Efficiency of Chitosan-Grafted Poly (Carboxymethyl Cellulose-Co-Acrylamide) Nano Hydrogel for Cadmium (II) Removal: Batch Adsorption Study

INTRODUCTION 
The extensive use of heavy metals (HMs) in modern world led to their increased levels in water sources resulting in deteriorating water quality and affecting all life forms on earth. Cadmium (Cd) is one of the toxic metals that enter water system via variety of routes such as galvanized pipes and metal fittings. When ingested, Cd is responsible for causing human organ dysfunction. Keeping in view the toxicity caused by Cd, Department of Environment, UK and European Economic Community placed this metal in red list of priority contaminants and black list of Dangerous Substance Directive respectively [1]. Researchers are now working on the development of effective and low-cost methods for removal of Cd (II) ions from water. Several physi-o-chemical and biological methods [2, 3] have been developed aiming to treat polluted water. However, high operational cost and secondary pollution associated with them make them ineffective for treating all kinds of pollutants in water [4-19]. Adsorption, however, an effective and efficient technique for water treatment takes precedence over all other conventional methods due to its simple design, low cost and easy availability of adsorbents. 
In adsorption method, selection of adsorbents for efficient pollutant removal is a crucial step. Various kinds of natural and synthetic adsorbents have been developed and used till date for Cd (II) adsorption [1, 20-29]. This study, however, investigated the adsorptive removal of Cd (II) ions from water using poly (carboxymethyl cellulose-co-acrylamide) (CS-g-P(CMC-co-AM)) nano hydrogel as an adsorbent. The nano hydrogel was characterized by FTIR, FESEM, and XRD techniques. Swelling properties of prepared nano hydrogels were also investigated under variable pH of solution. Influence of adsorption factors as contact time, pH, adsorbent dosage, salt concentration was also studied. The nature of adsorption process was investigated by applying the time data on two different kinetic models namely pseudo first and pseudo second order kinetic models.  

MATERIALS AND METHODS
Materials and chemicals used 
The experimental reagents used in study were acrylamide (AM), chitosan, hydrochloric acid, Carboxymethyl cellulose sodium salt (CMC), ethanol, potassium persulfate, bis-acrylamide, acrylic acid, nitrogen gas, acetic acid, tetramethylethylenediamine (TMEDA), sodium chloride, potassium chloride, and calcium chloride. These chemicals were essential components in conducting batch adsorption experiments and analyzing the results. 

Preparation of Cd (II) solution 
A stock solution (500 mg/L) of cadmium (II) ions was prepared by dissolving 1.37 grams of cadmium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O) in distilled water. The solution was quantitatively transferred to volumetric flask (1000 mL) and diluted to final volume by addition of water. Working solutions of variable concentrations were subsequently prepared from this stock solution for further experimentation. 

Synthesis and characterization of nano hydrogel CS-g-P(CMC-co-AM) 
The prepared nano hydrogel i.e., CS-g-P(CMC-co-AM) was synthesized via preparation of two different solutions [30, 31]. Firstly, solution 1 was prepared a 1% of acetic acid (AAC), dissolve 0.5 g of chitosan (CS) in 20 mL of this solution and dissolving 6g acrylamide in 10 mL deionized water which was then combined with solution of acrylamide (6g in 10 mL water). A solution of N, N’-methylenebisacrylamide (MBA) was prepared by dissolving its 0.05g in 5 mL deionized water. This solution was then added to mixture and stirred for 15 min with simultaneous introduction of nitrogen gas. Followed by preparation of solution 1, next step is to prepare solution 2. For this, 0.5 g Carboxymethyl cellulose sodium salt was dissolved in 10 mL deionized water that was stirred continuously for 15 min. Afterwards, potassium persulfate (KPS) solution (0.1g in 5 mL water) and a TEMED solution (0.05g in 5 mL water) were added to above mixture serially with continuous stirring. This was followed by addition of Solution 1 (in a dropwise manner) to Solution 2 at 50 °C while nitrogen gas was intermittently introduced for 30 sec. incubation of mixture was then carried out in a water bath at 60 °C for 2 hours that aid in completion of reaction. Obtained hydrogel was cut into smaller pieces that were then washed with water for one hour and oven-drying was done at 50 °C. The dried hydrogel was then ground into very fine particles having diameters varying from 50 to 250 nm. Fig. 1 shows prepared CS-g-P(CMC-co-AM) hydrogel.  
The successful synthesis of nano hydrogel was confirmed by Fourier Transform Infrared (FTIR) spectroscopy (Shimadzu 8400s spectrophotometer within range of 500 to 4000 cm⁻¹), Field Emission Scanning Electron Microscopy (FESEM) (TESCAN MIRA3 at 25 kV) and X-ray Diffraction (XRD) analysis (Shimadzu XRD-6000, within 2θ range of 10° to 80°) that helps in analysis of functional groups [2, 32, 33] surface morphology [34, 35] and crystallinity of adsorbent [36] respectively. 

Adsorption experiments 
Batch adsorption study was carried out for investigating the effect of equilibrium time (0 min to 220 min), pH (1.2 to 10), adsorbent dose (0.001g to 0.08g) and concentration of different salts i.e., NaCl, KCl as well as CaCl2 (0g/L to 0.2g/L). Temperature for all experiments was maintained at 20°C with shaking speed of 120 rpm, Cd (II) ions concentration of 200 mg/L and neutral pH (except pH study). The nano hydrogel dose used for each study was 0.05g and time of adsorption was 90 min unless otherwise specified. Once the adsorption equilibrium was achieved, % removal of adsorbate and adsorption capacity of adsorbent can be calculated using Eq. 1: 

where  and  refers to concentrations of Cd (II) ions in solution (mg/l) both before and after adsorption,  and  denote used volume (mL) of solution and adsorbent weight (g) correspondingly. For investigating effect of solution pH on swelling property of prepared nano hydrogel, 0.05g of it was immersed in solutions of varying pH i.e., 3 to 10. After immersion period, filtration of nano hydrogel was carried out for removing excess water and its weight was measured again. Afterwards, swelling ratios calculation was done by Eq. 2: 

here,  and  refers to weight of nano hydrogel that gets swelled and weight before swelling (dried form) correspondingly. 
Two different kinetic models namely pseudo first as well as pseudo second model applied to data obtained from kinetic study. Pseudo first order model assumes that adsorption rate has direct relation with number of available adsorption sites of adsorbent. In contrast, pseudo second model assumes that adsorption rate is directly related with square of available adsorption sites. Expressions for pseudo first and pseudo second order model are given in Eqs.3 and 4 respectively: 

here  (mg/g) and  (mg/g) refers to adsorbate amount adsorbed at time  (min) and at equilibrium, correspondingly. Further,  (1/min) and  (g/mgmin) are rate constant for pseudo-first and second model correspondingly [37]. 

   
RESULTS AND DISCUSSION 
Characterization study 
The FTIR spectra of CS-g-P(CMC-co-AM) nano hydrogel before and after Cd (II) ion adsorption revealed significant changes, providing insights into adsorption mechanism. Before adsorption (Fig. 2), the broad peak at 3400-3200 cm⁻¹ indicated the presence of hydroxyl and amide groups. After adsorption, a shift in these peak suggested interactions with Cd (II) ions. Furthermore, the peak at 2900 cm⁻¹, associated with C-H stretching, remained relatively unchanged. However, significant changes were observed in 1750-1500 cm⁻¹ region [5, 17, 19, 38, 39]. Results revealed that peaks related to C=O stretching of carboxylate groups undergo shifting in both intensity and position highlighting the presence of strong interactions between adsorbent functional groups and Cd (II) ions. Peaks around 1000-1200 cm⁻¹, attributed to C-O stretching, also undergo changes, that further corroborate the contribution of hydroxyl as well as carboxylate groups in Cd (II) ions removal. This variation in peaks highlight the contribution of hydroxyl, carboxylate, and amide functional groups of prepared nano hydrogels for cadmium ions adsorption [38, 40-46]. 
The crystallographic study of nano hydrogel (Fig. 3) revealed the presence of broad peak present nearly at 2θ of 20°. The peak corresponds to an amorphous structure of nano hydrogel due to the lack of long-range crystallinity. The highly amorphous nature of the nano hydrogel is mainly responsible for Cd (II) ions adsorption [47].    
The morphological analysis of nano hydrogel is shown in Fig. 4 highlighting the presence of numerous pores and heterogeneity in its structure. This porous structure is responsible for providing numerous adsorption sites for Cd (II) ions adsorption thereby results in improving its adsorption capacity. Study revealed that after adsorption of Cd (II) ions, the structure of nano hydrogel transit from heterogeneous to homogeneous one signifying saturation of active sites present on adsorbent surface with adsorbed Cd (II) ions [10, 48]. 

Kinetic modeling 
The results of contact time study in terms of % removal as well as adsorption capacity are shown in Fig. 5 and Table 1 highlighting the direct relation of adsorption time with Cd (II) ions removal. With increasing time from 1 min to 220 min, there was an increment in both percentage removal (from 89.81% to 93.357%) and adsorption capacity (increased from 35.92 mg/g to 37.343 mg/g). This can be better explained by accessibility of free active sites on adsorbent that allows maximum number of ions to get adsorb on adsorbent surface. However, no significant change was observed in Cd (II) ions adsorption after 90 min and 220 min where removal percentage was 93.32% and 93.35% and adsorption capacity observed to be 37.33 mg/g and 37.343 mg/g respectively. Due to this slight variation in results, 90 minutes was selected as the optimal contact time [5]. 
The results of kinetic study for pseudo first order and pseudo second order model are shown in Fig. 6a and 6b respectively. It was observed that there exists greater difference between calculated (0.02 mg/g) as well as experimental [37.34 mg/g) adsorption capacities for pseudo first model. Additionally, value of regression coefficient also observed to be lesser i.e., 0.6556 when compared with the results of pseudo second order model. These finding revealed the unsuitability of pseudo first model with the studied data. Owing to R² = 1 and less variation between experimental qe (37.34 mg/g) and calculated qe (37.31 mg/g), as summarized in Table 2, pseudo second order model fits best to studied adsorption process. The fitness of pseudo second model confirmed the chemical adsorption between adsorbent surface and Cd (II) ions [49].  

Effect of solution pH  
Solution pH is an important parameter that affect the charge of both adsorbent surface and adsorbate thereby affecting the overall adsorption capacity of adsorbent. To optimize best pH for study, experiments were carried out by varying pH (Fig. 7 and Table 3). Results showed that by increasing pH from acidic to basic, there was an increment in adsorption capacity of nano hydrogel. Adsorption capacity of 37.03 mg/g and 38.04 mg/g was observed by increasing pH from 1.2 to 10 respectively. Reason behind this was that at lower solution pH, i.e., acidic pH, the surface of adsorbent become positively charged that result in causing repulsions between positive Cd (II) ions and positively charged adsorbent surface. However, increasing pH from acidic to basic results in inducing native charge on adsorbent surface that thereby results in adsorbing cationic Cd (II) ions via electrostatic attraction [9].  
Swelling property of prepared nano hydrogel was investigated by varying solution pH from 3 to 10 (Fig. 8 and Table 4). It was observed that at lower solution pH i.e., 3, minimal swelling i.e., 60% can be observed. This swelling was increased to 920% when solution pH was 10. The reason behind this behaviour is that at lower pH values, water absorption was less due to the compact structure of nano hydrogel. However, an increase in solution pH allow for greater water uptake thus results in remarkable increase in swelling ratio [35].  \

Effect of nano hydrogel dose and salt concentration 
Effect of adsorbent dose on adsorption of Cd (II) both in terms of % removal and adsorption capacity were studied and results are presented in Fig. 9 and Table 5. It was observed that increasing nano hydrogel dose from 0.001g to 0.05g, there was an increment in % removal i.e., from 91.70% to 95.65% respectively. This was mainly due to the increased availability of active sites for adsorption of Cd (II) ions with an increase in adsorbent dose. However, with further increase in adsorbent dose to 0.08g, a decrease in % removal i.e., 94.53% was observed that might be due to agglomeration effect of adsorbent particles at higher dose. In terms of adsorption capacity, an inverse relation with an increase in adsorbent dose was observed i.e., by increasing dose from 0.001g to 0.08g, there was a continuous decrease in adsorption capacity from 1834.14 mg/g to 23.63 mg/g respectively. The reason of this trend was that with an increased % removal of Cd (II) ions, the availability of free actives sites for more adsorption decreased (since maximum adsorption sites are occupied by the adsorbate) thereby results in decreasing adsorption capacity [12, 35].
Adsorption capacity of prepared nano hydrogels was also investigated in presence of different salts i.e., NaCl, KCl and CaCl2. Findings of study (Fig. 10 and Table 6) revealed that the adsorption capacity of adsorbent increased with an increase in concentration of all salts being studied. However, the maximum results were obtained with NaCl when compared with KCl and CaCl2 due to the competitive effect of Cd (II) ions with ions of KCl and CaCl2. 
  
CONCLUSION 
Results of the study highlight the adsorptive potential of prepared poly (carboxymethyl cellulose-co-acrylamide) (CS-g-P(CMC-co-AM)) nano hydrogel to adsorb Cd (II) ions from water. Characterization of prepared nano hydrogels revealed presence of ionic functional groups within the nano hydrogel, chiefly hydroxyl, carboxylate, and amide groups that are mainly responsible for adsorption of Cd (II) ions on its heterogeneous and amorphous surface. Results of batch adsorption studies showed that adsorption efficiency increased with an increase in contact time and equilibrium was achieved within 90 minutes at higher pH levels (i.e., 10). The process followed pseudo-second kinetic model, suggestive of chemically controlled mechanism. The study highlights that the CS-g-P(CMC-co-AM) nano hydrogel is a low-cost, efficient adsorbent for removing cadmium ions from water. 

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