Kinetic, Isotherm, and Thermodynamic Study of Bismarck Brown Dye Adsorption onto Graphene Oxide and Graphene Oxide-Grafted-Poly (n-butyl methacrylate-co-methacrylic Acid)

The adsorption behavior of Bismarck brown (BB) dye from aqueous solutions onto graphene oxide GO and graphene oxide-g-poly (n-butyl methacrylate-co-methacrylic acid) GO-g-pBCM as adsorbents was investigated. The prepared GO and GO-g-pBCM were characterized by Fourier transform infrared spectroscopy FTIR, which confirmed the compositions of the prepared adsorbents. Adsorption of BB dye onto GO and GO-g-pBCM was explored in a series of batch experiments under various conditions. The data were examined utilizing Langmuir and Freundlich isotherms. The Langmuir isotherm was seen as increasingly reasonable from the experimental information of dye on formulating adsorbents. Kinetic investigations showed that the experimental data were fitted very well to the pseudo-second-order model, and the calculated positive values of the (∆H° and ∆S°) indicated that the adsorption of BB dye onto GO and GO-g-pBCM was endothermic and increasing of the adsorption process randomness. The negative values of (∆G°) imply that the adsorption process was spontaneous.


Introduction
Water contamination is one of the greatest dangers confronting humankind, which is ascribed to the fast industrialization and has been a significant subject of research overall 1 . Organic and synthetic dyes, the contaminant effluents cleared from different food and fabric industries, are one of the key sources of environmental contamination owed to their non-biodegradable behavior 2 . The enormous amount of industrialized organic dyes is discharged by the textile industry into waterways and other water resources, which eventually spoil water quality 3 . The vast majority of the manufactured dyes contain complex structures having azo groups, exhibit serious harmfulness when breathed in utilizing drinking dyecontaminated water, which in the end causes cancers of various portions of the human body 4 . Hence, the exclusion of dyes from wastewater is basic for a more protected environment. Different techniques have been accounted for in the literature for the clearing of dyestuffs, including photocatalysis and adsorption 5 .
Some approaches have been proven to expel dyes from wastewater, for example, flocculation, and ultra-filtration, precipitation, and so on. These procedures are costly and don't eliminate the color which prompts other contamination issues 6 . Because of these downsides, the expulsion of color from nature has faced more challenges. Adsorption is as yet a favored technique by investigators on account of its easy procedure, high effectiveness, simple recuperation, and adaptability. Moreover, the adsorbent can be reused many times and the risky byproducts produced through degradation can be removed 7 .
Graphene oxide (GO), presents excellent adsorbent performance for heavy metals, dyes, and pharmaceutical antibiotics because of the abundant oxygen-containing functional groups on the twodimensional 2D large surface, such as hydroxyl, carboxyl, and epoxy groups 8 . These oxygencontaining functional groups not only can complex pollutants in wastewater but also provide the reactive sites of chemical reactions for functionalizing graphene oxide 9 . Functionalization of GO composites using small molecules 10 or polymers containing abundant carboxylic groups like poly (acrylic acid) and other analog polymers 11 , which have a strong affinity to most positively, charged organic pollutants and the ability to enhance water dispersibility. Recently, numerous researches have been carried out to explore the environmental applications of GO and GO-based composite adsorbents, and these composite materials revealed outstanding adsorption capacities against various toxic compounds in an aqueous solution 12 .
Herein, we report for using the prepared graphene oxide (GO) and graphene oxide-g-poly (nbutyl methacrylate-co-methacrylic acid) (GO-g-pBCM) as adsorbents for removal of Bismarck Brown (BB) dye from wastewater by adsorption process. The current research work will explore the mechanism of adsorption and adsorption kinetics of the dye. The adsorption isotherms will be designated by using Langmuir and Freundlich isotherms. Further, this paper also discusses various thermodynamic parameters such as Gibbs free energy change (ΔG°), the heat of adsorption (ΔH°) and entropy change (ΔS°) respectively.

Materials and Methods:
All chemicals utilized are of the reagent grades. They were purchased from Sigma-Aldrich Company and used without any further treatment.
Branson Ultrasonic™, Type DHA-1000, Fisher Scientific (USA) and basic 20 pH-meter, Type E-08328 Alella-Barcelona, with a combined glass electrode, Crison Instruments, S. A. (EU) were used in the preparation and adsorption experiments.
Infrared spectra of the GO and GO-g-PBCM composite were recorded on Shimadzu, FTIR-8400S spectrometer/Japan as KBr pellets in the region 400-4000 cm -1 to investigate the chemical structure. The absorbance intensity of BB was recorded over the range of 200-800 nm on a PG Instruments Limited, model, T80 (EMC-LAB) a UV-Visible spectrophotometer/Germany using quartz cell of 1cm length at λ max 457 nm.

Synthesis of Graphene Oxide (GO)
GO was prepared by a modified Hummers method 13 . 2.0 g graphite powder and 1.0 g NaNO 3 were dissolved in 46.0 mL H 2 SO 4 (98%) under an ice bath. The mixture was left with stirring for 15 min, and then 6.0 g KMnO 4 was gradually added to the suspension with lowering the stirring speed as conceivable to control the temperature below 20°C. The suspension was left stirring for an additional 2 hrs. Afterward, 100.0 mL of deionized water was added to the suspension with raising the controlled temperature, and it ought to be limited lower than 98°C. After 15 min, the suspension was then additionally diluted to 280.0 mL with warm deionized water and 20.0 mL H 2 O 2 (30%), thus causing changing the suspension color to bright yellow. At that point, the suspension was filtered and washed with a warm aqueous 5% HCl solution and deionized water, separately, until no sulfate was found, and the pH of the filtrate was adjusted to 7.0. The GO was dried under vacuum at 50°C.

Copolymerization of n-butyl methacrylate/methacrylic acid (pBCM)
Butyl methacrylate monomer (BM) was washed twice with 5% NaOH to remove inhibitor and twice with water, then it was dried over anhydrous MgSO 4 , then with calcium hydride, and freshly vacuum distilled before copolymerization. Methacrylic acid monomer (MAA) was distilled under reduced pressure before use in the copolymerization using benzoyl peroxide (Bz 2 O 2 ) as an initiator. The copolymerization was carried out in the round bottom flask of equimolar amounts of freshly distilled n-butyl methacrylate and methacrylic acid under dry N 2 gas in a water bath after adding benzoyl peroxide (1x10 -4 mole/L) at 70℃ 13,14 . The copolymer was precipitated into methanol. It was vacuum dried at room temperature after the filtration and washing with methanol.

Synthesis of Graphene Oxide-poly (n-butyl methacrylate-co-methacrylic acid) (GO-g-pBCM)
The GO-g-pBCM composite was obtained by grafting polymerization. pBCM (0.3 g) and EDC (1ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride) (0.03 g) was added to the GO (0.3 g) dispersed in (7.5 mL) of phosphate buffer solution and the reaction mixture was sonicated for (30 min.). Then, it was left with stirring for 24 hrs. at room temperature. Finally, the formed composites were separated, washed with DI water over three times, and dried as a dark grey powder for 24 hrs. under vacuum 15 .

Preparation of Aqueous Dye Solution
Bismarck Brown BB (FW=419.31) was used without any purification in this study, and its chemical structure is shown in Fig. 1. A stock solution of the dye 1000.0 mg/L was prepared for the adsorption experiments and then the required

Batch Experiments
The adsorption process was done using a 0.1L of dye solution with an initial concentration of 400.0 mg/L as an initial concentration of adsorbents GO and GO-g-pBCM by contacting a 25.0 mg as a fixed weight of adsorbents for each of the initial concentrations, and the solutions were shaken at 200 rpm for 24 hrs. at 27℃. The equilibrium concentration was determined at λ max 457 nm for BB dye by using a UV-Visible Spectrophotometer.
The amounts of adsorbed dyes per gram of adsorbents GO and GO-g-pBCM (q e ) on the prepared adsorbents was calculated from the difference in dye concentration (mg/L) in the aqueous phase (C o -C e) before and after adsorption respectively, using the equation given below: Where Co and Ce (mg/L) are the initial and the equilibrium concentrations of dyes in the solution, V (L) is the volume of dye solution, m (g) is the mass of the used adsorbents in the experiment, and qe (mg/g) is the amount of adsorbed dyes per gram of adsorbents GO and GO-g-pBCM.

Adsorption Isotherms
There are various isotherm models to depict the isotherm results. In the present study, Langmuir and Freundlich's isotherm approaches are used to analyze the practical results 17 . Langmuir's approach 18 relies on the maximum adsorption that corresponds with the postulation of the liquid monolayer (adsorbate) on the solid (adsorbent) surface. The linearized type of Langmuir approach is shown below 19 Where C e (mg/L) the BB dye equilibrium concentration; q max (mg/g) the adsorption capacity required to complete monolayer on the adsorbent surface; q e (mg/g) the amount of adsorbate per unit mass of adsorbent at equilibrium (mg/g); K L Langmuir constant that relates to the energy of adsorption. Langmuir equation is valid for monolayer adsorption of the adsorbate onto the surface of the adsorbent and assumes there are restricted and homogenous adsorption sites. The fundamental attributes of a Langmuir model may be shown in terms of R L 20 which is characterized by: The other isotherm studied is Freundlich 21 ; the general Freundlich isotherm experiential equation is given by 22 : Where K F (L/mg) is adsorption constant and signifies the amount of adsorbed dye onto adsorbents at C e , and 1/n is the empirical parameter correlated to the intensity of adsorption, and its value varies according to the heterogeneity of the adsorbent. A favorable adsorption process occurs with a value between 0 and 1 23 .

Adsorption Kinetics
Kinetics results benefit to describe dye uptake rates, which control the staying time of adsorbate at the solid-liquid interface and give helpful data for adsorption process designing. The exploratory kinetic curves can be assessed using various models. In this study, the appropriateness of pseudo-first-order and pseudo-second-order were examined to interpret the mechanism of BB dye adsorption onto GO and GO-g-BCM. The linear form of the kinetic model equations (5 and 6 respectively) can be expressed as 24 : Where q t and q (1+2) are the dye adsorbed amounts at time t and equilibrium respectively; k 1 is the adsorption rate constant of the pseudo-first-order model; k 2 is the adsorption rate constant of the pseudo-second-order model.

Thermodynamic Studies
Thermodynamic parameters can be determined from the thermodynamic equilibrium constant (K) or known as a thermodynamic distribution coefficient 25. K is equal to: of thermodynamic findings helpful to determine the change of standard enthalpy (∆H°) and the change of standard entropy (∆S°), that they can be calculated from the following equation 26 : Where R is the universal gas constant (8.314 J/mol K), and T (K) is the absolute temperature, plotting of lnK L versus 1/T leading to calculate ∆H° (kJ/mol) from the slope and ∆S° (J K -1 mol -1 ) from the intercept. The standard Gibbs free energy ∆G° (kJ mol -1 ) values were computed for each temperature used in adsorption processes from equation 9 26 .

Results and Discussion: Characterization of Adsorbents by Fourier-Transform Infrared Spectrometry (FTIR)
The presence of the additional functional groups on the modified graphene oxide GO surface is studied using FTIR spectroscopy. The spectrum of layer graphite powder is almost featureless, whereas that of GO ( Fig. 2A) exhibits prominent bands at 3367 cm -1 , corresponds to hydroxyl stretching vibration, and at 1728 cm -1 corresponding to the stretching vibration of the carbonyl group (C=O), and the peak at 1620 cm -1 is attributed to aromatic C=C stretching vibrations. The FTIR spectra of GO and graphite are significantly different, where a high amount of oxygen-containing functional groups exist in the basal and edge planes of graphene oxide sheet 27 , and the differences are generally proportionate to those that have been described previously 28 , implying that graphene oxide was successfully prepared. The absorbance peaks of the GO-g-pBCM are observed at 1625, 1578, 1459, 1311, and 1160 cm -1 respectively as shown in Fig. 2B, combining those of GO and pBCM. Furthermore, the stretching vibration of the C=O peak shifts from 1728 cm -1 (GO) to 1625 cm -1 (GO-g-BCM), indicating interactions exist between GO and pBCM chains 29,30 .

Adsorption of BB Dyes onto GO and GO-g-pBCM
Adsorption experiments were run using the batch system technique for BB dye onto prepared adsorbents GO and GO-g-pBCM at optimum pH, contact time, and temperature.

Effect of pH
The pH of the dye solution plays a major role in the entire adsorption process, mainly on the adsorption capacity. The impact of pH on the adsorption capacities of prepared adsorbents was tested at the optimum initial concentration of BB dyes. Figure 3 shows the pH effect on the adsorption capacities at different pH values ranging from 3.0 to 12.0 for BB dye after adsorption onto GO and GOg-pBCM. The results achieved imply that the maximum pH of the adsorption process is dependent on the used adsorbate. Thus, the adsorption capacity seemed to be highly operative at pH 3.0 regardless of the nature of prepared adsorbents, where the q max recorded at 708.5 mg g -1 for GO and 741.2 mg g -1 for GO-g-pBCM. So, a decrease in adsorption capacity occurred with increasing pH values, and consequently, a pH 3.0 value was chosen for the followed adsorption experiments. Notably, the surface of the adsorbents contains different functional groups such as carboxylic and hydroxyl groups. So, the change in pH values of dye solutions affects the ionization of these groups in GO and GO-g-pBCM adsorbent 31,32 , and the optimum pH for adsorption depends on the types of adsorbate and adsorbent 33 .   Figure 4 shows the influence of contacting time on the adsorption of BB at pH 3.0 onto adsorbents GO and GO-g-pBCM respectively, at different temperatures. Seemingly, from Fig. 4A, the adsorption of BB dye rapidly increased from (1-45 min), then the equilibrium was attained within (45-75 min) for adsorbent GO. While the behavior of BB dye adsorption by GO-g-pBCM as shown in Fig. 4B has rapidly increased from (1-15 min.), then the equilibrium was reached within (15-45 min.). Thus, the optimum agitation times for all further experiments were chosen as 45 min for adsorption of BB dye by adsorbent GO, and 15 min for the adsorbent GO-g-pBCM. Also, it is found that the adsorption capacity for GO and GO-g-pBCM increased as the temperature rose from 27℃ to 60℃.

Adsorption Isotherms
The isotherm data are fitted to different isotherm models to find the most reasonable model that can be used to explain the adsorption process. The Langmuir isotherm depends upon the higher limit of adsorption coincides with a monolayer saturated surface of adsorbent by adsorbate. When plotting C e /q e versus C e , the slope of a plot equal to (1/q max ) and K L can be calculated from the intercept. Figure 5A shows the plots of Langmuir adsorption isotherms and Fig. 5B shows the Freundlich adsorption isotherms of BB dye adsorbed onto prepared adsorbents GO and GO-g-pBCM, and their parameters and the correlation coefficient R 2 are given in Table 1.
The obtained R 2 for the Langmuir is closer to one than that of the Freundlich isotherm equation as shown in Table 1, and are arranging between 0.9987≤R 2 ≤0.9992. This describes a monolayer adsorption process onto a surface containing a finite number of adsorption sites on the prepared adsorbents. Also, R L value for BB dye is less than one (0.0194 ≤R L ≤ 0.0963), which implies the favorability of the studied adsorbents for the adsorption process. This also suggests the creation of a BB monolayer on the GO and GO-g-pBCM adsorbent surface 34 .

Kinetic Study
Two kinetic models were tested to explain the adsorption mechanism of BB dyes onto GO and GO-g-pBCM. The first model was the pseudo-firstorder. Plotting of ln (q e -q t ) against t (according to eq. 5) will result in a straight line of the slope of k 1 and intercept of ln (qe). Figure 6 exhibits the pseudo-first-order equations for adsorption BB dye onto adsorbents GO and GO-g-pBCM respectively at temperatures 27, 40, and 60℃. The second kinetic model was pseudo-second-order. The q 2 and k 2 are determined from the plot of t/q t versus t (according to equation 6) from the slope and intercept, respectively. They are displayed in Fig. 7 for the adsorption of BB dye onto GO and GO-g-pBCM at different temperatures (27,40, and 60℃). All results obtained are summarized in Table 2.  The R 2 values from two kinetics models for BB dye adsorption are R 2 ≤0.9960 and R 2 ≤0.9993 for the pseudo-first-order and pseudo-second-order respectively. Furthermore, the calculated q max is highly effective with the experimental data. This suggests that the kinetic modeling of the temperature effect is more likely to fit the model of the pseudo-second-order adsorption 35 .

Thermodynamic Study
The enthalpy, entropy, and the free energy changes for the adsorption of BB dye onto GO and GO-g-pBCM are calculated as per the equilibrium data, and their values are shown in Table 3 and Fig.  8 demonstrates the ∆H° and ∆S° for adsorption of BB dye onto GO and GO-g-pBCM respectively. At all studied temperatures, ∆G° was negative implying that the adsorption process is spontaneous and reflecting the affinity of adsorbents towards BB dye 36 . The positive value of ∆H° verifies the endothermic character of the adsorption, also the positive value of ∆S° explains the increase of the adsorption process randomness and affinity of the adsorbents for BB dye 37 .

Conclusions:
In this work, GO and GO-g-pBCM synthesized based on graphite were investigated to evaluate the removing applicability of BB dye from the aqueous solutions. The highest adsorption occurred to BB dye with pH 3.0 for both adsorbents GO and GO-g-pBCM. It is found that the maximum adsorption capacity, q max , values are 833.3 and 1111.1 mg/g for BB dye adsorbed onto GO and GO-g-pBCM respectively. Depending on the R 2 values, the pseudo-second-order model accurately expressed the adsorption kinetics of the BB dye onto both adsorbents. Thermodynamic parameters showed that the spontaneous nature of adsorption systems of the adsorbents and confirm the endothermic nature of the adsorption.

Authors' contributions statement:
Hadi S. Al-Lami and Ali A. Abdulwahid suggested and planned the current research idea. Alaa A. Mizhr designed and carried out the experiments. All authors contributed in the analysis and discussing of the results and writing the manuscript.