Removal of Carcinogenic Dyes Congo red (CR) and Bismarck brown Y (BBY) by Adsorption onto Reusable Hydrogels Derived from Acrylamide

Hydrogels of acrylamide (AM), acrylamide\ 2-acrylamido-2-methyl-1-propane sulphonic acid (AMS), and acrylamide\ 2-acrylamido-2-methyl-1-propane sulphonic acid\graphene oxide (AMSGO) were prepared as adsorbents to remove carcinogenic dyes Congo red (CR) and Bismarck brown Y (BBY) from aqueous solutions. Hydrogels were characterized using FSEM and XRD analyses. For both dyes, the synthesized hydrogels demonstrated high adsorption capability at near-neutral pH. Experimental adsorption data were analyzed using the Langmuir and Freundlich isotherm models. It was found that the Langmuir model was more suitable for the experimental data. Kinetic studies found that the pseudo-second-order model demonstrated the best fitting to the experimental data. In addition, thermodynamic studies suggest that the adsorption process was spontaneous and endothermic. The prepared hydrogels were regenerated and reused in four consecutive cycles and it could be applied to remove anionic dyes from aqueous solutions as an effective adsorbent.


Introduction
Water is a vital component of both the natural ecosystem and human life. If there is any disturbance in the physicochemical characteristics of water would adversely affect both [1]. Water pollution is the contamination of water by an excess amount of a substance that can cause harm to human beings and/or the ecosystem. It's level subject to pollutant's abundance, pollutant's ecological impact, and water use [2] [3]. If these wastewaters are discharged without treatment, they will cause serious damage to the environment [4].
3rd international virtual conference of Chemistry (IVCC) 2021 Journal of Physics: Conference Series 2063 (2021) 012011 IOP Publishing doi: 10.1088/1742-6596/2063/1/012011 2 function such as hydroxyl (-OH), amine (-NH2), sulphonic (-SO3H), carboxyl (-COOH), and amide (-CONH2) groups [19]. The chemical structures, crosslinking densities, and compositions of the gels and their sizes have been important factors in their adsorption efficiency [20]. Also, due to their effortless handling, biocompatibility, environmentally friendly nature, and their associated reusability for the development of an effective desorption process [21], Polymeric hydrogels are very effective in dye adsorption. Porous hydrogel structure allows the dye solution to diffuse rapidly into the polymeric network and interact with its functional groups [22]. In past years, hydrogels derived from acrylamide (AM) have gained substantial attention for use as support carriers in biomedical engineering and as specific adsorbents. The introduction of the 2-acrylamido-2-methyl-1-propane sulphonic acid (AMPS) as a co-monomer along with AM can convey the pH responsiveness behavior in AM which can lead to and effectively remove the dye from the gel [23]. AMPS is an anionic monomer susceptible to pH, consisting of a highly ionizable sulfonate group where the acidic groups enhanced the polymeric network's hydrophilicity, which further increases the swelling behavior and may be used for the adsorption of dyes, and other organic contaminants [24]. Graphene oxide (GO) has been used in the hydrogel structure to improve the efficiency of adsorption and to enhance its physical properties. Dye adsorption can be improved by using GO due to its high specific surface area [25] and the presence of various oxygen-rich functionalities that allow either be covalent, non-covalent linkages, or a combination of both [26].
The present study reports an adsorption study of CR and BBY dyes by a hydrogel derived from acrylamide (AM), AM, and AMPS-based hydrogel, and its composite with GO as possible polymeric adsorbent was prepared via free radical polymerization. Here, our interest was to increase the adsorption capacity of AM hydrogels with highly hydrophilic functional groups containing chemical reagents such as AMPS with GO. This hydrogel is used as an adsorbent for the removal of CR and BBY dye from its aqueous solution. Batch adsorption studies are conducted.

Materials and measurements
All chemicals used were analytical reagent grades. They were used without further purification. Congo red (CR) and Bismarck brown Y (BBY) days were purchased from Sigma-Aldrich Company. The dye solutions were prepared in distilled water by dissolving accurately weighed dyes. Some characteristics of dyes were given in table 1. Table 1. Some characteristics of dyes.

Preparation of graphene oxide (GO)
Modified Hummers method for synthesis graphene oxide was carried out [27]. This was included dissolving 2.0 g of graphite and 1.0 g of NaNO3 in 46.0 ml of concentrated H2SO4 under an ice bath. After about 15 min of stirring, 6.0 g of KMnO4 was gradually added to the suspension by stirring as slowly as possible to control the reaction temperature below 20 °C. The suspension was stirred for two hours and then kept for 30 minutes at 35 ° C. 100.0 mL of deionized water was slowly poured into the suspension, allowing the temperature to rise steadily and maintaining the temperature below 98 °C. After 15 min., the suspension was further diluted with warm deionized water to approximately 280.0 ml. 20.0 mL H2O2 (30%) was added to remove the residual KMnO4 and MnO2 to change the color into luminous yellow. Then, the suspension was filtered and washed until no sulfates were found and the pH of the filtrate was changed to 7 percent aqueous HCl aqueous solution and deionized water, respectively. Graphene oxide GO was dried under vacuum at 50 °C.

Preparation of Acrylamide Hydrogel (AM)
A 1.0 g of Acrylamide (Am), and N, N\-methylene bisacrylamide as crosslinker agent (0.2 g, 20% compared to monomer) were dissolved in 5 mL distilled water with stirring. To this homogeneous solution was then added 1 mL ammonium persulfate (10% W/V) was added as an initiator of the polymerization reaction followed by the addition of 5 drops from N, N, N\, N\-tetramethyl ethylene diamine (TMEDA) as an accelerator to the decomposition of initiator with stirring at room temperature for three minutes to complete the polymerization reaction. The resulting hydrogel was dried and washed three times with water to remove any residue of monomer and initiator, filtered, and dried under vacuum at 30 °C.

Preparation of Acrylamide\ 2-Acrylamido-2-methyl-1-propane sulphonic acid (AMS)
A 1.0 g of AMPS was dissolved in 5 mL of distilled water at 35 °C with stirring then 1.0 g of Am and (0.4 g, 20% compared to monomers) N, N\-methylene bisacrylamide as crosslinker agent in 5 mL distilled water was added with stirring until the solution became homogenous. Then added 1 mL ammonium persulfate (10% W/V) was added as an initiator of the polymerization reaction followed by the addition of 5 drops from N, N, N\, N\-tetramethyl ethylene diamine (TMEDA) as an accelerator to the decomposition of initiator with stirring at room temperature for three minutes to complete the polymerization reaction. The resulting hydrogel (Scheme 2.5) was dried and washed three times with water to remove any residue of monomer and initiator, filtered, and dried under vacuum at 30 °C.

Preparation of Acrylamide\ 2-Acrylamido-2-methyl-1-propane sulphonic acid\ Graphene Oxide
Composite Hydrogels (AMSGO) A 1.0 g of AMPS was dissolved in 15 mL of distilled water at 45 °C with stirring then 1.0 g of Am and (0.4 g, 20% compared to monomers) of N, N\-methylene bisacrylamide as crosslinker agent in 5 mL distilled water were added with stirring until the solution becomes homogenous then 0.1 g GO was added to the monomer's solution. Then added 1 mL ammonium persulfate (10% W/V) was added as an initiator of the polymerization reaction followed by the addition of 5 drops from N, N, N\, N\-tetramethyl ethylene diamine (TMEDA) as an accelerator to the decomposition of initiator with stirring at room temperature for three minutes to complete the polymerization reaction. The resulting hydrogel (Figure 1) was dried and washed three times with water to remove any residue of monomer and initiator, filtered, and dried under vacuum at 30 °C.

Characterization
The morphology of the composite and hydrogels was measured by field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 450, FEI, USA). The patterns of X-ray diffraction (XRD) were obtained using a Rigaku X-ray Powder Diffractometer (Japan) at a scanning speed of 2 min -1 from 5 to 80° (Cu Kα radiation with a wavelength of 1.5406 Å). Absorption studies were determined by UV-Vis spectrophotometer (T80+, PG InL., UK) between (200-800) nm using quartz cuvettes with a path length of 1 cm.

Preparation of Aqueous Dyes Solutions
For the adsorption experiments, a stock solution containing 2000 mg L -1 of CR and BBY dyes was used. A dilution using deionized water was provided with the required concentrations.

Adsorption experiments
Equilibrium adsorption isotherms were determined using the batch studies. Batch adsorption experiments were conducted in stoppered flasks containing 25.0 mg adsorbents and 100 mL of dye solutions with different concentrations (100-2000 mg L -1 ). The adsorption experiments were performed on a thermostat shaker for a night at room temperature (27 °C) at 200 rpm. Then, the hydrogels were filtered off, and the concentration of the remaining dye was determined using a UV-Vis spectroscopy at 494 and 457 nm for CR and BBY, respectively. The effect of adsorption isotherms was studied by carrying out a series of isotherms at different temperatures (27,45, and 65 °C) and pH= (2-12). After adsorption, the adsorbent was separated from the solution by filtration and the concentration of remaining dyes was measured using UV-Visible Spectrophotometer. Adsorption processes were conducted at optimum conditions applying agitation time (60, 45, and 45 min), with a pH of 5.0 for AM, 7 for AMS, and AMSGO of the CR dye, respectively. While agitation time (60, 30, and 30 min), with a pH of 5.0 for AM, 6 for AMS, and AMSGO of the BBY dye, respectively. The amount of the CR and BBY dyes on the prepared adsorbents in the equilibrium (qe) were calculated from the following equation [28]: where Co and Ce (mg L -1 ) are the initial and equilibrium concentrations, respectively of the dye in solution, V (L) is the volume of dyes solution and W (g) is the weight of a hydrogel, and qe (mg/g) is the amount of adsorbed dyes per gram of adsorbents (adsorption capacity).
In the study of adsorption kinetics and the calculation of kinetic parameters, enthalpy (ΔH°), entropy (ΔS°) and free energy (ΔG°), experiments were conducted at (25,45 and 65 °C) with 25.0 mg of hydrogels at 0.1 L of (400.0, 1200.0 and 2000 mg L -1 ) CR solution for AM, AMS, and AMSGO, respectively. While 25.0 mg of hydrogels in 0.1 L of (300.0, 1200 mg L -1 ) and 0.2 L of 1000 mg L -1 , BBY solution for AM, AMS, and AMSGO, respectively. The maximum adsorption capacity was calculated at the optimum pH by changing the pH of the dye solution from 2.0 to 12.0; the pH of the dye solution was adjusted using 0.10 M HCl or NaOH solution.
Four adsorption/ desorption experiments were performed to desorb the dye from hydrogels under the following criteria: using the same adsorbents, maximum adsorption of dye conducted the optimum agitation time and the pH-value for each adsorbent in this work. The desorption experiments were carried out by immersing the dye-loaded adsorbent in 100.0 mL of distilled water and the mixture was continuously stirred for 60 min at room temperature, and the desorbed dyes were separated by centrifugation and filtration, then spectrophotometrically determined the concentration of dyes [29]. The efficiency of dye desorption removal (S) was determined by using the following equation [30]:

Field Emission Scanning Electron Microscopy (FESEM)
The external surface morphology of prepared hydrogels was analyzed by FESEM (Figure 2a-d). The hydrogel micrographs indicate that they have porous structures. From figure 2a, it can be seen that GO shows a layered and wrinkle-like structure which is due to graphite deformation upon the exfoliation process. Also, the surface appears somewhat bristly with edge planes which can enhance the interaction with dye [31]. On the other hand, as demonstrated in figure 2b, The AM hydrogel shown a smooth and neat surface morphology, the porosity was further enhanced with the incorporation of AMPS in AMS hydrogel ( Figure 2c). Also, after the addition of GO into the AMS hydrogel, the surface morphology of the AMSGO composite hydrogel becomes rougher and the AMSGO shows an irregular, plat-like structure ( Figure 2d). Furthermore, it can be seen that the GO platelets are well distributed as individual platelets in the hydrogel network and no apparent aggregation has been observed.

X-ray diffraction (XRD)
The XRD was used as an important method for studying the crystal structure of surface materials. XRD patterns of the GO, AM, AMS, and AMSGO are depicted in figure 3(a-d). As figure 3a for the GO, an intense peak was seen at 2θ= 10.04° was attributed to the diffraction of (001) crystal plane, which was the characteristic peak of GO. The corresponds to an interlayer distance (d) is 0.876 nm, which was attributed to the presence of oxygenated functional groups on GO sheets. After 12°, The curve progressively tends to be flat, suggesting that the successful oxidation of natural graphite [32]. However, both AM, AMS hydrogels, and AMSGO hydrogel composite exhibit amorphous characters, as shown in figure 2b and c, it also shows a peak at the angle range of (15-30 • ), which indicates the amorphous nature of the cross-linked chemical structure of both the hydrogels and its composite [33]. As it was observed in figure 2b and c, a broad non crystalline diffraction peak at (2θ= 22.677°), (2θ= 20.783°), corresponding to an interlayer spacing of (d= 3.9180A0), (d= 4.2706A0), respectively [34]. When comparing the XRD pattern of the GO with the composite hydrogel, clearly noticed the disappearance of the peak at 2θ = 10.04° in the XRD pattern of the AMSGO, and this implied that the GO sheets are completely exfoliated and dispersed evenly in the hydrogel network [32] [33].

Dye Adsorption by the Hydrogel
The CR and BBY dyes adsorption capacity of the prepared hydrogels were examined. The effects of various variables on dye removal were investigated, such as initial dye concentration, pH, time, and temperature, and also other kinetic, isothermal, and thermodynamic parameters.

Effect of pH on Adsorption
The solution's pH value plays an important factor in the adsorption capacity of the dyes onto the hydrogels. The effect of pH on dye's adsorption capacity by prepared hydrogels was investigated at an optimum initial concentration of both CR & BB dyes. The pH was adjusted at the range from 2.0 to 12.0 by 0.1 M NaOH and 0.1 M HCl solution. Figures 3 (a and b) show the pH effect of the adsorption capacities for CR and BBY dyes onto AM, AMS, and AMSGO at 27 °C, respectively. It is apparent that with the increase of pH values, the adsorption capacity of both dyes first increase and then decrease. The maximum adsorption capacity was reached as the pH value approaches 7. With the increasing pH values from 3 to 7, the sulfonic and carboxyl acid groups onto the hydrogels exist as -COOH and -SO3H, which was best suited to the formation of hydrogen bonds between dyes and hydrogels. when pH exceeds 9, adsorption levels of both CR and BBY dyes decrease due to the repulsion of anionic groups on the hydrogel [32].
The results suggest that the optimal pH value was based upon the type of adsorbate used for the adsorption process. Notably, the surface of all hydrogels includes various functional groups, such as sulfonic, carboxylic, and amine groups, these functional groups can ionization by changing the pH values of dye solution.

Effect of Time and Temperature on Adsorption
The effect of contact time on adsorption of CR and BB dyes onto the prepared hydrogels, at the initial concertation of each adsorbent and optimum pH at different temperatures was investigated. The results are shown in figure 6 and 7, reveals that the adsorption of CR and BBY has rapidly increased from 1 to 60 min, and then the equilibrium is attained within (60-120 min) for AM hydrogel. Also, the behavior of CR dye adsorption by adsorbents AMS and AMSGO as shown in figure 4 has increased rapidly from (1-45 min), and then the equilibrium is achieved within (45-90 min). While figure 5 shows a rapid increase for adsorbents AMS and AMSGO from 1 to 30 min, then the equilibrium is reached within (30-75 min). The trend for both dyes is more or less the same. Initially, the adsorption rate of both dyes was higher because of the availability of a wide number of active sites for adsorption in the hydrogel structure. The adsorption rate gradually decreased due to a reduction in the number of adsorption active sites until equilibrium was achieved. Also, the temperature increases from 25 to 65 ° C, the adsorption capacity for all hydrogels are raised. Since the adsorption of CR and BBY were endothermic, this pattern is predicted for the adsorption capability of the adsorbents. Therefore, for all further experiments, the optimal contact times were selected as 60 min for adsorption of CR dye by adsorbent AM and 45 min for both adsorbents AMS and AMSGO. Whereas 60 min for adsorption of BBY dye by AM adsorbents, and 30 min for adsorption of BBY dye by AMS and AMSGO.

Adsorption Isotherms
The adsorption isotherms indicate the interaction and equilibrium relationship between adsorbate and adsorbent at a constant temperature [35]. It is necessary to find best-fitting isotherms to describe the adsorption process [36] [37]. Adsorption data such as Langmuir and Freundlich isotherms are usually described by adsorption models [38] [39]. The Langmuir model describes the mechanism of the adsorption of substances on the surface uniformly [19]. It is based on the homogenous monolayer formation of dyes on the surface of hydrogels [24]. The Langmuir isotherm is represented by the following equation [40]: where qe is the amount adsorbed, Ce the equilibrium concentration, qmax the maximum adsorption capacity reflected on a complete monolayer, and KL represents the Langmuir constant that is related to the apparent energy of adsorption. A plot of Ce/qe versus Ce should indicate a straight line of slope (1/qmax) and an intercept of 1/(KL qmax). The essential characteristics of the Langmuir isotherms can be described by a separation factor or equilibrium parameter (RL), which is defined by the following equation [83]: where RL values indicate the type of adsorption to be irreversible (RL= 0), favorable (0 < RL< 1), liner (RL= 1) or unfavorable (RL> 1). This model is the most commonly used isothermal adsorption and has demonstrated good agreement with various experimental data [41] [42]. The plots of the Langmuir adsorption isotherms of CR and BBY adsorbed on hydrogels AM, AMS, and AMSGO are shown in figure 6 (a and b), respectively, and table 2 presents the isothermal constants, qmax, KL, RL, and the correlation coefficient (R 2 ). As listed in table 2, the KL offers information on the strength of adsorbateabsorbent interaction. High the KL value, the interaction is stronger and vice versa. In the present study, the KL value for all prepared hydrogels was very small, this demonstrates that the interaction between these materials and dyes is weak [24]. Besides, Since the RL value was found to be (0 < RL <1), the favorable isotherm has been confirmed and the R 2 was closed to unity, this confirmed that the Langmuir isotherm is applicable for the adsorption of dyes onto the prepared hydrogels [16] under the applied optimum conditions. While in the Freundlich model, which is based on adsorption on the surface of a heterogeneous multilayer, and assumes that the adsorption takes place at sites with varying energy of adsorption [43]. The Freundlich isotherm is represented as follow [44]: where, KF and 1/n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. KF indicates the adsorption capacity of the adsorbent toward the adsorbate and the 1/n is an indicator for the degree of the surface heterogeneity which describes the distribution of the adsorbed molecules on the adsorbent surface. Overall, a value of n higher than 1 indicates that the adsorbate is favorably adsorbed on an adsorbent, and the higher the n value the stronger the adsorption intensity [43]. Illustrated in figure 7 (a and b), the fitting of Freundlich adsorption isotherm of CR, and BBY adsorbed onto prepared hydrogels and the intercept and slope of the plots of ln qe against ln Ce were used to calculate the Freundlich isotherm constants KF, 1/n, and R 2 ( Table 2). The value of (1/n) indicates if the isotherm type is favorable for adsorption [16]. The adsorption process is favorable when the 1/n values are within the 0.1<1/n<1 range [45]. When the value of n is less than1, the adsorption involves a chemical process. Otherwise, the adsorption is a physical mechanism [45][46]. In all of these studies, the values of 1/n for dyes were between 0.1 and 1 suggesting that the adsorption of both dyes favored chemisorption [45]. Typically, if the value of the correlation coefficient (R 2 ) approaches unity, the adsorption mechanism is said to obey a certain isotherm model i.e., the Langmuir or Freundlich isotherm  [47]. Therefore, the dye adsorption by the synthesized hydrogels was consistent with the Langmuir isotherm model compare with that of the Freundlich isotherm, as indicated by the results shown in table 2.   Table 2. Isotherm parameters for adsorption of dyes by hydrogels.

Kinetics of Dyes Adsorption
For a better understanding of the adsorption mechanism, the adsorption kinetic curves of the CR and BBY dyes onto prepared hydrogels were studied by kinetic models. In three kinetic models, including pseudo-first-order, pseudo-second-order, and intraparticle diffusion, experimental values were introduced, using the following equations [48]: Where qt, q1, and q2 are the amounts of dye adsorbed by the prepared hydrogels at various time t, at equilibrium for the pseudo-first-order and pseudo-second-order, respectively. k1, k2, and kp are the adsorption rate constants of the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. C is the intraparticle diffusion constant, which was related to the boundary layer thickness. The experimental kinetic plots for the adsorption of CR and BBY dyes on prepared hydrogel are presented in figure 8-10 and figure 11-13, respectively. The kinetic parameters of kinetic models are summed up in tables 3 and 4: pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. Good matching of the experimental data to the kinetic models was assessed by a high correlation coefficient (R 2 ) and close accord of the calculated and experimental (q1 and q2) values [30]. For all the prepared hydrogels, for CR and BBY dyes, the strong and pronounced disparity between the experimental absorption capacity and the calculated absorption capacity shows the deviation from pseudo-first-order kinetics. While the theoretical equilibrium adsorption capacities obtained from the pseudo-second-order model correlated well with the experimental results, with better correlation coefficients (R2 > 0.999) values, thus indicating a well-fitting of the second-order model, compared to the pseudo-first-order kinetic model [49][50] Also, the adsorption rate k values were not always proportional to temperatures directly. The kinetic model of pseudo-second-order also notes that chemical adsorption is the rate-determining step. According to Eq. 8, if the key rate control step is intraparticle diffusion, the plot should be linear and pass through the origin. As illustrated in figures 10 and 13, the plots for CR and BBY dyes did not pass through the origin that suggests no intercept was found to be zero, which also implied that the diffusion intraparticle was not the only rate-determining step. From table 3 and 4, it is observed that values of kp did not always directly proportional with temperatures. Values of constants C were nonzero and they increased in increasing temperature.

Thermodynamic of Dyes Adsorption
To specify whether prepared hydrogels adsorption is an endothermic or exothermic process, the thermodynamic investigation was carried out at various temperatures by calculating thermodynamic parameters namely, standard free energy (ΔG°), standard entropy change (ΔH°) and standard entropy change (ΔS°) using the following equations [ (12) or (13). Also, From the Arrhenius equation, the activation energy (Ea) that can be defined as the minimum amount of energy needed to proceed with the adsorption process was determined by using the following equation [51]: where K is the rate constant of the pseudo-second-order kinetic model for adsorption system of CR and BBY dyes since the adsorption analyzes are based on the constant obtained from linearized plots R 2 closest to unity, A is the Arrhenius factor, a straight line with a slope of -Ea/R is obtained when ln K is graphed versus 1/T, as shown in figure 15. The calculated parameters for thermodynamics are shown in table 5. The positive value of ΔH° for the absorption process of CR and BBY dyes showed that the process was endothermic, which was supported by an increase in the absorption capacity of the prepared hydrogels. Similarly, the positive value of ΔS° for the adsorption of both dyes onto the prepared hydrogels suggests the high degree of randomness of the adsorption and affinity of the prepared hydrogels towards the dyes [11] [52]. Also, the ΔG° values at all temperatures studied were negative, which underpins adsorption's spontaneous nature and indicates that the adsorption was thermodynamically favorable [53]. As seen in Table 6, with an increasing temperature of (300. 15 -338.15) K, the removal of CR and BBY dyes was less spontaneous. Nevertheless, the spontaneity of sample AM is lower compared to AMS and AMSGO [24]. In addition, the Ea data for the adsorption of CR and BBY dyes were obtained and indicated in table 5., which confirmed the dominant reaction is the chemisorption phenomenon. Also, it can be observed that the adsorption of both dyes onto AM have lower Ea values (less than 40 kJ/mol), this result suggested that the adsorption mechanism was physical adsorption, but the modification of AM could have alternated the adsorption mechanism to chemisorption with a higher Ea value [54].  Table 5. Thermodynamic parameters for adsorption of CR and BBY dyes by the prepared hydrogels.

Desorption Studies
Desorption studies are essential to understand the mechanism of adsorption [30]. Also, Adsorbent regeneration might improve the adsorption process more economically. Thus, successive desorption processes on the dyes used to evaluate the reusability of the prepared hydrogels over four consecutive cycles were carried out [55] and results are given in table 6, indicate that the percentage desorption was good for the four cycles. Therefore, the prepared hydrogels could be used repeatedly with retaining their good adsorption capacity.