Performance Evaluation of Hybrid and Conventional Coagulants for the Removal of Sunset Yellow and Methylene Violet Dyes from Wastewater

Abstract

Textile industries release dyes into wastewater, and when present above certain levels, these dyes pose serious risks because of their high toxicity. This study investigates the removal of Sunset Yellow (SY) and Methylene Violet (MV) dyes from wastewater using chitosan (CS) and polysilicate acid (pSi) in the structure of aluminum-based coagulants, resulting in hybrid formulations (CS@Al, Al/pSi, and CS@Al/pSi). Among the various treatment methods that have been applied for the removal of dyes, the coagulation/flocculation process was chosen in the present study, as it is a cheap and effective method. Coagulation performance was optimized for pH, coagulant dosage, temperature and mixing time. The Al/pSi coagulant achieved nearly complete SY removal (98.8%) at 25 mg/L dosage and pH 3.0. MV removal in single-dye solutions was limited, with Al/pSi achieving only 26.6% removal at pH 3.0. However, in mixed-dye systems (SY/MV), synergistic interactions increased MV removal up to 94.4% and SY removal to 100%. Hybrid CS@Al/pSi showed lower SY removal (36.4%) for SY at 50 mg/L but provided stable floc formation, particularly in mixtures of anionic and cationic dyes. Application to real textile wastewater confirmed the high efficiency of the optimized coagulants, particularly with Al/pSi_20,A and AlCl₃, indicating their potential for industrial wastewater treatment. SEM, EDS, XRD, and FTIR analyses revealed structural consolidation, increased surface area, and successful dye adsorption, explaining the high removal efficiency.

1. Introduction

The rapid growth of textile, food, cosmetic, and pharmaceutical industries has led to the widespread discharge of synthetic dyes into aquatic environments [1]. Once present above certain concentrations, dyes become harmful due to their strong toxicity, persistence in the environment, non-biodegradability, and possible carcinogenic effects, posing risks to both ecosystems and human health [2]. Dyes are generally divided into three groups: non-ionic, anionic, and cationic. Cationic dyes are considered the most hazardous [3]. Among these, azo dyes (anionic in nature) constitute about 65% of industrial dyes. Their prevalence is linked to their low production cost, good stability, and high solubility in water, with their chemical structure typically containing one or more azo (-N=N-) chromophores [4].

This work investigates the removal of two representative dyes persistent in wastewater, due to their stable molecular structures and resistance to biodegradation, which have not been studied extensively in the literature to date: Sunset Yellow (SY) as anionic dye, and Methylene Violet (MV) as cationic dye. Sunset Yellow FCF (C₁₆H₁₀N₂Na₂O₇S₂, molecular weight 452.4 g/mol), formally named 6-hydroxy-5-(4-sulfonatophenylazo)-2-naphthalene-sulfonate (Figure 1a), is a synthetic azo dye widely used for decades in food and beverages [5]. Conversely, Methylene Violet, with the formula C₂₄H₂₈C_lN₃ (393.9 g/mol) (Figure 1b), belongs to the triphenylmethane class of synthetic dyes and is recognized as one of the most common and dangerous cationic colorants [6], employed in textile and printing processes. Its positive charge enables strong interactions with negatively charged substrates, making it highly applicable in textile dyeing, biological staining, inks, paints, and even antimicrobial formulations [3].

Numerous techniques have been explored for dye removal, each with specific strengths and drawbacks. Conventional treatment methods, including biological degradation, adsorption, and advanced oxidation, often face limitations in terms of cost, efficiency or generation of secondary pollutants. Specifically, biological treatments show potential for heavy metal and dye degradation but remain at an experimental stage [7]. Photocatalysis achieves rapid breakdown of dyes, although the use of UV light raises safety concerns [8]. Membrane filtration offers high efficiency and requires little space, but fouling significantly reduces its long-term performance [9]. Ultrasonication provides a compact and environmentally benign option, albeit with high energy demands [10]. Adsorption is one of the most widely studied methods due to its cost-effectiveness, operational simplicity, and high efficiency in removing organic pollutants, though it is limited by the need for frequent adsorbent regeneration [11]. Materials such as activated carbon and biochar have been successfully applied [12], yet issues remain regarding cost and disposal after use [13].

Among these approaches, coagulation/flocculation has emerged as a practical and widely applied approach for dye removal from wastewater, owing to its simplicity, rapid action, cost-effectiveness and ability to handle large volumes of effluent [14]. Coagulation has been employed for many years as a pre-treatment for dye-contaminated wastewater, requiring minimal time since only mixing and settling are involved. Despite challenges such as higher coagulant dosage requirements and sludge generation, it remains attractive due to its ease of operation, recyclability of some coagulants, and proven effectiveness in aggregating dye particles [12,13].

In addition, earlier studies focused largely on inorganic coagulants such as ferric chloride (FeCl₃), aluminum chloride (AlCl₃), magnesium chloride (MgCl₂) [15,16,17,18,19] and recently tin chloride (SnCl₄) [20], for removing direct dyes from aqueous solutions. In recent years, the use of inorganic polymeric coagulants has gained increasing attention as a promising alternative to traditional coagulants such as alum or ferric salts [21,22]. Furthermore, polysilicate aluminum-based coagulants [23,24], have demonstrated superior performance due to their unique polymeric structures, which enhance charge neutralization during coagulation. The incorporation of silicate into aluminum hydroxide species results in a stable, highly charged coagulant with improved destabilization capacity and floc-forming properties [25]. These materials not only exhibit higher dye removal efficiency but also produce denser and more easily settled flocs, leading to lower sludge volumes and improved water quality.

Alongside inorganic polymeric coagulants and natural coagulants such as chitosan (Cs) [26], have drawn attention for their eco-friendly, biodegradable, and non-toxic properties, making them environmentally approachable and economical alternatives to conventional coagulants [27]. Derived from chitin, chitosan is a cationic biopolymer capable of effectively removing anionic and reactive dyes through adsorption and charge neutralization [28].

This study presents the first systematic evaluation of the combined application of aluminum-based coagulants and chitosan for the removal of Sunset Yellow and Methylene Violet dyes from wastewater within a single, integrated coagulant system. Unlike previous research, which primarily investigated inorganic and natural coagulants separately or examined chitosan merely as an additive [29], the present work introduces a dual-coagulant strategy specifically designed to harness potential synergistic effects between the two components. This innovative approach not only enhances dye removal efficiency but also contributes to a reduction in chemical consumption, that is one of the major challenges in sustainable wastewater treatment [30]. Furthermore, a key novelty of this study lies in the subsequent evaluation of the prepared coagulants in both mixed dye systems and real textile wastewater samples, thereby demonstrating their practical applicability beyond controlled laboratory conditions. Furthermore, the study optimizes critical operational parameters including coagulant dosage, pH, initial dye concentration, temperature and contact time, while providing mechanistic insights into coagulant-dye interactions through advanced characterization techniques such as SEM, XRD and FTIR.

2. Materials and Methods

2.1. Materials

The anionic dye Sunset Yellow FCF (SY) (purity of ≥85%) and the cationic dye Methylene Violet (MV) (purity of ≥65%) supplied from Sigma-Aldrich-Merck KGaA (Darmstadt, Germany), whose structure is shown in Figure 1, are the dyes used in this study as target pollutants. In addition, Reactive Red 120 dye (RR120) (purity of ≥99%), was used as the second anionic dye in the binary dye systems. To prepare a 1000 mg/L stock solution, 1 g of dye was dissolved in distilled water. Furthermore, the materials used to synthesize the coagulants of these study, include AlCl₃·6H₂O (Sigma-Aldrich-Merck KGaA, Darmstadt, Germany), water glass solution (containing 10% NaOH and 27% SiO₂) (Sigma-Aldrich-Merck KGaA, Darmstadt, Germany), chitosan (CS) (310–375 kDa, DDA 75%) also supplied from Sigma Aldrich Co. (Sigma-Aldrich-Merck KGaA, Darmstadt, Germany), acetic acid (≥99%; Fisher Chemicals, Hampton, NH, USA) and NaOH (≥97.0% ACS NaOH pellets, Sigma-Aldrich-Merck KGaA, Darmstadt, Germany). The pH was regulated using diluted solutions of 37% HCl (Panreac, AppliChem, Barcelona, Spain) or NaOH, as previously described.

2.2. Synthesis of Coagulants

This study investigated different types of coagulants, such as aluminum chloride (AlCl₃), polysilicate acid (pSi), and chitosan (CS), as well as composites of theme, i.e., CS@Al, Al/pSi and CS@Al/pSi derivatives, to enhance the charge and molecular weight off the coagulants. The laboratory synthesis conditions were controlled in order to ensure consistency and reproducibility of the application and the related results. The synthesis conditions of the coagulants as well as their pH value are presented in

Table 1. Properties of coagulants.

Coagulant Type	Comments			pH
AlCl3	0.1 M with respect to Al			3.22
CS	CS in 2% v/v acetic acid			4.09
pSi	SiO2 in HCl 1 N → ageing → 0.38 M SiO2 respect to Si			2.01
CS@Al5%,A	${\displaystyle \frac{mAl}{mCS}}$ = ${\displaystyle \frac{1}{20}}$ = 5%	${\displaystyle \frac{\left[Al\right]}{\left[{OH}^{-}\right]}}$ = ${\displaystyle \frac{1}{1}}$= A		3.67
CS@Al10%,A	${\displaystyle \frac{mAl}{mCS}}$ $=$ ${\displaystyle \frac{1}{10 }}=10\%$	${\displaystyle \frac{\left[Al\right]}{\left[{OH}^{-}\right]}}$ = ${\displaystyle \frac{1}{1}}$= A		3.63
CS@Al50%,A	${\displaystyle \frac{mAl}{mCS}}$ $=$ ${\displaystyle \frac{1}{2}}$ = 50%	${\displaystyle \frac{\left[Al\right]}{\left[{OH}^{-}\right]}}$ = ${\displaystyle \frac{1}{1}}$ = A		3.90
CS@Al50%,B	${\displaystyle \frac{mAl}{mCS}}$ $=$ ${\displaystyle \frac{1}{2}}$ = 50%	${\displaystyle \frac{\left[Al\right]}{\left[{OH}^{-}\right]}}$ = ${\displaystyle \frac{1}{2}}$= B		4.13
Al/pSi10,A	${\displaystyle \frac{\left[Al\right]}{\left[Si\right]}}$ $=$ ${\displaystyle \frac{10}{1 }}=10$	${\displaystyle \frac{\left[Al\right]}{\left[{OH}^{-}\right]}}$ = ${\displaystyle \frac{1}{1}}$ = A		4.05
Al/pSi20,A	${\displaystyle \frac{\left[Al\right]}{\left[Si\right]}}$ $=$ ${\displaystyle \frac{20}{1}}=20$	${\displaystyle \frac{\left[Al\right]}{\left[{OH}^{-}\right]}}$ = ${\displaystyle \frac{1}{1}}$ = A		3.94
CS@Al/pSi50%,10,A	${\displaystyle \frac{mAl}{mCS}}$ $=$ ${\displaystyle \frac{1}{2}}$ = 50%	${\displaystyle \frac{\left[Al\right]}{\left[Si\right]}}$ $=$ ${\displaystyle \frac{10}{1 }}=10$	${\displaystyle \frac{\left[Al\right]}{\left[{OH}^{-}\right]}}$ = ${\displaystyle \frac{1}{1}}=\mathrm{A}$	3.81
CS@Al/pSi50%,10,B	${\displaystyle \frac{mAl}{mCS}}$ $=$ ${\displaystyle \frac{1}{2}}$ = 50%	${\displaystyle \frac{\left[Al\right]}{\left[Si\right]}}$ $=$ ${\displaystyle \frac{10}{1 }}=10$	${\displaystyle \frac{\left[Al\right]}{\left[{OH}^{-}\right]}}$ = ${\displaystyle \frac{1}{2}} =\mathrm{B}$	4.11

. As can be seen, all the coagulants examined have an acidic pH.

2.2.1. Preparation of AlCl₃ Solution

0.1 M AlCl₃ solution (with respect to Al) was prepared by dissolving 1.21 g (Mr = 241.43 g/mol) in deionized water and diluting to 50 mL in a volumetric flask.

2.2.2. Preparation of CS Solution

The CS solution was prepared following literature [31]. In a 100 mL beaker, 98 mL of distilled water and 1 g of CS were combined and stirred for 10 min. Then, 2 mL of 2% (v/v) acetic acid was added dropwise to prevent coagulation, and stirring continued for 30–45 min until fully dissolved. Finally, 1 mL of 0.5 M NaOH was added, and the mixture was stirred for 1 h.

2.2.3. Preparation of Polysilicate Acid Solution

Polysilicate acid (pSi) solution was prepared according to a previous study [32]. The water glass solution was diluted to 0.5 M SiO₂ and acidified with 1 N HCl under magnetic stirring to pH 4. After aging for 90 min at pH 4, the pH was lowered to 2 and maintained for 60 min, yielding a final SiO₂ (regarding Si) concentration of 0.37–0.38 M. It is worth noting that the initial pH of the diluted water glass solution was around 12.5. During the synthesis of polysilicic acid, the pH was gradually lowered, and the transition from pH 9.0 to 7.5 occurred relatively quickly, as silica polymerization is very rapid in this range [33]. Further, the pH of polysilicic acid was lowered from 4 to 2 by HCl addition to accelerate the condensation of silanol (Si–OH) groups into siloxane (Si–O–Si) bonds. This acid-induced polymerization promotes the growth of polysilicic chains, leading to a more stabilized network and influencing the structural and colloidal properties of the resulting silica material [34].

2.2.4. Preparation of Hybrid CS@Al Coagulants

CS@Al hybrid coagulants were synthesized to improve the coagulation efficiency of their individual constituents. To evaluate the influence of composition and determine the optimal AlCl₃ loading, three formulations were prepared with mass ratios of Al to chitosan (1/20, 1/10, and 1/2), denoted as CS@Al_5%, CS@Al_10% and CS@Al_50%. The selected ratios (1/20, 1/10, and 1/2) were determined based on preliminary synthesis experiments that optimized the interaction and stability of chitosan-based composites [35,36,37]. The synthesis procedure was consistent across all samples, with only minor adjustments in reagent quantities. In brief, AlCl₃ (2.42 g, 0.1 M regarding Al) was dissolved in 98 mL deionized water under agitation, until the salt completely dissolved, followed by the gradual addition of chitosan (respective mass addition), under vigorous stirring. After 10 min, acetic acid (2% v/v, 2 mL) was introduced to prevent gelation, and stirring continued for 30–45 min to ensure complete dissolution. NaOH (0.5 M) was subsequently added as a cross-linker, calculated to achieve [Al]/[OH] molar ratios of 1:1 (A) and 1:2 (B). The reaction was completed after 1 h of vigorous mixing.

2.2.5. Preparation of Hybrid Al/pSi Coagulants

The synthesis of Al/pSi coagulants followed a procedure similar to that described above. Briefly, 1.21 g of AlCl₃ (corresponding to 0.1 M Al at the end) was dissolved in 40 mL of deionized water under agitation for 10 min. Subsequently, an appropriate volume of freshly prepared pSi (see Section 2.2.3) was added to obtain the Al/pSi₁₀ and Al/pSi₂₀ formulations, followed by an additional 10 min of stirring. Finally, 10 mL of 0.5 M NaOH solution was added to each beaker to produce the Al/pSi_10,A and Al/pSi_20,A derivatives, after 1 h of continuous vigorous stirring.

2.2.6. Preparation of Hybrid CS@Al/pSi Coagulants

For the synthesis of composite CS@Al/pSi coagulants, a combined procedure was employed. Initial experiments conducted in this study, regarding the Al content, showed that the Al to chitosan mass ratio of 1/2 (50% according to Table 1) was optimal and was therefore adopted in this formulation. Briefly, 2.42 g of AlCl₃ (corresponding to 0.1 M Al) was dissolved in 98 mL of deionized water under agitation until fully dissolved, followed by the gradual addition of freshly prepared pSi to obtain an [Al]/[Si] ratio of 10. Chitosan (in the corresponding mass ratio) was then added under vigorous stirring. After 10 min, 2 mL of acetic acid (2% v/v) was introduced to prevent gelation, and stirring was continued for 30–45 min to ensure complete dissolution. Subsequently, 0.5 M NaOH was added as a cross-linking agent, with the required volume calculated to achieve [Al]/[OH] molar ratios of 1:1 (A) and 1:2 (B). The process was completed after 1 h of intense stirring, yielding the coagulants CS@Al/pSi_50%,10,A and CS@Al/pSi_50%,10,B.

2.3. Coagulation Performance

2.3.1. Jar-Tests

Coagulation tests were conducted using a jar-test apparatus (Aqualytic, VELP Scientifica, Usmate, Italy) with 1 L beakers. Each beaker contained 250 mL of 20 mg/L dye solution. Dye concentrations in textile wastewater are commonly reported in the range of 10 to 250 mg/L, depending on the dye class and industrial process [38,39]. A concentration of 20 mg/L was selected for this study as a representative level of typical textile effluents, enabling a realistic evaluation of the coagulation process under environmentally relevant conditions. The pH was adjusted to 3.0, 7.0, or 9.0 when studying pH effects, or kept at the optimal value for other factors (dosage, initial concentration, temperature, mixing time, mixed dyes, or real wastewater). Solutions were stirred rapidly at 200 rpm for 1 min (Rapid Mixing Period; coagulation), then slowly at 50 rpm for 15 min (Slow Mixing Period; flocculation), followed by 45 min of settling to allow floc sedimentation.

2.3.2. Analytical Determinations

After sedimentation, samples were taken ~5 cm below the beaker surface and filtered through a 0.45 μm nylon membrane filter. Samples are filtered through a 0.45 μm syringe filter to remove suspended particles that could interfere with analytical instruments, causing signal instability or clogging. This ensures that only the dissolved fraction is measured and improves the accuracy and reproducibility of results [40]. Hence, dye concentrations were measured by UV–vis spectrophotometry (WTW Spectroflex 6100, Xylem Analytics Germany Sales GmbH & Co. KG, Weilheim, Germany) at λ_max 418 nm for SY and 579 nm for MV using calibration curves [40].

2.3.3. Dye Removal Evaluation

The removal efficiency (R (%)) of each dye was determined using the following equation (Equation (1)):

$$\mathrm{R}\mathrm{\%}={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{f}}}{{\mathrm{C}}_0}}\mathrm{\ast }100\mathrm{\%}$$

(1)

here C₀ represents the initial dye concentration (mg/L), and C_f represents the dye concentration at the end of the experiment (mg/L).

2.4. Characterization Techniques

Coagulant powders were prepared by drying the relative aqueous solutions in an oven at about 40 °C. The surface morphology of the synthesized coagulants was analyzed using scanning electron microscopy (SEM), JEOL JSM-6390 LV, Tokyo, Japan, at EHT = 15 kV. Functional groups were identified through Fourier transform infrared spectroscopy (FTIR), PerkinElmer, New York, NY, USA, in the range of 4000–500 cm⁻¹. Structural crystallinity and phase composition were characterized by X-ray diffraction (XRD) analysis conducted with a Rigaku MiniFlex II diffractometer and a Bruker D8 FOCUS instrument, Karlsruhe, Germany, scanned from 10° to 60° (2θ) at a rate of 1°/min.

3. Results and Discussion

3.1. Effect of pH and Comparison of Coagulants

The effect of pH on coagulation efficiency for SY removal was first evaluated, along with the comparison of all synthesized coagulants. As shown in Figure 2a, maximum removal (~100%) occurred at pH 3.0 using Al/pSi_10,A and Al/pSi_10,B (99.79% and 100%, respectively) at a dosage of 50 mg/L (as Al concentration), compared to 78.9% with AlCl₃. This indicates that polysilicic acid enhances aluminum’s performance as a coagulant [24,32]. Therefore, Al/pSi₁₀ coagulants were selected for further study.

Regarding the role of chitosan (CS) in the coagulant structure, hybrid CS@Al coagulants showed reduced efficiency (5.8–18.2%), which improved with lower CS content and higher OH proportions. Among them, CS@Al_50% derivatives presented the best performance. Incorporating pSi (CS@Al/pSi) further enhanced efficiency, though not beyond that of the Al/pSi derivatives. Overall, removal efficiency decreased with increasing initial pH value of the solution. Moreover, in CS@Al/pSi composites, probably some aluminum is incorporated into the chitosan–silica network rather than remaining in solution (as confirmed in characterization part 3.8). The amino (–NH₂) and hydroxyl (–OH) groups of chitosan can chelate Al, reducing the formation of active polymeric Al species for coagulation [41]. Additionally, Al may be physically trapped within the composite, slowing its diffusion. These factors lower active coagulation sites and explain the reduced efficiency of CS@Al/pSi_50%,10,B compared with Al/pSi_20,A. Therefore, the materials selected for further study are the optimal Al/pSi_10,A and Al/pSi_10,B as well as AlCl₃, pSi and CS@Al/pSi_50%,10,B for comparative purposes.

In addition, Figure 2b shows the flocs that settled and formed agglomerates at the bottom of the beaker after coagulation/flocculation with the optimal Al/pSi_10,A and Al/pSi_10,B coagulants. It can be observed that higher OH content (higher degree of polymerization), as occurs in Al/pSi_10,B, leads to more intense aggregation, which is consistent with the higher effectiveness of this material.

In addition, the tested coagulants were evaluated for their ability to remove also MV dye. As shown in Figure 3, the coagulants developed in this study were less effective for cationic dyes compared to anionic dyes. Among all materials, Al/pSi_10,A exhibited the highest performance, achieving a maximum MV removal percentage of only 26.6% at the optimal pH of 3.0. It is noteworthy that when the flocculant structure contained CS, the optimum pH shifted to 9.0, although the removal efficiency remained low. This behavior is likely associated with the nature of the charged groups present in the material at this pH.

Regarding the fact that all applied coagulants provided an acidic pH value (Table 1), a slight decrease in pH was observed in all samples following coagulation, which is may attributed to the hydrolysis of metal-based coagulants and the consequent release of hydrogen ions (H⁺) into the solution [42].

To determine the surface charge of the coagulants, the point of zero charge (pH_pzc) of the main and most effective coagulants—namely AlCl₃, pSi, Al/pSi_20,A, and Cs@Al/pS_i50%,10,B—was measured (Figure 4) using the pH drift method [43]. The corresponding pH_pzc values were 2.64, 2.70, 3.13, and 3.90, respectively. When pH < pH_pzc, the coagulant surface carries a positive charge, whereas at pH > pH_pzc, the surface becomes negatively charged. Since the optimum pH (3.0) is below the pH_pzc, the coagulant surface remains positively charged and thus effectively attracts the negatively charged dye molecules. Therfore, the optimum Al/pSi_20,A can successfully destabilize the anionic dye by reducing its negative charge and forming insoluble complexes that can then be removed by sedimentation.

3.2. Effect of Dosage

The coagulant dosage is one of the key factors influencing the efficiency of the coagulation process. In this study, dosages ranging from 10 to 75 mg/L were tested at the optimal pH of 3.0 using different coagulants, including AlCl₃, pSi, Al/pSi_10,A, Al/pSi_20,A and CS@Al/pSi_50%,10,B, for SY removal. As illustrated in Figure 5, the application of 10 mg/L of Al/pSi_10,A and Al/pSi_20,A achieved removal of 71.5% and 74.6%, respectively. Increasing the dosage to 25 mg/L resulted in nearly complete SY removal, with efficiencies of 94.6% for Al/pSi_10,A and 98.8% for Al/pSi_20,A. Therefore, 25 mg/L was selected as the optimal dose for subsequent experiments. In contrast, CS@Al/pSi_50%,10,B limited performance, with a maximum of 36.4% removal at 75 mg/L. Meanwhile, AlCl₃ provided up to 82.9% removal at the same dosage. These results demonstrate that the Al/pSi coagulants are the most effective, even at relatively low dosages.

3.3. Effect of Initial Dye Concentration

Figure 6 describes the effect of initial Sunset Yellow (SY) dye concentration on the removal efficiency (%) under optimum conditions using the most favorable Al/pSi_20,A coagulant at pH 3.0, by adding 50 mg Al/L. Five different initial dye concentrations, in the range of 20–100 mg/L, where selected to be examined (20, 50, 100, 150 and 200 mg/L). As can be seen, removal efficiency remains above 95% at low concentrations (20–100 mg/L), indicating excellent performance of Al/pSi_20,A coagulant, but decreases sharply with increasing dye concentration, reaching ~45% at 200 mg/L In addition, higher dye loads can lead to reduced particle–particle collisions and incomplete floc formation, lowering overall coagulation efficiency [44]. Thus, Al/pSi shows maximum efficiency at low initial dye concentrations, but its performance decreases significantly when the dye concentration exceeds its optimal adsorption capacity.

During coagulation, aluminum hydrolysis products neutralize negative charges on dye molecules and facilitate aggregation into larger flocs that can be removed. At lower dye concentrations, the coagulant dose provides ample hydrolyzed Al species for charge neutralization and bridging, resulting in efficient floc formation and removal. However, as dye concentration increases, the constant amount of coagulant becomes insufficient to neutralize the larger number of dye molecules, leading to incomplete destabilization and weaker floc formation [45]. Furthermore, at high dye loadings, excessive negatively charged species can restabilize particles or prevent the development of compact flocs, further reducing removal efficiency. This demonstrates the typical limitation of coagulation at high pollutant concentrations, where charge neutralization and sweep flocculation mechanisms become less effective without increasing the coagulant dose [46].

3.4. Effect of Temperature

The effect of temperature (293, 303 and 313) on the SY removal using the optimum Al/pSi_20,A at pH 3.0, by adding 50 mg Al/L in 150 mg/L of dye (selected according the results illustrated in Figure 6), was investigated and represented in Figure 7. The results showed that the percentage removal was slightly increased with increasing temperature. The highest removal of SY was found to be 72.3%, at 313 K. This trend was also observed in the literature [47,48].

3.5. Effect of Mixing Time

The effects of fast and slow mixing conditions on the removal of SY from Al/pSi_20,A and CS@Al/pSi_50%,10,B are presented in Figure 8 and Figure 9, respectively. At constant slow mixing (Figure 8a–c corresponding to 15, 20 and 25 min), the increased speed to fast mixing (1, 1.5 and 2 min), in the case of Al/pSi_20,A, did not affect the material performance. However, in the case of CS@Al/pSi_50%,10,B, as the fast-mixing time increased, the removal efficiency decreased, probably due to agglomerate rupture. Therefore, Al/pSi_20,A exhibited higher shear strength of the flocs compared to CS@Al/pSi_50%,10,B, reflecting the formation of denser and more stable flocs. Therefore, when the rapid mixing time was 1 min, the removal rate was higher.

Furthermore, at constant rapid mixing (Figure 9a–c for 1, 1.5 and 2 min, respectively), by extending the slow mixing period (15, 20 and 25 min), once again, the performance of Al/pSi_20,A is not affected by time. When CS@Al/pSi_50%,10,B is used, extending the mixing duration increases the residual dye concentration (reducing the removal rates), as possibly excessive duration can destabilize the formed flocs, breaking them into smaller, less sedimentable aggregates [49]. This phenomenon seems to be more evident for CS@Al/pSi_50%,10,B, probably due to its higher surface modification, which increases the sensitivity of the floc structure. In conclusion, the 15 min of slow mixing combined with the 1 min of fast mixing, which emerged earlier, constitute the ideal conditions and confirm those used in this study in the previous experiments.

3.6. Mixed Dye Solutions

The coagulants were further evaluated in mixed-dye systems, one containing two anionic dyes (SY and RR120) and another consisting of an anionic (SY) and a cationic dye (MV) by applying 50 mg/L of the coagulant to a mixture of 10 mg/L of each dye. As illustrated in Figure 10a,b, the addition of RR120 slightly reduced the ability of pSi and AlCl₃ to remove SY due to competitive interactions. However, when applied as Al/pSi-based derivatives, the performance remained high, indicating a synergistic effect. Interestingly, for RR120, the use of Al/pSi_10,A and Al/pSi₂₀,_A coagulants showed improved removal efficiency in the presence of SY. This suggests that combining two anionic dyes in the same solution enhances the effectiveness of the studied coagulants. Such behavior can be attributed to the shared negative charges of the dyes, which strengthen their interactions with the positively charged coagulant species [50,51]. Furthermore, Figure 10c,d provides a qualitative comparison of the dye mixture before and after coagulation/flocculation. It is evident that, especially with the optimal Al/pSi_20,A coagulant, the formed flocs aggregate and settle at the bottom of the beaker, leaving the supernatant, later subjected to filtration, visibly clearer.

In contrast, the binary system composed of an anionic and a cationic dye exhibited a pronounced synergistic effect across all tested coagulants. For SY, the removal efficiency increased markedly from 18.2% to 81.2% with pSi and from 78.9% to complete removal (100%) with AlCl₃. Similarly, MV removal also showed significant enhancement: efficiency rose from 12.7% to 54.4% using pSi, from 5.6% to 55.3% with AlCl₃, from 26.6% to 79% with Al/pSi_10,A, and from 17.7% to 94.4% with Al/pSi_20,A. These strong improvements are consistent with previous findings reported in the literature [52].

This behavior can be explained by the charge interactions between the two dye species. The cationic dye partially screens the negative charge of the anionic dye, thereby reducing electrostatic repulsion and facilitating a more effective interaction with positively charged coagulant sites [53]. At the same time, the presence of opposite charges enables electrostatic pairing between SY and MV, further promoting flocculation and removal [54,55]. Under acidic conditions, these synergistic effects become stronger, which explains the substantial improvement in MV removal observed in the binary mixtures. Overall, the data demonstrate that coexisting anionic and cationic dyes can mutually enhance their removal efficiency through both charge neutralization and inter-dye interactions, offering a significant advantage for practical wastewater treatment applications.

3.7. Real Wastewater Samples

In this study, the proposed coagulants were also evaluated using real wastewater obtained from a textile dyeing facility in Langadas, Thessaloniki (Greece), prior to any on-site treatment. Each coagulant was applied at a dosage of 50 mg/L without adjusting the pH. Textile industries are well recognized for producing large volumes of wastewater with complex and variable composition, requiring efficient treatment approaches. Samples were collected in the spring (May) morning hours, which represent typical production hours. As shown in Figure 11, the untreated wastewater exhibits a distinct absorbance peak at approximately 597 nm, which is characteristic of residual dyes in the effluent. After coagulation, this peak was significantly reduced or nearly eliminated, particularly with Al/pSi_20,A and AlCl₃, indicating strong decolorization efficiency. Other coagulants, including pSi, CS, and composite formulations such as CS/Al/pSi_20,A, also decreased absorbance values, though to varying degrees, confirming their activity. These results demonstrate that the coagulants developed in this work, while highly effective for synthetic dye solutions, are also suitable for treating complex textile wastewater, thereby broadening their potential application to real industrial effluents.

3.8. Comparison with the Literature

The present study focuses on the coagulation/flocculation of Sunset Yellow (SY) and Methylene Violet (MV) using various aluminum-based coagulants synthesized in the laboratory.

Table 2. Comparison of the removal of various anionic yellow and cationic violet dyes by coagulation: performance of the present materials versus literature data.

Coagulant	Dye	C0 (mg/L)	pH	Dosage (mg/L)	Removal %	Dye Mixture 1	Real Wastewater 2	Ref.
	Violet cationic dyes
Avocado seed powder (ASP)	Crystal Violet	50	7.0	950	95.3	NO	NO	[56]
Palm petiole waste (PPW)	Crystal Violet	50	9.0	950	98.2	NO	NO	[57]
Alu- minum sulfate (AS) in	Methyl Violet 2B	30	10	60	45.5	YES	NO	[58]
Al/pSi10,A	Methylene Violet	20	3.0	50	26.6	YES	YES	This study
	Yellow anionic dyes
FeCl3	Eosin Yellow (EY)	100	2.0	100	98.0	NO	YES	[59]
FeCl3	Brilliant Yellow	200	4.0	200	90.5	NO	NO	[60]
Polyaniline-modified bioflocculant (PANi)	Direct Yellow 86 (DY86)	100	2.0	2500	90.0	NO	NO	[61]
Al/pSi20,A	Sunset Yellow	20	3.0	25	98.8	YES	YES	This study

¹ The coagulant was tested in mixed dye systems. ² The coagulant was tested in real wastewater sample.

presents a comparison between the findings of this work and those reported in the literature over the past two years (2024–2025). The comparison clearly confirms the novelty of the present research, as no previous studies were identified concerning the removal of these two specific dyes, Sunset Yellow and Methylene Violet. Instead, related works have primarily examined other cationic violet dyes (e.g., Crystal Violet and Methyl Violet 2B) and anionic yellow dyes (e.g., Eosin Yellow (EY), Brilliant Yellow, and Direct Yellow 86 (DY86)). Furthermore, none of the reviewed studies investigated the proposed coagulants comprehensively across single-dye, binary-dye, and real wastewater systems, as accomplished in the present work. Notably, the data in Table 2 demonstrate that this innovative approach not only improves dye removal efficiency but also reduces chemical consumption, as comparable or superior performance was achieved at lower coagulant dosages than those reported in the literature, addressing a key challenge in sustainable wastewater treatment.

3.9. Characterization of the Coagulants

Figure 12 presents the SEM images of the optimal (Al/pSi_20,A) and the selected coagulants (pSi, AlCl₃) for comparison purposes, in order to analyze the possible morphological differences in their structure. The SEM image of polysilicic acid (Figure 12a) shows irregular, plate-like particles with fragments and rough surfaces. The structure appears as a porous microstructure that provides a large surface area, making polysilicic acid useful in adsorption, catalysis and as a precursor for silicon-based materials [62,63]. On the other hand, the SEM image of AlCl₃ (Figure 12b) shows a more granular and agglomerated morphology compared to pSi. The surface appears covered by thin, compact aluminum chloride crystallites [64]. Furthermore, the morphology of the optimal Al/pSi_20,A (Figure 12c) indicates structural changes after the incorporation of aluminum into pSi. Compared to pure pSi, the surfaces appear more consolidated with fewer open voids, suggesting partial coverage of the pores by Al species [65]. The particles appear denser and more interconnected, which could reduce the pore size but increase the stability of the composite [33]. After SY adsorption (Figure 12d), the surface of Al/pSi_20,A becomes smoother and more uniform, indicating that the SY dye molecules have been adsorbed onto the Al/pSi composite. This morphological change confirms the successful adsorption and probably the partial occlusion of the pores due to the dye uptake.

Figure 13 presents the Energy-Dispersive X-ray Spectroscopy (EDS) analysis of four different samples: (a) porous silicon (pSi), (b) aluminum chloride (AlCl₃), (c) Al/pSi_20,A, and (d) Al/pSi_20,A after SY coagulation. The spectra show characteristic peaks corresponding to the elements present in each sample, along with their atomic percentages. For pSi the main elements detected are oxygen (O) (48.02%) and carbon (C) (20.29%), as well as silicon (Si) (8.56%) indicating surface oxidation and carbon contamination. Moreover, the AlCl₃ sample shows high oxygen (49.63%) and carbon (44.26%) content, along with aluminum (Al) (3.31%), reflecting the composition of the aluminum precursor. On the other hand, the Al/pSi_20,A complex before coagulation, displays a significant increase in oxygen (70.14%) and a decrease in carbon (8.68%), with the presence of aluminum (4.14%), nitrogen (N) (10.13%), sodium (Na) (1.36%), chlorine (Cl) (5.42%) and silicon (0.12%), suggesting successful incorporation of Al species onto the pSi surface. After SY coagulation (Figure 13d), there is a notable rise in carbon content (34.34%) and appearance of sulfur (S) (1.50%), as anionic dyes are typically rich in carbon, nitrogen, and sometimes sulfur, endorsing the presence of dyes. Furthermore, minor changes in other elements such as oxygen from 70.40 to 50.83% and aluminum from 4.14 to 1.4% after coagulation, confirming successful interaction and binding of the dye with the Al/pSi_20,A composite.

Moreover, the XRD patterns of pSi, AlCl₃, Al/pSi_20,A are illustrated in Figure 14a. The pSi pattern (red line) shows distinct diffraction peaks at 31.8°, 45.6°, and 56.6° corresponding to the crystalline planes of silicon (most likely Si (111), (220), (311)), indicating that the silicon framework is crystalline. Several characteristic peaks of reflections of crystalline AlCl₃ (black line) are presented around 14.8°, 24°, 27.2°, 38.8°, 41°, 43.8°, 54.8° 2-theta. In addition, multiple peaks including smaller angles (15.2°, 17.4°, 24.4°, 27.2°) and larger angles (31.8°, 35°, 39.2°, 41.4°, 45.6°, 46.8°, 52°) are presented in the pattern of Al/pSi_20,A (blue line). This pattern contains features from both AlCl₃ and pSi, plus additional/shifted peaks [33]. Therefore, it is observed that the peaks corresponding to Si remain, confirming the silicon structure is still present. The extra appeared peaks indicate structural changes after introducing Al into pSi.

In addition, FTIR spectra of tested coagulants including pSi, AlCl₃, Al/pSi_20,A before and after SY coagulation, are presented in Figure 14b. The spectrum of AlCl₃ (black line) and Al/pSi_20,A (blue line) exhibits a broad band around 3400–3100 cm⁻¹ corresponding to O–H stretching of surface hydroxyl groups, along with the H–O–H bending vibrations around 1620 cm⁻¹ [66]. Upon modification with aluminum species, the Al/pSi_20,A sample shows characteristic Al–O and Si–O–Al vibrations around 950–1255 cm⁻¹ confirming the presence of silicate in the structure. The intense absorbance valleys at 1200 cm⁻¹, for pSi samples, could be attributed to the symmetric stretching mode at Si-O-Si. These valleys also indicate the presence of SiO₂ [67]. After adsorption of SY dye, new peaks emerge at ~1600 cm⁻¹, ~1380 cm⁻¹, and ~1100 cm⁻¹, corresponding to C=C aromatic stretching [68], S=O stretching [69] from sulfonate groups, and C–O–C vibrations [70], respectively, suggesting successful interaction between the dye and the Al-modified surface. Concurrently, the reduction in Al–O and O–H band intensities further confirm the adsorption of SY onto the coagulant. These observations demonstrate that the functional groups of SY are effectively immobilized on the Al/pSi surface, confirming its adsorption capability during coagulation.

3.10. Proposed Mechanism

The removal of anionic Sunset Yellow (SY) dye by polysilicate aluminum chloride (Al/pSi_20,A) occurs, as proposed in Figure 15, through charge neutralization and interparticle bridging, as the used dosage was low and sweep flocculation may not occurs [71,72,73]. Upon hydrolysis, Al/pSi_20,A forms positively charged aluminum species (e.g., Al³⁺, Al(OH)²⁺, Al₁₃O₄(OH)₂₄⁷⁺) [33] that neutralize the negatively charged sulfonate groups (–SO₃⁻) of SY, reducing electrostatic repulsion and promoting aggregation. The polysilicate (pSi) component facilitates interparticle bridging between dye–Al complexes, enhancing floc size and stability [24,74].

4. Conclusions

This study investigated the removal of Sunset Yellow (SY) and Methylene Violet (MV) dyes from wastewater using chitosan (CS) and polysilicic acid (pSi) incorporated into the structure of aluminum-based coagulants formatting several types of hybrid coagulants, i.e., CS@Al, Al/pSi, and CS@Al/pSi. Among these, polysilicate aluminum-based coagulants, particularly the Al/pSi_20,A derivative, exhibited outstanding performance for anionic dye removal, achieving 98.8% SY removal at a dosage of 25 mg/L and pH 3.0. Notably, efficient removal was also achieved at lower dosages, with Al/pSi_10,A attaining 71.5% SY removal at 10 mg/L. Although MV removal was limited in single-dye systems (26.6% for Al/pSi_10,A), a pronounced synergistic effect was observed in mixed SY/MV systems, where removal efficiencies increased to 94.4% for MV and 100% for SY using Al/pSi_20,A.

On the other hand, the hybrid coagulant CS@Al/pSi_50%,10,B showed moderate SY removal (36.4%) at higher dosages (50 mg/L) but demonstrated improved floc stability and enhanced performance in mixed-dye systems. When applied to real textile wastewater, both Al/pSi_20,A and AlCl₃ effectively reduced absorbance at 597 nm, confirming their practical decolorization capability. The morphology of the optimal Al/pSi_20,A indicates clear structural changes following the incorporation of aluminum into pSi, further supporting its enhanced performance. Complementary EDS analyses confirmed both the modification of the complex coagulants and the aggregation of the anionic dye in them. while XRD and FTIR confirmed structural modifications and interactions between coagulant functional groups and dye molecules.

Collectively, these results underscore the potential of Al/pSi-based coagulants as highly efficient, cost-effective, and environmentally sustainable materials for industrial wastewater treatment applications.