Synthesis of an efficient adsorbent hydrogel based on biodegradable polymers for removing crystal violet dye from aqueous solution

Riham R. Mohamed, Mahmoud H. Abu Elella, Magdy W. Sabaa & Gamal R. Saad

Cellulose

ISSN 0969-0239

Cellulose DOI 10.1007/s10570-018-2014-x

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ORIGINAL PAPER

Synthesis of an efficient adsorbent hydrogel based on biodegradable polymers for removing crystal violet dye from aqueous solution

Riham R. Mohamed . Mahmoud H. Abu Elella . Magdy W. Sabaa . Gamal R. Saad

Received: 16 May 2018 / Accepted: 22 August 2018 Ó Springer Nature B.V. 2018

Abstract Water pollution with toxic dyes threatens Keywords Xanthan gum Á Poly (N-vinyl human health world-wide. Herein, we prepared an imidazole) Á Hydrogel Á Crystal violet Á Adsorption Á effective adsorbent hydrogel for removing toxic Regeneration cationic crystal violet (CV) dye from its aqueous solutions using biodegradable polymers such as; xanthan gum (XG) and poly (N-vinyl imidazole) (PVI). The structure and morphology of the prepared Introduction XG/PVI hydrogel and CV loaded hydrogel were characterized by FTIR, FE-SEM and XRD, while Water pollution is considered one of the most serious thermal stability of investigated hydrogel was charac- problems worldwide as it threatens human health due terized by TGA. Adsorption experiments were carried to rapid industrialization (Kumari et al. 2017). Water out as functions of initial concentration of CV dye, the is polluted with toxic dyes, toxic heavy metal ions and adsorbent dose, pH of solution, temperature, and organic contaminants (Ghorai et al. 2014). Recently contact time. Results were analyzed using Langmuir the use of synthetic dyes in different industries and Freundlich isotherm models. It is found that the increased such as; leather, ink, textile, food process- data were well fitted by Langmuir model. The ing, cosmetics, rubber and paper products because maximum adsorption capacity achieved was found to they are cheap and easily available (Mittal et al. 2014). be 453 mg g-1 due to electrostatic, H-bonding and Dyes are difficult to eliminate due to their synthetic dipole–dipole interactions between adsorbent surface origin and complex structure which makes them very and CV molecules. The adsorption kinetic studies stable (Kumari and Abraham 2007; Martins et al. showed that the adsorption followed pseudo-first order 2017; Mittal et al. 2016). and intraparticle diffusion kinetic models. Regenera- Moreover, dyes are extremely dangerous pollutants tion (desorption) studies showed that XG/PVI hydro- due to their toxicity and carcinogenic nature, poor gel is an interesting adsorbent for removing toxic dyes degradability and high solubility in water, so, they from waste water. accumulate in living cells and cause hazardous effects on human health (Mittal et al. 2014, 2016). Among various dyes, crystal violet (CV) dye (Fig. 1a)—which R. R. Mohamed (&) Á M. H. Abu Elella Á is the target of our research—is a kind of cationic M. W. Sabaa Á G. R. Saad triphenyl methane dye that is responsible for cancer Department of Chemistry, Faculty of Science, Cairo University, Giza 12613, Egypt and several eye irritation to human beings (Dil et al. e-mail: [email protected] 2016; Ghorai et al. 2014; Hameed 2008; Xu et al. 123 Author's personal copy

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Fig. 1 Chemical structures of a crystal violet dye CV, b XG and c PVI

2011). Various removal methods have been studied for (Benny et al. 2014; Bilanovic et al. 2016; Mittal et al. water purification from industrial drains by floccula- 2016; Talukdar and Kinget 1995). It was discovered tion, photocatalytic, membrane separation, ozonation, by Allene Jeanes and her co-workers in 1961 (Jeanes coagulation, ion exchange and electrolysis. However, et al. 1961). XG is a water soluble anionic polysac- these processes vary in their effectiveness, costs, and charide, it is known as microbial polysaccharide environmental impacts (Allegre et al. 2004; Hao et al. because it is extracted from Xanthomonas campestris. 2000; Mittal et al. 2016; Sanghi et al. 2007). Adsorp- XG structure (Fig. 1b) is composed of b-(1-4)-D- tion is one of the most effective techniques used for glucopyranose glucan in backbone chain that is linked pollutants removal from contaminated water as it is to side chain (a-D-mannopyranose-(2-1)-b-D-glu- simple, efficient, low cost, eco-friendly technique, curonic acid-(4-1)-b-D-mannopyranose) through b- (Al-Qodah 2000; Laaz et al. 2016; Mittal et al. 2014). (3-1) linkages. The molecular formula of its repeating

In the past decades, scientists became interested in unit is C35H49O29 (Benny et al. 2014; Garcıa-Ochoa natural polysaccharides as adsorbents for purifying et al. 2000; Pandey and Ramontja 2016). XG is rarely water from toxic dyes because polysaccharides are used alone as adsorbent due to its solubility in water, available, non-toxic, eco-friendly, biodegradable, so it should be modified to be used as adsorbent (Jalali inexpensive, and easily modified (Ghorai et al. 2014; et al. 2016). Mittal et al. 2016; Parker et al. 2012). Polysaccharides On the other hand, poly (N-vinyl imidazole) (PVI), adsorb dyes through the electrostatic interactions Fig. 1c, is a water soluble synthetic polymer, it is between adsorbent and dyes (Mittal et al. 2015, 2016). produced via free radical polymerization of N-vinyl Xanthan gum (XG) is one of the natural polysac- imidazole (Caner et al. 2007). PVI contains imidazole charides that are used in different applications such as; moiety that contains nitrogen atom, so it forms wastewater treatment, cosmetics, pharmaceuticals and complexes with different toxic heavy metal ions from food industry because it has good properties including their aqueous solutions through coordination bonds biodegradability, biocompatibility and non-toxicity (Broekema et al. 1982; El-Hamshary et al. 2014; Pekel

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Cellulose and Gu¨ven 2004). PVI has good properties such as; dissolved in 30 mL of benzene in two-neck round thermal stability, biocompatibility and biodegradabil- bottomed flask and stirred at 70 °C under N2 atmo- ity (Hu et al. 2017). sphere. Then 10 mL (0.1 M) of N-vinyl imidazole was Hydrogels are three-dimensional (3D) networks dropwisely added to the above solution. After that, the formed from physically or chemically crosslinked solution was stirred for 3 h at 70 ± 2 °C. After the polymers, so, they are insoluble in many different time was elapsed, the product was precipitated in solvents. They are capable of absorbing large amount acetone. PVI product was filtered and washed several of water or biological fluids due to the presence of times with acetone to remove unreacted monomer. hydrophilic groups such as; hydroxyl (–OH), car- The final product was dried in vacuum oven at 40 °C boxylic acid (–COOH), sulphuric acid (–SO3H) for 24 h and recovered with 80% yield. The viscosity groups and imidazole rings along chains of the average molecular weight of PVI was determined as hydrogels (Karadag˘ et al. 1995; Pekel and Gu¨ven 7.4 9 105 g mol-1. 2002). They function in water purification by remov- ing heavy metal ions, and then can be regenerated Synthesis of XG/PVI hydrogel easily (Pekel and Gu¨ven 2004; Pekel et al. 2000). Therefore, we expect that synthesized hydrogels 0.5 g of XG was dissolved in 50 mL of distilled water based on biodegradable polymers as XG and PVI under continuous stirring for 1 h then 0.5 g of PVI including different groups such as carboxylate, dissolved in 50 mL of distilled water then it was hydroxyl and imidazole groups along chains which dropwisely added. The mixture was kept under act as efficient adsorbents for removing toxic cationic constant stirring for 2 h at room temperature crystal violet dye from aqueous solutions. The dye (* 30 °C). The precipitated hydrogel was separated adsorption capacity of the prepared XG/PVI hydrogel via filtration using a G2 sintered glass funnel, was studied as a function of the initial concentration of collected by filtration, washed several times with dye, pH, temperature and contact time. The molecular distilled water and then dried in vacuum oven at 40 °C structure and morphology of XG/PVI hydrogel and for 24 h. XG/PVI loaded with CV are characterized via FTIR, XRD and FE-SEM. Swelling measurement

The ability of XG/PVI hydrogel for water uptake was Materials and experimental methods studied in phosphate buffered saline (PBS) (pH 7.4) at room temperature (* 30 °C). 50 mg of hydrogel was Materials immersed in 25 mL of PBS at different time periods until equilibrium water uptake was reached. The Xanthan gum was purchased from Alpha-Chemika, swollen hydrogel was removed from PBS then excess India. N-vinyl imidazole was purchased from Alfa water on the surface was removed with filter paper and AesarÒ-Germany. Crystal violet dye was obtained finally weighed. The percentage of the swelling was from Loba chemi Pvt. Ltd., Mumbai, India. Sodium calculated according to the following Eq. (1) (Ray hydroxide and hydrochloric acid were purchased from et al. 2010). Merck-Germany. Phosphate buffer saline tablets were ðÞW À W purchased from Sigma, USA. 2, 20-azobisisobutyroni- Swelling % ¼ s 0 Â 100 ð1Þ W trile (AIBN) and acetone were purchased from Sigma- 0

Aldrich, Germany. where Ws and W0 are the weights of swollen XG/PVI hydrogel and the dry sample, respectively. Synthesis of poly (N-vinyl imidazole) Adsorption of CV dye Poly (N-vinyl imidazole), PVI, was prepared by free radical polymerization of N-vinyl imidazole using The adsorption of CV dye from aqueous solution was AIBN as free-radical initiator in benzene (Unal et al. carried out in dark glass bottles at 30 °C for 6 h. The 2014). Briefly, 0.03 g (0.18 mM) of AIBN was effects of different experimental parameters such as; 123 Author's personal copy

Cellulose initial concentration of dye, adsorbent (XG/PVI on the hydrogel was recycled using 0.1 M HCl hydrogel) dose, solution pH, temperature and contact (desorption method). The percentage of desorption time were examined to obtain the best adsorption of dye was calculated according to Eq. 5 (Mittal et al. conditions. 1000 mg L-1 of CV as a stock solution 2016): was prepared then was diluted for adsorption methods. Cd The effects of pH on CV dye removal were studied % DesorptionðÞ¼ Regeneration  100 ð5Þ C over the pH range of 2–9; the pH of the solution was e adjusted with 0.1 M HCl and 0.1 M NaOH. All typical where Cd is the desorption concentration of CV dye batch experiments were done by soaking appropriate solution (mg L-1). amount of adsorbent in 50 mL of the dye solution for 6 h. After that the remaining dye was separated from the adsorbent by centrifugation (Centrifuge, 2-16PK, Characterization (instrumentation) Sigma, Germany) at 4000 rpm for 10 min and its absorbance was recorded using UV/Vis spectropho- Fourier Transform Infrared (FTIR) spectra of PVI, tometer at kmax = 578 nm. XG, XG/PVI hydrogel and hydrogel loaded with CV The percentage of CV dye removal (adsorption (tested samples) were done using Jasco FTIR 4100 efficiency) and the equilibrium adsorption (Qe) were spectrometer (Japan) through the frequency range of calculated according to the following Eqs. 2 and 3, 600–4000 cm-1. respectively (Ghorai et al. 2014). X-ray diffraction (XRD) spectra of tested samples were obtained using an X-ray powder diffractometer ðÞC À C % Removal of CV dye ¼ 0 e  100 ð2Þ (X’Pert PRO with Secondary Monochromator, Cu- C0 radiation (k = 1.542 A˚ ) at 45 K.V., 35 M.A. and scanning speed 0.02° s-1) between 2h =3°–60°. ðÞC0 À Ce V Q ¼ ð3Þ The surface morphologies were examined by a field e W emission scanning electron microscope, FE-SEM, where C0 is the initial concentration of CV dye (Quanta 250 FEG) attached with energy dispersive -1 solution (mg L ), Ce is the equilibrium concentration X-ray (EDX) unit and with a magnification ( 9 1000) -1 of CV dye solution (mg L ), qe is equilibrium and accelerating voltage was 30 kV. Samples were -1 adsorption of dye on adsorbent (mg g ), V is the prepared by placing a small part of the film on a carbon volume of CV dye solution taken (L) and W is the tape on a stub and were then coated with a thin layer of adsorbent weight (g). gold. The concentration of CV dye after recorded time The thermal stability of PVI, XG and XG/PVI intervals is determined; the adsorption capacity at time hydrogel was estimated using thermogravimetric -1 t, Qt (mg g ) was calculated using the following analysis (TGA) data that were recorded using TGA- equation: 50H Shimadzu Thermogravimetric Analyzer. The ðÞC À C V measurements were done over temperature ranges Q ¼ 0 t ð4Þ t W 30–700 °C with a dynamic heating rate 10 °C/min under N2 atmosphere and the reference material was where Ct is the remaining dye concentration in the alumina. solution at time. Adsorption studies of CV dye onto XG/PVI hydrogel were done using Unico 1200 UV–Vis Desorption (regeneration) method spectrophotometer at k max = 578 nm. To study the regeneration of XG/PVI hydrogel from the adsorbed CV dye, four adsorption–desorption cycles were carried out by soaking 30 mg of hydrogel into 50 mL of dye solution with 450 mg L-1 of CV for 6 h. Then the hydrogel loaded with the dye was separated by centrifugation and dried. The dye loaded

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Results and discussion 1229 cm-1 and 662 cm-1 referred to the stretching vibration of C–N groups. Also, two peaks for out-of- Characterization of XG/PVI hydrogel and XG/ plane bending vibration of C–H groups in imidazole PVI/CV matrix ring appeared at 822 cm-1 and 743 cm-1. FTIR characterization of PVI was similar to literature (Pekel FTIR and Gu¨ven 2002; Unal et al. 2014). The spectrum of XG/PVI hydrogel displayed the FTIR spectra of XG, PVI and XG/PVI hydrogel are characteristic peaks of both XG and PVI. The FTIR shown in Fig. 2. The XG IR spectrum was previously spectrum of XG/PVI showed the main characteristic discussed by the same authors (Abu Elella et al. 2017), peaks of both XG and PVI with slight change in their it is characterized by two absorption peaks at intensities. The absorption peak of -OH was shifted 3431 cm-1 and 2922 cm-1 related to the –OH and – from 3431 cm-1 to 3425 cm-1 and became broader CH stretching, respectively. The absorption peaks at which may be due to intermolecular H-bonding 1639 cm-1 and 1420 cm-1 are characteristic of interactions between –OH groups in XG and Nitrogen asymmetric and symmetric carboxylate groups, atom in imidazole ring in PVI, as shown in Scheme 1. respectively. The broad peak at 1065 cm-1 is assigned Also, the absorption peak of carbonyl group in –COO- for stretching vibration of glycosidic (C–O–C) bond. was shifted from 1639 to 1626 cm-1 due to The PVI spectrum showed two absorption peaks at intramolecular H-bonding interactions between XG 3109 cm-1 and 2922 cm-1 corresponded to the chains inside the hydrogel. stretching of = C–H groups in imidazole ring and C– Furthermore, the FTIR spectrum of CV showed all H groups in the main backbone chains, respectively. characteristic absorption peaks of disubstituted ben- The strong absorption peak at 1639 cm-1 is assigned zene rings. It showed absorption peak at 2938 cm-1 for C=C groups. In addition, two peaks are assigned referred to stretching vibration of SP3 hydrocarbon (– for C=N and C–C stretching vibrations of imidazole C–H) in methyl groups. Also, spectrum exhibited ring at 1498 cm-1 and 912 cm-1. Two sharp absorp- absorption peak at 1584 cm-1 corresponded to tion peaks at 1415 cm-1 and 1083 cm-1 corresponded stretching vibration of (C=C) groups of benzene ring to in-plane bending vibration of C-H groups. More- and the absorption peak for methyl group of quater- over, three absorption peaks appeared at 1282 cm-1, nized amine appeared at 1472 cm-1, while the

Fig. 2 FTIR of XG, PVI, XG/PVI hydrogel, CV and hydrogel loaded with CV Dye 123 Author's personal copy

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Scheme 1 The proposed mechanism for preparation of XG/PVI hydrogel absorption peak of C–H deformation in methyl groups X-ray diffraction (XRD) analysis of aromatic tertiary amine appeared at 1366 cm-1. Moreover, two absorption peaks appeared at The X-ray diffraction spectra of XG, PVI, XG/PVI 1166 cm-1 and 1132 cm-1 related to C-H bending hydrogel, CV dye and hydrogel loaded with CV (XG/ in benzene rings and C-N stretching vibration, PVI/CV) are illustrated in Fig. 3. The results showed respectively (Bajpai and Jain 2012; Cheriaa et al. that XG pattern had a broad weak diffraction peak at 2012). 2h =20°, suggesting that XG is amorphous in nature While, the FTIR spectrum of XG/PVI/CV, showed (Abu Elella et al. 2017). On the other hand, PVI that the characteristic absorption peaks of XG/PVI exhibited two pronounced diffraction peaks at hydrogel shifted after CV adsorption onto hydrogel. In 2h =12° and 21.7°, indicating a low partially crys- spectrum of XG/PVI/CV, the absorption peaks of –OH talline phase of PVI, which is consistent with previous stretching shifted from 3425 cm-1 to 3417 cm-1 and report (Kara et al. 2016). However, XRD pattern of became broader than before, carbonyl stretching XG/PVI hydrogel showed broad diffraction peak at shifted from 1626 cm-1 to 1617 cm-1, –COO- 2h = 20.6° located between the characteristic diffrac- stretching shifted from 1420 cm-1 to 1415 cm-1 and tion peaks of XG and PVI components. This suggested C=N (ring) groups shifted from 920 cm-1 to the presence of intermolecular interactions via 913 cm-1. These observations suggested that the H-bonding between XG and PVI (Scheme 1). The adsorption of CV dye occurred as a result of the XRD spectrum of CV showed several diffraction attractive forces between –OH, –COO-, carbonyl peaks from 2h =8° to 53.7° characteristic of the groups and imidazole rings on the surface of XG/PVI crystalline nature of CV dye (Kumari et al. 2017), hydrogel and CV structure. These observations were while the spectrum of CV dye loaded onto XG/PVI similar to methyl violet dye removal using h-XG/SiO2 hydrogel provided no information about dye crys- nanocomposite (Ghorai et al. 2014). The FTIR spec- tallinity, indicating a diffusion of CV into XG/PVI trum of (XG/PVI/CV) indicated that the interaction hydrogel. Similar observation was found for loading occurred between CV and the active sites (–OH, – CV dye on alginate-grafted poly (acrylic acid)/TiO2 COO-, carbonyl groups and imidazole rings) present nanocomposite (Thakur and Arotiba 2017) and onto in XG/PVI hydrogel. spent tea leaves (Bajpai and Jain 2012).

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Fig. 3 XRD charts of XG, PVI, XG/PVI hydrogels, CV and XG/PVI/CV

Field emission scanning electron microscope (FE- shown in Fig. 5. It can be seen that XG and PVI SEM) exhibited two main weight losses. One is in the range of 30–140 °C which was assigned for elimination of One of the most crucial properties of the adsorbents water adsorbed by hydrophilic polymers. The other is which can be considered is microstructure morphol- in the range of 220–500 °C for XG and 270–430 °C ogy. Figure 4 shows FE-SEM micrographs of the XG, for PVI were ascribed to a complex process including PVI, XG/PVI hydrogel and XG/PVI/CV samples. The dehydration of the saccharide rings, chain scission images showed that there was variation in surface with the formation of water, CO2 and CH4 for XG, morphology between XG and PVI components and while it was related to the elimination of imidazole their hydrogel. The XG surface was irregular and group and depolymerization of PVI. While the weight looked like lobules, similar to our previous work (Abu loss and residue for XG were found to be about 49% Elella et al. 2017). and 18%, respectively, the weight loss for PVI and While, the surface of the PVI exhibited highly residue were 73% and 13% at 600 °C. The decompo- irregular cavities and porous morphology. However, sition of XG started at about 230 °C, while it started at the surface of hydrogel was completely changed to about 375 °C for PVI, indicating that PVI had more rough and fibrous with many pores morphology. thermal stability than XG. Similar decomposition The hydrogel loaded with CV dye showed a behavior was previously observed for pure XG and relatively tight and smooth surface compared with PVI (Abu Elella et al. 2017; Fodor et al. 2012; Unal unloaded gel (Fig. 4d), indicating that many pores et al. 2014). occupied by CV dye. Typical EDX spectra of the XG/ However, TGA thermogram of XG/PVI hydrogel PVI gel and loaded gel with CV dye are shown in revealed that the degradation occurred via two degra- Fig. 4e, f. It is obvious that the gel loaded with CV dye dation steps. The first one, corresponding to the contains a signal for Cl which confirms the adsorption decomposition of the XG component, started at about of the CV dye onto the gel. 240 °C and the second started at about 345 °C due to the decomposition of PVI. These results indicated that Thermogravimetric analysis (TGA) the thermal stability of XG was enhanced by the presence of PVI component due to the formation of The thermal stability of XG, PVI and XG/PVI intermolecular hydrogen bonding between lone pair of hydrogel was studied by TGA and the results are

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Fig. 4 SEM images of a XG, b PVI, c XG/PVI hydrogel, d loaded XG/PVI/CV, and EDX of e XG/PVI, f loaded XG/PVI/CV

Swelling of the hydrogel

The swelling behavior of XG/PVI hydrogel was done in PBS buffer (pH 7.4) for various time intervals at room temperature (30 °C) and the results are repre- sented in Fig. 6. It was obvious that the swellability of the hydrogel increased with increasing the time interval to reach to equilibrium swelling at about 6 h. The high degree of swelling of XG/PVI hydrogel is due to presence of many hydrophilic groups along Fig. 5 TGA thermogram of XG, PVI and XG/PVI hydrogel chains of hydrogel such as hydroxyl, carboxylate and imidazole groups. The swell-ability of the XG/PVI imine group of the imidazole ring of PVI and both the hydrogel confirmed its ability to adsorb CV from hydroxyl and carboxylic groups of XG. aqueous solution.

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path length. Similar results were reported for methylene blue adsorption, CV and malachite green onto XG nanocomposites (Ghorai et al. 2014; Mittal et al. 2014, 2016) and CV on to TLAC/chitosan composites (Kumari et al. 2017).

Effect of pH of solution The pH of solution is considered one of the most important parameters in adsorption studies of cationic dyes onto bio adsorbents, because of its effects on both adsorption Fig. 6 Kitentic study of swell-ability of XG/PVI hydrogel in on active sites and dye molecules ionization (Mittal phosphate buffer saline (PBS) at different time et al. 2014). The effect of the solution pH on CV dye adsorption using XG/PVI hydrogel was studied using Adsorption studies of CV onto XG/PVI hydrogel pH range (2–9) and the results are illustrated in Fig. 7c. The results showed that the less adsorption of Optimization of the Adsorption Conditions CV was done in highly acidic solution (pH 2) because the concentration of H? ions is very high which is a Removal of toxic dyes from aqueous solution using an competitor for the adsorption of cationic CV dye onto adsorbent is well known to depend on the initial XG/PVI hydrogel due to its small size. XG and PVI are concentration of the dye, the adsorbent concentration, anionic polymers because they contain carboxylate temperature, the solution pH, added salt, the contact groups and imidazole moieties as anionic active sites time of adsorption. so, H? ions attached to them easier than CV molecules (CV is larger in size than H?) that led to less Effect of dye concentration Figure 7a illustrates the adsorption of CV in highly acidic solution. However, effect of the concentration of CV on the % adsorption CV adsorption % increased with increasing the using XG/PVI as an adsorbent. It was obvious that the solution pH to reach the maximum value (91%) at dye removal decreased from about 91 to 16% with an neutral pH because the H? ions concentration increase in CV concentration from 450 to decreased with increased pH. Afterwards, CV 600 mg L-1. This trend might be explained by the adsorption % remained constant, because, in basic possibility that at lower concentrations, a maximum medium, the negatively charged binding sites of number of dye molecules would be able to adsorb on hydrogel repulsed the OH- ions which led to the hydrogel. Conversely, at higher dye enhance CV dye adsorption. This behavior was concentrations, a lower % adsorption had been similar to the adsorption of malachite green on XG/ achieved due to the saturation of the active Fe3O4 nanocomposite (Mittal et al. 2014), methylene adsorption sites. These observations were similar to blue on unexpanded and expanded perlite (Dog˘an the adsorption of both methyl violet and methylene et al. 2000) and rhodamine B using bagasse fly ash blue on XG nanocomposites (Ghorai et al. 2014). (Gupta et al. 2000).

Effect of adsorbent concentration Figure 7b shows Effect of temperature Effect of temperature as a the effect of the hydrogel concentration on CV function of CV adsorption % is illustrated in Fig. 7d. adsorption % from aqueous solution. The maximum The results revealed an enhancement in the adsorption dye removal was found to be 91% at 30 mg adsorbent, efficiency from 52 to 91% with increasing temperature after which it reached a plateau. This can be explained from 10 to 30 °C. The increase in the adsorption % on the basis of the availability of the surface area and may be due to the increase in kinetic energy of dye active sites of the hydrogel. With higher adsorbent molecules and also increase in swelling which concentration above 30 mg, lower rate of adsorption enhanced CV molecules diffusion through hydrogel was observed which might be due to the aggregation of matrix (Chowdhury et al. 2011; Ghorai et al. adsorption sites, causing a decrease in the total 2013, 2014). However, at relatively high available surface area, and an enhanced diffusion temperatures (40–60 °C), the desorption of dye from 123 Author's personal copy

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Fig. 7 Effects of different adsorption parameters such as: a initial concentration of CV, b adsorbent concentration, c pH of dye solution, d adsorption temperature, e adsorption time, f ionic strength. Na ? ions) hydrogel surface may happen due to enhanced thermal increased with increasing the contact time and reached effects which weaken the bonds interactions between a maximum (91.0%) within 6 h, after that it leveled the CV molecules and the binding sites on the off. This suggests that a monolayer of dye molecules hydrogel (Chowdhury and Saha 2010; Ghorai et al. on the gel surface is formed. The formation of 2013, 2014). monolayer of CV dye at external interface on hydrogel resulting in increasing the adsorption driving force Effect of the adsorption time (Ghorai et al. 2013, 2014; Kumari et al. 2017; Wu et al. 2001). However, the removal of dye remained Figure 7e illustrates the effect of adsorption time on constant with increasing the adsorption time from 6 the percentage of the adsorption efficiency of the to 8 h, because the free adsorption active sites may hydrogel. The effect of adsorption time (equilibrium become saturated. So, increase the adsorption time time) is considered one of the important parameters in (above 6 h) did not affect the adsorption efficiency. design of efficient adsorbents for wastewater treatment Therefore, the adsorption equilibrium time was taken (Gupta and Saleh 2013; Kumari et al. 2017). The at 6 h). adsorption time was varied from 1 to 8 h while Therefore, it can be concluded that the maximum keeping all the other parameters constant; 30 mg of adsorption of CV dye molecules (adsorption efficiency -1 -1 adsorbent, 50 mL of dye solution (450 mg L ) with was 91% and Qmax = 453 mg g ) from aqueous neutral pH and temperature was kept at 30 °C. It can solution achieved at optimum conditions; adsorbent be seen that the adsorption efficiency of CV dye concentration at 30 mg, dye concentration at

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450 mg L-1 CV dye, pH 7, temperature 30 °C and the affinity of active sites and free energy of adsorption contact adsorption time 6 h. and KF and 1/n are Freundlich constant relates to the adsorption capacity and adsorption intensity. Effect of the ionic strength Figure 7f illustrated the Figure 8 illustrates adsorption isotherms fitted to effect of the ionic strength in presence of different Langmuir and Freundlich models. Table 1 displays NaCl concentrations (from 0.1 to 0.5 N) on the Langmuir and Freundlich equilibrium constants and adsorption efficiency of CV dye was studied at the corresponding fitting correlation coefficients (R2). optimum adsorption conditions; 30 mg of adsorbent/ The results revealed that Langmuir isotherm model 50 mL of dye solution (450 mg L-1), neutral pH (7), presented better correlation coefficient. The maximum temperature 30 °C and adsorption time 6 h. The adsorption capacity (Qm) was determined as results revealed that the adsorption of CV gradually 453 mg g-1, suggesting that that adsorption of CV decreased with the increase in Na? ions concentration. dye takes place preferably forming a monolayer. The

This observation showed the competitive effect value of the maximum adsorption capacity (Qm)ofthe between Na? ions and the cationic CV dye investigated dye was found to be 453 mg g-1 and molecules with anionic binding sites (negatively compared with other adsorbents (Table 2). It can be charged sites) on XG/PVI hydrogel. These seen from Table 2 that the Qm value of the XG/PVI observations were similar to the removal of hydrogel is larger than other pervious adsorbents. On methylene blue using XG nanocomposites and the other hand, the value of 1/n from Freundlich Cu(II)-exchanged montmorillonite (Ghorai et al. isotherm model was found less than 1.0 and indicated 2014; Ma et al. 2004). The ionic strength results that XG/PVI was suitable adsorbent for the adsorption confirmed that predominant attractions for adsorption of CV dye, these results were similar to other results of CV dye onto anionic XG/PVI hydrogel through mentioned for adsorption of CV (Bajpai and Jain electrostatic interactions. 2012; Ghorai et al. 2014; Mittal et al. 2016). The affinity of CV to XG/PVI hydrogel was further

Adsorption isotherm determined using the separation factor (RL) that was defined by the following Eq. (8): The adsorption isotherm models provide information R ¼ 1=1 þ bC ð8Þ about the interaction between the adsorbent surface L o -1 and dye molecules at equilibrium state (Bajpai and where Co is the initial concentration of CV (mg L ) Jain 2012). So, the adsorption isotherm of CV onto and b is Langmuir constant. XG/PVI hydrogel was studied at optimum conditions The adsorption process was defined according to RL that gave maximum adsorption efficiency; 0.03 g of value, so, it may be unfavorable if RL value was more hydrogel, 50 mL of neutral dye solution, 30 °C and than 1, linear if RL equal 1, irreversible if RL =0or adsorption time was 6 h and data was analyzed using favorable if 0 \ RL [ 1 (Bajpai and Jain 2012; Weber the linear Langmuir and Freundlich isotherm models. and Chakravorti 1974). In this study, the calculated The linear Langmuir and Freundlich isotherm models values of RL were found to be in range of 0.145–0.413, are represented according to the following Eqs. 6 and indicating favorable adsorption process of CV dye 7, respectively (Foo and Hameed 2010; Freundlich onto XG/PVI hydrogel. The previous results were 1906; Langmuir 1916; Tian et al. 2010). consistent with the previous findings for adsorption of CV on other adsorbents (Bajpai and Jain 2012; Ghorai 1=Q ¼ 1=Q þ 1=bQ C ð6Þ e m m e et al. 2014; Madhavakrishnan et al. 2009). ln Qe ¼ ln KF þ 1=nlnCe ð7Þ Adsorption kinetics where, Qe is the amount of CV dye adsorbed onto XG/ -1 PVI hydrogel surface (mg g ) at equilibrium, Qm The rate of adsorption of dye depends on both the refers to the maximum adsorption capacity corre- contact time of adsorbent with adsorbate and the -1 sponding to complete monolayer coverage (mg g ), diffusion processes (Pal et al. 2012). The adsorption Ce is the equilibrium concentration of CV dye (mg process is multi-step process which includes the -1 -1 L ), b is Langmuir constant (L mg ) that relates to migration of the dye molecules from aqueous solution 123 Author's personal copy

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Fig. 8 Linear Langmuir and Freundlich isotherm models for the absorption of CV dye onto XG/PVI hydrogel

Table 1 The adsorption isotherm parameters for adsorption CV dye onto XG/PVI hydrogel Langmuir isotherm model Freundlich isotherm model

-1 -1 2 -1 2 Qm (mg g ) b (L mg )R KF (L g ) 1/n R 453 0.011 0.9921 33.5 0.42 0.9843

Table 2 Comparison for the adsorption of CV dye with various adsorbents

-1 Adsorbents Adsorption capacity (Qmax) (mg g ) References

Soil-silver nanocomposite 1.9 Satapathy and Das (2014) Poly(AA-co-NVP)/Laponite RDS hydrogel 4.1 Zhang et al. (2006) Magnetic calcium ferrite nanoparticle 10.7 An et al. (2015) PAA-bentonite-Fe Co hydrogel 31.1 Shirsath et al. (2011) Coniferous pinus bark 32.8 Ahmad (2009) Macro-porous poly(AA-AM) hydrogel 40.3 Li et al. (2010) HPMC-g-PAM/MMT hydrogel 61.3 Mahdavinia et al. (2013) Magnetic nanocomposite 81.7 Singh et al. (2011) Modified cellulose with glycidyl methacrylate 218.8 Zhou et al. (2014) Carboxylate-functionalized cellulose nanocrystals 243.9 Qiao et al. (2015) Poly(AA-co-DMAM) hydrogel 310.0 Bekiari et al. (2008) XG nanocomposites 378.8 Ghorai et al. (2014) XG/PVI hydrogel 453 Present study

to boundary layer of the adsorbent (1st step) then the adsorption processes of CV dye on XG/PVI diffuse into the surface of adsorbent (2nd step) that hydrogel, pseudo-first-order and pseudo-second-order followed by the diffusion of dye molecules into kinetic models were used. The pseudo-first order, internal porous sites via the pore diffusion (Ghorai second order and pseudo-second order kinetic equa- et al. 2014; Mittal et al. 2016). In order to investigate tions can be represented by the following Eqs. 9, 10 123 Author's personal copy

Cellulose and 11, respectively (Ho and McKay 1998, 1999; The obtained adsorption kinetic parameters for

Lagergren 1898; Mittal et al. 2016; Pal et al. 2012). pseudo-first order k1 and qe were found to be 0.0045 min-1 and 444 mg g-1 and for second order lnðÞ¼ qe À qt ln qe À K1t ð9Þ -5 -1 k2 and qe were found to be 2.32 9 10 gmg - min-1 and 769 mg g-1. 1=qe À qt ¼ 1=qe þ K2t ð10Þ The diffusion of CV from boundary layer of the 2 adsorbent to porous sites of the XG/PVI hydrogel can t=qt ¼ 1=K3qe þ t=qe ð11Þ be explained using intraparticle diffusion kinetic where, Qe and Qt are the amounts of CV dye adsorbed model. The linear equation form of intraparticle on the adsorbent surface at equilibrium at time t diffusion model is represented by the following -1 (mg g ), respectively. While, k1,k2 and k3 are the Eq. 12 (Kumari et al. 2017; Thakur and Arotiba adsorption rate constants of pseudo-first order kinetic 2017; Weber and Morris 1963). model (min-1), second order and pseudo-second order -1 -1 0:5 kinetic model (g mg min ), respectively. The qt ¼ K4t þ C ð12Þ plots of ln (qe - qt) versus t, 1/qt versus t and t/qt where, k4 is the intraparticle diffusion rate constant versus t are shown in Fig. 9. The results reveal that (mg g-1 min-0.5) and C is the thickness of the there is no good fit between experimental data and the 0.5 boundary layer. The plot of qt against t is shown pseudo-second order model, while the data was well in Fig. 9d. Two linear segments can be distinguished described by the pseudo-first order model which confirmed the multi-step adsorption of CV dye (R2 = 0.974) than second order model (R2 = 0.915).

Fig. 9 Adsorption kitentic models a pseudo-first order, b second order, c pseudo-second order, d intraparticle diffusion for adsorption of CV onto XG/PVI hydrogel 123 Author's personal copy

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Scheme 2 Proposed mechanism for adsorption CV onto XG/PVI hydrogel onto XG/PVI hydrogel. The first linear section shows results, both the pseudo-first order model and intra- the quick increase in adsorption of CV dye due to the particle diffusion model are participating together in migration of CV molecules from aqueous dye solution adsorption of CV onto XG/PVI hydrogel. to boundary layer of the hydrogel by a strong electrostatic attraction. While, the second linear Mechanism of adsorption section refers to adsorption step, corresponding to diffuse CV molecules within the porous sites of the The maximum adsorption capacity may be rational- XG/PVI hydrogel via intraparticle diffusion. At this ized due to different bond interactions such as; step, all adsorption active sites on the hydrogel electrostatic, H-bonding and dipole–dipole interac- occupied by CV molecules. The presence of two tions between cationic (CV) dye molecules and linear sections in Fig. 9d and non-zero intercept anionic adsorbent (XG/PVI hydrogel) (Scheme 2). (thickness of the boundary layer) suggests that intra- The maximum CV dye removal from aqueous solution particle diffusion participates in the adsorption mech- was showed in Fig. 10. anism of CV onto the hydrogel surface in combination with another kinetic model. From adsorption kinetic

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Fig. 10 Adsorption steps of CV dye a CV dye before adsorption, b CV loaded onto hydrogel after 6 h, c remaining of CV dye solution after 6 h

Conclusion Ahmad R (2009) Studies on adsorption of crystal violet dye from aqueous solution onto coniferous pinus bark powder (CPBP). J Hazard Mater 171:767–773 In present work, novel physical crosslinking hydrogel Allegre C, Maisseu M, Charbit F, Moulin P (2004) Coagulation– with excellent CV dye adsorption efficiency has been flocculation–decantation of dye house effluents: concen- successfully prepared based on biodegradable poly- trated effluents. J Hazard Mater 116:57–64 mers (XG and PVI). The structure of prepared XG/PVI Al-Qodah Z (2000) Adsorption of dyes using shale oil ash. Water Res 34:4295–4303 hydrogel and loaded hydrogel with CV (XG/PVI/CV) An S, Liu X, Yang L, Zhang L (2015) Enhancement removal of was elucidated with different analysis tools such as; crystal violet dye using magnetic calcium ferrite FTIR, XRD and FE-SEM. Moreover, the maximum nanoparticle: study in single-and binary-solute systems. adsorption capacity of CV molecules onto hydrogel Chem Eng Res Des 94:726–735 -1 Bajpai S, Jain A (2012) Equilibrium and thermodynamic studies surface (Qmax = 453 mg g ) occurred at optimum for adsorption of crystal violet onto spent tea leaves (STL). conditions as; 30 mg of hydrogel, 50 mL of neutral Water 4:52–71 solution (pH 7) of 450 mg L-1 CV dye, adsorption Bekiari V, Sotiropoulou M, Bokias G, Lianos P (2008) Use of temperature 30 °C and adsorption time 6 h. The poly (N,N-dimethylacrylamide-co-sodium acrylate) hydrogel to extract cationic dyes and metals from water. adsorption data was fitted well with Langmuir adsorp- Colloids Surf A 312:214–218 tion isotherm model. Furthermore, the kinetic rate of Benny I, Gunasekar V, Ponnusami V (2014) Review on appli- adsorption of CV dye was controlled by pseudo-first cation of xanthan gum in drug delivery. Int J Pharm order and intraparticle diffusion kinetic model. Technol Res 6:1322–1326 Bilanovic D, Starosvetsky J, Armon R (2016) Preparation of Finally, the CV loaded hydrogel can be successfully biodegradable xanthan–glycerol hydrogel, foam, film, recycled using 0.1 M HCl for the successive four aerogel and xerogel at room temperature. Carbohydr adsorption–desorption cycles. Thus, the prepared XG/ Polym 148:243–250 PVI hydrogel can be used as promising adsorbent for Broekema R, Durville P, Reedijk J, Smit J (1982) The coordi- nation chemistry of N-vinylimidazole. Transit Met Chem removing toxic cationic dyes from wastewater. 7:25–28 Caner H, Yilmaz E, Yilmaz O (2007) Synthesis, characteriza- tion and antibacterial activity of poly (N-vinylimidazole) grafted chitosan. Carbohydr Polym 69:318–325 References Cheriaa J, Khaireddine M, Rouabhia M, Bakhrouf A (2012) Removal of triphenylmethane dyes by bacterial consor- tium. Sci World J 2012:1–9 Abu Elella M, Mohamed R, Abd ElHafeez E, Sabaa M (2017) Chowdhury S, Saha P (2010) Sea shell powder as a new Synthesis of novel biodegradable antibacterial grafted adsorbent to remove basic green 4 (malachite green) from xanthan gum. Carbohydr Polym 173:305–311 aqueous solutions: equilibrium, kinetic and thermody- namic studies. Chem Eng J 164:168–177

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