Adsorption of Methylene Blue Dye from aqueous solution using low cost adsorbent: Kinetic, Isotherm Adsorption and Thermodynamic Studies

doi:10.21203/rs.3.rs-2449414/v1

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Adsorption of Methylene Blue Dye from aqueous solution using low cost adsorbent: Kinetic, Isotherm Adsorption and Thermodynamic Studies

https://doi.org/10.21203/rs.3.rs-2449414/v1

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Fig leaf as an environmentally friendly byproduct of fruit plants, has been used for the first time to treatment of methylene blue dye. The fig leaf activated carbon (FLAC) was prepared successfully and used for adsorption of methylene blue dye (MB) purpose. The adsorbent was characterized by the Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscope (SEM) and the specific surface area by the Brunauer-Emmett-Teller (BET). In the present study, initial concentrations, contact time, temperatures, pH, carbon dose, volume solution and activation agent were covered. However, the initial concentration of MB was investigated at different concentrations 20, 40, 80, 120 and 200 mg/L. pH value was examined at these values (pH3, pH7, pH8 and pH 11). When pH is 11, the adsorption efficiency of MB retains at 84.3%. To determine how well FLAC removed MB, adsorption temperatures were changed at 20, 30, 40, and 50 °C. The adsorption capacity of FLAC was determined to be 22.7 mg/g for 0.08 g and 54.9 mg/g for 0.02 g. This adsorption inclined toward the Langmuir isotherm model (R²>0.95), where the adsorption created a monolayer covering the surface of the adsorbent, based on the curve-fitting using the Freundlich and Langmuir isotherm models. Additionally, it was discovered that the maximum adsorption capacity (Qm) was 69.93 mg/g and the Langmuir affinity constant (KL) was 0.08 L/mg. The FLAC as low-cost adsorbents for methylene blue dye has shown good cationic dye adsorption performance.

Fig leaf

methylene blue

adsorption

activated carbon

thermodynamic

kinetics

Water is the most essential element for life on earth and has a significant impact on the production of food and energy, industrial output, and the quality of our environment(Sharma and Bhattacharya 2017). Waterborne illnesses cause a considerable economic burden since they not only reduce worker productivity but also drive-up national health care expenses (Shannon et al. 2008; Sharma and Bhattacharya 2017). The production of textiles, plastic, paper, pharmaceuticals, and cosmetics all use methylene blue (Fig. 1), a cationic water-soluble dye (Stewart 2016).

However, due to its complex structure, natural degradation is challenging (it is nonbiodegradable), and above a certain concentration, it is toxic and carcinogenic. The removal of methylene blue from wastewater necessitates the employment of effective, environmentally friendly technology due to its presence in effluents (very apparent at modest levels of dyes 1 ppm), resulting from dying procedures or production of pharmaceuticals or cosmetics (Omer et al. 2018; Patra, Mukherjee, et al. 2021). Direct or indirect release of several sophisticated and powerful contaminants into water bodies results from domestic and industrial operations. One of these pollutants that is widely used in the textile, fabric, pharmaceutical, plastics, pulp and paper, printing, food, and printing industries is dye. The poisonous substances, acids, organics, bases, and other contaminants present in the discharged dyes have a negative impact on the natural habitats and metabolism of the aquatic and soil life (Patra, Mukherjee, et al. 2021; Patra, Nanda, et al. 2021a). Biological methods, such as anaerobic and aerobic digestion, physical methods, such as sedimentation, coagulation, filtration, ion exchange, adsorption, and reverse osmosis, and chemical methods, such as oxidation, ozonation, photochemical, and electrochemical degradation, are all used in conventional wastewater treatment to remove dyes (Patra, Nanda, et al. 2021b). Consequently, it is important to safeguard the current water resources. Insoluble chemical compounds, heavy metals, pharmaceutical micropollutants, and dyes are just a few of the contaminants that can be removed from water thanks to ongoing research efforts that have led to certain modified technologies and effective adsorbents (Huang and Shi 2014). The most effective method among them is adsorption because of its high efficiency and accessibility (Mohamed et al. 2019). The collection of solid wastes is addressed and prevented from contaminating the air and water during the natural decomposition process by converting biomass into beneficial activated carbon that is an environmentally friendly adsorbent (Singh et al. 2021). Additionally, the biomass adsorbents exhibit exceptional high surface area, substantially greater effectiveness, and a faster adsorption rate in water treatment when compared to traditional materials (L. Liu et al. 2019; Singh et al. 2021). The removal of azo dyes, and other harmful compounds from wastewater can be done with biochar nanoparticles since they are affordable, effective, and environmentally acceptable (Zhu et al. 2021). There are two different types of modification processes for biomass carbon materials now available: direct carbonization and chemical activation. Under the protection of an inert gas, high temperature carbonization of biomass carbon materials is carried out in order to break down large molecules in organic matter into smaller ones, such as carbon, water vapor, and carbon dioxide (Wu et al. 2020; Yağmur and Kaya 2021). Low specific surface area and ineffective adsorption effectiveness are drawbacks, though. The carbonization and activation processes can be finished in one step when using the chemical activation method. The use of activators such NaOH, KOH, K₂CO₃, H₃PO₄, H₂SO₄, and ZnCl₂ allows for the synthesis of carbon compounds with high specific surface area and large empty volume (Jawad et al. 2016; Kılıç et al. 2012; Rashid et al. 2016; Wu et al. 2020; Yağmur and Kaya 2021). Adsorption techniques are advantageous since they are economical and environmentally friendly (Gutub et al. 2013). Rice husk was altered by Liu et al. (Z. Liu et al. 2020) by chemical activation with KOH at 400°C for 30 min. and 850°C for 1 h. Due of its favorable effects on the environment and resource efficiency, modified rice husk was used to remediate wastewater that included mercury ions. Using date palm bark as the raw material, Haghbin et al. (Haghbin and Shahrak 2021) created a highly porous activated carbon using a thermal decomposition process at 400°C for 3 h followed by chemical activation with H₃PO₄. The materials as-prepared shown high porosity and efficient adsorption of diverse contaminants including heavy metals, dyes, and quercetin due to their large surface area and acidic functional groups on their surface (Haghbin and Shahrak 2021). Sunflower seed hulls, walnut shells, coconut shells, and corncob waste plant biomass might all be used using this straightforward process. In order to create extremely porous carbon material for the removal of phenolic compounds from aqueous medium, Prashanthakumar (Prashanthakumar et al. 2018) pyrolyzed coconut spathe in 800°C for 90 min while using KOH as an activating agent in a nitrogen atmosphere. By hydrothermally carbonizing coconut shell at 200°C for two hours and impregnating it in NaOH for four hours, Islam et al. (Islam et al. 2017) created mesoporous coconut shell activated carbon. The activated hydro-char was then heated for 1 hour at 600°C in a N₂ environment, acting as a more effective adsorbent to remove cationic dyes. KOH has been utilized extensively in comparison to other chemical activating agents to produce activated carbon that has a greater specific surface area (up to 3000 m²/g) and is effective at removing both organic and inorganic pollutants from wastewater (Choma et al. 2015; Li et al. 2017). Fallen leaves are a fantastic and promising option to make activated carbon. They are regarded as inexpensive adsorbents since they are abundant in nature, affordable, need little preparation, and serve as working materials as well (Faizal et al. 2019). Plant leaves are a promising choice for producing activated because of their high carbon contents, which will have a significant ecological impact (Faizal et al. 2019; Pérez-Ramírez et al. 2019). Chemical or gas activation can be used to create activated carbons from materials that contain carbon. They are the most widely utilized adsorbents because of their enormous surface area and exceptional adsorption capacity (Jawad et al. 2016).

There hasn't been any research on the use of fig leaves for the adsorption of the dye methylene blue, despite the fact that many papers on the preparation of activated carbon from various less expensive and substitute agricultural wastes and byproducts with chemical activation have been published recently. Regarding the chemical synthesis of activated carbon from fig leaves with H₃PO₄ or any other chemical agent, no investigations have been recorded. As a result, the goal of this research is to identify the ideal circumstances for producing activated carbon from fig leaf using H₃PO₄ activation. Investigations have been done into the effects of the activation temperature, pH, carbonization agents, and other variables. To characterize the activated carbon before and after adsorption, X-ray and FTIR, instruments were used. Moreover, the obtained activated carbon was compared with other fruit leaf activated carbon.

Materials collection and pretreatment

The fig leaves used in the current project came from waste at an Iraqi fig farm in Babylon, Iraq. After being cleaned with distilled water, fig leaves were allowed to air dry. The crushed, undersize particles from the sieving of the dried fig leaves through a 75 mesh screen were employed for the subsequent stages. Merck provided the H₃PO₄, H₂SO₄ and NaOH as activation agents. Methylene blue (C₁₆H₁₈ClN₃S), a basic (cationic) dye, is the adsorbate model utilized in adsorption studies; it was purchased from Dyestuffs and Chemicals Co. (china). Analytical grade chemicals were utilized exclusively; no further action was required.

Activated Carbon Preparation

The fig leaves (FL) were collected as crispy (i.e. dry), washed with water strongly to remove the dust and other solid matters then dried. Afterwards, the fig leaves were crushed using electric grinder and sieved for 20 mesh. A 15 mL deionized water was added in beaker contained 3 g fig leaves powder. And then, 5 mL of concentrated H₃PO₄, H₂SO₄ and NaOH was added, separately and stirred continuously until all powder immersed well. It was left for 2 hours then washed with water 3–5 times to remove all acid. After that sample was dried in normal conditions for 1–2 nights. Dried powder was then carbonized in furnace at 350°C for two hours. Full experimental methodology has been presented in Fig. 2.

Flac Surface Characterization

Fourier transform infrared (FTIR) spectroscopy was used to determine the presence of surface functional groups and chemical interaction for the treated fig leaf activated carbon using a Perkin Elmer Spectrum One spectrometer with universal attenuated total reflectance sampling for the range of 400–4000 cm^− 1. Cu K radiation with a wavelength of 1.54184, X-ray powder diffraction (XRD) in a Bruker D2 Phaser 2nd Gen apparatus, and a Lynxeye detector (ID mode). The adsorbent was characterized also by scanning electron microscope (SEM) and the specific surface area by the Brunauer-Emmett-Teller (BET).

Adsorption Experiments

A 25 mL of different concentrations of methylene blue dye solution (20, 40, 80, 120 and 200 mg/L) were mixed with 0.08 g of adsorbents and subjected to the shaker for 60 min at room temperature. The samples were centrifuged at 3000 rpm after adsorption, and the filtrates were examined. Using a UV-vis spectrophotometer and the calibration curves, the dye concentrations in the water were initially and residually measured at their highest absorbance wavelengths (650 nm). The following equations (1,2) were used to calculate the removal% and adsorption capacity of MB dye:

$$\text{R}\text{%}=\frac{{\text{C}}_{0}-{C}_{t}}{{\text{C}}_{\text{f}}}\times 100 \left(1\right)$$

$$\text{Q}=\frac{{(\text{C}}_{0}-{C}_{t})}{\text{m}}\times \text{V} \left(2\right)$$

where the starting dye concentration (C₀) (measured in mg/L for MB), the residual concentration (C_t) was measured for the MB dye in mg/L and the final dye concentration (C_f) (measured in mg/L for MB) are given.

R% is the percentage of MB dye solution removed after adsorption; Q is the adsorption capacity (mg/g) at various times; and m is the amount of FLAC (g). In order to create the necessary dye solutions, stock MB solution (1000 mg/L) was dissolved in deionized water. Adsorbent was added in 25 mL of wide spectrum of concentrations (20, 40, 80, 120 and 200 mg/L) MB solution together with 0.08 g to test the effects of contact time and initial concentration of dye on adsorption removal and adsorption capacity. According to the calibration curve, UV-vis spectrophotometer (UH4150, Hitachi) at their maximum absorption wavelengths. At a pH of (3, 7, 8, 11), with a 80 mg/L concentration of MB dye, the impact of the pH was shown. Using solutions of 1 M HCl and 1 M NaOH, the pH of the solution was changed. A 0.08 g of adsorbent was added to 25 mL of a 80 mg/L solution of MB dye at 60 min. The samples were centrifuged after adsorption then examined. Temperature has been investigated on adsorption of methylene blue dye in three different temperature values which are 20, 30, 40 and 50°C at 60 min. The concentration of MB dye was 80 mg/L and the adsorbent was 0.06 g. Different amounts of FLAC (0.02, 0.04, 0.06, 0.08 and 0.1 g) have been investigated on 80 mg/L of MB dye at 60 min. The effect of volume solution has been investigated at different values of 25, 50 and 100 mL at fixed conditions 80 mg/L MB and 0.08 g FLAC. Activation agent: three activating agents (H₃PO₄, H₂SO₄ and NaOH) have been applied to select the best agent for further experiments under same conditions.

Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy (FTIR) is the method that can be used to qualitatively assess the presence of the principal functional groups on the exterior surface of the bio-sorbent. Leaves are thought to be a biomass-rich lignocellulosic source display a lot of oxygen functional groups on its surface (Tran et al. 2017). However, from the searching on analysis and characterization the functional groups on the surface of waste leaf plants, it was observed the most common functional groups presents in leaf activated carbon are O-H group, C = O group, C-O group, C-H group and C = C group as shown in the previous studies (Guo et al. 2020; Jawad et al. 2017; Kushwaha et al. 2014). The FTIR characterization of fig leaf activated carbon (FLAC) is further performed in Fig. 3a to study the surface functional groups of these materials. It is possible to attribute the signal at 3065 cm^− 1 to vibrations of hydroxyl functional groups (Abdulhameed et al. 2021). The bands at 1622 cm^− 1 represent the vibrational stretching of carbonyl group C = O (Abdulhameed et al. 2021; Jawad et al. 2020). A significant band at 1317 cm^− 1 that was C-O band had a drop in intensity. Out of plane vibration of C-H band had observed at 781 cm^− 1.

X-ray Diffraction

The XRD spectrum of a FLAC sample activated in H₃PO₄ media is shown in Fig. 3b. A largely amorphous structure is revealed by the appearance of a broad diffraction background and the lack of a sharp peak. The outcome demonstrates that the FLAC sample has an amorphous structure, indicating that the organic components of fig leaf waste were mostly affected by the H₃PO₄ alteration.

Scanning Electron Microscopy

Fig leaf was activated with H₃PO₄ for scanning electron microscopy (SEM) evaluation, and it was found that this produced a more uniform surface and high porosity, which led to a high adsorption capacity (Fig. 3c). The exterior surfaces of the activated carbons feature various-sized voids. The presence of these holes may facilitate the facile diffusion and trapping of large numbers of MB molecules in the pore structure of activated carbon. As a result, the use of chemical treatment helps to thoroughly clean and remove the natural colors that are present in fig leaves, freeing up the pores that they occupy and improving the fig leaves' capacity to absorb things. The pores on the surfaces of activated carbon that are created after chemical treatment are caused by the evaporation of the activating agent during carbonization, which leaves behind the ruptured surface of activated carbon with pores formerly occupied by the activating agent (Bencheikh et al. 2020).

Nitrogen Adsorption-desorption Isotherms

Nitrogen adsorption-desorption tests were carried out to describe the porous FLAC architectures (Fig. 3d). The FLAC was estimated to have a Brunauer-Emmett-Teller (BET) specific surface area of 18.3 m²/g. Table 1 included a list of other porosity parameters. The high level of surface activity and vast surface area of porous carbons frequently lead to effective dye adsorption (Khangwichian et al. 2022). It was discovered that larger particles (327.9 nm) helped to boost adsorption capacity. Since heteroatoms can produce a redistribution of the surface charge of carbon materials, it has been demonstrated that the inclusion of heteroatoms in carbon materials greatly improves their performance.

BET surface area (m²/g)

Langmuir surface area

(m²/g)

Total pore volume (cm³/g)

Average particle size (nm)

Micropore surface area (m²/g)

Median pore width (nm)

18.2976

60.7756

0.0245

327.9

2.3137

1.2643

Effect of initial MB concentration

UV-vis measurement is used to assess the removal% and adsorption capabilities of prepared fig leaf activated carbon. In order to study the removal efficiencies and adsorption capacities of the MB dye, the initial MB concentrations were tested at these concentration 20, 40, 80, 120 and 200 mg/L at room temperature, as shown in Fig. 4.

Research on how different adsorbents respond to the initial concentration of adsorbate has shown consistent results. In their investigation on the adsorption of methylene blue using oil palm trunk nanocrystalline cellulose, it was observed that an increase in the initial concentration of methylene blue is related to an increase in the removal effectiveness of methylene blue (Mustikaningrum et al. n.d.). In order to overcome the solid-liquid mass transfer resistance, a greater starting concentration was necessary. A vacant active site on the surface of the adsorbent that is not occupied by the adsorbate molecule will exist at very low concentrations.

It can be regarded as a reduction in the system's adsorption capacity. Additionally, the active site on the surface of the adsorbent will decrease to slow down the adsorption process if an increase in initial concentration surpasses the optimal point. Figure 4 shows the graph of the fluctuation in methylene blue concentration on adsorption capacity (mg/g). In general, it may be said that the adsorption capacity increases with increasing starting concentration. The mass transfer resistance between the liquid (methylene blue) and the solid adsorbent is significantly overcome by the higher starting concentration (Al-Ghouti and Al-Absi 2020). The number of site FLAC might not be enough to absorb enough methylene blue molecules at high concentrations, which would lower the amount of color removed by the adsorption process. Methylene blue molecules enter the boundary layer and then diffuse to the surface of the adsorbent to start the adsorption process. The molecules continue to diffuse inside the adsorbent.

Effect Of Contact Time

The mass of dye adsorbed on the adsorbent and the cost of the adsorption process in wastewater treatment and water purification can both be affected by the contact time without a doubt. In a beaker filled with 25 mL of a different concentrations 20, 40, 80, 120 and 200 mg/L MB solution, 0.08 mg of FLAC was added. The mixture was then agitated strongly for a variety of times at room temperature. From Fig. 5a, the adsorption of MB by FLAC is > 77% when the concentration was 40, 80 and 120 mg/L, respectively at contact time of 10 minute. The removal efficiency rises to > 94% when the concentration was 20, 40, 80 and 120 mg/L, respectively at contact time of 60 minute which is the equilibrium time.

Adsorption capability was also taken into account and assessed, as seen in Fig. 5b. It is clear that removal percentage and adsorption capacity have the opposite relationships. However, the lowest adsorption capacity was 6.25 mg/g at equilibrium time with MB dye concentration of 20 mg/L. While the maximum adsorption capacity was 38.9 mg/g and the MB concentration was 200 mg/L. This pattern aligns with previously published findings. According to (Izan et al. n.d.), the percentage of MB removed decreased significantly as the initial concentration of methylene blue dye was raised on magnetic char. The MB intake, however, went from 42.6 to 63.7 mg/g. Blaga et al. (Blaga et al. 2022) used leftover biomass from the brewing sector to examine how the initial concentration of methylene blue affected adsorption capability. He discovered that raising the MB dye concentration from 5 to 70 mg/L caused the adsorption capacity to rise from 50 to 230 mg/g. This suggests that at greater initial MB concentrations, MB cation-adsorbent surface collisions take place more frequently, increasing MB adsorption capacity (Jiang et al. 2021).

Effect Of Temperature

Figure 6 shows the change in MB adsorption removal and capacity by fig leaf activated carbon at various temperatures. The elimination efficiency of FLAC is 77.3% at room temperature, or 30°C. At 50°C, the removal efficiency increases to up to 86.4%. The mobility of the dye molecules was dynamic as the temperature rose, and there were more active sites for adsorption as well (Bharathi and Ramesh 2013). The adsorption capacity was increased steadily with increasing temperature from 20°C to 50°C. For MB dye adsorption, the adsorption capacity reach to 28.8 mg/g at 50°C while it was the lowest value of 22.6 mg/g at 20°C. Based on the data presented, the temperature increase causes an increase in the adsorption capacity due to the swelling of the internal structure of the adsorbent, which allows methylene blue to penetrate further (Hu et al. 2018).

Effect Of Initial Solution Ph

Figure 7 shows how the fig leaf activated carbon performs in terms of MB adsorption at various initial solution pH ranges (pH3, pH7, pH8 and pH11). With acidic conditions, the MB absorption by FLAC is comparatively low, whereas the high adsorption property is realized under basic concentration. The cationic MB dye molecules experienced electrostatic mutual repulsion with greater H⁺ ions on the FLAC surface at lower pH levels. As we know, the OH group's active site was beneficial for adsorption of adsorbate on activated carbon surface (Islam et al. 2017). Therefore, at pH 3, the elimination effectiveness is just 88.5%. When the pH is increased from 3 to 7, the adsorption removal rises to 99.3%. The elimination effectiveness of FLAC is 95% when pH reaches 8 and 11. This finding was in agreement with other previous studies. According to a publication, decreased MB adsorption at pH3 may be caused by the adsorbent's predominantly protonated amino and carboxylic acid functional groups, which increase the cationic MB dye's electrostatic repulsion. When the initial pH rises till an alkaline medium, the high MB adsorption onto the adsorbents was observed (Shelke et al. 2022).

Additionally, it was found that the proton generation competes with the MB cation for adsorption on the active FLAC sites under acidic conditions, resulting in a reduction in adsorption capacity (Nordin et al. 2021). The high cation exchange capacity of FLAC is probably to blame for the high MB adsorption that becomes apparent as the pH of the solution rises to a high 11. The alkaline state of the solution suggests that the adsorbent surface has more negative charges than positive ones (Murthy et al. 2020). The high adsorption capacity of FLAC roughly 24.5 mg/g is caused by electrostatic attraction to cationic MB due to the negative charge on its surface. Thus, it is evident that FLAC requires a neutral or basic environment in order to achieve high adsorption efficiency.

Effect Of Flac Amount

Figure 8 displays the results of the evaluation of the dosage of fig leaf activated carbon for the adsorption of 80 mg/L of MB dye solution at pH 7. As can be seen, the plot demonstrates that at FLAC dosages of 0.02 and 0.1 g, respectively, the adsorption efficiency of MB rose from 67.5–100%. This is brought on by the rise in the number of available empty adsorption sites and the adsorbent's surface area for adsorption (Abuzerr et al. 2018). High removal efficiency may result from high adsorbent dose. But as the dosage is increased, the adsorption ability decreases. It might not be enough to give negative charges for MB adsorption if the adsorbent dosage is increased further since it could change the nature of the solution (Izan et al. n.d.). As can be seen, the adsorption capacity was decreased from 33.8 to 10 mg/g when the FLAC increased from 0.02 to 0.1 g.

Effect Of Solution Volume

With the exception of 100 mL, FLAC exhibits great removal efficiency in the adsorption of the methylene blue dye at different volumes of solution (25, 50, and 100 mL). The highest removal rate was 99.5% with a 25 mL dosage. As the volume of the MB dye solution increases, the removal efficiency declines. At 50 mL of dye solution, the greatest adsorption capacity of 44.5 mg/g is obtained. These findings are depicted in Fig. 9.

Effect Of Activation Agent

The activation process can undoubtedly influence the removal% and mass of adsorbed dye on the adsorbent. The effect of the activation process was presented in five aspects: AC1: only burn; AC2: pristine powder; AC3: impregnated in H₃PO₄; AC4: impregnated in NaOH; AC5: impregnated in H₂SO₄ on the removal% and adsorption capacity as presented in Fig. 10. 80 mg of adsorbent was added to 25 mL of an 80 mg/L MB solution in conical flasks and shaken for 60 min at room temperature. The elimination percent of MB by AC3 is 99.6%, and the adsorption capacity is 24.9 mg/g. By testing the others, the lowest removal efficiency is 75% when pristine powder is used without any further chemical treatment. The impregnation of fig leaf powder in NaOH solution has not improved the efficiency of adsorption as much as acid did.

Adsorption Isotherms

The Langmuir and Freundlich models equation method was used to determine the value of the adsorption equilibrium constant (Fig. 11). The Freundlich model has the lowest R² of the two models (0.851), but the Langmuir model has a high R² of 0.959. Adsorption kinetics models like the Langmuir isotherm model are frequently used to explain intricate adsorption dynamics. The maximal adsorption capacity in the current investigation was 69.93 mg/g, and the Langmuir affinity constant (KL) was 0.08 L/mg. This model accurately depicts the methylene blue adsorption by FLAC. In addition to the adsorbent's pores, the adsorption mechanism also depends on hydrogen bonds and Van der Waals interactions. The FLAC hydroxyl group, which binds the nitrogen element of methylene blue, contains hydrogen, which is what distinguishes the hydrogen bond in this adsorption method. Dipole ion interactions and electrostatic interactions are features of the Van der Waals force. An appropriate model to explain the chemical adsorption mechanism is the Langmuir isotherm one. The observable adsorption process, specifically the monolayer (Wang and Guo 2020), indicates this. This monolayer surface demonstrates that an active site, which can only be occupied by one molecule at a time, is responsible for carrying out the adsorption (Hasan et al. 2020). Other earlier investigations also supported the Langmuir isotherm model for the methylene blue adsorption procedure utilizing activated carbon from leaf waste plants (Guo et al. 2020; Jawad et al. 2017).

Thermodynamic Study

At 293, 303, 313, and 323 K, the impact of temperature on the adsorption of MB on FLAC adsorbent was examined. As the temperature rose from 293 to 323 K, it was found that the adsorption capacity increased from 29.2 to 30.9 mg/g. These results suggested that the MB dye may be pushed from the solution phase to the solid surface due to the increased feasibility of adsorption at higher temperatures caused by the rise in kinetic energy of dye molecules (Dural et al. 2011). In the study on the adsorption of MB onto FLAC, a related finding was also made. Using Eq. (3) and Eq. (4), the thermodynamic parameters change in enthalpy (∆H°), entropy (∆S°), and Gibbs free energy (∆G°) were calculated for the adsorption of MB on FLAC.

$$\text{ln}\left(\frac{{C}_{s}}{{C}_{e}}\right)= \frac{{\varDelta S}^{^\circ }}{R}-\frac{{\varDelta H}^{^\circ }}{RT} \left(3\right)$$

$${\varDelta G}^{^\circ }={\varDelta H}^{^\circ }-{T\varDelta S}^{^\circ } \left(4\right)$$

where Ce is the dye's equilibrium concentration in solution (mg/L) and Cs is its equilibrium concentration in the solid phase (mg/L). Temperature is T, and gas constant R is 8.314 J/mol/K. Changes in enthalpy (kJ/mol), entropy (J/mol/K), and Gibb's free energy (kJ/mol) are denoted by the symbols ∆H^°, ∆S^°, and ∆G^°, respectively. The slope (∆H^°/R) and intercept (∆S^°/R) of the plots of ln (Cs/Ce) vs. 1/T were used to get the values of ∆H^° and ∆S^°.

The coefficient of distribution is calculated using Eq. 5 which is named Kd.

$${K}_{d}=\frac{{C}_{s}}{{C}_{e}} \left(5\right)$$

Table 2 displays the thermodynamic parameter values. Negative values of ∆G^° demonstrated the viability and spontaneity of the adsorption process. As the temperature rose, the values of ∆G^° fell, indicating that the adsorption was more spontaneous at low temperatures. The increase in randomness at the adsorbent-solution interface during the adsorption was described by a positive value of ∆S^°. The overall endothermic nature of the MB adsorption on FLAC is confirmed by a positive value for ∆H^°.

Table 2

Thermodynamic parameters values for the adsorption of methylene blue onto FLAC at different temperatures.
Temperature (K)	Thermodynamic parameters
Temperature (K)	Kd	∆G° (kJ/mol)	∆H° (kJ/mol)	∆S° (J/mol.K)
293	13.93	-6.47	49.3	190.33
303	28.41	-8.37
313	54.56	-10.27
323	89.9	-12.18

Kinetic study

The kinetic data were investigated using the pseudo-first-order and pseudo-second-order linear models shown in equations (6) and (7), respectively (Mousavi et al. 2022).

$$\text{log}\left({Q}_{e}-{Q}_{t}\right)=\text{log}{Q}_{e}- \frac{{K}_{ads1}}{2.303} t \left(6\right)$$

$$\frac{t}{{Q}_{t}}=\frac{1}{{K}_{ads2}\times {Q}_{e}^{2}}+\frac{1}{{Q}_{e}}t \left(7\right)$$

Where Q_e and Q_t are the adsorption capacities (measured in mg/g of MB adsorbed on the material) at equilibrium and any time t (min), respectively. The rate constants for the pseudo-first-order (min^− 1) and pseudo-second-order (g/mg.min) adsorption processes are K_ad1 and K_ad2, respectively. The pseudo-first-order and pseudo-second-order models, respectively, for the kinetic processes of adsorption are shown in Fig. 12a,b. Each model's parameter values are displayed. When compared to the correlation coefficients obtained for the pseudo-first-order model, the data are demonstrated to suit the pseudo-second-order model well (R² = 0.9972) (Fig. 12b). Chemisorption is therefore shown to occur because the process is dependent on the adsorbent and the concentration of the adsorbate.

Selectivity Dye Adsorption

In the field of selective dye adsorption and separation, earlier studies have shown that leaf waste activated carbon displayed excellent affinity for organic dyes. The heterocyclic aromatic organic compound known as the cationic MB dye is frequently utilized as a target molecule for wastewater treatment and water purification. The Fig. 13 describe the chemical composition of the methylene blue organic dye in the study of selective dye adsorption. After 60 minutes of using 0.06 g FLAC as the MB adsorbent, the color of the 25 mL 40 mg/L MB solution is almost vanished to the human sight. The cationic MB is electrostatically bound to the anionic hydroxyl groups on the FLAC surface. To demonstrate the selective dye adsorption property, Fig. 13, UV-vis spectroscopy was used to record the solution's absorbance spectra. The MB peak is hardly discernible after adsorption.

The viability of using fig leaves as a novel, inexpensive precursor in the production of activated carbon is examined in this work (FLAC). The outcomes show that FLAC is a powerful adsorbent for the adsorption of the methylene blue dye. This study shown that H₃PO₄ treatment could improve FLAC's ability to adsorb MB and remove it from aqueous solutions compared to other activating agents NaOH and H₂SO₄. The results of the adsorption experiments showed that the pseudo-second-order model best described the kinetic uptake characteristics. The overall endothermic nature of the MB adsorption on FLAC is confirmed by a positive value for ∆H^°. The negative values of ∆G^° indicate that the adsorption was more spontaneous at low temperatures. The Langmuir model, on the other hand, does a good job of describing the adsorption isotherms. It was discovered that the maximum adsorption capacity (Qm) was 69.93 mg/g and the Langmuir affinity constant (KL) was 0.08 L/mg. The FLAC as low-cost adsorbents for methylene blue dye has shown good cationic dye adsorption performance.

Acknowledgments

The authors thank the College of Sciences for Women, University of Babylon, for facilitating this work.

Availability of data and materials

The dataset utilized/analyzed during the current study will be available from the corresponding author upon request.

Funding Declaration "No funding was obtained for this study”

Authors’ Contributions

Safaa Talib Al-Asadi: wrote the first draft of the manuscript, did the experiments and organized the structure of the manuscript. Fouad Fadhil Al-Qaim: analysed the data and edited the final draft of the manuscript

Declaration of Competing Interest The authors declare that they have no conflict of interest.

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No competing interests reported.

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Adsorption of Methylene Blue Dye from aqueous solution using low cost adsorbent: Kinetic, Isotherm Adsorption and Thermodynamic Studies

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Abstract

Figures

Introduction

Materials And Methods

Materials collection and pretreatment

Activated Carbon Preparation

Flac Surface Characterization

Adsorption Experiments

Results And Discussion

Fourier transform infrared spectroscopy

X-ray Diffraction

Scanning Electron Microscopy

Nitrogen Adsorption-desorption Isotherms

Effect Of Contact Time

Effect Of Temperature

Effect Of Initial Solution Ph

Effect Of Flac Amount

Effect Of Solution Volume

Effect Of Activation Agent

Adsorption Isotherms

Thermodynamic Study

Selectivity Dye Adsorption

Conclusion

Declarations

References

Additional Declarations

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