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Article

Citric Acid-Modified Sepiolite as an Efficient and Sustainable Adsorbent for the Removal of Methylene Blue from Aqueous Solutions

by
Zhuangzhuang Tian
1,
Ziyi Chen
2,
Qing Wang
2,
Xin Gao
2 and
Wei Wei
2,3,*
1
School of Telecommunications and Intelligent Manufacturing, Sias University, Zhengzhou 451150, China
2
Jiangsu Province Engineering Research Center of Environmental Risk Prevention and Emergency Response Technology, Jiangsu Engineering Lab of Water and Soil Eco-Remediation, School of Environment, Nanjing Normal University, Nanjing 210023, China
3
Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing 210023, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(20), 2998; https://doi.org/10.3390/w17202998
Submission received: 17 September 2025 / Revised: 14 October 2025 / Accepted: 15 October 2025 / Published: 17 October 2025
(This article belongs to the Special Issue Development of New Wastewater Treatments for the Efficient Removal of Micropollutants)
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Abstract

Eco-friendly clay-based adsorbents with low cost and high adsorption capacity for toxic dyes have attracted significant attention. In this study, a novel citric acid-modified sepiolite (CA-SEP) composite was developed for the efficient removal of methylene blue (MB) from aqueous solutions. The morphological, crystalline, and structural properties of the composite were characterized using XRD, FTIR, SEM, and BET analyses. Compared to pristine SEP, CA-SEP exhibited a 2.6-fold increase in adsorption capacity for MB and demonstrated excellent reusability. The effects of key parameters—including solution pH (2.0–10.0), contact time (0–300 min), adsorbent dosage (0.2–2.0 g/L), and initial MB concentration (10–150 mg/L)—on adsorption performance were systematically investigated. Modeling results indicated that the Sips isotherm provided the optimal fit for the equilibrium data. In kinetic studies, the adsorption process was best described by the pseudo-second-order model. The maximum adsorption capacity of CA-SEP for MB was estimated to be 40.61 mg/g. Moreover, the adsorbent retained high removal efficiency after five adsorption-desorption cycles, demonstrating good regenerability. These results indicate that CA-SEP is a highly efficient, sustainable, and economically viable adsorbent for the elimination of MB from contaminated water.

1. Introduction

Water pollution caused by organic dyes represents a significant environmental challenge [1]. Residual dyes in wastewater pose serious ecological threats due to their high visibility, persistence, and adverse effects on aquatic ecosystems and human health [2]. Methylene blue (MB), a widely used synthetic dye in textile, printing, and ink industries [3], exemplifies these concerns. Prolonged exposure to MB may cause symptoms including precordial pain, excessive sweating, vomiting, skin discoloration, anemia, hypertension, and mental disorientation [4]. Consequently, effective treatment of MB-containing industrial effluents prior to environmental discharge is imperative.
Recent approaches to MB removal include flocculation-coagulation, membrane separation, electrochemical oxidation, and biodegradation [5,6,7]. While effective, these methods suffer from substantial limitations: high energy consumption, significant chemical requirements, operational expenses, capital costs, and dependence on specialized personnel [8]. Moreover, advanced purification techniques often fail to degrade MB due to its complex molecular structure (C16H18N3SCl), which exhibits notable resistance to water, light, oxidants, and microbial degradation [9]. In contrast, adsorption technology using eco-friendly materials offers a promising alternative [10,11]. Activated carbon and engineered nanomaterials are particularly favored for their operational simplicity, ease of implementation, and high adsorption capacities—attributable to their large specific surface areas, tunable pore structures, and micro-mesoporous architectures [12,13,14,15]. However, these adsorbents face barriers to widespread adoption, including high synthesis costs, complex preparation protocols, and/or the use of environmentally incompatible additives [12,16]. Therefore, developing cost-effective, eco-friendly adsorbents remains an urgent priority.
Clay minerals have garnered significant attention as sustainable adsorbents due to their natural abundance, low cost, and environmental compatibility [17,18]. Studies confirm the efficacy of kaolinite [19], montmorillonite [20], zeolite [21], bentonite [22], palygorskite [23], and sepiolite (SEP) [1,24,25] in aqueous MB removal, highlighting their structural stability and ease of application. Nevertheless, pristine clay minerals exhibit limited adsorption capacity owing to insufficient porosity and scarcity of surface functional groups [22]. Surface modification is therefore essential to unlock their full potential by enhancing specific surface area and increasing active site density, thereby improving dye removal performance.
Unlike most clay minerals, SEP—a fibrous, hydrated Mg-rich aluminosilicate clay mineral with 2:1 ribbon-layer structure and unit-cell formula (Si12Mg8O30(OH)4(OH2)4·8H2O)—has received considerable interest in wastewater treatment due to its unique morphology, tunable structure, and cost-effectiveness [26]. To enhance SEP’s adsorption capacity toward MB, various physical/chemical modifications have recently been explored, including nitric acid treatment [27], microwave-assisted acid activation [28], 3-aminopropyltriethoxysilane grafting [29], alginate encapsulation [25], and magnetic iron oxide incorporation [30]. These strategies primarily improve MB removal by disaggregating SEP fiber bundles, enhancing surface reactivity, increasing specific surface area, and developing mesoporous structures. Although inorganic acids (e.g., nitric acid) effectively enhance SEP’s MB adsorption [27,28], their applicability is limited by significant drawbacks, particularly the generation of large volumes of acidic wastewater during activation, which pose serious environmental risks [27,28,31]. To address this issue, researchers have shifted focus to low-molecular-weight organic acids (LMWOAs) as eco-friendly modifiers for SEP. Citric acid (CA), a nontoxic and abundant LMWOA derived from natural biomass, offers dual advantages: its hydroxyl and carboxyl groups serve as effective anchors for binding to clay minerals while acting as chelating sites for organic dyes and metal ions [32]. For instance, CA-modified bentonite exhibits markedly higher adsorption capacities for Congo red and Methyl orange than pristine bentonite [32,33], achieved via an eco-friendly process that avoids secondary pollution. However, CA-modified SEP (CA-SEP) and its application in MB adsorption have remained unexplored.
Inspired by this gap, this study pioneers the use of CA as a green modifier for SEP to develop a novel adsorbent (CA-SEP) for MB removal. Unlike inorganic acid treatments that risk secondary pollution, CA offers an eco-friendly alternative rich in carboxyl groups. The principal novelty of this work lies in demonstrating that the enhanced MB adsorption capacity is predominantly attributed to the newly introduced oxygen-containing functional groups, rather than an increase in surface area—a finding that underscores a paradigm shift in the design of clay-based adsorbents. Furthermore, this study comprehensively evaluates the adsorbent’s performance in real textile wastewater, its reusability over multiple cycles, and its cost-effectiveness, thereby bridging the gap between laboratory synthesis and practical application. The objectives of this work are: (a) To synthesize a novel CA-SEP adsorbent and characterize its morphology, structure, and surface properties using multifaceted analytical techniques; (b) To evaluate the effects of key operational parameters on MB adsorption by CA-SEP and elucidate adsorption mechanisms by integrating kinetic/isotherm studies with FTIR analysis; (c) To assess CA-SEP’s recyclability and practical potential for MB removal in real wastewater matrices.

2. Materials and Methods

2.1. Materials

The materials and chemicals used in this study are listed in Table S1 in the Supplementary Materials.

2.2. Preparation and Characterization of CA-SEP

CA-SEP was synthesized through a simple impregnation method. Briefly, predetermined amounts of CA (C6H8O7·H2O) were accurately weighed and dissolved in 200 mL of deionized water to prepare solutions at concentrations of 0.1 M, 0.2 M, 0.5 M, and 1.0 M. Each solution was transferred into separate 250 mL beakers. Subsequently, 20 g of raw SEP, previously sieved through 300-mesh (particle size ≤ 48 μm) to ensure homogeneity, was dispersed into each CA solution. The mixtures were magnetically stirred (500 rpm) at ambient temperature (25 ± 2 °C) for 24 h to facilitate interaction between CA functional groups and SEP surface sites. The resultant suspensions were first centrifuged at 5000 rpm for 10 min to collect the solids. Subsequently, the supernatants were discarded, and the collected solids underwent three washing cycles with deionized water. Finally, the samples were placed in glass Petri dishes and oven-dried at 60 °C for 15 h to remove residual moisture while preserving their structural integrity [31]. The dried CA-SEP composites were gently ground using an agate mortar and stored in airtight containers at room temperature prior to characterization. The modified adsorbents were designated as xCA-SEP (where “x” represents CA concentration: 0.1, 0.2, 0.5, or 1.0 M), establishing a concentration-dependent series for comparative analysis.
Detailed descriptions of the characterization methodologies are provided in the Supplementary Materials. This includes X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) surface area analysis, and the determination of the point of zero charge (pHPZC) via the pH drift method.

2.3. Adsorption Experiments

Batch adsorption experiments for MB were conducted as follows: 0.05 g of adsorbent was accurately weighed into a 150 mL conical flask. Subsequently, 50 mL of MB solution at a specific concentration was added. The solution pH was adjusted to the desired value, and the flask was agitated using a reciprocating shaker (Zhicheng, Shanghai, China) at 180 rpm. A systematic assessment was conducted to evaluate the impact of key operational variables on the adsorption process. The parameters studied were as follows: solution pH (2–10), contact time (5–300 min), adsorbent dosage (0.2–5 g/L), initial MB concentration (10–150 mg/L), and temperature (283, 298, and 313 K). The effects of these parameters were investigated sequentially using a one-factor-at-a-time approach. Unless otherwise specified as the variable under study, the following baseline conditions were maintained: an initial MB concentration of 50 mg/L, an adsorbent dosage of 1.0 g/L, a solution pH of 8.0, a contact time of 240 min (to ensure equilibrium was reached), and a temperature of 298 K. For instance, during the pH effect study (pH 2–10), the other parameters (concentration, dosage, contact time, temperature) were kept constant at these baseline values. Similarly, when investigating the effect of temperature, the pH, concentration, dosage, and contact time were fixed at their baseline conditions.
After the designated shaking period, the mixture was centrifuged to separate the solid adsorbent. The residual MB concentration in the supernatant was immediately analyzed by measuring its absorbance at 664 nm using a UV-vis spectrophotometer (Metash UV-8000, Yuanxi Instruments, Shanghai, China). The adsorption capacity for MB was calculated based on the mass balance between the initial and equilibrium concentrations. All experiments were performed in triplicate.
Kinetic studies involved fitting the experimental data to pseudo-first-order (PFO), pseudo-second-order (PSO), and Weber-Morris (W-M) kinetic models. Isotherm data were analyzed using Langmuir, Freundlich, and Sips models. Thermodynamic parameters, including the standard Gibbs free energy change (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°), were calculated. Details of these models are provided in the Supplementary Materials.

2.4. Regeneration of Adsorbents

To assess the regeneration capability of the spent CA-SEP adsorbent, five consecutive adsorption-desorption cycles were conducted. Each cycle employed an initial MB concentration of 50 mg/L, pH 8.0, a CA-SEP dosage of 1.0 g/L, and agitation at 180 rpm for 240 min. Following adsorption, desorption was performed using a 1:1 (v/v) ethanol/water mixture as the eluent [34]. Subsequently, the CA-SEP was rinsed thoroughly with deionized water, dried, and reused in the next adsorption cycle.

2.5. Treatment of MB-Containing Textile Wastewater

To evaluate the effectiveness of the CA-SEP adsorbent for real-world applications, wastewater samples were collected from a textile dyeing factory within a textile industrial park (Changzhou, China). Two distinct wastewater samples were utilized in this study: Sample 1: Obtained directly from the production line, containing MB dye and inorganic salts; Sample 2: Collected from the secondary settling tank of the industrial park’s wastewater treatment plant and subsequently spiked with MB dye to a concentration of 50 mg/L. All wastewater samples were stored at 4 °C and equilibrated to room temperature prior to adsorption experiments. Immediately following collection, the physical and chemical characteristics of the wastewater were analyzed in the laboratory using standard methods. Due to the presence of suspended solids, the samples were filtered through glass microfiber filters before conducting adsorption studies. Batch adsorption experiments were performed under the predetermined optimal conditions to assess the applicability of the CA-SEP adsorbent for treating these real wastewater matrices.

3. Results and Discussion

3.1. Characterization of the Adsorbent

The characterization was focused on the pristine SEP and two representative CA-SEP samples modified with 0.1 M and 0.5 M CA (denoted as 0.1CA-SEP and 0.5CA-SEP). These samples were selected to elucidate the structural and chemical evolution from a lightly modified state (0.1CA-SEP) to the optimal performer identified by adsorption capacity tests (0.5CA-SEP), as shown in Section 3.2. This comparative approach efficiently captures the critical effects of the CA modification process.
The structural characteristics of the pristine SEP and the CA-SEP samples were determined by XRD, and the resulting patterns are depicted in Figure 1. The XRD pattern of the pristine SEP indicated that its primary crystalline phases consisted of SEP, dolomite, calcite, and quartz. The presence of these impurities is recognized as a typical characteristic of natural SEP from Chinese origins [35]. Treatment of the SEP with 0.1 and 0.5 M CA resulted in a notable decrease in the intensity of the calcite peaks, suggesting that the CA treatment led to the partial dissolution of calcite [31]. A concurrent decrease in the intensity of the dolomite peaks was also observed in both the 0.1CA-SEP and 0.5CA-SEP samples. These findings agreed with previous reports on the organic acid treated SEP [35,36].
FTIR spectra of the pristine SEP and the CA-SEP samples modified with 0.1 M and 0.5 M CA are shown in Figure 2. In the spectrum of pristine SEP, the weak band observed at 3676 cm−1 (which was also present in the CA modified samples) is assigned to the Mg-OH stretching vibration. The peak at 669 cm−1 is associated with the bending vibration of Mg-OH [37]. The stretching vibrations found at 1434 cm−1 are characteristic of carbonate impurities [38]. The peaks located at approximately 948 and 1016 cm−1 correspond to the stretching of Si-O in the Si-O-Si groups of the tetrahedral sheets of SEP, while the band near 463 cm−1 is attributed to the Si-O-Si bending vibration [39]. After modification with CA, the intensity of the CO32− group at 1434 cm−1 decreased accordingly due to the partly dissolution of carbonate phases [40], which was in good agreement with the XRD results. Furthermore, a new peak emerged at around 1620 cm−1 in the spectra of CA-SEP, which corresponds to the C=O stretching vibration of saturated fatty acid groups. This confirms the successful introduction of new functional groups onto the SEP surface following CA modification [33,41,42,43].
SEM-EDS characterization was conducted to determine the approximate chemical composition of the SEP samples and to confirm the successful modification with CA. SEM images of the pristine SEP and CA-SEP samples are presented in Figure 3. The pristine SEP exhibited a fibrous morphology with non-uniformly distributed impurities (Figure 3a). After CA modification (Figure 3b,c), the fibrous structure of SEP remained intact, indicating that the structural integrity was preserved. However, the surfaces of the CA-SEP samples appeared more irregular and rougher than those of the pristine sample. A similar morphological change has been reported in previous studies on surface-modified SEP [40,44,45]. The EDS data are shown in Figure 3d,e, with the corresponding elemental compositions provided in the inset tables. The EDS data confirmed that O, Si, and Mg were the major elements in SEP, while the presence of Ca indicated the calcium-rich nature of the Chinese SEP [35,46]. Compared with the pristine SEP, the CA-SEP sample showed a similar elemental profile but with reduced contents of Ca and O. The decrease in the relative amounts of O and Ca was primarily attributed to the partial dissolution of CaCO3 during the CA treatment process [35].
The nitrogen adsorption-desorption isotherms and pore size distribution curves of pristine SEP, 0.1CA-SEP, and 0.5CA-SEP are presented in Figure 4. The specific surface areas were determined to be 3.2727, 2.8908, and 2.6304 m2/g for SEP, 0.1CA-SEP, and 0.5CA-SEP, respectively. Correspondingly, the total pore volumes decreased from 0.016505 cm3/g for the pristine SEP to 0.009352 cm3/g and 0.008258 cm3/g for the 0.1 M and 0.5 M CA-modified samples, respectively (Table S2). The reduction in both specific surface area and total pore volume after CA modification suggests that the treatment may have resulted in partial pore blockage or structural alteration, potentially due to the dissolution of surface impurities or deposition of CA residues [47]. These textural changes are consistent with the removal of carbonate impurities as indicated by XRD and FTIR analyses, which likely contributes to the observed decrease in porosity. Despite the reduced surface area and pore volume, the modified adsorbents exhibited enhanced adsorption capacity for methylene blue, implying that the introduction of oxygen-containing functional groups through CA modification played a more dominant role in the adsorption process than the textural properties.

3.2. Optimization and Selection of Adsorbent

To determine the optimal concentration of CA for enhancing the adsorbed concentration at equilibrium (qe) of SEP toward MB, the performance of raw SEP and SEP modified with different CA concentrations (0.1 M, 0.2 M, 0.5 M, and 1.0 M) was evaluated under consistent experimental conditions. As illustrated in Figure 5, the qe of raw SEP was only 4.86 mg/g. In comparison, CA modification considerably improved MB uptake, with the qe increasing as the CA concentration rose to 0.5 M. The sample modified with 0.5 M CA (0.5CA-SEP) exhibited an adsorption capacity of 17.42 mg/g. This notable enhancement suggests that CA treatment effectively introduces functional groups that promote MB binding. However, further increasing the CA concentration to 1.0 M did not result in additional improvement in adsorption performance, indicating that the modification effect had reached a saturation point. Therefore, 0.5 M was identified as the optimal CA concentration for SEP modification. Based on these results, all subsequent experiments were performed using the 0.5 M CA-modified SEP (0.5CA-SEP), which is hereafter denoted as CA-SEP.

3.3. Effect of Solution pH

The adsorption performance of CA-SEP for MB was highly dependent on the solution pH, given its governing role in the adsorbent’s surface charge and the dye’s ionization state [1]. As illustrated in Figure 6a, the qe of CA-SEP was significantly dependent on the initial pH. The lowest adsorption capacity (1.66 mg/g) was observed at pH 2, which gradually increased to a maximum value of 17.43 mg/g at pH 8. Further increasing the pH to 9 and 10 resulted in a slight decrease in adsorption capacity to 15.61 mg/g and 13.88 mg/g, respectively. The low adsorption efficiency under highly acidic conditions (pH 2) can be ascribed to the protonation of functional groups on the CA-SEP surface, leading to a strong positive charge. This promotes electrostatic repulsion between the adsorbent and the cationic MB molecules [43]. Additionally, at low pH, excess H+ ions compete with MB+ for adsorption sites, further reducing the adsorption capacity [48]. As the pH increased, the surface of CA-SEP became less positively charged, thereby diminishing the electrostatic repulsion and enhancing MB uptake.
The pHPZC is a key factor determining the surface charge characteristics of the adsorbent. As shown in Figure 6b, the pHPZC values for pristine SEP and CA-SEP were determined to be 6.8 and 7.5, respectively. The increase in pHPZC after CA modification indicates the successful introduction of acidic functional groups (e.g., -COOH), which undergo deprotonation at higher pH, contributing to a more negatively charged surface [49]. At solution pH > pHPZC, the CA-SEP surface acquires a negative charge, favoring the adsorption of cationic dyes via electrostatic attraction. The highest adsorption capacity at pH 8—slightly above the pHPZC of CA-SEP—confirms the dominance of electrostatic interactions in the adsorption process. The slight decrease in adsorption at pH > 8 may be due to the partial hydrolysis or aggregation of MB molecules under strongly alkaline conditions, which could reduce their effective availability for adsorption [50]. Moreover, although electrostatic attraction is a primary mechanism, other interactions such as hydrogen bonding interactions between the -NH groups of MB and the functional groups on CA-SEP may also contribute to the adsorption process [48,51]. In conclusion, the optimal pH for MB adsorption onto CA-SEP was found to be 8. A similar pH-dependent trend in MB adsorption has also been reported for studies using both natural and artificial zeolites [52].

3.4. Effect of Contact Time and Adsorbent Kinetics

The adsorption kinetics of MB onto CA-SEP were studied at initial concentrations of 50 and 100 mg/L. As depicted in Figure 7a, the process was characterized by fast kinetics in the first hour, progressively reaching equilibrium at 240 min for both concentrations. The kinetic data were fitted using the PFO, PSO, and Weber-Morris (W-M) intraparticle diffusion models (Figure 7a–c), and the quantified parameters along with their R2 values from nonlinear regression are summarized in Table S3.
The R2 values for both PFO and PSO models were relatively high and close: 0.9039 and 0.9577 for PFO, and 0.9225 and 0.9672 for PSO, respectively. This suggests that the adsorption process might involve both physical and chemical interactions [53]. Although the PSO model yielded slightly higher R2 values, the calculated adsorbed concentration at equilibrium (qe, cal) from the PFO model were closer to the experimental values (qe, exp). This discrepancy further supports the coexistence of physical and chemical adsorption mechanisms in the MB uptake process [54].
In addition, the adsorption data were further analyzed using the W-M intraparticle diffusion model. The multi-linear plot (Figure 7c) suggests that both surface and intraparticle diffusion contribute to the overall adsorption process [4]. The initial steeper segment corresponds to rapid diffusion of MB to the external surface, while the subsequent gradual stage represents slower diffusion into the internal pores [55]. The higher rate constant kd1 compared to kd2 reflects the faster uptake at surface sites. The increase in the C value from the first to the second stage suggests a greater influence of boundary layer effects on the intraparticle diffusion step [56]. Furthermore, the non-zero intercept implies that intraparticle diffusion is not the sole rate-limiting step, supporting the involvement of a complex adsorption mechanism [57].

3.5. Effect of Adsorbent Dosage

The adsorption performance of CA-SEP for MB was investigated under varying adsorbent dosages, as illustrated in Figure 7d. It can be observed that as the adsorbent dosage increased from 0.2 to 10 g/L, the removal efficiency of MB showed a notable increase, approaching nearly 100% at higher dosages. This trend is attributed to the greater availability of active adsorption sites and increased surface area at elevated CA-SEP concentrations, which enhance the interaction between the CA-SEP and MB molecules [12,14].
Conversely, the adsorbed concentration at equilibrium, expressed as qe (mg/g), exhibited a significant decrease with increasing adsorbent dosage. This inverse relationship is commonly observed in adsorption systems and can be explained by the saturation of adsorption sites and the reduction in effective surface area per unit mass of adsorbent due to potential aggregation or overlapping active sites at higher dosages [16,18,19,20]. The results demonstrate that CA-SEP is an effective adsorbent for MB removal, particularly at moderate to high dosages, where both high removal rates and reasonable adsorption capacity can be achieved.

3.6. Adsorption Equilibrium Isotherms and Thermodynamic Study

The adsorption equilibrium isotherms of MB onto CA-SEP were evaluated at different temperatures, and the corresponding data were fitted with Langmuir, Freundlich, and Sips models, as shown in Figure 8a–c and Table S4. The maximum adsorption capacity (qm) derived from the Langmuir model increased slightly with temperature, from 39.66 mg/g at 283 K to 40.61 mg/g at 313 K, suggesting enhanced adsorption performance at elevated temperatures. The rise in the Langmuir constant KL with temperature further supports endothermic adsorption behavior. Although the Langmuir model yielded relatively high correlation coefficients (R2 > 0.95), suggesting monolayer adsorption may play a significant role, the Sips model provided the best fit across all temperatures (R2 > 0.98), implying a hybrid adsorption mechanism combining aspects of both monolayer and heterogeneous surface adsorption [58]. The calculated values of 1/n from the Freundlich model fell within the range of 0.1 to 1 across all temperatures, which is mathematically consistent with favorable adsorption of MB onto CA-SEP under the investigated conditions [43,57].
Thermodynamic parameters, presented in Table S5, offer further insight into the adsorption process. The positive values of ΔH° (ranging from 15.81 to 29.28 kJ/mol) suggest the endothermic nature of MB adsorption onto CA-SEP. The adsorption process was confirmed to be spontaneous, as indicated by the negative ΔG° values across all tested temperatures. The trend of these values becoming more negative with increasing temperature suggests a greater degree of spontaneity and thermodynamic favorability for adsorption at higher temperatures [37]. The observed positive ΔS° values point to a significant reorganization and an increase in randomness at the interface during adsorption. A plausible mechanism for this phenomenon is the displacement and release of pre-adsorbed water molecules from the CA-SEP surface by the incoming MB cations [41]. In conclusion, the adsorption of MB onto CA-SEP is well described by the Sips model, and thermodynamic results confirm the spontaneous and endothermic nature of the process.

3.7. Comparison of the Adsorption Capacity with Other Adsorbents

It is also valuable to compare the qm for MB adsorption on CA-SEP adsorbent with various clay adsorbents, because this comparison could imply the effectiveness of CA-SEP as an advantageous adsorbent for aqueous MB removal. Based on the comprehensive analysis of the adsorption performance of CA-SEP for MB removal and the comparative data provided in Table S6, a detailed comparison with various adsorbents reported in the literature is presented below. This comparison situates CA-SEP within the broader context of existing materials, highlighting its competitive advantages in terms of performance, cost, and sustainability.
As estimated by the Langmuir model, the qm of CA-SEP for MB was found to be 40.61 mg/g under optimal conditions. Compared to other adsorbents [59,60,61,62,63,64,65], CA-SEP exhibits significantly higher capacity than many low-cost natural and modified clays, such as natural zeolite (0.204 mg/g), artificial zeolite (0.3326 mg/g), bagasse-montmorillonite composite (11.94 mg/g), bentonite (22.131 mg/g), and chabazite (28.9 mg/g). It performs comparably to several composite and modified clay materials, including chitosan-modified zeolite (37.04 mg/g), chitosan/sepiolite composite (40.986 mg/g), and some NiO/sepiolite composites (22.73–50.00 mg/g). However, CA-SEP shows a lower qm than some advanced nanomaterials, such as multi-walled carbon nanotubes (55.18 mg/g) [55], silver nanoparticles decorated SEP (101 mg/g) [1], and g-C3N4-modified Na-Ca-magnesium silicate adsorbent (420 mg/g) [66].
Despite not achieving the highest adsorption capacity, CA-SEP offers a balanced profile of moderate performance, low cost, and environmental sustainability. Its synthesis is straightforward and eco-friendly, using abundant natural SEP and non-toxic CA, with a significantly lower production cost than commercial activated carbon and many engineered nanomaterials. Moreover, CA-SEP demonstrates excellent regenerability, retaining over 80% of its adsorption capacity after five consecutive cycles. In conclusion, while CA-SEP may not surpass all adsorbents in absolute adsorption capacity, its combination of competitive performance, low cost, ease of preparation, and green characteristics makes it a highly viable and sustainable option for practical wastewater treatment, particularly in scenarios where cost-effectiveness and environmental compatibility are prioritized.

3.8. Adsorption Mechanisms

To explore the adsorption mechanism, SEM-EDS and FTIR spectrum of the CA-SEP adsorbent after MB adsorption were measured. Figure 9a depicts that the morphology of the adsorbent is not changed obviously after adsorption, suggesting the stability of adsorbent. From the EDS data (Figure 9b), the presence of N, S, and Cl proved that MB was successfully adsorbed on the surfaces of CA-SEP adsorbent. Meanwhile, after the adsorption of MB onto the optimal CA-SEP adsorbent (from a 50 mg/L solution at pH 8.0 under baseline conditions), the FTIR spectrum (Figure 9c) exhibited several changes. The adsorption band at 1620 cm−1, corresponding to the C=O stretching of -COOH groups, diminished significantly. This suggests that the carboxyl groups on the surface of CA-SEP play a critical role in the adsorption of MB [33,41]. Therefore, the adsorption mechanism likely involves surface complexation between the positively charged NH+ groups in MB molecules and the negatively charged deprotonated COO− groups on the CA-SEP surface. Additionally, adsorption could be facilitated by hydrogen bonding between the -OH/-COOH groups of CA-SEP and the -NH groups of MB. Similar mechanisms have been reported for the adsorption of dyes onto CA-modified bentonite [32] and rhamnolipid-functionalized graphene oxide [43]. Moreover, the MB-loaded CA-SEP sample displayed several new small bands around 1600 cm−1, which are attributed to the aromatic C-C stretching vibrations of MB [44]. These results collectively demonstrate the successful adsorption of MB onto the CA-SEP adsorbent. Figure 9d shows the mechanism of CA-SEP adsorbent capturing MB.

3.9. Preliminary Cost Evaluation and Reusability of CA-SEP

The production cost is an initial screening factor for assessing an adsorbent’s potential for scale-up. In this work, CA-SEP was synthesized from low-cost and readily available starting materials: natural SEP and CA. As a natural clay mineral, SEP is abundant, with vast deposits existing in China, accounting for approximately one-fifth of the world’s reserves. This abundance makes it an inexpensive material, with an industrial price of around $112 per ton [40,46]. CA is also an inexpensive, commercially available organic acid, with an industrial price of approximately $350 per ton. Based on the synthesis protocol, the estimated production cost of CA-SEP was approximately $185 per ton. which is remarkably lower than that of many conventional adsorbents such as commercial activated carbon (~$1200 per ton) [67].
It should be noted that this is a preliminary cost estimate based solely on raw material inputs, and a full techno-economic analysis including energy, equipment, and operational costs would be required to accurately determine economic viability at scale. Nevertheless, this marked cost advantage suggests that CA-SEP holds promise as a low-cost adsorbent worthy of further development.
Furthermore, the regeneration and recyclability of an adsorption material are crucial parameters for assessing its overall economy and practical application. To evaluate this, the adsorption-desorption cycling performance of the CA-SEP adsorbent was investigated using a 1:1 (v/v) ethanol/water mixture as the desorbing agent [34]. As illustrated in Figure 10, the qe for MB remained above 80% of its original value even after five consecutive adsorption-desorption cycles. This demonstrates a relatively stable adsorption performance and confirms that the CA-SEP composite can be effectively regenerated for at least five cycles while maintaining stable adsorption efficiency. Meanwhile, the consistent adsorption performance over five regeneration cycles also suggests that the CA modifier is stably bound to the SEP surface, with no significant leaching observed that would lead to a rapid loss of capacity or secondary pollution. In conclusion, the combination of low initial production cost and good recyclability makes the CA-SEP composite an economically attractive and sustainable adsorbent for the elimination of MB from aqueous solutions.

3.10. Application of CA-SEP to Real Wastewater

To assess the practical applicability of the CA-SEP adsorbent, batch adsorption experiments were performed using real textile wastewater contaminated with MB. The wastewater samples were first characterized in terms of their physicochemical properties and color intensity (Table S7). The initial pH of the samples ranged from 8.2 to 11.8, which falls within the optimal range for MB adsorption onto CA-SEP, thus, no pH adjustment was necessary prior to treatment. After treatment with CA-SEP, a significant improvement in water quality was observed. The color dilution factor decreased to 180 for Sample 1 and to 5 for Sample 2. Meanwhile, the CODCr values were reduced from 1265 mg/L to 680 mg/L in Sample 1 and from 94.9 mg/L to 38 mg/L in Sample 2, demonstrating effective removal of both color and organic pollutants. The high removal efficiency can be attributed to the alkaline pH of the wastewater, which favors the adsorption of MB onto CA-SEP. Notably, after CA-SEP treatment, Sample 2—collected from the secondary settling tank of a textile wastewater treatment plant—met the requirements of the Chinese Discharge Standard of Water Pollutants for Dyeing and Finishing of Textile Industry (GB 4287-2012) [68]. These results underscore the strong potential of CA-SEP as a cost-effective and efficient adsorbent for the treatment of dye-laden industrial wastewater.

4. Conclusions

This study successfully developed an eco-friendly and cost-effective CA-SEP adsorbent for the removal of MB from aqueous solutions. The characterization results confirmed the successful incorporation of carboxyl groups onto the SEP surface, which significantly enhanced its adsorption performance. The optimal modification condition was achieved with 0.5 M citric acid, yielding an adsorption capacity of 17.42 mg/g—2.6 times higher than that of raw SEP. The adsorption process was highly dependent on solution pH, with optimal removal observed at pH 8.0. Kinetic and isotherm studies revealed that the adsorption process was well-described by the PSO kinetics model and the Sips isotherm, suggesting a combination of chemisorption and heterogeneous surface adsorption. Thermodynamic parameters were consistent with an endothermic and spontaneous nature of MB adsorption. The CA-SEP adsorbent exhibited excellent reusability, maintaining over 80% of its original adsorption capacity after five consecutive cycles. Cost analysis showed that CA-SEP is considerably more economical than commercial activated carbon. Furthermore, experiments using real textile wastewater demonstrated the effective applicability of CA-SEP under realistic conditions. These findings highlight the potential of CA-SEP as a sustainable, efficient, and regenerable adsorbent for the treatment of dye-containing wastewater, contributing to the development of green and scalable water purification technologies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17202998/s1; Table S1. The main experimental materials and chemicals used in this study. Table S2. BET surface area, pore volume and pore diameter values of the SEP, 0.1CA-SEP, and 0.5CA-SEP. Table S3. Kinetic model constants for MB adsorption by CA-SEP at different initial concentrations. Table S4. Parameters of thee isotherm models for MB adsorption on CA-SEP. Table S5. The thermodynamic parameters for adsorption of MB onto CA-SEP at various adsorption capacities and temperatures. Table S6. Comparison with other materials for MB adsorption. Table S7. Physicochemical characteristics of the real wastewater. References [1,14,15,21,22,52,59,60,61,62,63,64,65,66] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, Z.T. and W.W.; methodology, Z.C.; software, Q.W.; validation, X.G., Z.C. and Q.W.; formal analysis, X.G.; investigation, Z.T.; resources, W.W.; data curation, Z.T.; writing—original draft preparation, Z.T.; writing—review and editing, W.W.; visualization, Z.C.; supervision, W.W.; project administration, Z.T. and W.W.; funding acquisition, Z.T. and W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Key Technologies R&D Program of Henan Province (grant number: 242102230110) and PAPD (a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, grant number: 164320H101).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Fayazi, M.; Rezvannejad, E. Bio-inspired preparation of silver nanoparticles on nanostructured sepiolite clay: Characterization and application as an effective adsorbent for methylene blue removal. Inorg. Chem. Commun. 2024, 159, 111786. [Google Scholar] [CrossRef]
  2. Xu, Y.; Liu, Y.; Liu, S.; Tan, X.; Zeng, G.; Zeng, W.; Ding, Y.; Cao, W.; Zheng, B. Enhanced adsorption of methylene blue by citric acid modification of biochar derived from water hyacinth (Eichornia crassipes). Environ. Sci. Pollut. Res. 2016, 23, 23606–23618. [Google Scholar] [CrossRef]
  3. El Messaoudi, N.; Ulfa, M.; Hamzah, A.; Hamid, Z.A.A.; Ramadhani, D.V.; Suryanegara, L.; Mahardika, M.; Melenia, A.T.; Pratama, A.W.; Prasetyoko, D. Fabrication a sustainable adsorbent nanocellulose-mesoporous hectorite bead for methylene blue adsorption. Case Stud. Chem. Environ. Eng. 2024, 10, 100850. [Google Scholar] [CrossRef]
  4. Farasati Far, B.; Naimi-Jamal, M.R.; Jahanbakhshi, M.; Keihankhadiv, S.; Baradarbarjastehbaf, F. Enhanced methylene blue adsorption using single-walled carbon nanotubes/chitosan-graft-gelatin nanocomposite hydrogels. Scient. Rep. 2024, 14, 19217. [Google Scholar] [CrossRef] [PubMed]
  5. Al-Gethami, W.; Qamar, M.A.; Shariq, M.; Alaghaz, A.N.M.; Farhan, A.; Areshi, A.A.; Alnasir, M.H. Emerging environmentally friendly bio-based nanocomposites for the efficient removal of dyes and micropollutants from wastewater by adsorption: A comprehensive review. RSC Adv. 2024, 14, 2804–2834. [Google Scholar] [CrossRef] [PubMed]
  6. Piaskowski, K.; Świderska-Dąbrowska, R.; Zarzycki, P.K. Dye removal from water and wastewater using various physical, chemical, and biological processes. J. AOAC Int. 2018, 101, 1371–1384. [Google Scholar] [CrossRef]
  7. Bal, G.; Thakur, A. Distinct approaches of removal of dyes from wastewater: A review. Mater. Today Proceed. 2022, 50, 1575–1579. [Google Scholar] [CrossRef]
  8. Zain, N.S.; Mahmoud, M.H.H.; Khan, M.I.; Zafar, F.; Manzoor, S.; Akhtar, N.; Khan, M.A.; El Azab, I.H.; El-Bahy, Z.M. Machine learning-assisted optimization and evaluation of methylene blue adsorption kinetics on citrus aurantifolia leaves: Insights from isotherm and thermodynamic studies. J. Taiwan Inst. Chem. Eng. 2024, 164, 105696. [Google Scholar] [CrossRef]
  9. Khan, I.; Saeed, K.; Zekker, I.; Zhang, B.; Hendi, A.H.; Ahmad, A.; Ahmad, S.; Zada, N.; Ahmad, H.; Shah, L.A.; et al. Review on methylene blue: Its properties, uses, toxicity and photodegradation. Water 2022, 14, 242. [Google Scholar] [CrossRef]
  10. Kandel, D.R.; Kim, H.J.; Lim, J.M.; Poudel, M.B.; Cho, M.; Kim, H.W.; Oh, B.T.; Nah, C.; Lee, S.H.; Dahal, B.; et al. Cold plasma-assisted regeneration of biochar for dye adsorption. Chemosphere 2022, 309, 136638. [Google Scholar] [CrossRef]
  11. Radoor, S.; Kandel, D.R.; Park, K.; Jayakumar, A.; Karayil, J.; Lee, J. Low-cost and eco-friendly PVA/carrageenan membrane to efficiently remove cationic dyes from water: Isotherms, kinetics, thermodynamics, and regeneration study. Chemosphere 2024, 350, 140990. [Google Scholar] [CrossRef]
  12. Lee, H.; Fiore, S.; Berruti, F. Adsorption of methyl orange and methylene blue on activated biocarbon derived from birchwood pellets. Biomass Bioenergy 2024, 191, 107446. [Google Scholar] [CrossRef]
  13. Velusamy, S.; Roy, A.; Sundaram, S.; Kumar, M.T. A review on heavy metal ions and containing dyes removal through graphene oxide-based adsorption strategies for textile wastewater treatment. Chem. Record 2021, 21, 1570–1610. [Google Scholar] [CrossRef]
  14. Essa, W.K. Methylene blue removal by copper oxide nanoparticles obtained from green synthesis of Melia azedarach: Kinetic and isotherm studies. Chemistry 2024, 6, 249–263. [Google Scholar] [CrossRef]
  15. Ceroni, L.; Benazzato, S.; Pressi, S.; Calvillo, L.; Marotta, E.; Menna, E. Enhanced adsorption of methylene blue dye on functionalized multi-walled carbon nanotubes. Nanomaterials 2024, 14, 522. [Google Scholar] [CrossRef] [PubMed]
  16. Parastar, S.; Asl, F.B.; Poureshgh, Y.; Rashtbari, Y.; Nazari, S.; Asgari, E.; Hayati, B. Assessment of the efficiency of methylene blue removal from aqueous solutions using Iron magnetic nanoparticles immobilized on clinoptilolite zeolite. Int. J. Environ. Res. 2025, 19, 35. [Google Scholar] [CrossRef]
  17. Serikbayeva, A.M.; Roman, F.F.; Gomes, H.T.; Kalmakhanova, M.S. Development and characterization of organically grafted clay minerals for the removal of methylene blue from water. Clays Clay Miner. 2024, 72, e34. [Google Scholar] [CrossRef]
  18. Saad, E.M.; Wagdy, M.; Orabi, A.S. Advanced nano modification of ecofriendly glauconite clay for high efficiency methylene blue dye adsorption. Scient. Rep. 2024, 14, 23614. [Google Scholar] [CrossRef]
  19. Hamri, N.; Imessaoudene, A.; Hadadi, A.; Cheikh, S.; Boukerroui, A.; Bollinger, J.C.; Amrane, A.; Tahraoui, H.; Tran, H.N.; Ezzat, A.O.; et al. Enhanced adsorption capacity of methylene blue dye onto kaolin through acid treatment: Batch adsorption and machine learning studies. Water 2024, 16, 243. [Google Scholar] [CrossRef]
  20. Shiferraw, B.T.; Mengesha, D.N.; Kim, H. Enhanced adsorption capacity of phosphoric acid-modified montmorillonite clay and ground coffee waste-derived carbon-based functional composite beads for the effective removal of methylene blue. Colloids Surf. A 2024, 695, 134243. [Google Scholar] [CrossRef]
  21. Hayfron, J.; Jääskeläinen, S.; Tetteh, S. Synthesis of zeolite from rice husk ash and kaolinite clay for the removal of methylene blue from aqueous solution. Heliyon 2025, 11, e41325. [Google Scholar] [CrossRef] [PubMed]
  22. Hamad, H.N.; Idrus, S.; Yusuf, B.; Jamali, N.S.; Ahsan, A.; Suhartini, S.; Wahab, A.M.A. Optimized bentonite clay adsorbents for methylene blue removal. Processes 2024, 12, 738. [Google Scholar] [CrossRef]
  23. Figueiredo, V.V.D.; Vianna, E.L.F.; Lima, B.S.; Jesus, T.C.L.D.; García-Villén, F.; Bertolino, L.C.; Spinelli, L.S.; Viseras, C. Brazilian palygorskite as an alternative to commercial adsorbents for methylene blue: A discussion about composition, morphology and pore profile. Microp. Mesop. Mater. 2024, 366, 112957. [Google Scholar] [CrossRef]
  24. Acar, E.T. An experimental and theoretical investigation of cationic azine dye adsorption on natural sepiolite in single and multi-component systems. Chem. Eng. Res. Design 2022, 187, 507–515. [Google Scholar] [CrossRef]
  25. Chinoune, K.; Mekki, A.; Boukoussa, B.; Mokhtar, A.; Sardi, A.; Hachemaoui, M.; Iqbal, J.; Ismail, I.; Abboud, M.; Aboneama, W.A. Adsorption behavior of MB dye on alginate-sepiolite biocomposite beads: Adsorption, kinetics, and modeling. Inorg. Chem. Commun. 2024, 165, 112558. [Google Scholar] [CrossRef]
  26. Yang, F.; Wang, A. Recent researches on antimicrobial nanocomposite and hybrid materials based on sepiolite and palygorskite. Appl. Clay Sci. 2022, 219, 106454. [Google Scholar] [CrossRef]
  27. Cheng, Z.; Yang, R.; Zhu, X. Adsorption behaviors of the methylene blue dye onto modified sepiolite from its aqueous solutions. Desalin. Water Treat. 2016, 57, 25207–25215. [Google Scholar] [CrossRef]
  28. Pardo, L.; Cecilia, J.A.; López-Moreno, C.; Hernández, V.; Pozo, M.; Bentabol, M.J.; Franco, F. Influence of the structure and experimental surfaces modifications of 2:1 clay minerals on the adsorption properties of methylene blue. Minerals 2018, 8, 359. [Google Scholar] [CrossRef]
  29. Moreira, M.A.; Ciuffi, K.J.; Rives, V.; Vicente, M.A.; Trujillano, R.; Gil, A.; Korili, S.A.; de Faria, E.H. Effect of chemical modification of palygorskite and sepiolite by 3-aminopropyltriethoxisilane on adsorption of cationic and anionic dyes. Appl. Clay Sci. 2017, 135, 394–404. [Google Scholar] [CrossRef]
  30. Grządka, E.; Godek, E.; Le, T.A.; Maciołek, U.; Galaburda, M.; Orzeł, J.; Leskinen, T.; Huynh, T.P. Synthesis, properties and applications of new hybrid material based on native/magnetically modified sepiolite and chitosans. Sep. Purif. Technol. 2024, 348, 127671. [Google Scholar] [CrossRef]
  31. Yonar, S.; Ugwu, E.L.; Sabah, E. Modelling and optimization of sepiolite activation with citric acid using factorial experimental design and response surface methodology. Silicon 2021, 13, 2185–2194. [Google Scholar] [CrossRef]
  32. Zhang, H.; Zhou, J.; Muhammad, Y.; Tang, R.; Liu, K.; Zhu, Y.; Tong, Z. Citric acid modified bentonite for Congo red adsorption. Front. Mater. 2019, 6, 5. [Google Scholar] [CrossRef]
  33. Wen, L.; Ye, C.; Gan, F.; Ao, F.; Luo, Y.; Deng, J.; Chen, X. Citric acid-modified bentonite for enhanced dye removal in cutoff walls. Mater. Chem. Phys. 2025, 332, 130286. [Google Scholar] [CrossRef]
  34. Le, T.P.; Luong, H.V.T.; Nguyen, H.N.; Pham, T.K.T.; Le, T.L.T.; Tran, T.B.Q.; Ngo, T.N.M. Insight into adsorption-desorption of methylene blue in water using zeolite NaY: Kinetic, isotherm and thermodynamic approaches. Results Surf. Interf. 2024, 16, 100281. [Google Scholar] [CrossRef]
  35. Miura, A.; Nakazawa, K.; Takei, T.; Kumada, N.; Kinomura, N.; Ohki, R.; Koshiyama, H. Acid-, base-, and heat-induced degradation behavior of Chinese sepiolite. Ceram. Int. 2012, 38, 4677–4684. [Google Scholar] [CrossRef]
  36. Wu, C.R.; Hong, Z.Q.; Zhan, B.J.; Tang, W.; Cui, S.C.; Kou, S.C. Effect of acid treatment on the reactivity of natural sepiolite used as a supplementary cementitious material. Constr. Build. Mater. 2022, 316, 125860. [Google Scholar] [CrossRef]
  37. Li, Y.; Wang, M.; Sun, D.; Li, Y.; Wu, T. Effective removal of emulsified oil from oily wastewater using surfactant-modified sepiolite. Appl. Clay Sci. 2018, 157, 227–236. [Google Scholar] [CrossRef]
  38. Ma, Y.; Zhang, G. Sepiolite nanofiber-supported platinum nanoparticle catalysts toward the catalytic oxidation of formaldehyde at ambient temperature: Efficient and stable performance and mechanism. Chem. Eng. J. 2016, 288, 70–78. [Google Scholar] [CrossRef]
  39. Zhang, Q.T.; Li, S.X.; Hu, X.P.; Wang, P.J.; Zeng, J.B.; Wang, X.L.; Wang, Y.Z. Structure, morphology, and properties of LDPE/sepiolite nanofiber nanocomposite. Polym. Adv. Technol. 2017, 28, 958–964. [Google Scholar] [CrossRef]
  40. Ma, S.; Wei, S.; Li, S.; Wei, W.; Huang, Y. Facile activation of natural calcium-rich sepiolite with oxalic acid for selective Pb(II) removal: Highly-efficient performance, mechanisms and site energy distribution. Chemosphere 2023, 342, 140201. [Google Scholar] [CrossRef]
  41. Sajab, M.S.; Chia, C.H.; Zakaria, S.; Jani, S.M.; Ayob, M.K.; Chee, K.L.; Khiew, P.S.; Chiu, W.S. Citric acid modified kenaf core fibres for removal of methylene blue from aqueous solution. Bioresour. Technol. 2011, 102, 7237–7243. [Google Scholar] [CrossRef]
  42. Sun, C.; Qiu, J.; Zhang, Z.; Marhaba, T.F.; Zhang, Y.; Zhang, W. Characterization of citric acid-modified clam shells and application for aqueous lead(II) removal. Water Air Soil Pollut. 2016, 227, 298. [Google Scholar] [CrossRef]
  43. Wu, Z.; Zhong, H.; Yuan, X.; Wang, H.; Wang, L.; Chen, X.; Zeng, G.; Wu, Y. Adsorptive removal of methylene blue by rhamnolipid-functionalized graphene oxide from wastewater. Water Res. 2014, 67, 330–344. [Google Scholar] [CrossRef]
  44. Zhang, Y.; Wang, W.; Zhang, J.; Liu, P.; Wang, A. A comparative study about adsorption of natural palygorskite for methylene blue. Chem. Eng. J. 2015, 262, 390–398. [Google Scholar] [CrossRef]
  45. Zou, Y.; Ren, B.; He, Z.; Deng, X. Enhanced removal of Sb(III) by hydroxy-iron/acid–base-modified sepiolite: Surface structure and adsorption mechanism. Water 2022, 14, 3806. [Google Scholar] [CrossRef]
  46. Song, N.; Hursthouse, A.; McLellan, I.; Wang, Z. Treatment of environmental contamination using sepiolite: Current approaches and future potential. Environ. Geochem. Health 2021, 43, 2679–2697. [Google Scholar] [CrossRef] [PubMed]
  47. Zhang, Z.; Wang, W.; Kang, Y.; Zong, L.; Wang, A. Tailoring the properties of palygorskite by various organic acids via a one-pot hydrothermal process: A comparative study for removal of toxic dyes. Appl. Clay Sci. 2016, 120, 28–39. [Google Scholar] [CrossRef]
  48. Chouaybi, I.; Ouassif, H.; Matbout, O.; Bettach, M.; Moujahid, E.M. Highly efficient removal of alizarin yellow R dye from aqueous solution using a synthetic hydrocalumite-type LDH (CaAl-NO3). J. Inorg. Organomet. Polym. Mater. 2023, 33, 1517–1526. [Google Scholar] [CrossRef]
  49. Kim, H.; Kim, H.R. Production of coffee-dyed bacterial cellulose as a bio-leather and using it as a dye adsorbent. PLoS ONE 2022, 17, e0265743. [Google Scholar] [CrossRef]
  50. Ataei-Germi, T.; Nematollahzadeh, A. Bimodal porous silica microspheres decorated with polydopamine nano-particles for the adsorption of methylene blue in fixed-bed columns. J. Colloid Interface Sci. 2016, 470, 172–182. [Google Scholar] [CrossRef]
  51. Yao, J.; Yang, H.; Zuo, D.; Xu, J.; Zhang, H. Facile preparation and adsorption behavior studies of poly(acrylic acid)-based hydrogels reinforced by hydrogen bonds for methylene blue dye. J. Polym. Environ. 2023, 31, 552–564. [Google Scholar] [CrossRef]
  52. Turp, S.M.; Turp, G.A.; Ekinci, N.; Özdemir, S. Enhanced adsorption of methylene blue from textile wastewater by using natural and artificial zeolite. Water Sci. Technol. 2020, 82, 513–523. [Google Scholar] [CrossRef] [PubMed]
  53. Zhou, P.; Li, X.; Zhou, J.; Peng, Z.; Shen, L.; Li, W. Insights of the adsorption mechanism of methylene blue on biochar from phytoextraction residues of Citrus aurantium L.: Adsorption model and DFT calculations. J. Environ. Chem. Eng. 2023, 11, 110496. [Google Scholar] [CrossRef]
  54. Pei, P.; Xu, Y.; Wang, L.; Liang, X.; Sun, Y. Thiol-functionalized montmorillonite prepared by one-step mechanochemical grafting and its adsorption performance for mercury and methylmercury. Sci. Total Environ. 2022, 806, 150510. [Google Scholar] [CrossRef]
  55. Alam, S.; Ullah, B.; Khan, M.S.; Rahman, N.U.; Khan, L.; Shah, L.A.; Zekker, I.; Burlakovs, J.; Kallistova, A.; Pimenov, N.; et al. Adsorption kinetics and isotherm study of basic red 5 on synthesized silica monolith particles. Water 2021, 13, 2803. [Google Scholar] [CrossRef]
  56. Luoyang, Y.; Wang, H.; Li, J.; Chen, B.; Li, X.; Guotao, Z. Microstructural tuning and high-efficiency adsorption performance of carbonaceous porous adsorbents from coal gasification fine slag for methylene blue removal. Sep. Purif. Technol. 2025, 357, 130135. [Google Scholar] [CrossRef]
  57. Huang, Q.; Chai, K.; Zhou, L.; Ji, H. A phenyl-rich β-cyclodextrin porous crosslinked polymer for efficient removal of aromatic pollutants: Insight into adsorption performance and mechanism. Chem. Eng. J. 2020, 387, 124020. [Google Scholar] [CrossRef]
  58. Obayomi, K.S.; Lau, S.Y.; Zahir, A.; Meunier, L.; Zhang, J.; Dada, A.O.; Rahman, M.M. Removing methylene blue from water: A study of sorption effectiveness onto nanoparticles-doped activated carbon. Chemosphere 2023, 313, 137533. [Google Scholar] [CrossRef]
  59. Marrakchi, F.; Khanday, W.A.; Asif, M.; Hameed, B.H. Cross-linked chitosan/sepiolite composite for the adsorption of methylene blue and reactive orange 16. Int. J. Biol. Macromol. 2016, 93, 1231–1239. [Google Scholar] [CrossRef]
  60. Yao, Y.; Gao, B.; Fang, J.; Zhang, M.; Chen, H.; Zhou, Y.; Creamer, A.E.; Sun, Y.; Yang, L. Characterization and environmental applications of clay–biochar composites. Chem. Eng. J. 2014, 242, 136–143. [Google Scholar] [CrossRef]
  61. Zeng, L.; Xie, M.; Zhang, Q.; Kang, Y.; Guo, X.; Xiao, H.; Peng, Y.; Luo, J. Chitosan/organic rectorite composite for the magnetic uptake of methylene blue and methyl orange. Carbohydr. Polym. 2015, 123, 89–98. [Google Scholar] [CrossRef]
  62. Aysan, H.; Edebali, S.; Ozdemir, C.; Karakaya, M.C.; Karakaya, N. Use of chabazite, a naturally abundant zeolite, for the investigation of the adsorption kinetics and mechanism of methylene blue dye. Microporous Mesoporous Mater. 2016, 235, 78–86. [Google Scholar] [CrossRef]
  63. Xie, J.; Li, C.; Chi, L.; Wu, D. Chitosan modified zeolite as a versatile adsorbent for the removal of different pollutants from water. Fuel 2013, 103, 480–485. [Google Scholar] [CrossRef]
  64. Gao, S.; Wang, D.; Huang, Z.; Su, C.; Chen, M.; Lin, X. Recyclable NiO/sepiolite as adsorbent to remove organic dye and its regeneration. Sci. Rep. 2022, 12, 2895. [Google Scholar] [CrossRef]
  65. Su, C.; Wang, L.; Chen, M.; Huang, Z.; Lin, X. Adsorptive removal of methylene blue by CuO-acid modified sepiolite as effective adsorbent and its regeneration with high-temperature gas stream. Water Sci. Technol. 2016, 74, 844–851. [Google Scholar] [CrossRef]
  66. Soyer-Uzun, S.; Yu, P.; Oner, F.K.; Sen, S. Mechanistic Understanding of Superior Methylene Blue Adsorption Capacity in a Novel g-C3N4 Modified Amorphous Na–Ca–Mg Silicate Adsorbent: Insights from Multinuclear Solid-State NMR Spectroscopy. J. Phys. Chem. B 2024, 128, 12638–12650. [Google Scholar] [CrossRef]
  67. Yasir, H.A.; Zein, S.H.; Holliday, M.C.; Jabbar, K.J.; Ahmed, U.; Jalil, A.A. Comparison of activated carbon and low-cost adsorbents for removal of 2,4-dichlorophenol from wastewater using Aspen Adsorption and response surface methodology. Environ. Technol. 2024, 45, 3029–3047. [Google Scholar] [CrossRef] [PubMed]
  68. GB 4287-2012; Discharge Standards of Water Pollutants for Dyeing and Finishing of Textile Industry. Ministry of Ecology and Environment: Beijing, China, 2013. Available online: https://english.mee.gov.cn/Resources/standards/water_environment/Discharge_standard/201301/t20130107_244749.shtml (accessed on 14 October 2025).
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Figure 1. The XRD patterns of SEP before and after treated with 0.1 and 0.5 M CA.
Figure 1. The XRD patterns of SEP before and after treated with 0.1 and 0.5 M CA.
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Figure 2. The FTIR spectra of SEP before and after treated with 0.1 and 0.5 M CA.
Figure 2. The FTIR spectra of SEP before and after treated with 0.1 and 0.5 M CA.
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Figure 3. The SEM images and EDS data of SEP (a,d), 0.1CA-SEP (b,e), and 0.5CA-SEP (c,f).
Figure 3. The SEM images and EDS data of SEP (a,d), 0.1CA-SEP (b,e), and 0.5CA-SEP (c,f).
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Figure 4. The N2 adsorption/desorption isotherm and pore size distributions of SEP and CA-SEP.
Figure 4. The N2 adsorption/desorption isotherm and pore size distributions of SEP and CA-SEP.
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Figure 5. Comparison of MB adsorption capacity of the SEP and CA-SEP (Experimental conditions: initial MB concentration = 50 mg/L, pH = 8.0, adsorbent dosage = 1.0 g/L, contact time = 240 min, temperature = 298 K).
Figure 5. Comparison of MB adsorption capacity of the SEP and CA-SEP (Experimental conditions: initial MB concentration = 50 mg/L, pH = 8.0, adsorbent dosage = 1.0 g/L, contact time = 240 min, temperature = 298 K).
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Figure 6. (a) Effect of pH on MB adsorption by CA-SEP; (b) comparison of the pHPZC of SEP and CA-SEP.
Figure 6. (a) Effect of pH on MB adsorption by CA-SEP; (b) comparison of the pHPZC of SEP and CA-SEP.
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Figure 7. Adsorption kinetics of MB on CA-SEP as fitted by the PFO (a), PSO (b), and W-M (c) kinetic models; (d) Effect of adsorbent dosage on MB adsorption on CA-SEP (qt: the amount of MB adsorbed at time t).
Figure 7. Adsorption kinetics of MB on CA-SEP as fitted by the PFO (a), PSO (b), and W-M (c) kinetic models; (d) Effect of adsorbent dosage on MB adsorption on CA-SEP (qt: the amount of MB adsorbed at time t).
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Figure 8. Nonlinear Langmuir (a), Freundlich (b), and Sips (c) isotherm models at different temperatures for the adsorption of MB on CA-SEP. (d) Plots of ln 1/Ce versus 1/T for the MB adsorption on CA-SEP at different adsorption capacities.
Figure 8. Nonlinear Langmuir (a), Freundlich (b), and Sips (c) isotherm models at different temperatures for the adsorption of MB on CA-SEP. (d) Plots of ln 1/Ce versus 1/T for the MB adsorption on CA-SEP at different adsorption capacities.
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Figure 9. The SEM-EDS (a,b) and FTIR spectrum (c) of the CA-SEP adsorbent after MB adsorption; (d) the schematic representation of proposed MB adsorption mechanism on CA-SEP.
Figure 9. The SEM-EDS (a,b) and FTIR spectrum (c) of the CA-SEP adsorbent after MB adsorption; (d) the schematic representation of proposed MB adsorption mechanism on CA-SEP.
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Figure 10. MB adsorption on virgin and regenerated CA-SEP adsorbent with five adsorption-regeneration cycles.
Figure 10. MB adsorption on virgin and regenerated CA-SEP adsorbent with five adsorption-regeneration cycles.
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Tian, Z.; Chen, Z.; Wang, Q.; Gao, X.; Wei, W. Citric Acid-Modified Sepiolite as an Efficient and Sustainable Adsorbent for the Removal of Methylene Blue from Aqueous Solutions. Water 2025, 17, 2998. https://doi.org/10.3390/w17202998

AMA Style

Tian Z, Chen Z, Wang Q, Gao X, Wei W. Citric Acid-Modified Sepiolite as an Efficient and Sustainable Adsorbent for the Removal of Methylene Blue from Aqueous Solutions. Water. 2025; 17(20):2998. https://doi.org/10.3390/w17202998

Chicago/Turabian Style

Tian, Zhuangzhuang, Ziyi Chen, Qing Wang, Xin Gao, and Wei Wei. 2025. "Citric Acid-Modified Sepiolite as an Efficient and Sustainable Adsorbent for the Removal of Methylene Blue from Aqueous Solutions" Water 17, no. 20: 2998. https://doi.org/10.3390/w17202998

APA Style

Tian, Z., Chen, Z., Wang, Q., Gao, X., & Wei, W. (2025). Citric Acid-Modified Sepiolite as an Efficient and Sustainable Adsorbent for the Removal of Methylene Blue from Aqueous Solutions. Water, 17(20), 2998. https://doi.org/10.3390/w17202998

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