Document Type : Research Paper
Author
Faculty of Pharmacy, Jabir Ibn Hayyan University for Medical and Pharmaceutical Sciences, Najaf Ashraf, Iraq
Abstract
Keywords
INTRODUCTION
Pollution of water by synthetic dyes is still a serious problem for industrialized and developing countries alike. Every year the textile, leather, paper, and food-processing sectors release large quantities of coloured effluent into rivers and lakes, and the resulting harm runs from eutrophication to outright aquatic toxicity [1,2]. An example of such a dye is Safranin O (Saf; C₂₀H₁₉ClN₄; λmax = 520 nm), a cationic phenazine dye used for biological staining, textile finishing and as a redox indicator. The aromatic framework that renders it chemically stable and light-fast, also explains its recalcitrance to biodegradation, its toxicity to aquatic life and to humans in whom it has been associated with mutagenic, carcinogenic and DNA-damaging effects [3,4]. The phenazine skeleton of Saf belongs to the wider family of nitrogen-containing heterocycles, a class of compounds that is extensively synthesised and screened for biological activity, in line with the toxicological concerns noted above [5,6]. There are several ways proposed for the removal of dyes from water including advanced oxidation, membrane filtration, electrochemical treatment and biological process [7,8]. Each of them has its advantages, but adsorption is still of interest due of its simplicity, low cost of installation and wide application to pollutants [9]. Among these adsorbents, carbons obtained from natural, low-cost precursors, including activated and nano-structured forms, have been widely reviewed as effective materials for water treatment [10]. Much of the recent work has focused on adsorbents derived from renewable biodegradable polymers, particularly hydrogels, because their three-dimensional crosslinked networks swell in water and offer functional groups (hydroxyl, carboxyl, amide) which bind cationic species [11,12].
Carboxymethyl cellulose (CMC) is a water-soluble derivative of cellulose, which is biocompatible, low cost and has a high density of carboxylate groups, making it an ideal backbone for hydrogels [13]. CMC is grafted with vinyl monomers such as acrylic acid (AAc) and hydroxyethyl acrylamide (HEAM) via free-radical polymerisation to create additional ionisable sites and make the gel mechanically stronger [14]. In contrast, the purely organic hydrogels usually have a small surface area and moderate temperature stability only. The addition of an inorganic filler, in particular zeolite (a microporous aluminosilicate, which is an ion exchanger and has a high specific surface area) can overcome these drawbacks, leading to an organic-inorganic nanocomposite with improved adsorption properties than the separate components [15,16].
Publications on hydrogel adsorbents are growing rapidly, but there are comparatively few publications on a ternary system of CMC, two-monomer graft (AAc + HEAM) and natural zeolite for safranin O. Published work has focused on methylene blue and crystal violet, thus Saf has been less studied, regardless of the fact that its rigid structure and nitrogen-rich framework may react differently on a mixed-functional surface [17,18]. Another disadvantage is that many articles give isotherm and kinetic data, but do not carry out a proper nano-characterization or any test of regeneration, so it is difficult to determine how practical the material actually is. With that in mind, the aims of this study were to (i) prepare the (CMC-AAC-co-HEAM)-g-zeolite composite hydrogel by free-radical graft polymerization; (ii) characterize it in detail by FTIR, XRD, FESEM, TEM, BET, TGA, and zeta potential; (iii) measure its batch adsorption of Saf while varying contact time, pH, temperature, adsorbent dose, and ionic strength; (iv) fit the equilibrium, kinetic, and thermodynamic data to suitable models; and (v) check how well it holds up over repeated use.
MATERIALS AND METHODS
Chemicals and reagents
Sodium carboxymethyl cellulose (CMC, M. wt ≈ 250,000, DS ≈ 0.7), hydroxyethyl acrylamide (HEAM, 99.0%), acrylic acid (AAc, 99.0%), and safranin O (Saf, C₂₀H₁₉ClN₄, M. wt = 350.85 g mol⁻¹, purity > 95%) were all supplied by Himedia (India). N, N′-Methylenebisacrylamide (MBA, 99.0%) and potassium persulfate (KPS, 99.5%) came from CDH (India). The natural zeolite was of the clinoptilolite type and obtained from a commercial supplier. Hydrochloric acid (37%), sodium hydroxide, NaCl, KCl, and CaCl₂ were all analytical grade, and every solution was made up in deionized water.
Preparation of the hydrogel composite
The composite was made in a single pot by free-radical graft polymerization. In a representative run, 1.0 g of CMC powder was dissolved in 50 mL of deionized water and stirred magnetically (120 rpm) at 50 °C for 30 min in a 250 mL three-neck round-bottom flask equipped with a reflux condenser, a thermometer, and a nitrogen inlet. After complete dissolution, 4 mL of HEAM was added and agitated until homogenous, then 4 mL of AAc was added dropwise at 25 °C. The crosslinker MBA (0.01 g in 2 mL water) was added slowly with stirring over 5 min and then the initiator KPS (0.01 g in 2 mL water) was added to initiate the production of radicals. 0.01 g of zeolite (dispersed in water (10 mL) by sonication for 10 min) was added dropwise. The mixture was then heated up to 70 °C in a water-bath, kept at this temperature for 3 h under nitrogen. The gel obtained was washed multiple times with deionised water to remove unreacted monomer, dried at 60 °C for 48 h then crushed and sieved. As a control, a plain hydrogel was made in the same way but without zeolite.
Characterization techniques
The FTIR spectra were acquired on KBr pellets in the range 400-4000 cm⁻¹ using a Shimadzu 8400S. X-ray diffraction patterns (XRD, Shimadzu XRD-6000, Cu-Kα, λ = 1.5406 Å) were measured in the range of 2θ = 8-80°. The morphology was studied by using TESCAN MIRA3 field emission scanning electron microscope (FESEM) and LEO 912AB transmission electron microscope (TEM). The specific surface area and BJH pore-size distribution were measured by using nitrogen adsorption-desorption isotherms obtained on a Quantachrome NOVA 2200e. Thermal stability was checked by TGA (Perkin Elmer TGA4000) under N₂ at a heating rate of 10 °C min⁻¹ from room temperature up to 983 °C. Zeta potentials were evaluated in the pH range of 2-12 using a Malvern Zetasizer Nano ZS (Malvern Panalytical Ltd., Malvern, UK. Saf concentrations were quantified by UV-Vis absorption with a Shimadzu UV-1800 spectrophotometer at λmax = 520 nm.
Batch adsorption experiments
All adsorption experiments were conducted in batch mode. A fixed concentration of Saf solution was prepared in a glass vial containing a weighed amount of composite (often 0.02 g) and shaken at 120 rpm. After the contact period, the suspension was centrifuged (6000 rpm, 10 min) and the absorbance of the supernatant was measured at 520 nm. The adsorption capacity (qe, mg/g) and the removal efficiency (%R) were then calculated from the conventional expressions:
qe = (Co – Ce) * V / m (1)
%R = [(Co – Ce)/ Co] * 100 (2)
where Co and Ce are the initial and equilibrium dye concentrations (mg/ L), V is the volume of solution (L), and m is the mass of adsorbent (g).
Contact time was studied from 0 to 120 min (C₀ = 300 mg L⁻¹, pH 7, 25 °C), while the adsorbent dose was changed from 0.01 to 0.09 g. The pH was set anywhere between 2 and 11 with 0.1 M HCl or NaOH. For the isotherms, the temperature was varied over 5-35 °C using initial concentrations of 200-500 mg L⁻¹. To probe ionic strength, NaCl, KCl, and CaCl₂ were each used at 0.01-0.20 mol L⁻¹. Reusability was checked over four successive adsorption-desorption cycles with 0.1 M HCl as the eluent.
Isotherm and kinetic modeling
The equilibrium data were treated with the Langmuir, Freundlich, and Temkin isotherm models, and the kinetic data with the pseudo-first-order (PFO) and pseudo-second-order (PSO) models. Thermodynamic quantities (ΔG, ΔH, ΔS) were obtained from the van’t Hoff equation by plotting ln Xm against 1/T [19,20].
RESULTS AND DISCUSSION
Structural and morphological characterization
The FTIR spectra of the composite before and after Saf uptake are shown in Fig. 1. In the pristine material a broad band sat at 3409 cm⁻¹, which we assign to overlapping O-H stretching of CMC and HEAM together with N-H vibrations from MBA and HEAM [21]. The clear absorption at 1720 cm⁻¹ belongs to the carbonyl stretch of the ester, amide, and carboxyl groups brought in by the grafted monomers, and the band at 2885 cm⁻¹ to aliphatic C-H₂ stretching; the features at 1249 and 1157 cm⁻¹ correspond to C-N and C-O stretches, respectively. The bands at 1064 and 555 cm⁻¹ are the ones that confirm zeolite has been built into the network, arising from asymmetric Si-O-Si stretching and Al-O-Si bending [22]. That no vinyl C=C peak appears near 1600 cm⁻¹ tells us the monomer double bonds were fully consumed during polymerization. Adsorption of Saf brought a few changes worth noting: the broad O-H/N-H band moved to 3448 cm⁻¹ and lost intensity, which is consistent with hydrogen bonding between the composite’s functional groups and the amino groups of the dye. The aromatic C=C vibrations of the bound Saf resulted in the appearance of a new band at 1627 cm−1, and the carbonyl peak shifted from 1720 to 1689 cm−1, suggesting the direct involvement of the carboxylate groups in the binding of the dye [23].
The XRD trace of plain hydrogel displayed a broad diffuse hump at 2θ = 21.4° which is typical of amorphous crosslinked polymer (Fig. 2). The halo still presented in the composite with weak peaks at 2θ = 9.85°, 22.1° and 29.2° which are in agreement with the crystalline clinoptilolite phase [24]. These zeolite reflections were substantially weaker than those of pure zeolite, suggesting that the zeolite is dispersed as a distinct phase throughout the amorphous gel rather than as discrete clumps. In fact, it is beneficial to keep the amorphous nature for adsorption, since a disordered, open network allows dye molecules to seep in more easily.
Before adsorption, the FESEM images (Fig. 3a, b) showed a rough, very porous surface dotted with cavities and folds of irregular size. This kind of texture, which the zeolite helps create, opens several routes for the adsorbate to travel and enlarges the area actually available for contact. Once Saf had been taken up (Fig. 3c, d) the surface looked distinctly smoother and the pores were harder to make out, in line with pores being filled and the surface being covered by dye [25]. The TEM images (Fig. 3e, f) showed zeolite nanoparticles scattered through the polymer, clustered here and there, which confirms they are genuinely embedded. It is likely the combination of the zeolite’s microporous framework with the hydrogel’s larger channels that gives the composite its good adsorption behaviour.
Surface area, pore structure, and surface charge
The nitrogen adsorption-desorption isotherms were Type IV with H3 hysteresis, which marks both the hydrogel and its composite as mesoporous [26]. Adding zeolite raised the BET surface area from 8.243 m² g⁻¹ for the plain hydrogel to 9.801 m² g⁻¹ for the composite, and the total pore volume from 0.00567 to 0.00675 cm³ g⁻¹ (Fig. 4a and Table 1). These numbers are small next to those of activated carbons, but they reflect only the polymer matrix; in hydrogels it is really the density of functional groups, not the surface area on its own, that governs how much dye is taken up [27]. BJH analysis put the mean pore diameter near 2.75 nm, within the mesopore range (2-50 nm by the IUPAC scheme), wide enough for the fairly bulky Saf molecule (MW = 350.85 g mol⁻¹) to move into the interior of the gel.
The point of zero net charge (pHzpc) of the composite was found to be 3.8. Above this pH the surface grows steadily more negative as the carboxyl and hydroxyl groups ionize, which pulls the cationic Saf toward it. Below 3.8 the groups are protonated and the surface carries a net positive charge that pushes the cationic dye away, and this fits the low uptake seen under strongly acidic conditions (Fig. 4b). The zeta potential results thus back up the pH-dependent adsorption behaviour described in Section 3.4 [28].
Gel content and swelling behavior
The gel fraction worked out at 99.95% for the plain hydrogel and 99.93% for the composite, which means monomer conversion was almost total and the network tightly crosslinked. Values this high show that the KPS/MBA pair propagated radicals efficiently and that the small amount of zeolite did nothing to hold back crosslinking [29]. In the swelling tests the composite took up far more water at basic pH than at acidic pH, with a maximum around pH 8. The reason is that above their pKa the carboxyl groups ionize to COO⁻; these charged groups repel one another, the network expands, and more water, along with dissolved pollutant, can work its way inside [30].
Effect of contact time and adsorption kinetics
Fig. 5a follows Saf uptake on the composite over time at 25 °C (C₀ = 300 mg L⁻¹, 0.02 g adsorbent, pH 7). The dye was taken up quickly in the first 20 min, more slowly between 20 and 45 min, and had essentially levelled off by about 60 min, so 60 min was adopted as the equilibrium time. There are numerous empty sites early on and a big concentration gradient, which explains the quick start; when those sites fill and diffusion inside the particles becomes limiting, the rate falls off [31].
The kinetic data were fitted to the PFO and PSO models to determine the rate controlling process. This is clearly presented in Fig. 6 and Table 2. The PSO model was fitted almost perfectly (R 2 = 1.000) and its calculated equilibrium capacity (qe, calc = 149.254 mg/g) was close to the measured value. The PFO fit was bad (R2 = 0.891) and substantially overestimated qe. A PSO fit of this grade indicates the prevalence of chemisorption type interactions, mainly electrostatic attraction and hydrogen bonding between the anionic groups of the composite and the cationic dye [32]. The initial rate h was 312.5 mg g⁻¹ min⁻¹ suggesting easy affinity of Saf to the surface.
Effect of pH
Solution pH has a strong effect on how much cationic dye is adsorbed, because it changes both the charge on the adsorbent and the form the dye takes in solution. Fig. 5b shows Saf uptake climbing steadily as the pH went from 2 to 9 and then dropping back a little past that. At low pH the carboxyl and hydroxyl groups are protonated, the surface carries a positive charge, and the cationic Saf is repelled. Once the pH climbs above the pHzpc (3.8), stepwise deprotonation leaves the surface negatively charged and it now attracts the dye, which accounts for the rising curve [33]. The little dip above pH 9 might be because the dye’s own amino groups are losing protons, reducing the coulombic attraction, or it might just be the torrent of OH⁻ ions contending for the surface. Based on that, pH 9 was selected as best, while the rest of the trials done at pH 7 to be close to near neutral circumstances of real water.
Adsorption isotherms
The equilibrium values at 25 °C were fitted to the Langmuir, Freundlich and Temkin equations for starting Saf concentrations in the range of 200-500 mg/L. The linearised fits are compared in Fig. 7 and the parameters are listed in Table 3. Freundlich provided the best correlation followed by Temkin and Langmuir with R2 = 0.9869, 0.9648 and 0.9374 respectively. The best fitting to Freundlich model indicates a heterogeneous surface where the sites are not all energetically equivalent and the covering occurs in more than one layer, which is in agreement with the mixed organic-inorganic nature of the composite [34]. Adsorption was favourable with an intensity parameter n = 2.206 (> 1) and the capacity factor KF = 99.51 mg/g (mg/L) (1/n) showed a high affinity. The Langmuir fit, although weaker, still produced a maximum monolayer capacity of qm = 271.19 mg g⁻¹, a convenient number to compare with other adsorbents (Table 3). The Temkin constant B = 66.45 J mol⁻¹ is related to the heat of adsorption and, given the context of the thermodynamic data (Section 3.7), is compatible with adsorption that is physical but with an electrostatic component [35,36].
Temperature effect and thermodynamic evaluation
The adsorption was conducted at four different temperatures (5, 15, 25, 35 °C) and different beginning concentrations (200-500 mg/L). The capacity slowly decreased with the increase in temperature (Fig. 8a). The maximum absorption was decreased from 248.8 mg/g at 5 °C to 235.9 mg/g at 35 °C. Such an inverse dependence is common for an exothermic process, in which the added heat energy disrupts the weak physical connections between the dye and the surface, and by increasing solubility of dye, shifts equilibrium back toward solution [37].
The thermodynamic parameters were obtained from the Van’t Hoff plot of ln Xm vs. 1/T (Fig. 8b) and are presented in Table 4. The exothermic nature is quantified by the negative enthalpy (ΔΗ = −58.76 kJ/mol) and the negative ΔG for the temperatures tested, confirms the spontaneity of the activity. The negative entropy shift (ΔS = −165.1 J/ K.mol) indicates that the solid-liquid interface becomes more ordered after the Saf binding, as expected for the formation of an ordered dye-surface complex. Note that the value of ΔH is greater than the 40 kJ/mol figure commonly used to distinguish between physisorption and chemisorption, so the binding is unlikely to be purely van der Waals but a mixture of electrostatic attraction, hydrogen bonding and possibly π-π stacking between the aromatic rings of Saf and the polymer backbone [38,39]. This mixed mechanism interpretation is consistent with the FTIR alterations seen upon adsorption and the PSO kinetics.
Ionic strength effect and reusability
The competing cations were evaluated by adding 0.01-0.20 mol/L of NaCl, KCl or CaCl2 (Fig. 9a). In all cases salt decreased capacity, CaCl₂ being the most damaging followed by NaCl and then KCl. The divalent Ca2+ ion has a greater contribution to the ionic strength per mole than the monovalent one and a better competition for the negatively charged sites, pushing Saf off the surface. This sensitivity of the primary adsorption mechanism to ionic strength is itself a proof of the electrostatic nature of the process [40].
The material was well regenerated in four adsorption-desorption cycles with 0.1 M HCl, and the removal efficiency was only reduced slightly from 99.1% in the first cycle to 87.4% in the fourth cycle (Fig. 9b). That modest reduction is due to loss of a portion of the accessible sites, and slight structural relaxation in the gel, but the remainder is still high enough for genuine column or batch application. HCl worked better than NaOH or ethanol, most likely because in acid the COO⁻ and NH groups pick up protons, the electrostatic hold on the cationic dye weakens, and the dye comes off more easily [41].
Comparison with other adsorbents
Table 5 sets the capacity of the present composite against a selection of adsorbents from the recent literature. Its qm of 271.19 mg g⁻¹ holds up against, and in a few cases beats, figures reported for carbon-based, clay-based, and other polymeric adsorbents, which speaks to the value of pairing a swellable hydrogel with the ion-exchange capacity of zeolite [42-45].
CONCLUSION
A zeolite-grafted biopolymer composite, (CMC-AAC-co-HEAM)-g-zeolite, was prepared by free-radical graft polymerization and characterized in some detail for its nano-scale structure and surface chemistry. It removed safranin O from water effectively, reaching a Langmuir maximum capacity of 271.19 mg g⁻¹ and coming to equilibrium inside 60 min. The uptake followed pseudo-second-order kinetics and the Freundlich isotherm, which places the rate-limiting step at surface interactions on an energetically uneven surface. The thermodynamics were exothermic and spontaneous, driven by electrostatic attraction, hydrogen bonding, and probably some π-π interaction between the aromatic part of the dye and the polymer backbone. Dilute HCl regenerated the material easily, and about 87% of the original removal efficiency survived four cycles. On the whole, then, this zeolite-enhanced hydrogel is a cheap, reusable, and effective adsorbent for pulling cationic dyes out of contaminated water. Sensible next steps would be to test it in a fixed-bed column, to look at competition when several pollutants are present, and to weigh up scale-up under genuine wastewater conditions.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.
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