Introduction

The increasing demand for manufactured products worldwide and the use of synthetic dyes in various industries such as textile, leather, paper, rubber, plastic, cosmetic, etc. led to the proportionate release of a large quantity of effluent into the environment. In addition, this effluent contained non-biodegradable toxic and carcinogenic dye substances into the environment [1,2,3,4,5]. Rhodamine B (RhB) and chromotrope 2R (C2R) are synthetic dyes that are commonly used in leather, textile, and paper industries and cause various health hazards [6, 7].

The composites and nanocomposite of polymer–clay have been gaining increase attention by researchers globally due to the hybrid properties which they exhibit when compared with either the polymer or clay separately [8]. A wide range of polymer–clay composite/nanocomposite has been produced and used for a variety of applications such as water treatment [5, 8], dye adsorption [2, 9,10,11], etc.

Several physical and chemical methods like chemical precipitation, ion exchange, membrane separation, chemical reduction, chemical oxidation, advanced oxidation processes (AOPs), etc. [12] have been employed in the removal of toxic substances from the environment. However, these methods are ineffective in removing most of the dyes molecules and are time-consuming, not cost-effective, 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 described as one of the effective and promising techniques for removal of pollutants due to its simplicity, inexpensiveness, etc. [13,14,15,16,17,18].

Several biopolymer-based hydrogels such as Gum gatti [19], Gur gum [14], Kappa carrageenan [20,21,22], chitosan [23], and Guaran [24] were studied as adsorbents for the removal of dyes from aqueous solution. Casey and Wilson [25] reported the adsorption of Methylene blue (MB) dye on Chitosan-PVA composite films and direct relationship between film composition (Chitosan-PVA) with solution pH and the uptake of MB were observed. Similarly, Datskevich et al. [26] synthesized cationic starch and sodium alginate-based composite and studied the adsorption of Methyl orange and MB under different conditions. The adsorption of congo red on Chitosan/Montmorillonite composite has been studied by Wang and Wang [1]. Vasugi and Girija [4] reported the adsorption of reactive blue dye on hydroxyapatite-alginate composite.

The composite materials consisting of clay and a biopolymer are very effective in removal of dyes due to the availability of numerous functional groups on the biopolymer and the clay for binding with the dye molecules, rendering the materials useful as adsorbents.

The aim of the present study is to obtain a functional composite hydrogel consisting of clay, a biopolymer, and a synthetic polymer to be evaluated as an adsorbent for dyes. This has been achieved by polymerizing AMPS, DMAPMAm, and MBA in the presence of gellan gum and montmorillonite (MMT) clay in water and its effectiveness as an adsorbent for removal of dyes has been studied using chromotrope 2R and rhodamine B as model ionic dyes.

Materials and methods

Materials

Gellan gum (GG) was purchased from Sigma-Aldrich Chemicals Pvt Ltd., Bangalore, India. 2-Acrylamidomethyl-2-propane sulfonic acid (AMPS), dimethylaminopropyl methacrylamide (DMAPMAm), N, N-methylene-bis-acrylamide (MBA), and montmorillonite (MMT) were obtained from Sigma-Aldrich Chemie, GmbH, Germany. Ammonium peroxodisulphate (APS) was obtained from Spectro Chem Pvt. Ltd., Mumbai, India. Rhodamine B (RhB) was obtained from s.d. Fine Chemical Limited, Mumbai, India. Chromotrope 2R (C2R) was purchased from Loba Chemie Limited Mumbai, India. Acetone was obtained from Nice Chemicals Pvt Ltd., Kerala, India. Methanol was obtained from Himedia Laboratories Pvt Ltd., Mumbai, India. NaCl, KCl, FeCl3, CaCl2, and Na2SO4 were obtained from Merck Ltd., Mumbai, India. DMAPMAm was purified by passing through column containing alumina gel before use. All other reagents were used as received. Distilled water was used throughout the experiments.

Methods

Synthesis of GG-g-AMPS

GG-g-AMPS was prepared via free-radical polymerization process as follows: 0.15 g GG was dissolved in distilled and stirred overnight. To the resultant solution, varying amounts (0.1–0.30 g) of AMPS were added followed by APS (0.05) under continues stirring. The temperature was raised to 40 °C under continues stirring for 2 h. The gel was precipitated with acetone and washed with methanol several times, and dried in an oven at 50 °C for 24 h.

Synthesis of GG-g-poly(AMPS-co-DMAPMAm)

The graft copolymer GG-g-poly(AMPS-co-DMAPMAm) gel was synthesized based on the established methods reported by Nie et al. [27] with a little modification as follows: a known amount of GG (0.1 g) was dissolved in distilled water and stirred overnight at room temperature. A specified amount of AMPS (0.1–0.30 g) and DMAPMAm (0.15–0.50 g) were added to the above solution. To the mixture above, APS (0.05 g) and MBA (0.05 g) were added and stirred by raising the temperature to 60 °C slowly for 4 h maintaining the temperature at 60 °C until a gel-like solution was formed. It was then allowed to cool for an hour to complete the polymerization and added to excess acetone to remove un-reacted components. The gels obtained were then washed with 50% ethanol and placed in a hot oven at 50 °C until constant weight was obtained. The GG-g-poly(AMPS-co-DMAPMAm) gel formation was optimized. The percentage yield and grafting percentage (GP) were calculated by the following equation:

$$\% {\text{Yield}} = \frac{\text{Experimental yield }}{\text{Theoretical yield}} \times 100,$$
(1)
$${\text{GP}}\left( \% \right) = \frac{{(w_{1} - w_{\text{o}} )}}{{w_{\text{o}} }} \times 100,$$
(2)

where w 0 and w 1 are the weight of grafted gels and monomers, respectively.

Synthesis of GG-g-poly(AMPS-co-DMAPMAm)/MMT

The GG-g-poly(AMPS-co-DMAPMAm)/MMT composite hydrogel was made following the same procedure as in 2.2.2 with the addition of MMT (0.01–0.03 g) after adding DMAPMA under continuous stirring slowly during 1 h.

Characterization

The GG, GG-g-AMPS, GG-g-poly(AMPS-co-DMAPMA)-8, and GG-g-poly(AMPS-co-DMAPMA)/MMT-3 samples were characterized using FTIR, TGA, SEM, and XRD techniques. The FTIR were recorded using FTIR-Prestige-21, Shimadzu Japan, in the range of 4000–400 cm−1 wavenumber during 40 scans, with a resolution of 2 cm−1. Thermograms were recorded using standard DSC-TGA (Q600 V20.9 model) Japan, by heating the samples in the ranges of 30–700 °C, under a nitrogen atmosphere at 10 °C/min. Surface morphology of the samples was obtained on gold coating JOEL JSM-6380LA analytical Scanning electron microscope (SEM) under magnification of 2000 at 20 kV. XRD pattern was recorded on X-ray diffractometer (Rigatu Miniflex 600-XRD instrument, USA) using Cu Kά radiation generated at 35 kV and 35 mA in the differential angle 2θ at a range of 0°–80° in steps of 0.020/s.

Swelling studies

Swelling experiments of the GG-g-poly(AMPS-co-DMAPMA)-8 and GG-g-poly(AMPS-co-DMAPMA)/MMT-3 samples were carried out in different media (pH and salts solution). A known amount of the samples were weighed and immersed into swelling media at room temperature. After specified interval of time, the samples were removed and the excess surface water was wiped away gently using blotting (tissue) paper and re-weighed. This procedure was repeated until equilibrium is reached. The data were reported as the mean of three different measurements. The effects of nature of different salts solution (0.1 M) on the swelling ratio were also studied in the same manner.

The swelling ratio (SR) and swelling equilibrium (S eq) were calculated by the following equations:

$${\text{SR}} \left( {{\raise0.7ex\hbox{$g$} \!\mathord{\left/ {\vphantom {g g}}\right.\kern-0pt} \!\lower0.7ex\hbox{$g$}}} \right) = \frac{{(W_{\text{t}} - W_{\text{o}} )}}{{W_{\text{o}} }},$$
(3)
$$S_{\text{eq}} \left( {{\raise0.7ex\hbox{$g$} \!\mathord{\left/ {\vphantom {g g}}\right.\kern-0pt} \!\lower0.7ex\hbox{$g$}}} \right) = \frac{{(W_{\text{e}} - W_{ 0} )}}{{W_{\text{o}} }},$$
(4)

where W 0, W t, and W e are the weight of the gel at time t = 0, t = t, and at equilibrium, respectively [28].

Dyes adsorption studies

A known amount of GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 samples were left immersed in 100 mg/L solutions of RhB and C2R dyes. At different time intervals, 2.5 mL of the supernatant solution were withdrawn and the absorbance values were measured using UV–visible spectrophotometer (UV-1800 Shimadzu, Japan) λ max of 554 and 510 nm for RhB and C2R, respectively. Calibration curves were used to convert the absorbance measured into concentration using standard solutions of 2, 4, 6, 8, and 10 mg/L of the dyes. Different initial concentrations (10, 30, 50, 70, and 100 mg/L) were used for equilibrium adsorption studies by immersing varied amount of the adsorbent and allowed to stand for 14 h and the resultant solutions were decanted and the absorbance were recorded. The amount of dyes adsorbed at time t (q t) and at equilibrium (q e) in mg/g was calculated using the following equations [29, 30]:

$$q_{\text{t}} = \frac{{(C_{0} - C_{\text{t}} ) }}{M} \times V,$$
(5)
$$q_{\text{e}} = \frac{{(C_{ 0} - C_{\text{e}} ) }}{M} \times V ,$$
(6)

where q t and q e are the amount of dyes adsorbed (mg/g) at time t = t and at equilibrium, respectively. C 0, C t, and C e are dyes concentration (mg/L) at time t = 0, t = t, and at equilibrium, respectively, M is the weight of the gel (g) and \(V\) is the volume (L) of the dye solution.

Results and discussion

The composite hydrogels were prepared by crosslink copolymerization of AMPS, DMAPMAm, and MBA in water in the presence of GG. MMT was incorporated in situ in the copolymer network. During the polymerization reaction, the bi-functional MBA copolymerizes with AMPS and DMAPMAm to form a network, while GG takes part in the free-radical polymerization reaction by forming macroradicals [31]. Thus, a composite gel is formed by entrapment of MMT clay in the copolymer network. The free-radical reaction mechanism along with the formation of the gel network is shown in Scheme 1.

Scheme 1
scheme 1

Proposed scheme for the formation of GG-g-poly(AMPS-co-DMAPMAm) gel

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The composition of the gels/composites and the percentage yield is presented in Table 1. The optimized product [GG-g-poly(AMPS-co-DMAPMAm)-8] was used for composite formation and used as representative sample for swelling and dye adsorption studies. The grafting conditions were optimized by varying monomer (DMAPMAm and AMPS) contents and keeping all other parameters constant.

Table 1 Composition of hydrogels/composite and percentage yield
Full size table

The GP increases as the AMPS content increases from 0.1 to 0.25 g, and decreases as AMPS content increases to 0.30 g (Table 1). For DMAPMAm, the GP follows a similar pattern as in AMPS. The decreases in GP as the content of monomers increases could be attributed to the less reactive side on the GG as its content remains constant, and hence, there are more molecules of DMAPMAm and AMPS than GG and this could lead to the formation of homopolymer and hence low yield.

FTIR

FTIR Spectra of GG, GG-g-AMPS, GG-g-poly(AMPS-co-DMAPMAm)-8, and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite gels are shown in Fig. 1. The spectrum (Fig. 1a) showed a broad absorption band at 3290 cm−1 which is due to stretching of O–H and a medium absorption peak at 2926 cm−1 corresponding to the C–H stretching of CH2 groups. The absorption at 1605 cm−1 is related to the C=O stretching of COO− of the GG. The peak at 1016 cm−1 is assigned to C–O bond stretching frequencies [32]. Comparing the GG and GG-g-AMPS (Fig. 1b) spectra, new characteristic peaks were observed at 1643, 1438, and 923 cm−1 which are attributed to asymmetric stretching vibration of C=O, C–N stretching, and S–O stretching of the SO3H, respectively [33]. The additional characteristic absorption bands of 1537 and 2771 cm−1 for N–H stretching and C–H stretching of –N(CH3)2 of DMAPMAm [25] were observed in the spectra of GG-g-poly(AMPS-co-DMAPMAm)-8 (Fig. 1c). Similarly, in addition to the peaks on the spectra (Fig. 1a–c), peaks at 1040, 814, and 621 cm−1 for Si–O–Si, Al–Al–OH, and Si–Al–OH [34], respectively, were observed on the spectrum of GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 (Fig. 1d) indicating the entrapment of MMT on the gel matrices.

Fig. 1
figure 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

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TGA

The thermograms of GG, GG-g-AMPS, GG-g-poly(AMPS-co-DMAPMAm)-8, and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 are presented in Fig. 2. GG (Fig. 2a) shows three degradation steps. The first step of degradation occurs between temperatures of 35–100 °C with the weight loss of 14%, and is attributed to the loss of moisture content in the polysaccharide. The second step of the decomposition occurs in the range of 210–260 °C with a major weight loss of 36% due to the breaking of the glycosidic linkage of the GG. The final decomposition of GG occurs around 270–540 °C with the weight loss of 32%. About 14% of the GG sample remains as residual matter at 550 °C. For GG-g-AMPS (Fig. 2b), four decomposition steps occur. With the first step in the range of 30–140 °C with a weight loss of 13% due to loss of moisture in the sample and 10% weight loss between 182 and 265 °C, this might be due to the dissociation of GG from AMPS. Between 320 and 520 °C, a major weight loss (17%) occurred and is linked to the degradation of the polysaccharide and final step decomposition at 555 °C with 25% weight loss leaving 4% as the residue. The thermogram of GG-g-poly(AMPS-co-DMAPMAm)-8 (Fig. 2c) shows four degradation steps with the first step losing 16% of the weight between 35 and 160 °C due to the elimination of moisture from the system. The second steps occurred in the range of 324–510 °C with the loss of 18% which could be due to the breaking of the grafting between GG and the copolymers. The third degradation step results in weight loss of 28% between the temperature range of 324–510 °C. The complete degradation of the grafted gel occurred around 580 °C. For the GG -g-poly(AMPS-co-DMAPMAm)/MMT-3 (Fig. 2d), the degradation steps are similar to that of GG-g-AMPS and are in four stages. The first was the elimination of water molecule from the composite gel in the range of 33–180 °C. The second steps of the degradation occurred in the temperature range of 205–256 °C with a weight loss of 16% and could be due to the degradation of GG. The third step occurred between 317 and 535 °C with major weight loss of 20% and the final step of degradation occurred at 633 °C leaving around 13% as residual matter. The GG-g-AMPS, and GG-g-poly(AMPS-co-DMAPMAm)-8 showed low thermal stability compared GG and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3. The high stability of GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 toward heat is attributed to the incorporation of MMT in the system [35].

Fig. 2
figure 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

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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. 3
figure 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

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XRD

The XRD patterns of MMT, GG, GG-g-AMPS, GG-g-poly(AMPS-co-DMAPMAm)-8, and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 samples are shown in Fig. 4. MMT diffractogram (Fig. 4a) shows a complete amorphous nature of the clay MMT. GG diffractogram (Fig. 4b) showed two major peaks at 2θ values of 21.31° (medium) and 30.47° (sharp). The exhibition of this sharp peak at lower 2θ value indicated the crystalline nature of the GG. In GG-g-AMPS sample (Fig. 4c); the disappearance of some peaks and appearing of new one at a 2θ value of 5.82° indicated the grafting of AMPS on GG. The appearance of many sharp peaks at low 2θ (9.92°, 14.12° and 16.32°) values (Fig. 4d) indicated the incorporation of DMAPMAm on GG-g-AMPS and it further indicates the semi-crystalline nature of the gel network [36]. The intercalation of MMT within the polymer gel network is evidence of the major shift of the peak from the 2θ value of 18.79° (Fig. 4d) to 22.18°. Similarly, the increases of d-spacing for MMT to 11.4527 Å from the normal 9.8 Å [34] are another evidence to show the intercalation of MMT into the gel network. The 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 are further evidences to prove the intercalation of MMT into the gel matrix. The disappearance of a small broad peak at a 2θ value of 31.69° in Fig. 4e also showed the intercalation of MMT into the gel network which resulted in decreases in the degree of crystallinity [37].

Fig. 4
figure 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

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Swelling responsive in different salt solution

The effect of different salt solution (Fig. 5) on the swelling ratio of GG-g-poly(AMPA-co-DMAPMAm)-8 and GG-g-poly(AMPA-co-DMAPMAm)/MMT-3 samples shows higher swelling capacity in NaCl solution compared to other salt solution with the least swelling response in FeCl3. The swelling behavior is affected by many factors such as nature of cations (charge and radius of cations). The greater the charge of cations, the greater the cross linking degree which results in decrease in swelling [28]. The swelling of the gel is due to osmotic pressure difference developed between the gel and the external salt solution due to the charge screening effect of the salt solution. The composite hydrogels exhibited salt sensitivity as reported in the literature [38, 39].

Fig. 5
figure 5

Effects of different salt solution (0.1 M) on swelling ratio of GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3

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Effect of pH on the swelling ratio

The swelling ratios of the gel in different pH media were studied and the results reported in Fig. 6a. Figure 6 showed higher swelling ratio (SR) in basic medium (pH 9.0) with lower SR in acidic medium (pH 1.2) by GG-g-poly(AMPS-co-DMAPMAm)-8. The presence of free amino group on DMAPMAm leads to the formation of many hydrogen bonds in an alkaline medium which will restrict the relaxation of network chain. While in acidic medium, the free amino group is expected to ionize which may result in the breakage of hydrogen bond and generate electrostatic repulsion on the polymer chain [40]. In acidic medium, the attraction between SO3 2− and quaternary ammonium group restricts the swelling [41]. Furthermore, the carbonyl group of the AMPS and DMAPMAm forms hydrogen bonding with the GG which also aids in reducing the swelling [42, 43].

Fig. 6
figure 6

Swelling curves for GG-g-poly(AMPS-co-DMAPMAm)-8 gel

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Swelling kinetics

The mechanisms of swelling studies for the gel/composite were carried out based on the standard methods reported in the literature [44, 45]. The swelling parameters of the gel/composite were determined from the various plots (Figs. 6, 7). The parameters such as swelling equilibrium ratio (S eq) in gram of water per gram of gel/composite, initial swelling rate (R i) in g of water/g gel/composite min−1, swelling rate constant (K s), in g gel/composite/g water min−1), swelling exponent (n), and maximum equilibrium swelling (S max) in g of water g−1 of sample were calculated using the dynamic swelling data obtained from various plots shown in Figs. 6 and 7. The swelling mechanism of the gel/composite was experimentally determined by employing a second-order kinetic equation as follows:

$$\frac{{{\text{d}}s}}{{{\text{d}}t}} = k_{\text{s}} (s_{\text{eq}} - s)^{2} ,$$
(7)

where k s and s eq are the swelling rate constant and degree of swelling at equilibrium, respectively. The above equation on integration over the limit S = S 0 at t = t o and S = S at t = t gives

$$\frac{t}{\text{SR}} = \frac{1}{{k_{\text{s}} s_{\text{eq}}^{2} }} + \frac{1}{{s_{\text{eq}} }}t,$$
(8)

where \(k_{\text{s}} s_{\text{eq}}^{2}\) is equal to R i which is the initial swelling rate, \(s_{\text{eq}}\) is the equilibrium swelling, and k s is the swelling rate constant. The plot of t/SR vs t (Figs. 6b, 7b) produced a linear straight line with a slope of 1/s eq and intercept of \(\frac{1}{{k_{\text{s}} s_{\text{eq}}^{2} }}.\) This indicates that the second-order kinetics is followed by the swelling process. Furthermore, the calculated seq from the slope are in good agreement with the experimental value as shown in Table 2. The initial swelling rate (R i) of the GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 decreases drastically from acidic to basic medium. A similar finding was reported [45]. However, the swelling rate constant (k s) increases as the pH values increase.

Fig. 7
figure 7

Swelling curves for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3

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Table 2 Kinetics swelling and diffusion parameters of GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm/MMT-3
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The mechanism of diffusion is one of the factors that govern the applicability of materials [42]. The absorption process involves the diffusion of water molecules into the free spaces of the materials which increase the segmental mobility and consequently result in an expansion of chain segment between crosslink and later result in swelling. The dynamics of water sorption process was studied using the simple empirical equation called power law equation which is used mostly in determining the mechanism of diffusion in the polymeric network [46]:

$$F = {\text{Kt}}^{\text{n}} .$$
(9)

The above equation can be rewritten in form of ln as follows:

$${ \ln }F = { \ln }K + n { \ln }t.$$
(10)

The values of K and n were calculated from the intercept and slope of lnF vs t plots (Figs. 6c, 7c) and tabulated in Table 2, where F, n, and K are swelling power, swelling exponent, and swelling rate constant, respectively.

Depending on the diffusion rate of the material relaxation, three different diffusion mechanisms are proposed [42,43,44,45, 47]:

  1. 1.

    Fickian diffusion in which the diffusion rate is less than the relaxation rate (n = 0.50);

  2. 2.

    Diffusion which is rapid compared the relaxation processes (n = 1); and

  3. 3.

    Non-Fickian or anomalous diffusion which occurs when the rate of diffusion and that of relaxation are comparable (0.50 < n < 1).

The Fickian diffusion, actually, refers to a situation where water penetration rate in the gels is less than the polymer chain relaxation rate. Therefore, n = 0.5 indicates a perfect Fickian process. Nevertheless, when the water penetration rate is much below the polymer chain relaxation rate, it is possible to record the n values below 0.5. This situation is still regarded as Fickian diffusion or “Less Fickian diffusion” behavior [46]. In this study, the values of n (Table 2) are all below 0.5. Hence, it is said to follow a less Fickian diffusion mechanism.

Dye adsorption studies

The dye adsorption studies were carried out using rhodamine B and chromotrope 2R as model dyes and their structures are given in Fig. 8.

Fig. 8
figure 8

Structures of the dyes used: a rhodamine B and b chromotrope 2R

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Effects of contact time on the adsorption capacity

The effect of contact time on the adsorption of dyes RhB and C2R on both GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-Dmapmam)/MMT-3 (Fig. 9) showed an increase in the adsorption capacity slowly with increase in time. RhB showed higher adsorption compared to C2R; this could be attributed to the presence of many negative (acidic groups) site on the adsorbent which could result in the formation of an electrostatic attraction with the positive (basic groups) part of the adsorbate. The adsorption of C2R on both the adsorbents is proceeded at slower rate especially on the GG-g-poly(AMPS-co-DMAPMAm)/MMT-3. This could be due to the repulsion between the anionic (basic site) groups on both C2R and the composites [48].

Fig. 9
figure 9

Amount of dyes (rhodamine B and chromotrope 2R) adsorbed (mg/g) on GG-g-poly(AMPS-co-DMAPMAm)-8 gel and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite over time

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Adsorption isotherm

Adsorption isotherms usually describe the performance of adsorbents in adsorption processes by describing the surface interaction between the adsorbent and adsorbate [49]. There are various isotherm models used to describe the adsorption processes. In this study, the two most common used adsorption isotherms, namely, Langmuir isotherm [50, 51] and Freundlich isotherm [52], are employed.

The Langmuir isotherm is a model which quantitatively describes equilibrium monolayer adsorbate formation on the surface of the adsorbent, and is expressed as follows:

$$\frac{{C_{\text{e}} }}{{q_{\text{e}} }} = \frac{1}{{q_{\text{m}} }} \times C_{\text{e}} + \frac{1}{{K_{\text{L }} q_{\text{m}} }},$$
(11)

where C e and q e are the equilibrium concentration of dye (mg/L) and the amount of dye adsorbed (mg/g), respectively, q m, is the maximum adsorption corresponding to complete monolayer coverage on the surface (mg/g), K L is the Langmuir constant which is related to the energy of adsorption (L/mg). K L and q m are determined from the intercept and slope of the linear plot of C e/q e versus Ce (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 L [53, 54], which can be expressed as follows:

$$R_{\text{L}} = \frac{1}{{1 + K_{\text{L}} C_{\text{o}} }},$$
(12)

where C 0 is the initial concentrations of dyes, K L is the constant related to the energy of adsorption (Langmuir Constant). RL value indicates the favorability nature of adsorption. If R L > 1, the adsorption is unfavorable; if R L = 1, the adsorption is linear; if 0 < R L < 1, the adsorption is favorable; and if R L = 0, then the adsorption is irreversible. From the data reported in Table 3, the R L is greater than 0 but less than 1 indicating the favorability of Langmuir isotherm for the adsorption of RhB and C2R. Similarly, comparing the q m calculated (33.33 and 16.13 mg/g) with the experimental q m (35.7 and 17.73 mg/g), respectively, for RhB and C2R on GG-g-poly(AMPS-co-DMAPMAm)-8. This indicated the formation of a monolayer of RhB and C2R on the surfaces of GG-g-poly(AMPS-co-DMAPMAm)-8.

Fig. 10
figure 10

Adsorption isotherms for rhodamine B dye. a Langmuir isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 gel, b Langmuir isotherm for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite gel, c Freundlich isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 gel, and d Freundlich isotherm for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite

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Fig. 11
figure 11

Adsorption isotherms for chromotrope 2R dye. a Langmuir isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 gel, b Langmuir isotherm for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite gel, c Freundlich isotherm for GG-g-poly(AMPS-co-DMAPMAm)-8 gel, and d Freundlich isotherm for GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite

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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
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The Freundlich adsorption isotherm is based on the assumption that encompasses the heterogeneity of the surface and the adsorption capacity related to the equilibrium concentration of the adsorbate. The Freundlich isotherm is commonly expressed as follows:

$$\ln q_{\text{e }} = \ln k_{\text{f}} + \frac{1}{n}{ \ln }C_{\text{e}} ,$$
(13)

where q e and C e are the amount of dyes adsorbed (mg/g) and the equilibrium concentration of dyes (mg/L), respectively, K f and n are Freundlich adsorption isotherm constants that represent the adsorption capacity and the degree of nonlinearity between the dye concentration and the adsorption, respectively. The values of K f and n were calculated from the intercept and slope of the plot between ln q e and ln C e (Figs. 10c, d, 11c, d) and are presented in Table 3. The value of n indicates whether the adsorption is favorable or otherwise. If it lies within the range of 1–10, then the adsorption is considered favorable. In this case, the value of n lies between 1.23 and 4.83 which shows a favorable adsorption. Similarly, the R 2 values for the adsorption of RhB and C2R on GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 are 0.994 and 0.996, respectively, which are higher when compared with 0.989 and 0.990, respectively, for RhB and C2R on GG-g-poly(AMPS-co-DMAPMAm)-8. Hence, we can say that adsorption of RhB and C2R on GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 base fits into the Freundlich model.

Kinetic studies

The adsorption capacity of RhB and C2R dyes as a function of time by the adsorbents [GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3] is shown in Fig. 9. The rate of adsorption of the dye uptake was little slow especially with respect to C2R compared to RhB adsorption. The maximum adsorption observed in C2R was 17.72 and 16.99 mg/g, respectively, for GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 after 12 h. While higher adsorption capacity of 35.70 and 31.20 mg/g of RhB was recorded for GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3, respectively, at 12 h. The adsorption of RhB on different adsorbents has been reported [7, 49, 55, 56].

To investigate the mechanism of adsorption, the adsorption data obtained in this work were subjected to various kinetics models. The models employed in this work are Lagergren’s pseudo-first-order [55, 57] and pseudo-second-order kinetic models.

Pseudo-first-order kinetics

The Lagergren’s pseudo-first-order kinetic model is based on assumption that the rate of adsorption of adsorbate with time is directly proportional to difference in equilibrium concentration and concentration with time and this can be represented as follows:

$$\log \left( {q_{\text{e}} - q_{\text{t}} } \right) = { \log }\;q_{\text{e}} - \frac{{K_{1} }}{2.303}t,$$
(14)

where q e and q t are the amount of dyes adsorbed (mg/g) at equilibrium and at time t, respectively. K 1 is the rate constant (h−1) for the pseudo-first-order kinetics and t is the time (h) of adsorption. The value of K 1 and R 2 were calculated from the plot of log(q e − q t) versus t and tabulated in Table 4. The q e cal for the first-order rate equation was found to be in sharp disagreement with the q e exp in all cases. Furthermore, the values of correlation coefficients are low, which is an indication of bad quality linearization. Hence, the adsorption cannot be said to be of first order. It has been suggested that the differences in experimental and theoretical qe values are that there is a time lag due to external resistance controlling at the beginning of the adsorption [57].

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
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Pseudo-second-order kinetics

The kinetics studies of RhB and C2R adsorption on the adsorbents was carried out using the initial dyes concentration of 100 mg/L in all cases. The pseudo-second-order rate expression of Ho and McKay [58] was adopted in this study and the best model that fit the adsorption was selected based on the values of the linear regression correlation co-efficient (R2). The pseudo-second-order equation is given as follows:

$${\raise0.7ex\hbox{$t$} \!\mathord{\left/ {\vphantom {t q}}\right.\kern-0pt} \!\lower0.7ex\hbox{$q$}} = \frac{1}{{k_{2} q_{\text{e}}^{2} }} + \frac{1}{{q_{\text{e}} }} \times t,$$
(15)

where k 2 is the adsorption rate constant for pseudo-second-order kinetics (gmg−1 h−1), and qe is the adsorption capacity calculated from pseudo-second-order kinetic model (mgg−1), q e is the equilibrium adsorption (mgg−1), and t is the adsorption time (h). The linear form of the pseudo-second-order kinetic model is given in Fig. 12. The values of k 2, and q e were calculated from the slope and intercept of the linear plot of t/q vs t. The values of R 2 were higher than those in the pseudo-first-order model and they approach unity in all cases. Hence, the adsorption is of the second-order kinetics. In addition, the qe cal values are in agreement with the q e exp. Therefore, the experimental results support the assumption behind the model that the rate-limiting step in the adsorption of dyes are chemisorptions involving valence forces through the exchange of electrons between adsorbent and dyes [7]. A similar finding was reported in the literature [59, 60].

Fig. 12
figure 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

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Desorption studies

The re-usability of GG-g-poly(AMPS-co-DMAPMAm)-8 gel and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 suspending about 15 mg of the adsorbent in 25 mL of solution (pH 1.2 and pH 13.0) allowed to stand for 8 h at room temperature. The solutions were filtered and diluted appropriately and the absorbance of the desorbed RhB and C2R were measured using UV–Vis spectrophotometer. The amounts of the two dyes were calculated using Eq. (6). The percentage of desorption for the dyes was calculated using the following equation [61]:

$${\text{Desorption}}\left( \% \right) = \frac{\text{Amount of dye desorbed}}{\text{Amount of dye adsorbed}} \times 100.$$
(16)

It was clear from Table 5 that RhB can be efficiently removed by regenerating the adsorbent at least twice under pH 1.2 and 13.0, whereas desorption of C2R under the same conditions is low. In all cases, the % desorption is high at pH 1.2 and this could be attributed to the reaction between H+ and a lone pair of electrons in both the secondary amine groups of the adsorbent and the tertiary nitrogen of RhB. Hence, more of RhB will go into solution [62].

Table 5 Desorption capacity (%) of GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 for RhB and C2R under pH 1.2 and pH 13.0
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The adsorption capacity of GG-g-poly(AMPS-co-DMAPMAm)-8 gel and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 on RhB and C2R is compared with other previously developed adsorbents [7, 23, 31, 49, 59,60,61, 63, 64] towards different adsorbates and presented in Table 6.

Table 6 Comparison of adsorption capacity of dyes onto different adsorbents
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Conclusion

In this work, an amphoteric terpolymer consisting of gel GG, AMPS, and DMAPMAm, and its clay composite containing MMT have successfully been synthesized and studied. The GG-g-poly(AMPS-co-DMAPMAm)-8 gel showed maximum swelling at pH of 9.0 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 composite shows maximum swelling at neutral pH. The adsorption of RhB and C2R dyes on the gels/composite is higher for RhB compared to C2R. The adsorption kinetic studies revealed a second-order adsorption process. Furthermore, adsorption of RhB and C2R on to GG-g-poly(AMPS-co-DMAPMAm)-8 and GG-g-poly(AMPS-co-DMAPMAm)/MMT-3 was found to best fit into the Langmuir and Freundlich models, respectively.