Utilization of Spent Yerba Mate as an Unconventional Sorbent for the Removal of Acid and Basic Dyes from Aqueous Solutions

Abstract

This study investigated the potential use of Yerba Mate (YM) residue as an unconventional sorbent for removing acid dyes—Acid Red 18 (AR18) and Acid Yellow 23 (AY23)—and basic dyes—Basic Violet 10 (BV10) and Basic Red 46 (BR46)—from aqueous solutions. The research included characterization of YM (FTIR, BET specific surface area, porosity, pH_PZC), examination of the effect of pH on dye sorption efficiency, analysis of sorption kinetics (pseudo-first-order and pseudo-second-order models, intraparticle diffusion model), and determination of maximum sorption capacity (Langmuir I and II models, and the Freundlich model). The sorption efficiency of the dyes onto YM was highest at pH 2 for AR18 and AY23, at pH 3 for BV10, and at pH 6 for BR46. The sorption equilibrium time for all dyes onto YM mainly depended on their initial concentration, ranging from 180 min (at 50 mg/L) to 210 min (at 500 mg/L). The kinetics of dye sorption were best described by the pseudo-second-order model. The maximum sorption capacity (Q_max) of YM for the acid dyes AR18 and AY23 was 24.95 mg/g and 22.86 mg/g, respectively. The sorption capacities of the tested sorbent for the basic dyes were higher, with Q_max = 46.24 mg/g for BV10 and Q_max = 60.54 mg/g for BR46.

1. Introduction

Wastewater from the textile, tannery, or paper industries can contain high concentrations of synthetic dyes. The dyes used in industry are typically organic compounds with complex chemical structures, rich in aromatic rings. These substances are generally characterized by high stability and low susceptibility to biodegradation [1]. Furthermore, a significant portion of the dissolved dyes may exhibit toxic properties to most living organisms [2]. Consequently, the purification of colored wastewater using traditional, biological methods based on activated sludge or biofilm technology is often ineffective.

Dyes that are not removed from wastewater during purification can enter the natural environment. Aquatic ecosystems near dyeing facilities are particularly vulnerable to dye contamination. Colorants, even at very low concentrations of a few mg/L, are highly visible in water and compromise the aesthetic appeal of the landscape [3]. A greater concern is that dyes in the water limit sunlight penetration to deeper layers of the reservoir, inhibiting photosynthesis in underwater flora [4]. Additionally, some colorants react with dissolved oxygen in the water, which, combined with reduced photosynthesis, can lead to hypoxia, especially in hypolimnetic waters [5]. Oxygen deficits, as well as dye toxicity, can cause a significant reduction in biodiversity and even the collapse of the local ecosystem.

To minimize the risk of environmental contamination by colorants, facilities that generate colored wastewater should implement effective wastewater decolorization technologies. Because biological methods for removing dyes from solutions are typically ineffective, physicochemical methods are much more frequently employed. These include coagulation, advanced oxidation, and membrane processes. Coagulation uses metal salts or polyelectrolytes to destabilize colloids and aggregate dye particles. The dyes, precipitated as sludge, can be easily separated from the wastewater through sedimentation or filtration. This method is fast, but it is not effective for all dyes [6]. Major drawbacks include the large amounts of sludge produced, which require disposal, as well as the salinization of the treated wastewater [7]. Advanced oxidation involves the complete mineralization of dyes using hydroxyl radicals, which are generated in the system through the chemical activation of simple oxidizers (H₂O₂, O₃), often with the use of catalysts [8]. Advanced oxidation is effective for most dyes, but due to high energy consumption (ozonation) or expensive chemical reagents (Fenton’s reaction), it is also quite costly [9]. Another issue with advanced oxidation is the risk of accumulation of toxic intermediate oxidation products in the system [10]. Wastewater decolorization through membrane processes relies on the physical separation of dye molecules from water based on the sieving effect. Depending on the membrane pore diameter, there are ultrafiltration (10–100 nm), nanofiltration (1–10 nm), and reverse osmosis (<1 nm) [11]. Reverse osmosis allows for nearly 100% removal of dyes [12]. However, this process is quite costly due to the need to generate high operating pressures (5–10 bar) and requires management of the concentrate, the volume of which can be up to 25% of the original wastewater volume [13].

An alternative to the previously mentioned physicochemical methods of wastewater decolorization is sorption. The process is easy to perform and, importantly, unlike precipitation and membrane methods, sorption does not produce large quantities of sludge or concentrate that require further management. Moreover, compared to advanced oxidation methods, sorption does not pose the risk of generating toxic intermediate products. For this reason, sorption is widely accepted as an environmentally friendly method [14]. The effectiveness of this process depends on the conditions under which it is carried out (pH, temperature, mixing speed). The efficiency of sorption also largely depends on the type of sorbent used.

Activated carbons are commonly used sorbents in colored wastewater treatment processes. These materials consist mainly of elemental carbon in an amorphous form. They are most often produced from fossil coals or lignocellulosic plant biomass. They are characterized by high porosity (typically above 0.5 cm³/g) and a large specific surface area (500–3000 m²/g) [15], which enables them to exhibit high sorption capacity for both anionic and cationic dyes [16]. Despite their superior performance, activated carbons face significant economic limitations due to high production costs from intensive energy consumption during raw material activation and regeneration [17,18]. These economic and energy constraints necessitate the search for sustainable, low-cost alternatives that can bypass the costly activation process while maintaining sufficient sorption efficiency.

Raw materials for producing low-cost sorptive materials are increasingly sourced from waste products. In particular, there is significant interest in waste from the agri-food industry. In addition to their low acquisition cost, these materials are typically widely available. The unconventional sorbents based on agro-industrial materials tested so far can be divided into those of animal and plant origin.

The first group includes chitinous materials (obtained from crab or shrimp shells), keratinous materials (fish scales, bird feathers), and materials based on calcium carbonate (mollusk shells, bird eggshells). In this group, sorbents based on chitin exhibited the best sorption capabilities, which, in the case of anionic dyes, can show a higher sorption capacity than commercial activated carbons [19]. However, a drawback of chitin-based sorbents is their low effectiveness toward cationic dyes [20]. Furthermore, raw materials for chitin production are scarce in countries where marine crustaceans are not caught. Additionally, chitin-containing crustacean shells have recently been increasingly purchased by pharmaceutical companies for the production of dietary supplements and drug carriers (chitins, chitosan, and glucosamine) [21].

Plant-derived waste materials constitute a much broader group, most often consisting of the inedible parts of cultivated plants. Examples include stems and leaves [22], seed husks [23], fruit seeds [24], nut shells [25], vegetable peels [26], and fruit peels such as those of banana [27].

The sorption capacity of plant biomass largely depends on its polysaccharide and lignin content. The carboxylic groups in hemicellulose and the phenolic groups directly linked to the aromatic structures of lignin are responsible for the slightly acidic nature of these materials, which favors the sorption of cationic dyes [28]. For example, certain lignocellulosic materials with high polysaccharide and lignin content, such as pumpkin or rapeseed seed husks, can bind basic dyes as effectively as some activated carbon-based materials [29]. Good capacity for cationic dye sorption does not automatically preclude the possibility of sorbing acid-type dyes. Theoretically, anionic dyes could be effectively bound to biomass containing basic functional groups, such as amine groups [30]. In plant biomass, these groups primarily originate from the protein content.

A literature review indicates that the most suitable raw materials for producing low-cost, unconventional sorbents are widely available plant wastes containing polysaccharides, lignin, and proteins, which possess diverse functional groups capable of binding both anionic and cationic dyes. Spent Yerba Mate biomass appears to meet these criteria. Yerba Mate is consumed globally, and its waste is available in large quantities [31,32]. Importantly, spent Yerba Mate has a high content of polysaccharides, lignin, and proteins, which contain significant amounts of functional groups such as hydroxyl, carboxyl, and amine groups. The presence of both acidic (carboxyl) and basic (amine) functional groups suggests a unique potential for effective simultaneous sorption of cationic and anionic dyes. Despite this promising chemical profile and vast availability, there is still a significant lack of research in the literature on the ability of Yerba Mate waste to sorb various types of dyes.

This study aims to assess the feasibility of using spent Yerba Mate as an unconventional, low-cost sorbent for removing representative anionic (acidic) dyes—Acid Red 18 (AR18) and Acid Yellow 23 (AY23)—and cationic (basic) dyes—Basic Violet 10 (BV10) and Basic Red 46 (BR46)—from aqueous solutions. The study focuses on characterizing the tested sorbent using FTIR, BET, and pH_PZC, followed by a thorough analysis of the sorption process for both dye classes by determining kinetics and equilibrium isotherms.

2. Materials and Methods

2.1. Yerba Mate

The adsorbent precursor, consisting of ground leaves and small stems of Paraguayan holly (Ilex paraguariensis), was purchased at the Auchan supermarket in Poland under the designation “YERBA MATE Amanda Elaborada.” According to the producer, this material originated from the Misiones province of Argentina.

Yerba Mate biomass underwent a preparatory extraction process. The dry mass was infused three consecutive times with boiled water maintained at 75 °C. Each extraction lasted at least 5 min, closely following traditional preparation methods to remove soluble organic compounds. The spent Yerba Mate (YM) has the following chemical composition by mass fraction: cellulose, 32–39%; hemicellulose, 26–32%; lignin, 25–30%; proteins, 4–12% [33,34,35,36,37,38].

2.2. Sorbates (Dyes)

The dyes used in this investigation were obtained from the Boruta-Zachem SA manufacturing plant in Zgierz, Poland. The study focused on four synthetic dyes: two acid dyes (Acid Red 18-AR18; Acid Yellow 23-AY23) and two basic dyes (Basic Violet 10-BV10; Basic Red 46-BR46). The chemical structures of these compounds are shown in Figure 1, and their physicochemical properties are provided in

Table 1. Dye properties and classification.

Dye Name	Acid Red 18 (AR18)	Acid Yellow 23 (AY23)	Basic Violet 10 (BV10)	Basic Red 46 (BR46)
Other trade names	Acid Brilliant Red 3R, Acid Scarlet 3R	Tartrazine, Acid Tartrazine	Rhodamine B, Basic Red RB	Anilan Red GRL, Basic Red X-GRL
Chemical formula	C20H11N2Na3O10S3	C16H9N4Na3O9S2	C28H31ClN2O3	C18H21BrN6
Molecular weight	604.5 g/mol	534.4 g/mol	479.0 g/mol	321.4 g/mol
Dye class	single azo dye	single azo dye	xanthene dye	single azo dye
Dye type	anionic (acidic)	anionic (acidic)	cationic (basic)	cationic (basic)
λmax	509 nm	428 nm	554 nm	530 nm
Uses	dyeing wool, silk, polyamide fiber	dyeing wool, silk, polyamide fiber	dyeing textiles, paper, leather	dyeing leather, paper, wool,

.

2.3. Chemical Reagents

Sodium hydroxide (NaOH), supplied as micropellets with a purity greater than 99.9%, and hydrochloric acid (HCl) (37% aqueous solution) were used to adjust the pH of the solutions. Acetone (C₃H₆O, purity > 99.5%) was used as a cleaning solvent for the diamond crystal in the ATR spectrometer attachment. All chemical reagents were obtained from POCH S.A., Gliwice, Poland, and met analytical purity specifications.

2.4. Laboratory Equipment

Structural characterization of the sorbent material was conducted by FTIR spectroscopy using an FT/IR-4700LE FTIR Spectrometer equipped with a single reflection ATR attachment (JASCO International, Tokyo, Japan). The textural properties of the sorbent, including porosity and specific surface area, were determined using the ASAP 2020 analyzer (Micromeritics, Norcross, GA, USA). The pH of all solutions was monitored and adjusted using a HI 110 pH meter (HANNA Instruments, Olsztyn, Poland). Mechanical agitation during the sorption process was provided by a SK-71 Laboratory Shaker (JEIO TECH, Daejeon, Republic of Korea) or an MS-53M Multi-Channel Stirrer (JEIO TECH, Daejeon, Republic of Korea). Quantitative analysis of dye concentrations in the liquid phase was performed spectrophotometrically with a UV-3100 PC spectrophotometer (VWR International LLC, Mississauga, ON, Canada).

3. Methods

3.1. Preparation and Conditioning of Spent Yerba Mate (YM)

Following the initial extraction phase (described in Section 2.1), the spent Yerba Mate biomass was thoroughly rinsed with deionized water. Washing continued until the collected filtrate was completely colorless, indicating the removal of residual soluble components. The purified biomass was then dried in a laboratory dryer at 105 °C. The dried material was subsequently fractionated using laboratory sieves with nominal apertures of 3 mm and 1 mm. The specific particle fraction of Yerba Mate biomass, with a diameter range of 1–3 mm (YM), was prepared for adsorption experiments. This prepared YM was stored in sealed polyethylene containers at an ambient temperature of 20 °C prior to use.

3.2. Sorbent Properties Investigation

3.2.1. FTIR Spectroscopic Analysis of the YM

The surface functional groups of spent Yerba Mate (YM) were characterized using an FTIR spectrometer (Model FT/IR-4700LE) equipped with a single-reflection diamond ATR crystal. Spectral data for YM were collected across the infrared range from 4000 to 400 cm⁻¹ with a spectral resolution of 1 cm⁻¹. To ensure high spectrum quality, the final result was obtained by averaging 64 scans.

3.2.2. Measurement of Specific Surface Area (BET) of YM

The BET specific surface area and porosity of YM were examined using an ASAP 2020 analyzer. The measurement was based on the low-temperature nitrogen physisorption technique (adsorption/desorption analysis). Prior to analysis, the YM sample was meticulously conditioned by degassing under vacuum at 100 °C for 4 h to remove adsorbed moisture and residual volatiles.

The detailed procedures and methodology for both FTIR and BET analyses of YM were consistent with those previously published in our work [30,39].

3.2.3. Measurement of pH_PZC of YM

The pH_PZC of YM was determined using the “drift” method [40]. First, 100 mL of 0.01 M NaCl solution was dispensed into each of 10 beakers. The initial pH of these solutions was adjusted to values of 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11 using standardized HCl and NaOH solutions. A fixed mass of the sorbent (1.00 g d.m.) was then added to each solution. The beakers were continuously mixed on magnetic stirrers at an agitation rate of 200 rpm for 12 h. After this period, the final pH of each suspension was accurately measured. The pH_PZC value was determined as the point where the line intersects the X-axis on a plot (with the X-axis representing the initial pH and the Y-axis representing the difference between the final and initial pH).

3.3. Studies on the Effect of pH on the Efficiency of Dye Sorption on YM

Into ten 250 mL beakers, a precisely measured mass of YM (2.50 g dry matter) was added. Then, 250 mL of the pre-prepared dye solution, with a concentration of 50 mg/L and a pH ranging from 2 to 11, was added to the corresponding beakers. The beakers were subjected to mechanical agitation for 120 min. Stirring was performed using a multi-position magnetic stirrer set at 200 rpm, employing a 6 × 30 mm magnetic bar. After the sorption period, 10 mL were withdrawn from each beaker using an automatic pipette and transferred to dedicated test tubes. These samples were reserved for subsequent spectrophotometric quantification of the residual dye concentration. The final pH of each solution was also noted.

3.4. Studies on the Kinetics of Dye Sorption on YM

A precise quantity of YM (20.00 g d.m.) was accurately placed into two beakers. Then, 2000 mL of dye solution (concentration 50/500 mg/L) at the optimal pH for sorption (determined in Section 3.3) was added. The beakers were continuously agitated using a multi-position magnetic stirrer operating at 200 r.p.m. with a 9 × 80 mm magnetic bar. During the process, at 0, 10, 20, 30, 45, 60, 90, 120, 150, 180, 210, 240, 270, and 300 min, 5 mL samples were withdrawn from the solutions into dedicated test tubes using an automatic pipette for later analysis.

3.5. Determination of the Maximum Sorption Capacity

A fixed mass of YM (2.50 g d.m.) was precisely added to ten beakers (250 mL vol.). Then, 250 mL of dye solution (concentration 25–600 mg/L) at the optimal pH for sorption (Section 3.3) was added to each beaker. The beakers were mechanically agitated with a multi-position magnetic stirrer (200 rpm, 6 × 30 mm magnetic bars). After the sorption equilibrium time (Section 3.4), 10 mL of solution was taken from each beaker and transferred to previously prepared test tubes for subsequent analysis of the residual concentration.

General Comments on Section 3.3, Section 3.4 and Section 3.5

• All sorption experiments on YM, including dye solution preparation, sorption process, and sample pre-treatment, followed the established methodology described in detail in our previous publications [30,41], with modifications specific to this study.
• Throughout all stages of the investigation, the sorption properties of YM were assessed against AR18, AY23, BV10, and BR46.
• The range of dye concentrations in the studies was from typical levels for textile wastewater (50 mg/L) to very high levels (500–600 mg/L) [42].
• Deionized water was used exclusively for the preparation of all dye stock and working solutions.
• In every experiment, the YM dose was 10.00 g dry matter (s.m.)/L. The YM dose of 10 g/L ensured a significant removal of each tested dye (at least several dozen percent) and allowed for an observation of differences in dye sorption efficiency at each stage of the research.
• YM portions were prepared using a precision balance calibrated to an accuracy of 0.001 g.
• Mixing parameters ensured a homogeneous distribution of YM throughout the entire volume of the solutions.
• The concentration of the dyes in the collected liquid samples was determined spectrophotometrically using a UV-VIS spectrometer equipped with a quartz cuvette with a standard optical path length of 10 mm.
• Calibration curves, essential for the spectrophotometric quantification of dye concentrations, were established at the λ_max specific to each dye. The standard concentration range used for generating these curves was 0.0–50.0 mg/L, except for BV10, which used the 0–10 mg/L range. If necessary, samples were diluted appropriately with deionized water prior to measurement.
• All experimental series were conducted in triplicate, and the reported data represent the arithmetic mean of these measurements.
• The laboratory environment was thermostated at 20 °C throughout the study, ensuring that the temperature of the reacting solutions remained constant at ambient conditions.

3.6. Computation Methods

The quantity of dye adsorbed onto YM was determined using the mass balance defined by Equation (1):

$$\mathbf{Q}=({\mathbf{C}}_{\mathbf{0}}-{\mathbf{C}}_{\mathbf{S}})\times {\displaystyle \frac{\mathbf{V}}{\mathbf{m}}}$$

(1)

• Q—mass of sorbed dye [mg/g];
• C₀—initial concentration of dye [mg/L];
• C_S—concentration of dye after sorption [mg/L];
• V—volume of the solution [L];
• m—mass of the sorbent [g].

For the kinetic evaluation of the adsorption process, the experimental data were analyzed by fitting them to three models: the pseudo-first-order model (2), the pseudo-second-order model (3), and the intraparticle diffusion model (4).

$$\mathbf{q}={\mathbf{q}}_{\mathbf{e}}\times (\mathbf{1}-{\mathbf{e}}^{\left(-{\mathbf{k}}_{\mathbf{1}}\times \mathbf{t}\right)})$$

(2)

$$\mathbf{q}={\displaystyle \frac{({\mathbf{k}}_{\mathbf{2}}\times {{\mathbf{q}}_{\mathbf{e}}}^{\mathbf{2}}\times \mathbf{t})}{(\mathbf{1}+{\mathbf{k}}_{\mathbf{2}}\times {\mathbf{q}}_{\mathbf{e}}\times \mathbf{t})}}$$

(3)

$$\mathbf{q}={\mathbf{k}}_{\mathbf{i}\mathbf{d}}\times {\mathbf{t}}^{\mathbf{0.5}}$$

(4)

• q—instantaneous value of the sorbed dye [mg/g];
• q_e—the amount of dye sorbed at the equilibrium state [mg/g];
• t—time of sorption [min];
• k₁—pseudo-first-order adsorption rate constant [1/min];
• k₂—pseudo-second-order adsorption rate constant [g/(mg × min)];
• k_id—intraparticle diffusion model adsorption rate constant [mg/(g × min^0.5)].

Additionally, the equilibrium data obtained from the study on maximum sorption capacity were interpreted using established adsorption isotherm models. The experimental results were successfully correlated with the Langmuir models (Type 1 (5) and Type 2 (6)) and the Freundlich model (7).

$$\mathbf{Q}={\displaystyle \frac{({\mathbf{Q}}_{\mathbf{m}\mathbf{a}\mathbf{x}}\times {\mathbf{K}}_{\mathbf{C}}\times \mathbf{C})}{(\mathbf{1}+{\mathbf{K}}_{\mathbf{C}}\times \mathbf{C})}}$$

(5)

$$\mathbf{Q}={\displaystyle \frac{({\mathbf{b}}_{\mathbf{1}}\times {\mathbf{K}}_{\mathbf{1}}\times \mathbf{C})}{(\mathbf{1}+{\mathbf{K}}_{\mathbf{1}}\times \mathbf{C})}}+{\displaystyle \frac{({\mathbf{b}}_{\mathbf{2}}\times {\mathbf{K}}_{\mathbf{2}}\times \mathbf{C})}{(\mathbf{1}+{\mathbf{K}}_{\mathbf{2}}\times \mathbf{C})}}$$

(6)

$$\mathbf{Q}=\mathbf{K}\times {\mathbf{C}}^{{\displaystyle \frac{\mathbf{1}}{\mathbf{n}}}}$$

(7)

• Q—mass of the sorbed dye at the equilibrium state [mg/g];
• Q_max—maximum sorption capacity in the Langmuir equation [mg/g];
• b₁—maximum sorption capacity of sorbent (type I active sites) [mg/g];
• b₂—maximum sorption capacity of sorbent (type II active sites) [mg/g];
• K_C—constant in Langmuir equation [L/mg];
• K₁,K₂—constants in Langmuir 2 equation [L/mg];
• K—the equilibrium sorption constant in the Freundlich model;
• C—concentration of dye remaining in the solution [mg/L];
• n—constant in the Freundlich model.

4. Results and Discussion

4.1. Characterization of YM

4.1.1. FTIR Analysis

The FTIR spectrum of Yerba Mate biomass shows bands typical of lignocellulosic plant material (Figure 2). Peaks in the 1400–900 cm⁻¹ range are characteristic of polysaccharides. The presence of cellulose and hemicellulose in YM is indicated by peaks at 1232 cm⁻¹ and 1028 cm⁻¹, which result from the stretching vibrations of C–O bonds at the C6 carbon atom of the saccharide ring [43], as well as a peak at 1153 cm⁻¹ corresponding to the asymmetrical stretching vibration of the C–O–C glycosidic linkage [44]. Additionally, typical for these polysaccharides is the peak at 1370 cm⁻¹, resulting from the deformation vibrations of C–H bonds at the C6 carbon of the saccharide ring [45]. The peak at 1420 cm⁻¹ is attributed to the bending vibrations of CH₂ in the cellulose ring and suggests a crystalline form of this polysaccharide [46]. In contrast, the peak at 898 cm⁻¹ indicates the C1–H vibration of the β-glycosidic ring, which is characteristic of amorphous cellulose [47]. The similar height of the 1420 cm⁻¹ and 898 cm⁻¹ bands suggests the presence of both cellulose forms in YM, which is typical for plant biomass. The peak at 1733 cm⁻¹ indicates the presence of hemicellulose and corresponds to the stretching vibration of the C=O carbonyl bond in the acetyl and carboxyl groups of this polysaccharide.

The presence of lignin in YM biomass is indicated by peaks at 1630 cm⁻¹ and 1510 cm⁻¹, which represent the stretching vibration of the C=C bonds within its aromatic rings [48]. Additionally, typical for the aromatic structures of this component is the peak at 1455 cm⁻¹, resulting from C–H bond vibrations [49]. The band at 1317 cm⁻¹ is attributed to the C–O stretching vibrations of the syringyl rings of lignin [50] (Figure 2).

The presence of proteins in the tested material is indicated by the peak at 1545 cm⁻¹, which signifies the C-N stretching vibration and the N-H bending vibration of the peptide bond (Amide II band) as well as the peak at 3280 cm⁻¹, which corresponds to the N-H stretching vibration (Amide A) [51].

The broad absorption band at 3600–3000 cm⁻¹ is attributed to the O-H stretching vibration, commonly present in many key YM components (cellulose, hemicellulose, lignin). The peaks at 2920 cm⁻¹ and 2850 cm⁻¹, corresponding to the asymmetrical and symmetrical stretching vibrations of the -CH₂ groups, are also non-specific, as they may originate from aliphatic fragments of cellulose, hemicellulose, lignin, or terminal protein groups [52].

4.1.2. pH_PZC of YM

The determined pH_PZC (pH at the Point of Zero Charge) for YM was 4.75 (Figure 3). At a solution pH below the isoelectric point, the sorbent has a net positive charge, while at a pH above pH_PZC, the material has a net negative charge.

The value of pH_PZC = 4.75 indicates the acidic nature of the sorptive material. This is due to the predominance of acidic functional groups (e.g., carboxyl groups) on the material’s surface compared to basic groups (e.g., amine groups). The acidic character of YM suggests higher effectiveness in removing basic dyes (e.g., BV10, BR46) than acid dyes (e.g., AR18, AY23). A similar pH_PZC value for Yerba Mate biomass (pH_PZC = 4.70) was also obtained in other studies on unconventional sorbents [53].

4.1.3. BET Surface Area and Porosity

The BET specific surface area of YM was quite small, at 1.682 ± 0.113 m²/g. The average pore volume of the tested sorbent was 0.0018 cm³/g, and the average pore size on its surface was 22.21 Å (2.221 nm), which qualifies the material as mesoporous (2–50 nm range). Similar values for the specific surface area and pore diameter of YM were obtained in other studies on the sorption properties of Yerba Mate [54].

The relatively low BET surface area of the tested material suggests that multilayer physical adsorption, which depends on a large sorbent surface area, will not be the dominant mechanism for dye removal. pH_PZC studies indicate the presence of readily ionizable functional groups, such as carboxyl or amine, on the YM surface. FTIR spectral analysis also suggests the presence of hydroxyl groups in the material. This predicts that electrostatic interactions and hydrogen bonds will play a significant role in the process of dye binding to YM.

4.2. Effect of pH on the Effectiveness of Dye Sorption on YM

The sorption efficiency of the acid dyes AR18 and AY23 onto YM was highest in a strongly acidic environment with pH 2 (Figure 4a). As the initial pH increased, the effectiveness of binding these dyes to the tested sorbent decreased. The most significant drop in sorption efficiency for AR18 and AY23 on YM occurred when the pH increased from 2 to 4. Conversely, in the initial pH range of 5–9, the binding efficiency of the acid dyes on YM remained at a similar level. The lowest sorption results for the anionic dyes were obtained at pH 11 (Figure 4a).

The binding of anionic dyes onto YM primarily occurs through electrostatic interactions between the ionized functional groups of the dyes and the sorbent. The tested anionic dyes, AR18 and AY23, possess characteristic sulfonic groups (-SO₃H) of an acidic nature, which generate a negative charge (-SO₃⁻) in an aqueous environment.

• -SO₃H + H₂O → -SO₃- + H₃O+ (deprotonation in the wide pH range 2–11)

In contrast, the YM biomass contains groups that can be protonated in an acidic environment. Under low pH conditions, which indicate a high concentration of hydronium ions (H₃O⁺), the functional groups on the YM surface—specifically the -OH groups (from polysaccharides and lignins) and -NH₂ groups (from proteins)—are protonated.

• -NH₂ + H₃O⁺ → -NH₃⁺ + H₂O (protonation effective even at pH < 9)
• -OH + H₃O⁺ → -OH₂⁺ + H₂O (low efficiency even at pH < 3)

Furthermore, at pH < 4, hydronium ions can attach to the sorbent surface through the formation of a hydrogen bond between the hydrogen atoms of the YM hydroxyl groups and the hydrogen atom of the H₃O⁺ cation [55].

• -OH + H₃O⁺ → –OH···H-OH₂⁺ (“···”—indicates hydrogen bond formation)

Moreover, at low pH (pH < 3), acidic groups present on the YM surface, such as -COOH (originating from hemicellulose, proteins, or partially degraded lignin), may undergo protonation in this context.

• -COO⁻ + H₃O⁺ → -COOH + H₂O (protonation by proton attachment pH < 3)

The reactions described above result in the sorbent surface acquiring a strong positive charge, which favors the sorption of anionic dyes [56]. This explains the high efficiency of AR18 and AY23 removal on YM at low pH.

As the pH increased, the efficiency of functional group protonation decreased. At pH > 4, nearly all hydroxyl groups existed in a non-ionized form, and all carboxyl groups were deprotonated. Additionally, at pH > 4, hydronium ion adsorption onto the sorbent practically did not occur. This led to a significant decrease in the net positive charge on the YM surface. This clarifies the significant drop in acid dye binding efficiency observed on the tested sorbent in the pH 2–4 range (Figure 4a). Presumably, in the pH 5–9 range, in addition to electrostatic interactions with the protonated amine groups, π–π interactions between the aromatic structures of lignin and the aromatic rings of the dyes may have contributed to the sorption of AR18 and AY23 onto YM [57].

At pH ≥ 5 (pH_PZC = 4.75), the surface of YM began to acquire a negative charge. This resulted from deprotonated carboxyl groups, which exist in their ionized form even at pH > 3. At pH > 10, the net charge on the sorbent surface increased further due to the deprotonation of some hydroxyl groups.

• -OH + OH⁻ → -O⁻ + H₂O (deprotonation at pH > 10)

In a strongly alkaline environment, the YM surface, having acquired a strong net negative charge, effectively repelled the acid dyes electrostatically, hindering their sorption. This explains the negligible effectiveness of AR18 and AY23 binding onto YM at pH 11 (Figure 4a).

A positive effect of an acidic environment on the sorption efficiency of anionic (acid) dyes was also observed in studies on the sorption of AR18 onto sunflower husks [58], as well as in studies on the sorption of AY23 onto sawdust [59].

The binding of the basic dyes BV10 and BR46 onto YM, similar to the acid dyes, occurred through electrostatic interactions between the ionized functional groups of the dyes and the sorbent. However, the sorption of these dyes via the formation of hydrogen bonds (e.g., between the hydroxyl groups of YM and the tertiary amine groups of the dyes) or through π–π interactions cannot be ruled out.

The efficiency of basic dye BV10 binding onto YM was highest at pH 3 (Figure 4b). As with the acid dyes, the effectiveness of binding this dye to the tested sorbent decreased as the initial pH increased, with the lowest efficiency observed at pH 11. A significant reduction in BV10 binding efficiency was also observed when the pH was lowered from 3 to 2 (Figure 4b).

BV10 contains a carboxyl group (-COOH), which is unusual for a basic dye and imparts acidic character. This group generates a local negative charge in an aqueous environment, causing BV10 to behave similarly to anionic dyes under certain conditions. This is reflected in the high sorption efficiency at low pH (pH 3) and the decrease in sorption effectiveness as pH increases (Figure 4b). At very low pH (pH 2), deionization (protonation) of the carboxyl group occurs, resulting in the loss of the local negative charge and a greater net positive charge for BV10. Consequently, the positively charged YM surface strongly repels the cationic BV10 dye, which explains the sharp drop in sorption efficiency at pH 2 (Figure 4b).

A similar effect of pH on BV10 sorption efficiency has also been observed in studies on the removal of this dye using sorbents such as spent coffee and green tea grounds [60] and rapeseed husks [29].

The sorption mechanism of BR46 onto YM is presumably based on ionic interactions between the negatively charged functional groups of the sorbent (such as deprotonated carboxyl groups, -COO-) and the protonated tertiary amine groups of the dye. As mentioned, at very low pH (pH 2), the carboxyl groups are in a non-ionized form, and the YM surface acquires a strong positive charge that electrostatically repels the cationic BR46. This explains the low efficiency of its sorption at low pH.

At pH 4, the net positive charge on the YM surface was already much smaller than at pH 2. Additionally, at pH > 3, most of the sorbent’s carboxyl groups were in their deprotonated form. This explains the ‘jump’ in the sorption efficiency of BR46 onto YM when the initial pH in the system increased from pH 2 to pH 4 (Figure 4b). At pH ≥ 5, the YM surface acquired a net negative charge (pH_PZC = 4.75), creating very favorable conditions for the sorption of the cationic dye BR46. In the pH 5–9 range, the net electrical charge on the YM surface did not undergo significant changes. The amount of deprotonated functional groups was also similar, which explains the comparable sorption efficiency of BR46 within this pH range (Figure 4b). At pH > 7, the cationic rings of some BR46 molecules could undergo neutralization, which might have caused lower effectiveness of dye binding to the sorbent surface. At pH > 9, the BR46 solutions spontaneously decolorized due to degradation of the dye’s chromophore system by excess OH⁻ ions in the system. Due to the risk of data misinterpretation, the results for BR46 sorption onto YM in the pH 9–11 range are not included in Figure 4b.

Similar results for BR46 sorption, with low efficiency at low pH and optimal efficiency at pH~6, were also observed in studies on BR46 sorption onto waste paper [30] and conifer cones [39]. In these articles, the authors also reported spontaneous decolorization of BR46 solutions only at high pH (pH > 9).

The YM significantly influenced the change in pH of the dye solutions during sorption. Regardless of the dye tested, in the initial pH range of 4–10, the pH after sorption stabilized between 4.6 and 5.0 (Figure 5). This phenomenon resulted from the tested sorbent containing a significant quantity of ionizable functional groups.

The basic functional groups of the sorbent (e.g., amine groups) extract a proton from the aqueous solution, resulting in a decrease in hydronium ion concentration (at pH < 7) or an increase in hydroxide ion concentration (at pH > 7), and consequently, an increase in solution pH. Conversely, acidic functional groups (e.g., carboxyl groups) release a proton into the solution, increasing hydronium ion concentration (at pH < 7) or decreasing hydroxide ion concentration (at pH > 7), ultimately leading to a decrease in solution pH. The final pH of the solution mainly depends on the ratio of acidic to basic groups within the sorbent structure. The system always tends to reach a solution pH value close to the pH_PZC of the sorbent used (pH_PZC = 4.75).

The subsequent stages of the study, described in Section 4.3 and Section 4.4, were conducted at the most favorable pH values for dye sorption (pH 2 for AR18 and AY23, pH 3 for BV10, and pH 6 for BR46).

4.3. Kinetics of Dye Sorption on YM

The sorption equilibrium time for AR18, AY23, BV10, and BR46 onto YM depended mainly on the initial dye concentration in the solution. Despite the different binding mechanisms for acid and basic dyes, the equilibrium time for all tested sorbates ranged from 180 min at concentrations of 50 mg/L to 210 min at concentrations of 500 mg/L (Figure 6,

Table 2. Kinetic parameters of sorption of dyes onto YM determined from the pseudo-first-order and pseudo-second-order models. Dose of the sorbent: 10 g/L, optimal sorption pH (pH 2 for AR18/AY23; pH 6 for BR46; pH 3 for BV10), mixing speed: 200 r.p.m., sorption time: 0–300 min, temp. 20 °C.

Dye	Dye Conc.	Pseudo-First-Order Model			Pseudo-Second-Order Model			Exp. Data	Equilibrium Time
		k1	qe, cal.	R2	k2	qe, cal.	R2	qe, exp.	[min]
	[mg/L]	[1/min]	[mg/g]	-	[g/mg × min]	[mg/g]	-	[mg/g]
AR18	50	0.0377	2.82	0.9816	0.0141	3.28	0.9974	2.99	180
	500	0.0393	16.26	0.9760	0.0025	18.90	0.9908	17.32	210
AY23	50	0.0293	2.34	0.9839	0.0118	2.80	0.9942	2.35	180
	500	0.0332	14.54	0.9850	0.0023	17.21	0.9981	15.37	210
BV10	50	0.0292	4.28	0.9851	0.0064	5.14	0.9954	4.38	180
	500	0.0233	30.35	0.9760	0.0006	37.73	0.9905	32.78	210
BR46	50	0.0338	3.86	0.9896	0.0086	4.56	0.9983	4.01	180
	500	0.0300	34.56	0.9849	0.0008	41.55	0.9980	36.44	210

). In every test series, dye sorption intensity was highest at the beginning of the process. Regardless of dye type and concentration, over 50% of the q_e value (q_e—the amount of sorbate bound to the sorbent after the sorption equilibrium time) was achieved within the first 30 min of the process.

The slightly longer time required to reach sorption equilibrium at higher initial concentrations resulted from the higher concentration gradient, which is the driving force for sorption. At higher concentrations, the particle pressure of the sorbate facilitated dye penetration into the deeper layers of YM and binding to more difficult-to-access active sites, which extended the time required to achieve sorption equilibrium.

Similar times to reach sorption equilibrium for the removal of anionic dyes were also reported in studies on AR18 removal onto chitosan flakes (180 min) [61] and in studies on AY23 sorption onto cotton fibers (240 min) [62]. For cationic dyes, similar sorption times were observed during experiments on BV10 sorption onto waste paper (210 min) [30], and in studies on BR46 sorption onto rapeseed husks (180 min) [29], larch cones (180 min) [39], and spent green tea grounds (240 min) [60].

The experimental data from the dye sorption kinetics studies on YM were analyzed using the pseudo-first-order and pseudo-second-order models. Based on the determination coefficient (R²) values, it was found that in every test series, regardless of dye type or initial concentration, the process is best described by the pseudo-second-order model (Table 2). This is a typical result for the sorption of organic dyes onto plant biomass-based materials.

The analysis of experimental data from the sorption kinetics studies was extended by describing it using the intraparticle diffusion model (Figure 7,

Table 3. Dye diffusion rate constants, determined from the intraparticle diffusion model. Dose of the sorbent: 10 g/L, optimal sorption pH (pH 2 for AR18/AY23; pH 6 for BR46; pH 3 for BV10), mixing speed: 200 r.p.m., sorption time: 0–240 min, temp. 20 °C.

Dye	Dye Conc.	Phase I			Phase II
		kd1	Time Duration	R2	kd2	Time Duration	R2
	[mg/L]	[mg/(g × min0.5)]	[min]	-	[mg/(g × min0.5)]	[min]	-
AR18	50	0.3126	60	0.9887	0.1086	120	0.9558
	500	1.9610	45	0.9969	0.5536	165	0.9714
AY23	50	0.2356	90	0.9969	0.1135	90	0.9225
	500	1.5758	60	0.9958	0.5130	150	0.9810
BV10	50	0.4171	90	0.9952	0.1114	90	0.9846
	500	2.8178	60	0.9980	1.6062	150	0.9902
BR46	50	0.4280	60	0.9951	0.1261	120	0.9975
	500	3.6180	60	0.9947	1.4686	150	0.9794

). This model allows for the identification of the transport mechanism of dye molecules from the solution to the internal structure of the sorbent. The arrangement of data points on the graphs (Figure 7) and the parameters determined from the intraparticle diffusion model (Table 3) indicate that the sorption of the tested dyes onto YM occurred in two distinct phases.

In most test series, the first sorption phase had a shorter duration than the second phase and significantly higher sorption intensity (Table 3). During this phase, dye molecules diffused from the solution to the sorbent surface and occupied easily accessible active sites on the surface. The second phase began when most active sites on the sorbent surface were saturated. In this phase, dye molecules competed for the remaining vacant sorption centers and attached to active sites in the deeper layers of the YM. As a result, this stage was characterized by significantly lower efficiency and, in most cases, a longer duration compared to the first stage (Table 3).

In all test series, the straight line generated from the data points of the first sorption phase passes through, or is very close to, the origin of the coordinate system (Figure 7). This suggests that the mass transfer resistance of dye molecules from the solution to the external surface of the YM (film diffusion resistance) is close to zero, and the only process that significantly influences the adsorption rate is intraparticle diffusion [63].

The parameter values k₂, q_e, k_d1, and k_d2, determined from the pseudo-second-order model and the intraparticle diffusion model, are highly dependent on the initial concentration of the dyes in the system. This may suggest a rather low affinity of the tested dyes for the YM sorption centers.

4.4. Maximal Sorption Capacity of YM

The experimental data on the maximum sorption capacity of YM were described using the Langmuir 1, Langmuir 2, and Freundlich models (Figure 8,

Table 4. Constants determined from Langmuir 1, Langmuir 2, and Freundlich models.

Dye	Langmuir 1 Model			Langmuir 2 Model						Freundlich Model
	Qmax	Kc	R2	Qmax	b1	K1	b2	K2	R2	k	n	R2
	[mg/g]	[L/mg]	-	[mg/g]	[mg/g]	[L/mg]	[mg/g]	[L/mg]	-	-	-	-
AR18	24.95	0.0072	0.9912	24.95	12.49	0.0072	12.46	0.0072	0.9912	0.892	0.515	0.9634
AY23	22.86	0.0052	0.9958	22.86	11.34	0.0052	11.52	0.0052	0.9958	0.552	0.561	0.9737
BV10	46.24	0.0143	0.9961	46.24	25.51	0.0143	20.73	0.0143	0.9961	2.376	0.507	0.9648
BR46	60.54	0.0103	0.9941	60.54	32.86	0.0103	27.68	0.0103	0.9941	1.754	0.605	0.9650

). For all tested dyes, the Langmuir models provided a better fit to the data than the Freundlich model, as indicated by higher R² values. This result is typical for sorbents with a low BET surface area. This also suggests a sorbate binding mechanism in which each dye molecule attaches to a single sorption site on the sorbent. As a result, most of the adsorbed dye mass forms a monolayer on the sorbent surface [64]. However, within this monolayer, the dyes may exchange positions.

In all test series, the K_C constant (Langmuir 1) values are identical to the K₁ and K₂ values (Langmuir 2). The Q_max and R² values determined from both Langmuir models are also the same (Table 4). This indicates that only one type of active site is primarily responsible for the sorption of the tested dyes onto YM. Presumably, for the acid dyes AR18 and AY23, as well as the basic dye BV10, this sorption center consists of protonated hydroxyl functional groups, which are much more prevalent in YM than amine groups. In contrast, for BR46, deprotonated carboxyl groups may serve as the key sorption centers.

The maximum sorption capacity of YM determined from the Langmuir models for the acid dyes AR18 and AY23 was Q_max = 24.95 mg/g and Q_max = 22.86 mg/g, respectively. For the basic dyes, the sorption capacity of YM was significantly higher, with Q_max = 46.24 mg/g and Q_max = 60.54 mg/g for BV10 and BR46, respectively. The Langmuir constants K_C/K₁/K₂ determined from the models for the basic dyes also have higher values (>0.01) compared to the anionic dyes, indicating a higher affinity of the cationic dyes for the YM functional groups. However, it should be noted that K_C/K₁/K₂ values below 0.1 usually classify the sorbates as having a low affinity for the sorbent’s active sites (what was already mentioned at the end of Section 4.3).

The generally greater sorption capacity of YM for basic dyes compared to acid dyes presumably results from the acidic nature of the sorbent. This is confirmed by the YM’s pH_PZC (pH_PZC = 4.75). YM, being rich in functional groups of an anionic nature, is potentially a good sorbent for most cationic pollutants (those carrying a positive charge in aqueous solution). The lower efficiency of BV10 sorption onto YM compared to BR46 presumably resulted from different sorption conditions. Additionally, BV10 sorption based on electrostatic attraction may have been limited by the dye possessing both a positive and a negative charge.

Table 5. Comparison of the sorption properties of various sorbents towards AR18 and AY23 dyes.

Dye	Sorbent	Qmax [mg/g]	pH of Sorption	Time of Sorption [min]	Source
AR18	Coconut Shells	0.7	2	45	[65]
	Sunflower seed hulls	1.8	3	90	[58]
	Corrugated cardboard (used)	6.6	2	150	[30]
	Newsprint paper (used)	7.8	2	90	[30]
	Sargassum glaucescens biomass	15.0	6	60	[66]
	Yerba Mate residues	25.0	2	210	This work
	Carboxymethyl cellulose	29.7	6	120	[61]
	Activated carbon from poplar wood	30.3	5	120	[67]
	Chitosan flakes	39.9	4	180	[61]
	Granular activated carbon	45.5	9	120	[68]
	Activated carbon WG-12	100.0	–	–	[69]
AY23	Coconut Shells	0.5	2	45	[65]
	Sunflower seed hulls	2.3	3	90	[58]
	Cotton fibers	3.6	3	240	[62]
	Sawdust	4.7	3	70	[59]
	Corrugated cardboard (used)	6.6	2	150	[30]
	Newsprint paper (used)	7.2	2	90	[30]
	Yerba Mate residues	22.9	2	210	This work
	Chitin flakes	24.2	2	120	[61]
	Deoiled soya	24.6	2	–	[70]
	Chitin	30.5	3	240	[71]
	Commercial activated carbon	56.5	8	120	[72]
	Activated carbon of Lantana camara	58.8	2	30	[73]

and

Table 6. Comparison of the sorption properties of various sorbents towards BR46 and BV10 dyes.

Dye	Sorbent	Qmax [mg/g]	pH of Sorption	Time of Sorption [min]	Source
BV10	Powdered coffee	2.5	2	180	[74]
	Coal-fired coconut fiber	2.6	6.5	150	[75]
	Chitin from the molts of mealworms	3.2	6	120	[20]
	Mango leaves (powder)	3.3	–	50	[76]
	Calotropis procera leaf biomass	4.1	–	60	[77]
	Office paper (used)	5.9	2	120	[30]
	Sugar cane fiber	10.4	–	–	[78]
	Coconut fiber	14.9	9.2	90	[79]
	Municipal solid waste compost	19.3	3	1440	[80]
	Banana peels	20.6	7	1440	[81]
	Rapeseed husks	20.9	3	180	[29]
	Corrugated cardboard (used)	24.7	2	210	[30]
	Spent green tea leaves	26.7	3	240	[60]
	Activated carbon from jute fiber	28.0	8	220	[82]
	Coconut shells	28.5	3	180	[65]
	Activated carbon (palm shell-based)	30.0	3	–	[83]
	Yerba Mate residues	46.2	3	210	This work
	Activated carbon (from carbon fossils)	119.0	12	240	[84]
BR46	Wood sawdust	19.2	–	120	[85]
	Office paper (used)	19.6	6	90	[30]
	Natural sugarcane stalk powder	21.0	7.2	60	[86]
	Nut sawdust	30.1	7	–	[87]
	Biochar from Chrysanthemum morifolium straw	32.3	10	60	[88]
	Spent green tea leaves	58.0	6	240	[60]
	Rapeseed hulls	59.1	6	180	[29]
	Yerba Mate residues	60.5	6	210	This work
	Activated carbon from Cerbera odollam	65.7	7	90	[89]
	Activated carbon “Chemviron”	106.0	7.4	120	[90]

compare the sorption capacities of YM with those of other unconventional biomass-based sorbents and activated carbons.

The sorption capacity of YM for acid dyes is higher than that of many plant-based sorbents, such as sawdust, sunflower husks, and cocoa shells. The sorption efficiency of AR18 and AY23 on YM is also comparable to that of some types of activated carbon (Table 5).

For basic dye sorption, YM is more efficient than most plant-derived materials (e.g., sawdust, citrus peels, seed husks, tea leaves, waste paper) (Table 6). The sorption efficiency of BV10 and BR46 on YM is also comparable to that of many types of activated carbons.

Unlike typical lignocellulosic sorbents based on plant biomass (e.g., coconut shells [65]), YM exhibits good sorption properties for both acidic and basic dyes. The versatility of YM, along with its performance comparable to some activated carbons (Table 5 and Table 6), suggests the potential for using YM to treat various types of colored industrial wastewater, regardless of composition. This underscores the innovative nature of YM as a biosorbent.

It is important to address the management of YM after dye sorption. Users of wastewater treatment systems that rely on activated carbon sorption typically send the spent sorbent for regeneration and reuse. For plant biomass such as YM, there is a theoretical possibility of chemical regeneration using a solvent (eluent) to desorb the dye from the material. However, YM, as unmodified plant biomass, is relatively unstable and may be severely damaged during repeated sorption and desorption cycles. Damaged biomass could release biogenic compounds into the solution, leading to secondary contamination. Furthermore, chemical regeneration almost never restores full sorption capacity. The process is usually expensive (due to the cost of the eluent) and generates wastewater that also requires treatment. Since YM is a waste product and is inherently inexpensive, its regeneration for reuse may be economically unjustified. According to the authors, a better solution is to dry the spent YM and co-incinerate it in a combined heat and power plant. The potential to use YM as a fuel is due to the high calorific value of plant biomass and the dyes themselves. Alternatively, spent YM can serve as a substrate in biogas production (anaerobic digestion). It has been shown that contamination of the fermentation feedstock with dyes (e.g., Basic Red 46) does not significantly inhibit methanogenesis [91]. Another option is to carbonize the spent YM and then activate it to produce activated carbon. The activated carbon produced in this way, for example, in granular form, could then be used for wastewater treatment. The spent sorbent from rapeseed husks may be used as a good raw material for the production of activated carbon, which can also be used for wastewater treatment.

5. Conclusions

YM can be used to sorb both acidic dyes (AR18, AY23) and basic dyes (BV10, BR46) from aqueous solutions. The maximum sorption capacity of YM is Q_max = 24.95 mg/g for AR18 and Q_max = 22.86 mg/g for AY23, while for BV10 and BR46, it is Q_max = 46.24 mg/g and Q_max = 60.54 mg/g, respectively. The greater efficiency of YM for basic dyes is likely due to the acidic nature of the sorbent (pH_PZC = 4.75). The acidic character of YM results from the quantitative predominance of acidic groups (e.g., -COOH) on the sorbent surface compared to basic groups (e.g., NH₂).

Only one type of sorption site plays a key role in dye sorption onto YM. For the acidic dyes AR18 and AY23, as well as the basic dye BV10, this sorption center is likely protonated hydroxyl functional groups, while for BR46, deprotonated carboxyl groups may be key.

Solution pH is crucial in the sorption of anionic and cationic dyes onto YM. The sorption efficiency of AR18 and AY23 onto YM is greatest at pH 2. The binding of the cationic dye BV10 is most effective at pH 3, and sorption of BR46 occurs best at pH 6. The time to reach sorption equilibrium depends primarily on the initial dye concentration and, regardless of dye type, ranges from 180 min (at 50 mg/L) to 210 min (at 500 mg/L). The slightly longer time to reach equilibrium at higher initial concentrations results from the higher concentration gradient, which drives sorption. At higher concentrations, the pressure of the sorbate particles allows the dye to penetrate deeper layers of YM, which delays the process of reaching equilibrium.

Sorption of the tested dyes on YM occurred in two phases, differing in intensity and duration. In the first phase, dye molecules diffused from the solution to the sorbent surface and occupied easily accessible external active sites. In the second phase, dye molecules competed for the remaining free active sites and attached to sorption sites in the deeper layers of YM.