A Keratin-Waste Derived Multifunctional Gel System: Reusable Activated Carbon/Alginate Microbeads for Simultaneous Dye and Pharmaceutical Adsorption

Abstract

The resource utilization of keratin waste has garnered significant attention, yet the processing of yak hair keratin in underdeveloped regions such as Tibet and Qinghai in China remains challenging. This study addresses these concerns by carbonizing yak hair keratin waste using a steam flash explosion (SFE) technique for 150 s, which is followed by activation with KOH at various ratios and subsequent to produce activated carbon (AC) samples. The AC was then combined with sodium alginate (Alg) at different ratios, pH and applied voltage to yield AC−Alg gel microbeads using an electrospinning method. The characterization of the AC and AC–Alg gel microbeads was conducted using SEM, BET, TG, and FT-IR analysis. In adsorption studies, AC−Alg_0.5U gel microbeads prepared with optimized conditions (pH 7, 11 kV, 19 G needle) were used to remove dyes (methylene blue (MB) and methyl orange (MO)) and antibiotic minocycline hydrochloride (MH). Various parameters such as temperature, pH, and adsorbent dose were optimized to obtain the maximum adsorption performance under model concentrations. The experimental results showed that the AC−Alg_0.5U gel microbeads can effectively adsorb MB and MO with adsorption capacities of 1038.9 mg/g and 206.2 mg/g, respectively. Moreover, the microbeads had the best adsorption performance for MH (1694.2 mg/g), with the kinetics most accurately represented by the pseudo-second-order kinetic model (R² = 0.999), and the isotherm followed the Langmuir model (R² = 0.984). The microbeads maintained a high adsorption capacity of 75% after six cycles. The composite gel microbeads not only utilize yak hair keratin waste but also will be used as durable and favorable adsorbents for the removal of pollutants.

1. Introduction

Keratin waste, like wool, hair, horns, hooves, and feathers, has been a particularly abundant biopolymer from the industries of slaughterhouses and meat processing plants [1]. A total quantity of approximately 5 million tons of keratin waste is produced annually around the world [2,3]. The keratin-rich wastes are abundantly available, and the yak breeding industries in China has contributed mostly, as about 15 million yaks breeding in China account for roughly 95% of the world’s yak population [4]. The abundant and inexpensive keratin-rich yak resources remain underutilized, since large quantities of yak hair are landfilled or incinerated annually [5]. Hence, the treatment and utilization of this waste are therefore of great significance for environmental protection [6,7].

Due to its high carbon content, keratin holds potential for the production of biochar [8,9] and composite-activated carbon (AC) [10,11]. Previous studies have shown that keratin waste-derived AC with a high adsorption capacity is acknowledged as an efficient and renewable adsorbent material [12] because of its well-matched porous structure and extensive specific surface area, offering considerable economic advantages and making it pivotal for removing water pollutants [13,14,15]. Therefore, utilizing keratin waste to produce AC not only promotes resource upcycling and reduces environmental pollution but also offers cost advantages owing to its high adsorption efficiency [16]. However, the application of powdered AC as an adsorbent has been reported to be limited by the difficulties associated with its separation and regeneration [14,17]. Powdered AC could only be recovered and reused at a lower efficiency through filtration or centrifugation [18], resulting in potential particle loss during water treatment [19] and leading to significant waste and an increase in additional environmental concerns. Therefore, research on the development of AC in a microbead or bead form, achieved by combining AC with other materials [20,21], has focused on enhancing the efficiency of AC separation.

The composite materials used to form beads with AC primarily include chitosan [22], pectin [23], and sodium alginate (Alg) [24]. Alg, a natural anionic polymer commonly obtained from brown algae [25], is one of the most widely employed materials for the preparation of composite AC microbeads due to its easy of hydrogel formation [26], ideal biocompatibility [27], low toxicity, environmental friendliness, biodegradability [28], and economic viability [29]. Nasrullah et al. [30] prepared mangosteen fruit peel-based AC–Alg beads for the efficient removal of methylene blue (MB) with an adsorption capacity of 230 mg/g for a concentration of 100 mg/L MB. Similarly, Liu et al. [31] developed Alg/polyacrylate/AC hydrogel beads that adsorbed Pb(II) as a photocatalyst to excellently degrade Rhodamine B. However, these studies employed the direct injection method for bead preparation, which does not account for the controlled size production of microbeads, particularly smaller ones. Furthermore, the adsorption capacity of the composite beads requires improvement. Although Makhado et al. [32] demonstrated that nanosized AC–SA hydrogel exhibits a higher adsorption capacity, exceeding 1596.0 mg/g, the separation of nanosized gel microspheres also poses a challenge. Therefore, there is a scarcity of both effective adsorption capacity and preparation processes for smaller-sized AC–Alg gel microbeads that can be separated.

Currently, electrospinning has emerged as a highly effective technique for the fabrication of microbeads [33]. This method provides precise control over the size and morphology of microbeads, enabling the production of uniform and spherical particles with tailored properties [34]. The application of the electrospinning method may facilitate the encapsulation of AC and Alg into small-sized AC–Alg gel microbeads, which could enhance the recovery of AC. Additionally, this technique will allow for the controlled production of AC–Alg gel microbeads with improved adsorption performance.

In this study, keratin-rich yak hair waste was rapidly carbonized using a steam flash explosion (SFE) technique for 150 s [35], which was followed by activation with potassium hydroxide (KOH) to produce AC within 1 h. The AC was then crosslinked with Alg to create AC−Alg gel microbeads using the electrostatic spinning method with the optimization of the ratios of AC to Alg and solution pH. The adsorption capacity of the AC−Alg gel microbeads was systematically evaluated for the removal of various pollutants, including cationic dye methyl blue (MB), anionic dye methyl orange (MO), and minocycline hydrochloride (MH).

2. Results and Discussion

2.1. Characterization Results

2.1.1. Characterization Result of SFEM

Figure 1 illustrates the SEM image and XRD patterns of the carbonized yak hair material (SFEM) prepared using the SFE at 1.5 MPa for 150 s. The SEM image reveals that the surface of the SFEM exhibited significant roughness, which was characterized by the presence of irregular cracks and minor defects following SFE treatment. Additionally, the XRD patterns were analyzed to assess the crystallinity of the samples, showing two distinct broad diffraction peaks at 19.36° and ~43°, corresponding to the (002) [36], (100) and (101) [37,38] crystal planes of graphitic carbon, respectively. The lack of sharp diffraction peaks near these two characteristic peaks typically indicates that the SFEM has an amorphous structure, further confirming the complete carbonization of the yak hair waste achieved through the SFE method over a duration of 150 s.

2.1.2. Characterization Result of AC

To characterize the surface properties of the AC derived from yak hair keratin waste, the FT-IR spectra of the synthesized material were scanned in the range of 500−4000 cm⁻¹. As illustrated in Figure S1, the FT-IR analysis indicated that the AC samples prepared at an activation temperature of 600 °C exhibit the strongest tensile vibrational intensity of C-O bonds in the carboxyl group with both carbonyl and hydroxyl groups significantly contributing to the adsorption properties [39]. Consequently, an activation temperature of 600 °C was selected for AC preparation.

Subsequently, the mass ratio between SFEM and KOH was optimized, and the morphologies of the ACs prepared with varying SFEM/KOH mass ratios were characterized using SEM, as shown in Figure 2A. The results indicate that all the AC samples exhibited irregular pore shapes and structures along with the varying degrees of collapse.

A thermogravimetric (TG) analysis of AC was conducted to investigate its structural properties, as illustrated in Figure 2B. The initial mass loss observed below 200 °C was attributed to the loss of water molecules in the samples, while the subsequent mass loss at the temperatures between 200 °C and 400 °C is primarily due to the volatilization of ammonia, carbon dioxide, water, and sulfur-containing compounds [40]. As shown in Figure 2B, the addition of KOH may reduce the thermal stability of the AC [41], resulting in increased weight loss at higher temperatures of 400–800 °C with a greater proportion of KOH. The weight loss for AC₃ and AC₅ is pronounced, whereas AC₂ and AC₄ show greater stability. The surface functional groups of the AC samples (AC₂–AC₅) were characterized using FT-IR spectroscopy, as shown in Figure 2C. The weak peak in the 1000−1200 cm⁻¹ range corresponded to the -C-O bonds [42]. The peak at 3411 cm⁻¹ was associated with the stretching vibration of the hydroxyl groups in AC [42], while the peak at 1500−1580 cm⁻¹ suggested the presence of -COOH functional groups. The FT-IR spectral analysis indicated that hydroxyl groups are the most abundant functional groups on the AC surface, significantly contributing to the adsorption properties [43]. Among the AC samples, AC₄ exhibited a consistent zeta potential in relation to the functional group of this material, as illustrated in Figure S2.

The adsorption capability of AC prepared with varying SFEM/KOH mass ratios was also evaluated with cationic MB dye as the model adsorbate, as shown in Figure 2D. The results illustrated that AC₄ had the highest adsorption performance among all of the AC materials. The adsorption properties of AC₄ at different pH values and temperatures were further investigated and illustrated with the adsorption kinetic and isothermal simulations (as shown in Figure S3, Tables S1 and S2). The results suggested that the adsorption of MB by AC₄ showed a better fit with the pseudo-first-order model (R² = 0.999) and the Langmuir model (R² = 0.934), while the maximum adsorption capacity of AC₄ for MB was measured at 331.13 mg/g. The stronger adsorption performance exhibited by AC₄ might be related to its surface area and pore size. The N₂ adsorption–desorption isotherm analyses of AC₄ (Figure 2E) indicated that the surface area and volumes of AC₄ were 412.1 m²/g and 3.41 nm, respectively, as shown in Figure 2E.

Based on the above results of SEM, TG, FT-IR, and adsorption performance with MB as the model adsorbate, the formulation of AC₄ was chosen as the optimized AC material for microbead preparation in the subsequent adsorption experiments.

2.1.3. Optimization of the Preparation of AC−Alg Microbeads

In the subsequent adsorption study, AC₄ was utilized to prepare AC−Alg microbeads. As illustrated in Figure 3A, the SEM results of AC–Alg microbeads with varying AC/Alg mass ratios exhibited a rougher texture with increasing AC content with AC−Alg_0.5 exhibiting the roughest surface. Additionally, the FT-IR spectra of the AC−Alg microbeads, as shown in Figure S4, confirmed the successful crosslinking between the particles of AC and Alg.

The TG analysis of the AC−Alg microbeads at varying AC/Alg mass ratios, depicted in Figure 3B, indicates that the initial weight loss observed between 0 °C and 100 °C was attributed to the evaporation of both bound and free water [44]. The thermal degradation of Alg typically occurred at elevated temperatures, specifically between 200 °C and 300 °C, resulting in further weight loss during the degradation of polysaccharide chains of Alg [45]. Among the AC−Alg microbead samples, AC−Alg_0.5 microbeads exhibited the least mass loss.

The N₂ adsorption/desorption isotherm for AC−Alg_0.5 microbeads is illustrated in Figure 3C with the corresponding pore structure parameters listed in

Table 1. BET parameters of AC−Alg microbeads synthesized at varying ratios of AC/Alg.

Materials	SBET (m2/g)	Pore Size (nm)	Vt (cm3/g)
AC−Alg3	16.23	15.61	0.066
AC−Alg2	22.34	16.17	0.093
AC−Alg1	30.36	12.05	0.096
AC−Alg0.5	381.27	2.99	0.287

. The presence of mesopores (2.99 nm) in the AC−Alg_0.5 microbeads was indicated by the type-IV isotherms of the H₂ hysteresis loop behavior [46]. Furthermore, the specific surface area of AC−Alg_0.5 microbeads was measured to be 381.27 m²/g. The result indicated that the specific surface area of the AC−Alg microbeads increased, while the porosity values decreased with the increasing AC content in the AC−Alg microbeads, which aligns with the SEM results discussed earlier. Additionally, compared to the AC−Alg mixed materials reported in previous studies [47], the AC–Alg_0.5 microbeads exhibited a higher specific surface area (as listed in Table S3). Therefore, the mass ratio of AC/Alg (1: 0.5) was identified as optimal for further refinements of the electrospinning preparation conditions, including pH and applied voltage.

The images of AC–Alg_0.5 microbeads prepared at different pH levels are presented in Figure 4A, demonstrating that pH variation could affect the surface charge of the Alg molecular chains. This, in turn, affects the intermolecular interactions and the degree of crosslinking between AC and Alg molecules. At higher pH levels, the deprotonation of carboxyl and hydroxyl groups enhances intermolecular and intramolecular electrostatic repulsion, and the size of the microbeads, resulting in an increase in microbead size from 665 ± 52 μm (pH 7.0) to 743 ± 201 μm (pH 11.0), as illustrated in Figure 4B. Conversely, at lower pH levels, the carboxyl groups on Alg become protonated, affecting the dissolution of Alg in the solution. This leads to a reduction in both the intermolecular and intramolecular interactions between Alg and AC, resulting in an increase in microbead size (696 ± 82 μm) and size distribution (11.8%). At an even lower pH of 3.0, Alg molecules precipitate from the aqueous solution, significantly influencing the formation of AC–Alg microbeads via the electrospinning method. Thus, the smallest size and narrowest size distribution of electrostatically sprayed AC−Alg microbeads (665 ± 52 μm, with a distribution of 7.7%) were achieved at pH 7.0. This suggests that the interaction between AC and Alg is more stable under neutral conditions, promoting the formation of microbeads with uniform size.

Furthermore, the effect of the electrospinning voltage on the preparation of AC–Alg microbeads was investigated under the optimized conditions of pH 7.0 and AC/Alg ratios (AC–Alg_0.5) with a 19G needle. As shown in Figure 4C, increasing the electrospinning from 9 kV to 13 kV resulted in a significant exponential decrease in the size of the AC–Alg microbeads from 1544 ± 60 μm to 610 ± 62 μm. Concurrently, the error range of the prepared microbeads increased from 3.9% to 10.2%. Moreover, an exponential equation (Equation (1)) with R² = 0.980 was derived (Figure S5), which describes the relationship between the diameter of the AC–Alg microbeads and electrospinning voltage (kV) as follows:

$$y=\mathrm{568,660}{e}^{-0.7x}+520$$

(1)

where y represents the diameter of the microbeads (μm) and x represents the electrospinning voltage (kV).

The adsorption capacity results of AC−Alg microbeads toward MB dye, shown in Figure S6, indicated that smaller AC−Alg_0.5 microbeads exhibited better adsorption performance. Therefore, the optimal preparation conditions of AC–Alg microbeads were determined to be at pH 7.0 and an electrospinning voltage of 11 kV, henceforth denoted as AC–Alg_0.5U, for subsequent adsorption measurements.

2.2. Adsorption Capacity of AC−Alg Microbeads

This paper investigated the adsorption characteristics and behavior of AC−Alg_0.5U microbeads in the removal of various types of pollutants, including anionic and cationic dyes (methylene blue (MB) and methyl orange (MO)), as well as an antibiotic (minocycline hydrochloride (MH)).

2.2.1. Adsorption Capacity for Dye Pollutants

Figure 5 illustrates the adsorption of MB and MO onto the AC−Alg_0.5U microbeads under varying pH levels, temperatures and amounts of microbeads. Increasing the temperature raises the molecular kinetic energy [48], thereby enhancing the interactions between the AC−Alg_0.5U microbeads and the dye molecules of MB or MO. Elevated temperatures also promote the swelling of microbead pores, accelerating the internal diffusion of MB or MO molecules on the surface within the pores of the microbeads, ultimately resulting in an increased adsorption of MB or MO onto AC−Alg_0.5U microbeads (see Figure 5A1,B1).

Additionally, the pH of the solution significantly affects the surface charge of the AC−Alg_0.5U microbeads, influencing the availability of binding sites and the electrostatic interactions between the AC−Alg_0.5U microbeads and MB molecules. As indicated in Figure 5A2, the adsorption capacity of MB onto AC−Alg_0.5U microbeads increased with increasing solution pH, reaching a maximum at pH = 10.0. As the pH increased, the AC−Alg_0.5U microbeads demonstrated enhanced electrostatic attraction to MB due to the deprotonation of functional groups, which increased the negative charge on the microbeads (as corroborated by the zeta potential data of AC shown in Figure S2). Conversely, at a lower pH levels, a higher concentration of hydrogen ions leads to the protonation of binding sites, thereby reducing the affinity of the AC−Alg_0.5U microbeads for MB molecules [49]. In contrast, with the anionic MO dye, electrostatic repulsion occurs at pH levels exceeding the dissociation constant of MO, which hinders adsorption. Nonetheless, the removal efficiencies of MO across different pH levels remained relatively consistent, indicating that AC−Alg_0.5U microbeads can effectively adsorb MO dye effectively from water solution over a wide pH range [50].

Figure 5A3,B3 illustrate the effect of dosage of AC−Alg_0.5U microbeads on adsorption performance, which showed a gradual decrease as the adsorbent dosage increased. This phenomenon occurs because although the number of adsorption sites increases with higher adsorbent dosages, the utilization of these sites per unit mass of adsorbent decreases. As the adsorbent dosage continues to rise, the adsorption sites on the surface become increasingly occupied, leading to a decline in the adsorbent’s unit adsorption capacity.

To further analyze the adsorption performance of MB and MO onto AC−Alg_0.5U microbeads, both pseudo-first-order and pseudo-second-order kinetic models were applied to the experimental data (see Figure 5A4,A5,B4,B5). The results revealed that the pseudo-first-order model (R² = 0.877) fitted better for MB, indicating that the adsorption rate may be more closely related to the concentration of the reactants than to that of the product. Conversely, the adsorption of MO at various temperatures and pH levels is well described by the pseudo-second-order model with an R² value of 0.999 (as listed in

Table 2. The adsorption kinetics parameters of MB or MO on AC−Alg_0.5U microbeads.

Kinetic Model		Kinetic Parameters (MB)	Kinetic Parameters (MO)
Pseudo-first-order	Qe,exp (mg/g) k1 (min−1) Qe,cal (mg/g) R2	861.1 3.24 × 10−4 859.6 0.877	203.3 0.0212 60.43 0.905
Pseudo-second-order	k2 (g·min·mg−1) Qe,cal (mg/g) R2	7.65 × 10−5 105.26 0.856	9.28 × 10−4 207.04 0.999

). This finding suggests the possibility of a chemisorption process occurring between MO and AC–Alg_0.5U microbeads.

Furthermore, the Langmuir model (R² = 0.984 for MB; R² = 0.999 for MO) demonstrated a better fitting compared to the Freundlich model, as listed in

Table 3. The adsorption isotherm parameters of MB or MO onto AC−Alg_0.5U microbeads.

Isotherm Model		Isotherm Parameters (MB)	Isotherm Parameters (MO)
Langmuir	KL (L/mg) Qm (mg/g) R2	0.0119 1038.9 0.984	0.526 206.2 0.999
Freundlich	KF [(mg/g) (L/mg) (1/n)] 1/n R2	19.33 0.702 0.949	75.56 0.286 0.851

. This suggests that a monolayer of homogeneous adsorption occurs on the surface of AC−Alg_0.5U microbeads (see Figure 5A6,A7,B6,B7). These results indicated that diffusion may be a significant factor influencing the adsorption of both MB and MO onto AC–Alg_0.5U microbeads.

2.2.2. Adsorption Capacity for Minocycline Hydrochloride

As shown in Figure 6, the effects of temperature, pH, and adsorbent dosage on the adsorption capacity of MH onto AC−Alg_0.5U microbeads were investigated. Within the temperature range of 15 °C to 25 °C, the adsorption capacity increased, which can be attributed to the enhanced movement of the MH molecules at higher temperatures. As the temperature rises, the acceleration of molecular diffusion facilitates increased interactions between MH molecules and the pore sites of AC−Alg_0.5U microbeads, thereby enhancing adsorption [51]. However, at higher temperatures above 25 °C, excessive molecular movement may lead to collisions that hinder effective adsorption on the microbead surface, ultimately reducing adsorption efficiency. This decrease might be further exacerbated by the thermal instability of MH at elevated temperatures.

Additionally, increasing the microbead dosage (Figure 6C) resulted in a gradual decrease in adsorption capacity, which is similar to the trends observed in previous studies on the adsorption of MB and MO by AC–Alg_0.5U microbeads.

To analyze the adsorption process, pseudo-first-order and pseudo-second-order kinetic models were applied to fit the experimental data (Figure 6D,E) [52]. The results indicated that the pseudo-second-order model (R² = 0.999) provided a better fit (as listed in

Table 4. The adsorption kinetics parameters of MH onto AC−Alg_0.5U microbeads.

Kinetic Model		Kinetic Parameters (MH)
Pseudo-first-order	Qe,exp k1 (min−1) Qe,cal (mg/g) R2	977.03 0.70 × 10−4 845.62 0.643
Pseudo-second-order	k2 (g·min·mg−1) Qe,cal (mg/g) R2	0.00285 144.1 0.999

) with the theoretical adsorption value closely aligning with the experimental values. This finding suggests that the adsorption may be consistent with a chemisorption mechanism.

Moreover, the Langmuir model (R² = 0.984) provided a slightly better fit compared to the Freundlich model (R² = 0.980), as listed in

Table 5. The adsorption isotherm parameters of MH onto AC−Alg_0.5U microbeads.

Isotherm Model		Isotherm Parameters (MH)
Langmuir	KL (L/min) Qm (mg/g) R2	0.00716 1694.2 0.984
Freundlich	KF [(mg/g) (L/mg) (1/n)] 1/n R2	16.15 0.815 0.980

. This suggests that it is highly likely that a monolayer of homogeneous adsorption occurred on the surface of the microbeads.

2.3. Reusability of the Microbeads

With MB as the adsorbent, the reusability of the AC−Alg_0.5U microbeads was evaluated mainly based on the decrease in adsorption capacity, as shown in Figure 7. After the first adsorption cycle, the AC−Alg_0.5U microbeads were regenerated with 0.1 M HCl or 0.1 M NaOH to desorb the adsorbed dye molecules, rinsed to neutrality, and re-employed in the consecutive adsorption–regeneration cycles.

After six consecutive cycles, the adsorption capacity of AC–Alg_0.5U microbeads declined from 182.7 mg/g to 132.0 mg/g when 0.1 M HCl was used as eluent, whereas an equivalent protocol with 0.1 M NaOH reduced the adsorption capacity from 179.2 mg/g to 106.4 mg/g (Figure 7). The superior performance of HCl is attributed to protonation of surface sites. H⁺ ions bind to the adsorption sites of AC−Alg_0.5U microbeads, promoting ion exchange interactions and facilitating the release of MB molecules from the AC−Alg_0.5U microbeads during acidic desorption [53]. In contrast, the high Na⁺ concentration in NaOH solution would occupy the negatively charged group (-COO⁻) sites of Alg, thereby displacing the adsorbed MB into the solution. However, Na⁺ showed weak competitiveness and the -COO⁻ group of Alg remains, and some of the adsorption sites may be retained, resulting in incomplete desorption. Consequently, 0.1 M HCl provides a stronger desorption ability than 0.1 M NaOH. Overall, the above results suggest that AC–Alg_0.5U microbeads were an adsorbate capable of adsorbing various water pollutants—mainly, the microbeads could be easily separated and regenerated.

3. Conclusions

In this study, keratin-rich yak hair waste was rapidly carbonized by SFE treatment for 150 s at 1.5 MPa to obtain carbonized SFEM, which was followed by activation with KOH, yielding yak hair-based AC within 1 h at 600 °C. The synthesized yak hair-based AC was combined with Alg in various mass ratios, pH, and applied voltage to prepare small-sized AC−Alg microbeads using the electrospinning method, which was utilized for pollutant removal in water. With the optimized AC−Alg_0.5U microbeads (665 ± 52 μm, wet state), the adsorption capacity of AC−Alg_0.5U was systematically studied with different kinds of pollutants, including MB, MO, and antibiotic MH, which supported that the AC−Alg_0.5U microbeads exhibit favorable adsorption capacity for various pollutants with model concentration. This paper identifies the following optimum parameters for AC–Alg_0.5U: for MB—35 °C, pH 10.0, dosage 0.1 g L⁻¹; for MO—15 °C, pH 2–10, dosage 0.1 g L⁻¹; and for MH—25 °C, pH 8.0, dosage 0.1 g L⁻¹ under the model concentrations used in this experiment. Six adsorption–desorption cycles with 0.1 M HCl restore 72% of the original adsorption capacity, confirming good reusability. This paper highlights the use of keratin-rich yak hair waste into the AC−Alg microbeads as potential adsorbents, which has a certain degree of reusability in terms of adsorption performance.

4. Materials and Methods

4.1. Materials and Reagents

Yak hair was supplied by the Gannan region of Gansu Province, China. The reagents used in this study were analytical grade reagents and purchased from Tianjin Bailens Biotechnology Co., Ltd. (Tianjin, China), Tianjin Damao Chemical Reagent Factory (Tianjin, China), Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China), Shanghai Zhongtai Chemical Reagent Co., Ltd. (Shanghai, China), Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China), Tianjin Best Chemical Co., Ltd. (Tianjin, China), Shanghai Zhongqin Chemical Reagent Co., Ltd. (Shanghai, China), and Agilent Technologies Ltd. (Santa Clara, CA, USA) without undergoing additional purification. All solutions were prepared with ultrapure water from a Milli-Q water purification system (Millipore, Burlington, MA, USA).

4.2. Preparation of Activated Carbon

In this experiment, AC was prepared from yak hair sourced from the Gannan region of China. The preparation process involved the following steps, as illustrated in Scheme 1: (1) Carbonization: After thorough cleaning and drying, the yak hair was rapidly carbonized using steam flash explosion (SFE) technology at 1.5 MPa for 150 s with a QB-300B model continuous feeder (Tsing-Gentle Eco-technology (Suzhou) Co., Ltd., Suzhou, China), and then washed thoroughly with ultrapure water, following drying to remove impurities. The resulting carbonized material was termed SFEM. (2) Activation: The SFEM was activated using KOH as the reference [54]. SFEM was mixed with KOH at various ratios (g/g, SFEM/KOH, 1: 2, 1: 3, 1: 4, and 1: 5) to prepare AC at 600 °C (see Figure S1 for the specific temperature selection) for 1 h under N₂ atmosphere with a heating rate of 5 °C/min. After activation, the materials were washed with ultrapure water until neutralization. Subsequently, they were dried, sieved, and ground to yield the final AC products (designated as AC₂, AC₃, AC₄, and AC₅).

4.3. Preparation of AC−Alg Microbeads

In this experiment, AC–Alg microbeads were prepared with an electrostatic spinning method. The preparation process of the AC−Alg microbeads involved three steps. (1) Dissolution: Alg was dissolved in 100 mL water and stirred at 80 °C for 1 h. Subsequently, AC was added to Alg solution in predetermined ratios (AC/Alg, g/g). The mixture was then stirred continuously and sonicated to ensure homogenization and eliminate air bubbles. (2) Electrostatic spinning: Utilizing the principle that the high viscosity of Alg was subjected to high voltage during electrostatic spinning, which could effectively counteract the aggregation effects associated with high concentrations of Alg, allowing for the formation of smaller-sized microbeads, the mixed solution (AC and Alg) was dripped into 2% CaCl₂ solution using an electrostatic spinning device and stirred for 30 min to achieve stable curing. (3) Post-preparation processing: After preparation, the microbeads were thoroughly washed with ultrapure water to remove impurities and unreacted CaCl₂ and then freeze-dried to obtain AC−Alg microbeads.

During the preparation of the AC–Alg microbeads, the effects including the ratios of AC/Alg (g/g 1: 3, 1: 2, 1: 1, and 1: 0.5, designated as AC−Alg₃, AC−Alg₂, AC−Alg₁, and AC−Alg_0.5, respectively, electrospinning at 10 kV with 19G needle, the inner diameter of 19G needle is 0.75 μm), the pH of the mixed solution of AC/Alg (3.0, 5.0, 7.0, 9.0, and 11.0, electrospinning at 11 kV with 19G needle), and the applied voltage (9, 10, 11, 12, and 13 kV) were investigated. The images of the microbeads were taken using a fluorescence microscope (Olympus IX73, Tokyo, Japan), 4×.

4.4. Characterization of AC and AC−Alg Microbeads

The crystal structure of the SFEM samples was analyzed using the X-ray diffraction (XRD) measurement (PANalytical Empyrean, Almelo, The Netherlands), employing Cu Kα radiation (λ = 1.54056 Å) over a 2θ range of 10° to 80°. The morphology of SFEM, AC, and AC−Alg microbeads was characterized using scanning electron microscopy (SEM, Hitachi SU8220, Tokyo, Japan). The surface charge of AC was detected by a Nano ZSE MAL instrument (Malvern, UK) at different pH levels. The adsorption capacity was evaluated through N₂ adsorption–desorption isotherms at 77 K using a surface area and porosity analyzer (Mike ASAP 2460, Norcross, GA, USA). During the desorption phase, the pore size distribution was estimated using the Barrett–Joyner–Halenda (BJH) analysis method [55]. Fourier transform infrared (FT-IR) spectra of the AC and AC−Alg microbeads were characterized using a Fourier transform infrared spectroscopy (Thermo Fisher Nicolet iS10, Waltham, MA, USA). The thermal characterization of AC and AC−Alg microbeads was conducted with thermogravimetric (TG) analysis (NETZSCH TG 209 F3, Selb, Germany) to assess the weight changes during the heating.

4.5. Adsorption Experiment

The adsorption capacity of the optimized AC−Alg_0.5U microbeads was evaluated using various model adsorbates including the cationic dye MB, anionic dye MO, and the antibiotic MH. For the adsorption experiments, 0.1 g/L of AC−Alg_0.5U microbeads was added to the adsorbate solution and agitated on a shaker at 200 rpm (IS-RDV1, Crystal, MN, USA). The adsorption capacity of the AC−Alg_0.5U microbeads for these adsorbates was investigated at varying temperature (15–35 °C) and pH (2.0–10.0). The adsorption capacity was calculated using Equation (2) [56].

$$Q_t=\left(C_0-C_t\right)V/m$$

(2)

where C₀ represents the initial concentration (mg/L) of the adsorbates, C_t (mg/L) represents the concentration of the adsorbates at reaction time t, V represents the volume of the solution (L), m represents the mass of the adsorbent, and Q_t represents the adsorption capacity (mg/g) at the reaction time t.

When equilibrium is reached, the equilibrium adsorption capacity (Q_e, mg/g) is calculated by substituting C_t with the equilibrium concentration C_e, and performed in triplicate, the results are expressed as mean ± standard deviation (SD).

The adsorption kinetics of the above pollutants onto AC–Alg_0.5U microbeads were studied with the batch adsorption experiment at room temperature with samples collected at 5 min intervals for analysis using a UV-Vis spectrophotometer (GENESYS 10S UV-VIS, Madison, WI, USA) (the initial wavelength for MH was set at 355 nm [57], the initial wavelength for MO was set at 466 nm [58], and the initial wavelength for MB was set at 664 nm [59]). The experimental data were fitted using both the pseudo-first-order model (Equation (3)) and pseudo-second-order model (Equation (4)) [60].

$$ln\left(Q_e-Q_t\right)=ln{Q}_e-k_{1}t$$

(3)

$$t/Q_t=1/k_2{Q}_e^2+1/Q_e \times t$$

(4)

where Q_e and Q_t represent the adsorption capacity (mg/g) at equilibrium and specific moments t, respectively, and $k$₁ and $k$₂ represent the rate constants associated with the quasi-primary kinetic model equation (min⁻¹) and quasi-secondary kinetic model equation (g·min·mg⁻¹), respectively.

Adsorption isotherms described the interactions between the adsorbate and adsorbent at the equilibrium state during the adsorption process. In this experiment, Langmuir (Equation (5)) and Freundlich adsorption isotherms (Equation (6)) [61] were employed to fit the equilibrium adsorption data across various concentrations of MB, MO, and MH, respectively.

$${\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$Q_e$}\right.}={\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$K_L{Q}_m{C}_e$}\right.}+{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$Q_m$}\right.}$$

(5)

$$\mathit{ln}{Q}_e=\mathit{ln}{K}_F+1/n\mathit{ln}{C}_e$$

(6)

where K_L represents the Langmuir constant (L/mg), Q_m represents the maximum adsorption capacity (mg/g), 1/n represents an empirical constant indicating the strength of adsorption, and K_F represents the adsorption rate constant ((mg/g) (L/mg) ^(1/n)).

For cycling experiments, saturated AC−Alg_0.5U microbeads were regenerated via the addition of 0.1 M HCl or 0.1 M NaOH followed by stirring at room temperature for 1 h (200 r/min). After regeneration, the AC−Alg_0.5U microbeads were rinsed thoroughly with deionized water to remove any residual acid or base until the solution reaches neutrality. This cycle was repeated six times to assess the reusability of the AC−Alg_0.5U microbeads.