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CC BY 4.0 · Sustainability & Circularity NOW 2025; 02: a26302599
DOI: 10.1055/a-2630-2599
Original Article

Methyl Blue Dye Adsorption Using Activated Eggshell: Kinetics, Isotherm, and Phytotoxicity Analysis

Al Shariar Hasan
1   Chemistry Discipline, Khulna University, Khulna, Bangladesh
,
Sagar Kumar Dutta 
1   Chemistry Discipline, Khulna University, Khulna, Bangladesh
,
Abul Bashar
1   Chemistry Discipline, Khulna University, Khulna, Bangladesh
,
Palash Kumar Dhar
1   Chemistry Discipline, Khulna University, Khulna, Bangladesh
,
Rezaul Haque
1   Chemistry Discipline, Khulna University, Khulna, Bangladesh
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Abstract

Aesthetic and health concerns have been raised about removing color from wastewater. Industrial activity is a major source of water contamination. Dyes can be hazardous, carcinogenic, and mutagenic to wildlife, plants, and humans. Consequently, it is essential to process wastewater prior to its release into the environment. Fabrics, leather tanning, paper, plastics, and printing all employ synthetic dyes like methyl blue (MB). MB is one of the triphenylmethane acid dyes and anionic dyes. In this investigation, refuse eggshell powder was carbonized and subsequently activated with lemon juice extract as an activating agent. The activated carbon eggshell (ACE) that was prepared was subjected to Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and zero point charge (ZPC). The adsorption rate was significantly influenced by several parameters, including pH, adsorbent dosage, initial concentration, contact time, and temperature. Additionally, isotherm models (including Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) isotherm models), kinetic models (including pseudo-first-order, pseudo-second-order, Elovich, intraparticle diffusion, and liquid-film diffusion models), and phytotoxicity studies were investigated. The optimum adsorption was achieved at a pH of 2, adsorbent dosage of 1.0 g, contact time of 70 min, and initial (MB) dye concentration of 40 mg/L. The maximal (MB) dye removal efficiency was initiated at 98.94%. This adsorbent (ACE) is expected to be well received as a more cost-effective alternative to other adsorbents.


#

Keywords

Eggshell - Adsorption - Methyl blue - Kinetics - Isotherm - Phytotoxicity
Significance

This study presents a sustainable approach to wastewater treatment by converting eggshell waste into low-cost activated carbon using lemon juice as a green activator. The adsorbent effectively removes methyl blue dye from water, offering an eco-friendly alternative to chemical adsorbents. This work contributes to the UN Sustainable Development Goals, particularly SDG 6 (Clean Water and Sanitation) and SDG 12 (Responsible Consumption and Production).

1

Introduction

Our environmental community faces several issues, including the proper disposal of effluents containing dyes. The unregulated release of dye substances, whether they are of natural or synthetic origin, is a significant cause of nonaesthetic pollution, which subsequently results in the degradation of ecosystems. However, dye goods are still essential for enhancing the aesthetic appeal of our environment.[1] [2] The chromatic component of the dye reflects and absorbs sunlight that penetrates contaminated wastewater, hence inhibiting photosynthesis and restricting the proliferation of aquatic organisms.[3] [4] The synthetic origin and intricate molecular compositions of dye-based effluents hinder their biodegradability and frequently make them unsuitable for conventional biological wastewater treatment methods.[5] The substantial quantities of industrial waste that are being released into water sources worldwide have brought attention to the evident need to treat wastewater to tackle the scarcity of drinkable water.[6] [7] Given the vital role of water in sustaining all living organisms, the issue of water pollution has received considerable global recognition.[8]

Every year, the global textile industry manufactures around 70 million tonnes of synthetic dyes. Of these, 10–15% are released into the atmosphere through industrial channels, resulting in substantial pollution of aquatic environments.[9] [10] Dyes seriously threaten both marine life and human health.[11] This is because many dyes are poisonous, cannot be broken down naturally, and are the main causes of mutagenesis, carcinogenesis, and respiratory toxicity.[12] [13] Effluent contaminated with dyes poses a challenging and expensive task for treatment, primarily because dyes consist of aromatic compounds resistant to light, heat, microbial destruction, and oxidizing agents.[6] [14] Currently, there is significant discussion on the disposal of effluents containing hazardous dyes. Various techniques for removing pigments have been studied to minimize their ecological footprint.[15] The mentioned approaches include adsorption, biofilm utilization, coagulation-flocculation, bacterial therapy, electrochemical oxidation, photocatalytic oxidation, membrane filtering, and solvent extraction.[9] [10]

Adsorption is the commonly used and effective method for removing dyes, yielding positive results[16] [17] because of its simple design, wide applicability, and minimal production of harmful byproducts.[18] Because of its cost-effectiveness and simplicity, it is frequently used to treat wastewater contaminated with both inorganic and organic hazardous compounds, without the need for expensive equipment or highly trained staff.[19] Adsorption is a very efficient technique for taking color out of water-based solutions.[20] The principal adsorbent employed in the elimination of dyes is activated carbon.[21] However, a more affordable substitute adsorbent must be created because of its high cost. Colors have been removed from water solutions using a range of inexpensive adsorbents, such as zeolites, siliceous materials, activated carbon made from coffee husks, deteriorated coffee beans, marine algae, chitosan, and ion exchange resin.[22] [23] [24] [25] We have developed a cost-effective adsorbent by utilizing discarded eggshells.[26] The eggshell waste was used to create activated carbon eggshells (ACE).[27] This method is ecologically sound and highly efficient for extracting pigments from their aqueous solution.[28]

Methyl blue (MB) is an anionic toxic dye that is used to color histology samples for structures made of collagen and fungi.[8] [29] In this investigation, MB was utilized as a conventional dye. MB is a triphenylmethane-derived acid dye.[6] [30] Its complex aromatic components and high solubility in water make MB challenging to extract from water.[31] This shows that when exposed to light, water, and specific chemicals, it has great resistance to fading.[32] Eliminating the presence of MB in wastewater is a compelling field of study within environmental science that has the potential to bring about significant enhancements to the natural surroundings.[33]

This study aimed to investigate the effectiveness of ACE adsorbents in eliminating the detrimental MB dye from contaminated water while considering the challenges mentioned earlier. Batch adsorption tests were conducted to identify the most favorable conditions for removing MB dye using ACE. The factors examined included pH, contact time, beginning dye concentration, and temperature. An analysis was conducted to understand the precise adsorption process by investigating isotherms, kinetics, and thermodynamics. It is an extremely efficient and reasonably priced adsorbent to eliminate MB dye.


# 2

Experimental Section

2.1

Chemicals

The reagents and chemicals utilized are all analytical grade, and no additional purification was performed. The anionic dye MB (molecular formula: C37H27N3Na2O9S3, molecular weight: 799.814 g/mole) was purchased from Sigma-Aldrich. The dye was precisely dissolved in the weight amount of deionized water to produce the stock solution (1000 mg/L). The original solution was diluted with deionized water to make several dye solutions at varying concentrations. The dye exhibited a maximum absorbance, known as lambda max, at a wavelength of 596 nm under acidic circumstances. Depending on the situation, 0.1 M NaOH or HCl solutions were used to adjust the pH of the various solutions.


# 2.2

Adsorbent (ACE) Preparation

2.2.1

Carbonized Eggshell (CE) Preparation

Eggshells were obtained from domestic and culinary refuse. The items were given many tap water washes before being utilized, followed by deionized water to get rid of any pollutants and contaminants. Afterward, they underwent a drying process in an electric oven for 13 h at a temperature of 105 °C. After blending the eggshells entirely, they were sieved according to the ASTM mesh standard to create a fine powder with particles as small as 100 μm. The powder underwent a drying process in a muffle furnace at a temperature of 400 °C for 2 h, culminating in the formation of the carbonized product. Then, the carbonized product was placed in a plastic container.[26] [28] [34] [35] The procedure is presented in [Scheme 1].


# 2.2.2

Preparation of ACE

The CE was activated using freshly extracted lemon juice as a natural and eco-friendly activating agent. A total of 200 mL of filtered lemon juice (pH ~ 2.3) was thoroughly mixed with the CE powder in a 2:1 (v/w) ratio. The mixture was stirred to ensure uniform coating and left to soak at ambient temperature for 24 h. After impregnation, the material was filtered and oven-dried at 110 °C for another 8 h until it reached a constant weight.[27] [36] [37] The dried composite was then calcined at 400 °C for 1 h in a muffle furnace. Following thermal treatment, the material was repeatedly washed with deionized water until the rinsing solution reached neutral pH and finally dried again at 110 °C for 8 h to obtain the ACE.

Lemon juice was selected as a green activating agent due to its high citric acid content, which acts as a mild acid capable of chelating calcium and modifying the surface structure of the eggshell-derived carbon. This approach was compared to traditional activation using strong acids like phosphoric acid (H₃PO₄) or hydrochloric acid (HCl), which are commonly employed to enhance surface area and porosity. For example, H₃PO₄-treated eggshell carbon has been reported to achieve adsorption capacities of up to 120 mg/g for methylene blue.[28] [38]

However, these chemical agents are highly corrosive, require post-treatment neutralization, generate hazardous waste, and raise safety and environmental concerns. A schematic diagram illustrating this technique is presented in [Scheme 1].

Zoom Image
Scheme 1 Overall formation process of activated eggshell.

# 2.2.3

Characterization of ACE

The functional groups, morphological aspect, and elemental composition of the adsorbent surface were examined using Fourier transform infrared spectroscopy (FTIR, Model: Shimadzu FTIR Prestige 21 spectrometer), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) before and after the adsorption process. The Fourier transform infrared (FTIR) spectra of eggshells were obtained using the KBr disc technique at ambient temperature, covering the wavenumber range of 400–4000 cm−1. Analysis of surface morphology and elemental characterization was conducted using a Hitachi S-4800 electron microscope from Japan, equipped with a 15 kV accelerating voltage. A comprehensive addition technique investigation was conducted on the synthesized adsorbent to ascertain its surface zero charges.[39]


# 2.2.4

Batch Adsorption Studies

The efficiency of ACE in eliminating MB from the chemically contaminated aqueous solution was determined using batch mode adsorption assessments. An investigation was conducted on a 100 mL MB solution containing varying adsorbent dosages (0.5–3 g/L), pH values (2–12), initial concentrations (10–40 mg/L), contact times (30–70 min), and solution temperatures (303 K and 323 K). The pH of the solution was changed using 0.1 M HCl and NaOH. The absorption of the dye was ultimately detected using a UV/vis spectrophotometer that had a peak wavelength of 596 nm. The dye removal efficiency (%), adsorption capacity at time t, (qt ) in milligrams per gram (mg/g), and equilibrium adsorption capacity (qe ) in milligrams per gram (mg/g) were determined for each adsorbent using a set of equations:

Removal efficiency % = C 0 − C t C 0 × 100 %
q t = C 0 − C t m × V
q e = C 0 − C e m × V

where the dye concentrations at a time interval are denoted by Ct (mg/L) and the initial and equilibrium dye concentrations are denoted by C 0 (mg/L). Moreover, the weight of the adsorbent and the volume of the dye solution are indicated by V (L) and m (g), respectively.[9] [12]


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#
# 3

Discussion and Results

3.1

Analysis of ACE Adsorbent

3.1.1

The Zero Point Charge

The point of zero charge (pHZPC) is the pH at which the surface charge density of a solution attains neutrality, signifying an equal quantity of positively and negatively charged centers. If the pH is below the pHZPC, the adsorbent surface has a positive charge. If the pH exceeds the pHZPC, the adsorbent surface has a negative charge.[40] The pH at which there is no net charge is known as the zero point load (pHZPC). The objective is to evaluate the surface charge of the adsorbent and comprehend the interaction between the binding sites of the adsorbent and the adsorbate molecules. The pH at the point of zero charge (pH) of the ACE was assessed by adding 0.1 g of adsorbent to 50 mL of 0.1 mol/L NaCl solution. The pH values varied from 2 to 12 (pHInitial). The pH of the suspension was adjusted by adding 0.1 M hydrochloric acid (HCl) or sodium hydroxide (NaOH) solutions, followed by pH measurement after agitating the solution at 150 rpm for 1 h. The pH was measured at ambient temperature (25 °C). The graph illustrates the variation in pH (ΔpH) between the initial and final values relative to the initial pH. The pH at the zero point of charge (pHZPC) was ascertained at the intersection of the curve with the x-axis. The eggshell’s point of zero charge (pHZPC) was established at 7.74 ([Fig. 1]), indicating that its total surface charge is positive at pH levels below 7.74 and negative at pH levels beyond this threshold.[26] [37]

Zoom Image
Fig. 1 Determination of the point-zero charge (pHPZC) of ACE.

# 3.1.2

FTIR Analysis

The activated eggshell ash was subjected to FTIR measurement to characterize its properties both before and after the adsorption of MB dye, as depicted in [Fig. 2]. The broad transmission bands in the IR spectra at approximately 870, 1425, 1800, 2515, and 2900 cm−1 are linked to the CaCO3 molecule’s absorption. According to the study, the primary component detected in regular eggshells, both before and after MB dye adsorption, was predominantly CaCO3. Additionally, the stretching vibration of the CO3 2− group was ultimately identified at a frequency of 1063 cm−1, indicating that the eggshell absorbent’s inherent chemical composition is carbonate (CO3 2−). The formation of Ca(OH)2 is responsible for the functional group’s presence in eggshell ash at 3600 cm−1, both before and after MB dye adsorption. This formation is likely a result of the adsorption of CaO with H2O. It is important to note that this compound was not present in nonpyrolyzed eggshells. Moreover, only after adsorption was the eggshell ash found to have the OH group-associated bending band at 3400 cm−1 and a weak band at 2863 cm−1, indicating the existence of Ca(OH)2 in the ash. However, a peak was observed around 710 cm−1, which suggests that the activated eggshell ash surface is the only place where a Ca–O bond is present. This finding confirms that CaO was added to the eggshell solely through heating.[27] [41] This signifies a vital optimization procedure to alter the eggshell to adsorb MB dye. Furthermore, the distinctive Ca(OH)2 band on the eggshell ash surface appeared to decrease following adsorption, although the stretching vibration peak of CaO (710 cm−1) before adsorption was enhanced.[39] The adsorption process, which converts Ca(OH)2 into CaO, could be the cause of this outcome.[36]

Zoom Image
Fig. 2 FTIR spectrum of ACE (a) before and (b) after adsorption.

# 3.1.3

Energy-Dispersive X-ray Spectroscopy

The powdered ACE was analyzed using EDS. The elements calcium (Ca), oxygen (O), magnesium (Mg), carbon (C), and others were present in the activated eggshell powder under analysis, as depicted in [Fig. 3]. The peaks verified that calcium carbonate (CaCO3) constituted the majority of the eggshell powder.[28] [42]

Zoom Image
Fig. 3 EDS analysis of the ACE.

# 3.1.4

SEM Analysis

The SEM method was used to examine the structural morphology of AC. SEM pictures of ACE were obtained to examine the surface structure of the adsorbent, both before and during the adsorption of MB. These images are shown in [Fig. 4a,b]. [Fig. 4a] illustrates how the adsorbent has an uneven and porous surface before dye uptake. [Fig. 4b] shows that pores have disappeared from the surface of MB-loaded ACE after dye adsorption, indicating that MB has been adsorbed onto ACE. Moreover, the fact that a molecular cloud forms on the dye-loaded adsorbent’s surface indicates that the dye has adsorbed to the ACE.[38] [43] [44]

Zoom Image
Zoom Image
Fig. 4 (a) Before SEM micrograph of ACE. (b) After the SEM micrograph of ACE.

#
#
# 4

Batch Studies

4.1

Effect of pH on Adsorption

The adsorption properties of ACE have been examined in batch experiments using a 10 mg/L solution of MB dye. The initial pH of the mixture was varied from 2 to 12. The studies were carried out for 60 min at 30 °C with a stirrer speed of 150 rpm. The adsorbent concentration used was 1 g/L. The graph in [Fig. 5] illustrates the percentage of dye removal effectiveness of ACE throughout a pH range of 2–12. At low pH values, adsorption was more effective, and no noticeable difference was observed between pH values of 6–12.[19] [37] Similar optimal pH ranges (2–3) have been reported in the literature, such as pH 2.5 for MB adsorption using a biodegradable polymer adsorbent,[11] pH 3 for dye removal using calcined eggshell,[28] [38] and pH 2.0 using pea peel-derived carbon.[24] [25]

Zoom Image
Fig. 5 Effect of pH on % removal of MB by ACE (C 0 = 10 mg/L, ACE dosage 1.0 g/L, contact time 60 min, temp. = 303 K).

# 4.2

Effect of Adsorbent Dosage

Adsorption investigations require careful consideration of the adsorbent dose because it immediately impacts the adsorbent’s efficiency in absorbing a given starting concentration of dye solution. [Fig. 6].[19] demonstrates that the dose of ACE adsorbent affects the adsorption of MB dye. In [Fig. 6], as adsorbent dosages increased, there was a positive connection observed in the percentage of dye removal. The connection between the rise in surface area and adsorption sites may represent the reason for this association. Following a precise dosage, the elimination was shown to be nearly consistent. In this experiment, the quantity of MB dye removed was increased from 0.5 g to 3.0 g. However, it was observed that the removal rate became virtually constant after reaching 3.0 g. The adsorption sites’ saturation could be the cause of this. Therefore, the 1.0 g dosage was tested and found to be the most effective, as it resulted in the highest elimination of MB dye (79.2%). This observation was reported in refs. [22], [40].

Zoom Image
Fig. 6 Effect of adsorbent dosage on the (a) percentage removal of MB dye and (b) adsorption capacity of ACE (C 0 = 40 mg/L, contact time 60 min, pH 2.0, T = 303 K).

# 4.3

Effect of Initial Concentration and Contact Time

The dye concentration data were measured at different intervals, ranging from 30 to 70 min, over a dye concentration range of 20–40 mg/L with an ACE loading of 1 g/L, at 303 °C, and a pH of two in the initial solution. This was done to assess the influence that adsorption time has on the efficacy of dye removal. Additionally, [Fig. 7] shows the effect of initial concentration on ACE’s adsorption capability and the percentage of MB removed. By raising the initial concentration, the quantity of adsorbent that was used per unit mass of adsorbent was raised, and the rate of MB that was removed was also increased.[19] [45]

After analyzing the results presented in [Tables 7], [8], [9], the dye removal efficiency reached its maximum value of 98.94% when the initial dye concentration was 40 mg/L sustained for 70 min. It was also found that the adsorption of the MB with various adsorbents had a behavior comparable to that explained before.[46] As the adsorption time was increased, the slope of the curve dropped, showing that the adsorption rate was larger at first and then progressively decreased as time approached equilibrium. This was explained by a rise in the quantity of accessible active adsorption sites on the adsorbent’s surface.[47]

Zoom Image
Fig. 7 Effect of initial dye concentration and contact time on the (a) percentage removal of MB dye and (b) adsorption capacity of ACE (adsorbent dosage 1 g/L, pH 2.0, T = 303 K).

The adsorption capacity exhibited a significant rise of 1.98 mg/g within 70 min when the dosage was increased from 20 to 40 mg/L at first, as seen in [Tables 7], [8], [9]. This is due to the direct relationship between the initial concentration differential between the solution and the adsorbent surface and the number of MB molecules that were adsorbed.[48] We observed a similar outcome, indicating that the adsorption capability increased in proportion to the dye concentration.


# 4.4

Effect of Temperature

In [Fig. 8], the effect of temperature on the absorbent’s equilibrium adsorption capacity is illustrated. Increasing the temperature resulted in a reduction in the adsorption capacity, which is evidence that the adsorption process of MB onto ACE is exothermic. The decrease in adsorption with increasing temperature can be attributed to the characteristics of physical adsorption, which is typically exothermic in nature.[49] At higher temperatures, the MB molecules undergo a greater amount of thermal motion, which in turn reduces the adsorptive interactions between the molecules that make up the adsorbed phase’s surrounding space and the active sites of the adsorbent and adsorbate species.[50] This is something that is predicted to occur because the MB molecules experience more thermal motion. In several different pieces of literature, a similar observation has been documented. After 70 min at 303 °C, the highest possible adsorption level of 1.98 mg/g was reached in this investigation. At two distinct temperatures (303 K and 323 K), the experiment was carried out with an ACE dose of 1 g/L, a starting dye concentration of 40 mg/L, a pH of 2.0, and a stirring speed of 250 revolutions per minute.[40]

Zoom Image
Fig. 8 Effect of temperature on (a) adsorption capacity and (b) dye removal efficiency.

# 4.5

Economic and Scalability Analysis

To assess the practical viability of using ACE for wastewater treatment, an approximate cost estimation and scalability discussion were performed. The production of ACE in this study utilizes waste eggshells readily available and cost-free in most regions and lemon juice as a natural activating agent. Table S1 (Supplementary Information) presents a breakdown of the estimated costs involved in producing 1 g of ACE, including energy, materials, and water. The total cost per gram of ACE was estimated to be approximately $0.12, which is significantly lower than commercial activated carbon (typically $0.50–$3.00/g). The major costs arise from energy consumption during drying, thermal activation, and lemon juice acquisition. Washing the eggshells required around 5 L of water per 100 g of raw material, but this step can potentially be optimized or scaled with recycled water.

In terms of scalability, the method is adaptable to small- or medium-scale decentralized water treatment systems. Since the raw material is waste and the activation process avoids hazardous chemicals, the method aligns well with green chemistry and circular economy principles. The process could be implemented in rural or industrial settings with minimal modification, making it accessible and environmentally sustainable.


#
# 5

Evaluation of Adsorption Kinetics

The adsorption kinetic investigations involved obtaining the adsorbent–adsorbate solution at predetermined intervals and then monitoring the solution concentration. By doing kinetic studies, one can get insight into the pace as well as the mechanism of the adsorption process. There are two independent processes that may be used to explain dye adsorption on solid surfaces [50]: (1) the dye molecules’ first, quick attachment to the adsorbent’s surface, and (2) a relatively slow intraparticle diffusion. The adsorption kinetics of dye on the adsorbent was investigated using pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion.[51]

5.1

Pseudo First-Order Kinetic Model (Lagergren Model)

The following is the standard form of Lagergren’s first-order equation (Lagergren, 1898):

d q d t = q e − q t K 1

Here, K 1 represents the pseudo-first-order sorption rate constant (min−1), qe and qt are the equilibrium and time t adsorbate amounts (mg/g). [Equation (4)] integrated becomes

log q e − q t = log q e − K 1 2.303 × t

The natural logarithm on a graph of (qe − qt ) plotted against time (t) should exhibit a linear correlation, with the slope representing the value of K 1 and the intercept representing the logarithm of qe . The graph of the linearized version of the pseudo-first-order equation is seen in [Fig. 9a]. [Table 1] displays the pseudo-first-order rate constant K 1, the estimated projected values qe (cal) acquired from the model, and the correlation coefficients R 2. The table indicates that the correlation coefficients for the pseudo-first-order model were below 0.85. This suggests that the pseudo-first-order model is adequate for fitting the kinetic data.[19] [47] [52]

Zoom Image
Fig. 9 (a) Pseudo-first-order kinetics for MB at multiple concentrations. (b) Pseudo-second-order kinetics for MB at multiple concentrations.
Table 1

Kinetic parameters of MB adsorption onto ACE for pseudo-first-order kinetics

Concentration (ppm)

q e(exp) (mg/g)

q e(cal) (mg/g)

K 1 (min−1)

R 2

20

1.95

0.30

0.03

0.2998

30

17.47

8.88

0.11

0.8587

40

47.98

21.10

0.13

0.9371

Likely, this method will correctly predict the behavior throughout the entire adsorption range. The linear plot demonstrates a strong correlation between the experimental and computed qe values, as seen in [Table 1]. In addition, the correlation coefficient (R) values for the pseudo-second-order kinetics were highly proximate to 1, suggesting that pseudo-first-order kinetics were followed by the adsorption process. The adsorption process is believed to be regulated by physisorption. Upon observation, the adsorption capacity qe (exp) at various concentrations was found to be more accurately predicted by the pseudo-first-order model, as indicated by the value of qe (cal). This confirms that the pseudo-second-order model was more suitable and applicable compared to the pseudo-first-order model. Since the value of R 2 was roughly 0.85, which is much lower than 0.99, the adsorption process of MB onto ACE was found to be well explained by the pseudo-first-order equation.


# 5.2

Pseudo-Second-Order Kinetic Model

The following is the definition of the pseudo-second-order chemisorption kinetic rate equation: if the pseudo-second-order mechanism determines the adsorption rate. The rate of solute absorption is precisely proportional to the square of the concentration difference between the solute and the equilibrium saturation concentration on the adsorbent, based on the interpretation of this model. The following is an example of the form of the rate equation for a pseudo-second-order kinetic model[19] [28]:

d q d t = q e − q t 2 × K 2

Here, K 2 (g mg−1 min−1) is the pseudo-second-order rate constant. The integration of this equation with the initial condition qt = 0 at t = 0 gives the following expression qe 2.

t qt = 1 K 2 q e 2 + t q e

By plotting t/qt versus t ([Fig. 9b]), qe and K 2 can be determined from slope and intercept, respectively, and listed in [Table 2].

Table 2

Kinetic parameters of MB adsorption onto ACE for pseudo-second-order kinetics

Concentration (ppm)

q e(exp) (mg/g)

q e(cal) (mg/g)

K 2 (g mg−1 min−1)

R 2

20

1.95

1.30

4.45 × 10−6

0.9991

30

17.47

18.88

1.31 × 10−2

0.9893

40

47.98

51.10

1.13 × 10−2

0.9978

The relationship between t/qt and t is linear, as seen in [Fig. 9b]. The values of K 2 and qe , obtained from the slope and intercept of the plot, are mentioned in [Table 2], along with the related correlation coefficients. Since the value of R 2 was roughly 0.3, which is much lower than 0.99, it was determined that the pseudo-second-order equation is unable to effectively clarify the adsorption process of MB onto ACE.


# 5.3

Intraparticle Diffusion Model

Weber and Morris (1962) expressed the intraparticle diffusion model as shown in [Eq. (8)]:

q t = K I t 1 / 2 + C I

where the intraparticle diffusion rate constant (mg/g min1/2) is denoted by K I, and the intercept by C I. The half-adsorption time (min1/2) is denoted by t 1/2, and the quantity of solute absorbed per unit weight of adsorbent per time (mg/g) by qt . Plot intercepts provide details on boundary layer thickness. The regression of qt versus t 1/2 has to be linear and go through the origin if intraparticle diffusion is to be the sole rate-determining step. Should it be otherwise, it suggests that there are other rate-controlling processes besides intraparticle diffusion.[23] [53]

Zoom Image
Fig. 10 Intraparticle diffusion model for adsorption of MB onto ACE.

Within a bulk adsorption system, intraparticle pore diffusion of adsorbate ions may occur, potentially resulting in a step that restricts the rate. Consequently, the intraparticle diffusion model delineated in [Eq. (8)] was employed to investigate the potential impact of intraparticle diffusion resistance on the adsorption process. When the adsorption mechanism is governed by the intraparticle diffusion phenomenon, a straight line through the origin can be drawn from a depiction of qt versus t 1/2. [Fig. 10] illustrates the adsorption of MB onto ACE at three distinct concentrations (20 ppm, 30 ppm, and 40 ppm), as predicted by the intraparticle diffusion model.[51]

Table 3

Intraparticle diffusion parameters for the adsorption of methyl blue onto ACE

Concentration (ppm)

C I (mg/g)

K I (mg/g min1/2)

R 2

20

0.36

0.15

0.9492

30

0.18

0.21

0.9001

40

0.06

0.31

0.8167

The first linear segment is attributed to the phenomenon of external surface adsorption, in which the adsorbate penetrates the solution and reaches the outer surface of the adsorbent substrate. The model parameters obtained for the intraparticle diffusion model are displayed in [Table 3]. The intercept values (C I) of the first linear segments suggest that external diffusion is rapid and poses minimal hindrance to mass transfer. Conversely, the rising values for the intercepts (C I) of the second linear segments suggest a stronger boundary layer impact.[54] This study illustrates that pore diffusion exhibits a certain level of hindrance to the overall rate.


# 5.4

Elovich Kinetic Model

The system is stated as a linear equation:

q t = 1 β ln αβ + 1 β ln t

for nonlinear:

q t = 1 β ln 1 + αβt

We define α as the initial desorption rate, quantified in milligrams per gram per minute. The desorption constant, denoted as β, is measured in units of g/mg. The value of 1/β indicates the quantity of accessible sites for adsorption, whereas the value of (1/β) implies the reciprocal of β. The term (αβ) indicates the amount of adsorption when the value of In t is zero.[55] [Fig. 11] displayed a graph illustrating the connection between the natural logarithm of time (In t) and the quantity (qt ) at various concentrations (20, 30, and 40 ppm). The values of α, β, and R 2 were computed and presented in [Table 4] based on the intercept and slope of the curve. The graph of qt vs. In t is used to ascertain whether the kind of adsorption occurring on the surface of the adsorbent is not uniform, namely, whether it is chemisorption or not.[53] [56]

Zoom Image
Fig. 11 Elovich kinetic model for MB at different concentrations.
Table 4

Elovich kinetic parameters for the adsorption of methyl blue onto ACE

Concentration (ppm)

α (mg/g min)

β (mg/g min1/2)

R 2

20

1.36

6.15

0.9159

30

1.4

5.83

0.9367

40

0.91

11.19

0.8058

#
# 6

Adsorption Isotherms

The most crucial piece of information for analyzing and designing an adsorption process is the information provided by the equilibrium isotherms. Adsorption isotherms are widely categorized into two types: Freundlich and Langmuir. When designing adsorption systems, it is helpful to have the parameters of these equilibrium isotherms. The Langmuir isotherm is based on the simple premise that the adsorbent’s surface is equally distributed with active sites (binding sites) that adsorb a single layer of adsorbate molecules in a species-independent fashion.[57] However, the Freundlich isotherm model, which is an exponential formula, applies to heterogeneous systems where adsorbed molecules interact with one another and is not limited to monolayer formation. Once all of the binding sites are filled, the sorption energy drops exponentially, and the concentration of the adsorbate on the adsorbent surface rises in tandem with the concentration of the adsorbate in the liquid. The adsorption mechanism, surface characteristics, and affinity can be inferred from the equilibrium parameters and the thermodynamic assumptions of isotherm models.[18] [54]

6.1

Langmuir Isotherm

According to the Langmuir isotherm, the strength of intermolecular forces diminishes dramatically with increasing distance. This model makes it easier to forecast whether an adsorbate monolayer will exist on the adsorbent’s exterior surface. The Langmuir isotherm assumes that:

  • The adsorption energy is the same at every site,

  • Atoms or molecules that have been adsorbed are concentrated at particular sites,

  • Only one molecule or atom can fit at a time in each site, and

  • The adsorbates do not interact with one another.

The state known as adsorption equilibrium occurs when a saturation point is reached and further adsorption is not possible. The isotherm equation, first stated by Langmuir in 1918, can be expressed in a linear form:

C e / q e = C e / q m + 1 / K L · q m

where Ce , is the concentration of the adsorbate at equilibrium (mg/L), qe , is the quantity of adsorbate adsorbed per unit mass of adsorbent (mg/g), qm is the maximal monolayer adsorption capacity of the adsorbent (mg/g), K L represents the Langmuir adsorption constant (L/mg).[51] [53]

Zoom Image
Fig. 12 Langmuir adsorption isotherm for the MB–ACE system.

The adsorption coefficients might be assessed by plotting Ce /qe vs Ce at various temperatures ([Fig. 12]). [Table 5] lists the qm , K L, R L, and R 2 calculated from the Langmuir isotherm. The dimensionless separator factor R L can be used to express the key features of the Langmuir equation, and it is described as:

R L = 1 / 1 + K L C 0

where K L is the Langmuir constant (L/mg) and C 0, is the initial MB concentration (mg/L).

If R L > 1 is the unfavorable adsorption

0 ˂ R L ˂ 1 is the favorable adsorption

R L = 0 is the irreversible adsorption

R L = 1 is the linear adsorption

Table 5

Langmuir isotherm’s parameter for MB adsorption onto ACE

Temperature (K)

q m (mg/g)

K L (L/mg)

R 2

303

45.23

2.78

0.9976

323

32.51

0.50

0.9983
Table 6

Determination of R L (separator factor)

Temperature (K)

K L (L/mg)

Concentration (mg/L)

R L

303

2.78

20

0.018

30

0.012

40

0.009

323

0.50

20

0.091

30

0.063

40

0.048

It is obvious from [Table 5] that, upon raising of solution temperature from 303 K to 323 K, the value of qm declined from 0.62 mg/g to 0.42 mg/g. A similar scenario was noticed for the value of K L. The decrease in the values of qm and K L with the increase of temperature demonstrates that the MB is favorably adsorbed by ACE at reduced temperatures, which reveals that the MB adsorption phenomenon is exothermic.

The obtained R values from [Table 6], which ranged from 0.018 to 0.048, show that the adsorption procedure is advantageous. These results indicate that the adsorption takes place on the structurally homogeneous ACE, where all of the adsorption sites are identical and energetically similar, and that it is monolayer adsorption at specific homogeneous sites within the adsorbent.


# 6.2

Freundlich Isotherm

The Freundlich model is obtained from the process of adsorption occurring on a heterogeneous surface, meaning it consists of several types of sites with varying affinities. The assumption is that the more powerful binding sites are filled before the weaker ones and that the strength of binding diminishes as the rate at which the sites are occupied rises. The Freundlich isotherm[36] [53] is mathematically represented as follows:

ln q e = 1 n ln C e + ln K F

where qe is the quantity of adsorbate adsorbed per unit mass of adsorbent (mg/g), K F, the Freundlich isotherm constant mg/g, Ce , the equilibrium concentration of the adsorbate (mg/L); n, the heterogeneity factor. The Freundlich isotherm ([Fig. 13]) fits the adsorption data, if the plot of Inq, against InC, gives a linear form. Other constants can be computed. from 1/n (the slope) and log K F (the intercept) of the graph.[18] [51] [58]

Zoom Image
Fig. 13 Freundlich adsorption isotherm for the MB–ACE system.
Table 7

Freundlich isotherm’s parameter for MB adsorption onto ACE

Temperature (K)

n

K F (mg/g)

R 2

303

0.94

1.06

0.9884

323

0.63

8.04

0.9847

Adsorption capacity, or K F, is a measure of the adsorbent’s favorability in the adsorption process. Good, challenging, and bad adsorption properties are indicated by values of 2 < n < 10, l < n < 2, and < 1.


# 6.3

Temkin Isotherm

The Temkin isotherm considers the adsorbent–adsorbate interactions. The adsorbent–adsorbate connections are assumed to cause a linear decrease in the heat of the adsorption of molecules increasing coverage. According to Temkin and Pyzhev (1940), the Temkin model is shown:

q e = B ln K T C e

Rearranging this will allow it to:

q e = B ln K T + B ln C e

where B = RT/B T is the constant associated with the adsorption heat (L/mg); qe is the quantity of adsorbate adsorbed at equilibrium (mg/g); B T is the slope of the curve; Ce is the equilibrium concentration of adsorbate (mg/L); T is the absolute temperature; R is the universal gas constant (8.314 J/mol K); and K T is the equilibrium binding constant (L/mg).[51] [58] [59]

Zoom Image
Fig. 14 Temkin adsorption isotherm for the MB–ACE system.
Table 8

Temkin isotherm’s parameter for MB adsorption onto ACE

Temperature (K)

B T (J/mol)

K T (L/mg)

R 2

303

1.489

1.04

0.9973

323

1.925

0.08

0.9175

A plot of qe versus In Ce ([Fig. 14]) at different temperatures produces a linear line, and the values of B T and K T can be computed from the slope and the intercept, respectively. The values of R ([Table 8]) indicate that this isotherm is less fitted than the Langmuir isotherm. Based on the correlation coefficients (R 2), the Langmuir isotherm model provided the best fit for the experimental data compared to the Freundlich and Temkin models, indicating monolayer adsorption on a homogeneous ACE surface.


#
# 7

Phytotoxicity

Agricultural production may be negatively impacted by the discharge of dye-loaded adsorbent, treated dye, and dye-containing water into the environment. These actions may also negatively affect aquatic life and soil fertility.[60] [61] Therefore, to guarantee environmental safety, it is important to evaluate the phytotoxicity of these media. The phytotoxicity study used chickpea (Cicer arietinum) seeds toward experimental MB dye, treated dye, dye-loaded ACE adsorbent, and freshwater. The findings are depicted in [Fig. 15] and summarized in [Table 9]. Remarkably, no germination was observed in the experimental MB solution (10 mg/L), whereas treated dye, dye-loaded ACE adsorbent, and freshwater showed 73%, 87%, and 100% germination, respectively. Analogously, in the treated dye, dye-loaded ACE adsorbent, and freshwater, the mean root and shoot lengths rose regularly.[19] [62] The acquired results provide compelling evidence for the dye-loaded ACE adsorbent and the treated dye’s nontoxic behavior toward seed germination. The percentage of phytotoxicity and tolerance index was determined by applying the formulas below [63] [64] [65]:

Percentage of phytotoxicity % = Radicle length of control − Redicle length of the test Radicle length of the control sample × 100
Index of tolerance % = Mean length of the longest root in the treatment Mean length of the longest root in the control sample × 100
Zoom Image
Fig. 15 Phytotoxicity study on Cicer arietinum seeds toward (a) fresh water, (b) experimental MB dye, (c) treated MB solution, and (d) MB-loaded ACE adsorbent.
Table 9

Phytotoxicity study on chickpea (Cicer arietinum) seeds

Study area

Germination (%)

Mean root length (cm)

Mean shoot length (cm)

Phytotoxicity (%)

Index of tolerance (%)

Experimental MB dye

No germination

0

0

-

-

Treated MB solution

73

4.17

2.96

29.92

70.08

MB-loaded ACE adsorbent

87

5.24

4.8

11.93

88.07

Freshwater

100

5.95

6.38

0

0
Table 10

Comparison of various adsorbents for MB removal

Adsorbent material

Activation method

Maximum adsorption capacity (qm , mg/g)

Best fit isotherm model

Reference

Notes:

  • Costs are approximate and reflect raw material accessibility and processing needs.

  • Langmuir model fit suggests monolayer adsorption on homogeneous surfaces.

  • Freundlich model fit indicates adsorption on heterogeneous surfaces.

Activated carbon eggshell (ACE)

Lemon juice (citric acid)

45.23

Langmuir (R 2 = 0.9976)

This study

Commercial activated carbon

Commercial chemical

150–400

Langmuir/Freundlich

[21]

H₃PO₄-activated eggshell

Phosphoric acid

120

Langmuir

[28], [38]

Bombax ceiba fruit shell activated carbon

H₃PO₄

52.8

Langmuir

[37]

Marine algae (Gracilaria sp.)

NaOH

37.0

Freundlich

[24], [25]

Pumice powder

None

32.47

Langmuir

[66], [67]

Cuttlefish bone (green-synthesized)

Biogenic route

41.20

Langmuir

[68]

Fe-hydroxyapatite composite

Chemical precipitation

60-70

Langmuir

MB adsorption by Fe-HAp composite

# 8

Conclusions

According to the current research findings, ACE can be utilized as an inexpensive adsorbent to effectively remove MB from hydrocarbon solutions. Several parameters that influence removal efficiency were investigated. Regarding ACE loading, beginning pH, and dye concentration, the ideal values are 1 g/L, 250 rpm, pH 2.0, and 40 mg/L, respectively. All of these values are optimal. In 70 min, the equilibrium of the adsorption process was attained, and it proceeded with pseudo-first-order kinetics. At a temperature of 303 K, the maximal adsorption capacity (q) was determined to be 0.42 mg/g. This value was found to be a perfect fit for the Langmuir isotherm, with the correlation coefficient R being precisely 0.9656. There is a possibility that physical adsorption is responsible for the decrease in adsorption capacity that occurs when the temperature is raised. Compared to other natural and synthetic adsorbents, it is easily discernible that ACE possesses a better capacity for MB adsorption. After the ACE has been used, it can also be disposed of securely. Based on these observations, we can therefore conclude that ACE might be a suitable contender to serve as a nontoxic adsorbent throughout the process of eliminating methyl blue from the aqueous solution.


#

Abbreviations

ACE: Activated carbon eggshell
CE: Carbonized eggshell
MB: Methyl blue
FTIR: Fourier transform infrared spectroscopy
SEM: Scanning electron microscopy
EDS: Energy-dispersive X-ray spectroscopy
ZPC: Zero point charge
C 0 : Initial dye concentration
Ce : Equilibrium dye concentration
Ct : Dye concentration at a time interval
qt : Adsorption capacity at t time
qe : Equilibrium adsorption capacity
q max : Maximum adsorption capacity
K 1 : Pseudo-first-order sorption rate constant
K 2 : Pseudo-second-order rate constant
K F : Freundlich isotherm constant
K I : Intraparticle diffusion rate constant
K L : Langmuir adsorption constant
R L : Separator factor


#

Contributorsʼ Statement

A.S.H.: validation, investigation, formal analysis, writing—original draft. S.K.D.: conceptualization, methodology, formal analysis, writing—original draft, visualization, software, supervision. M.A.B.: validation, investigation, writing-review and editing. P.K.D.: visualization, writing—review and editing. M.R.H.: writing—review and editing.

Conflict of Interest

The authors declare that they have no conflict of interest.

Acknowledgments

The authors are grateful to the Chemistry Discipline, Khulna University, Khulna–9208, for providing the necessary laboratory facilities.

Supplementary Material


Correspondence

Sagar Kumar Dutta
Chemistry Discipline, Khulna University
9208 Khulna
Bangladesh   

Publication History

Received: 11 March 2025

Accepted after revision: 06 June 2025

Article published online:
02 July 2025

© 2025. This is an open access article published by Thieme under the terms of the Creative Commons Attribution License, permitting unrestricted use, distribution, and reproduction so long as the original work is properly cited. (https://creativecommons.org/licenses/by/4.0/).

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Bibliographical Record
Al Shariar Hasan, Sagar Kumar Dutta, Abul Bashar, Palash Kumar Dhar, Rezaul Haque. Methyl Blue Dye Adsorption Using Activated Eggshell: Kinetics, Isotherm, and Phytotoxicity Analysis. Sustainability & Circularity NOW 2025; 02: a26302599.
DOI: 10.1055/a-2630-2599

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Zoom Image
Scheme 1 Overall formation process of activated eggshell.
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Fig. 1 Determination of the point-zero charge (pHPZC) of ACE.
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Fig. 2 FTIR spectrum of ACE (a) before and (b) after adsorption.
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Fig. 3 EDS analysis of the ACE.
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Fig. 4 (a) Before SEM micrograph of ACE. (b) After the SEM micrograph of ACE.
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Fig. 5 Effect of pH on % removal of MB by ACE (C 0 = 10 mg/L, ACE dosage 1.0 g/L, contact time 60 min, temp. = 303 K).
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Fig. 6 Effect of adsorbent dosage on the (a) percentage removal of MB dye and (b) adsorption capacity of ACE (C 0 = 40 mg/L, contact time 60 min, pH 2.0, T = 303 K).
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Fig. 7 Effect of initial dye concentration and contact time on the (a) percentage removal of MB dye and (b) adsorption capacity of ACE (adsorbent dosage 1 g/L, pH 2.0, T = 303 K).
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Fig. 8 Effect of temperature on (a) adsorption capacity and (b) dye removal efficiency.
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Fig. 9 (a) Pseudo-first-order kinetics for MB at multiple concentrations. (b) Pseudo-second-order kinetics for MB at multiple concentrations.
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Fig. 10 Intraparticle diffusion model for adsorption of MB onto ACE.
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Fig. 11 Elovich kinetic model for MB at different concentrations.
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Fig. 12 Langmuir adsorption isotherm for the MB–ACE system.
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Fig. 13 Freundlich adsorption isotherm for the MB–ACE system.
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Fig. 14 Temkin adsorption isotherm for the MB–ACE system.
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Fig. 15 Phytotoxicity study on Cicer arietinum seeds toward (a) fresh water, (b) experimental MB dye, (c) treated MB solution, and (d) MB-loaded ACE adsorbent.