1 Introduction

The effluent dye industry has caused severe hazards to the environment and biology due to its complex structure and chromogenic functional groups [1,2,3]. Among many pollutants that the mill discharges millions of gallons into the water contain naphthol, vat dyes, sulfur, nitrates, acetic acid, chromium compounds and heavy metal ions like arsenic, lead, cadmium, copper, and many other pollutants [4,5,6]. Most of these contaminants are hydrocarbon-based softeners, formaldehyde-based dye fixing agents and non-biodegradable dyeing chemicals [1, 2, 6]. Methylene blue dye (MB) is harm to the respiratory system and skin. It can cause vomiting and even cancer for living organisms [4]. MB absorbs light, and it affects the growth of the aquatic organisms [5]. Moreover, the aromatic structure is hardly biodegradable which means the remaining of MB for a long time if there is no adopting method for treatment [7]. Various physicochemical techniques such as adsorption, photodegradation, and membrane separation have been applied to treat the dye effluents [8]. The ideal material for efficient adsorption should possess the properties of porosity, high surface area, and physicochemical stability [9]. Bio-based adsorbents are considered one of the most promising materials in water treatment  [10].

One of the most interesting materials for scientists is Chitosan (CS). It is a biodegradable, renewable, and inexpensive natural polymer  [11, 12]. It has many environmentally friendly qualities and has high efficiency in water purification  [13, 14]. Chitosan is used for the removal of dyes and metals [15, 16] as it has active adsorption sites like amino (-NH2) and hydroxyl (-OH) functional groups and metals. However, it has some disadvantages for industrial applications [3], wastewater technologies, such as the too soft structure, which creates problems  [17,18,19], its high solubility in the organic solvent, high swelling percent in water, low mechanical strength, and low surface area [20]. Many studies have been applied to improve CS properties and efficiency [21,22,23]. The combination of inorganic [24] and polymeric materials [25] is a successful strategy for the improvement of the restricted properties and developing new properties by introducing new functional groups that can be enhancing the ability of dye-binding [26,27,28]. Also improving the chemical stability of chitosan toward the acidic environment and reduce its the hydrophobicity chemical cross-linking reaction used [29]. Graft polymerization of vinyl monomers onto chitosan using gamma irradiation have been reported [30] for examples butyl acrylate onto chitosan [31], it was revealed that grafting percentage increased as the monomer concentration and the total irradiation dose were increased. Radiation grafting of acrylamide and maleic acid on chitosan and its effective application for removal of Co(II) from aqueous solutions has been studied by Saleh et al.; [32] it was found that chitosan-P(acrylamide) and chitosan-P(maleic acid) show high sorption capacity toward Co(II) as. 150 mg/g and 421 mg/g, respectively, that makes them potential sorbents of Co(II) for wastewater treatment.

In this study, (CS- PAAc/TiO2) nanocomposite hydrogel was prepared using the gamma irradiation technique. This technique has the advantage of liberty from toxic impurities such as crosslinking agents and initiators [33, 34]. Characterization and properties the prepared nanocomposites were studied using different techniques. The influence of TiO2 nanoparticle content on the adsorption was studied towards methyl blue dye (MB). The effect of different parameters was studied to estimate the best condition for adsorption of methyl blue dye (MB) dye that was included; pH of dye solution; adsorbent dose and temperature of the feed solution.

2 Materials and methods

Chitosan (CS) has average molecular weight 100.000–300.000 (Acros, Belgium), acrylic acid (AAc) of purity 99.9% (Aldrich) and titanium dioxide nanoparticle (TiO2) of powder size 15 ± 3 nm (nano-gate; Egypt) were used without further purification. Other chemicals, such as buffers were purchased from El-Nasr Co. for Chemical Industries, Egypt and used without further purification.

2.1 Preparation of CS-PAAc hydrogel

CS-PAAc hydrogels were prepared by adding AAc to CS in different copolymer composition wt.%, to obtain a solution of total concentration 20 wt.%. The solution was transferred into a glass tube to be irradiated by γ-rays from Co-60 source at radiation dose of 30 kGy. The hydrogels were obtained in a long cylindrical shape and were cut into small pieces. All samples were washed in excess water to remove the unreacted component then dried in air to constant weight.

2.2 Preparation of CS-PAAc/TiO2 nanocomposite

A solution of CS-PAAc of equal ratio was prepared as mentioned above and different content of TiO2, 0.0, 1.0, 2.0 & 3.0 wt.% of the total polymer concentration, was added to the solution. The mixtures were sonicated in a bath sonicator for 15 min to obtain homogenous solutions. The solutions were transferred into small glass vials and were subjected to 60Co-gamma rays at irradiation dose of 30 kGy. After that, the above steps of preparation CS-PAAc hydrogel were followed.

2.3 Gamma irradiation

Irradiation to the required doses was carried out in the cobalt-60 (60Co) gamma cell (India) at a dose rate range from 1.77 to 2.23 kGy/min in air. This source was installed at the National Center for Radiation Research and Technology (NCRRT).

2.4 FTIR spectroscopy measurement

The infrared spectra were investigated by FTIR spectrophotometer, Spectrum One, Perkin Elmer, USA, over the range of 4000–400 cm−1.

2.5 X–ray diffraction analysis (XRD)

XRD analysis was carried out using a Shimadzu Diffractometer D6000 series Kyoto, Japan. (30 mA and 40 kv) at Cu Kα (λ = 1.54 Å) radiation at room temperature in a 2θ range of 4–90 scan speed 8 deg/min. The average particle size was also calculated using the Scherer formula.

$$L_{\theta } = \frac{k\lambda }{\beta cos\theta }$$
(1)

where, Lθ is average grain size of formed crystallite and λ (1.54 Å) is the wavelength of used X-ray. β denotes the full width at half maximum (FWHM) for corresponding diffraction peak and θ is diffraction angle. k is constant (K: 0.9–1).

2.6 Scanning electron microscope (SEM)

Scanning Electron Microscope analysis (SEM) Surface of the samples was investigated by Jasco JSM-5200 scanning electron microscope (SEM), Japan with voltage accelerated at 25 kV after gold deposition in vacuum for 3 min.

2.7 Transmission electron microscopy (TEM)

The morphology of the nanocomposite was observed using transmission electron microscope (TEM) (JEOL-JEM 1400CX ELECTRON MICROSCOPE, Japan,) at acceleration voltage 80 kV. The nanocomposite sample was prepared by grinding carefully, mixed with acetone, and accompanied by a sonication process. A drop of the solution was deposited on a microgrid, to prevent the high-voltage electron beam from damaging the sample; the chamber of the sample inside the device was placed in a bath of liquid nitrogen to stabilize the temperature.

2.8 Thermal properties

Thermal Gravimetric Analysis (TGA) and differential Thermal Analysis DTA. The thermogravimetric analysis (TGA) was performed by using TGA-30 (Shimadzu, Japan) at a heating rate of 10 °C/min under the nitrogen atmosphere in the temperature range from room temperature up to 600 °C. The relative thermal stability of the samples was evaluated in terms of decomposition temperature.

2.9 Swelling properties

CS-PAAc hydrogels and CS-PAAc/TiO2 nanocomposite hydrogels of known weights were immersed in distilled water at definite interval times until the equilibrium. The swollen samples were re-weighed after the excess surface water was removed immediately with a filter paper. The degree of swelling was determined according to the following equation:

$$Degree\; of\; Swelling\;\left( \% \right) = \frac{{ W_{t} - W_{d} }}{{W_{d} }} \times 100$$
(2)

where Wt is the weight of the sample after swelling at time t and Wd is initial of dry sample.

2.10 Point of zero charge

The pH that the charge of the adsorbent surface is zero has known as the zero point of charge (pHZPC). Zero-point charge pH of CS-PAAc/TiO2(3.0 wt.%) was determined by the pH drift method as follows [25]:25 ml of 0.01 M NaCl solutions were poured into the beakers. The pH was adjusted 2, 4, 6, 8, 10, and 12 by adding 0.1 M HCl or 0.1 M NaOH. 0.1 g of adsorbent was then added to each beaker, and the beakers were agitated for 72 h at room temperature. Finally, the pH of solutions was measured, and the final pH was plotted against the initial pH. The zero-point charge pH is the point of intersection of the resulting curve.

2.11 Adsorption study

Batch adsorption experiments were carried out at different temperatures; 25, 35, and 50 °C. Exactly 20 ml of dye solution of known initial concentration (20 mg/L) at adjusted pH was shaken at a certain agitation speed of 250 rpm of a known dose of dried sample for 24 h.

The absorption capacity (Qe, mg.g−1) and MB removal percent were calculated using the following equation:

$$Q_{e} = \frac{{\left( {C_{0} - C_{e} } \right)V}}{m}$$
(3)
$$MB \;Removal \left( \% \right) = \frac{{C_{0} - C_{e} }}{{C_{0} }} \times 100$$
(4)

where, C0 and Ce (both in mg/L) are the initial dye concentration and the dye concentration at equilibrium, respectively. V is the initial solution volume (L); and m is the adsorbent dry weight (g). MB dye concentration was calculated using a UV/VIS spectrometer, model UV-Jasco V-530 made by Japan at λmax of 664 nm with a quartz cell of 1.0 cm optical length.

3 Results and discussion

3.1 FTIR spectroscopy

The radiation-induced copolymerization mechanism of acrylic acid onto chitosan is shown in Scheme 1. Infrared spectroscopy analysis is an important tool to confirm the occurrence of the functional groups of the prepared hydrogel.

Scheme 1
scheme 1

Synthesis of CS-PAAc/TiO2 nanocomposite hydrogel

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Figure 1 explains the FTIR spectra of CS-PAAc hydrogel and CS-PAAc/TiO2(3.0 wt.%) nanocomposite hydrogel. For CS-PAAc hydrogel, there is broadband that appears in the range of 3678–3003 cm−1 due to O–H and N–H bands of AAc and CS which overlapped with the stretching band of C–H group at 2928 cm−1. The band at 1702 cm−1 corresponds to the C = O of the carbonyl group and amide I. The band at 1590 cm−1 due to the N–H deformation of the amino group (amine II) [33]. For CS-PAAc/TiO2 nanocomposite a new peak was observed at 505 cm−1 due to Ti–O–Ti vibration [35].

Fig. 1
figure 1

FTIR spectra of CS-PAAc hydrogel and CS-PAAc/TiO2 nanocomposite hydrogel; TiO2 content 3.0 wt.%

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3.2 X-ray diffraction (XRD)

XRD diffractograms of CS-PAAc/TiO2 nanocomposite hydrogels were investigated in a 2θ range of 4°–90° and the results are shown in Fig. 2. In the pattern of CS-PAAc hydrogel (0.0% TiO2), a broad peak was observed at 2θ = 20° due to the CS-PAAc copolymer hydrogel network and confirmed the formation of an amorphous phase in the hydrogel [34]. In CS-PAAc/TiO2 nanocomposite hydrogels patterns, sharp peaks appear at 2θ = 25° compared with the XRD pattern of the CS-PAAc hydrogel. The intensity of peaks increases as TiO2 content increases. Moreover, the presence of low-intensity diffraction peaks at 2θ values of about 46.2°, 52.9°, and 72.2° assigned to TiO2 nanoparticles. These diffraction peaks confirmed the presence of crystalline TiO2 nanoparticles in the nanocomposite hydrogels [36]. The average particle size of TiO2 nanoparticles was calculated by the Scherrer equation (Eq. 1). It is estimated as; 5.8, 5.7, and 5.9 nm for 1.0, 2.0, and 3.0 wt % TiO2 content; respectively in the CS-PAAc/TiO2 nanocomposite hydrogels.

Fig. 2
figure 2

XRD patterns of CS-PAAc/TiO2 nanocomposites of different TiO2 content

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3.3 Thermal properties

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were evaluated as shown in Fig. 3. TGA determines the changes in weight as a function of temperature [37]. The TGA results [Fig. 3a] seem similar thermal behaviors and maximum thermal temperature in each stage of all investigated samples. In general, the TG curves resolved four main decomposition stages. The first decomposition stage in the range of ~ 50–180 °C due to the evaporation of the physically bonded water. This stage accompanies by an endothermic peak centered at 121 °C [Fig. 3b]. The second decomposition stage in the range of 180–293 °C due to the destruction of the side groups and vaporization of voltaic compounds [38]. A broad endothermic peak centered at 253 °C accompanies this stage. The third decomposition stage occurs in the range of 293–423 °C due to due to thermal degradation of the backbone polymeric chains and accompanies by endothermic peak centered at 354 °C. The last decomposition stage was detected in the range of 423–558 °C accompanies with a sharp exothermic peak [Fig. 3b] corresponds to the transformation of the TiO2 nanoparticles phase.

Fig. 3
figure 3

a TGA and b DTA analysis of CS-PAAc hydrogel and CS-PAAc/TiO2 nanocomposites hydrogel; TiO2 content (0.0, 1.0, 2.0 & 3.0 wt.%)

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3.4 The swelling properties

The swelling of the polymeric material is important in several aspects. Water molecules may act as plasticizers relaxing interaction between the polymeric chains. Moreover, it may have a dramatic change in the permeability properties, especially in the hydrophilic polymers. Figure 4a represents the effect of CS-AAc composition on the swelling percentage. It can be seen that the swelling percentage enhances as the content of AAc is increased. It was reported that as the polymer chain backbone is built using a hydrophilic type monomer, here is (AAc), the water uptake may increase [39]. Figure 4b shows the effect of TiO2 content on the swelling percentage of CS-PAAc/TiO2 nanocomposite hydrogels. It can be noted that the swelling percentage increases with the increase of TiO2 content in the matrix. The swelling percentage enhances nearly fourth times by adding 1 wt.% TiO2 nanoparticle in the nanocomposite hydrogel compared with CS-PAAc hydrogel. The increase of TiO2 content in the nanocomposite matrix may reduce the crosslink density. Where a large free volume is available in the polymeric network consequently, more water can be absorbed [40].

Fig. 4
figure 4

a Effect of CS-AAc composition on the swelling percentage of CS-AAc hydrogel; b Effect of TiO2 content on the swelling percentage of CS-AAc/TiO2 nanocomposite hydrogel, CS: AAc (50:50 wt.%)

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3.5 Surface morphology

The surface morphology of the prepared CS-PAAc hydrogel and CS-PAAc/TiO2 nanocomposites hydrogel was examined by SEM as shown in Fig. 5. The SEM image of CS-PAAc hydrogel [Fig. 5a] appears as a net-like structure with pores due to the hydrophilicity of both CS and PAAc, that have a great affinity to swell in water. CS-PAAc hydrogel morphology is changed drastically when loaded with TiO2 [Fig. 5b]. The surface morphological structure of CS-PAAc/TiO2 nanocomposite, 3 wt.% of TiO2, appears as a compact surface with very large pores and this result illustrates the increase of the swelling percentage in the presence of TiO2 nanoparticle.

Fig. 5
figure 5

a SEM of CS-PAAc hydrogel; b SEM of CS-PAAc/TiO2 nanocompositeand c TEM of CS-PAAc/TiO2 nanocomposites; TiO2 content 3 wt.%

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Figure 5c shows the TEM image of CS-PAAc/TiO2 nanocomposite hydrogel. It is clear that TiO2 nanoparticles appears as black irregular spherical particles. The particle size is in the range of 32-59 nm. This means there is some agglomerations where the calculated particle size by the Scherrer equation is about 6 nm.

3.6 Adsorption study

The adsorption characteristics of the prepared CS-PAAc/TiO2 nanocomposites towards methylene blue dye (MB) were examined. Furthermore, the adsorption behavior of CS-PAAc hydrogel (TiO2; 0 wt.%) was also evaluated to know the influence of the presence of TiO2 nanoparticles in the copolymeric matrix. The factors affect the adsorption behavior such as pH, adsorbent dose, and temperature were studied.

As it is clear in Fig. 6a, CS-PAAc hydrogel and CS-PAAc/TiO2 nanocomposite hydrogel have negatively charge surface in pH range 2 to 12. It has been reported that, at pH less than pHzpc, due to the presence of the high concentration of H+ ions in the environment, the surface charge of adsorbent is positive [41] while at pH greater than pHpzc the surface is negatively charged. The acidic character of CS-AAc/TiO2 surface is attributted to the Acrylic acid COOH- and the acidic feature of titanium dioxide.

Fig. 6
figure 6

a The pH drift method for CS-PAAc and CS-PAAc/TiO2 nanocomposite; blue line indicates that (pHi) = (pHf). b The effect of pH on the removal percentage of MB dye by CS-PAAc/TiO2 nanocomposites at ambient temperature, initial dye concentration; 20 mg/L and contact time; 24 h

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The influence of pH of the medium on the removal percentage of MB by CS-PAAc/TiO2 nanocomposites was examined over pH range values between 2.0 through 10.5 and the results are shown in Fig. 6b. It was found that the removal percentage increases with the increase in pH of the medium of all investigated samples and the maximum value are got at pH 10.5. It is well known that the extent of adsorption is highly varied according to the adsorbent functional groups [42]. MB is cationic dye. At low pH values, the amino group of CS is protonated, the existence of partially positive charges on the adsorbent reduces the removal percentage by the electrostatic reparation between them and the cationic dye molecules. As the pH value increases over the pKa of –COOH which is approximately 4.6 [43] the carboxylic acid groups are ionized. An electrostatic attraction between the positively charged cationic dye and the negatively charged carboxylate anion is done which responsible for enhancing the removal percentage. On the other hand, it can be noted that CS-PAAc hydrogel (TiO2; 0 wt) has the lowest removal percentage. An enhancement in the removal percentage was observed by using CS-PAAc/TiO2 nanocomposites. However, no remarkable change in the adsorption was detected by increasing the TiO2 content in the nanocomposite matrix. The presence of TiO2 in the copolymeric matrix enhances the adsorption by increasing the physical interaction between the dye molecules and the adsorbent surface. As mentioned before in the examination of the surface morphology by SEM, the surface of the nanocomposite has very large pores compared with CS-PAAc hydrogel, Fig. 5a, b. The large pores permit the possibility of the connection between the adsorbent surface and MB molecules through the large surface area.

The influence of the adsorbent dosage on the removal percentage is shown in Fig. 7. It can be obtained that the removal percentage values increase with increasing the dosage of the sorbent from 0.05 to 0.15 g. Above this value a decrease in adsorption was observed for CS-PAAc hydrogel. In addition, a little increase in adsorption was done at 0.2 g of the two investigated nanocomposites. The maximum adsorbent dosage for the investigated CS-PAAc/TiO2 nanocomposites is 0.20 g per liter of adsorbate and for CS-PAAc hydrogel is 0.15 g per liter of adsorbate. With increasing adsorbent dosage, more adsorption active sites are avaiable for adsorption of MB dye and consequently the dye removal raises. However, at higher dosage huge available functional groups on the adsorbent compared with the number of available dye molecules [44].

Fig. 7
figure 7

The effect of the adsorbent dosage on the removal percentage of MB dye by CS-PAAc/TiO2 nanocomposites at ambient temperature, initial dye concentration; 20 mg/L and contact time; 24 h

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The influence of TiO2 content on the removal percentage of MB dye at different temperatures is shown in Fig. 8. It can be obtained that TiO2 does a remarkable increase in the removal parentage. The removal percentage of MB by CS-PAAc hydrogel (TiO2; 0.0 wt.%) is 73% at 25 °C while it becomes 86% by using CS-PAAc/TiO2 nanocomposite (TiO2; 1.0 wt.%) at the same temperature. Above this TiO2 content value, a little increase in the removal percentage was observed with increasing TiO2 content in the nanocomposite.

Fig. 8
figure 8

The effect of TiO2 content (wt.%) on the removal percentage of MB dye by CS-PAAc/TiO2 nanocomposites at different temperatures, initial dye concentration; 20 mg/L and contact time; 24 h

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On the other hand, an enhancement in the removal percentage was obtained by rising the temperature. This result indicated to that the adsorption reaction is endothermic.

The thermodynamics of adsorption of MB dye onto CS-PAAc and CS-PAAc/TiO2 nanocomposite hydrogels were analyzed by the thermodynamic parameters, such as Gibb’s free energy change (ΔG), enthalpy change (∆H), and entropy change (∆S) that can be calculated by the following equations:

$$\Delta G = - RT \ln K_{C}$$
(5)
$$K_{C} = \frac{{Q_{e} }}{{C_{e} }}$$
(6)
$$\ln K_{C} = \frac{\Delta S}{R} - \frac{\Delta H}{RT}$$
(7)

where, Kc is the thermodynamics parameter, T is the absolute temperature (K), R is the universal gas constant (8.314 J mol−1K−1). The values of ∆H and ∆S can be obtained from the slope and intercept of the linear graph about \({\text{lnK}}_{\text{C}}\) versus 1/T, as shown in Fig. 9, respectively as listed in Table 1.

Fig. 9
figure 9

Van’t Hoff Plot for adsorption of MB dye onto CS-PAAc and CS-PAAc/TiO2

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Table 1 Thermodynamic parameters of MB dye removal with CS-PAAc and CS-PAAc/TiO2
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For all samples, the values of ∆G at all investigated temperatures are negative during the adsorption process. This implies that the adsorption is thermodynamically favorable and spontaneous nature of the adsorption process. Moreover, the increase of the negative values of ∆G as the temperature increases indicated that the adsorption of MB onto CS-AAc and CS-AAC/TiO2 nanocomposite hydrogel is more spontaneous at higher temperature. The positive values of ∆H confirmed the physisorption process and the endothermic nature of adsorption. ∆S also has positive values demonstrated increasing the randomness at the solid-solution interface.

4 Conclusions

In the present study, CS-PAAc/TiO2 nanocomposite hydrogel was prepared by gamma irradiation technique. Characterization and properties were investigated. It was found that XRD diffractograms of CS-PAAc/TiO2 nanocomposite hydrogels confirmed the presence of crystalline TiO2 nanoparticles in the nanocomposite hydrogels. The average particle size of TiO2 nanoparticles is estimated as; 5.8, 5.7, and 5.9 nm for 1.0, 2.0, and 3.0 wt.% TiO2 content; respectively. The TGA results seem similar thermal behaviors and maximum thermal temperature in each stage of CS-PAAc/TiO2 nanocomposite by varying the TiO2content with very little change. However, the swelling percentage enhances nearly fourth times by adding 1 wt.% TiO2 nanoparticle in the nanocomposite hydrogel compared with CS-PAAc hydrogel and there are further increases in the swelling percentage with increasing TiO2 content in the matrix. The adsorption characteristics of the prepared CS-PAAc/TiO2 nanocomposites were studied for removal of MB dye. It was obtained that the maximum removal percentage is obtained at pH = 10.5. The maximum adsorbent dosage for CS-PAAc/TiO2 nanocomposites is 0.20 g and for CS-PAAc hydrogel is 0.15 g per liter of the adsorbate. The removal percentage of MB by CS-PAAc/TiO2 nanocomposite is higher than CS-PAAc hydrogel however a little increase in the removal percentage was observed with increasing TiO2 content in the nanocomposite. The removal percentage is enhanced with rising the temperature this means the adsorption reaction is endothermic. This study revealed that the loading of TiO2 nanoparticles into the polymeric matrix of CS-PAAc does a remarkable increase in the removal parentage of MB dye from its aqueous solution.