Introduction

Organic dyes have become one of the main sources of toxins for the environment in general and water in particular. Various industries, such as plastics, cosmetics, paints, varnish, and textiles use synthetic dyes extensively1. Most processes in these industries consume large amounts of water, and a sizable portion of the stained water is released into the environment as wastewater2. Most of the dyes are stable against photo-degradation and biodegradation1,2,3. Consequently, colored wastewater poses a challenge to conventional wastewater treatment processes.

Methylene blue (MB) is a cationic dye commonly used in dyeing cotton, wood and silk as well as medical and agricultural applications. MB has the potential to induce ocular burns, which could lead to irreversible damage to the eyes in both humans and animals. Inhalation of MB can result in brief episodes of rapid or labored breathing, whereas oral ingestion may cause a burning sensation accompanied by nausea, vomiting, excessive sweating, cognitive impairment, and methemoglobinemia3,4. Consequently, the removal of dyes from wastewater has become a critical environmental concern. Recent studies have revealed that adsorption-based dye removal is superior all treatment processes because its availability, simple operation, fast andefficient results, versatility for different dyes5,6,7,8.

One gram of adsorbent has the capacity to eliminate several grams of contaminants, indicating a high efficiency in dye removal. Additionally, during the adsorbent regeneration process, the adsorbed molecules can be released, allowing the solid phase to be reused for subsequent adsorption cycles9.

Natural polymeric materials have garnered significant interest due to their renewability, non-toxicity, biodegradability, and potential as eco-friendly alternatives10.

Chitosan (CTS), N-deacetylated form of chitin, is the second most abundant natural biopolymer. Renowned for its adsorption capabilities, CTS is extensively employed in the extraction of heavy metals and dyes6. Due to its cationic properties, chitosan exhibits limited adsorption capacity for cationic dyes. Consequently, chemical modifications are often required to enhance its efficacy in dye adsorption.

Composites made from biopolymers are gaining attention due to their small size, biodegradability, and their useful structural and functional properties. Alqarni et al.11 developed a biocomposite based on modified chitosan by the addition of chicken bone waste-derived hydroxyapatite and TiO2. The biocomposite exhibited strong fluoride adsorption capabilities with a maximum capacity of 115 mg/g and showed resilience in the face of coexisting anions, making it a viable solution for water purification in regions affected by fluoride contamination.

A new chitosan-based composite, namely magnetic chitosan with calcium phosphate rock (MCs@CPR), was synthesized to be an efficient adsorbent for lead (II) from wastewater with an adsorption capacity of 234.19 mg g−1 at 25 °C12.

Carboxymethylation of chitosan is an effective modification, as it introduces carboxyl (–COOH) groups into the chitosan molecule, rendering it anionic and thereby improving its ability to adsorb cationic dyes9. However, the high-water solubility of this compound complicates its reuse and increases its cost. Recently, clay materials have been developed as economically effective and efficient adsorbents for dyeing wastewater treatment. Among these, montmorillonite (MMT) is frequently utilized for organic dye removal due to its lamellar structure, fine particle size, and negatively charged surface13. Nevertheless, enhancing the dispersion and suspension properties of clay in aqueous solutions remains necessary for practical applications.

According to previous reports, chitosan could intercalate into the layers of MMT to modify and enhance their adsorption capacity13,14,15.

The adsorption capacity of MMT increased after the interaction of chitosan (CTS) with MMT. CTS consists of primary amino (–NH2) and hydroxyl (–OH) groups that facilitate the adsorption of cationic dyes and also increase the spacing between the layers of MMT clay. Another work by Mitra et al.16 investigated the intercalation of carboxymethyl chitosan with MMT. This adsorbent showed much higher adsorption capacity compared to CTS and MMT. Carboxymethylation of chitosan introduces new active anionic carboxylic acid group (–COO), which enhance the adsorption capacity of CMCTS-MMT compared to other adsorbents.

The economic viability of adsorbent preparation and utilization is a crucial determinant in evaluating its appropriateness for water remediation compared to alternative technologies. The cost of adsorbents can be assessed through various approaches, i.e., expense of raw materials, discounted cash flow analysis, cost indices, cost of adsorbent per gram of the adsorbate removed, Annual Capital Expenditure (CAPEX) and Operating Expenditures (OPEX), and the cost associated with the application of the adsorbent in the adsorption process17. A large number of adsorbents and biosorbents have been successfully employed to remove hazardous pollutants from wastewater18,19,20,21,22,23. Mostly biosorbents can be synthesized from green sources24. However, there is a need to reduce the cost of using these adsorbents for wastewater treatment without compromising the quality of water treatment25.

Therefore, the core novelty of this study is integrating an economic analysis with the practical dye adsorption study using CMC/MMT for industrial wastewater treatment which considered fairly unique.

The goal of the present study is to investigate the CMC/MMT nanocomposite biosorbentfor the removal of cationic dye (MB) and to provide a comprehensive cost-effectiveness evaluation at industrial scale and to compare it to commercial adsorbents which aren’t deeply covered in previously found studies.

The study examined the influence of the initial pH of the dye solution, initial dye concentration, and temperature on the adsorption process. Additionally, the adsorption kinetics and isotherm behavior of MB dye onto the composite material were analyzed, as well as comparing the prepared ion exchanger and a commercial resin namely, Amberlite IR 120 H, was conducted. Also, a detailed techno-economic study, including the design of the process line for the target bio-sorbent, description of the manufacturing equipment and its utilities based on the basic results obtained from comprehensive material and energy balances throughout the process phases was conducted.

Materials and methods

Materials

  • The adsorbate used in the sorption experiments is the Methylene blue dye. Its molecular formula is C16H18ClN3S·3H2O having a structure as shown below:

figure aThe alternative text for this image may have been generated using AI.

The basic dye, Methylene Blue (MB), obtained from RFCL LIMITED with 99% purity and with maximum wave length (λmax) equal to 670 nm, was used without further purification. The experimental solution was prepared by diluting a stock solution of 1000 mg/l with distilled water to different concentrations ranging from50 to 250 mg/ L. The maximum wave length of MB (λmax) is 670 nm.

  • N, O-CMC-MMT was prepared according to the optimum procedure previously established and reported by the authors15.

  • HCl analytical grade was obtained from Aldrich Company with concentration of 33–36%.

Methods

Experimental procedure

Batch adsorption

The adsorption of MB onto the prepared CMC-MMT nanocomposite was performed using a batch technique. For each experiment, 100 mL of an MB solution at a predetermined concentration was combined with 0.05 g of the biosorbent in a 250 mL glass-stoppered flask. The mixture was agitated in a temperature-controlled water bath shaker (Julabo SW21) at specified pH levels, temperatures, and contact times. Prior to adsorption, the pH of each solution was adjusted using 0.1 N HCl or NaOH. To establish the equilibrium time, 1 mL aliquots were withdrawn at predetermined intervals, filtered, and analyzed.

Determination of point of zero charge

A 0.1 M NaCl solution (50 mL) was prepared in a series of conical flasks. The initial pH of each solution was adjusted to values between 2 and 10 using 1 M HCl or 1 M NaOH. A precise mass of the resin (0.1 g) was added to each flask, and the mixtures were agitated on a shaker for 24 h. The final pH of each suspension was then measured using a digital pH meter. The ΔpH (final pH–initial pH) was calculated and plotted against the initial pH. The point of zero charge (pHpzc) was identified as the point where ΔpH = 0.

Analysis

  • The main functional group presence in the synthetic nanocomposite was determined using Fourier-transform infrared spectroscopy (FT-IR). FT-IR spectrums were recorded on a Jasco FTIR-6100 system, using a pellet made with dehydrated KBr and an instrument in the reflectance mode. The FTIR spectrums were determined between 400 and 4000 cm− 1.

  • Scanning electron microscope (SEM) analysis was accomplished to determine the prepared adsorbent morphology using JEOL5410 apparatus.

  • -Energy dispersive X-ray analysis (EDX) was determined using JEOL JX 2840 apparatus.

  • -The adsorption capacity was quantified by measuring the adsorbate concentrations before and after treatment with a HACH DR-2800 spectrophotometer (The DR 2800 spectrophotometer is a VIS spectrophotometer with a wavelength range of 340 to 900 nm, with 13-mm and 16-mm round cuvettes/vials). All the measurements were undertaken at the wavelength that corresponds to the maximum absorbance. Figure 1 represents the calibration curve of MB at the maximum wave length in the range of 0–10 ppm. The CSV data(0,0); (0.369, 1); (0.908, ); (1.393, 5); (2.388,10)

Fig. 1
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Calibration curve of methylene blue (MB).

The removal efficiency, R, and the adsorption capacity, q, were determined using the following Eqs. (1 and 2).

$$R\left( \% \right){\text{ }}={\text{ }}\frac{{\left( {{C_o}--{\text{ }}{C_e}} \right)}}{{{C_o}}}{\text{ }} \times 100$$
(1)
$$q{\text{ }}={\text{ }}\frac{{\left( {{C_o} - {\text{ }}{C_e}} \right)}}{m} \times V$$
(2)

where Co is the initial concentration of MB solution in (mg/L), Ce is the concentration at final time in (mg/L), V is the volume of the solution sample (L), m is the amount of the adsorbent in (g).

Regeneration/reusability test

Following the determination of optimal removal efficiency, the adsorbent (CMC/MMT) was subjected to a regeneration study. The spent adsorbent was immersed in a regeneration solution at the predetermined optimal pH and other conditions. The regenerated material was then reused for five consecutive adsorption-desorption cycles to assess its stability. Between cycles, the composite was recovered by filtration, washed five times with 99% methanol, and dried. The adsorption capacity and removal efficiency for MB were calculated for each cycle.

Results and discussions

Characterization of the adsorbent

SEM and EDX analysis

Scanning electron microscopy (SEM) was used to characterize the CMC-MMT nanocomposite before and after MB adsorption (Fig. 2). The pre-adsorption microstructure (Fig. 2a) exhibits a highly heterogeneous and porous architecture, featuring interlayer clay galleries and polymer-clay interfaces that form a network of micro- and mesopores. This hierarchical porosity is responsible for the material’s high surface area and enhanced barrier properties, which facilitate efficient adsorption. Following MB uptake, the post-adsorption image (Fig. 2b) reveals the occlusion of these pores by dye aggregates, providing direct morphological evidence of successful adsorption.

Fig. 2
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SEM analysis for CMC-MMT before adsorption (a) and after adsorption with MB (b).

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EDX analysis (a) before adsorption, (b) after adsorption of MB.

Energy-dispersive X-ray spectroscopy (EDX) analysis of the native CMC-MMT nanocomposite is presented in Fig. 3a. The elemental composition (in wt%) is 21.64% C, 0.83% N, 15.95% Si, 6.90% Al, and 44.28% O. Sulfur (S) and chlorine (Cl) were not detected. The resulting Si/Al atomic ratio is 2.3. In contrast, the EDX spectrum of the nanocomposite after MB adsorption (Fig. 3b) shows a distinct compositional change. Key observations include an increase in nitrogen content to 4.86 wt%, the appearance of sulfur (S) and chlorine (Cl) peaks, and adjusted concentrations of carbon (13.85 wt%), silicon (15.04 wt%), aluminum (8.01 wt%), and oxygen (52.87 wt%). These changes specifically the increased nitrogen and the presence of sulfur and chlorine are consistent with the successful adsorption of MB molecules onto the composite surface.

IR analysis

FTIR analysis of the CMC/MMT nanocomposite (Fig. 4) confirms the presence of characteristic functional groups originating from both CMC and MMT. The spectrum exhibits broad hydroxyl (–OH) stretching vibrations, carboxyl (–COOH/–COO−) functional groups associated with the CMC matrix, and siloxane-related bands (Si–O–Si and Si–O–Al) corresponding to the clay structure. Following adsorption of MB, noticeable shifts and intensity variations in these characteristic bands are observed, indicating strong interactions between the dye molecules and the composite surface. In addition, new absorption bands attributed to aromatic C=C and C–N stretching vibrations arising from the thiazine ring of MB appear in the post-adsorption spectrum. These spectral changes collectively confirm the successful adsorption of MB onto the CMC/MMT nanocomposite, predominantly governed by electrostatic interactions between negatively charged carboxylate groups and cationic MB molecules, as well as hydrogen bonding with surface hydroxyl groups. Table 1 demonstrates the characteristic functional groups of nanocomposite before and after adsorption.

Fig. 4
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IR analysis of CMC/MMT nano-composite before adsorption (a) and after adsorption with MB (b).

Table 1 FTIR analysis nanocomposite (before and after MB adsorption).
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Effect of main operating parameters

Effect of pH

Figure 5b illustrates the effect of pH on the adsorption capacity of N, O-CMC-MMT for MB removal. The experiments were conducted under constant conditions: a temperature of 30 °C, an initial MB concentration of 100 mg L−1, a solid-to-liquid ratio of 0.5 g L−1 (0.05 g per 100 mL), and a shaking rate of 120 rpm.

Fig. 5
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pHpzc for CMC-MMT (a) and the effect of pH on the adsorption capacity of methylene blue onto CMC-MMT (b).

While pH is a critical parameter in adsorption processes, its effect on MB removal by the CMC-MMT composite was relatively minor under the tested conditions. As shown in Fig. 5, the adsorption capacity exhibited only slight variation, fluctuating between 192 and 199 mg/g across the entire pH range studied. To elucidate the adsorption mechanism, the point of zero charge (pHpzc) the pH at which the adsorbent surface has a net neutral charge was determined. Adsorption of cationic species like MB is typically enhanced at pH values above the pHpzc, due to favorable electrostatic attraction to the negatively charged surface. Conversely, anion adsorption is favored at pH values below the pHpzc.In this study, the pHpzc for the N, O-CMC-MMT composite was determined to be 6.67 (Fig. 5a), compared to 2.6 for unmodified MMT. N, O-carboxymethyl chitosan (N, O-CMC) is an amphoteric polyelectrolyte whose conformation and charge depend on solution pH. At pH values exceeding 6.67, the N, O-CMC-MMT surface acquires a net negative charge31. Since MB exists as a cation in aqueous solution, this creates a favorable electrostatic attraction that enhances dye adsorption onto the composite surface16.

MMT possesses a permanent negative layer charge arising from isomorphic substitution within its structure. Conversely, the edges of the clay platelets feature pH-dependent hydroxyl groups (e.g., Si–OH, Al–OH). Consequently, pHpzc for raw MMT is typically low (pH ≈ 2–3). At solution pH values below this pHpzc, these edge hydroxyl groups become protonated, imparting a localized positive charge to the platelet edges. However, significant adsorption capacity at low pH can be attributed to chemical interactions between the MB dye and the MMT component. This is supported by the low pHpzc (≈ 2.6) reported for unmodified MMT32, which suggests the retention of positively charged edge sites at pH > 2.6.

Effect of temperature

Figure 6 illustrates the effect of temperature (30–50 °C) on the sorption equilibrium of methylene blue onto the chelating resin (CMC/MMT) at 120 rpm, 0.05 g resin/100 ml solution, 100 ppm initial dye concentration and pH 8.5.

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Isotherms of MB dye onto CMC/MMT at different temperatures.

An increase in the temperature (from 30 to 40 °C) leads to an increase in the adsorption capacities in the low range of MB concentrations. This may be due to an increase in dye mobility and the temperature enhances the chemical reaction step between the dye molecule and the reactive groups of the bio-sorbent. However, the increase of temperature from 40 to 50 °C has no effect on the adsorption capacity in this range of concentration. At high concentrations, the effect of temperature is reversed. The high adsorption capacity was observed at 30 °C and there was a decrease in the adsorption capacity by increasing the temperature from 30 to 50 °C.

Effect of time

Figure 7 illustrates the temporal decrease in MB concentration under the following conditions: 30 °C, 120 rpm, 0.5 g L−1 adsorbent dosage (0.05 g per 100 mL), an initial dye concentration of 100 mg L−1, and pH 8.5. Adsorption proceeded rapidly during the initial phase, followed by a slower approach to equilibrium over the next 20–30 min. Beyond this period, the concentration decreased at a much slower rate, stabilizing after approximately 2 h, indicating equilibrium attainment. Therefore, the majority of the adsorption capacity was achieved within the first 30 min. Such rapid kinetics are advantageous for practical application, as they allow for smaller reactor volumes, enhancing both process productivity and cost-effectiveness.

Fig. 7
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Effect of time on the removal of MB onto CMC/MMT nano-composite resin.

Sorption isotherms models

Adsorption isotherms describe the equilibrium relationship between the amount of adsorbate on the solid (qe) and its concentration in the solution (Ce) at constant temperature and pH. In this study, the Langmuir and Freundlich isotherm models were applied to the equilibrium data for methylene blue (MB) removal by the CMC/MMT nanocomposite.

Langmuir isotherm

The Langmuir isotherm model is applicable to adsorption on a homogeneous surface, assuming negligible intermolecular interactions in the adsorbed monolayer. Its equilibrium expression for a single solute is:

$${{\text{q}}_{\text{e}}}{\text{= }}\left[ {{\text{ }}{{\text{K}}_{\text{L}}}{{\text{C}}_{\text{e}}}} \right]{\text{ / }}\left[ {{\text{1}}\,{\text{+}}\,{{\text{a}}_{\text{L}}}{{\text{C}}_{\text{e}}}} \right]$$
(3)

where qe is the equilibrium solid phase concentration (mg/g resin), Ce is the equilibrium liquid phase concentration (mg/L), KL (L/g) and aL (L/mg) are the Langmuir constants, which are related to the system physical properties: KL reflects the solute adsorptivity and aL is related to the energy of adsorption. (KL/aL) is defined as the monolayer adsorbent capacity. KL and aL are evaluated by linearization of Eq. (1) as follows:

$$\left[ {{{\text{C}}_{\text{e}}}{\text{/}}{{\text{q}}_{\text{e}}}} \right]{\text{ = }}\left[ {\left( {{{\text{a}}_{\text{L}}}{\text{/}}{{\text{K}}_{\text{L}}}} \right){{\text{C}}_{\text{e}}}} \right]{\text{ + }}\left[ {{\text{1/ }}{{\text{K}}_{\text{L}}}} \right]$$
(4)

Langmuir isotherms were constructed by plotting Ce/qe against Ce for MB sorption at 30, 40, and 50 °C under otherwise constant conditions. The resulting plots were linear across the studied concentration range (Fig. 8). The slope (aL/KL), intercept (1/KL), and calculated maximum adsorption capacity (Qmax) for each temperature were determined by linear regression and are presented in Table 2.

Fig. 8
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Langmuir plots of MB onto CMC/MMT at different temperatures.

The tabulated results indicate that the adsorption capacity decreased at higher temperatures. Specifically, the maximum capacity (Qmax​) decreased by 54.54% as temperature increased from 30 to 50 °C. The sorption behavior conformed well to the Langmuir isotherm model, which suggests that MB molecules form a uniform monolayer on the adsorbent surface without significant intermolecular interactions at adjacent sites33.

Table 2 Parameters of the Langmuir adsorption model for MB dye onto the CMC/MMT nano-composite.
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The values of the separator factor (RL= 1/[1 + KL Co]) for MB were calculated, and plotted against initial dye concentrations (Fig. 9). RLvalue indicates the adsorption nature to be either unfavourable if RL>1), linear if RL =1, favourable if 0 < RL<1, and RL=0 is irreversible. From the calculated data RL is greater than 0 but less than 1 indicating that Langmuir isotherm is favorable.

Fig. 9
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Calculated separation factor profile for the MB sorption onto CMC/MMT nanocomposite, function of dye concentrations.

Freundlich isotherm

The Freundlich isotherm describes equilibrium on heterogeneous surfaces and hence does not assume monolayer capacity. Mathematically, it is expressed by:

$${{\text{q}}_{\text{e}}}{\text{= F}}{{\text{C}}_{\text{e}}}^{{{\text{1/n}}}}$$
(5)

where F (l/g)and n are the Freundlich constants, and can be evaluated by linearization of Eq. (3):.

$${\text{Log}}{{\text{q}}_{\text{e}}}{\text{= log F + }}\left( {{\text{1/n}}} \right){\text{ log }}{{\text{C}}_{\text{e}}}$$
(6)

The equilibrium adsorption data for methylene blue (MB) on the CMC/MMT composite were also analyzed using the Freundlich isotherm model, as shown in Fig. 10. The calculated Freundlich parameters KF (the adsorption capacity constant) and n (the intensity constant) are presented in Table 3. The value of n > 1 indicates a favorable physical adsorption process34,35. However, the initial data points deviated from the Freundlich model and were therefore excluded from the linear regression.

Fig. 10
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Freundlich plots of MB onto CMC/MMT at different temperatures.

Table 3 Parameters of Freundlich model for MB basic dye onto CMC-MMT.
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The fitting data to the non-linear form of the models are presented in Fig. 11 which indicate that Langmuir model is more suitable for representing the experimental data.

Fig. 11
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Non linear fits for Langmuir/Freundlich plots of MB onto CMC/MMT at 30 °C.

Sorption kinetics models

Mathematical models capable of describing batch biosorption performance across a range of experimental conditions are essential for process scale-up and optimization. In this work, three kinetic models were evaluated against the experimental data. Model selection was based on the quality of fit and the mechanistic plausibility of the underlying sorption process.

Pseudo-first-order model

The sorption kinetics may be described by a pseudo-first-order reaction as follows36:

$${\text{d}}{{\text{q}}_{\text{t}}}{\text{/dt}}\,{\text{=}}\,{{\text{k}}_{\text{1}}}\left( {{{\text{q}}_{\text{e}}}{\text{-- }}{{\text{q}}_{\text{t}}}} \right)$$
(7)

In this context, qe​ is the equilibrium adsorption capacity (mg/g), qt​ is the adsorption capacity at time t (mg/g), and k1​ is the adsorption rate constant. Equation (5) is solved by applying the initial conditions (q = 0 at t = 0) and integrating from t = 0 to t. Subsequent rearrangement yields the following linearized, time-dependent form:

$${\text{log }}\left( {{{\text{q}}_{\text{e}}}{\text{-- q}}} \right)\,{\text{=}}\,{\text{log}}\left( {{{\text{q}}_{\text{e}}}} \right){\text{ -- }}\left( {{{\text{k}}_{\text{1}}}{\text{/2}}{\text{.303}}} \right){\text{t}}$$
(8)

Pseudo-second-order model

This model characterizes the sorption kinetics, as described by the differential equation:36,37:

$${\text{dq/dt}}\,{\text{=}}\,{{\text{k}}_{\text{2}}}{\left( {{{\text{q}}_{\text{e}}}{\text{-- q}}} \right)^{\text{2}}}$$
(9)

where k2 is the adsorption constant (min− 1). Integrating Eq. (7) for the boundary conditions t = 0 to time t and q = 0 to q and rearranging, a linear form is obtained as follows:

$${\text{t/q}}\,{\text{=}}\,{\text{1/}}{{\text{k}}_{\text{2}}}{{\text{q}}_{\text{e}}}^{{\text{2}}}\,{\text{+}}\,{\text{t/}}{{\text{q}}_{\text{e}}}$$
(10)

\({\text{h}}\,{\text{=}}\,{{\text{k}}_{\text{2}}}{{\text{q}}_{\text{e}}}^{{\text{2}}}\)

In these equations, h is the initial sorption rate (mg/g min).

Intra-particle diffusion model

The fundamental analysis of diffusion is based on Fick’s second law. This law models mass transport driven by a concentration gradient. For a radial diffusion system where the spatial coordinate (x) is analogous to the pore radius (r) the governing equation takes the following form38:

$${\text{dC/dt}}\,{\text{=}}\,{\text{D }}\left[ {{{\text{d}}^{\text{2}}}{\text{C/d}}{{\text{X}}^{\text{2}}}} \right]$$
(11)

where D is the diffusivity coefficient.

One solution reveals a linear correlation between (Co–Ct)/(Co–Ce) and (Dt/r2)0.5 throughout a significant portion of the adsorption process. Given that D and rare constants for a particular system, the amount adsorbed qt, which is proportional to (Co–Ct)/(Co–Ce), will exhibit a linear dependence on t0.5. The slope of the linear region in the plot of qt vs. t0.5 is denoted as kid and is referred to as the rate parameter.

By applying the three kinetic models with experimental results, at 30 °C temperature and pH 8.5, it can be observed that:

For the pseudo-first-order model, as calculated, the regression correlation factor (R2) value is 0.988 suggesting a good agreement with the model for the obtained kinetic data (Fig. 12).

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Pseudo-first-order plot of MB onto CMC/MMT.

As it is observed from Fig. 13, the experimental results exhibit an excellent compliance with the pseudo-second-order kinetic model, with higher value of the correlation coefficient (R2 = 0.9969).The pseudo-second order model assumes that the sorption follows a sorption mechanism where the influence of mass transfer is limited to film diffusion, and the rate controlling step can be the chemical sorption involving valence links or covalent links between sorbent and adsorbate.

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Pseudo-second-order plot of MB onto CMC/MMT.

The intraparticle diffusion model describes sorption as a multi-step process in which the rate-controlling step is the diffusion of the adsorbate into the internal pores of the adsorbent. As shown in Fig. 14, rapid initial surface saturation occurs within approximately 20 min. Thereafter, the linear relationship between the adsorption capacity (q) and the square root of time (t0.5) indicates that intraparticle diffusion becomes the dominant, rate-limiting mechanism.

Fig. 14
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Intra-particle diffusion model plot of MB onto CMC/MMT.

The kinetic parameters for the three models are summarized in Table 4. The correlation factors strongly suggest that while both mechanisms are applicable, the sorption process is predominantly governed by pseudo-second-order kinetics.

Table 4 Kinetic parameters of MB onto CMC/MMT resin at 30 °C.
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Thermodynamic parameters

Thermodynamic parameters were assessed for the removal of MB dye by using the prepared nano-composite (CMC/MMT) in the temperature range of 30 to 50 °C. Figure 15 illustrates the Van’t Hoff plot of MB adsorption on CMC/MMT. Table 5 listed the calculated parameters from the thermodynamic equation (ΔG, ΔS° and ΔH°). The Kd values were obtained from the adsorption data at different temperatures (Kd = qe/Ce). The ΔG° values shown in Table 5 confirm that the adsorption process was spontaneous at the selected temperature range (30–50 °C), which is in coincidence with the positive value of ΔS° (ΔS > 0 for the spontaneous process). The calculated value of ΔH° (− ve139 kJ/mol) is implicit in the exothermic characteristics of the process. The ΔH value of the adsorption of MB onto synthesized CMC/MMT resin was 139 kJ mol− 1, indicating the adsorption by a chemical bond39.

Fig. 15
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Linear relationship of Van’t Hoff equation at different adsorption temperatures.

Table 5 Thermodynamic parameters of MB onto CMC/MMT resin.
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Mechanism of the adsorption process

The adsorption of methylene blue (MB) by carboxymethyl chitosan/montmorillonite (CMC/MMT) composites is governed by multiple synergistic mechanisms: (As represented in Fig. 16)

  1. i.

    Electrostatic attraction between the cationic MB species and the anionic sites of the composite, specifically the carboxylate groups (− COO⁻) of CMC and the negatively charged silicate surfaces of MMT.

  2. ii.

    Ion exchange, wherein MB cations displace exchangeable interlayer cations (e.g., Na⁺ and Ca²⁺) within the MMT structure.

  3. iii.

    Hydrogen bonding between donor groups on the adsorbent (e.g., hydroxyl (− OH) and carboxyl (− COOH) groups of CMC) and acceptor atoms (e.g., nitrogen or sulfur) within the MB molecular structure.

As resulted from the kinetic study and isotherms of the proposed system, the overall process typically follows pseudo-second-order kinetics and the Langmuir adsorption isotherm model, suggesting a chemical and monolayer adsorption process which confirmed by the above mechanism.

Fig. 16
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Mechanism of the adsorption process between CMC/MMT and MB.

Desorption and reusability

The nanocomposite (CMC/MMT) was regenerated via acid washing using 0.1 M HCl. During this process, the solution color changed from transparent to blue, indicating the desorption of MB from the spent adsorbent. After washing and drying, the regenerated composite was reused in the next adsorption cycle. As shown in Fig. 17a, the MB removal percentage decreased slightly from 99.6 to 92.8% over five consecutive cycles at an initial dye concentration of 50 ppm. The successful regeneration over five cycles can be attributed not only to the reactivated adsorption sites but also to the presence of vacant sites on the adsorbent surface. In Fig. 17b, however, at an initial dye concentration of 100 ppm, the removal efficiency decreased from 98.97 to 80.36% across the first three cycles, followed by a sharper decline to 73% and 54% in the fourth and fifth cycles, respectively. This more significant decline is attributed to the accumulation of residual MB molecules on the adsorbent surface. As discussed in the proposed adsorption mechanism, while electrostatic repulsion is favored at lower pH, hydrophobic interactions and electron donor–acceptor (EDA) interactions still facilitate MB adsorption.

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Regeneration of the nano-composite CMC/MMT (a) at Co = 50 ppm and (b) at Co = 100 ppm of MB dye.

Comparison between the prepared nano-composite and other adsorbents

A comparative analysis of the adsorption capacity for basic dyes between the present composite and other recently developed adsorbents is presented in Table 6.

Table 6 Comparison of adsorption performance, isotherm, and kinetic models for MB removal.
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The Table demonstrates that the prepared nano-composite surpasses literature values for maximum adsorption capacity while also achieving equilibrium more rapidly than other adsorbent types.

The comparison between the new synthesized CMC/MMT nanocomposite resin and a commercial ion exchanger, namely Amberlite IR 120 H, was illustrated in Fig. 18. From this figure, it is clear that the CMC/MMT nanocomposite has greater sorption capacity than the commercial resin for basic dye. The percent increase in capacity is about 27%.

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Basic dye (MB) removal comparison.

Economic evaluation for CMC/MMT in nano form production line

Process description

Synthesis of the target anion-exchange resin/chelating agent involved three principal steps: formation of medium-molecular-weight chitosan, its subsequent derivatization to carboxymethyl chitosan (CMC), and finally, composite formation with montmorillonite (MMT).

As detailed in a prior report15, the process is illustrated by a qualitative block-flow diagram and a mass balance for 2 ton/day production (Figs. 19 and 20).

Fig. 19
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Qualitative block flow sheet for the CMC/MMT nanocomposite production.

Fig. 20
Fig. 20The alternative text for this image may have been generated using AI.
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Mass balance for 2 ton/day CMC/MMT nano-composite production.

Fixed capital cost

Total capital investment (T.C.I.) for the plant is calculated by adding direct fixed capital (DFC) cost, working capital cost, and startup cost. DFC is based on direct costs, indirect costs, and contractor’s fee and project contingency, which are taken as 5 and 10% of the sum of direct and indirect costs. The purchased-equipment costs (E) and accordingly the major components of total capital investment (T.C.I.).Costs of equipment locally purchased are estimated according to current costs as provided by local market, while imported equipment are estimated according to international firm suppliers included freight and customs charges (Based on year 2024).

Direct cost is made up of equipment purchase cost and costs related to instrumentation, piping, installation, electrical facilities, buildings, yard improvement and land which are 20, 32, 32, 10, 20, 40 and 6% of total equipment purchase cost, respectively. The indirect cost includes engineering and construction costs, which are 12% of direct costs. Working capital is estimated as 15% of FCI. Based on the above description the equipment costs (E) $1,729,500, the direct cost $4,497,750, the indirect cost $1,099,200, the working capital investment $987,000 and the total capital investment $6,583,950.

Annual production cost

The production cost is calculated as the aggregate of annual operating expenses, detailed in Table 7, and the depreciation cost per unit of product. The annual depreciation is determined using a useful life of 10 years for fixed capital assets, while depreciation for buildings and concrete structures is based on a 30-year lifespan. Accordingly, the total depreciation rate is 571,245 $/Year, giving an estimated production cost (PC) of 21.15 $/kg.

Table 7 Annual operating costs.
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Comparative cost for pollutants removal

The quantities of the prepared target chitosan resin derivative, required to remove 1 kg of MB dye, compared to the commercial resin were calculated for the sake of cost comparison and the results are illustrated in Table 8.The comparison reveals that the new synthesized resin under investigation exhibits a lower cost per 1 kg removed dye.

Table 8 Cost comparison between the target product and the commercial resin for 1 kg pollutants removal.
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Conclusions

The carboxymethyl chitosan-montmorillonite (CMC/MMT) nanocomposite biosorbent demonstrates effective adsorption of methylene blue (MB) from aqueous solution. Equilibrium data were well described by the Langmuir isotherm, yielding a maximum adsorption capacity (Qmax) of 435 ± 5 mg/g under optimized conditions (pH 8.5, 30 °C, 60 min). Kinetic studies revealed that adsorption kinetics follow pseudo-second-order behavior, consistent with a chemisorption-controlled mechanism.

The prepared resin shows superior adsorption performance, with an increased capacity and faster kinetics relative to existing adsorbents.

Furthermore, the prepared CMC/MMT ion exchanger shows a favorable combination of higher adsorption capacity and lower cost compared to the commercial Amberlite IR 120 resin.