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Amphoteric gellan gum-based terpolymer–montmorillonite composite: synthesis, swelling, and dye adsorption studies

Amphoteric gellan gum-based terpolymer–montmorillonite composite: synthesis, swelling, and dye... Int J Ind Chem (2017) 8:345–362 DOI 10.1007/s40090-017-0126-z RESEARCH Amphoteric gellan gum-based terpolymer–montmorillonite composite: synthesis, swelling, and dye adsorption studies 1 1 Sirajo Abubakar Zauro B. Vishalakshi Received: 5 October 2016 / Accepted: 14 June 2017 / Published online: 6 July 2017 © The Author(s) 2017. This article is an open access publication Abstract A terpolymer gel, Gellan gum-graft-poly(2- Introduction acrylamido-2-methyl-1-propanesulfonic acid-co-dimethy- laminopropyl methacrylamide) and its composite with the The increasing demand for manufactured products world- clay, Montmorillonite, was prepared by free-radical poly- wide and the use of synthetic dyes in various industries merization and crosslinking reactions in solution. The such as textile, leather, paper, rubber, plastic, cosmetic, etc. terpolymer gel and the clay composite were characterized led to the proportionate release of a large quantity of using FTIR, TGA, SEM, and X-ray diffraction techniques. effluent into the environment. In addition, this effluent Swelling studies were carried out in different pH and salt contained non-biodegradable toxic and carcinogenic dye solutions. The gel showed maximum swelling capacity in substances into the environment [1–5]. Rhodamine B alkaline medium, while the composite showed higher (RhB) and chromotrope 2R (C2R) are synthetic dyes that swelling in neutral medium. The swelling of the gel and the are commonly used in leather, textile, and paper industries composite followed second kinetics model and water and cause various health hazards [6, 7]. transport is found to be a less Fickian diffusion process. The composites and nanocomposite of polymer–clay The terpolymer gel and the composite were evaluated for have been gaining increase attention by researchers the adsorption of rhodamine B (RhB) and chromotrope 2R globally due to the hybrid properties which they exhibit (C2R) dyes. Rhodamine B is found to be adsorbed to a when compared with either the polymer or clay separately higher extent than chromotrope 2R and the adsorption [8]. A wide range of polymer–clay composite/nanocom- isotherm studies suggested that adsorption of both RhB and posite has been produced and used for a variety of C2R on the terpolymer gel was best explained by Langmuir applications such as water treatment [5, 8], dye adsorption model, while the adsorption on the Composite fitted best [2, 9–11], etc. into Freundlich model. Similarly, the adsorption kinetics Several physical and chemical methods like chemical data for both RhB and C2R dyes followed the second-order precipitation, ion exchange, membrane separation, chemi- kinetics. cal reduction, chemical oxidation, advanced oxidation processes (AOPs), etc. [12] have been employed in the Keywords Gellan gum · 2-Acrylamido-2-methyl-1- removal of toxic substances from the environment. How- propanesulfonic acid · Dimethylaminopropyl ever, these methods are ineffective in removing most of the methacrylamide · Montmorillonite · Swelling · dyes molecules and are time-consuming, not cost-effective, Dye adsorption and sometimes generate large amount of sludge that are toxic to the biotic organisms in the environment. Hence, adsorption using biopolymer-based composites has been & B. Vishalakshi described as one of the effective and promising techniques vishalakshi2009@yahoo.com for removal of pollutants due to its simplicity, inexpen- siveness, etc. [13–18]. Department of Post-Graduate Studies and Research in Several biopolymer-based hydrogels such as Gum gatti Chemistry, Mangalore University, Mangalagangothri, [19], Gur gum [14], Kappa carrageenan [20–22], chitosan Dakshina Kannada, Mangalore, Karnataka 574199, India 123 346 Int J Ind Chem (2017) 8:345–362 [23], and Guaran [24] were studied as adsorbents for the Methods removal of dyes from aqueous solution. Casey and Wilson [25] reported the adsorption of Methylene blue (MB) dye Synthesis of GG-g-AMPS on Chitosan-PVA composite films and direct relationship GG-g-AMPS was prepared via free-radical polymerization between film composition (Chitosan-PVA) with solution pH and the uptake of MB were observed. Similarly, process as follows: 0.15 g GG was dissolved in distilled and stirred overnight. To the resultant solution, varying Datskevich et al. [26] synthesized cationic starch and sodium alginate-based composite and studied the adsorp- amounts (0.1–0.30 g) of AMPS were added followed by APS (0.05) under continues stirring. The temperature was tion of Methyl orange and MB under different conditions. The adsorption of congo red on Chitosan/Montmorillonite raised to 40 °C under continues stirring for 2 h. The gel was composite has been studied by Wang and Wang [1]. Vasugi precipitated with acetone and washed with methanol sev- and Girija [4] reported the adsorption of reactive blue dye eral times, and dried in an oven at 50 °C for 24 h. on hydroxyapatite-alginate composite. The composite materials consisting of clay and a Synthesis of GG-g-poly(AMPS-co-DMAPMAm) biopolymer are very effective in removal of dyes due to the The graft copolymer GG-g-poly(AMPS-co-DMAPMAm) availability of numerous functional groups on the biopolymer and the clay for binding with the dye mole- gel was synthesized based on the established methods reported by Nie et al. [27] with a little modification as cules, rendering the materials useful as adsorbents. The aim of the present study is to obtain a functional follows: a known amount of GG (0.1 g) was dissolved in composite hydrogel consisting of clay, a biopolymer, and distilled water and stirred overnight at room temperature. A a synthetic polymer to be evaluated as an adsorbent for specified amount of AMPS (0.1–0.30 g) and DMAPMAm dyes. This has been achieved by polymerizing AMPS, (0.15–0.50 g) were added to the above solution. To the DMAPMAm, and MBA in the presence of gellan gum mixture above, APS (0.05 g) and MBA (0.05 g) were added and montmorillonite (MMT) clay in water and its effec- and stirred by raising the temperature to 60 °C slowly for tiveness as an adsorbent for removal of dyes has been 4 h maintaining the temperature at 60 °C until a gel-like studied using chromotrope 2R and rhodamine B as model solution was formed. It was then allowed to cool for an hour to complete the polymerization and added to excess ionic dyes. acetone to remove un-reacted components. The gels obtained were then washed with 50% ethanol and placed in Materials and methods a hot oven at 50 °C until constant weight was obtained. The GG-g-poly(AMPS-co-DMAPMAm) gel formation was Materials optimized. The percentage yield and grafting percentage (GP) were calculated by the following equation: Gellan gum (GG) was purchased from Sigma-Aldrich Experimental yield %Yield ¼  100; ð1Þ Chemicals Pvt Ltd., Bangalore, India. 2-Acry- Theoretical yield lamidomethyl-2-propane sulfonic acid (AMPS), ðw  w Þ 1 o dimethylaminopropyl methacrylamide (DMAPMAm), N, GPðÞ % ¼  100; ð2Þ N-methylene-bis-acrylamide (MBA), and montmorillonite (MMT) were obtained from Sigma-Aldrich Chemie, where w and w are the weight of grafted gels and 0 1 GmbH, Germany. Ammonium peroxodisulphate (APS) monomers, respectively. was obtained from Spectro Chem Pvt. Ltd., Mumbai, India. Rhodamine B (RhB) was obtained from s.d. Fine Chemical Synthesis of GG-g-poly(AMPS-co-DMAPMAm)/MMT Limited, Mumbai, India. Chromotrope 2R (C2R) was purchased from Loba Chemie Limited Mumbai, India. The GG-g-poly(AMPS-co-DMAPMAm)/MMT composite Acetone was obtained from Nice Chemicals Pvt Ltd., hydrogel was made following the same procedure as in Kerala, India. Methanol was obtained from Himedia Lab- 2.2.2 with the addition of MMT (0.01–0.03 g) after adding oratories Pvt Ltd., Mumbai, India. NaCl, KCl, FeCl , DMAPMA under continuous stirring slowly during 1 h. CaCl , and Na SO were obtained from Merck Ltd., 2 2 4 Mumbai, India. DMAPMAm was purified by passing Characterization through column containing alumina gel before use. All other reagents were used as received. Distilled water was The GG, GG-g-AMPS, GG-g-poly(AMPS-co-DMAPMA)- used throughout the experiments. 8, and GG-g-poly(AMPS-co-DMAPMA)/MMT-3 samples 123 Int J Ind Chem (2017) 8:345–362 347 were characterized using FTIR, TGA, SEM, and XRD varied amount of the adsorbent and allowed to stand for techniques. The FTIR were recorded using FTIR-Prestige- 14 h and the resultant solutions were decanted and the −1 21, Shimadzu Japan, in the range of 4000–400 cm absorbance were recorded. The amount of dyes adsorbed at −1 wavenumber during 40 scans, with a resolution of 2 cm . time t (q ) and at equilibrium (q ) in mg/g was calculated t e Thermograms were recorded using standard DSC-TGA using the following equations [29, 30]: (Q600 V20.9 model) Japan, by heating the samples in the ðC  C Þ 0 t q ¼  V ; ð5Þ ranges of 30–700 °C, under a nitrogen atmosphere at 10 ° C/min. Surface morphology of the samples was obtained ðC  C Þ 0 e on gold coating JOEL JSM-6380LA analytical Scanning q ¼  V ; ð6Þ electron microscope (SEM) under magnification of 2000 at 20 kV. XRD pattern was recorded on X-ray diffractometer where q and q are the amount of dyes adsorbed (mg/g) at t e (Rigatu Miniflex 600-XRD instrument, USA) using Cu Kά time t = t and at equilibrium, respectively. C , C , and C 0 t e radiation generated at 35 kV and 35 mA in the differential are dyes concentration (mg/L) at time t = 0, t = t, and at ° ° angle 2θ at a range of 0 –80 in steps of 0.020/s. equilibrium, respectively, M is the weight of the gel (g) and V is the volume (L) of the dye solution. Swelling studies Swelling experiments of the GG-g-poly(AMPS-co- Results and discussion DMAPMA)-8 and GG-g-poly(AMPS-co-DMAPMA)/MMT- 3 samples were carried out in different media (pH and salts The composite hydrogels were prepared by crosslink solution). A known amount of the samples were weighed and copolymerization of AMPS, DMAPMAm, and MBA in immersed into swelling media at room temperature. After water in the presence of GG. MMT was incorporated in situ specified interval of time, the samples were removed and the in the copolymer network. During the polymerization excess surface water was wiped away gently using blotting reaction, the bi-functional MBA copolymerizes with (tissue) paper and re-weighed. This procedure was repeated AMPS and DMAPMAm to form a network, while GG until equilibrium is reached. The data were reported as the takes part in the free-radical polymerization reaction by mean of three different measurements. The effects of nature of forming macroradicals [31]. Thus, a composite gel is different salts solution (0.1 M) on the swelling ratio were also formed by entrapment of MMT clay in the copolymer studied in the same manner. network. The free-radical reaction mechanism along with The swelling ratio (SR) and swelling equilibrium (S ) the formation of the gel network is shown in Scheme 1. eq were calculated by the following equations: The composition of the gels/composites and the per- centage yield is presented in Table 1. The optimized ðW  W Þ t o SR = ¼ ; ð3Þ g product [GG-g-poly(AMPS-co-DMAPMAm)-8] was used for composite formation and used as representative sample ðW  W Þ e 0 for swelling and dye adsorption studies. The grafting S = ¼ ; ð4Þ eq conditions were optimized by varying monomer (DMAP- MAm and AMPS) contents and keeping all other where W , W , and W are the weight of the gel at time 0 t e parameters constant. t = 0, t = t, and at equilibrium, respectively [28]. The GP increases as the AMPS content increases from 0.1 to 0.25 g, and decreases as AMPS content increases to Dyes adsorption studies 0.30 g (Table 1). For DMAPMAm, the GP follows a similar pattern as in AMPS. The decreases in GP as the A known amount of GG-g-poly(AMPS-co-DMAPMAm)-8 content of monomers increases could be attributed to the and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 samples less reactive side on the GG as its content remains constant, were left immersed in 100 mg/L solutions of RhB and C2R dyes. At different time intervals, 2.5 mL of the supernatant and hence, there are more molecules of DMAPMAm and AMPS than GG and this could lead to the formation of solution were withdrawn and the absorbance values were measured using UV–visible spectrophotometer (UV-1800 homopolymer and hence low yield. Shimadzu, Japan) λ of 554 and 510 nm for RhB and max C2R, respectively. Calibration curves were used to convert FTIR the absorbance measured into concentration using standard FTIR Spectra of GG, GG-g-AMPS, GG-g-poly(AMPS-co- solutions of 2, 4, 6, 8, and 10 mg/L of the dyes. Different DMAPMAm)-8, and GG-g-poly(AMPS-co-DMAPMAm)/ initial concentrations (10, 30, 50, 70, and 100 mg/L) were MMT-3 composite gels are shown in Fig. 1. The spectrum used for equilibrium adsorption studies by immersing 123 348 Int J Ind Chem (2017) 8:345–362 O. OH OH OH COOH COOH O O O (NH ) S O [APS] O 4 2 2 8 O O CH CH 3 O OH O OH O OH OH OH OH H H H H OH OH OH OH OH OH OH OH Macroradical (RO ) Gellan gum (ROH) OCH H C CH 3 3 ONH N NH NH MBA CH CH 3 2 DMAPMAm H C SO H MMT H C AMPS NH CH CH CH CH CH C OR RO CH CH 2 2 CO HC O C CO NH NH H C HN (CH ) CH 2 3 H C CH SO H HN CH H C CH HC CH CH OR RO C CH CH n 2 2 C O C O NH NH CH H C CH (CH ) 3 2 2 3 SO H N 3 CH H C 3 Scheme 1 Proposed scheme for the formation of GG-g-poly(AMPS-co-DMAPMAm) gel −1 (Fig. 1a) showed a broad absorption band at 3290 cm the SO H, respectively [33]. The additional characteristic −1 which is due to stretching of O–H and a medium absorption absorption bands of 1537 and 2771 cm for N–H −1 peak at 2926 cm corresponding to the C–H stretching of stretching and C–H stretching of –N(CH ) of DMAPMAm 3 2 −1 CH groups. The absorption at 1605 cm is related to the [25] were observed in the spectra of GG-g-poly(AMPS-co- − −1 C=O stretching of COO of the GG. The peak at 1016 cm DMAPMAm)-8 (Fig. 1c). Similarly, in addition to the is assigned to C–O bond stretching frequencies [32]. peaks on the spectra (Fig. 1a–c), peaks at 1040, 814, and −1 Comparing the GG and GG-g-AMPS (Fig. 1b) spectra, new 621 cm for Si–O–Si, Al–Al–OH, and Si–Al–OH [34], characteristic peaks were observed at 1643, 1438, and respectively, were observed on the spectrum of GG-g-poly −1 923 cm which are attributed to asymmetric stretching (AMPS-co-DMAPMAm)/MMT-3 (Fig. 1d) indicating the vibration of C=O, C–N stretching, and S–O stretching of entrapment of MMT on the gel matrices. 123 Int J Ind Chem (2017) 8:345–362 349 Table 1 Composition of hydrogels/composite and percentage yield Gel Code GG (g) AMPS DMAPMAm APS MBA MMT GP (%) Yield (g) (g) (g) (g) (%) GG-g-poly(AMPS-co-DMAPMAm) 1 0.1 0.1 0.15 0.05 0.05 63.18 57.87 2 0.1 0.1 0.20 0.05 0.05 64.08 60.32 3 0.1 0.15 0.20 0.05 0.05 72.91 58.73 4 0.1 0.15 0.25 0.05 0.05 73.81 69.80 5 0.1 0.20 0.25 0.05 0.05 79.13 50.91 6 0.1 0.20 0.30 0.05 0.05 79.72 62.32 7 0.1 0.25 0.30 0.05 0.05 89.12 68.92 8 0.1 0.25 0.40 0.05 0.05 89.82 79.56 9 0.1 0.30 0.40 0.05 0.05 88.12 64.39 10 0.1 0.30 0.50 0.05 0.05 87.65 63.87 Composite GG-g-poly(AMPS-co-DMAPMAm)/MMT 1 0.1 0.25 0.40 0.05 0.05 0.01 59.23 69.21 2 0.1 0.25 0.40 0.05 0.05 0.02 59.87 67.12 3 0.1 0.25 0.40 0.05 0.05 0.03 58.94 74.18 Fig. 1 FTIR spectra of a GG, b GG-g-APMS, c GG-g-poly (AMPS-co-DMAPMAm)-8, and d GG-g-poly(AMPS-co- DMAPMAm)/MMT-3 TGA loss of 14%, and is attributed to the loss of moisture con- tent in the polysaccharide. The second step of the The thermograms of GG, GG-g-AMPS, GG-g-poly decomposition occurs in the range of 210–260 °C with a (AMPS-co-DMAPMAm)-8, and GG-g-poly(AMPS-co- major weight loss of 36% due to the breaking of the gly- DMAPMAm)/MMT-3 are presented in Fig. 2. GG (Fig. 2a) cosidic linkage of the GG. The final decomposition of GG shows three degradation steps. The first step of degradation occurs around 270–540 °C with the weight loss of 32%. occurs between temperatures of 35–100 °C with the weight About 14% of the GG sample remains as residual matter at 123 350 Int J Ind Chem (2017) 8:345–362 DMAPMAm)/MMT-3 toward heat is attributed to the incorporation of MMT in the system [35]. SEM The surface morphology of GG, GG-g-AMPS, GG-g-poly (AMPS-co-DMAPMAm)-3, and GG-g-poly(AMPS-co- DMAPMAm)/MMT-8 is presented in Fig. 3. It could be deduced from Fig. 3b that grafting of AMPS on the GG changes the fibrous homogeneous surface of GG (Fig. 3a) into heterogeneous. Likewise, Fig. 3c shows a very distinct crystalline-like morphology suggesting the grafting of GG on poly(AMPS-co-DMAPMAm)-8. On the other hand, exfoliating MMT in the system also shows a considerable change in the surface morphology producing cotton like accumulation with an irregular shape that appears fibrous (Fig. 3d). Fig. 2 Thermograms of a GG, b GG-g-APMS, c GG-g-poly(AMPS- co-DMAPMAm)-8, and d GG-g-poly(AMPS-co-DMAPMAm)/ MMT-3 XRD The XRD patterns of MMT, GG, GG-g-AMPS, GG-g-poly 550 °C. For GG-g-AMPS (Fig. 2b), four decomposition (AMPS-co-DMAPMAm)-8, and GG-g-poly(AMPS-co- steps occur. With the first step in the range of 30–140 °C DMAPMAm)/MMT-3 samples are shown in Fig. 4. MMT with a weight loss of 13% due to loss of moisture in the diffractogram (Fig. 4a) shows a complete amorphous nat- sample and 10% weight loss between 182 and 265 °C, this ure of the clay MMT. GG diffractogram (Fig. 4b) showed might be due to the dissociation of GG from AMPS. two major peaks at 2θ values of 21.31 (medium) and Between 320 and 520 °C, a major weight loss (17%) 30.47 (sharp). The exhibition of this sharp peak at lower occurred and is linked to the degradation of the polysac- 2θ value indicated the crystalline nature of the GG. In GG- charide and final step decomposition at 555 °C with 25% g-AMPS sample (Fig. 4c); the disappearance of some weight loss leaving 4% as the residue. The thermogram of peaks and appearing of new one at a 2θ value of 5.82 GG-g-poly(AMPS-co-DMAPMAm)-8 (Fig. 2c) shows four indicated the grafting of AMPS on GG. The appearance of ° ° ° degradation steps with the first step losing 16% of the many sharp peaks at low 2θ (9.92 , 14.12 and 16.32 ) weight between 35 and 160 °C due to the elimination of values (Fig. 4d) indicated the incorporation of DMAP- moisture from the system. The second steps occurred in the MAm on GG-g-AMPS and it further indicates the semi- range of 324–510 °C with the loss of 18% which could be crystalline nature of the gel network [36]. The intercalation due to the breaking of the grafting between GG and the of MMT within the polymer gel network is evidence of the copolymers. The third degradation step results in weight major shift of the peak from the 2θ value of 18.79 (Fig. 4d) loss of 28% between the temperature range of 324–510 °C. to 22.18 . Similarly, the increases of d-spacing for MMT to ˚ ˚ The complete degradation of the grafted gel occurred 11.4527 A from the normal 9.8 A [34] are another evidence around 580 °C. For the GG -g-poly(AMPS-co-DMAP- to show the intercalation of MMT into the gel network. The MAm)/MMT-3 (Fig. 2d), the degradation steps are similar decrease in the intensity of the peak around 2θ value of 9.1 and the disappearance of the peak at 2θ of 7.2 from Fig. 4d to that of GG-g-AMPS and are in four stages. The first was the elimination of water molecule from the composite gel are further evidences to prove the intercalation of MMT in the range of 33–180 °C. The second steps of the into the gel matrix. The disappearance of a small broad degradation occurred in the temperature range of 205– peak at a 2θ value of 31.69 in Fig. 4e also showed the 256 °C with a weight loss of 16% and could be due to the intercalation of MMT into the gel network which resulted degradation of GG. The third step occurred between 317 in decreases in the degree of crystallinity [37]. and 535 °C with major weight loss of 20% and the final step of degradation occurred at 633 °C leaving around 13% Swelling responsive in different salt solution as residual matter. The GG-g-AMPS, and GG-g-poly (AMPS-co-DMAPMAm)-8 showed low thermal stability The effect of different salt solution (Fig. 5) on the swelling compared GG and GG-g-poly(AMPS-co-DMAPMAm)/ ratio of GG-g-poly(AMPA-co-DMAPMAm)-8 and GG-g- poly(AMPA-co-DMAPMAm)/MMT-3 samples shows MMT-3. The high stability of GG-g-poly(AMPS-co- 123 Int J Ind Chem (2017) 8:345–362 351 Fig. 3 SEM images of a GG, b GG-g-AMPS, c GG-g-poly(AMPS-co-DMAPMAm)-8, and d GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 Fig. 4 XRD diffractograms of a MMT, b GG, c GG-g-AMPS, d GG-g-poly(AMPA-co- DMAPMAm)-8, and e GG-g- poly(AMPA-co-DMAPMAm)/ MMT-3 higher swelling capacity in NaCl solution compared to such as nature of cations (charge and radius of cations). other salt solution with the least swelling response in The greater the charge of cations, the greater the cross FeCl . The swelling behavior is affected by many factors linking degree which results in decrease in swelling [28]. 123 352 Int J Ind Chem (2017) 8:345–362 ratio (S ) in gram of water per gram of gel/composite, eq initial swelling rate (R ) in g of water/g gel/composite −1 min , swelling rate constant (K ), in g gel/composite/g −1 water min ), swelling exponent (n), and maximum equi- −1 librium swelling (S ) in g of water g of sample were max calculated using the dynamic swelling data obtained from various plots shown in Figs. 6 and 7. The swelling mech- anism of the gel/composite was experimentally determined by employing a second-order kinetic equation as follows: ds ¼ k ðs  sÞ ; ð7Þ s eq dt where k and s are the swelling rate constant and degree s eq of swelling at equilibrium, respectively. The above equa- tion on integration over the limit S = S at t = t and 0 o S = S at t = t gives t 1 1 Fig. 5 Effects of different salt solution (0.1 M) on swelling ratio of ¼ þ t; ð8Þ SR k s s GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co- s eq eq DMAPMAm)/MMT-3 where k s is equal to R which is the initial swelling eq rate, s is the equilibrium swelling, and k is the swelling eq s The swelling of the gel is due to osmotic pressure differ- rate constant. The plot of t/SR vs t (Figs. 6b, 7b) pro- ence developed between the gel and the external salt duced a linear straight line with a slope of 1/s and eq solution due to the charge screening effect of the salt intercept of : This indicates that the second-order solution. The composite hydrogels exhibited salt sensitivity k s eq kinetics is followed by the swelling process. Furthermore, as reported in the literature [38, 39]. the calculated s from the slope are in good agreement eq with the experimental value as shown in Table 2. The Effect of pH on the swelling ratio initial swelling rate (R ) of the GG-g-poly(AMPS-co- DMAPMAm)/MMT-3 decreases drastically from acidic to The swelling ratios of the gel in different pH media were basic medium. A similar finding was reported [45]. studied and the results reported in Fig. 6a. Figure 6 showed However, the swelling rate constant (k ) increases as the higher swelling ratio (SR) in basic medium (pH 9.0) with pH values increase. lower SR in acidic medium (pH 1.2) by GG-g-poly(AMPS- The mechanism of diffusion is one of the factors that co-DMAPMAm)-8. The presence of free amino group on govern the applicability of materials [42]. The absorption DMAPMAm leads to the formation of many hydrogen process involves the diffusion of water molecules into the bonds in an alkaline medium which will restrict the free spaces of the materials which increase the segmental relaxation of network chain. While in acidic medium, the mobility and consequently result in an expansion of chain free amino group is expected to ionize which may result in segment between crosslink and later result in swelling. The the breakage of hydrogen bond and generate electrostatic dynamics of water sorption process was studied using the repulsion on the polymer chain [40]. In acidic medium, the 2− simple empirical equation called power law equation which attraction between SO and quaternary ammonium group is used mostly in determining the mechanism of diffusion restricts the swelling [41]. Furthermore, the carbonyl group in the polymeric network [46]: of the AMPS and DMAPMAm forms hydrogen bonding with the GG which also aids in reducing the swelling F ¼ Kt : ð9Þ [42, 43]. The above equation can be rewritten in form of ln as Swelling kinetics follows: lnF ¼ lnK þ nlnt: ð10Þ The mechanisms of swelling studies for the gel/composite The values of K and n were calculated from the intercept were carried out based on the standard methods reported in and slope of lnF vs t plots (Figs. 6c, 7c) and tabulated in the literature [44, 45]. The swelling parameters of the Table 2, where F, n, and K are swelling power, swelling gel/composite were determined from the various plots exponent, and swelling rate constant, respectively. (Figs. 6, 7). The parameters such as swelling equilibrium 123 Int J Ind Chem (2017) 8:345–362 353 Fig. 6 Swelling curves for GG-g-poly(AMPS-co-DMAPMAm)-8 gel Depending on the diffusion rate of the material relax- Fickian diffusion” behavior [46]. In this study, the values ation, three different diffusion mechanisms are proposed of n (Table 2) are all below 0.5. Hence, it is said to follow a [42–45, 47]: less Fickian diffusion mechanism. 1. Fickian diffusion in which the diffusion rate is less Dye adsorption studies than the relaxation rate (n = 0.50); 2. Diffusion which is rapid compared the relaxation The dye adsorption studies were carried out using rho- processes (n = 1); and damine B and chromotrope 2R as model dyes and their 3. Non-Fickian or anomalous diffusion which occurs structures are given in Fig. 8. when the rate of diffusion and that of relaxation are comparable (0.50\ n\ 1). Effects of contact time on the adsorption capacity The Fickian diffusion, actually, refers to a situation where water penetration rate in the gels is less than the The effect of contact time on the adsorption of dyes RhB polymer chain relaxation rate. Therefore, n = 0.5 indicates and C2R on both GG-g-poly(AMPS-co-DMAPMAm)-8 a perfect Fickian process. Nevertheless, when the water and GG-g-poly(AMPS-co-Dmapmam)/MMT-3 (Fig. 9) penetration rate is much below the polymer chain relax- showed an increase in the adsorption capacity slowly with ation rate, it is possible to record the n values below 0.5. increase in time. RhB showed higher adsorption compared This situation is still regarded as Fickian diffusion or “Less to C2R; this could be attributed to the presence of many 123 354 Int J Ind Chem (2017) 8:345–362 Fig. 7 Swelling curves for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 Table 2 Kinetics swelling and PH S (g/g) R (g of water/g of sample) K (g/g) NK S eq i s max diffusion parameters of GG-g- poly(AMPS-co-DMAPMAm)-8 GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co- 1.2 2.45 1.69 0.099 0.10 1.03 2.38 DMAPMAm/MMT-3 7.0 3.68 0.20 0.379 0.04 7.48 3.65 9.0 6.85 2.07 0.010 0.24 2.40 6.32 GG-g-poly(AMPS-co-DMAPMAm/MMT-3 1.2 8.93 10.33 0.0012 0.48 3.733 6.86 7.0 13.70 2.71 0.0020 0.31 4.383 11.93 9.0 6.67 2.12 0.0106 0.2 3.679 6.36 negative (acidic groups) site on the adsorbent which could rate especially on the GG-g-poly(AMPS-co-DMAPMAm)/ result in the formation of an electrostatic attraction with the MMT-3. This could be due to the repulsion between the positive (basic groups) part of the adsorbate. The adsorp- anionic (basic site) groups on both C2R and the composites tion of C2R on both the adsorbents is proceeded at slower [48]. 123 Int J Ind Chem (2017) 8:345–362 355 Fig. 8 Structures of the dyes used: a rhodamine B and b chromotrope 2R intercept and slope of the linear plot of C /q versus C e e e (Figs. 10a, b, 11a, b) and presented in Table 3. The essential feature of the Langmuir isotherm can be represented in terms of separation factor (dimensionless equilibrium parameter) R [53, 54], which can be expressed as follows: R ¼ ; ð12Þ 1 þ K C L o where C is the initial concentrations of dyes, K is the 0 L constant related to the energy of adsorption (Langmuir Constant). RL value indicates the favorability nature of adsorption. If R [ 1, the adsorption is unfavorable; if R = 1, the adsorption is linear; if 0 \ R \ 1, the L L adsorption is favorable; and if R = 0, then the adsorption is irreversible. From the data reported in Table 3, the R is greater than 0 but less than 1 indicating the favorability of Langmuir isotherm for the adsorption of RhB and C2R. Fig. 9 Amount of dyes (rhodamine B and chromotrope 2R) adsorbed Similarly, comparing the q calculated (33.33 and (mg/g) on GG-g-poly(AMPS-co-DMAPMAm)-8 gel and GG-g-poly 16.13 mg/g) with the experimental q (35.7 and 17.73 mg/ (AMPS-co-DMAPMAm)/MMT-3 composite over time g), respectively, for RhB and C2R on GG-g-poly(AMPS- co-DMAPMAm)-8. This indicated the formation of a Adsorption isotherm monolayer of RhB and C2R on the surfaces of GG-g-poly (AMPS-co-DMAPMAm)-8. Adsorption isotherms usually describe the performance of The Freundlich adsorption isotherm is based on the adsorbents in adsorption processes by describing the sur- assumption that encompasses the heterogeneity of the face interaction between the adsorbent and adsorbate [49]. surface and the adsorption capacity related to the equilib- There are various isotherm models used to describe the rium concentration of the adsorbate. The Freundlich adsorption processes. In this study, the two most common isotherm is commonly expressed as follows: used adsorption isotherms, namely, Langmuir isotherm [50, 51] and Freundlich isotherm [52], are employed. The Langmuir isotherm is a model which quantitatively ln q ¼ ln k þ lnC ; ð13Þ e f e describes equilibrium monolayer adsorbate formation on where q and C are the amount of dyes adsorbed (mg/g) the surface of the adsorbent, and is expressed as follows: e e and the equilibrium concentration of dyes (mg/L), respec- C 1 1 ¼  C þ ; ð11Þ e tively, K and n are Freundlich adsorption isotherm q q K q e m L m constants that represent the adsorption capacity and the where C and q are the equilibrium concentration of dye degree of nonlinearity between the dye concentration and e e (mg/L) and the amount of dye adsorbed (mg/g), respec- the adsorption, respectively. The values of K and n were tively, q is the maximum adsorption corresponding to calculated from the intercept and slope of the plot between m, complete monolayer coverage on the surface (mg/g), K is L ln q and ln C (Figs. 10c, d, 11c, d) and are presented in e e the Langmuir constant which is related to the energy of Table 3. The value of n indicates whether the adsorption is adsorption (L/mg). K and q are determined from the favorable or otherwise. If it lies within the range of 1–10, L m 123 356 Int J Ind Chem (2017) 8:345–362 Fig. 10 Adsorption isotherms for rhodamine B dye. a Langmuir gel, c Freundlich isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 gel, b Langmuir gel, and d Freundlich isotherm for GG-g-poly(AMPS-co-DMAP- isotherm for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite MAm)/MMT-3 composite then the adsorption is considered favorable. In this case, the MMT-3] is shown in Fig. 9. The rate of adsorption of the value of n lies between 1.23 and 4.83 which shows a dye uptake was little slow especially with respect to C2R favorable adsorption. Similarly, the R values for the compared to RhB adsorption. The maximum adsorption adsorption of RhB and C2R on GG-g-poly(AMPS-co- observed in C2R was 17.72 and 16.99 mg/g, respectively, DMAPMAm)/MMT-3 are 0.994 and 0.996, respectively, for GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly which are higher when compared with 0.989 and 0.990, (AMPS-co-DMAPMAm)/MMT-3 after 12 h. While higher respectively, for RhB and C2R on GG-g-poly(AMPS-co- adsorption capacity of 35.70 and 31.20 mg/g of RhB was DMAPMAm)-8. Hence, we can say that adsorption of RhB recorded for GG-g-poly(AMPS-co-DMAPMAm)-8 and and C2R on GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 GG-g-poly(AMPS-co-DMAPMAm)/MMT-3, respectively, base fits into the Freundlich model. at 12 h. The adsorption of RhB on different adsorbents has been reported [7, 49, 55, 56]. Kinetic studies To investigate the mechanism of adsorption, the adsorption data obtained in this work were subjected to The adsorption capacity of RhB and C2R dyes as a func- various kinetics models. The models employed in this work tion of time by the adsorbents [GG-g-poly(AMPS-co- are Lagergren’s pseudo-first-order [55, 57] and pseudo- DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/ second-order kinetic models. 123 Int J Ind Chem (2017) 8:345–362 357 Fig. 11 Adsorption isotherms for chromotrope 2R dye. a Langmuir gel, c Freundlich isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 gel, b Langmuir gel, and d Freundlich isotherm for GG-g-poly(AMPS-co-DMAP- isotherm for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite MAm)/MMT-3 composite found to be in sharp disagreement with the q in all Pseudo-first-order kinetics e exp cases. Furthermore, the values of correlation coefficients are low, which is an indication of bad quality linearization. The Lagergren’s pseudo-first-order kinetic model is based on assumption that the rate of adsorption of adsorbate with Hence, the adsorption cannot be said to be of first order. It has been suggested that the differences in experimental and time is directly proportional to difference in equilibrium concentration and concentration with time and this can be theoretical q values are that there is a time lag due to represented as follows: external resistance controlling at the beginning of the adsorption [57]. logðÞ q  q ¼ log q  t; ð14Þ e t e 2:303 Pseudo-second-order kinetics where q and q are the amount of dyes adsorbed (mg/g) at e t equilibrium and at time t, respectively. K is the rate con- The kinetics studies of RhB and C2R adsorption on the −1 stant (h ) for the pseudo-first-order kinetics and t is the adsorbents was carried out using the initial dyes concen- time (h) of adsorption. The value of K and R were cal- tration of 100 mg/L in all cases. The pseudo-second-order culated from the plot of log(q − q ) versus t and tabulated e t rate expression of Ho and McKay [58] was adopted in this in Table 4. The q for the first-order rate equation was e cal study and the best model that fit the adsorption was 123 358 Int J Ind Chem (2017) 8:345–362 Table 3 Isotherm model parameters for the adsorption of rhodamine B and chromotrope 2R dyes on GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 Dyes RhB C2R Adsorbent GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- DMAPMAm)-8 DMAPMAm)/MMT-3 DMAPMAm)-8 DMAPMAm)/MMT-3 C 10–100 10–100 10–100 10–100 (mg/L) Langmuir R 0.43–0.88 0.797–0.975 0.045–0.320 0.19–0.165 model K 0.013 0.0026 0.212 0.509 q 33.33 90.91 16.13 3.66 (mg/ g) R 0.841 0.969 0.999 0.996 Freundlich k 10.91 2.76 1.21 0.89 model n 1.61 1.23 1.54 4.83 R 0.989 0.994 0.990 0.996 R linear regression correlation co-efficient Table 4 Pseudo-first-order and pseudo-second-order kinetics model data for the adsorption of rhodamine B and chromotrope 2R dyes on GG-g- poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 Dye RhB C2R Adsorbent GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- DMAPMAm)-8 DMAPMAm)/MMT-3 DMAPMAm)-8 DMAPMAm)/MMT-3 Q (mg/ 35.70 31.92 17.73 17.56 e exp g) Pseudo-first-order q g/ 45.42 15.34 5.84 2.88 e cal (m kinetic model g) −1 K (hr ) 0.15 0.198 0.32 0.124 R 0.935 0.856 0.875 0.777 Pseudo-second-order q mg/ 37.04 34.48 18.18 17.24 ecal ( kinetic model g). K (g/ 0.016 0.028 0.016 0.01 −1 mghr ) R 0.991 0.984 0.999 0.996 R linear regression correlation co-efficient higher than those in the pseudo-first-order model and they selected based on the values of the linear regression cor- relation co-efficient (R ). The pseudo-second-order approach unity in all cases. Hence, the adsorption is of the second-order kinetics. In addition, the q values are in equation is given as follows: e cal agreement with the q . Therefore, the experimental e exp 1 1 = ¼ þ  t; ð15Þ results support the assumption behind the model that the k q q 2 e rate-limiting step in the adsorption of dyes are where k is the adsorption rate constant for pseudo-second- chemisorptions involving valence forces through the −1 −1 order kinetics (gmg h ), and qe is the adsorption exchange of electrons between adsorbent and dyes [7]. A capacity calculated from pseudo-second-order kinetic similar finding was reported in the literature [59, 60]. −1 −1 model (mgg ), q is the equilibrium adsorption (mgg ), and t is the adsorption time (h). The linear form of the Desorption studies pseudo-second-order kinetic model is given in Fig. 12. The values of k , and q were calculated from the slope and The re-usability of GG-g-poly(AMPS-co-DMAPMAm)-8 2 e intercept of the linear plot of t/q vs t. The values of R were gel and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 123 Int J Ind Chem (2017) 8:345–362 359 Fig. 12 Pseudo-second-order kinetics for the adsorption of a rhodamine B and b Chromotrope 2R Dyes on GG-g-poly(AMPS-co-DMAPMAm)- 8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 Table 5 Desorption capacity PH Desorption (%) (%) of GG-g-poly(AMPS-co- DMAPMAm)-8 and GG-g-poly GG-g-poly(AMPS-co-DMAPMAm)-8 GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 (AMPS-co-DMAPMAm)/ RhB C2R RhB C2R MMT-3 for RhB and C2R under pH 1.2 and pH 13.0 First cycle 1.2 91.57 48.50 94.25 48.34 13.0 83.18 41.85 91.69 43.92 Second cycle 1.2 89.23 42.23 83.17 44.03 13.0 74.12 38.11 64.98 39.62 Table 6 Comparison of adsorption capacity of dyes onto different adsorbents Adsorbate Adsorbent Q (mg/g) References RhB GG-g-poly(AMPS-co-DMAPMAm)-8 35.70 Present work RhB GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 31.92 Present work C2R GG-g-poly(AMPS-co-DMAPMAm)-8 17.73 Present work C2R GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 17.56 Present work RhB Acid activated mango leaf powder 3.85 [7] RhB Palm shell-based activated carbon 2.92 [49] RhB Coffee powder 4.018 [59] Methyl orange Gellan gum-graft-poly(DMAEMA) 25.8 [31] C2R Carbons modified with lanthanum 164 [60] Methylene blue Poly(acrylic acid-co-acrylamide) 1313 [61] Methylene blue and Direct blue Polyacrylamide/chitosan 6.744 [63] RhB Rice husk activated carbon 275.2 [59] Crystal violet Car/poly(AAm-co-Na-AA)-MMT 46.15 [64] Crystal violet CarAlg/MMT 88.8 [23] 123 360 Int J Ind Chem (2017) 8:345–362 suspending about 15 mg of the adsorbent in 25 mL of References solution (pH 1.2 and pH 13.0) allowed to stand for 8 h at 1. 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Amphoteric gellan gum-based terpolymer–montmorillonite composite: synthesis, swelling, and dye adsorption studies

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Chemistry; Industrial Chemistry/Chemical Engineering; Polymer Sciences; Nanochemistry; Environmental Chemistry
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10.1007/s40090-017-0126-z
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Abstract

Int J Ind Chem (2017) 8:345–362 DOI 10.1007/s40090-017-0126-z RESEARCH Amphoteric gellan gum-based terpolymer–montmorillonite composite: synthesis, swelling, and dye adsorption studies 1 1 Sirajo Abubakar Zauro B. Vishalakshi Received: 5 October 2016 / Accepted: 14 June 2017 / Published online: 6 July 2017 © The Author(s) 2017. This article is an open access publication Abstract A terpolymer gel, Gellan gum-graft-poly(2- Introduction acrylamido-2-methyl-1-propanesulfonic acid-co-dimethy- laminopropyl methacrylamide) and its composite with the The increasing demand for manufactured products world- clay, Montmorillonite, was prepared by free-radical poly- wide and the use of synthetic dyes in various industries merization and crosslinking reactions in solution. The such as textile, leather, paper, rubber, plastic, cosmetic, etc. terpolymer gel and the clay composite were characterized led to the proportionate release of a large quantity of using FTIR, TGA, SEM, and X-ray diffraction techniques. effluent into the environment. In addition, this effluent Swelling studies were carried out in different pH and salt contained non-biodegradable toxic and carcinogenic dye solutions. The gel showed maximum swelling capacity in substances into the environment [1–5]. Rhodamine B alkaline medium, while the composite showed higher (RhB) and chromotrope 2R (C2R) are synthetic dyes that swelling in neutral medium. The swelling of the gel and the are commonly used in leather, textile, and paper industries composite followed second kinetics model and water and cause various health hazards [6, 7]. transport is found to be a less Fickian diffusion process. The composites and nanocomposite of polymer–clay The terpolymer gel and the composite were evaluated for have been gaining increase attention by researchers the adsorption of rhodamine B (RhB) and chromotrope 2R globally due to the hybrid properties which they exhibit (C2R) dyes. Rhodamine B is found to be adsorbed to a when compared with either the polymer or clay separately higher extent than chromotrope 2R and the adsorption [8]. A wide range of polymer–clay composite/nanocom- isotherm studies suggested that adsorption of both RhB and posite has been produced and used for a variety of C2R on the terpolymer gel was best explained by Langmuir applications such as water treatment [5, 8], dye adsorption model, while the adsorption on the Composite fitted best [2, 9–11], etc. into Freundlich model. Similarly, the adsorption kinetics Several physical and chemical methods like chemical data for both RhB and C2R dyes followed the second-order precipitation, ion exchange, membrane separation, chemi- kinetics. cal reduction, chemical oxidation, advanced oxidation processes (AOPs), etc. [12] have been employed in the Keywords Gellan gum · 2-Acrylamido-2-methyl-1- removal of toxic substances from the environment. How- propanesulfonic acid · Dimethylaminopropyl ever, these methods are ineffective in removing most of the methacrylamide · Montmorillonite · Swelling · dyes molecules and are time-consuming, not cost-effective, Dye adsorption and sometimes generate large amount of sludge that are toxic to the biotic organisms in the environment. Hence, adsorption using biopolymer-based composites has been & B. Vishalakshi described as one of the effective and promising techniques vishalakshi2009@yahoo.com for removal of pollutants due to its simplicity, inexpen- siveness, etc. [13–18]. Department of Post-Graduate Studies and Research in Several biopolymer-based hydrogels such as Gum gatti Chemistry, Mangalore University, Mangalagangothri, [19], Gur gum [14], Kappa carrageenan [20–22], chitosan Dakshina Kannada, Mangalore, Karnataka 574199, India 123 346 Int J Ind Chem (2017) 8:345–362 [23], and Guaran [24] were studied as adsorbents for the Methods removal of dyes from aqueous solution. Casey and Wilson [25] reported the adsorption of Methylene blue (MB) dye Synthesis of GG-g-AMPS on Chitosan-PVA composite films and direct relationship GG-g-AMPS was prepared via free-radical polymerization between film composition (Chitosan-PVA) with solution pH and the uptake of MB were observed. Similarly, process as follows: 0.15 g GG was dissolved in distilled and stirred overnight. To the resultant solution, varying Datskevich et al. [26] synthesized cationic starch and sodium alginate-based composite and studied the adsorp- amounts (0.1–0.30 g) of AMPS were added followed by APS (0.05) under continues stirring. The temperature was tion of Methyl orange and MB under different conditions. The adsorption of congo red on Chitosan/Montmorillonite raised to 40 °C under continues stirring for 2 h. The gel was composite has been studied by Wang and Wang [1]. Vasugi precipitated with acetone and washed with methanol sev- and Girija [4] reported the adsorption of reactive blue dye eral times, and dried in an oven at 50 °C for 24 h. on hydroxyapatite-alginate composite. The composite materials consisting of clay and a Synthesis of GG-g-poly(AMPS-co-DMAPMAm) biopolymer are very effective in removal of dyes due to the The graft copolymer GG-g-poly(AMPS-co-DMAPMAm) availability of numerous functional groups on the biopolymer and the clay for binding with the dye mole- gel was synthesized based on the established methods reported by Nie et al. [27] with a little modification as cules, rendering the materials useful as adsorbents. The aim of the present study is to obtain a functional follows: a known amount of GG (0.1 g) was dissolved in composite hydrogel consisting of clay, a biopolymer, and distilled water and stirred overnight at room temperature. A a synthetic polymer to be evaluated as an adsorbent for specified amount of AMPS (0.1–0.30 g) and DMAPMAm dyes. This has been achieved by polymerizing AMPS, (0.15–0.50 g) were added to the above solution. To the DMAPMAm, and MBA in the presence of gellan gum mixture above, APS (0.05 g) and MBA (0.05 g) were added and montmorillonite (MMT) clay in water and its effec- and stirred by raising the temperature to 60 °C slowly for tiveness as an adsorbent for removal of dyes has been 4 h maintaining the temperature at 60 °C until a gel-like studied using chromotrope 2R and rhodamine B as model solution was formed. It was then allowed to cool for an hour to complete the polymerization and added to excess ionic dyes. acetone to remove un-reacted components. The gels obtained were then washed with 50% ethanol and placed in Materials and methods a hot oven at 50 °C until constant weight was obtained. The GG-g-poly(AMPS-co-DMAPMAm) gel formation was Materials optimized. The percentage yield and grafting percentage (GP) were calculated by the following equation: Gellan gum (GG) was purchased from Sigma-Aldrich Experimental yield %Yield ¼  100; ð1Þ Chemicals Pvt Ltd., Bangalore, India. 2-Acry- Theoretical yield lamidomethyl-2-propane sulfonic acid (AMPS), ðw  w Þ 1 o dimethylaminopropyl methacrylamide (DMAPMAm), N, GPðÞ % ¼  100; ð2Þ N-methylene-bis-acrylamide (MBA), and montmorillonite (MMT) were obtained from Sigma-Aldrich Chemie, where w and w are the weight of grafted gels and 0 1 GmbH, Germany. Ammonium peroxodisulphate (APS) monomers, respectively. was obtained from Spectro Chem Pvt. Ltd., Mumbai, India. Rhodamine B (RhB) was obtained from s.d. Fine Chemical Synthesis of GG-g-poly(AMPS-co-DMAPMAm)/MMT Limited, Mumbai, India. Chromotrope 2R (C2R) was purchased from Loba Chemie Limited Mumbai, India. The GG-g-poly(AMPS-co-DMAPMAm)/MMT composite Acetone was obtained from Nice Chemicals Pvt Ltd., hydrogel was made following the same procedure as in Kerala, India. Methanol was obtained from Himedia Lab- 2.2.2 with the addition of MMT (0.01–0.03 g) after adding oratories Pvt Ltd., Mumbai, India. NaCl, KCl, FeCl , DMAPMA under continuous stirring slowly during 1 h. CaCl , and Na SO were obtained from Merck Ltd., 2 2 4 Mumbai, India. DMAPMAm was purified by passing Characterization through column containing alumina gel before use. All other reagents were used as received. Distilled water was The GG, GG-g-AMPS, GG-g-poly(AMPS-co-DMAPMA)- used throughout the experiments. 8, and GG-g-poly(AMPS-co-DMAPMA)/MMT-3 samples 123 Int J Ind Chem (2017) 8:345–362 347 were characterized using FTIR, TGA, SEM, and XRD varied amount of the adsorbent and allowed to stand for techniques. The FTIR were recorded using FTIR-Prestige- 14 h and the resultant solutions were decanted and the −1 21, Shimadzu Japan, in the range of 4000–400 cm absorbance were recorded. The amount of dyes adsorbed at −1 wavenumber during 40 scans, with a resolution of 2 cm . time t (q ) and at equilibrium (q ) in mg/g was calculated t e Thermograms were recorded using standard DSC-TGA using the following equations [29, 30]: (Q600 V20.9 model) Japan, by heating the samples in the ðC  C Þ 0 t q ¼  V ; ð5Þ ranges of 30–700 °C, under a nitrogen atmosphere at 10 ° C/min. Surface morphology of the samples was obtained ðC  C Þ 0 e on gold coating JOEL JSM-6380LA analytical Scanning q ¼  V ; ð6Þ electron microscope (SEM) under magnification of 2000 at 20 kV. XRD pattern was recorded on X-ray diffractometer where q and q are the amount of dyes adsorbed (mg/g) at t e (Rigatu Miniflex 600-XRD instrument, USA) using Cu Kά time t = t and at equilibrium, respectively. C , C , and C 0 t e radiation generated at 35 kV and 35 mA in the differential are dyes concentration (mg/L) at time t = 0, t = t, and at ° ° angle 2θ at a range of 0 –80 in steps of 0.020/s. equilibrium, respectively, M is the weight of the gel (g) and V is the volume (L) of the dye solution. Swelling studies Swelling experiments of the GG-g-poly(AMPS-co- Results and discussion DMAPMA)-8 and GG-g-poly(AMPS-co-DMAPMA)/MMT- 3 samples were carried out in different media (pH and salts The composite hydrogels were prepared by crosslink solution). A known amount of the samples were weighed and copolymerization of AMPS, DMAPMAm, and MBA in immersed into swelling media at room temperature. After water in the presence of GG. MMT was incorporated in situ specified interval of time, the samples were removed and the in the copolymer network. During the polymerization excess surface water was wiped away gently using blotting reaction, the bi-functional MBA copolymerizes with (tissue) paper and re-weighed. This procedure was repeated AMPS and DMAPMAm to form a network, while GG until equilibrium is reached. The data were reported as the takes part in the free-radical polymerization reaction by mean of three different measurements. The effects of nature of forming macroradicals [31]. Thus, a composite gel is different salts solution (0.1 M) on the swelling ratio were also formed by entrapment of MMT clay in the copolymer studied in the same manner. network. The free-radical reaction mechanism along with The swelling ratio (SR) and swelling equilibrium (S ) the formation of the gel network is shown in Scheme 1. eq were calculated by the following equations: The composition of the gels/composites and the per- centage yield is presented in Table 1. The optimized ðW  W Þ t o SR = ¼ ; ð3Þ g product [GG-g-poly(AMPS-co-DMAPMAm)-8] was used for composite formation and used as representative sample ðW  W Þ e 0 for swelling and dye adsorption studies. The grafting S = ¼ ; ð4Þ eq conditions were optimized by varying monomer (DMAP- MAm and AMPS) contents and keeping all other where W , W , and W are the weight of the gel at time 0 t e parameters constant. t = 0, t = t, and at equilibrium, respectively [28]. The GP increases as the AMPS content increases from 0.1 to 0.25 g, and decreases as AMPS content increases to Dyes adsorption studies 0.30 g (Table 1). For DMAPMAm, the GP follows a similar pattern as in AMPS. The decreases in GP as the A known amount of GG-g-poly(AMPS-co-DMAPMAm)-8 content of monomers increases could be attributed to the and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 samples less reactive side on the GG as its content remains constant, were left immersed in 100 mg/L solutions of RhB and C2R dyes. At different time intervals, 2.5 mL of the supernatant and hence, there are more molecules of DMAPMAm and AMPS than GG and this could lead to the formation of solution were withdrawn and the absorbance values were measured using UV–visible spectrophotometer (UV-1800 homopolymer and hence low yield. Shimadzu, Japan) λ of 554 and 510 nm for RhB and max C2R, respectively. Calibration curves were used to convert FTIR the absorbance measured into concentration using standard FTIR Spectra of GG, GG-g-AMPS, GG-g-poly(AMPS-co- solutions of 2, 4, 6, 8, and 10 mg/L of the dyes. Different DMAPMAm)-8, and GG-g-poly(AMPS-co-DMAPMAm)/ initial concentrations (10, 30, 50, 70, and 100 mg/L) were MMT-3 composite gels are shown in Fig. 1. The spectrum used for equilibrium adsorption studies by immersing 123 348 Int J Ind Chem (2017) 8:345–362 O. OH OH OH COOH COOH O O O (NH ) S O [APS] O 4 2 2 8 O O CH CH 3 O OH O OH O OH OH OH OH H H H H OH OH OH OH OH OH OH OH Macroradical (RO ) Gellan gum (ROH) OCH H C CH 3 3 ONH N NH NH MBA CH CH 3 2 DMAPMAm H C SO H MMT H C AMPS NH CH CH CH CH CH C OR RO CH CH 2 2 CO HC O C CO NH NH H C HN (CH ) CH 2 3 H C CH SO H HN CH H C CH HC CH CH OR RO C CH CH n 2 2 C O C O NH NH CH H C CH (CH ) 3 2 2 3 SO H N 3 CH H C 3 Scheme 1 Proposed scheme for the formation of GG-g-poly(AMPS-co-DMAPMAm) gel −1 (Fig. 1a) showed a broad absorption band at 3290 cm the SO H, respectively [33]. The additional characteristic −1 which is due to stretching of O–H and a medium absorption absorption bands of 1537 and 2771 cm for N–H −1 peak at 2926 cm corresponding to the C–H stretching of stretching and C–H stretching of –N(CH ) of DMAPMAm 3 2 −1 CH groups. The absorption at 1605 cm is related to the [25] were observed in the spectra of GG-g-poly(AMPS-co- − −1 C=O stretching of COO of the GG. The peak at 1016 cm DMAPMAm)-8 (Fig. 1c). Similarly, in addition to the is assigned to C–O bond stretching frequencies [32]. peaks on the spectra (Fig. 1a–c), peaks at 1040, 814, and −1 Comparing the GG and GG-g-AMPS (Fig. 1b) spectra, new 621 cm for Si–O–Si, Al–Al–OH, and Si–Al–OH [34], characteristic peaks were observed at 1643, 1438, and respectively, were observed on the spectrum of GG-g-poly −1 923 cm which are attributed to asymmetric stretching (AMPS-co-DMAPMAm)/MMT-3 (Fig. 1d) indicating the vibration of C=O, C–N stretching, and S–O stretching of entrapment of MMT on the gel matrices. 123 Int J Ind Chem (2017) 8:345–362 349 Table 1 Composition of hydrogels/composite and percentage yield Gel Code GG (g) AMPS DMAPMAm APS MBA MMT GP (%) Yield (g) (g) (g) (g) (%) GG-g-poly(AMPS-co-DMAPMAm) 1 0.1 0.1 0.15 0.05 0.05 63.18 57.87 2 0.1 0.1 0.20 0.05 0.05 64.08 60.32 3 0.1 0.15 0.20 0.05 0.05 72.91 58.73 4 0.1 0.15 0.25 0.05 0.05 73.81 69.80 5 0.1 0.20 0.25 0.05 0.05 79.13 50.91 6 0.1 0.20 0.30 0.05 0.05 79.72 62.32 7 0.1 0.25 0.30 0.05 0.05 89.12 68.92 8 0.1 0.25 0.40 0.05 0.05 89.82 79.56 9 0.1 0.30 0.40 0.05 0.05 88.12 64.39 10 0.1 0.30 0.50 0.05 0.05 87.65 63.87 Composite GG-g-poly(AMPS-co-DMAPMAm)/MMT 1 0.1 0.25 0.40 0.05 0.05 0.01 59.23 69.21 2 0.1 0.25 0.40 0.05 0.05 0.02 59.87 67.12 3 0.1 0.25 0.40 0.05 0.05 0.03 58.94 74.18 Fig. 1 FTIR spectra of a GG, b GG-g-APMS, c GG-g-poly (AMPS-co-DMAPMAm)-8, and d GG-g-poly(AMPS-co- DMAPMAm)/MMT-3 TGA loss of 14%, and is attributed to the loss of moisture con- tent in the polysaccharide. The second step of the The thermograms of GG, GG-g-AMPS, GG-g-poly decomposition occurs in the range of 210–260 °C with a (AMPS-co-DMAPMAm)-8, and GG-g-poly(AMPS-co- major weight loss of 36% due to the breaking of the gly- DMAPMAm)/MMT-3 are presented in Fig. 2. GG (Fig. 2a) cosidic linkage of the GG. The final decomposition of GG shows three degradation steps. The first step of degradation occurs around 270–540 °C with the weight loss of 32%. occurs between temperatures of 35–100 °C with the weight About 14% of the GG sample remains as residual matter at 123 350 Int J Ind Chem (2017) 8:345–362 DMAPMAm)/MMT-3 toward heat is attributed to the incorporation of MMT in the system [35]. SEM The surface morphology of GG, GG-g-AMPS, GG-g-poly (AMPS-co-DMAPMAm)-3, and GG-g-poly(AMPS-co- DMAPMAm)/MMT-8 is presented in Fig. 3. It could be deduced from Fig. 3b that grafting of AMPS on the GG changes the fibrous homogeneous surface of GG (Fig. 3a) into heterogeneous. Likewise, Fig. 3c shows a very distinct crystalline-like morphology suggesting the grafting of GG on poly(AMPS-co-DMAPMAm)-8. On the other hand, exfoliating MMT in the system also shows a considerable change in the surface morphology producing cotton like accumulation with an irregular shape that appears fibrous (Fig. 3d). Fig. 2 Thermograms of a GG, b GG-g-APMS, c GG-g-poly(AMPS- co-DMAPMAm)-8, and d GG-g-poly(AMPS-co-DMAPMAm)/ MMT-3 XRD The XRD patterns of MMT, GG, GG-g-AMPS, GG-g-poly 550 °C. For GG-g-AMPS (Fig. 2b), four decomposition (AMPS-co-DMAPMAm)-8, and GG-g-poly(AMPS-co- steps occur. With the first step in the range of 30–140 °C DMAPMAm)/MMT-3 samples are shown in Fig. 4. MMT with a weight loss of 13% due to loss of moisture in the diffractogram (Fig. 4a) shows a complete amorphous nat- sample and 10% weight loss between 182 and 265 °C, this ure of the clay MMT. GG diffractogram (Fig. 4b) showed might be due to the dissociation of GG from AMPS. two major peaks at 2θ values of 21.31 (medium) and Between 320 and 520 °C, a major weight loss (17%) 30.47 (sharp). The exhibition of this sharp peak at lower occurred and is linked to the degradation of the polysac- 2θ value indicated the crystalline nature of the GG. In GG- charide and final step decomposition at 555 °C with 25% g-AMPS sample (Fig. 4c); the disappearance of some weight loss leaving 4% as the residue. The thermogram of peaks and appearing of new one at a 2θ value of 5.82 GG-g-poly(AMPS-co-DMAPMAm)-8 (Fig. 2c) shows four indicated the grafting of AMPS on GG. The appearance of ° ° ° degradation steps with the first step losing 16% of the many sharp peaks at low 2θ (9.92 , 14.12 and 16.32 ) weight between 35 and 160 °C due to the elimination of values (Fig. 4d) indicated the incorporation of DMAP- moisture from the system. The second steps occurred in the MAm on GG-g-AMPS and it further indicates the semi- range of 324–510 °C with the loss of 18% which could be crystalline nature of the gel network [36]. The intercalation due to the breaking of the grafting between GG and the of MMT within the polymer gel network is evidence of the copolymers. The third degradation step results in weight major shift of the peak from the 2θ value of 18.79 (Fig. 4d) loss of 28% between the temperature range of 324–510 °C. to 22.18 . Similarly, the increases of d-spacing for MMT to ˚ ˚ The complete degradation of the grafted gel occurred 11.4527 A from the normal 9.8 A [34] are another evidence around 580 °C. For the GG -g-poly(AMPS-co-DMAP- to show the intercalation of MMT into the gel network. The MAm)/MMT-3 (Fig. 2d), the degradation steps are similar decrease in the intensity of the peak around 2θ value of 9.1 and the disappearance of the peak at 2θ of 7.2 from Fig. 4d to that of GG-g-AMPS and are in four stages. The first was the elimination of water molecule from the composite gel are further evidences to prove the intercalation of MMT in the range of 33–180 °C. The second steps of the into the gel matrix. The disappearance of a small broad degradation occurred in the temperature range of 205– peak at a 2θ value of 31.69 in Fig. 4e also showed the 256 °C with a weight loss of 16% and could be due to the intercalation of MMT into the gel network which resulted degradation of GG. The third step occurred between 317 in decreases in the degree of crystallinity [37]. and 535 °C with major weight loss of 20% and the final step of degradation occurred at 633 °C leaving around 13% Swelling responsive in different salt solution as residual matter. The GG-g-AMPS, and GG-g-poly (AMPS-co-DMAPMAm)-8 showed low thermal stability The effect of different salt solution (Fig. 5) on the swelling compared GG and GG-g-poly(AMPS-co-DMAPMAm)/ ratio of GG-g-poly(AMPA-co-DMAPMAm)-8 and GG-g- poly(AMPA-co-DMAPMAm)/MMT-3 samples shows MMT-3. The high stability of GG-g-poly(AMPS-co- 123 Int J Ind Chem (2017) 8:345–362 351 Fig. 3 SEM images of a GG, b GG-g-AMPS, c GG-g-poly(AMPS-co-DMAPMAm)-8, and d GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 Fig. 4 XRD diffractograms of a MMT, b GG, c GG-g-AMPS, d GG-g-poly(AMPA-co- DMAPMAm)-8, and e GG-g- poly(AMPA-co-DMAPMAm)/ MMT-3 higher swelling capacity in NaCl solution compared to such as nature of cations (charge and radius of cations). other salt solution with the least swelling response in The greater the charge of cations, the greater the cross FeCl . The swelling behavior is affected by many factors linking degree which results in decrease in swelling [28]. 123 352 Int J Ind Chem (2017) 8:345–362 ratio (S ) in gram of water per gram of gel/composite, eq initial swelling rate (R ) in g of water/g gel/composite −1 min , swelling rate constant (K ), in g gel/composite/g −1 water min ), swelling exponent (n), and maximum equi- −1 librium swelling (S ) in g of water g of sample were max calculated using the dynamic swelling data obtained from various plots shown in Figs. 6 and 7. The swelling mech- anism of the gel/composite was experimentally determined by employing a second-order kinetic equation as follows: ds ¼ k ðs  sÞ ; ð7Þ s eq dt where k and s are the swelling rate constant and degree s eq of swelling at equilibrium, respectively. The above equa- tion on integration over the limit S = S at t = t and 0 o S = S at t = t gives t 1 1 Fig. 5 Effects of different salt solution (0.1 M) on swelling ratio of ¼ þ t; ð8Þ SR k s s GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co- s eq eq DMAPMAm)/MMT-3 where k s is equal to R which is the initial swelling eq rate, s is the equilibrium swelling, and k is the swelling eq s The swelling of the gel is due to osmotic pressure differ- rate constant. The plot of t/SR vs t (Figs. 6b, 7b) pro- ence developed between the gel and the external salt duced a linear straight line with a slope of 1/s and eq solution due to the charge screening effect of the salt intercept of : This indicates that the second-order solution. The composite hydrogels exhibited salt sensitivity k s eq kinetics is followed by the swelling process. Furthermore, as reported in the literature [38, 39]. the calculated s from the slope are in good agreement eq with the experimental value as shown in Table 2. The Effect of pH on the swelling ratio initial swelling rate (R ) of the GG-g-poly(AMPS-co- DMAPMAm)/MMT-3 decreases drastically from acidic to The swelling ratios of the gel in different pH media were basic medium. A similar finding was reported [45]. studied and the results reported in Fig. 6a. Figure 6 showed However, the swelling rate constant (k ) increases as the higher swelling ratio (SR) in basic medium (pH 9.0) with pH values increase. lower SR in acidic medium (pH 1.2) by GG-g-poly(AMPS- The mechanism of diffusion is one of the factors that co-DMAPMAm)-8. The presence of free amino group on govern the applicability of materials [42]. The absorption DMAPMAm leads to the formation of many hydrogen process involves the diffusion of water molecules into the bonds in an alkaline medium which will restrict the free spaces of the materials which increase the segmental relaxation of network chain. While in acidic medium, the mobility and consequently result in an expansion of chain free amino group is expected to ionize which may result in segment between crosslink and later result in swelling. The the breakage of hydrogen bond and generate electrostatic dynamics of water sorption process was studied using the repulsion on the polymer chain [40]. In acidic medium, the 2− simple empirical equation called power law equation which attraction between SO and quaternary ammonium group is used mostly in determining the mechanism of diffusion restricts the swelling [41]. Furthermore, the carbonyl group in the polymeric network [46]: of the AMPS and DMAPMAm forms hydrogen bonding with the GG which also aids in reducing the swelling F ¼ Kt : ð9Þ [42, 43]. The above equation can be rewritten in form of ln as Swelling kinetics follows: lnF ¼ lnK þ nlnt: ð10Þ The mechanisms of swelling studies for the gel/composite The values of K and n were calculated from the intercept were carried out based on the standard methods reported in and slope of lnF vs t plots (Figs. 6c, 7c) and tabulated in the literature [44, 45]. The swelling parameters of the Table 2, where F, n, and K are swelling power, swelling gel/composite were determined from the various plots exponent, and swelling rate constant, respectively. (Figs. 6, 7). The parameters such as swelling equilibrium 123 Int J Ind Chem (2017) 8:345–362 353 Fig. 6 Swelling curves for GG-g-poly(AMPS-co-DMAPMAm)-8 gel Depending on the diffusion rate of the material relax- Fickian diffusion” behavior [46]. In this study, the values ation, three different diffusion mechanisms are proposed of n (Table 2) are all below 0.5. Hence, it is said to follow a [42–45, 47]: less Fickian diffusion mechanism. 1. Fickian diffusion in which the diffusion rate is less Dye adsorption studies than the relaxation rate (n = 0.50); 2. Diffusion which is rapid compared the relaxation The dye adsorption studies were carried out using rho- processes (n = 1); and damine B and chromotrope 2R as model dyes and their 3. Non-Fickian or anomalous diffusion which occurs structures are given in Fig. 8. when the rate of diffusion and that of relaxation are comparable (0.50\ n\ 1). Effects of contact time on the adsorption capacity The Fickian diffusion, actually, refers to a situation where water penetration rate in the gels is less than the The effect of contact time on the adsorption of dyes RhB polymer chain relaxation rate. Therefore, n = 0.5 indicates and C2R on both GG-g-poly(AMPS-co-DMAPMAm)-8 a perfect Fickian process. Nevertheless, when the water and GG-g-poly(AMPS-co-Dmapmam)/MMT-3 (Fig. 9) penetration rate is much below the polymer chain relax- showed an increase in the adsorption capacity slowly with ation rate, it is possible to record the n values below 0.5. increase in time. RhB showed higher adsorption compared This situation is still regarded as Fickian diffusion or “Less to C2R; this could be attributed to the presence of many 123 354 Int J Ind Chem (2017) 8:345–362 Fig. 7 Swelling curves for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 Table 2 Kinetics swelling and PH S (g/g) R (g of water/g of sample) K (g/g) NK S eq i s max diffusion parameters of GG-g- poly(AMPS-co-DMAPMAm)-8 GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co- 1.2 2.45 1.69 0.099 0.10 1.03 2.38 DMAPMAm/MMT-3 7.0 3.68 0.20 0.379 0.04 7.48 3.65 9.0 6.85 2.07 0.010 0.24 2.40 6.32 GG-g-poly(AMPS-co-DMAPMAm/MMT-3 1.2 8.93 10.33 0.0012 0.48 3.733 6.86 7.0 13.70 2.71 0.0020 0.31 4.383 11.93 9.0 6.67 2.12 0.0106 0.2 3.679 6.36 negative (acidic groups) site on the adsorbent which could rate especially on the GG-g-poly(AMPS-co-DMAPMAm)/ result in the formation of an electrostatic attraction with the MMT-3. This could be due to the repulsion between the positive (basic groups) part of the adsorbate. The adsorp- anionic (basic site) groups on both C2R and the composites tion of C2R on both the adsorbents is proceeded at slower [48]. 123 Int J Ind Chem (2017) 8:345–362 355 Fig. 8 Structures of the dyes used: a rhodamine B and b chromotrope 2R intercept and slope of the linear plot of C /q versus C e e e (Figs. 10a, b, 11a, b) and presented in Table 3. The essential feature of the Langmuir isotherm can be represented in terms of separation factor (dimensionless equilibrium parameter) R [53, 54], which can be expressed as follows: R ¼ ; ð12Þ 1 þ K C L o where C is the initial concentrations of dyes, K is the 0 L constant related to the energy of adsorption (Langmuir Constant). RL value indicates the favorability nature of adsorption. If R [ 1, the adsorption is unfavorable; if R = 1, the adsorption is linear; if 0 \ R \ 1, the L L adsorption is favorable; and if R = 0, then the adsorption is irreversible. From the data reported in Table 3, the R is greater than 0 but less than 1 indicating the favorability of Langmuir isotherm for the adsorption of RhB and C2R. Fig. 9 Amount of dyes (rhodamine B and chromotrope 2R) adsorbed Similarly, comparing the q calculated (33.33 and (mg/g) on GG-g-poly(AMPS-co-DMAPMAm)-8 gel and GG-g-poly 16.13 mg/g) with the experimental q (35.7 and 17.73 mg/ (AMPS-co-DMAPMAm)/MMT-3 composite over time g), respectively, for RhB and C2R on GG-g-poly(AMPS- co-DMAPMAm)-8. This indicated the formation of a Adsorption isotherm monolayer of RhB and C2R on the surfaces of GG-g-poly (AMPS-co-DMAPMAm)-8. Adsorption isotherms usually describe the performance of The Freundlich adsorption isotherm is based on the adsorbents in adsorption processes by describing the sur- assumption that encompasses the heterogeneity of the face interaction between the adsorbent and adsorbate [49]. surface and the adsorption capacity related to the equilib- There are various isotherm models used to describe the rium concentration of the adsorbate. The Freundlich adsorption processes. In this study, the two most common isotherm is commonly expressed as follows: used adsorption isotherms, namely, Langmuir isotherm [50, 51] and Freundlich isotherm [52], are employed. The Langmuir isotherm is a model which quantitatively ln q ¼ ln k þ lnC ; ð13Þ e f e describes equilibrium monolayer adsorbate formation on where q and C are the amount of dyes adsorbed (mg/g) the surface of the adsorbent, and is expressed as follows: e e and the equilibrium concentration of dyes (mg/L), respec- C 1 1 ¼  C þ ; ð11Þ e tively, K and n are Freundlich adsorption isotherm q q K q e m L m constants that represent the adsorption capacity and the where C and q are the equilibrium concentration of dye degree of nonlinearity between the dye concentration and e e (mg/L) and the amount of dye adsorbed (mg/g), respec- the adsorption, respectively. The values of K and n were tively, q is the maximum adsorption corresponding to calculated from the intercept and slope of the plot between m, complete monolayer coverage on the surface (mg/g), K is L ln q and ln C (Figs. 10c, d, 11c, d) and are presented in e e the Langmuir constant which is related to the energy of Table 3. The value of n indicates whether the adsorption is adsorption (L/mg). K and q are determined from the favorable or otherwise. If it lies within the range of 1–10, L m 123 356 Int J Ind Chem (2017) 8:345–362 Fig. 10 Adsorption isotherms for rhodamine B dye. a Langmuir gel, c Freundlich isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 gel, b Langmuir gel, and d Freundlich isotherm for GG-g-poly(AMPS-co-DMAP- isotherm for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite MAm)/MMT-3 composite then the adsorption is considered favorable. In this case, the MMT-3] is shown in Fig. 9. The rate of adsorption of the value of n lies between 1.23 and 4.83 which shows a dye uptake was little slow especially with respect to C2R favorable adsorption. Similarly, the R values for the compared to RhB adsorption. The maximum adsorption adsorption of RhB and C2R on GG-g-poly(AMPS-co- observed in C2R was 17.72 and 16.99 mg/g, respectively, DMAPMAm)/MMT-3 are 0.994 and 0.996, respectively, for GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly which are higher when compared with 0.989 and 0.990, (AMPS-co-DMAPMAm)/MMT-3 after 12 h. While higher respectively, for RhB and C2R on GG-g-poly(AMPS-co- adsorption capacity of 35.70 and 31.20 mg/g of RhB was DMAPMAm)-8. Hence, we can say that adsorption of RhB recorded for GG-g-poly(AMPS-co-DMAPMAm)-8 and and C2R on GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 GG-g-poly(AMPS-co-DMAPMAm)/MMT-3, respectively, base fits into the Freundlich model. at 12 h. The adsorption of RhB on different adsorbents has been reported [7, 49, 55, 56]. Kinetic studies To investigate the mechanism of adsorption, the adsorption data obtained in this work were subjected to The adsorption capacity of RhB and C2R dyes as a func- various kinetics models. The models employed in this work tion of time by the adsorbents [GG-g-poly(AMPS-co- are Lagergren’s pseudo-first-order [55, 57] and pseudo- DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/ second-order kinetic models. 123 Int J Ind Chem (2017) 8:345–362 357 Fig. 11 Adsorption isotherms for chromotrope 2R dye. a Langmuir gel, c Freundlich isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 gel, b Langmuir gel, and d Freundlich isotherm for GG-g-poly(AMPS-co-DMAP- isotherm for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite MAm)/MMT-3 composite found to be in sharp disagreement with the q in all Pseudo-first-order kinetics e exp cases. Furthermore, the values of correlation coefficients are low, which is an indication of bad quality linearization. The Lagergren’s pseudo-first-order kinetic model is based on assumption that the rate of adsorption of adsorbate with Hence, the adsorption cannot be said to be of first order. It has been suggested that the differences in experimental and time is directly proportional to difference in equilibrium concentration and concentration with time and this can be theoretical q values are that there is a time lag due to represented as follows: external resistance controlling at the beginning of the adsorption [57]. logðÞ q  q ¼ log q  t; ð14Þ e t e 2:303 Pseudo-second-order kinetics where q and q are the amount of dyes adsorbed (mg/g) at e t equilibrium and at time t, respectively. K is the rate con- The kinetics studies of RhB and C2R adsorption on the −1 stant (h ) for the pseudo-first-order kinetics and t is the adsorbents was carried out using the initial dyes concen- time (h) of adsorption. The value of K and R were cal- tration of 100 mg/L in all cases. The pseudo-second-order culated from the plot of log(q − q ) versus t and tabulated e t rate expression of Ho and McKay [58] was adopted in this in Table 4. The q for the first-order rate equation was e cal study and the best model that fit the adsorption was 123 358 Int J Ind Chem (2017) 8:345–362 Table 3 Isotherm model parameters for the adsorption of rhodamine B and chromotrope 2R dyes on GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 Dyes RhB C2R Adsorbent GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- DMAPMAm)-8 DMAPMAm)/MMT-3 DMAPMAm)-8 DMAPMAm)/MMT-3 C 10–100 10–100 10–100 10–100 (mg/L) Langmuir R 0.43–0.88 0.797–0.975 0.045–0.320 0.19–0.165 model K 0.013 0.0026 0.212 0.509 q 33.33 90.91 16.13 3.66 (mg/ g) R 0.841 0.969 0.999 0.996 Freundlich k 10.91 2.76 1.21 0.89 model n 1.61 1.23 1.54 4.83 R 0.989 0.994 0.990 0.996 R linear regression correlation co-efficient Table 4 Pseudo-first-order and pseudo-second-order kinetics model data for the adsorption of rhodamine B and chromotrope 2R dyes on GG-g- poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 Dye RhB C2R Adsorbent GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- GG-g-poly(AMPS-co- DMAPMAm)-8 DMAPMAm)/MMT-3 DMAPMAm)-8 DMAPMAm)/MMT-3 Q (mg/ 35.70 31.92 17.73 17.56 e exp g) Pseudo-first-order q g/ 45.42 15.34 5.84 2.88 e cal (m kinetic model g) −1 K (hr ) 0.15 0.198 0.32 0.124 R 0.935 0.856 0.875 0.777 Pseudo-second-order q mg/ 37.04 34.48 18.18 17.24 ecal ( kinetic model g). K (g/ 0.016 0.028 0.016 0.01 −1 mghr ) R 0.991 0.984 0.999 0.996 R linear regression correlation co-efficient higher than those in the pseudo-first-order model and they selected based on the values of the linear regression cor- relation co-efficient (R ). The pseudo-second-order approach unity in all cases. Hence, the adsorption is of the second-order kinetics. In addition, the q values are in equation is given as follows: e cal agreement with the q . Therefore, the experimental e exp 1 1 = ¼ þ  t; ð15Þ results support the assumption behind the model that the k q q 2 e rate-limiting step in the adsorption of dyes are where k is the adsorption rate constant for pseudo-second- chemisorptions involving valence forces through the −1 −1 order kinetics (gmg h ), and qe is the adsorption exchange of electrons between adsorbent and dyes [7]. A capacity calculated from pseudo-second-order kinetic similar finding was reported in the literature [59, 60]. −1 −1 model (mgg ), q is the equilibrium adsorption (mgg ), and t is the adsorption time (h). The linear form of the Desorption studies pseudo-second-order kinetic model is given in Fig. 12. The values of k , and q were calculated from the slope and The re-usability of GG-g-poly(AMPS-co-DMAPMAm)-8 2 e intercept of the linear plot of t/q vs t. The values of R were gel and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 123 Int J Ind Chem (2017) 8:345–362 359 Fig. 12 Pseudo-second-order kinetics for the adsorption of a rhodamine B and b Chromotrope 2R Dyes on GG-g-poly(AMPS-co-DMAPMAm)- 8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 Table 5 Desorption capacity PH Desorption (%) (%) of GG-g-poly(AMPS-co- DMAPMAm)-8 and GG-g-poly GG-g-poly(AMPS-co-DMAPMAm)-8 GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 (AMPS-co-DMAPMAm)/ RhB C2R RhB C2R MMT-3 for RhB and C2R under pH 1.2 and pH 13.0 First cycle 1.2 91.57 48.50 94.25 48.34 13.0 83.18 41.85 91.69 43.92 Second cycle 1.2 89.23 42.23 83.17 44.03 13.0 74.12 38.11 64.98 39.62 Table 6 Comparison of adsorption capacity of dyes onto different adsorbents Adsorbate Adsorbent Q (mg/g) References RhB GG-g-poly(AMPS-co-DMAPMAm)-8 35.70 Present work RhB GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 31.92 Present work C2R GG-g-poly(AMPS-co-DMAPMAm)-8 17.73 Present work C2R GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 17.56 Present work RhB Acid activated mango leaf powder 3.85 [7] RhB Palm shell-based activated carbon 2.92 [49] RhB Coffee powder 4.018 [59] Methyl orange Gellan gum-graft-poly(DMAEMA) 25.8 [31] C2R Carbons modified with lanthanum 164 [60] Methylene blue Poly(acrylic acid-co-acrylamide) 1313 [61] Methylene blue and Direct blue Polyacrylamide/chitosan 6.744 [63] RhB Rice husk activated carbon 275.2 [59] Crystal violet Car/poly(AAm-co-Na-AA)-MMT 46.15 [64] Crystal violet CarAlg/MMT 88.8 [23] 123 360 Int J Ind Chem (2017) 8:345–362 suspending about 15 mg of the adsorbent in 25 mL of References solution (pH 1.2 and pH 13.0) allowed to stand for 8 h at 1. 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Journal

International Journal of Industrial ChemistrySpringer Journals

Published: Jul 6, 2017

References