INTRODUCTION
The rapid expansion of industrial activities, especially in the textile, leather, paper, cosmetic, and pharmaceutical sectors, has led to the release of huge amounts of coloured wastewater into natural water bodies [1,2]. Even at very low concentrations, synthetic dyes can impair light penetration, disturb aquatic ecosystems, and pose serious risks to human health due to their mutagenic and, in some cases, carcinogenic nature [3]. Fuchsin Basic (FB) — also known as basic fuchsin or rosaniline hydrochloride — is a triphenylmethane cationic dye which is commonly used for staining bacteria and for colouring textile fibres [4]. It is highly stable, water-soluble, and difficult to remove by conventional biological treatment processes [5].
Adsorption is still one of the best ways to remove dye, even if there are many others, such as coagulation-flocculation, membrane filtering, advanced oxidation, photocatalysis, and electrochemical therapy. This is mainly due to its simplicity of operation, low running cost, and relatively high efficiency [6,7]. There are many various kinds of adsorbents that have been written about in the literature. Some examples are activated carbon, clays, charcoal, zeolites, and different kinds of polymeric hydrogels [8–10]. Hydrogels, in particular, have a customisable three-dimensional polymeric network and a lot of functional groups that can strongly interact with dye molecules [11].
Graphene oxide (GO) has recently emerged as an excellent adsorbent precursor thanks to its very large specific surface area, its abundant oxygen-containing functional groups (–OH, –COOH, epoxy), and its ability to form π–π interactions with aromatic pollutants [12,13]. However, pure GO tends to aggregate in aqueous media and is usually difficult to recover after use. Embedding GO into a polymer network, for instance a copolymer of acrylic acid and maleic anhydride, can provide additional anionic sites for capturing cationic dyes such as FB, while also offering mechanical stability to the adsorbent [14,15]. In this context, the present work was designed to prepare a GO/P(AA-MA) nanocomposite hydrogel and to investigate its behaviour for the removal of FB from aqueous solutions, with particular attention given to the kinetics, isotherms, thermodynamics, and reusability.
MATERIALS AND METHODS
Materials
Acrylic acid (AA, ≥ 99%), maleic anhydride (MA, ≥ 98%), N,N′-methylenebisacrylamide (MBA, used as cross-linker), potassium persulfate (KPS, initiator), sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium chloride (NaCl), and Fuchsin Basic dye (molecular formula C₂₀H₂₀ClN₃; molecular weight 337.86 g mol⁻¹) were purchased from Sigma-Aldrich and used without further purification. Graphene oxide was prepared in our laboratory from graphite powder by a modified Hummers method, as described in an earlier report [16]. All solutions were prepared using distilled water.
Preparation of the GO/P(AA-MA) nanocomposite
The nanocomposite hydrogel was synthesized by free radical polymerization in an aqueous medium. In a typical procedure, 10 mL of AA was partially neutralized (~60%) with NaOH solution under continuous stirring and then mixed with 1.5 g of MA in a three-necked flask. An aqueous dispersion of GO (0.05 g in 10 mL of distilled water) was sonicated for 20 min and added dropwise to the monomer mixture under vigorous stirring in order to promote good dispersion. After that, 0.1 g of MBA and 0.15 g of KPS (previously dissolved in 5 mL of water) were added, and the mixture was transferred to a water bath set at 70 °C for 4 h under a nitrogen atmosphere. The obtained gel was cut into small pieces, washed several times with distilled water and then with ethanol to remove any unreacted species, dried in an oven at 50 °C until constant weight, and finally ground and stored in a desiccator.
Characterization
The functional groups present in GO, P(AA-MA), and the GO/P(AA-MA) nanocomposite were identified using a Shimadzu IR Prestige-21 FTIR spectrometer in the wavenumber range 4000–400 cm⁻¹ (KBr pellet method). The crystalline structure of the samples was analysed by X-ray diffraction using a Shimadzu XRD-6000 instrument with Cu Kα radiation (λ = 1.5406 Å) over the 2θ range 5–60° at a scanning rate of 2° min⁻¹. The surface morphology was examined using a TESCAN MIRA3 field-emission scanning electron microscope after sputter-coating the samples with a thin gold layer.
Batch adsorption experiments
A stock solution of FB (1000 mg L⁻¹) was prepared by dissolving an appropriate amount of the dye in distilled water, and the working solutions were obtained by successive dilution. In a typical experiment, 0.05 g of the nanocomposite was added to 50 mL of dye solution of known concentration in a 100 mL conical flask, and the mixture was shaken at 200 rpm in a thermostatic water-bath shaker. Samples were withdrawn at pre-set time intervals, centrifuged for 5 min at 4000 rpm, and the residual dye concentration was determined spectrophotometrically at λmax = 545 nm.
The removal efficiency (R %) and the adsorption capacity (qt, mg g⁻¹) were calculated according to:
R % = [(C₀ – Cₜ)/C₀] × 100 (1)
qₜ = [(C₀ – Cₜ) × V] / m (2)
where C₀ and Cₜ (mg L⁻¹) are the dye concentrations at the initial time and at time t, respectively, V is the volume of the solution (L), and m is the mass of the adsorbent (g).
Systematic investigations were conducted on the effects of contact time (5-180 min), starting dye concentration (25-300mg.L⁻¹), adsorbent dosage (0.01-0.1 g), pH (2-10), ionic strength (0-0.5M NaCl), and temperature (288-318K). HCl (0.1M) and NaOH (0.1M) solutions used to change the pH. For regeneration tests, the utilised nanocomposite was washed with 0.1M HCl and then with distilled water until the pH was neutral. It was then dried and used for five more cycles.
Swelling study
The swelling behaviour of the nanocomposite was measured gravimetrically. A known mass of dry hydrogel (md) was immersed in distilled water at room temperature. At given time intervals, the swollen sample was removed, gently blotted with filter paper to eliminate excess surface water, and weighed (ms). The swelling ratio (SR) was calculated as:
SR = (ms – md) / md (3)
RESULTS AND DISCUSSION
Characterization of the nanocomposite
Fig. 1 shows the key characterisation data for the materials that were made. These include the FTIR spectra, XRD patterns, and FESEM micrographs of GO, pure P(AA-MA), and the final GO/P(AA-MA) nanocomposite. The FTIR spectrum of GO shows a broad band around 3400 cm⁻¹ that is caused by the stretching vibration of –OH groups. The peaks at 1720, 1620, and 1050 cm⁻¹ are due to the C=O of carboxylic groups, the aromatic C=C skeletal vibration, and the C–O of epoxy/alkoxy groups, respectively [17]. The spectrum of P(AA-MA) displays typical absorption bands of carboxylic groups near 1710 cm⁻¹ and a broad –OH band centred around 3200 cm⁻¹. In the composite, most of the characteristic peaks of both components are still visible, though with some slight shifts and broadening, which is consistent with the formation of hydrogen bonds and possible esterification reactions between the oxygen-containing groups of GO and the –COOH/–OH groups of the copolymer [18].
The XRD pattern of GO (Fig. 1b) shows the characteristic sharp peak at 2θ ≈ 10.5° corresponding to the (001) reflection, which gives an interlayer spacing close to 0.84 nm [19]. Pure P(AA-MA) exhibits a broad amorphous halo centred near 2θ ≈ 22°, which is typical of polymeric hydrogels. In the nanocomposite, the sharp peak of GO is strongly weakened and shifted to a slightly lower angle, suggesting that GO sheets are exfoliated and well dispersed inside the polymer network rather than forming aggregated stacks [20]. FESEM images (Fig. 1c) confirm these observations: pure GO shows the expected wrinkled sheet-like morphology, P(AA-MA) appears relatively smooth with few surface features, whereas the composite displays a rougher and more porous texture, in which GO sheets seem embedded within the polymer. Such a surface is expected to increase the number of accessible adsorption sites and to improve dye diffusion inside the matrix.
Effect of contact time and adsorption kinetics
The influence of contact time on the uptake of FB was evaluated at an initial dye concentration of 100mg.L⁻¹, keeping all other parameters constant. As illustrated in Fig. 2a, the adsorption was rapid during the first 30min., then slowed gradually, and finally reached equilibrium at around 90 min. This type of profile, which has been observed for many hydrogel-type adsorbents, is generally attributed to the abundance of free active sites in the early stage of the process, which become progressively saturated with time [21].
In order to describe the mechanism of adsorption, four well-known kinetic models were applied to the experimental data: the pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and the intraparticle diffusion (Weber–Morris) models, expressed as follows:
ln(qₑ – qₜ) = ln qₑ – k₁ t (4)
t/qₜ = 1/(k₂ qₑ²) + t/qₑ (5)
qₜ = (1/β) ln(αβ) + (1/β) ln t (6)
qₜ = kid t0.5 + C (7)
The linear plots corresponding to these four models are shown in Fig. 2b-e, and the estimated kinetic parameters together with the correlation coefficients are summarized in Table 1. Among the tested models, the pseudo-second-order model gave the highest correlation coefficient (R² > 0.99) and the calculated qe values were in excellent agreement with the experimental ones. This outcome suggests that the adsorption of FB on GO/P(AA-MA) is governed by a chemisorption-type mechanism, which most likely involves electrostatic attraction and hydrogen bonding between the positively charged dye and the deprotonated -COO⁻ groups of the nanocomposite [22]. The Weber-Morris plot (Fig. 2e) was not a single straight line passing through the origin; instead, at least two linear segments were observed, which indicates that more than one step controls the overall rate, namely external (film) diffusion during the first stage and intraparticle diffusion at longer times [23].
Effect of initial concentration and adsorption isotherms
The equilibrium uptake increased from about 48 to 265 mg g⁻¹ when the initial dye concentration was raised from 25 to 300 mg L⁻¹, while the removal percentage showed the opposite trend. This behaviour, which is often reported in the literature, can be explained by the fact that a higher concentration provides a stronger driving force that helps to overcome the mass-transfer resistance between the solution and the solid surface, although the finite number of active sites cannot capture all the dye molecules at high loadings, which leads to a decrease of the percentage removal [24].
The equilibrium data were then analysed using four classical isotherm models — Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D-R) — whose linearized forms are given below:
Cₑ/qₑ = 1/(qmax KL) + Cₑ/qmax (8)
ln qₑ = ln KF + (1/n) ln Cₑ (9)
qₑ = B ln KT + B ln Cₑ (10)
ln qₑ = ln qD – β ε² (11)
Table 2 shows the computed isotherm parameters and correlation coefficients. The Langmuir model fit the experimental data best, with a R² value of almost 0.998. It also showed that the maximum adsorption capacity (qmax) was about 285 mg g⁻¹ at 298 K. This means that FB typically sticks to places where the energy is the same, which makes a single layer of coverage on the surface of the nanocomposite [25]. The dimensionless separation factor RL, calculated from RL = 1/(1 + KL C₀), was between 0 and 1 in all cases, indicating favourable adsorption. The Freundlich exponent 1/n was smaller than unity, which is again consistent with a favourable process [26]. The mean adsorption energy E, estimated from the D-R model, was in the range 8–12 kJ mol⁻¹, suggesting that the interaction between FB and the nanocomposite is of an intermediate nature, somewhere between pure physical adsorption and ion-exchange/chemisorption.
Effect of temperature and thermodynamic study
The role of temperature in the adsorption process was investigated between 288 and 318 K, while keeping the other parameters constant. As shown in Fig. 3a, the amount of dye adsorbed at equilibrium increased with increasing temperature, which is typical of an endothermic process and may be related to an enhanced mobility of the dye molecules and/or an opening of the polymer network at higher temperatures [27].
Thermodynamic parameters — the Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) — were estimated using the following equations:
ΔG° = -RT ln Kd (12)
ln Kd = (ΔS°/R) – (ΔH°/RT) (13)
where Kd = qₑ/Cₑ is the distribution coefficient, R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), and T is the absolute temperature (K). The computed values are presented in Table 3, and the corresponding van’t Hoff plot (ln Kd vs 1/T) is displayed in Fig. 3b.
The ΔG° values were negative at all temperatures and became more negative as the temperature increased, which confirms that the adsorption of FB on GO/P(AA-MA) is a spontaneous and thermodynamically favourable process. A positive ΔH° value (+24.6 kJ mol⁻¹) indicates that the process is endothermic in nature. The positive value of ΔS° (+96 J mol⁻¹ K⁻¹) reflects an increase in the randomness at the solid–liquid interface, which is usually associated with the release of hydration water molecules surrounding the charged sites of the adsorbent once the dye molecules are fixed onto them [28].
Effect of pH, ionic strength, and swelling behaviour
The solution pH is a crucial variable in dye adsorption, because it controls both the surface charge of the adsorbent and the ionization state of the dye molecules [29]. As illustrated in Fig. 4a, the removal of FB was low in strongly acidic media (pH 2) and increased considerably when the pH was raised up to 7–8, after which a slight decrease was noticed. At low pH values, the carboxylic groups of the nanocomposite are mostly in their protonated form (–COOH), so the electrostatic attraction toward the cationic FB is suppressed; in addition, the excess of H⁺ competes with the dye cations for the available sites. With increasing pH, –COOH groups are progressively converted into –COO⁻, which strongly attract the positively charged FB molecules through electrostatic interaction [30]. The small drop observed above pH 9–10 may be attributed to competition with OH⁻ ions and to a possible change in the ionization state of the dye itself.
The effect of ionic strength was examined by varying the NaCl concentration from 0 to 0.5 M (Fig. 4b). A clear decrease of the adsorption capacity was noticed when the salt concentration was increased. Two main reasons can be advanced to explain this behaviour. First, Na⁺ ions compete with FB cations for the negatively charged sites of the nanocomposite; second, the high ionic strength partly screens the electrostatic interaction between the dye and the adsorbent. The fact that this effect is relatively pronounced supports the idea that electrostatic interactions play an important role in the adsorption mechanism, although they are not the only contribution since a significant uptake is still observed even at high salt concentrations [31].
The swelling behaviour of the nanocomposite is shown in Fig. 4c. The swelling ratio increased sharply during the first 3 h, then rose more slowly, and reached a plateau after about 6 h with an equilibrium value of ~18 g g⁻¹. Such a pronounced swelling is directly related to the presence of hydrophilic –COOH/–COO⁻ groups in the polymer network and to the oxygen-containing functionalities on GO sheets, which together favour the diffusion of water into the matrix [32]. A high swelling degree is a valuable feature for adsorption, since it facilitates the diffusion of dye molecules through the hydrogel and enhances the contact with the internal active sites.
Regeneration and reusability
From a practical and economic point of view, the ability to regenerate and reuse an adsorbent is a key feature that determines its suitability for real applications [33]. In this work, the spent nanocomposite was regenerated by washing with 0.1 M HCl, followed by rinsing with distilled water until neutral pH, and finally drying before being used again. The results obtained over five consecutive adsorption–desorption cycles are presented in Fig. 5. The removal efficiency decreased only slightly from about 94 % in the first cycle to about 82 % in the fifth cycle, which corresponds to a total drop of less than 13 %.
The small decrease observed after several cycles can be ascribed to the incomplete desorption of strongly adsorbed dye molecules from the internal sites of the network, and possibly to a minor loss of active sites and/or of mass during the handling steps. Overall, these results clearly indicate that GO/P(AA-MA) maintains a good adsorption capacity after several cycles and can therefore be regarded as a reusable and economically reasonable adsorbent for the removal of FB dye from contaminated water.
Comparison with other adsorbents
To place the present results in a broader perspective, the maximum adsorption capacity obtained for FB on GO/P(AA-MA) was compared with the values reported in the literature for other adsorbents towards Fuchsin Basic and similar cationic dyes (Table 4). The capacity obtained in this study (~285 mg g⁻¹) is comparable to, or higher than, that of many previously reported materials, which confirms that the combination of GO with the P(AA-MA) network provides an effective platform for the uptake of cationic dyes.
CONCLUSION
This paper made a new nanocomposite hydrogel called GO/P(AA-MA) via free radical polymerisation. They then used it to remove Fuchsin Basic dye from water. FTIR, XRD, and FESEM tests showed that GO was well integrated into the polymer network and that the final material had a rough and porous surface with a lot of functional groups. Batch adsorption investigations demonstrated that the absorption of FB is significantly affected by contact time, starting concentration, pH, ionic strength, and temperature. The kinetic study showed that the process followed the pseudo-second-order model. The Langmuir isotherm best characterised the equilibrium data, with a maximum adsorption capacity of roughly 285 mg g⁻¹ at 298 K. Thermodynamic calculations showed that the process happens on its own, takes in energy, and is driven by entropy. The composite also had a high swelling ratio and worked well during five regeneration cycles, with only a small drop in efficiency. Taken together, these findings indicate that GO/P(AA-MA) is a promising, low-cost, and reusable adsorbent that could be developed further for the treatment of real dye-containing wastewater.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.
