their interactions, surface homogeneity, and the power of the sorbent surface. 3.5. Thermodynamic studies The thermodynamic parameters were evaluated from the slope and intercept of the Van’t Hoff plot (Figure 11) and are presented in Table 5. With the increase in temperature, the magnitude of Gibbs free energy change shifted to a high negative value, suggesting that adsorption was a rapid and spontaneous process. The positive value of ΔH° confirmed the endothermic nature of adsorption. The positive value of ΔS° suggested an increase in randomness at the solid-solution interface during the adsorption of MB onto the hybrid materials. 4. Conclusions New poly(methacrylate)/silica nanohybrid materials were synthesized by the emulsifier–free emulsion polymerization. These were found to be effective and selective methylene blue adsorbents with maximum adsorption at pH 9.0 and at 45 °C. The 13

adsorption process follows pseudo–second order model and Langmuir adsorption isotherm with a high qm value of 91.324 mg/g and 56.625 mg/g, respectively, for the HMMA and HHPMA- based hybrid materials. The cumulative adsorption capacities were obtained after repeated cycles as the hybrid materials are reusable. The materials exhibited high reusability when studies up to six cycles; hence, these hybrid materials can be prospective candidates for the removal of cationic dyes from the polluted water. Author Information Notes The authors declare no competing financial interest. Acknowledgments Department of Chemistry, Himachal Pradesh, University, Shimla, HP, India, is acknowledged for providing research facilities.

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20

833.4 749.7 962.9

519.6

796.3 963.6

HMMA

Si-O-Si stch(sym)

2000

C-O stch

1500

1166.6

1264.4

2500

1716.2

C=O stch

3000

1270.6

1486.4 1451.1 1390.2

2360.7 2341.8

3500

2999.2

3436.1

4000

Si-O-Si stch(asym

Si-O-Si stch(sym)

C-N stch

-CH stch -NH stch

HHPMA

668.8

1718.6

2988.3

C-O stch

C=O stch

%T

3475.2

-NH stch

1175.1

-CH stch

1390.2

1487.9

C-N stch

Si-O-Si stch(asym)

1000

-1

Wavenumber (cm ) Figure 1: FTIR spectra of the as-synthesized hybrid materials

21

500

(a)

(b)

(c)

(d)

Figure 2: SEM images of the hybrid materials HMMA (a), HHPMA (b), MB-loaded-HMMA (c) and MB-loaded-HHPMA (d)

22

(a)

Element C N O Si Total

Weight % 59.00 13.18 24.69 3.13 100

(b)

Atomic % 66.81 12.03 19.73 1.43

Element C N O Si Total

Weight % 68.47 1.34 21.44 8.75 100

Atomic % 76.54 1.29 17.99 4.18

Figure 3: EDX images of HMMA (a) and HHPMA (b)

(a)

(b)

Figure 4: TEM images of hybrid materials HMMA (a) and HHPMA (b)

23

400

400

HHPMA

HMMA 350

350

300

Intensity (a.u.)

250 200 150

250 200 150 100

100

50

50

0

0 10

20

30

40

50

60

70

80

20



40

60



Figure 5: XRD spectra of the hybrid materials

1 0

HMMA HHPMA

-1

pHf-pHi

Intensity (a.u.)

300

-2 -3 -4 -5 -6 -7 2

4

6

8

10

pHi

Figure 6: Plot of pHf –pHi vs. pH 24

80

(a)

(b)

6.0

HMMA

5.5

9.5

MB MG CR

5.0 4.5

HMMA HHPMA

9.0

4.0

8.5

3.5 3.0

8.0

2.5

q(mg/g)

2.0

7.5 0

50

100

150

200

250

300

350

q(mg/g)

1.5 400

6.0 5.5

HHPMA

5.0

7.0 6.5

4.5 4.0

6.0

3.5 3.0

5.5

2.5 2.0

5.0

1.5 0

50

100

150

200

250

300

350

400

2

4

6

8

10

Time(min)

pH

(c)

(d)

30

9

q(mg/g)

8

7

q(mg/g)

HMMA HHPMA

25

HMMA HHPMA

20

15

6 10

5 5

4 0

5

10

15

20

25

30

35

40

45

Concentration(ppm)

3 25

30

35

40

45

50

o

Temp ( C)

Figure 7: Effect of (a) contact time on the uptake of MB,MG and CR by HMMA and HHPMA (initial concentration,10 mg/L; shaking rate, 200 rpm; pH 7.0;30 °C), (b) pH (initial concentration, 10 mg/L; contact time,300 min; shaking, 200 rpm; 45 °C), (c) temperature (initial concentration, 10 mg/L; contact time, 300 min; shaking, 200 rpm; pH 7.0), and (d) concentration (contact time, 300 min; shaking, 200 rpm; pH 9.0; 45 °C) on the uptake of Methylene Blue

25

12

30

HMMA HHPMA

25

q(mg/g)

20

15

10

5

0 1

2

3

4

5

6

No. of cycles

Figure 8: Reusability studies after adsorbent regeneration (initial concentration, 30 mg/L; contact time, 300 min; shaking rate, 200rpm; pH 9.0; 45 °C)

( a)

(b)

0.5

90

HMMA HHPMA

0.0

HMMA HHPMA

80 70 60

t/qt(min.g/mg)

log (qe-qt)

-0.5

-1.0

50 40 30

-1.5

20 10

-2.0

0 0

50

100

150

200

250

0

300

50

100

150

200

Time (min)

Time (min)

26

250

300

350

400

(c)

(d) 7

4.4 4.2

5 4

qt (mg / g)

4.0 3.8

qt(mg/g)

Experimental Pseudo-first order Pseudo-second order Elovich equation

HMMA

6

HMMA HHPMA

3.6

3 2 1 0

50

100

150

200

250

300

350

400

100

150

200

250

300

350

400

7

3.4

HHPMA

6

3.2 5

3.0

4

2.8

3 2

2.6 1

3.0

3.5

4.0

4.5

5.0

5.5

0

6.0

50

Time (min)

ln t

Figure 9: Plot of different kinetic models: (a) pseudo-first order model, (b) pseudosecond order model, (c) Elovich equation, and (d) their comparison with experimental data for HMMA and HHPMA.

(a)

(b)

0.25

1.8

HMMA HHPMA

HMMA HHPMA

1.6

0.20

log qe

1/qe

1.4 0.15

0.10

1.2

1.0

0.8

0.05

0.6 0.00 0.0

0.5

1.0

1.5

2.0

1/Ce

-0.4

-0.2

0.0

0.2

0.4

0.6

log Ce

Figure 10: Plot of different adsorption isotherm models: (a) Langmuir isotherm, (b) Freundlich isotherm. 27

0.8

1.0

1.2

2.0

HMMA HHPMA

1.5

lnKc

1.0

0.5

0.0

-0.5

-1.0 0.00310

0.00315

0.00320

0.00325

0.00330

0.00335

0.00340

1/T

Figure 11: Thermodynamic plot of lnKc vs. 1/T for the hybrid materials CH3

CH3

CH3

CH 3

KPS H 2C

C

n +

H 2C

C

H 2C

n

COOCH3

COOH

methacrylic acid (MAA)

CH 2

C

C

n

COOCH 3

COOH

methylmethacrylate (MMA) APTES

CH 3 H 2C

C

CH3 CH 2

C

CH 3

O

n

O

i)TEOS

Si COOCH3

C O

N H

O

O

(CH 2)3Si

O

Si

O Si

O

H 2C

C

CH2

C

n

COOCH 3

C

O

O

ii) Condensation

CH3

O NH

O

(CH 2)3Si (OC2H5) 3

O

O

poly(MAA-MMA)-silica hybrid [HMMA]

Scheme 1: Proposed mechanism for the synthesized hybrid materials

28

Table 1: Composition of hybrid materials

Hybrid HMMA PVA (4.0g) HHPMA PVA (4.0g)

Composition MAA (2.0g) + MMA ( 2.0g)

+

APTES (0.2 g)

MAA (2.0g) + HPMA (2.0g) + APTES

(0.2g)

+ TEOS (0.50g) + + TEOS (0.50g) +

Table 2: Comparison of maximum adsorption capacities for the adsorption of MB onto different hybrid materials Hybrid material GNS/Fe3O4 composite PProDOT/MnO2 PANI nanotube base/silica composite TiO2–GNs (MAA-co-MMA)/SiO2 [HMMA] (MAA-co-HPMA)/SiO2 [HHPMA]

Maximum adsorption capacity (mg/g) 35.73 13.94 10.3 83.3 85.33 87.37

References 37 51 52 53 Present work Present work

Table 3: Comparison of rate constants and correlation coefficients for different kinetic models applied Kinetic model applied

Pseudo-first-order

Pseudo-second-order

Elovich equation

Kinetic parameter

HMMA

HHPMA

qe k1 R2 χ2 qe k2 R2 χ2 α β R2

2.8054 17.04 × 10-3 0.9758 0.7163 4.4662 11.82 × 10-3 0.9997 0.0132 6.2007 1.9387 0.9911

2.272 11.05 × 10-3 0.9944 1.5972 4.5146 7.72 × 10-3 0.9993 0.0252 1.9450 2.3255 0.9954

29

Table 4: Comparison of isotherm constants and correlation coefficients for different isotherm models applied Adsorption isotherms

Isotherm constants

HMMA

HHPMA

qm KL R2

91.324 11.2 ×10-2 0.9703

56.625 12.5 × 10-2 0.9843

n KF R2

1.915 10.094 0.8696

1.732 7.175 0.9334

Langmuir

Freundlich

Table 5: Thermodynamic parameters for adsorption of MB by the hybrid material

HMMA Temp. (°K)

-ΔG° (KJ/mol)

ΔH° (KJ/mol)

298 303 308 313 318 323

-1.574 -0.789 1.134 3.609 4.854 4.933

90.582

HHPMA ΔS° (J/mol/K) 298.25

30

-ΔG° (KJ/mol)

ΔH° (KJ/mol)

ΔS° (J/mol/K)

-1.574 -0.837 1.100 3.427 4.751 4.828

89.12

293.30