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Removal of some most hazardous cationic dyes using novel poly (NIPAAm/AA/N-allylisatin) nanohydrogel
⁎Corresponding author. Tel.: +91 02692 226856. patelmanish1069@yahoo.com (Manish P. Patel)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Synthesis of nanoparticles by microemulsion method is an interesting research area of current years. By accepting this opinion, N-isopropylacrylamide was polymerized with different amounts of acrylic acid (AA) and N-allylisatin using aerosol (AOT) as a surfactant, ethylene glycol dimethacrylate (EGDMA) as a cross linker and 2,2′-azobisisobuteronitrile (AIBN) as a surface active initiator. The chemical structure of nanohydrogel was characterized by FT-IR, DSC and TGA analysis. SEM photographs demonstrate the surface morphology of nanohydrogel before and after the dye adsorption. TEM micrographs confirm the particle size distribution in the range between 5 and 10 nm. Specific surface area and pore volume of the synthesized nanohydrogel were determined by BET and BJH analysis. The nanohydrogels were used in experiments on swelling behavior and adsorption of some water-soluble cationic dyes such as Methylene Blue (BB-9), Auramine O (BY-2) and Chrysoidine G (BO-2). Furthermore, the Langmuir and Freundlich adsorption isotherm models were applied which showed a favorable adsorption. From the results, removal of dyes within the nanohydrogel increased in the following order: BB-9 > BY-2 > BO-2.
Keywords
N-Isopropylacrylamide
Inverse microemulsion polymerization
Polymeric nanomaterials
Swelling equilibrium
Adsorption
Cationic dyes
- NIPAAm
-
N-isopropylacrylamide
- AA
-
acrylic acid
- AOT
-
aerosol ((2-hydroxyethyl sulfosuccinate) sodium salt)
- EGDMA
-
ethylene glycol dimethacrylate
- AIBN
-
2,2′-azobisisobuteronitrile
- BB-9
-
Basic Blue-9
- BY-2
-
Basic Yellow-2
- BO-2
-
Basic Orange-2
Abbreviations
1 Introduction
With the increasing use of synthetic dyes in industrialization and urbanization, the world has been polluted and all living creatures experience this threat. One of the major pollution problems is contamination of serviceable waters by toxic pollutants. Pollutants like toxic metal ions, textile dye molecules, phenols, insecticides, pesticides and a wide spectrum of aromatics in wastewater possess severe problems as they are toxic to water animal and human beings, and also damage the visual nature of the environment. (Kyzas and Lazaridis, 2009; Dadhaniya et al., 2007; Santhi et al., 2011). In contact with BB-9, BY-2 and BO-2 dyes, it may cause some harmful effects such as heart beat increase, vomiting, shock, cyanosis, jaundice, and tissue necrosis in humans. Also, several health and environmental organizations confirmed that both Auramine O and Chrysoidine G were responsible for carcinogenicity (bladder cancer) in animals and also in humans (Gupta et al., 2010, 2011). Thus, it is crucial to remove these dyes from wastewater before they are discarding in unpolluted natural water bodies.
Researchers are interested toward working with hydrogels due to their spacious applications in areas like drug delivery systems, contact lenses, solute separation, diapers, feminine napkins, soil for agriculture and horticulture, water-blocking tapes, absorbent pads, etc. (Okano, 1998). Poly (N-isopropylacrylamide) is the most well known synthetic polymer among the thermo-responsive polymers since it displays a sharp change close to 32–34 °C. By introducing more hydrophilic monomers into PNIPAAm network with specific functional groups it can be used in variety of applications, including chromatography, affinity precipitation, controlled biocatalysis, viable cell immobilization, and biomimetic actuators, in particular, modulated drug delivery (Li and Antony, 2003; Schild, 1992).
One of the most used novel methods to obtain nanoparticles is the microemulsion polymerization (Jayakrishnan and Shah, 1984). In comparison to ordinary emulsions, microemulsion forms upon simple mixing of the components and do not require the high shear conditions generally used in the formation of ordinary emulsions (Maqsood et al., 2012). Microemulsion polymerization achieved hydrogel nanoparticles (5–10 nm) and increased specific surface area which can provide faster responsiveness. However, the synthesis required high quantity of surfactant and long reaction times which is an inconvenient feature of the method. The utilization of this route as a polymerization method to obtain uniform hydrogel nanoparticles is a novel resource that exploits the various characteristics of these systems.
In general, synthesis and applications of poly (NIPAAm/AA/N-allylisatin) nano hydrogels (5–10 nm) in waste water treatment are not explained in detail yet. The goal of our present study is that by adsorption of cationic dyes and equilibrium swelling of hydrogel is increasing by decreasing the particle size of hydrogel to increase surface area. For this purpose, N-isopropylacrylamide was polymerized with different amounts of AA and N-allylisatin using inverse microemulsion polymerization. The chemical structure, swelling and shrinking, effect of temperature and pH on the synthesized nanohydrogels were investigated. Then the prepared nanohydrogels were used in adsorption of some water-soluble cationic dyes such as Methylene Blue (Basic Blue-9), Auramine O (Basic Yellow-2) and Chrysoidine G (Basic Orange-2).
2 Experimental
2.1 Materials
N-Allylisatin was synthesized and characterized by previously reported method (Ukrainets et al., 2009). N-isopropylacrylamide (NIPAAm) (Aldrich, 97%) was used as received and no need to purify further. Acrylic acid (AA) (Himedia, 99%) was used as received. The surface active radical initiator 2,2′-azobisisobutyronitrile (AIBN) (Himedia, 98%) was recrystallized twice from methanol prior to use. Aerosol AOT ((2-hydroxyethyl sulfosuccinate) sodium salt) (Aldrich) was as an anionic surfactant and the cross linking agent and ethylene glycol dimethacrylate (EGDMA) (Aldrich, 98%) was used without further purification. Methylene Blue (BB-9, >96%) was obtained from SD Fine Chemicals Limited (SDFCl), Mumbai. Auramine O (BY-2, 80%) and Chrysoidine G (BO-2, 90%) were obtained as a gift from Nir Colour Chem, Ahmedabad. Extra pure grade toluene (Merck, >99%) and analytical grade ethanol, acetone and diethyl ether were used as received. Double distilled water was used throughout the investigation.
2.2 Instruments
Spectrophotometric analysis were carried out with a Shimadzu 160-A UV–Visible spectrophotometer. The chemical structure of the poly (NIPAAm/AA/N-allylisatin) hydrogel nanoparticles was directly analyzed by a Perkin–Elmer GX-FTIR spectrophotometer using KBr pellets. The cloud point or LCST of the synthesized nanohydrogel was determined by a differential scanning calorimeter (DSC) with Perkin–Elmer Pyris-1 instrument. Thermal behaviors of the nanohydrogel were investigated with Perkin–Elmer Pyris-1 Thermo Gravimetric Analyzer (TGA). Surface morphology of the nanohydrogel was observed using a Philips XL30 ESEM (Environmental Scanning Electron Microscope). TEM micrographs of the hydrogel nanoparticles were taken using a Philips Tecnai-20 Transmission Electron Microscope (TEM). BET and BJH analysis for specific surface area and pore volume were measured on Nova 1000e. The pH of solutions was adjusted with very small amounts of 0.01 M hydrochloric acid and sodium hydroxide and determined using Eco Tester pH 1 instrument.
2.3 Synthesis of nanohydrogels
Inverse microemulsion polymerization was employed to synthesize the superabsorbent nanohydrogels with a specified amount of N-isopropylacrylamide and by varying the AA and N-allylisatin content. The detailed polymerization procedure for microemulsion and recipes are reported elsewhere (Elsaeed et al., 2011; Guerrero-Ramirez et al., 2008). The polymerization was carried out in a four-necked round bottomed flask having a reflux condenser, an inlet for N2, a thermometer pocket (TP) and fourth neck was used to add the reactants. Before the polymerization proceeds the reaction medium was purged with dry N2 to remove oxygen during the entire reaction. Initially, for the continuous phase, 0.5 g of AOT was added to 5 mL of toluene and stirred in dry N2 for 30 min. Temperature of the flask was maintained at 60 °C using a temperature controller.
The disperse phase was prepared by dissolving the required amount of NIPAAm with differing amounts of AA and N-allylisatin. The solution was stirred under N2 until a homogeneous solution was obtained. Now the disperse phase was added dropwise into continuous phase to form a W/O microemulsion. The reaction medium was stirred at a speed of 300 rpm for 30 min. A cross linking agent, EGDMA, was added, followed by the addition of AIBN as a surface active initiator. Total conversion was obtained after 7 h of reaction.
The obtained hydrogels were purified by dialysis. The formed hydrogels were then transferred to a 1 L beaker containing double distilled water and left for 2–3 days by changing water at every 4 h of interval in order to remove the unreacted monomers and other reactants. The swollen gel was dried using solvent acetone (solvent dried) in order to ensure that the desired porosity in hydrogel was generated during solvent drying. The process was repeated until the dry hydrogel was obtained. Finally the hydrogel was kept in a vacuum oven to constant weight. The feed compositions and relatively swelling percentage of the nanohydrogels are given in Table 1.
Sr. no.
Sample code
NIPAAm (mol%)
AA (mol%)
NAI (mol%)
AIBN (mol%)
EGDMA (mol%)
Swelling% after 4 h
Swelling% after 24 h
Swelling% after 48 h
1
VNH 01
79.5
20
0.5
3
1
11,480
16,010
16,345
2
VNH 02
79
20
1
3
1
14,850
17,510
17,726
3
VNH 03
77.5
20
2.5
3
1
17,664
24,980
25,310
4
VNH 04
75
20
5
3
1
20,200
29,405
29,856
5
VNH 05
70
25
5
3
1
18,875
33,200
33,480
6
VNH 06
65
30
5
3
1
31,925
36,405
36,565
7
VNH 07
60
35
5
3
1
30,040
38,240
38,310
8
VNH 08
50
45
5
3
1
30,240
38,900
40,125
9
VNH 09
70
20
10
3
1
26,456
32,380
32,596
10
VNH 10
55
35
10
3
1
30,390
33,980
34,050
2.4 Swelling behaviors of the nanohydrogel
The swelling behaviors of poly (NIPAAm/AA/N-allylisatin) nanohydrogels were determined by gravimetry method at 25 °C after 24 h by adopting a tea-bag (Uzum and Karadag, 2007). To determine the swelling behavior, 100 mg of the dry nanohydrogels was taken in a tea-bag and fully immersed in beakers containing DD water. The nanohydrogels were taken out at different time intervals, hung up for 20–30 min to drain the excess solution and excess surface water was removed by wiping with filter papers. The weights of the swollen hydrogels were measured and then kept to the respective beakers. The procedure until the constant weight was obtained was repeated i.e., nanohydrogel achieved its equilibrium swelling when it did not absorb water further. The swelling percentage was calculated by the following Eqs. (1) and (2).
Effect of temperature and pH on swelling percentage of the nanohydrogel was also investigated. Dry nanohydrogel sample was immersed in double distilled water in the temperature range from 5 to 80 °C (5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80 °C) and in different pH solutions (pH range = 2–14). After 24 h, the hydrogel sample was weighted and equilibrium swelling percentage (Qe) was calculated using Eq. (2) for a given temperature.
Shrinking study of the nanohydrogel was carried out at 80 °C using a thermo-stated water-bath by gravimetric method. A given amount of swollen hydrogel at 25 °C was immersed in water at 80 °C. The water loss versus time (10, 20, 40, 60, 80, 100, 120,140, 160, 180, 200 min) of the hydrogels was recorded during deswelling. The water loss value was calculated using Eq. (1). All the swelling/shrinking experiments were repeated twice for nanohydrogel sample and the average of the results was reported.
2.5 Adsorption experiments
The adsorption kinetics of three basic dyes, namely Methylene Blue (BB-9), Auramine O (BY-2) and Chrysoidine G (BO-2) on the nanohydrogel was carried out in batch experiments. Adsorption kinetics experiments were carried out in 50 mL of synthetic dye solution at room temperature. For the determination of maximum adsorption capacity, about 0.025 g of sample respectively was added to the BB-9, BY-2, and BO-2 solutions with concentrations of 250 mg/L (Aly, 2012). During the experiments, approximately 1–2 mL of dye solution was taken out at various time intervals (2, 4, 8, 16, 24, 36, and 48 h) and analyzed calorimetrically with a spectrometer at the specific absorption wavelengths 664, 434, and 449 nm for BB-9, BY-2 and BO-2, respectively. All of the experiments were done at the natural pH value of dye solutions.
Adsorption isotherms of basic dyes were also investigated in batch experiments. In order to determine adsorption isotherms, constant weight of the nanohydrogel in dye solutions for 24 h with various initial concentrations (50, 100, 150, 200, 250 mg/L), effect of pH (pH range = 2–14) and adsorption dose (10, 20, 30, 40, 50 mg) were measured. The amount of residual dye in aqueous solution was determined with a UV–Visible spectrophotometer after 24 h. The maximum adsorption capacity (mg of dye/g of nanohydrogel) and % removal efficiency (RE) of the nanohydrogels were calculated using the following Eqs. (3) and (4), respectively.
3 Results and discussion
3.1 Synthetic optimization of preparation parameters
Inverse microemulsion polymerization was employed to synthesize the superabsorbent nanohydrogels with a specified amount of N-isopropylacrylamide and by varying the AA and N-allylisatin content. Most optimization studies during the development of a novel hydrogel involve variation of one factor at a time, keeping all other factors constant. This traditional optimization method is time consuming and expensive. Today, a statistical-based technique of response surface methodology (RSM) is used extensively to elucidate the interaction between reaction parameters for optimization. However, to the best of our knowledge, little information was reported using RSM to optimize the process parameters for obtaining hydrogels with desired properties specific to a particular application. The goal of our present study is that by adsorption of cationic dyes and equilibrium swelling of hydrogel is increasing by decreasing the particle size of hydrogel to increase surface area. In the present study, we have chosen the swelling behavior of nanohydrogel as main criteria for the removal of dyes. We have synthesized nanohydrogels by changing different amounts of described monomers as in Table 1. Not only, swelling and adsorption properties, but also the physical nature of the nanohydrogel is important. VNH-06 may comply with all the required characteristics for above mentioned properties and therefore, the ratio obtained for the preparation of nanohydrogel VNH 06 taking as an optimum condition for the further swelling, characterization and dye adsorption studies.
3.2 Characterization
Poly (N-isopropylacrylamide/acrylic acid/N-allylisatin) nanohydrogels (VNH 01–VNH 10) were synthesized using different compositions of monomer, surfactant, initiator, and cross linker. Schematic representation of the synthesis of nanohydrogel is shown in Scheme 1. VNH 06 nanohydrogel exhibited optimum swelling after 4 h, and had good mechanical strength, stability as well as fast responsive properties than others and thus VNH 06 is utilized for characterization and dye removal study.
Reaction scheme of poly (NIPAAm/AA/N-allylisatin) nanohydrogel.
3.2.1 FTIR analysis
The FTIR spectrum of the VNH 06 nanohydrogel is shown in Fig. 1. The FTIR spectrum obtained for nanohydrogel has characteristic peaks at 3437 cm−1 and 3090 cm−1 due to –OH stretching of carboxylic acid and –NH stretching from amide. The spectrum exhibits characteristic –C⚌O stretching at 1650 cm−1. In addition, a stretching peak appeared at 1722 cm−1, belonging to the ester carboxyl group. The band appeared at 1556 cm−1 is for N–H in plane bending vibration. The peak at 1172 cm−1 was assigned for C–N of the amide group. Also, the characteristic peak at 928 cm−1 is for C–N of N-allylisatin. The double peak at 1385 and 1368 cm−1 formed by symmetrical bending vibration and coupling split originating from bimethyl of isopropyl group is easily observed. The characteristic peaks at 2974 and 2919 cm−1 are due to the aromatic –CH stretching of the nanohydrogel (Zhang et al., 2009).
FT-IR spectrum of VNH 06 nanohydrogel.
3.2.2 DSC analysis
In order to determine the LCST (Lower Critical Solution Temperature) of the nanohydrogel DSC technique was employed. Fig. 2 shows the DSC analysis of the synthesized nanohydrogel. In general, the solubility of most polymers increases with an increase in temperature. However, in the case of polymers exhibiting LCST, an increase in temperature decreases the polymer’s water solubility due to predominating hydrophobic interactions. LCST is defined as, “the temperature at which the polymer displays a sharp change from a soluble to an insoluble state on increasing temperature”.
DSC analysis of nanohydrogel.
Modification in LCST of poly (NIPAAm) with more hydrophilic monomer will favor hydrogen bonding in preference to hydrophobic interactions and will increase the LCST of the nanohydrogel. From the figure, it is observed that the nanohydrogel presents a broad peak located at around 78 °C. This is because of the higher content of acrylic acid and N-allylisatin was taken in the feed composition for the synthesis of nanohydrogel. The more hydrophilic the moiety of AAc and N-allylisatin in the nanohydrogels, the stronger the hydrogen bond interaction is in aqueous solutions. This stronger bond requires more energy to destroy it and results in the increase of LCST. Thus, LCST of nanohydrogels could be adjusted by taking wt% of hydrophilic monomers (Deng et al., 2009).
3.2.3 TGA analysis
The TGA curve for poly (NIPAAm/AA/N-allylisatin) nanohydrogel is shown in Fig. 3. The nanohydrogel shows fair thermal stability until about 180 °C and then in the temperature range of 150–200 °C a weight loss corresponding to first degradation process is observed. Above 200 °C no relevant thermal event occurs till 300 °C, when a second weight loss, corresponding to the decomposition of polymer starts. First the degradation process is related to the loss of water molecules through the formation of intra-molecular and inter-molecular linkages and also to the decarboxylation of a fraction of the –COOH groups by which CO2 is produced. In the second degradation stage, the polymer decomposes with the elimination of CO and CO2 by way of abundant backbone scission and formation of a small concentration of unsaturation. The final weight loss at above 400 °C represents the degradation of the main chain of nanohydrogel. The thermal stability of the nanohydrogel was increased due to the high concentration of hydrophilic monomers in the polymer network (McNeill et al., 1995).
TGA curve of nanohydrogel.
3.2.4 SEM analysis
The surface morphology of VNH 06 and MB chelated with VNH 06 was examined and shown in Fig. 4(a) and (b), respectively. From the figure, it is observed that the surface of nanohydrogel was porous and uniform in nature. Before adsorption, various pores were generated on the surface of nanohydrogel due to solvent drying method which were filled up with the Methylene Blue adsorption (Bajpai et al., 2012).
SEM photographs of VNH 06 and MB adsorbed onto VNH 06 nanohydrogel.
3.2.5 TEM analysis
The TEM micrographs of poly (NIPAAm/AA/N-allylisatin) nanohydrogel are shown in Fig. 5. The particle size distribution of the nanohydrogel was measured in acetone at 25 °C by Transmission Electron Microscope showing particle size distribution between 5 and 10 nm. With an increasing concentration of AIBN, the resulting hydrogel particle sizes decreased up to concentration of AIBN 3% and EGDMA 1% with respect to the total monomer concentration (Batich et al., 1993; Mittal, 2011). After then the high concentration of AIBN in the feed composition has irrelevant effect on hydrogel nanoparticles. It is well-known that the swelling of polymer network is dependable not only on the pores but also on the surface area of polymer chain (Roosta et al., 2014a,b). In this study, the swelling of nanohydrogel depends on the surface area of polymer network.
TEM micrographs of reactive nanohydrogel particle.
3.2.6 BET and BJH analysis
It is well known that BET (Brunauer–Emmett–Teller) and BJH (Barrett–Joyner–Halenda) analysis provides precise surface area and pore volume of adsorbent, respectively. In general, it is interesting to determine the specific surface area and pore volume of the nanohydrogel due to its role in adsorption of water as well as dyes (Roosta et al., 2014a,b). The synthesized nanohydrogel exhibits a large surface area and pore volume acquires higher adsorption of dyes on its surface. Analysis was carried out by nitrogen multilayer adsorption and desorption technique measured as a function of relative pressure using a fully automated analyzer. Specific surface area and pore volume of the synthesized nanohydrogel are given in Table 2.
BET analysis
BJH analysis
Specific surface area
Pore volume
BJH adsorption summary
BJH desorption summary
114.266 m2/g
0.040 cc/g
0.023 cc/g
3.3 Swelling studies of nanohydrogel
The swelling study of nanohydrogel was affected by several factors such as hydrophilic and hydrophobic nature of the network, pH, temperature, ionization degree of hydrophilic groups, and dynamic volume. It is well known that a hydrophilic/hydrophobic balance exists in the PNIPAAm network because of its hydrophilic and hydrophobic nature. The synthesized nanohydrogels show both thermo and pH sensitive nature because new functional monomers are introduced in the polymer network. Therefore, pH and temperature play an important role in swelling of the nanohydrogels.
The different factors affecting the swelling studies of nanohydrogels are plotted as graphs and illustrated in Figs. 6–9.
Swelling properties of nanohydrogel in double distill water.
Effect of temperature on% swelling of nanohydrogel.
Shrinking of nanohydrogel in double distill water at 80 °C.
The effect of pH on% swelling of hydrogel.
3.3.1 Equilibrium swelling percentage
Hydrogen bond that occurs between water molecules and hydrophilic groups of monomers leads to good solubility of the nanohydrogel at low temperatures. When the external temperature is increased to the LCST, the hydrophobic interactions among the hydrophobic groups overcome the hydrogen bonds, and phase separation occurs. Here, as the hydrophilic monomers AA and N-allylisatin were introduced into the backbone of the gel, the hydrophilic/hydrophobic ratio of the gel network was increased, and the hydrophilicity of the nanohydrogel as a whole improved, leading to an increasing swelling percentage at room temperature (Emik and Gurdag, 2005; Bengi et al., 2011).
Fig. 6 shows that effect of the contact time on swelling of the nanohydrogel. From the figure, it is observed that fast swelling of the nanohydrogels after 4 h of contact time and equilibrium swelling percentage obtained after 24 h. Even after increasing the contact time up to 48 h, there is no more affective swelling is observed.
3.3.2 Effect of temperature
Equilibrium swelling percentage of the nanohydrogel in distilled water is plotted as a function of temperature in Fig. 7. The incorporation of hydrophilic groups such as AA and N-allylisatin monomers bearing the polymer framework caused the LCST of nanohydrogel to increase, as a higher temperature was needed to drive the disruption of strengthened hydrogen bonds. The nanohydrogel presents a single peak located at around 78 °C. Nanohydrogel exhibits a higher swelling degree in swollen state below the LCST and lower in collapsed state above the LCST. It is clearly observed from the plot that the decrease in swelling percentage of the nanohydrogel is more even when the temperature raises up to 60 °C and then after the swelling percentage shows a dramatic decrease between 60 and 80 °C.
3.3.3 Shrinking kinetics of nanohydrogel
Fig. 8 demonstrated the shrinking kinetics of nanohydrogel after temperature jumps from the equilibrium state in double distill water at 0 °C to above the LCST (78 °C). Holding the nanohydrogel at 80 °C causes a dramatic decrease in swelling percentage of the nanohydrogel. From the figure, it is concluded that the% weight loss of swollen nanohydrogel are continuously diminishing by increasing time in double distill water at 80 °C. The nominal change in water loss was obtained after 180 min.
3.3.4 Effect of pH
The effect of pH on the swelling percentage of nanohydrogel was investigated by measuring equilibrium swelling percentage of nanohydrogel in different solutions with pH ranging from 2 to 14 at 25 °C. Effect of pH has great influence on the swelling percentage of nanohydrogel. The results, as shown in Fig. 9, indicate that swelling percentage increases with pH of the swelling medium, attains an optimum value of about 44, 954% at pH 12 and then begins to decrease up to pH 14. This sharp change on swelling percentage was observed due to incorporation of ionizable –COOH groups of poly (acrylic acid) chains. However, as the pH of the swelling medium increases, the –COOH groups initiate to ionize into –COO− groups which may repel each other remaining to similar charges thus causing the polymer chains to relax or unfold. This finally, results in greater swelling percentage of the nanohydrogel. However, as the pH is increased further, the swelling percentage begins to decrease. This might be due to the formation of uncross linked polymer networks which must have a fair tendency to dissolve in concentrated alkaline as well as acidic medium.
3.3.5 Determination of pH of zero charge for nanohydrogel
For the determination of pH of zero charge (pzc) or isoelectric pH (IEP) of adsorbent, four methods namely, fast titration, salt addition, mass titration, and ζ potentiometry, were employed. The pH of zero charge (pzc) is the pH of the suspension at which the net charge on the surface of an insoluble adsorbent is zero. Pzc plays an important role in surface characterization of adsorbent. In the environmental science field, pzc determines how easily an adsorbent can absorb potentially harmful ions.
In the present studies, salt addition method was applied for the investigation of pH of zero charge or isoelectric pH for the behavior of nanohydrogel. This consists of a simple titration that requires a smaller amount of solid sample than other methods. Here, 0.200 g of nanohydrogel was added to 40.0 mL of 0.1 M NaNO3 in fourteen 50-mL beakers. The pH was adjusted using a Eco Tester pH 1 instrument to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 (±0.1 pH units) with 0.1 M HNO3 and 0.1 M NaOH as needed in each beaker. These were then shaken for 24 h in a revolving water bath to reach equilibrium. After this time, each resulting pH was measured and the initial pH (pHo) vs. the difference between the initial and final pH values (pH) was plotted. The pzc was taken as the point where pH = 0. After thoroughly studies, the solution of pH 12 was nearest to zero (±0.1) which means that the nanohydrogel has pH of zero charge or isoelectric pH was 12. This value also supports our later studies, adsorption of dyes in different pH solutions in which maximum adsorption of dyes on nanohydrogel occurred at pH 12.
3.4 Dye adsorption studies
It is well known that mainly two factors responsible for the adsorption of dyes on hydrogels are electrostatic interaction and hydrophobic interaction. Electrostatic interaction occurs between ionizable groups of hydrogel and specific charge of the dye molecule. The presence of carboxyl (–COO−) groups in polymer network favors the electrostatic interaction. The other types of interaction between the hydrogel and the monovalent cationic dyes may be hydrophobic and hydrogen bond. Hydrophobic effects are specifically aqueous solution interactions, which in the present case will involve the aromatic rings and the methyl group on the dye molecules and the methine groups on the nanohydrogel. Hydrogen bond will be expected to occur between amine group and nitrogen atoms on the dye molecules and the amine and carbonyl groups on the monomer unit of the nanohydrogel. The possible complexation process between poly (NIPAAm/AA/N-allylisatin) nanohydrogel and dyes molecule is shown in Scheme 2 (Solpan et al., 2009).
Possible complexation process between nanohydrogel and dye molecule.
The adsorption experimental data of BB-9, BY-2 and BO-2 on the nanohydrogel are given in Figs. 10–17 and Tables 3 and 4 in the following sections.
The effect of treatment time on adsorption.
Effect of initial dye concentration on removal efficiency of nanohydrogel.
The effect of pH on adsorption.
The effect of adsorption dose on adsorption.
The effect of temperature on the adsorption.
The effect of ionic strength on the adsorption.
Langmuir isotherm for adsorption of dyes onto nanohydrogel.
Freundlich isotherm for the adsorption of dyes onto nanohydrogel.
Dyes adsorbed onto nanohydrogel
Percent desorption efficiency (%)
1st cycle
2nd cycle
BB-9
80
∼100
BY-2
75
92
BO-2
63
86
Sr. no.
Dye name
Langmuir isotherm
Freundlich isotherm
R2
q0
b
R2
Kf
n
1
BB-9
0.9765
392.1569
1.1591
0.9667
0.8022
2.6603
2
BY-2
0.9538
337.8378
0.0208
0.9794
0.1245
1.5659
3
BO-2
0.9955
961.5385
0.0023
0.9918
−0.7234
1.1207
3.4.1 Effect of contact time
The adsorption of three cationic dyes onto the nanohydrogel was investigated as a function of treatment time to determine the required time for maximum adsorption, and the results are illustrated in Fig. 10. In this adsorption study, 25 mg of nanohydrogel was agitated with 50 mL solution (250 mg/L each) of three cationic dyes at their natural pH value. The binding capacity was calculated at different time intervals until the equilibrium has reached. As can be seen, the adsorption of dyes onto the nanohydrogel was rapid initially, then uptake increased gradually, and finally equilibrium was reached within 24 h. After equilibrium, the adsorption of dyes increased so slowly even after time increased up to 48 h. From this experiment, it is evident that the time has significant authority on the adsorption of dye.
3.4.2 Effect of Initial dye concentration
To explore the applicability of the nanohydrogel, it was informative to obtained knowledge on its sorption capacity toward cationic dyes. This is carried out by equilibrating a fixed amount of the nanohydrogel with a series of dye solutions of gradually increasing concentration. Fig. 11 illustrates the removal efficiency of nanohydrogel toward BB-9, BY-2 and BO-2 with different initial feed concentrations. From the figure, it is observed that the removal efficiency of BB-9 on nanohydrogel is extensively greater than compared to BY-2 and BO-2. The increase in the qe (mg/g) with an increase in the initial concentration of dye may be due to the higher adsorption rate and the utilization of all the activated sites available for the adsorption at higher concentrations. After maximum adsorption, all activated sites were packed with dye molecule and there were no sites available for binding and therefore, irrelevant effect was observed for adsorption of dyes onto nanohydrogel.
3.4.3 Effect of pH on adsorption
One of the most vital factors in adsorption studies is the effect of pH on the medium. It is well known that the pH of the medium has great influence on the performance of the hydrogels that it influences its swelling and also adsorption of dyes. The adsorption behavior of cationic dyes on nanohydrogel at various pH ranges (2–14) was investigated in the batch process and the results are presented in Fig. 12.
For this experiment, all the dye solutions were prepared keeping the concentration at 150 (mg/L) for BO-2, 200 (mg/L) for BY-2 and 300 (mg/L) for BB-9. To 50 mL solutions of each dye was added nanohydrogel (0.025 g) and stirred for 24 h at 100–150 rpm at room temperature. By looking at Fig. 12, we can say that the pH has profound effect on dye adsorption i.e., the nanohydrogel can absorb maximum dyes at specific pH value and then after it decreases at higher pH values. At basic pH, –COOH groups ionize to –COO− ions and therefore, dye adsorption occurs via strong ionic interaction between positive charge of dye molecule and negative charge of –COO− groups of the nanohydrogel. At acidic pH, the ionic interaction is comparatively weak due to partial positive charge of –COO− groups of nanohydrogel and also at a higher pH the diffusion of H+ ions occurs in dye molecule.
3.4.4 Effect of adsorption dose
For this experiment, the adsorbent doses were increased from 10 to 50 mg for dye concentration of 150 mg/L for BO-2 mg/L, 200 mg/L for BY-2 and 300 mg/L for BB-9. The results are shown in Fig. 13. It is well known that the adsorption occurs on the activated sites of dye molecule. Figure explains that an increase in adsorbent dosage leads to a decrease in the adsorption capacity of dye solutions. These decreases in adsorption are due to the sites remaining unsaturated during the adsorption process.
3.4.5 Effect of temperature
The effect of temperature on the adsorption of dyes was investigated by carrying out equilibrium adsorption studies at 10–80 °C and is shown in Fig. 14. In the present study, it was observed that as the temperature of medium increases the adsorption capacity of nanohydrogel also decreases slowly at temperature up to 60 °C and after that, a sharp decrease in adsorption was observed. Almost 50% decrease in the adsorption of dyes showed that the nanohydrogel is not capable of swelling in water as well as in dye solution after 60 °C. These also support our observation for swelling behavior and LCST of nanohydrogel. This may be attributed to the fact that the kinetic energy of adsorbate molecules increases with temperature, thus weakening the adsorbate–adsorbent interactions. It is to be noted that adsorption of dyes was stable up to 60 °C due to development of new pores in the adsorbent and reduction in the viscosity of the medium.
3.4.6 Effect of ionic strength
Dye adsorption was strongly affected by electrostatic parameters such as surface charge, pH, and ionic strength. Consequently, taking VNH-06 nanohydrogel as representative, six different concentrations of sodium chloride (i.e., 0, 0.01, 0.02, 0.04, 0.08, and 0.16 mol/L) were investigated to determine the effect of ionic strength on adsorption of dyes, as illustrated in Fig. 15. The initial concentration of dyes was 250 ppm and the pH was not adjusted. The temperature was kept at 25 °C for 24 h. The adsorption of dyes decreased as sodium chloride concentration increased. It was largely because of the screening effect that Na+ partially neutralized the negative surface charge, and then resulted in a compression of the electrical double layer, which led to a reduction in the attractive forces between the surfaces of the adsorbent and the cationic species of the dyes.
3.4.7 Desorption and reusability of nanohydrogel
Cationic dyes adsorbed onto nanohydrogel have been desorbed in a batch wise mode, in two steps:
-
0.2 M HCl, in a ratio of 50 mL/0.05 g dry dyes adsorbed nanohydrogel have been added for 120 min in this step, followed by washing at neutral pH.
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0.2 M NaOH, the same ratio like HCl, added to the remaining dyes adsorbed solution for 120 min. After washing at neutral pH, the gels have been ready for a new cycle of sorption/desorption process. The detail desorption process for dyes adsorbed onto nanohydrogel is given in Table 3. Desorption studies can help elucidating the mechanism of an adsorption process. In the present case, the strong acid or base, such as HCl or NaOH can desorb the dyes adsorbed onto nanohydrogel in a high percent. Therefore, it can be said that maximum adsorption was by ion exchange or electrostatic attraction.
3.5 Adsorption isotherms
The mathematical correlation, which constitutes an important role toward the modeling analysis, operational design, and applicable practice of the sorption systems, is usually explained by graphically expressing the solid-phase against its residual concentration. In general, the adsorption isotherms explain the correlation between the amount of dye adsorbed and the residual concentrations of dyes at equilibrium. It was well known that several models were useful for the phenomenon of governing the retention or mobility of a substance from the aqueous media to a solid-phase (Ghaedi et al., 2014).
In this study, Langmuir and Freundlich isotherm models were applied to establish the adsorption isotherm of nanohydrogel. The Langmuir isotherm is a well-known isotherm model being applied for homogeneous performance of different adsorbents. The Langmuir model assumes that all the sites are identical, contain one molecule and are all energetic and sterical. The linear form of the Langmuir isotherm is described by Eq. (5).
The Freundlich isotherm assumes a heterogeneous surface with a non uniform distribution of adsorption and describes the adsorption process as “the ratio of the amount of solute adsorbed onto nanohydrogel to residual concentration is not same at different concentrations”. Its linear form is expressed by the following Eq. (6).
Table 4 represents the Langmuir and Freundlich parameters for cationic dye adsorption onto the nanohydrogel. These parameters are determined from the slope and intercept of the linear plots of 1/qe vs. 1/Ce (Fig. 16), and the values of Kf and n (Fig. 17), as determined from the intercept and slope of the linear plot of log qe vs. log Ce. According to these studies, it was concluded that both isotherm models do not explain the good adsorption of cationic dyes onto nanohydrogel but have reliable effect on adsorption of cationic dyes toward nanohydrogel. This was due to the more decreasing the particle size of nanohydrogel different type of adsorption was observed on nanohydrogel. It is well known that n > 1 indicates favorable adsorption; since all the calculated n values of Freundlich isotherm are in the range 1–3. Therefore, it can be easily concluded that we are getting maximum adsorption and high removal efficiency of three cationic dyes toward nanohydrogel even though kinetic and equilibrium adsorption isotherms are failed to explain the adsorption.
4 Conclusion
In this study, we have synthesized a new superabsorbent poly (NIPAAm/AA/N-allylisatin) nanohydrogel by inverse microemulsion polymerization. The success of the synthesis was confirmed by different analyses techniques such as FTIR, DSC, TGA and TEM. The particle size distribution of nanohydrogel was in range between 5 and 10 nm. The LCST of nanohydrogel was greatly increased due to more hydrophilic groups introduced in the polymer network. The effect of pH, temperature and time was investigated to determine the maximum swelling percentage of the nanohydrogel. The effect of treatment time, initial dye concentration, adsorption dose and pH was investigated to determine the maximum adsorption of cationic dyes onto the nanohydrogel. Effect of pH on swelling percentage as well as adsorption of cationic dye onto the nanohydrogel was marvelous. Maximum removal efficiency of BB-9, BY-2 and BO-2 in 300 (mg/L), 200 (mg/L) and 150 (mg/L) is 98.24%, 87.42% and 70.12%, respectively. The equilibrium adsorption data were analyzed by the Langmuir and Freundlich isotherm equations, and the results showed that both the isotherm could not well describe the adsorption process due to the extremely small size of nanohydrogel. Feasible improvements in the adsorption behaviors encourage efforts for the nanohydrogel to be used in wastewater treatment.
Acknowledgments
The authors gratefully acknowledge the Head of Department of Chemistry at Sardar Patel University for research facilities. Appreciation is expressed for the studies in the Sophisticated Instrument Center for Applied Research and Testing [SICART], Vallabh Vidyanagar for FTIR, DSC, TGA and TEM analysis. Also, authors gratefully acknowledge Heubach Colour PVT. LTD, at Ankleshwar for free of charge for BET and BJH analysis. Financial support for this work was provided by University Grant Commission, New Delhi (Project Number: F.39-685/2010(SR)) to whom researches are gratefully acknowledged.
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