INTRODUCTION

In today’s world, pharmaceutically active compounds (PhACs) i.e., NSAIDs, antibiotics, antidepressants, and hormones appeared as an emerging water pollutant [1] that are not fully metabolized and get discharged into water. Inappropriate disposal of unused medications and inefficient working of wastewater treatment plants (WWTPs) for PhACs removal further aggravates the issue [2]. These compounds are responsible for severe health problems and environmental pollution. Among all PhACs, diclofenac sodium (DS) is one of the common non-steroidal anti-inflammatory drugs (NSAID) with numerous applications all over the world [3, 4]. This drug contains two aromatic rings and different functional groups (as phenylacetic, chlorine, secondary amino) in its structure [5]. It finds widespread uses as a cyclooxygenase inhibitor owing to its anti-inflammatory, analgesic, as well as antipyretic characteristics [6]. The remarkable proportion of this drug, however, get discharged into the environment via urine and feces. Owing to its resistant nature toward degradation, this drug stays for a longer time in water posing serious harms to aquatic life mainly fishes by damaging their kidney, gills, as well as their endocrine system. Furthermore, this is responsible for inducing oxidative stress and lowering the testosterone levels in aquatic organisms [7-15]. For mitigating the problems caused by DS drug pollution, there is a need for developing some effective water treatment methods aiming to remove DS drug from wastewater and industrial effluents to reduce its associated health risks both to humans and environment [8, 15-17].

Different procedures for treating polluted water have been developed as advanced oxidation, membrane filtration, membrane-sorption hybrids and adsorption. Among these techniques, adsorption process is widely employed method due to its low energy consumption, easy operation, and high effectiveness [18-30]. In adsorption process, selecting the most appropriate adsorbent is the crucial step and numerous adsorbents have been developed till date. Hydrogels are widely used adsorbents owing to their hydrophilic nature, large surface area available for adsorption and diversity of functional groups in its structure [31, 32]. These materials are biocompatible and biodegradable in nature [33-35]. Sodium carboxymethyl cellulose (CMC) is a polymer that finds extensive uses in different fields mainly due to its unique chemical, physical, and biological characteristics. Presence of amine (-NH₂) and carboxyl (-COOH) functional groups in CMC make it pH sensitive as its swelling is more in basic solutions rather than in acidic ones. This property of CMC make it effective adsorbing material for controlled drug delivery systems [36].

However, to further improve the adsorbing potential of CMC, different polymers are grafted in it. Poly acrylic acid (PAAc) and polyacrylamide (PAAm) possess high efficiency for adsorbing variety of pollutants [37] mainly due to presence of ionic functional groups in their structure [38, 39]. This study deals with use of carboxymethyl cellulose-g-poly (acrylic acid-co-acrylamide) i.e., CMC-g-P(AAc-AAm) nano-hydrogel for adsorptive removal of DS drug from water. The adsorbent’s synthesis was done by method of free radical polymerization and further analyzed via FTIR, SEM and XRD. Adsorption of drug was studied at variable temperatures and concentrations. Data from adsorption study was applied to Langmuir, Freundlich and Temkin models for describing the adsorption mechanism. The thermodynamic behaviour of DS drug adsorption on prepared nano-hydrogel was also analyzed by applying Van’t Hoff plot. Findings revealed adsorptive potential of prepared adsorbent toward DS drug adsorption.

MATERIALS AND METHODS

Reagents and chemicals

The reagents used in current study were sodium carboxmethyl cellulose (NaCMC, C₂₈H₃₀Na₈O₂₇), acrylamide (AAm, C₃H₅NO), N, N’-methylene-bis-acrylamide (MBA, C₇H₁₀N₂O₂) and Mueller-Hinton agar that were procured from Himedia. Furthermore, potassium per sulfate (KPS, K₂S₂O₈) and sodium chloride (NaCl) were obtained from Fluka. From Thomas maker, acrylic acid (AAc, C₃H₄O₂) and from Alpha Chemika, potassium chloride (KCl) were purchased. Additionally, from B.D.H, hydrochloric acid (HCl) was obtained. Nitrogen gas (N₂) was also used in study. The purity percentages of these chemicals vary, but most of them have purity greater than 98%.

Preparation of CMC-g-P(AAc-AAm) nano-hydrogel

To synthesize CMC-g-P(AAc-AAm), free radical polymerization method was used. Potassium persulfate (KPS) was utilized as an initiator while methylene bisacrylamide (MBA) served as a crosslinking agent. Initially, 1g of sodium carboxymethyl cellulose (CMC) was dissolved in 20 mL deionized distilled water in a three-necked round-bottom flask that was equipped with a condenser, separating funnel, and nitrogen gas inlet. This was followed by heating the solution with continuous stirring in a magnetic stirrer-heater maintained at temperature of 50 °C till a homogeneous and transparent solution was obtained. Cooling of solution was carried out at normal temperature. In the next step, the initiator solution (0.1g KPS in 2 mL water) was added to above solution with stirring using separating funnel. Subsequently, an acrylamide (AAm) solution (1g AAm in 1 mL of deionized distilled water) was introduced in above mixture with constant stirring followed by addition of 4 mL acrylic acid (AAc) with ongoing stirring. Next, the crosslinker solution of MBA (0.05g MBA in 1 mL deionized distilled water) was added, ensuring thorough mixing. All additions were performed under a nitrogen (N₂) atmosphere. Finally, mixture was added to test tubes and placed in a water bath at 70°C for time period of two hours that allow the formation of nano-hydrogel [40, 41] as depicted in Fig. 1.

Activating the surface of prepared nano-hydrogel and characterization study

The prepared nano-hydrogel was cut into small pieces followed by washing with deionized distilled water for one hour. The washing process was repeated several times to remove any unreacted materials. After thorough washing, the nano-hydrogel was dried at 60°C and subsequently ground to small particles (Fig. 2).

Successful synthesis of nano-hydrogel was confirmed by its analysis via different techniques namely Fourier Transform Infrared (FTIR, Shimadzu 8400s spectrophotometer from 500 to 4000 cm⁻¹), Field Emission Scanning Electron Microscopy (FESEM, TESCAN MIRA3 at voltage of 25 kV) and X-ray Diffraction (XRD, Shimadzu XRD-6000, with 2θ range of 10° to 80°).

Calibration curve and adsorption study 

For plotting the calibration curve, variable solutions of DS drug (1 mg/L to 30 mg/L) prepared and measurement of absorbance was carried out. Results of calibration graph (Fig. 3) revealed the higher correlation coefficient value highlighting linear relation between drug concentration and its respective absorbance.

For studying the adsorption of DS drug from water, solutions of variable concentrations were prepared varying from 10 mg/L to 180 mg/L at variable temperatures i.e., from 10 °C to 30 °C. All experiments were performed at solution pH of 9 and shaking speed of 120 rpm. The amount of nano-hydrogel used in each experiment was 0.06g. After an equilibrium time of 120 min, adsorption efficiency calculation was carried out employing Eq. 1:

Here C₀  and C_e, V (mL) and M (g) denotes initial, equilibrium dye concentrations (mg/l), solution volume and weight of adsorbent correspondingly. Three isotherm models i.e., Langmuir, Freundlich, Temkin were studied to better understand mechanism of adsorption. While results from experiment of temperature study were analyzed by applying thermodynamic model highlighting the feasibility of process.

RESULTS AND DISCUSSION

Characterization results

The functional group study of nano-hydrogel (Fig. 4) revealed the presence of numerous functional groups in it. The peak obtained within range 3600-3100 cm⁻¹ correspond to the presence of -N-H and -O-H stretching vibrations while the -C-H stretching vibrations were observed by the presence of small peaks from 2900 cm^-1 to 2800 cm^-1. Peaks at 1720 cm^-1 and 1630 cm^-1 to 1560 cm^-1 are mainly due to the vibrations of -C=O bonds in acrylic acid (AAc) and acrylamide (AAm) respectively. In addition to that, asymmetric and symmetric stretching vibrations of -C=O in carboxylate ions are observed by the peaks at 1488 cm^-1 and 1380 cm^-1 correspondingly. Peak obtained at 1164 cm^-1 and 995 cm^-1 corresponds mainly to the presence of -C-N and -C-O-C bond vibrations correspondingly. Post adsorption analysis of FTIR revealed remarkable change in both intensity and position of -N-H, -O-H, -C=O, -C-N and -C-O-C peaks obtained. This reflects the interactions between functional groups of studied nano-hydrogel and adsorbed drug molecules [17, 36, 42-52].

Crystallographic results of XRD (Fig. 5) revealed the presence of a broad peak i.e., at 2θ = 20° to 30°. This peak confirmed the amorphous nature of prepared nano-hydrogel i.e., absence of long-range order and less crystallinity that is one of the key properties of majority of hydrogels due to presence of inter-linked polymers in it. This amorphous nature of nano-hydrogel is responsible for its high swelling property i.e., high water retention ability that aid in effective drug adsorption [36].

Morphological analysis of prepared nano-hydrogel revealed the presence of numerous pores and heterogeneity on its surface before drug adsorption (Fig. 6a). The presence of heterogeneity is the key feature for any adsorbent. The small size particles of nano-hydrogel are responsible for adsorbing drug molecules on its surface [53]. Post adsorption FESEM results (Fig. 6b) revealed the conversion of heterogeneous surface to somewhat homogeneous surface due to filling of empty active sites by drug molecules. In summary, the surface characteristics of nano-hydrogel varied after drug adsorption due to interactions taking place between drug molecules and surface of nano-hydrogel [21, 36, 54-57].

Isotherm and thermodynamic study

Effect of drug concentration on adsorption capacity of prepared nano-hydrogel was studied at variable concentrations and temperatures. Results of study (Fig. 7 and Table 1) showed that an increase in solution concentration led to an increase in adsorption capacity while on the other hand, increasing temperature resulted in decrement in adsorption capacity (Q_e). Maximum adsorption can take place at the highest concentration i.e., 180 mg/L but at the lowest temperature i.e., 10 °C where 26.66 mg of drug adsorbed per gram of adsorbent surface. This capacity continues to decrease with increase in temperature and reaches 24.61 mg/g while using 180 mg/L solution at 30 °C. The results are further confirmed by the thermodynamic study wherein the value of enthalpy change was negative highlighting the exothermic nature of adsorption. This can be attributed to the weakening of attractive forces between studied drug molecules and adsorbent surface at higher temperatures [58].

Data from concentration study was applied to different isotherm models. The Langmuir model mainly deals with single layer adsorption of pollutant on adsorbing material [18]. Freundlich isotherm, on the other hand, involves the adsorption of pollutant on adsorbent surface in multilayer manner [25]. For better understanding of interactions taking place between adsorbent and the adsorbate, Temkin model is widely employed that provides information regarding adsorption heat changes taking place throughout the adsorption process [59]. Linearized forms of Langmuir, Freundlich as well as Temkin model are expressed by Eqs. 2, 3 and 4 correspondingly.

The Langmuir constant and maximum adsorption capacity are represented by b (L/mg) and q_max (mg/g) correspondingly.

Constant and exponent of Freundlich model (mg/g) are represented by k_f and ‘n’correspondingly [56].

here,  and B refers to constant of universal gas (J/mol K) and adsorption heat (J/mol), ,  and  refers to constant of Temkin equilibrium binding (L/ g), Temkin constant (J/ mol) and absolute temperature in Kelvin correspondingly [59]. Results (Fig. 8a-c and Table 2) revealed that data fits best to Freundlich model that has the highest regression coefficient value i.e., 0.9582 when compared with Langmuir (R² = 0.8773) and Temkin (R² = 0.8113) models. Furthermore, data revealed the maximum adsorption capacity of 1.872 mg/g from Freundlich model revealing the multilayer adsorption of drug on heterogenous adsorbent surface [58].

Effect of temperature on adsorption capacity of adsorbent was investigated by varying temperature from 10 °C to 30 °C for 500 mg/L drug concentration solution for an equilibrium time of 120 min. Results (Fig. 9 and Table 3) revealed that with increasing temperature, adsorption capacity of adsorbent for drug adsorption decreases continuously. With increasing temperature from 10 °C to 30 °C, there was a decrement in adsorption capacity from 26.66 mg/g to 24.61 mg/g respectively. Findings showed exothermic nature of process suggesting breakdown of weak adsorption forces (van der Waals forces or dipole-dipole interactions) between drug molecules and adsorbent surface at higher temperatures [58].

Data from temperature study was analyzed thermodynamically for examining the feasibility of process in terms of thermodynamic parameters (i.e., ΔH, ΔS, and ΔG). For calculating value of ∆G (Eq. 5), distribution coefficient, k_c, was employed (Eq. 6):

where C_ad (mg/L), R , T denotes to drug concentration that get adsorbed, ideal gas constant (8.314 J/mol K) and absolute temperature in Kelvin correspondingly. For calculating the change in Gibbs free energy (ΔG), Eq. 7 can be used:

When we substitute Equation (5) into Equation (7), an expression for ln k_c can be obtained as Eq. 8:

From the graph (Van’t Hoff graph) plotted between ln (K_c) and 1/T, values of ΔH and ΔS were determined from its slope and intercept, correspondingly. Results of Van’t Hoff plot are shown in Fig. 10 and parameters calculated from its slope and intercept are elaborated in Table 4. Data revealed the non-spontaneous and exothermic behaviour of process due to positive and negative values obtained for ΔG and ΔH respectively. Furthermore, study revealed the decreased randomness of system with increasing temperature revealing feasibility of studied process [60, 61].

CONCLUSION

This research was devoted to investigate the adsorptive potential of CMC-g-P(AAc-AAm) nano-hydrogel towards diclofenac sodium (DS) drug removal from water. The nano-hydrogel synthesis was carried out via free radical polymerization method and characterized by different techniques namely FTIR, SEM, and XRD. The analysis revealed the highly porous, heterogenous adsorbent surface that possess numerous ionic functional groups favoring the adsorption process. Concentration study showed fitness of data to Freundlich model suggesting multilayer adsorption with maximum adsorption capacity of 1.872 mg/g. From temperature study, feasibility of the process and higher adsorption capacity at the lowest temperature was confirmed. Overall, the study revealed the potential of prepared adsorbent for DS drug adsorption from water.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.