Application of oak charcoal-based activated carbon for the removal of methylene blue dye from aqueous solutions: kinetics, equilibrium, and reusability studies

The quality of many water bodies has deteriorated due to a variety of factors such as population growth, rapid and unplanned urbanization, industrialization, technological expansion, energy consumption, and waste generation from domestic and industrial sources. This deterioration has rendered many water bodies unwholesome and hazardous to the health of both humans and other living organisms. In developing countries, contaminated water is responsible for 80% of all diseases, as stated by the World Health Organization. Industrial effluents are often major contributors to a wide range of water pollution problems (Amuda & Ibrahim 2006; Sharifian et al. 2017).

Synthetic dyes and pigments are widely used in various industries worldwide, producing over 7 × 10⁷ tons of dyestuff annually (Al-Tohamy et al. 2022). However, the discharge of colored wastewater carrying residual dyes from these industries poses significant environmental problems. The presence of even trace amounts of dyes in water is remarkably visible, objectionable, and undesirable and can negatively affect the public perception of water quality. Moreover, some synthetic dyes and their metabolites are highly toxic, potentially carcinogenic, mutagenic, and allergenic to the exposed organism (Rafatullah et al. 2010; Rehman et al. 2012; Chequer et al. 2013; Gholami-Borujeni et al. 2013; Zhou et al. 2014).

Methylene blue (MB) is a cationic dye commonly used in the colorization of cotton, silk, and wood. When dissolved in water, it produces a deep blue color. However, MB is harmful in nature and can cause allergies, irritation, vomiting, breathing difficulties, diarrhea, and nausea when ingested or inhaled (Ullah et al. 2022).

Based on current scientific knowledge, it is imperative to treat dye-contaminated effluents using appropriate methods prior to discharge into the receiving water bodies. Failure to do so can result in adverse impacts on aquatic life and the environment. Therefore, it is crucial to implement effective treatment strategies to mitigate the harmful effects of dye pollution on the ecosystem (Afshin et al. 2019; Dolas 2023).

Various treatment methods such as physical, biological, chemical oxidation (such as ozonation), advanced oxidation (such as photocatalytic decomposition), electrocoagulation, nanoparticles, ion exchange, and membrane separation processes are available to remove dyes from industrial effluents (Ullah et al. 2022; Dolas 2023). However, due to high cost and operational problems associated with these methods, their use is not cost-effective for many countries. For instance, the coagulation and filtration method produces a high amount of secondary waste. Dyes are also resistant to aerobic biodegradation and are not eliminated by conventional biological processes. Membrane processes are costly and require specialized personnel to operate. The large-scale application of chemical oxidation methods is not feasible in the industry (Al-Tohamy et al. 2022).

Among the available options, the adsorption process is widely used due to its high efficiency, ease of operation, insensitivity to toxic compounds, and the availability of a wide range of adsorbents. The adsorption method produces a high-quality effluent that does not contain harmful substances such as ozone and free radicals (Shirmardi et al. 2016; Putranto et al. 2022; Ullah et al. 2022). Activated carbon (AC), sugarcane pulp ash, rice husk ash, coconut husk, magnesium chloride, and chitosan are some of the adsorbents tested for removing dyes. AC, in particular, is widely used due to its high adsorption capacity and regenerability. However, commercial ACs are generally expensive, and researchers are exploring the production of ACs using sustainable and cost-effective resources. One promising avenue is the use of agricultural resources, waste materials, and by-products to produce AC. AC derived from waste residues has received significant attention due to its renewability, low cost, and eco-friendliness (Gholami Borujeni et al. 2013; Xue et al. 2022; Bouchelkia et al. 2023). Numerous studies have been carried out by researchers to develop low-cost AC using various materials for the adsorption of dyes. For instance, Weng et al. (2009) used pineapple leaf powder as an adsorbent for the removal of MB dye from aqueous solutions. In addition, comprehensive lists and reviews are available that cover various types of adsorbents for the removal of dyes (Sharma et al. 2009; Weng et al. 2009; Pathania et al. 2017; Naushad et al. 2019; Nizam et al. 2021).

Despite the fact that many low-cost adsorbents have been studied for the removal of MB dye and decontamination purposes, studies on the application of oak charcoal-based AC (OC-AC) as a cheap and economical adsorbent for MB dye removal are limited. Therefore, this study aims to investigate the use of oak charcoal, a cheap, readily available, and natural adsorbent, for removing MB dye from aqueous solutions after preparation, activation, and surface functionalization. The study will also evaluate the effects of pH, contact time, adsorbent dose, initial dye concentration, and temperature on the efficiency of the adsorption process. The adsorption kinetic and equilibrium data were also fitted to the nonlinear form of the related models.

The oak charcoals used in this study as the starting raw materials were prepared by the natives of Lordegan county, Chaharmahal and Bakhtiari province, Iran, who live in the foothills of the Zagros mountain range. The oak charcoals were produced by a traditional procedure. The purchased charcoals were washed several times with distilled water, were boiled in distilled water for 1 h to remove impurities and ashes, and then dried in an oven at 110°C overnight.

The dried charcoals were ground and sieved to discrete smaller particles with the size of ≤300 μm. After separating the particles, a chemical method was used to prepare and activate the adsorbent. A mixture of FeCl₃, ZnCl₂, KOH, and lime (a weight ratio of 35:35:15:15) was used for the chemical activation. The prepared sample had an inorganic:organic weight ratio of 1:1. After adding the chemicals to the raw oak charcoals, 200 mL of distilled water was added to the resulting mixture to form a homogeneous composition and dissolve the chemicals. The mixture was refluxed at 100°C under magnetic stirring conditions for 1 h, filtered, and then dried overnight at 100°C. After drying, it was stored in a desiccator for the final activation step.

To complete the final activation of the pretreated charcoal, it was activated in a furnace under nitrogen atmosphere at the temperature range of 30–700°C. The temperature was increased by 10°C min⁻¹, and the rate of nitrogen gas flow was 150 mL min⁻¹. The mixture was kept at 700°C for 1 h and then cooled under a nitrogen atmosphere. After cooling, the charcoal was washed with 6 M HCl to remove impurities and mineral residues. Finally, the activated charcoals were rinsed several times with distilled water to adjust the pH to a range of 6–7, dried overnight in an oven, and kept in an appropriate glass container for further applications (Shirmardi et al. 2016; Takdastan et al. 2016). The OC-AC adsorbent was characterized by several techniques, and the associated details are presented in the Supplementary Material.

Batch adsorption experiments were carried out using laboratory bottles containing 50 mL of MB aqueous solution. The effects of the most important operational parameters including solution pH (range 3–11), contact time (5–300 min), adsorbent dosage (0.5–3 g L⁻¹), initial MB concentration (10–150 mg L⁻¹), and temperature (20–45 °C) were evaluated and optimized. All experiments were conducted on a magnetic stirrer or in an incubator shaker at a constant agitation speed of 250 rpm. To run an experiment, the other variables were kept constant at the predetermined value. Blank experiments were also run in parallel with no adsorbent added under the same conditions to determine if the pH of the MB solution affected MB adsorption and to examine the possibility of the adsorption of MB onto the bottles and to exclude the effects of other variables on the MB adsorption.

The final concentration of MB in the aqueous solution was determined using a UV–Vis spectrophotometer (DR5000, Hach Company, USA) at a wavelength of 665 nm. First, a calibration curve was prepared using a series of standard solutions of known MB concentrations. The absorbance of each standard solution was measured at 665 nm, and a calibration curve was plotted using the absorbance values and corresponding concentrations. The concentration of MB in the aqueous solution was then determined by measuring its absorbance and interpolating its concentration from the calibration curve. The measurement was repeated three times for each sample, and the mean value was used for further analysis. The accuracy of the spectrophotometric measurements was verified by comparing the measured values with the known concentrations of the standard solutions. The percent recovery of the standard solutions was found to be within the acceptable range of 95–105%. After each run, the sample was immediately filtered using a 0.22-μm membrane filter to remove adsorbent particles from the solution.

In these equations, is the value of q obtained from experimental measurements, while is the predicted value of q derived from the model. The average value of all q obtained through experimental measurements is represented by ⁠; n denotes the total number of experiments conducted, and p refers to the number of parameters in the fitted model (dos Santos et al. 2015; Saucier et al. 2015c).

Following the adsorption process, the OC-AC adsorbent was dispersed in a solution of HCl (0.1 mol L⁻¹) and agitated for 1 h to regenerate and reuse the exhausted adsorbent. After the desired time, the adsorbent was separated from the solution by decanting process and rinsed with deionized water to remove any residual acid and dye completely. Finally, OC-AC was placed inside an oven at 100 °C overnight. The regeneration procedure was repeated for five cycles. The efficiency of the regenerated adsorbent was assessed through adsorption experiments, measuring the dye removal efficiency and comparing it to the initial performance of the adsorbent (Buelvas et al. 2023).

To determine some characteristics of the OC-AC adsorbent, such as the phases on the adsorbent structure, surface morphology, elemental composition (chemical composition), functional groups on the adsorbent surface, and specific surface area, X-ray diffraction (XRD), field emission scanning electron microscopy, energy-dispersive X-ray spectrometry, Fourier transform infrared, and Brunauer–Emmett–Teller (BET) analyses were carried out to characterize the OC-AC adsorbent.

The XRD pattern of the prepared AC is shown in Fig. 1S. The XRD pattern of the AC shows a wide peak at 2θ between 13 and 35°. This broad peak can be related to amorphous carbon (Saucier et al. 2015a). In addition, four peaks associated with other crystalline phases left in the XRD pattern of the AC even after leaching with 6 M HCl (Saucier et al. 2015b). These peaks can be indexed to calcium carbide (CaC₂: JCPDS card 01-075-1558) and calcium chloride carbide (Ca₃Cl₂C₃: JCPDS card 076-0298). For more details about the characterization of OC-AC, please see the Supplementary Material.

The presence and abundance of anionic and/or cationic ions in the surrounding environment can influence the interactions between the adsorbent and the target molecule or ion for adsorption. Therefore, the adsorption process is significantly influenced by the pH of the adsorbate solution, as it can affect the chemistry of both the adsorbent and the adsorbate.

The results of current research are in line with the findings of other similar studies (De Castro et al. 2018; Ivanets et al. 2022). To avoid the inclusion of extra ions resulting from pH adjustment using acids or alkalis, further adsorption tests were conducted under the original or natural pH conditions.

Adsorption kinetics is an important phenomenon that describes the rate at which adsorbate molecules are taken up by the adsorbent material over time (Farooq et al. 2022). The selection of an appropriate kinetic model is crucial for accurate analysis and understanding of the adsorption behavior of a target pollutant (dos Reis et al. 2023). In this study, we investigated the kinetic behavior of MB dye adsorption onto the OC-AC adsorbent using the pseudo-first-order, pseudo-second-order, AFO, and intraparticle diffusion kinetic models.

The adsorption capacity (q_t) increased from 23 to 44 mg g⁻¹ with increasing time from 5 to 300 min (for MB concentration of 50 mg L⁻¹), indicating that OC-AC was effective in removing the dye from the solutions. In our study, we evaluated the goodness of fit of the nonlinear kinetic models to the experimental data by analyzing the adjusted R² and the SD values. The SD values indicate the degree of deviation between the theoretically calculated q value and the experimentally measured q value. A higher SD value suggests a greater degree of discrepancy between the two values (dos Reis et al. 2023).

In this study, SD values were calculated for the three different kinetic models. The SD ratio was then used to compare the fitness of each individual model. As presented in Table 1, for an MB concentration of 25 mg L⁻¹, SD values for the pseudo-first-order, pseudo-second-order, and AFO kinetic models were 1.88, 0.93, and 0.38, respectively. The corresponding values for the MB concentration of 50 mg L⁻¹ were 5.02, 3.35, and 1.35, respectively. In addition, for the MB concentration of 25 mg L⁻¹, the SD ratio values for the pseudo-first-order, pseudo-second-order, and AFO kinetic models were found to be 4.95, 2.45, and 1, respectively (Table 1). For the MB concentration of 50 mg L⁻¹, the corresponding values were 3.72, 2.48, and 1, respectively. Based on Figure 3, it is obvious that for both MB concentrations, the AFO kinetic model provided the best fit to the experimental data, as evidenced by the lowest SD ratio values and the highest adjusted coefficient of determination (R²_adj) values (Table 1). The AFO model proposes that the adsorption is a complex process with numerous potential pathways. Changes in the adsorption mechanism occur during the process, and the kinetics may follow multiple orders that vary over time as the adsorbent and adsorbate interact. Typically, the n_AV exponent takes on a fractional value, in agreement with the predictions of the AFO model (Cimirro et al. 2022; dos Reis et al. 2023).

To further investigate the effect of mass transfer resistance on the adsorption process, we applied the intraparticle diffusion model. The plots of the amount adsorbed (q_t) vs. the square root of time (t^0.5) showed multilinearity, indicating that the adsorption process involved more than one adsorption rate (dos Reis et al. 2023; Singh et al. 2023). Each linear section was ascribed to a particular stage of the adsorption process. The first linear section corresponded to the external surface adsorption, which involved the diffusion of MB molecules from the bulk solution to the external surface of the OC-AC adsorbent. The second linear section was attributed to the intraparticle diffusion, which involved the diffusion of MB molecules from the external surface to the interior pores of the OC-AC adsorbent. The third linear section corresponded to the diffusion through smaller pores (dos Reis et al. 2023; Singh et al. 2023).

The results of the kinetic studies revealed that the minimum contact time required to reach equilibrium for the adsorption of MB onto the OC-AC adsorbent was about 120 min. To ensure that equilibrium was attained even at higher concentrations, we fixed the contact time at 180 min for the rest of our experimental work.

Adsorbent dosage is a crucial factor that affects the efficiency of the adsorption process. The amount of adsorbent added to the system has a direct impact on the removal efficiency of the adsorbent and optimization of the process. Increasing the adsorbent dosage provides more surface area for adsorption, resulting in higher contact between the adsorbate and the adsorbent (Wu et al. 2022). In the present study, the adsorbent dosage was varied from 0.5 to 3 g L⁻¹, and its effect on the removal percentage and adsorption capacity (q_e) of the adsorbent was investigated for two initial MB concentrations of 50 and 100 mg L⁻¹.

In this study, the adsorption of MB dye onto the OC-AC adsorbent was evaluated using nonlinear equations of three popular adsorption isotherm models: Langmuir, Freundlich, and Liu. The experimental data were collected at different MB concentrations (ranging from 10 to 150 mg L⁻¹) and temperatures (room temperatures of 20–22, 35, and 45 °C) under previously optimized experimental conditions. Our results revealed that the removal percentage of MB decreased as the initial concentration of MB increased. For example, at room temperature, the removal percentage decreased from 100 to 75% as the initial dye increased from 10 to 150 mg L⁻¹. This decrease is due to the fact that as the initial concentration of MB increases, the amount of available adsorption sites on the adsorbent decreases because of the increased number of dye molecules in the solution.

Moreover, our findings showed that the amount of MB adsorbed (q_e) onto the surface of OC-AC increased with increasing the temperature from 20 to 35 °C. This could be attributed to the increased mobility of MB molecules at a higher temperature of 35 °C, which enhances the interaction between the dye molecules and the solid adsorbent. However, at 45 °C, compared to 35 °C, the removal percentage and q_e were not changed significantly and slightly decreased for higher concentrations (90–150 mg L⁻¹), indicating that there may be a limit to the reduction of MB concentration at high temperatures. We compared the goodness of fit of the models based on the SD values and adjusted R². Our results indicated that the Langmuir model did not accurately describe the equilibrium data as it showed the highest SD and the lowest adjusted R² values compared to the other models studied. Conversely, the Freundlich and Liu models provided a good fit for the equilibrium data. However, the Freundlich model exhibited the lowest SD values, indicating that it provided the best fit to the experimental data across all studied temperatures.

The present study investigated the potential of OC-AC as an adsorbent for the removal of MB dye from aqueous solutions. The results showed that OC-AC was effective in removing MB, with a high removal percentage achieved under optimized conditions. The adsorption kinetics and isotherm behavior of OC-AC were also analyzed, and the AFO kinetic model and the Freundlich isotherm model provided the best fit to the experimental data. In addition, the OC-AC adsorbent was found to be reusable for up to five cycles without significant loss of its removal percentage (range: 90.2–99.9%). Therefore, OC-AC can be considered a low-cost environmentally friendly adsorbent for the removal of MB dye from contaminated effluents with a removal percentage in the range of 90–97% when using an adsorbent dosage of 1.5 g L⁻¹. This study provides useful information for researchers and practitioners in the field of water and wastewater treatment, particularly in the textile and dyeing industries.

This research is approved and supported by Student Research Committee, Babol University of Medical Sciences (IRCT code: IR.MUBABOL.HRI.REC.1401.029). The authors sincerely thank the Vice-Chancellor for Research and Technology of Babol University of Medical Sciences for the financial support. The authors also acknowledge the School of Public Health for providing necessary facilities to accomplish this research.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.