1 Introduction

Pollutants can cause eye sickness, cyanosis, vomiting, shortness of breath, burns, and other health problems. Some of the industries that make these pollutants are paper, printing, leather, textiles, electroplating, and cosmetics [1,2,3,4]. They have a huge impact on human health and the environment because the majority of them are toxic and non-degradable. As a result, it's critical to get rid of these harmful chemicals from the water supply. Adsorption technology, the easiest and most cost-effective treatment method among chemical, physical, and biological treatments, is widely used [5]. A common type of direct dye is anionic and water-soluble, making it easy to apply and cost-effective in the textile industry. Viscose dyeing, silk and silk printing, leather dying, the production of organic pigment sediment color, etc. all employ methylene blue, a commercial dye [6,7,8,9]. As a result, scientists are focusing their efforts on developing new technologies for removing MB from contaminated water. To date, several strategies have been used to extract or degrade MB from contaminated water systems, including biological, physical, and chemical approaches [1, 10, 11]. Adsorption, coagulation, biological processes, membrane separation, ion exchange methods, and flocculation techniques are some of the ways used to remove harmful compounds from wastewater. Other approaches include photocatalytic degradation and electrochemical procedures that are used for these purposes [12,13,14,15,16]. Separating hazardous compounds from industrial effluents can be accomplished using any of these methods. However, new solutions are constantly being looked at because current procedures are either ineffective or produce harmful waste. This method of dye separation has become popular because of its superior performance. Adsorbents are readily available, the operation is simple, and reusability and regeneration are cost-effective. Therefore, it is widely utilized around the world. Due to its pilot-scale use, the adsorption approach can be used to offer exact feedback for industrial-scale operations [17,18,19]. A carbon nanoparticle (CNP) is a form of carbon that is non-graphite and can be produced by activating carbon obtained from agricultural wastes, such as coconut shells, bamboo, wood chips, or sawdust. Two steps are involved in the activation process. The raw material is first charred, then activated chemically, thermally, or with steam to drive the highly porous material [20]. The high porosity, huge surface area, and thermal stability of CNP improve its adsorption efficiency by making it a flexible adsorbent. That led to the usefulness of CNP for wastewater treatment. Its high production costs make it prohibitively expensive, which leads to the limits of its use most of the time. As a result, companies are scrambling to find raw materials for making CNP that are less expensive, more readily available, and more efficient [21,22,23]. Various doping approaches, such as the addition of additional functional groups and cross-linking, have been used to improve the efficiency of alginate-beads for MB adsorption. The use of lignocellulosic biomass is suggested as an alternative carbon precursor because it is abundant, renewable, and low in cost [1, 24, 25]. The surfactant-impregnated AC-alginate beads ability to remove MB has not been studied. In light of the above [26, 27]. From these findings and in continuation of our efforts to produce several matrices of biodegradable composites for industrial and environmental applications [1, 28,29,30,31], we have constructed a composite from alginate and CNP in the form of microsphere beads with a highly porous structure for efficient removal of methylene blue as an example of an industrial dye.

2 Experimental

2.1 Materials

All chemicals such as calcium chloride (CaCl2, 98%), MB Sodium alginate (98%), sodium hydroxide (NaOH, 99%), activated charcoal and hydrochloric acid (37%) having molecular were provided by Sigma Aldrich. As well as sodium hydroxide, hydrochloric acid, and activated charcoal were purchased from Merck, Germany.

2.2 Preparation of Alginate/Activated Charcoal Nanocomposite Beads

Sodium alginate and charcoal nanoparticle composite beads (SA/AC) were prepared by dissolving 2 g sodium alginate in 100 mL distilled water for 3 h. To remove any remaining gas bubbles, the resultant solution was degassed by sonication for 2 h. 0.2 g of active charcoal was added to the alginate solution at room temperature and stirred for 3 h. Using the NRC-5 Pro microencapsulation device (developed and built by our group), we made SA/Ac composite beads by dropping an alginate Active charcoal solution into a magnetically stirred 3% (w/v) CaCl2 solution. The proposed chemical structure of SA/AC is illustrated in Scheme 1.

Scheme 1
scheme 1

Schematic presentation of alginate activated charcoal structure

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2.3 Characterizations

Shimadzu FT-IR Spectrophotometer (8007S) spectra were measured from 500 to 4000 cm−1 at 4 cm−1 resolution. SEM was carried out using JEOL JEM-2100 electron microscope equipped with 100 × magnification and 120 kV acceleration voltage was used to obtain an image of the SA and AC/SA beads. Thermal analysis (STA6000, Perkinelmer) was performed at 10 °C/min.

2.4 Adsorption Studies

A stock solution of MB with a concentration of 2000 ppm was diluted with water to produce a series of aqueous solutions with varying concentrations. The following measures were taken in general: SA/Ac nanocomposite beads are prepared in MB solution using appropriate starting concentrations and time intervals. A spectrophotometer (UNICO UV-2000) was used to measure the aliquots at maximum absorbance (668 nm). It is possible to conduct batch adsorptions by adsorbing to SA/Ac composite beads regularly, in a variety of pH solutions, and at regular intervals. The following equation was used to determine the amount of MB dye adsorbed on SA/Ac composite beads at the adsorption equilibrium (in mg/g).

$$q_{e} = \, \left( {\left( {{\text{C}}_{{\text{o}}} - {\text{C}}_{{\text{e}}} } \right){\text{V}}} \right)/{\text{W}}$$
(1)

Concentrations of dye at initial and equilibrium (Co and Ce respectively) in SA/Ac nanocomposite bead solution (mg/L), dye solution volume in L (V), and nanocomposite weight (W) (g).

In order to investigate the recyclability of beads, we mixed 10 mg with 20 mL of MB solution using a batch concentration of 100 mg/L and a pH of 8. The beads were then agitated with 0.5 M sodium chloride in C2H5OH (50:50 v/v) solution as an adsorbing reagent. The beads were utilised four times, with Eq. (2).

$$Desorption =\frac{Mass\,Desorbed}{Mass\,Adsorbed}$$
(2)

3 Results and Discussion

3.1 Adsorbent Characterization

3.1.1 SEM

To characterize the surface structure of beads produced, scanning electron microscopy (SEM) was used. At various magnifications, SEM images indicated a high degree of porosity and a number of voids in the morphology. As illustrated in Fig. 1, the surface of the beads was extremely porous, providing additional active sites for MB adsorption. Because the porous structure of alginate active charcoal provides high surface area of 650 m2/g for MB adsorption, its capacity to absorb MB molecules has improved.

Fig. 1
figure 1

SEM images of (A) sodium alginate and (B, C) sodium alginate/activated nanocomposite with different magnifications

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3.1.2 FTIR Spectra

The FTIR spectra for SA, and SA/AC beads before and after MB adsorption are shown in Fig. 2. The peak at 3400 cm−1 corresponds to the hydroxyl group (–OH group). The peak shifts to 3300 cm−1 after MB adsorption which represents the H-bonding interaction between AS/AC beads and MB. Also, significant peaks were appeared in the spectra after MB adsorption such as at 1314 cm−1 (C–N stretching), 1407 cm−1 (N–H), and 1592 cm−1 (C=N of aromatic ring).

Fig. 2
figure 2

FTIR analysis of (A) SA, (B) SA/AC and SA/AC after dye adsorption

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3.1.3 TGA

In Fig. 3, the thermogravimetric analysis (TGA) and its derivatives are shown for SA and SA/AC beads with a temperature range of 25 to 700 °C and a heating rate of 10 °C/min. Weight loss between 50 and 200 °C is attributed to moisture content. The degradation of SA and SA/AC beads above 200 °C is caused by the thermal degradation of sodium alginate, and the final weight loss is 46 and 49%, respectively. It seems that there is no significant difference in the thermal stability with the presence of AC.

Fig. 3
figure 3

(A) TGA and (B) DTG analysis of SA and SA/AC beads

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3.2 Application of the Alginate/Activated Charcoal Nanocomposite for Dye Adsorption

3.2.1 Effect of pH on the Adsorption

An adsorbent's and an adsorbate's chemical characteristics are greatly influenced by the pH of the solution in the adsorption process. A range of pH values were assessed in the current study for the adsorption of the MB dye, as shown in Fig. 4. Variation in pH can result in the ionization of the MB molecule, which then interacts with the adsorbent's surface. The MB (cationic dye) has a positive charge and interacts strongly with negatively charged surfaces. The adsorption capacity increased with pH, reaching 94 mg/g at neutral pH. Thus, pH influences adsorption behavior affecting the absorbent's surface quality and dye ionization. In acidic pH ranges, free protons compete with MB molecules for surface adherence. Conversely, an increase in the hydroxyl group, which might give the beads' surface a negative charge, increases the basic area's adsorption capability. This hydroxide ion increases the bead-MB molecule contact. Because pH 8 had the best MB dye adsorption capacity on the beads that were made, it was chosen as the best pH.

Fig. 4
figure 4

A Picture of alginate/AC before and after dye adsorption. B Effect of pH on the adsorption capacities of SA/AC for MB dye. Dyes at a concentration of 100 mg/L; samples at a concentration of 0.05 g/50 mL for 120 min

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3.2.2 Contact Time and Adsorption Kinetics

Figure 5 shows MB dye adsorption capacity of alginate/active charcoal based on contact time. The figure shows that MB adsorption increases from 10 to 180 min of contact time. The results showed that removing MB from beads in 10–60 min was fast. Synthesized beads have more active centers on their surface, which become saturated when dye molecules adsorb. This quick increase suggested significant electrostatic force causing rapid MB cation transport towards the surface of beads. In around 120 min, the MB adsorption reached equilibrium (94 mg/g).

Fig. 5
figure 5

Effect of contact time on MB adsorption on SA/AC beads

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Different kinetic models, such as the Pseudo first and Pseudo second models, were used to determine the adsorption appliance. The linear equation for the Pseudo first kinetic model is as follows:

$$\mathrm{log} \left({q}_{e }- {q}_{t }\right)=\mathrm{log}({q}_{e })-\frac{{K}_{1}}{2.303}t$$
(3)

Because the values of \(\mathrm{log} \left({q}_{e }- {q}_{t}\right)\) and t have a direct relationship, A linear plot's intercept and slope can be used to calculate K1 and qe. The R-squared value and the expected qe value are typically used to choose the better fit (Fig. 6).

Fig. 6
figure 6

Kinetic models for MB adsorption on SA/AC beads. a pseudo first and b pseudo second order

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K2 is the equilibrium rate constant for the pseudo-second kinetic model, qe is the quantity of MB loaded on beads at equilibrium, and qt is the amount of MB loaded on beads at any time t. The kinetics of pseudo-seconds are linear.

$$\frac{t}{{q}_{t}}=\frac{t}{{q}_{e}}+\frac{1}{{K}_{2}{q}_{e}^{2}}$$
(4)

qt (mg/g) is adsorption time, qe (mg/g) is equilibrium adsorption, k1 (min−1) and k2 (g mg−1 min−1) are pseudo first and second model rate constants respectively.

We used pseudo first and second kinetics to analyse the experimental data for MB uptake by manufactured beads. Figure 5 shows simulated charts for the Pseudo first and second models (a, b). Also, Table 1 displays the experimentally determined MB uptake factors by alginate/AC. Compared to the pseudo first model, the pseudo second model yields a better regression coefficient for MB. The estimated adsorption capability (qcal = 204 mg/g) is close to the experimentally obtained adsorption capacity (qexp = 194 mg/g), demonstrating that the second models kinetic accurately captures MB absorption by beads. Adsorption of MB on synthesised beads appears to be dominated through chemisorption.

Table 1 Kinetic parameters of SA/AC nanocomposites adsorbing MB
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3.2.3 Adsorption Isotherms

Studies of adsorption isotherms are typically used to illustrate how dye molecules diffuse across the interfaces between solids and liquids. In this study, the adsorption isotherms of nanocomposite have been calculated at various initial levels of MB concentration after reaching saturation at ambient conditions. It has been found that the Langmuir isotherm provides accurate data on homogenous adsorption and predicts the formation of monolayers on surfaces. Freundlich isotherms describe heterogeneous and multilayer surfaces. Isotherm models by Langmuir and Freundlich (Eqs. 5 and 6) are as follows:

$$\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{K}_{s}{q}_{max}}+\frac{{C}_{e}}{{q}_{\mathrm{max}}}$$
(5)

MB adsorbed by nanocomposites is qe (mg/g), dye equilibrium concentration is Ce (mg/l), maximum MB is qmax (mg/g), and Langmuir constant is Ks (mg/l).

The Freundlich equation is given by:

$$\log q_{e} = \frac{1}{n}\log C_{e} + \log P$$
(6)

In this equation, P refers to adsorption capacity (mg/g) and n is a constant that represents adsorption intensity (dimensionless).

According to Table 2, these isotherm adsorption factors are reported for isotherm models. Langmuir isotherms of MB dye molecules adsorbed on a prepared nanocomposite showed a higher correlation coefficient (R2). The high correlation coefficient indicated in the Langmuir isotherm indicates that identical adsorption sites are directly connected to the adsorbed MB-shaped monolayer. Adsorption efficiency according to the Langmuir isotherm was 555 mg/g. Figure 7 displays the results of the study for varying concentrations from 25 to 1000 mg/L. An increase in MB concentration promotes the force that moves from the solid phase to the aqueous phase, resulting in an increase in MB adsorption to the active sites on the nanocomposite. Table 2 parameters for MB adsorption by SA/Activated charcoal nanocomposites according to different equilibrium models.

Table 2 Parameters for MB adsorption by SA/Activated charcoal nanocomposites according to different equilibrium models
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Fig. 7
figure 7

MB adsorption based on (a) concentration (non-linear fitting), (b) Langmuir model (linear fitting), and (c) Freundlich model (linear fitting)

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3.2.4 Effect of Temperature and Thermodynamic Parameters

The temperature also affects the adsorption process. Figure 8 and Table 3 show the results of the study of the variation of the MB's adsorption capacity on A SA/AC as a function of temperature. Results show that the temperature variation influences slightly the adsorption capacity of MB dye on SA/AC, so, the adsorption is endothermic.

Fig. 8
figure 8

(A) Effect of temperature on the adsorption capacity of MB on SA/AC, (B) Van’t Hoff plot of ln K vs. 1/T

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Table 3 Values of thermodynamic parameters for the removal of MB on SA/AC
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Studying temperature effects on MB adsorption on SA/AC, the thermodynamic parameters including ΔH° (enthalpy), ΔS° (entropy) and ΔG° (Gibbs free energy) were evaluated. Based on Van't Hof (Eq. 7), ΔG° was calculated.

$$\mathrm{\Delta G}^\circ =-\mathrm{RT}\, {\text{ln}}\, {K}_{c}$$
(7)
$${K}_{c}=\frac{{q}_{e}}{{C}_{e}}$$
(8)
$$\mathrm{ln }\,{K}_{c}=\frac{\mathrm{\Delta S}^\circ }{R}-\frac{\mathrm{\Delta H}^\circ }{RT}$$
(9)

Kc is the distribution coefficient constant; R is the general gas constant (8.314 J mol−1 K−1) and T is the absolute temperature (Kelvin). ΔH° and ΔS° values were obtained by plotting Ln Kc vs 1/T and calculating their slopes and intercepts (Table 4).

Table 4 Comparison of adsorption capacities of some reported in literature and several reported AC/alginate beads
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3.3 Desorption Study

The reuse of adsorbent materials is important for industry. In Fig. 9, the results of four repeated adsorption and desorption cycles were shown for SA/AC. With regenerated nanocomposites, after four regeneration cycles, both samples had a slightly lower adsorption capacity, 93 instead of 98. Overall, regenerated SA/AC is an effective adsorbent for removing cationic dyes.

Fig. 9
figure 9

Reusability of SA/AC for MB adsorption

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4 Mechanism of the Adsorption

Scheme 2 show the adsorption mechanism of MB onto the surface of SA/AC adsorbent. The mechanism of the adsorption depends on the presence of different functional groups and charges on both adsorbent and adsorbates. The electrostatic interaction will occur between the cationic groups of MB with the negative groups on the surface of SA/AC groups.

Scheme 2
scheme 2

Proposed mechanism of adsorption of SA/AC with MB dye

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5 Conclusions

This work proved the feasibility of producing novel low-cost and sustainable adsorbent composite materials to obtain greater MB dye removal from wastewater. Sodium alginate and charcoal nanoparticles (SA/AC beads) were successfully produced and analyzed utilizing a variety of analytical techniques. Batch adsorption testing showed a maximum capacity of 555 mg/g at pH 8. Kinetic analysis and isotherm data evaluation suggest that the Langmuir model best fits the experimental data. A dye molecule is transferred from the main solution to the external surface of the beads during the first stage, while dye intraparticle are diffused inside the pores of the beads during the second stage. Beads that have been made do not need to be thrown away after four cycles of adsorption. As a result, they have a high efficiency in removing MB, which is important for long-term and industrial-scale operations.