Comparison of methylene blue removal efficiency of activated carbon and carbon nanosheets derived from olive stone from aquatic environment

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Comparison of methylene blue removal efficiency of activated carbon and carbon nanosheets derived from olive stone from aquatic environment

https://doi.org/10.21203/rs.3.rs-5196609/v1

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In recent years, the increase in population growth and the rapid development of industries have given way to a rise in the usage of natural resources and waste production. Solid waste management (SWM) is an essential component of an environmental management system. SWM approaches are being adapted to make them more practical and effective by environmental regulations and to promote sustainability through the application of the “reduce”, “reuse”, and “recycle” (3R) principles. Therefore, the present study focuses on the reuse of waste as an adsorbent agent with a low cost for removing methylene blue. To achieve such feat, olive stones were milled, pyrolyzed, and sonicated to synthesize activated carbon and carbon nanosheets. Additionally, the study investigated the impact of five parameters (dose of adsorbent, pH, primary pollutant concentration, and temperature) on the adsorption process. FESEM and TEM analyses were carried out in order to make clear what the characteristics of the adsorbent were. The isotherm assessments show that the data is quite fitting for the Langmuir model for adsorption by activated carbon and carbon nanosheets. Additionally, the adsorbent obtained through kinetic modeling indicate that the experimental data is well-matched by the pseudo-second-order kinetic model for activated carbon and carbon nanosheets. Synthesized activated carbon and carbon nanosheets from olive stones are low-cost adsorbents with good adsorption properties. They reduce solid waste generation and are efficient adsorbents when it comes to removing methylene blue that is present in aquatic environments.

Physical sciences/Nanoscience and technology/Other nanotechnology

Physical sciences/Engineering

activated carbon

carbon nanosheets

olive stones

decolorization

kinetic

wastewater

Environmental pollution is a significant issue linked to swift industrialization, urbanization, and improved living standards [1]. The substantial increase that we have witnessed in world population, the fast pace with which industries have developed, along with the development in urban living spaces, have all led to a significant rise in the amount of waste that is produced [2]. Industrial operations produce solid, liquid, and gaseous emissions, as well as residual and unwanted waste. Waste management effectively reduces waste generation, improves environmental conditions, and lowers water and energy consumption. Throughout production, waste is generated in the form of air emissions, liquid discharge, and solid waste [3]. Activated carbon has garnered extensive attention in the past decades as a result of its usefulness as an adsorbent. In recent years, the rising cost and decreasing accessibility of commercial activated carbon have highlighted how important it is to use waste materials [4]. The choice of precursor for low-cost adsorbent development depends on various factors [5].

Because of human activities that lead to pollution, fast pace of industrialization, and development of urban areas without careful planning, the threats of air, land, and water pollution are more serious than ever. Water pollution is a significant environmental problem that threatens the humankind in the current century, with severe consequences. Other than the lack of water resources caused by drought and misappropriation of such resources, huge sums of wastewater that are produced have put significant pressure on people’s access to clean water. There are various types of water contaminants [6, 7].

The dye industry produces a significant amount of wastewater during its manufacturing processes. This wastewater contains various organic and inorganic compounds, heavy metals, and other contaminants which might cause harm to the environment and human health if not properly treated.

Activated carbon is the most effective method for treating this wastewater.

Olive stone is an agricultural/industrial waste, easily attainable, and could be collected from an olive farm and olive oil industries in Tarom, Zanjan, Iran. Methylene blue powder could be purchased from Pajoohesh Gostar Zanjan Company (Zanjan, Iran) in purity.

Preparation of activated carbon and carbon nanosheets

Olive stones were chosen as cost efficient adsorbent for eliminating Methylene blue, an inorganic pollutant. To prepare the stones, distilled water was used to wash them and remove any interferences and then they were dried in an oven at 150 ºC for one hour. The dried stones were milled into a powder and then pyrolyzed for two hours at 700 ºC under N2 flow using a vertical tubular furnace (Nobertherm) to produce activated carbon. Activated carbon powder was sonicated in distilled water for one hour using a Sonics & Materials Inc. 500Watt ultrasonic processor equipped with a ½” diameter probe operating at a frequency of 20 kHz and an amplitude of 40%. The resulting suspension contains carbon nanosheets.

Characterization

Characteristics of synthesized activated carbon and carbon nanosheets were identified by FESEM and TEM analysis which is been implemented by Beem Gostar Taban.

Preparation of dye and analysis

The stock solution of Methylene blue with a concentration of 1000 mg/l was prepared and then it was diluted to the preferred concentration (20, 50, 100, 150, and 200 mg/l). After adsorption, the dye concentration in the supernatant liquor was calculated with the help of a UV-visible spectrophotometer. The parameters were optimized using 20 mg/l of dye and 500 mg/l of adsorbent.

Adsorption studies

This study examines the batch adsorption experiments of methylene blue on activated carbon and carbon nanosheets synthesized from olive stones. This study measures the consequence of preliminary dye concentration, adsorbent dose, contact time, pH, and temperature on the amount of dye adsorbed per unit mass of activated carbon and carbon nanosheets. The study measures the effect of initial dye concentration, adsorbent dose, contact time, pH, and temperature on the amount of dye adsorbed per unit mass of activated carbon and carbon nanosheets

$$\:{q}_{e}=v\left(\frac{{C}_{o}-{C}_{e}}{w}\right)$$

Where v is the volume of solution (L), w is the mass of adsorbent (g), C₀ is the initial concentration (mg/l) and C_e is the equivalent concentration (mg/l).

Furthermore, isotherm models, kinetic models, and thermodynamic studies have been implemented for both adsorbents.

Characterization

FESEM analysis

The impact of sonication on activated carbon can be observed through Field Emission Scanning Electron Microscopy (FESEM) analysis. This technique enables the observation of surface morphology and structural features of materials. Upon sonication, the resulting activated carbon nanosheets (Fig. 1b) are expected to exhibit distinct surface morphology compared to untreated activated carbon (Fig. 1a). In the synthesis of carbon nanosheets from activated carbon, sonication is employed to break down the activated carbon particles into smaller sizes and disperse them in the liquid medium [21]. This process can lead to carbon nanosheets with a larger surface area being formed, which can potentially be applied in the domain of energy storage and conversion, catalysis, and water treatment. The FESEM analysis results for activated carbon show that the particle size ranges from 100–150 nm. Additionally, the size of carbon nanosheets ranges from 80–100 nm.

TEM analysis

Activated carbon and carbon nanosheets are two types of carbon materials that have unique features in their structures and properties. TEM analysis, which is a powerful technique, can provide valuable insights into their morphologies and structures at the nanoscale level. Activated carbon is considered among the most porous materials that has a large surface area, which makes it an excellent choice for adsorption and purification applications. TEM analysis has revealed that activated carbon (Fig. 2a) has an amorphous structure with irregularly shaped pores and a disordered arrangement of carbon atoms [22], which is responsible for its excellent adsorption properties. In contrast, carbon nanosheets are thin layers of graphene that are only a few nanometers thick. They have a well-defined two-dimensional structure with a significant aspect ratio and outstanding mechanical, electrical, and thermal features. The TEM analysis of carbon nanosheets (Fig. 2b) shows a flat and regular structure with a hexagonal arrangement of carbon atoms [23]. This unique arrangement of carbon atoms gives carbon nanosheets their exceptional properties, making them ideal for a large number of use cases, such as energy storage, catalysis, and electronics.

Effect of contact time on adsorption

The contact time denotes the duration for which the adsorbate directly interacts with the adsorbent. The impact of contact time on the adsorption of methylene blue on activated carbon (Fig. 3a) and carbon nanosheets (Fig. 3b) can be studied by monitoring the quantity of dye adsorbed at various time intervals (25, 45, 65, 85, 105, and 120 min). Typically, the adsorption of methylene blue on activated carbon follows moderate uplift and reached a peak at 120 min, which removed 63% of the dye. Furthermore, as is shown in Fig. 3b the removal followed by a gradual decrease using carbon nanosheets can be the result of a lack of available adsorption spots on the surface of the adsorbent.

Effect of temperature on adsorption

Temperature is regarded as a significant factor that can significantly affect the adsorption of methylene blue on activated carbon and carbon nanosheets. The adsorption capacity of activated carbon and carbon nanosheets for methylene blue is usually affected by the temperature of the system. Following Fig. 4, removing of methylene blue by activated carbon and carbon nanosheets declined by increasing temperature which is 27% and 40% at 55ºC respectively.

Effect of pH on adsorption

The supplied graphs in Fig. 5, depict that range of pH (3, 5, 7, and 10) has a significant effect on methylene blue adsorption on activated carbon and carbon nanosheets. By increasing pH, removing of methylene blue by activated carbon and carbon nanosheets escalated and reached the vertex at pH = 10.

Effect of initial dye concentration on adsorption

The preliminary concentration of methylene blue in a solution can have a striking impact on the adsorption of methylene blue on activated carbon and carbon nanosheets. In general, the higher the initial concentration of methylene blue, the lower adsorption capacity of the adsorbent. The reason is that at high initial concentrations, the available adsorption sites on the adsorbent become saturated more quickly, leaving fewer sites available for further adsorption. As a result, the adsorption capacity of the adsorbent decreases as the initial concentration of dye increases. The effect of initial dye concentration ranges from 20, 50, 100, 150, and 200 ppm has been determined. As is observed in Fig. 6, the removal of methylene blue with both adsorbents has moderately declined by increasing methylene blue concentration.

Effect of adsorbent dose on adsorption

The adsorbent dose is regarded as a salient factor in the adsorption process and can affect the efficiency with which methylene blue is adsorbed by activated carbon and carbon nanosheets. An increase in the adsorbent dose can improve the adsorption capacity of activated carbon for methylene blue. However, beyond a certain point, the adsorption capacity may reach a plateau or even decrease due to pore blockage, reducing the available surface area for adsorption. Similarly, carbon nanosheets have shown excellent adsorption properties for dyes due to their unique structural features. The adsorption capacity of carbon nanosheets for methylene blue can also grow by increasing the adsorbent dose. Figure 7 gives data about the rising removal of methylene blue by increasing adsorbent dose also, the adsorption capacity has an upward tendency.

Isotherm studies

Adsorption isotherms describe the relationship that is present in the concentration of a pollutant in a solution and the amount of adsorbent required to remove it. Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm models were studied for methylene blue removal (Table 2) with activated carbon and carbon nanosheets collected from olive stone. The linear equation of isotherms is mentioned in Table 1.

Table 1

Equation of isotherm models
	Isotherm model	Linear equation	parameters
1	Langmuir [26]	$\:\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{K}_{L}{Q}_{m}}+\frac{{C}_{e}}{{Q}_{m}}$	where Q_m (mg/g or mol/g) and C_e (mol/L) are the maximum adsorption capacity and the concentration at equilibrium, in that order, and K_L is the Langmuir constant, which stands for the energy of adsorption or the equilibrium constant of adsorbate-absorbent equilibrium (L/g or L/mol which is dependent on the unit of Q_m and C_e).
2	Freundlich [26]	$\:\text{log}\left({q}_{e}\right)=log{K}_{F}+\left(\frac{1}{n}\right)log{C}_{e}$	where C_e is the equilibrium concentration of the adsorbate (mol/L). K_F and 1/n are the Freundlich constants that are representative of the adsorption capacity and the intensity of adsorption, respectively.
3	Temkin [27]	$\:{q}_{e}=\frac{RT}{{b}_{T}}ln{A}_{T}+\left(\frac{RT}{{b}_{T}}\right)ln{C}_{e}$	By plotting q_e vs. ln (C_e), both A_T and b_T constants are determined
4	Dubinin-Radushkevich [27]	$\:ln{q}_{e}={q}_{s}-{K}_{D-R}{Ɛ}^{2}$	Where is Dubinin-Radushkevich adsorption constant and q_s is lnQ_m (Q_m is adsorption capacity)

Table 2

Correlation coefficient of isotherm models
Isotherm models		Langmuir	Freundlich	Temkin	Dubinin-Radushkevich
parameters	Activated carbon	R² = 0.92	R² = 0.49	R² = 0.44	R² = 0.70
parameters	Carbon nanosheets	R² = 0.99	R² = 0.61	R² = 0.56	R² = 0.57

The results of Table 4 show that the correlation coefficient (R²) for the Langmuir isotherm model for activated carbon and carbon nanosheets were 0.92 and 0.99 respectively which is just beyond than other three models. As the results show in Figs. 8 and 9, the Langmuir isotherm proved to be a better fit for the data for both of the adsorbents. The Langmuir isotherm model is extensively applied for the description of the adsorption of molecules onto a solid surface. This model forms the basis of a simple theoretical framework for understanding the behavior of adsorbates on surfaces. The Langmuir isotherm model describes the adsorption of molecules onto a solid surface by assuming a monolayer coverage. It is based on the following assumptions [27, 28]:

Adsorption occurs on a homogeneous surface with a fixed number of identical adsorption sites.
The adsorption process is reversible, meaning molecules can both adsorb and desorb from the surface.
No cross interaction is observed between adsorbed molecules.
The adsorption process reaches equilibrium at a constant temperature.

Adsorption kinetics

Pseudo-first-order and pseudo-second-order kinetic models are mathematical representations that are implemented to describe the rate of chemical reactions. These models are often applied in the field of chemical kinetics to analyze and predict the behavior of reactions, particularly in the context of adsorption processes (Fig. 10).

The adsorption kinetics of methylene blue onto activated carbon and carbon nanosheets were investigated in optimum conditions using two kinetic models. the Lagergren pseudo-first order and pseudo-second order models whose equations are mentioned below [29]:

Pseudo-first order:

$$\:Ln\:\left({q}_{e}-{q}_{t}\right)=Ln{q}_{e}-{k}_{1}t$$

Pseudo-second order:

$\:\frac{t}{{q}_{t}}=\frac{1}{{k}_{2}{q}_{e}^{2}}+\frac{1}{{q}_{e}}t$

(3)

where q_t (mg g^− 1) is the adsorption at time t (min); q_e (mg g^− 1) is the adsorption capacity at adsorption equilibrium; and k₁ (min^− 1) and k₂ (g mg^− 1 min^− 1) are the kinetic rate constants for the pseudo-first-order and pseudo-second-order models, respectively.

Based on Table 3 information, the adsorption of methylene blue onto activated carbon may be well illustrated by pseudo-second-order as the correlation coefficient is just over than pseudo-first-order. Moreover, for carbon nanosheets pseudo-second-order with a higher correlation coefficient (0.98) was found to supply a better fit for the data.

Table 3

Kinetic models correlation coefficients
Type of kinetic models	pseudo-first-order	pseudo-second-order
Activated carbon	R² = 0.61	R² = 0.98
Carbon nanosheets	R² = 0.61	R² = 0.98

Thermodynamic study

The effect of temperature on the adsorption of methylene blue on activated carbon and carbon nanosheets can be explained by the thermodynamics of the adsorption process. The adsorption of methylene blue on the adsorbent is a spontaneous process which includes transferring methylene blue ions from the solution phase to the adsorbent surface. The energy required for this process is made possible through the difference in free energy between the two phases. The thermodynamic parameters that describe the adsorption process consist of the Gibbs free energy (∆G°), enthalpy (∆H°), and entropy (∆S°) [30]. The adsorption of methylene blue on activated carbon and carbon nanosheets is generally a physisorption process, which means that the energy of adsorption is low and the adsorbent-adsorbate interaction is mostly the result of the Van der Waals forces. The enthalpy of adsorption (∆H°) onto activated carbon and carbon nanosheets was negative, indicating that the adsorption process is exothermic also the entropy of adsorption (∆S°) for both adsorbents was negative, representing the emergence of order (Fig. 11).

$\:K=\frac{{C}_{s}}{{C}_{e}}$	(4)
$\:\varDelta\:G={\varDelta\:G}^{○}+RTlnK$	(5)
$\:\varDelta\:G={\varDelta\:G}^{○}+RTlnK$	(6)

Table 4

Thermodynamic parameters
parameters		∆H°	∆S°
adsorbents	Activated carbon	-21841	-88
adsorbents	Carbon nanosheets	-36748	-119

Considering the results of activated carbon and carbon nanosheets synthesized from olive stones as a cost effective adsorbent that has desirable features, in addition to being eco-friendly materials that can considerably drive down the production of waste, they are also efficient adsorbents for methylene blue elimination with a removal percentage of 60 and 81 for activated carbon and carbon nanosheets respectively, from aquatic solution in optimum condition.

Disclouser statement

The authors declare that they have no conflict of interest.

Funding

No funding has been allocated for this study.

Author Contributions Statement

Negar Hariri: performing experiment, data analysis, writing - original draft

Hossein Danafar: conceptualization, methodology and reviewing manuscript

Zohre Farahmandkia: advisor

Mehran Mohammadian fazli: conceptualization, methodology and reviewing manuscript

data availability statement

The datasets used during the current study available from the corresponding author on reasonable request

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Comparison of methylene blue removal efficiency of activated carbon and carbon nanosheets derived from olive stone from aquatic environment

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	Isotherm model	Linear equation	parameters
1	Langmuir [26]	\(\:\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{K}_{L}{Q}_{m}}+\frac{{C}_{e}}{{Q}_{m}}\)	where Q_m (mg/g or mol/g) and C_e (mol/L) are the maximum adsorption capacity and the concentration at equilibrium, in that order, and K_L is the Langmuir constant, which stands for the energy of adsorption or the equilibrium constant of adsorbate-absorbent equilibrium (L/g or L/mol which is dependent on the unit of Q_m and C_e).
2	Freundlich [26]	\(\:\text{log}\left({q}_{e}\right)=log{K}_{F}+\left(\frac{1}{n}\right)log{C}_{e}\)	where C_e is the equilibrium concentration of the adsorbate (mol/L). K_F and 1/n are the Freundlich constants that are representative of the adsorption capacity and the intensity of adsorption, respectively.
3	Temkin [27]	\(\:{q}_{e}=\frac{RT}{{b}_{T}}ln{A}_{T}+\left(\frac{RT}{{b}_{T}}\right)ln{C}_{e}\)	By plotting q_e vs. ln (C_e), both A_T and b_T constants are determined
4	Dubinin-Radushkevich [27]	\(\:ln{q}_{e}={q}_{s}-{K}_{D-R}{Ɛ}^{2}\)	Where is Dubinin-Radushkevich adsorption constant and q_s is lnQ_m (Q_m is adsorption capacity)

\(\:K=\frac{{C}_{s}}{{C}_{e}}\)	(4)
\(\:\varDelta\:G={\varDelta\:G}^{○}+RTlnK\)	(5)
\(\:\varDelta\:G={\varDelta\:G}^{○}+RTlnK\)	(6)