Isotherms, kinetics and thermodynamic mechanism of methylene blue dye adsorption on synthesized activated carbon

The treatment of methylene blue (MB) dye wastewater through the adsorption process has been a subject of extensive research. However, a comprehensive understanding of the thermodynamic aspects of dye solution adsorption is lacking. Previous studies have primarily focused on enhancing the adsorption capacity of methylene blue dye. This study aimed to develop an environmentally friendly and cost-effective method for treating methylene blue dye wastewater and to gain insights into the thermodynamics and kinetics of the adsorption process for optimization. An adsorbent with selective methylene blue dye adsorption capabilities was synthesized using rice straw as the precursor. Experimental studies were conducted to investigate the adsorption isotherms and models under various process conditions, aiming to bridge gaps in previous research and enhance the understanding of adsorption mechanisms. Several adsorption isotherm models, including Langmuir, Temkin, Freundlich, and Langmuir–Freundlich, were applied to theoretically describe the adsorption mechanism. Equilibrium thermodynamic results demonstrated that the calculated equilibrium adsorption capacity (qe) aligned well with the experimentally obtained data. These findings of the study provide valuable insights into the thermodynamics and kinetics of methylene blue dye adsorption, with potential applications beyond this specific dye type. The utilization of rice straw as an adsorbent material presents a novel and cost-effective approach for MB dye removal from wastewater.

In 2021, Jawad et al. 39 introduced a new approach to produce a novel active carbon, using dragon fruit peel, to treat MB dye from a desired solution.Ighalo et al. 40 studied the effect of the pore size on the kinetics of MB dye adsorption.However, despite many studies on activated carbon with different precursors, no in-depth research has been carried out to thermodynamically understand the mechanism of adsorption.Every year, millions of tons of straw are discarded globally as waste, which in addition to being environmentally unsustainable, destroys a huge source of precious materials 41,42 .Rice is among the globally cultivated crops, and Fathy et al. 43 studied the effect of rice straw modification by alkali acid on MB dye treatment in binary and single solutions.
The production of straw-based adsorbents is a promising way to use this valuable bioresource for the adsorption of MB from industrial wastewater.The purpose of the present study was to evaluate the effects of operational conditions, including pH, temperature, and MB dye concentration, on the removal efficiency.Thus, three different models of adsorption isotherms, Langmuir, Temkin, and Freundlich models, which differ in their assumptions, nature of the adsorbent surface, and shape of the isotherm, were used to correlate the process of adsorption.In this study, an analytical isotherm expression was developed to simulate the effects of pH and temperature on surface adsorption.The proposed model, which is based on the Langmuir isotherm, is a suitable alternative for complex simulations that require numerical solution methods.It was shown that for the obtained experimental data, there is a linear correlation between the isotherm coefficients and pH or temperature.
Several studies correlate the surface adsorption isotherm equations with the experimental data at different pH values.Eswad-El et al. 44 correlated the experimental data of the surface adsorption of Ni(II) and Cu(II) on humic acid adsorbents at different pH values using the Langmuir isotherm.Silber et al. 45 obtained experimental data for the surface adsorption of Zn ions on two different types of adsorbents, assuming a constant pH during the adsorption process, and correlated the surface adsorption model with the developed Langmuir model.Youcef et al. 46 applied the Langmuir isotherm model to study the adsorption of MB dye on Algerian palygorskite.
Mustafa et al. 47 correlated the experimental data for cadmium ion adsorption at different pH values using the Freundlich isotherm.Arora et al. 48also investigated the removal of MB dye from wastewater using the Freundlich isotherm.Regelink et al. 49 studied surface adsorption and precipitation of Ni ions.They compared the experimental data at five different pH values using different multilevel adsorption models.They also reported the amount of nickel ion adsorption with changes in pH.Han et al. 50correlated the experimental data obtained from the surface adsorption of uranium using the Langmuir and Redlich-Peterson models at different pH values.
Hu et al. 30 fitted the experimental surface adsorption data at different pH values using the Langmuir and Freundlich isotherm models and obtained a linear relationship between the isotherm parameters and pH.They also presented a modified Langmuir-Freundlich relationship and measured its validity using the obtained experimental data, which suggested that this model can predict surface adsorption at different pH values.
The use of statistical physics models in the adsorption of dye molecules on bioadsorbents has been a topic of interest in recent years.Several studies have investigated the application of statistical physics models to understand the adsorption mechanism and optimize the process.In this literature review, we will discuss some of the recent studies that have used statistical physics models to investigate the adsorption of dye molecules on bioadsorbents.
Khalfaoui et al. 51 used statistical physics modeling to investigate the adsorption of dyes on modified cotton.The study estimated all the mathematical parameters in the model, including the receptor site density and the half-saturation concentration.The results showed that the statistical physics model provided a good fit to the experimental data and could be used to optimize the adsorption process.
In another study, Raval et al. 52 used statistical physics modeling to evaluate the adsorption properties of chitosan-zinc oxide nanocomposites for the removal of an anionic dye.The study demonstrated an effective synthesis route that could be implemented to prepare new adsorbents using chitosan at a large-scale for water treatment.
In a study by Hanafy et al. 53 , statistical physics modeling was used to interpret the adsorption of dye remazol black B on natural and carbonized biomasses.The study employed natural and carbonized pine-fruit shells to study the adsorption of remazol black B dye from aqueous solution.The results showed that the statistical physics model could be used to predict the adsorption capacity of the adsorbent and optimize the process.
In a recent study, Zhang et al. 54 established an advanced statistical physics adsorption model to investigate the adsorption of Congo Red and Methylene Blue onto nanopore-structured Ashitaba waste and walnut shellbased activated carbons.The study demonstrated that the statistical physics model could provide a good fit to the experimental data and could be used to optimize the adsorption process.
Finally, in a study by Sangon et al. 55 , waste rice straw was used to produce highly effective carbon-based adsorbents for dyes removal.The pre-carbonized potassium hydroxide activated carbon was demonstrated to be highly effective at the adsorption of methylene blue and Congo red dyes.The study provides valuable insights into the use of waste materials for the production of effective adsorbents.
In this study, using a cheap material (i.e.rice straw) and a simple and economical method, an adsorbent that is selective and robust in adsorbing MB has been produced.In addition to the experimental study of MB dye adsorption on the fabricated adsorbent, the adsorption isotherms were investigated under different process conditions (e.g.temperature and pH).The goal of previously published papers in this field was to adsorb MB dye, whereas this study aims to optimize the process while understanding the thermodynamics and kinetics of the process.

Materials and chemicals
Rice straw was collected from irrigated and rain-fed fields in Jordan.Permission to collect rice straw was obtained from the relevant authorities.Experimental research and field studies on plants, including the collection of plant material, comply with relevant institutional, national, and international guidelines and legislation.Sulfuric acid Vol:.(1234567890 ) with purity of 99.999% and reagent grade anhydrous potassium hydroxide (KOH) pellets (≥ 98% purity) were obtained from Sigma-Aldrich.MB was also purchased from Sigma-Aldrich, and all chemical reagents were of analytical grade.The rice straw was cut into 0.5-cm pieces and dried at 90 °C.

Characterization
Scanning Electron Microscopy (SEM, Thermo Fisher, VolumeScope 2) was performed to characterize the morphology and structure of the pores on the rice straw surface.Moreover, Energy Dispersive X-ray Spectroscopy (EDX, Hitachi, Quantax75) was applied to detect the elemental content on the adsorbent surface, and the functional groups of the adsorbent surface were analyzed using Fourier transform infrared (FTIR, JASCO FT/IR-4X) analysis over the wavenumber range of 500-4000 cm −1 .

Experimental procedure
A novel and simple procedure was used to fabricate a low-cost adsorbent from rice straw.First, the rice straw was dried and soaked in an H 2 SO 4 solution (0.1 M) for 24 h, washed using deionized water, and dried overnight at 95 °C.Subsequently, the dried substances were soaked in KOH (0.2 M) for 24 h at 25 °C; it was then washed with deionized water and dried at 95 °C for 24 h.To prepare synthetic wastewater, solutions were prepared using an aqueous solution of MB dye.All experiments were performed in a 5-L flask at different pH and temperature values; pH adjustment of the solutions to the desired value was carried out using H 2 SO 4 or NaOH.
To calculate the thermodynamic adsorption isotherm parameters, the equilibrium adsorption process was investigated at different pH values (3-9) and temperatures (293-333 K).The adsorbent material was added to water, pH of the mixture was adjusted, and it was allowed to stand for approximately 30 min.The pH was measured using a pH meter (Milwaukee, MW802, Germany) before and after adsorption.Subsequently, different volumes of 4 mg/L MB dye solution were added to the adsorbent system.Finally, the mixtures were stirred for 20 h to reach equilibrium, and all the experiments were performed several times.
To analyze the efficiency of the treatment process, the concentration of the MB dye was measured at a wavelength of 664 nm using a Shimadzu 2570 UV-visible spectrophotometer.The adsorption capacity of MB was calculated using Eq. ( 1), where C 0 (mg/L) and C e (mg/L) are the initial and equilibrium concentration of the adsorbed dye, respectively, V is the MB dye solution volume in liters, and m is the dry weight of rice straw in grams.
The adsorption of the control group can be measured as the difference between the total MB amount and residual MB.HCl was applied to desorb MB at the end of the experiment.Therefore, the adsorption amount can be evaluated using Eq. ( 2), where C d (mg/L) is the MB concentration, V d is the volume of the dye solution, and m d is the dry weight of the rice straw.The adsorbed amount of MB dye was obtained as the difference between the total treatment amount and adsorbed amount after removal.
The concentration equilibrium relationship in the system was investigated using adsorption isotherms at a constant temperature.The maximum adsorption capacity was evaluated using equilibrium adsorption isotherms.Thus, three different adsorption isotherm models were used in this study, Langmuir, Temkin, and Freundlich isotherms, which differ in their assumptions, nature of the adsorbent surface, and shape of the isotherm.
Similar methods have also been used by other researchers to perform adsorption isotherms at different pH values 44,45,47,48 .
The Langmuir isotherm model 56 is applied to explain the nonlinear adsorption and proposes that take place on a homogeneous surface without interactions between the adsorbed species.In this model, each binding position accepts only one molecule and has the same energy.The Langmuir isotherm model is given by Eq. ( 3), which is in a linear form.
where q e and C e are the adsorption capacities at the equilibrium and equilibrium concentrations, respectively; K l and q m are the Langmuir constant and maximum adsorption capacity, respectively.According to Eq. (3), the Langmuir giving correlation coefficients (R 2 ) validity can be confirmed using a linear graph of C e q e versus C e .The reversible adsorption on different adsorption energy sites can be reported using Freundlich isotherm model as an empirical equation.
Freundlich equation describes the adsorption of a solute from a source to solid surface; it assumes that stronger binding sites are occupied first, and other sites are occupied in the decreasing order of energy.This model presents monolayer adsorption with an inhomogeneous energy distribution in the active sites.The Freundlich isotherm model, in linear form, is shown in Eq. ( 4), where K F is the Freundlich constant, and n is related to the adsorption intensity and called the heterogeneity factor.
( The third isotherm model is the Temkin isotherm model, which is described in Eq. ( 5), where T is the temperature, K T and b are adjustable constants, and R is the universal gas constant.

Results and discussion
An SEM image of the rice straw adsorbent is shown in Fig. 1.As can be seen from the SEM analysis, the surface pores of the adsorbent were reasonably uniform in the range of 45-55 μm, which caused dye adsorption.
Table 1 shows the contents of the different elements on the adsorbent surface, which were detected via EDX analysis.The adsorbent surface is dominated by C, Ca, O, Si, which were involved in the adsorption experiments, with values of 47.7, 1.6, 41.7, and 8.7 wt.%, respectively.
Figure 2 shows the functional group identification of the manufactured adsorbent, and the FTIR results are summarized in Table 2.The peak at 1060 cm −1 is attributed to the C-O group.The absorbance bands at 1370 cm −1 and 1630 cm −1 are assigned to the C-C and C=O groups, respectively; the peaks at 2800 cm −1 and 3450 cm −1 are related to C-OH and O-H groups, respectively.
To obtain various densities at different MB dye concentrations in the solution, the calibration curve for the MB dye was plotted in terms of concentration in ppm vs. wavelength reading, and the k factor was calculated as shown in Fig. 3.   Figure 4 shows the effects of temperature on the adsorption of MB dye.As shown in Fig. 4, a decrease in adsorption can be seen with increasing temperature, which can be attributed to MB adsorption being an exothermic process.The linearized model of the Langmuir isotherm at different temperatures is shown in Fig. 5.In all the experiments, the initial solution concentration of MB dye was 4 mg/L, and the amount of adsorbent was 1 g in 300 mL based on the MB dye concentration.
To correlate the adsorption process using isotherm models, three different adsorption isotherm models, Langmuir, Temkin, and Freundlich isotherms, were applied, and the isotherm parameters were determined using  www.nature.com/scientificreports/linearized experimental data.The measured parameters of the isotherms for MB dye treatment using modified rice straw at various temperatures are listed in Table 3. Adsorption isotherm constants can determine the characteristics of the surface, attraction of the synthesized adsorbent, and capacity for pollutant adsorption.The EDX analysis reveals that the adsorbent contained different surface functional groups that could be affected by the pH value in terms of adsorbing the substances.
Clearly, with an increase in the pH value, the amount of hydroxyl (OH -) in the system increases and surface charge of the adsorbent particles becomes more negative; therefore, MB adsorption on the adsorbent is predicted to increase.However, the pH domain is among the key characteristics of wastewater, which can affect the surface characteristics of the adsorbent by changing the ionization degree.The results showed that at acidic pH values, owing to the protonation of active sites and an increase in the charge density, the removal efficiency of MB dye is lower; at alkaline pH values, the electrostatic attraction between the ions and dye is observed.Figure 6 shows the effect of pH on the adsorption of MB dye.As shown in Fig. 6, an increase in pH causes an increase in adsorption, which can be attributed to the bonding between the MB and rice straw adsorbent being disrupted by H + ions at higher acidity.The linearized model of the Langmuir isotherm for different acidities is shown in Fig. 7.
To correlate the adsorption process using isotherm models, three different adsorption isotherm models, Langmuir, Temkin, and Freundlich isotherms, were used, and the isotherm parameters were determined by linearizing the experimental data.The isotherm parameters are listed in Table 4.
Because the Langmuir and Freundlich models both correlated well with the experimental adsorption data, the combined Langmuir-Freundlich model was used.Therefore, in the present work, the Langmuir-Freundlich isotherm was also applied to correlate the obtained data.The Langmuir-Freundlich equation, also known as the SIP equation, is a multidimensional equation that can predict the behavior of both Langmuir and Freundlich isotherms 57 .
Some studies have reported the adsorption capacity of adsorbent materials at different pH values and temperatures.For example, Xu et al. 58 reported experimental Pb (II) adsorption data on an MX-80 bentonite adsorbent at different pH values.Regelink et al. 49 reported Ni adsorption on a specific type of adsorbent at different   8 shows the relationship between the Langmuir and Freundlich isotherm constants and pH for MB dye removal using the modified rice straw adsorbent.As shown in Fig. 8, a linear relationship with pH was obtained.With an increase in the pH value, the equilibrium coefficient in the Langmuir-Freundlich relationship increases linearly.Figure 9 shows the relationship between the Langmuir and Freundlich isotherm constants and temperature for MB dye removal using the modified rice straw adsorbent.As shown in Fig. 9, a linear relationship with temperature was obtained; with an increase in temperature, the equilibrium coefficient in the Langmuir-Freundlich relationship increases linearly.The constants of the Langmuir-Freundlich relationship at various temperatures in presence of a similar adsorbent show a behavior similar to that at different pH values.

Adsorption thermodynamics
The free energy of adsorption can be obtained using Eq. ( 6), where K D can be evaluated at q e = 0 in the q e /C e vs. q e plot.
Therefore, ΔH and ΔS, as the thermochemical parameters, can be calculated by applying the van't Hoff equation (Eq.7).
The free energy, ΔG, of rice straw adsorption of MB is shown in Table 5.The calculated free energy values of MB adsorption on rice straw are negative, which indicates that the adsorption process can be carried out spontaneously.These results have been previously confirmed by other researchers 59,60 .
To analyze the obtained results, the free energy, ΔG, of MB adsorption on rice straw must be obtained; it is obtained using Eq. ( 6), and the results are listed in Table 5.To obtain ΔH and ΔS as the thermochemical parameters, Eq. ( 7), which is known as the van't Hoff equation, is used.Using Eq. ( 7), ΔH/R is −6167 kg K/mol and ΔS/R is equal to −17.3 kg/mol.

Conclusions
In recent years, various materials, including agricultural residues, have been explored for MB dye adsorption from wastewater.However, a comprehensive thermodynamic understanding of the adsorption mechanism has been lacking.This study introduced the use of rice straw for the novel and cost-effective removal of MB dye from wastewater.The investigation of different operational conditions, such as temperature, pH, and MB dye concentration, aimed to optimize the adsorption process's removal efficiency.The results indicated that this novel adsorption method is strongly pH-and temperature-dependent and effective for treating MB dye in aqueous wastewater.The study also explored the equilibrium thermodynamic models of adsorption using various isotherm models, revealing the impact of pH and temperature variations on the constants of different isotherm models.In this study, the equilibrium thermodynamic models of adsorption were also investigated using three different adsorption isotherm models, namely, Langmuir, Temkin, and Freundlich models; moreover, Langmuir-Freundlich model was used to correlate the adsorption results.The results show that the constants of different isotherm models can change significantly by varying the pH and temperature values.A decrease in adsorption can be observed owing to an increase in the temperature, which can be attributed to the fact that MB adsorption is an exothermic process; an increase in pH causes an increase in adsorption, which can be attributed to the fact that, at higher acidity, the bonding between the MB and rice straw as an adsorbent can disrupted by H + ions.The findings contribute to a deeper understanding of the adsorption process and its potential applications in wastewater treatment.

Figure 2 .
Figure 2. FTIR results of the rice straw absorbent.

Figure 5 .
Figure 5. Linearized model of Langmuir isotherm at different temperatures (293-333 K) and pH 6; the amount of adsorbent = 1 g in 300 mL methylene blue.

Figure 6 .Figure 7 .
Figure 6.Effect of pH on the MB dye adsorption on modified rice straw at different pH values (2-10) and T = 303 K; the amount of adsorbent = 1 g in 300 mL methylene blue.

Figure 8 .Figure 9 .
Figure 8. Relationship between pH and equation constants for methylene blue dye adsorption on modified rice straw adsorbent.

Table 1 .
EDX study of rice straw adsorbent.

Table 2 .
Functional groups of the rice straw adsorbent.

Table 3 .
Parameters of isotherms for methylene blue dye treatment using modified rice straw at different temperatures (293-333 K) and pH 6.

Table 4 .
Parameters of isotherm for methylene blue dye treatment using modified rice straw at different pH values (2-10) and T = 303 K.

Table 5 .
Free energy, ΔG, of methylene blue adsorption on rice straw at various temperatures and pH 6.