Enhanced Stability and Adsorption of Cross-Linked Magnetite Hydrogel Beads via Silica Impregnation

Abstract

Hydrogel-based adsorbents have gained increasing recognition in recent years due to their promising potential for pollutant removal. However, conventional hydrogels often suffer from low mechanical strength over prolonged use. Therefore, this study explores the incorporation of silica extracted from bamboo culm (Dendrocalamus asper) to enhance the mechanical stability of hydrogel beads composed from carboxymethyl cellulose (CMC), chitosan (CS), and magnetite ferrofluid (Fe₃O₄), through cross-linking. We hypothesize that silica enhances the mechanical properties of magnetite hydrogel beads without compromising their adsorption capacity. The extracted silica was confirmed with FTIR and EDS analysis. The synthesized CMC-CS-Fe₃O₄-Si hydrogel beads were characterized using FTIR and SEM. Its stability was assessed through dry weight loss measurements, while its adsorption efficiency was evaluated using batch adsorption experiments. The silica-incorporated hydrogel exhibited enhanced mechanical and thermal stability under various pH and temperature conditions, without negatively affecting its adsorption performance, achieving maximum adsorption capacities of 53.00 mg/g for Cr (VI) and 85.06 mg/g for Cu (II). Desorption and regeneration studies confirmed the reusability of the hydrogel for more than four cycles. Overall, the interaction between the hydrogel and silica resulted in excellent adsorption performance, improved mechanical properties, and long-term reusability, making this a promising hydrogel adsorbent for wastewater remediation.

1. Introduction

The current trends of rapid economic and industrial development are important to maintain a sustainable economy. Nevertheless, it is believed that this growth is closely connected to the increasing issues of chemical pollution, especially heavy metals in the environment. Heavy metals such as lead, chromium, cadmium, copper, and arsenic are dangerous contaminants, which can cause detrimental effects if exposed to water, as this can contaminate the environment and pose major health hazards to both aquatic life and humans [1]. These metals typically originate from a variety of sources, including mining, industrial discharges, agricultural runoff, and inappropriate disposal of waste [2,3]. The accumulative properties of heavy metals typically lead to their build up in water and human bodies, inflicting neurological impairments, cancer, and organ damage [4,5]. Therefore, it is critical to tackle this issue by employing effective techniques to reduce toxic heavy metals such as Cr (VI) and Cu (II) in water for a safer and more sustainable environment.

Several approaches have been utilized to treat heavy metal pollution, including chemical precipitation, ion exchange, membrane filtration, and electrochemical treatments [6,7,8,9]. Although efficient, these methods frequently struggle with operational challenges such as high energy consumption, excessive operating cost, and generation of secondary pollution [10]. Therefore, compared to the aforementioned approaches, adsorption techniques utilizing green technologies such as bio-sorbents or composite hydrogels have emerged as promising solutions to remove heavy metals, owing to their simple operation. Moreover, current studies have highlighted the significant potential of hydrogel technologies, due to their good water retention, tunable structural properties, and chemical functional groups [11].

Hydrogels are 3D polymer networks that are capable of absorbing and retaining large volumes of water while remaining structurally intact [12,13]. These hydrogels, which are commonly derived from non-toxic material such as chitosan, cellulose, starch, and alginate, have been extensively employed in various industries such as agriculture and medicine, as well as in commercial products and wastewater treatments. Our previous study [14] developed a magnetic hydrogel by integrating chemical functional groups, specifically the carboxyl groups from carboxymethyl cellulose (CMC) and the amine groups from chitosan (CS), for the removal of Cr (VI). However, these magnetite hydrogel beads exhibited poor mechanical strength over prolonged use, as excessive swelling compromised their stability, eventually causing the hydrogel to break into smaller fragments. Similarly, İlktaç et al. [15] has highlighted a common limitation faced by composite hydrogel, which is the poor stability due to its highly swollen structure. Hydrogels have a tendency to swell excessively, resulting in a soft and brittle texture, making them prone to fragmentation when exposed to environmental stress [16]. This creates significant challenges in practical application when separating the solid from the aqueous phase during the post-adsorption process.

Therefore, many researchers have attempted to develop sophisticated hydrogels with superior mechanical properties, high elasticity, and a robust structure. For instance, Yang et al. [17] developed a double network of hydrogels with enhanced strength and elastic properties by cross-linking polyvalent anions with short-chain chitosan. This system disperses energy more effectively, improving its durability, and recovery after stress is alleviated. Other attempts, including fiber-reinforcement, supramolecular interaction, and well-aligned microstructure, have also been explored to enhance the mechanical stability of hydrogels [18]. However, they often require complicated procedures with energy-intensive processing. A simpler and more straightforward method to improve hydrogel’s mechanical properties can be achieved by incorporating fillers or nanoparticles to produce strong hydrogel. Silica (SiO₂) particles, for example, have been found to enhance the mechanical strength of hydrogel through their role as filler and their interaction with the hydrogel network. A previous study has also discovered the function of silica in enhancing mechanical strength by effectively dissipating energy through hydrogen bonding and the Van der Waals interaction between mesoporous silica and alginate/poly(acrylamide) hydrogel [19]. Moreover, another study [20] has reported an improved stability in hydrolytic polyacrylamide hydrogel by incorporating 2 wt% of SiO₂, attributed to the interaction between the OH group of SiO₂ and the amide group in PAM.

Our previous study [14] has demonstrated the potential use of composite hydrogel for heavy metal adsorption, but mechanical instability remains a challenge. Therefore, this study aims to improve hydrogel stability through the simple process by incorporating silica, maintaining high adsorption performance while improving its stability. This study intends to extract silica from natural sources such as bamboo, because it is one of the plant-based sources that is rich in silica. It has been reported that many parts of bamboo are composed of valuable amounts of silica [21,22]. The extracted silica will then be characterized using FTIR, EDS, and SEM. Following the successful extraction and incorporation of silica into the hydrogel matrix, the mechanical and thermal stabilities, as well as the adsorption capacity of the newly improved magnetite hydrogel beads (CMC-CS-Fe₃O₄-Si) were investigated to broaden their potential in practical application for pollutant removal.

2. Materials and Methods

2.1. Chemical and Reagents

The raw material of bamboo culm (Dendrocalamus asper) was supplied from West Sumatra, Indonesia. Other chemicals, such as sodium hydroxide (NaOH, 99.99%) and hydrochloric acid (HCl, 35–37%), were purchased from Kanto Chemical Co. (Tokyo, Japan). Chitosan (molecular weight: 100,000–300,000 Da) was purchased from Acros Organics (Shanghai, China). Carboxymethyl cellulose (CMC) powder was supplied by Katayama Chemical (Osaka, Japan). Iron (III) chloride (FeCl₃, 38%) was purchased from Hayashi Pure Chemical Ind. (Osaka, Japan). Acetic acid (CH₃COOH, 99.7%), potassium dichromate (K₂Cr₂O₇, 99.5%), copper standard solution (Cu, 1000 mg/L), methylene blue (C₁₆H₁₈N₃SCI, 98.5%), iron (II) sulfate heptahydrate (FeSO₄·7H₂O, 99–102%), ammonium fluoride (NH₄F, 97%), boric acid (H₃BO₃, 99.5%), ethanol (C₂H₆O, 99.5%), ammonium molybdate (NH₄)₆Mo₇O₂₄), oxalic acid (C₂H₂O₄, 99.5–102%), and ascorbic acid (C₆H₈O₆, 99.6%) were obtained from Kanto Chemical Co. (Tokyo, Japan). Tri-sodium citrate (C₆H₅O₇Na₃·2H₂O, 99%) was purchased from Nacalai Tesque (Kyoto, Japan). All the chemicals and reagents used in this study were of chemical grade.

2.2. Preparation and Extraction of Silica

Primarily, the raw material of the bamboo culm (BC) was cleaned with distilled water and dried in an oven at 60 °C until fully dry. Then, the samples were ground into small granules or powder using a force mill. Around 30 g of sample was subjected to thermal treatment in a muffle furnace (FO 100, Yamato Scientific Co., Tokyo, Japan) at a high temperature of 750 °C for 3 h at a heating rate of 10 °C. The carbonized sample of BC underwent several procedures for the extraction of silica, as reported by [22,23], with modifications. The carbonized BC was treated with 50 mL of 3N NaOH by heating to 100 °C while being magnetically stirred for 2 h. Subsequently, the mixture was filtered and washed thoroughly with boiling water. The filtrate was then allowed to cool to room temperature before the pH was adjusted to 7 using 3N HCl under constant stirring. Once a white gelatinous precipitate formed, the solution was left to age overnight. The sol–gel was centrifuged at 5000 rpm for 15 min to separate the gel from the solution. The gel was then washed with distilled water to remove any chemical residues and dried in a drying oven at 60 °C for 24 h. The extracted product is referred to as bamboo culm silica.

2.3. Characterization of Extracted Silica

The yield of the extracted silica was determined using Equation (1). The properties of the extracted silica compound were characterized by analyzing the functional groups present in its structure using Fourier-transform infrared spectroscopy (ATR-FTIR, Thermo Scientific Nicolet iS10, Waltham, MA, USA). The result obtained was then compared with silica spectra from the reference method described in [22] for preliminary verification. The successful extraction of silica was further confirmed by performing elemental composition analysis via energy-dispersive X-ray spectroscopy (EDS, JIED-2300, Shimadzu, Kyoto, Japan). The morphology and structural shape of the extracted silica were examined using scanning electron microscopy (SEM, Miniscope TM3000, Hitachi-hitech, Tokyo, Japan).

$$\mathrm{Silica}\mathrm{yield}\left(\%\right)={\displaystyle \frac{Weight of silica \left(g\right)}{Weight of BC ash \left(g\right)}}\times 100$$

(1)

2.4. Synthesis and Incorporation of Silica into Hydrogel Beads

By employing similar procedures to those described in [14], we prepared newly synthesized magnetite hydrogel beads with the addition of extracted silica to enhance the structural properties of the hydrogel. Firstly, the hydrogel was synthesized by cross-linking a mixture of chitosan and carboxymethyl cellulose (CMC) in a ratio of 1:3 (w/w), respectively. The mixture was dissolved in 20 mL of 2% acetic acid (v/v) and mixed for 2 h at room temperature before the pH was adjusted to 2 using HCl (1 N) to enhance the homogeneity of the polymers. Simultaneously, a magnetic solution of FeCl₃ and FeSO₄ was prepared in another beaker by combining 1.0 M FeSO₄·7H₂O and FeCl₃ (1:2 molar ratio) in 20 mL of distilled water. In a separate beaker, silica solution was prepared by dissolving extracted silica in 0.1 M NaOH solution to achieve 100 ppm. Subsequently, 2 mL of the prepared magnetite (Fe) solution and 2 mL of the silica solution were added into the CS/CMC mixture. The sample was magnetically stirred for 1 h at room temperature. Using a 10 mL syringe, the sample was slowly added dropwise into 200 mL of a pre-prepared NaOH solution (4% w/v) containing 0.05 M tri-sodium citrate to create magnetite–silica hydrogel beads through in situ co-precipitation. The small beads were left in the NaOH solution for 24 h to ensure uniformity of the precipitation reaction, before being thoroughly washed with distilled water and 0.1 M HCl to remove any remaining alkali. Finally, the beads (CMC-CS-Fe₃O₄-Si) were dried in a drying oven overnight at 60 °C before being used in the adsorption experiment. For comparison, the magnetite hydrogel without silica (CMC-CS-Fe₃O₄) was prepared following similar procedures, excluding the addition of silica.

2.5. Characterization of CMC-CS-Fe₃O₄-Si Hydrogel Beads

ATR-FTIR analysis was employed to identify changes in the functional groups present in the hydrogel beads upon the incorporation of silica using a Thermo Scientific Nicolet iS10 (Waltham, MA, USA). Additionally, the morphological structures of the magnetite hydrogel beads were examined via scanning electron microscopy using a Miniscope TM3000, Hitachi-hitech, Tokyo, Japan. The hydrogel beads were dried in an oven at 60 °C for 24 h prior to these analyses.

2.6. Determination of Swelling Properties of CMC-CS-Fe₃O₄-Si Hydrogel Beads

The swelling properties of the magnetite hydrogel beads were assessed by immersing dry hydrogel beads with a known weight ($m_d$) in distilled water with various pH values of 2, 4, 6, 8, and 10 until the hydrogel beads reached equilibrium swelling. Prior to measuring the final swollen weight ($m_s$) of the hydrogel beads, the remaining excess water on the surface of the swollen hydrogel was removed using blotting paper. The swelling properties of the hydrogel were determined according to the equation below, Equation (2):

$$\mathrm{Swelling}\mathrm{ratio},\mathrm{SWR}\left(\%\right)={\displaystyle \frac{m_s-m_d}{m_d}}\times 100$$

(2)

2.7. Assessment of Mechanical Stability of CMC-CS-Fe₃O₄-Si Hydrogel Beads

To assess the performance of the improved hydrogel formulation, both hydrogel beads (with and without silica) were subjected to mechanical stability tests to compare their results. In brief, known weights for both hydrogel beads were quantified and recorded. The hydrogel beads were then shaken (100 rpm) in distilled water at various pH values (2, 4, 6, 8, and 10) for 60 min at constant temperature of 30 °C. Subsequently, the hydrogel beads were separated from the solution using an external magnet and dried in the oven for 24 h. All the experiments were conducted in duplicate, and the statistical significance tests were conducted as described in Section 2.14. The changes in the dry weight of both hydrogel beads were recorded and quantified following Equation (3) below

$$\mathrm{Dry}\mathrm{weight}\mathrm{loss}\left(\%\right)={\displaystyle \frac{W_i-W_f}{W_i}}\times 100$$

(3)

where $W_i$ represents initial dry weight and $W_f$ is the final dry weight after the beads were shaken in deionized water.

2.8. Assessment of Thermal Stability of CMC-CS-Fe₃O₄-Si Hydrogel Beads

The hydrogel beads (with and without silica) underwent thermal stability tests to assess their stability and durability at different temperatures. Initially, both hydrogel beads (dry) were weighed using analytical balance and the initial weights were recorded ($W_i$). Then, the beads were placed in a conical flask containing distilled water at 40 °C and shaken (100 rpm) for 1 h using a mechanical shaking water bath (BW101, Yamato Scientific Co., Ltd., Tokyo, Japan). The above process was repeated for temperatures of 60 °C and 80 °C. After that, the hydrogel beads were separated and dried in an oven before being weighed to determine the final dry weight ($W_f$). This experiment was conducted in duplicate and subjected to Tukey’s test. The percentage of changes in the weight (dry) was calculated using Equation (4):

$$\mathrm{Dry}\mathrm{weight}\mathrm{changes}\left(\%\right)={\displaystyle \frac{W_i-W_f}{W_i}}\times 100$$

(4)

2.9. Adsorption of Heavy Metals in Aqueous Solution

To confirm the effect of silica impregnation on the adsorption performance of the newly synthesized hydrogel beads (CMC-CS-Fe₃O₄-Si), we tested their performance against the original, CMC-CS-Fe₃O₄, for adsorption of Cr (VI) and Cu (II). Firstly, the experiments were conducted under different pH ranges (from 2 to 10) for 60 min, while keeping other conditions constant to determine the optimum pH for the removal of these metals. Hydrochloric acid (0.1 M) or sodium hydroxide (0.1 M) was used to adjust the pH of the solution. A fixed amount of magnetite hydrogel (50 mg) was placed in 50 mL of a solution of contaminants under constant temperature (30 °C) and subjected to adsorption experiments. After the adsorption experiments, the magnetite hydrogel beads were separated from the aqueous solutions using an external magnet. The remaining metal concentrations of Cr (VI) and Cu (II) in the solutions were quantified using a spectrophotometric heavy metal test kit by Kyoritsu Chemical-Check Lab (Yokohama, Japan). Subsequently, other variables such as contact time and initial metal concentration (10–100 mg/L) were tested to obtain maximum adsorption capacity. All the experiments were carried out in duplicate, and the experimental standard deviations are reported in the graph. The capacities of the magnetite hydrogel beads to remove and adsorb heavy metals were computed using the following equations, Equations (5) and (6)

$$\mathrm{Removal}\left(\%\right)={\displaystyle \frac{C_i-C_f}{C_i}}\times 100$$

(5)

$$\mathrm{Adsorption}\mathrm{capacity},q_e\left(\mathrm{mg}/\mathrm{g}\right)={\displaystyle \frac{C_i-C_f}{W_H}}\times V$$

(6)

where $C_i$ and $C_f$ are the initial and final concentrations of the metal solution, respectively. $q_e$ is the adsorption capacity of the magnetite hydrogel beads in mg/g, $W_H$ is the weight of the hydrogel beads, and $V$ is the volume of the metal solution.

2.10. Desorption Experiment and Regeneration

For the desorption experiment, firstly, the CMC-CS-Fe₃O₄-Si hydrogels were initially loaded with Cr (VI) and Cu (II) ions at initial concentrations of 50 mg/L and 100 mg/L, respectively, performed under optimal adsorption conditions obtained earlier. After separation and drying, the hydrogel beads were shaken in 0.3 M NaOH solution for 5 h to release Cr (VI) ions from the hydrogel matrix. For the desorption of Cu (II), the hydrogel beads were immersed in 0.2 M thiourea solution, adjusted to pH 3 using sulfuric acid, and shaken for 5 h. The concentrations of Cr (VI) and Cu (II) released into the solution were quantified using a heavy metal test kit (Kyoritsu Chemical-Check Lab, Yokohama, Japan). The regeneration process was carried out by immersing the hydrogel in 0.3 M NaOH solution for 1 h, followed by drying in an oven before subjecting to subsequent adsorption experiments. Desorption percentage (%) was calculated using Equation (7)

$$\mathrm{Desorption}\left(\%\right)={\displaystyle \frac{M_2}{M_1}}\times 100$$

(7)

where M₁ is the amount of Cr (VI) initially adsorbed and M₂ represents the released amount of Cr (VI).

2.11. Simultaneous Removal of Cr (VI) and Cu (II)

Simultaneous adsorption of Cr (VI) and Cu (II) in a binary system was performed to explore the potential of CMC-CS-Fe₃O₄-Si and investigate factors affecting their adsorption affinity. In this experiment, 50 mg of CMC-CS-Fe₃O₄-Si hydrogels were added into an aqueous solution containing 100 mg/L of both Cr (VI) and Cu (II), adjusted to pHs 2, 3, and 4.5. The adsorption procedures were performed following the optimum conditions for 5 h at 30 °C, as described previously. The remaining concentrations of the heavy metals were quantified using the test kit by Kyoritsu Chemical-Check Lab (Yokohama, Japan). This experiment was performed in duplicate, and the average values are reported in the graph.

2.12. Effect of Coexisting Ions or Complex Compounds on Adsorption Efficiency

The CMC-CS-Fe₃O₄-Si hydrogel beads (0.05 g) were mixed with 50 mL of Cr (VI) and Cu (II) solutions containing a common anion or cation such as ${\mathrm{SO}}_4^{2-}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{Na}}^{+}$, or citric acid, each at a concentration of 50 mg/L. The concentrations of the Cr (VI) solutions were maintained at 50 mg/L and the pH of the solutions was fixed at 2. Similar procedures were repeated to determine the effect of coexisting ions or complex compounds on Cu (II) removal, at pH 4.5. The adsorption experiments and quantification of heavy metal concentrations were carried out in duplicate as conducted previously.

2.13. Adsorption of Methylene Blue

The hydrogel beads were also evaluated for their ability to remove methylene blue (MB). A stock solution of methylene blue dye was prepared and diluted to the designated concentration for both the removal experiment and the preparation of a standard calibration curve. Adsorption experiments were performed by shaking 0.05 g CMC-CS-Fe₃O₄-Si hydrogel beads in a flask containing 50 mL of MB solution at a pH level of 6 for a predetermined period at a constant temperature of 30 °C [24]. After the separation of the magnetite hydrogel beads, the remaining methylene blue concentration was measured. The MB solution was subjected to UV–Vis spectrophotometry analysis (JASCO, V-530) at 660 nm, and the MB concentration was determined using the pre-prepared standard calibration curve of MB. The removal of MB was assessed using Equation (5).

2.14. Statistical Analysis

The data analyses were performed using MINITAB 21 software. The means values were subjected to one-way and two-way analysis of variance and Tukey’s test (p-value < 0.05).

3. Results and Discussion

3.1. Characteristics of Extracted Silica

The yield of extracted silica from bamboo culm was calculated using Equation (1). The silica obtained from bamboo culm ash was found to be 21.2%. This result indicates that bamboo culm contains a high percentage of silica, in accordance with the literature that highlights bamboo as a plant-based material that is rich in silica [22]. Based on Figure 1, FTIR spectrum of bamboo culm silica shows the formation of strong and intense absorption peaks at 1054 and 795 cm⁻¹. These peaks indicate the asymmetric stretching and bending vibrations of Si−O−Si (siloxane groups) and, respectively, [25,26]. Moreover, a broad band adsorption formed between 3200–3390 cm⁻¹ corresponding to the stretching vibration of Si−OH, which overlapped with the O−H bond formed between the water molecules and the silanol group [21]. The extracted silica exhibited multiple characteristic adsorption peaks for silica components, thereby providing a preliminary verification that the extracted compound from the bamboo culm was silica.

In this study, the elemental composition of the extracted bamboo culm silica was determined using energy-dispersive spectrometry (EDS). Based on the mapping figures (Figure 2), the intensities of energy reveal a significant presence of both Si and O elements, confirming that the main component of the extracted sample is silica. Furthermore, the EDS results in

Table 1. Elemental data of silica from EDS analysis.

Elements	Mass %	Atom %
Si	47.47	33.99
O	52.53	66.01

show two prominent peaks assigned to Si and O elements, with their atomic percentages measured at 33.99% and 66.01%, respectively. This ratio closely aligns with the theoretical atomic ratio of SiO₂, indicating a successful extraction of high purity silica. As reported in [27], bamboo-derived silica typically has a high percentage of SiO₂ content, with minimal values of other constituents, including K₂O and CaO. From the results, it can be deduced that the extracted compound from bamboo culm ash is a high purity silica, with a substantial silica component percentage.

Figure 3 presents scanning electron microscopy (SEM) images of the extracted silica. It is observed that the silica particles exhibit irregular shapes, with variation in their sizes. The precipitation process of the silica granules may have slightly affected the formation of pores and contributed to the observed size variations in the silica structure [23]. In addition, the extracted silica particles show a crystalline structure, with some silica granules appearing as agglomerated fine particles. Based on the SEM micrographs, the extracted silica’s morphology demonstrates well-developed crystallization, forming interconnected Si-O-Si networks.

3.2. Characterization of CMC-CS-Fe₃O₄ Hydrogel Beads Impregnated with Silica

The FTIR spectra of newly synthesized magnetite hydrogel beads containing silica (CMC-CS-Fe₃O₄-Si) and the original formulation are shown in Figure 4. It is observed that the broad peak of O−H bending and N−H vibration at 3288 cm⁻¹ in CMC-CS-Fe₃O₄ shifted to 3290 cm⁻¹ after silica incorporation [28]. On the other hand, the successful interaction between CMC and chitosan was indicated by the formation of amide II (−NH bending) and the COO⁻ peak at 1586 cm⁻¹ in CMC-CS-Fe₃O₄ hydrogel [29], and the number is reduced to 1579 cm⁻¹ after the addition of silica. It is worth noting that the presence of an adsorption peak at 1411 cm⁻¹ in CMC-CS-Fe₃O₄ is attributed to carboxylate (symmetric), C−H stretching vibration, and C−N vibration [28,30,31]. Upon the addition of silica, the number changed to a lower wavenumber of 1405 cm⁻¹ and it was predicted that the shifting of the band could be due to the increased interaction of the hydrogel polymers with SiO₂ particles.

The peak at 1036 cm⁻¹ in CMC-CS-Fe₃O₄ indicates the C−O vibration [32], and it shifted to 1055 cm⁻¹ after silica addition. The distinctive peak at 590 cm⁻¹ is assigned to the Fe−O stretching band in the CMC-CS-Fe₃O₄ formulation, and after interaction with silica, the peak transformed to a lower wavenumber at 580 cm⁻¹ [33]. In addition, the adsorption bands of −OH vibration at 1322 cm⁻¹ significantly decreased after the addition of silica, suggesting that the O−H may be chemically bonded to silicon dioxide. A similar observation was also reported in [34]. Moreover, in the CMC-CS-Fe₃O₄-Si spectrum, the formation of a new peak has been detected at 877 cm⁻¹, which could be identified as the silanol functional group [26]. At the same time, the adsorption peak at 1055 cm⁻¹ observed in CMC-CS-Fe₃O₄-Si may also correspond to the Si−O−Si functional group [28]. This finding suggests that the silica was successfully incorporated into the network of the hydrogel beads matrix.

The SEM images of the magnetite hydrogel beads (before and after the incorporation of silica) are presented in Figure 5. The newly formulated hydrogel showed notable changes in its structure after the incorporation of silica. As seen in Figure 5a,c, the silica-containing hydrogel exhibited a slightly smoother surface with agglomerated particles attached to it, whereas the original hydrogel had a wrinkled texture with a fairly random surface pattern. The incorporation of silica contributed to a uniform distribution and denser surface, which is due to the silica functionality that acts as a filler for better mechanical properties. Furthermore, the cross-sectional view in Figure 5b,d reveals that hydrogel beads containing silica have a rough surface, with the presence of pores on the structure, compared to the hydrogel without silica. This indicates that silica incorporation slightly improves the visibility of void pores by improving the permeability while retaining the three-dimensional network of the hydrogel shape [35,36]. A previous study [37] also supports this finding, mentioning that silica has been utilized to enhance the mechanical characteristics of chitosan hydrogels and build extremely porous membrane systems.

3.3. Swelling Properties of CMC-CS-Fe₃O₄-Si Hydrogel Beads

Swelling percentage generally signifies the amount of water penetration into the pores of the hydrogel beads, reflecting their ability to retain a large amount of water. Figure 6 displays the swelling percentages of CMC-CS-Fe₃O₄ and CMC-CS-Fe₃O₄-Si hydrogel beads at different pH levels. It can be observed that CMC-CS-Fe₃O₄-Si has a notable increase in the swelling rate compared to the hydrogel without silica. This is likely due to silica’s hydrophilic properties, which allow it to bond with water molecules and improve water retention. Although some studies hypothesize that silica reduces swelling by increasing polymer cross-linking density, there are many studies which suggest separate findings on this matter. Previous studies have supported our results that silica incorporation contributes to higher swelling performance in CMG-Bt-Si (carboxymethyl guar gum-bentonite-silica) and polyacrylamide (PAM)/hydroquinone (HQ)– HMTA containing silica, as silanol groups promote binding with water molecules [38].

In addition to that, the swelling percentage of the CMC-CS-Fe₃O₄-Si hydrogel beads was observed to be varied across pHs, as shown in Figure 6. The swelling percentage increased as the pH increased from 2 to 8, but declined once the pH reached 10. These conditions are due to the abundant carboxyl groups in CMC, which remain protonated, reducing the electrostatic repulsion between polymer chains. This leads to a more compact structure, limiting water uptake. Above neutral pH, ionization of the carboxyl groups occurred, enhancing the repulsion between polymer chains and resulting in a higher swelling ratio. However, a decrease in swelling percentage was observed at pH 10, consistent with the findings from a previous study [39]. This is explained by the effect of Fe ions, which interferes with the repulsion between carboxyl groups, reducing the swelling of the magnetite hydrogel beads. Additionally, abundant sodium cations at high pH may also constitute a shielding effect that weakens the electrostatic repulsion and limits swelling [38].

3.4. Assessment of Mechanical Stability of CMC-CS-Fe₃O₄-Si

The mechanical stability of the newly formulated CMC-CS-Fe₃O₄-Si beads and the magnetite hydrogel beads without silica (CMC-CS-Fe₃O₄) were compared under various pH conditions. Based on Figure 7, the addition of silica significantly ($p$ < 0.05) improved the stability of the magnetite hydrogel beads by reducing the percentage of dry weight loss by 69.75–79.90% compared to the original formulation. This improvement aligns with other studies that have utilized microcrystalline cellulose/nano-SiO₂ [40] and chitosan/poly(methyl methacrylate)/silica [41]. The enhanced mechanical stability of the magnetite hydrogel beads may be attributed to the strong hydrogen bonding between silica and the hydroxyl group on the hydrogel polymer, resulting in a more elastic network [42]. Silica can also act as an adhesive filler by promoting greater interfacial adhesion between the hydrogel matrix, providing structural integrity and leading to increased mechanical strength and a resistance to deformation [43,44].

It is clearly observed that CMC-CS-Fe₃O₄-Si has the highest stability at pH 2, as indicated by the lowest dry weight loss (7.69%), compared to 38.24% for CMC-CS-Fe₃O₄. This can be explained by the acidic condition which causes protonation of carboxylate ions, reducing repulsive force and increasing cross-linking interaction between polymers, resulting in a denser and more robust hydrogel integrity. The mechanical properties of CMC-CS-Fe₃O₄-Si gradually declined as the pH increased, reaching the highest dry weight loss (12.86%) at pH 8. However, this value was greatly improved to 9.98% at pH 10. At a higher pH, increased carboxylate ionization in the hydrogel matrix leads to a stronger electrostatic repulsion, expanding the mesh dimension of the hydrogel and promoting a higher swelling ratio. This phenomenon indirectly signifies a weakened cross-linking density which causes breakage of some cross-link chains and structural degradation, leading to higher dry weight loss. This has been validated by [45], who reported that higher pH disrupts the cross-link between polymers and accelerates polymer deformation. However, at pH 10, swelling of the hydrogel becomes reduced due to the screening effect of Na⁺, as previously mentioned in [38], thereby strengthening the polymer interaction and minimizing the degradation of the hydrogel structure.

3.5. Assessment of Thermal Stability of CMC-CS-Fe₃O₄-Si

An ideal hydrogel should exhibit great thermal stability across various temperature ranges. To evaluate this characteristic, a series of experiments was performed to measure the percentage of dry weight loss of CMC-CS-Fe₃O₄-Si hydrogel beads after immersion in solutions at 40 °C, 60 °C, and 80 °C. The results were compared with those for the CMC-CS-Fe₃O₄ beads. As presented in Figure 8, all the hydrogels demonstrated an increased trend in dry weight loss with rising temperature. As the temperature increased, the rate of degradation also increased due to the breakdown of cross-link bonds, which weakens the hydrogel network. Interestingly, it can be clearly observed that the incorporation of silica positively enhanced the thermal stability of the magnetite hydrogel beads, resulting in a lower percentage of dry weight loss by 50.86–59.14% ($p$ < 0.05). This has been proven by [46], who demonstrated that interaction of the polymer with nano-SiO₂ have greatly increased the thermal stability of the gel. The hydrogen bonds and covalent interactions between silica and polymer chains generate a more stable chains and networks of hydrogel, reinforcing the hydrogel structure against thermal degradation. Additionally, silica particles can act as a thermal barrier by slowing down the penetration of thermal energy [47], while also improving interfacial stability and crystallinity, resulting in enhanced thermal stability [48]. Since silica has relatively high thermal conductivity, it can dissipate the heat more efficiently, increasing the thermal stability of the hydrogels and reducing structural breakdown. The results obtained validate our hypothesis in this research, as the mechanical stability of CMC-CS-Fe₃O₄ was significantly enhanced after the addition of silica particles.

3.6. Comparative Study on Adsorption Performance of Hydrogel After Silica Incorporation

3.6.1. Influence of pH

According to Figure 9 and Figure 10, notable differences in the removal percentage of Cr (VI) and Cu (II) were observed with the changes in initial pH. Based on Figure 9, both types of hydrogels showed similar trends of decreasing performance over increasing pH, with the highest removal of Cr (VI) observed at pH 2. This is mainly due to the protonation of amine functional groups in chitosan, resulting in strong electrostatic interactions with dominant structures of Cr (VI) anions such as HCrO₄⁻ and Cr₂O₇²⁻. Ref. [49] has reported that HCrO₄⁻ and Cr₂O₇²⁻ are more easily adsorbed by a protonated polymer at an acidic pH, compared to CrO₄²⁻ which is more prevalent at a higher pH. At a higher pH, the deprotonation of hydrogel’s functional group takes place, leading to repulsion with anionic Cr (VI), causing the adsorption of Cr (VI) to decline.

For Cu (II), the results demonstrate a rapid increase in the removal percentage from pH 2 to 4.5, followed by a fluctuating trend until it reaches a final pH of 10 (Figure 10). The hydrogel beads demonstrated the lowest removal percentage at pH 2, likely due to the abundant presence of H⁺, which competes with cations (Cu²⁺) for binding sites. Additionally, at a low pH, the functional groups of the hydrogel, particularly carboxyl (-COOH), become more prone to protonation, thereby reducing their ability to bind with Cu²⁺ [50]. As the pH increases to 4.5, a rapid increase in the removal percentage is observed due to the reduced availability of H⁺ ions, which allows for the deprotonation of the functional groups and creates optimal conditions for the interaction with Cu²⁺. Conversely, at pH 6 and above, the Cu (II) removal decreases dramatically and becomes ineffective since the copper precipitates into its hydroxide forms. Overall, this finding signifies that the pH of a solution is important in the adsorption process as it strongly affects the ionization state and the charge of the metals and adsorbent.

Simultaneously, the incorporation of silica was observed to enhance the uptake of heavy metals, with an 8.72% increase for Cr and 34.79% increase for Cu compared to CMC-CS-Fe₃O₄. To be specific, the mechanism for Cr (VI) and Cu (II) adsorption on these magnetite hydrogels mainly relies on the functional group properties of chitosan and CMC present in the composite hydrogel. Silica molecules generally have weak electrostatic affinity toward anionic pollutant, as validated in [37], resulting in reduced enhancement of Cr (VI) adsorption. Although silica has a limited capacity for metal binding affinity, it contains certain functional groups such as hydroxyl, silanol, and siloxane, which provide additional sites for the hydrogel to interact with metal ions. Furthermore, metal ion properties such as charge and electronegativity exert influence on the adsorption efficiency of silica. A previous study [51] found that application of nano-silica can reduce the concentration of metals, mainly Pb, Cu, and Zn, in the soil through the binding of cationic metals to the siloxane oxygen. Other silica-based substances have been successfully used for heavy metal remediation in the environment [52].

3.6.2. Influence of Contact Time

As observed in Figure 11 and Figure 12, the adsorption capacities of hydrogel for Cr (VI) and Cu (II) increased steadily with time. The adsorption amount increased rapidly in the first 120–180 min before gradually slowing down until it reached equilibrium adsorption at 300–360 min. This phenomenon can be explained by the availability of numerous adsorption sites at the initial stage; the adsorption process gradually slows down as the surface sites become saturated. From Figure 11, it is evident that CMC-CS-Fe₃O₄-Si showed a faster uptake of Cr (VI) with 42 mg/g, while CMC-CS-Fe₃O₄ adsorbed only 37 mg/g in the first 60 min of contact time. In the case of Cu (II), both magnetic hydrogels reached equilibrium within 360 min of exposure, with faster kinetics observed in CMC-CS-Fe₃O₄-Si during the first 60 min. Similar to the adsorption trends obtained previously, impregnation of silica simultaneously improved the adsorption capacity of the magnetite hydrogel. Ref. [41] also reported that modification of chitosan/poly(methyl methacrylate) with silica increased the ability of the copolymer to remove Cr (VI). Aside from the reactive functional groups in silica, the porosity introduced by silica onto the hydrogel structure also provides space for metal ion diffusion, as confirmed by the SEM results.

3.6.3. Effect of Initial Metal Concentration

Figure 13 and Figure 14 display the adsorption amounts of Cr (VI) and Cu (II) with different initial concentrations of metal solution. These experiments were conducted under a fixed mass of hydrogels (0.05 g) in 50 mL of metal solution. Based on the findings, the metal uptake increased proportionally with the concentration of metal ions. Higher concentrations of metal ions created a saturated environment around the adsorbent, leading to a greater driving force from the higher concentration gradient of Cr and Cu ions to diffuse and occupy the adsorption sites of the hydrogel beads [50]. In particular, the driving force played a crucial role in overcoming mass transfer resistance for the metal ions, thus facilitating their movement from the solution to the surface of the composite hydrogel. The hydrogel beads containing silica demonstrated a higher adsorption capacity (53.00 mg/g) for Cr (VI) compared to the hydrogel beads without silica (48.00 mg/g). Similarly, CMC-CS-Fe₃O₄-Si exhibited greater performance in adsorbing Cu (II) compared to CMC-CS-Fe₃O₄ (65.70 mg/g). The highest adsorption values observed in this study, of 53.00 mg/g for Cr (VI) and 85.06 mg/g for Cu (II), surpass the 14.45 mg/g and 56.79 mg/g reported in [53,54], respectively, demonstrating the effectiveness of silica incorporation.

3.7. Adsorption Kinetic and Isotherm Studies of CMC-CS-Fe₃O₄-Si

The adsorption data of Cr (VI) and Cu (II) by CMC-CS-Fe₃O₄-Si hydrogel beads in Section 3.6.2 were fitted to kinetic isotherm models of pseudo-first-order (Equation (8)) and pseudo-second-order (Equation (9)). These kinetic models were used to obtain a better understanding of the mechanism and kinetic parameters of metal adsorption. Based on the results presented in Figure 15 and

Table 2. Parameters of kinetic models for Cr (VI) and Cu (II) adsorption by CMC-CS-Fe₃O₄-Si hydrogel beads.

Adsorption Kinetic Models	Parameters	Cr (VI)	Cu (II)
Pseudo-first-order	$q_e$	25.9119	63.1684
	$K_1$	0.0078	0.0081
	R2	0.9341	0.9678
Pseudo-second-order	$q_e$	57.8035	103.0928
	$K_2$	0.0007	0.0017
	R2	0.9988	0.9977

, the adsorption data show the greatest fit for pseudo-second-order, with maximum R² values of 0.9988 and 0.9977 for Cr (VI) and Cu (II), respectively. Therefore, this result suggests that the adsorption of these metals was controlled by chemical sorption [55]. In contrast, the lowest value of R² in pseudo-first-order shows that physical adsorption was not the rate-limiting factor for Cr and Cu adsorption.

Equations (8) and (9) show the kinetic isotherm models

$$\mathit{log}\left(q_e-q_t\right)=\mathit{log}{q}_e-\left({\displaystyle \frac{k_{1}t}{2.303}}\right)$$

(8)

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{1}{q_e}}$$

(9)

where k₁ is the rate constant for the pseudo-first-order (min⁻¹), k₂ is the rate constant for the pseudo-second-order (g mg⁻¹ min⁻¹), q_e refers to the amount of metals adsorbed (mg/g), and t is the time.

In order to evaluate the adsorption performance of the hydrogel, two adsorption isotherms (Langmuir and Freundlich) were studied by adopting the experimental data from Section 3.6.3. The results are presented in Figure 16 and

Table 3. Parameters of isotherm models for adsorption of Cr (VI) and Cu (II) by CMC-CS-Fe₃O₄-Si.

Adsorption Isotherm	Parameters	Cr (VI)	Cu (II)
Langmuir	${\mathrm{q}}_{\mathrm{m}}$	94.3396	113.6364
	${\mathrm{K}}_{\mathrm{L}}$	0.0241	0.0260
	${\mathrm{R}}_{\mathrm{L}}$	0.2932	0.2780
	R2	0.9298	0.6420
Freundlich	${\mathrm{K}}_{\mathrm{f}}$	3.721	1.9519
	1/n	0.6877	1.3324
	R2	0.9798	0.9592

. Isotherm analysis helps to provide valuable information on the interaction between the adsorbate and adsorbent [56]. According to the calculations based on the isotherm model equations, the Langmuir (Equation (10)) and the Freundlich equations (Equation (11)), the Langmuir model exhibited a poor fitting toward adsorption data as evident from the low correlation coefficient (R²) values in Table 3, whereas the Freundlich equation showed significantly higher R² values approaching 1. This indicates that the Freundlich model fits the adsorption data well for both Cr (VI) and Cu (II), signifying that a multilayer adsorption process has occurred, with an uneven distribution of affinities of chromium and copper ions on the heterogeneous surface of the adsorbent [57].

Equations (10) and (11) show the adsorption isotherm models

$${\displaystyle \frac{C_e}{q_e}}={\displaystyle \frac{1}{q_m.K}}+{\displaystyle \frac{C_e}{q_m}}$$

(10)

$$\mathit{ln}{q}_e=\mathit{ln}{K}_f+{\displaystyle \frac{1}{n}}\mathit{ln}{C}_e$$

(11)

where q_e (mg/g) represents the adsorption capacity, C_e (mg/L) denotes the equilibrium concentration, q_max (mg/g) signifies the maximum adsorption capacity, R_L indicates a coefficient reflecting the stronger of adsorption, K_L (L/mg) and K_F (mg/g) represent the equilibrium constants of adsorption, and 1/n signifies the adsorption intensity.

3.8. Desorption of Cr (VI) and Cu (II) and Regeneration of CMC-CS-Fe₃O₄-Si

Desorption and regeneration studies are essential for determining the efficiency and reusability of this adsorbent for wastewater application. Based on the results depicted in Figure 17, CMC-CS-Fe₃O₄-Si showed a stable performance for four repeated adsorption desorption cycles of Cr (VI) using 0.3 M NaOH. The desorption efficiency ranged from 74% to 82%, indicating a sufficiently high amount of Cr (VI) ions recovered after the adsorption process. The desorption efficiency was 74–82%, which is sufficiently high. Moreover, the adsorption performance remained stable over four cycles with only a slight decrease of 7.69% in adsorption capacity. To release the adsorbed copper ions from the surface of the CMC-CS-Fe₃O₄-Si hydrogel, a desorbing solution of 0.1 mol/L urea at pH 2 was utilized [58]. The desorption efficiency was 82.5–87%, which indicates a relatively high recovery rate (Figure 18). The hydrogel beads were then regenerated using 0.2 mol/L NaOH before being recycled in subsequent adsorption experiments. After the regeneration treatment, the hydrogel exhibited a good adsorption performance for several cycles with minimal reduction in adsorption capacity. In addition, the hydrogel’s structure remained intact with no visible damage observed, demonstrating a good reusability of these materials.

3.9. Performance of CMC-CS-Fe₃O₄-Si for Simultaneous Adsorption of Cr (VI) and Cu (II)

The adsorption performance of CMC-CS-Fe₃O₄-Si in a binary system (Cr (VI) + Cu (II)) was investigated at pH 2, 3, and 4.5 to determine its practical functionality in an actual wastewater environment where different types of heavy metals coexist. As shown in Figure 19, the simultaneous adsorption of Cr (VI) achieved the highest capacity at pH 2, whereas Cu (II) adsorption remained very low in that pH condition. Meanwhile, at pH 4.5, the adsorption of Cu (II) reached the highest capacity, with negligible interference from Cr (VI). These results are in accordance with the earlier experimental results, which show that CMC-CS-Fe₃O₄-Si favors pH 2 for the removal of Cr (VI), and pH 4.5 for the adsorption of Cu (II).

Moreover, the simultaneous presence of these metals made a very slight impact on the adsorption capacities of Cr (VI) and Cu (II) at pH 2 and 4.5, respectively, if compared to the metal adsorption in a single system. These findings posit weak competitive behavior between Cr (VI) and Cu (II) for binding toward the hydrogel due to their distinct adsorption preferences. This was confirmed by the negligible q_e changes and the very minimal inhibitory effect, as shown in the results.

Interestingly, the adsorption of Cu (II) at pH 3 was comparably higher than that of Cr (VI). The adsorption behavior of heavy metals is influenced by various factors, such as electronegativity and ionic radius. The smaller atomic radius and higher electronegativity possessed by Cu (II) can contribute to the stronger adsorption affinity [59]. Moreover, the higher CMC content in this hydrogel helps to increase the availability of carboxyl (–COO‾) groups, providing more adsorption sites for binding with Cu²⁺ ion compared to anionic Cr (VI). A previous study [60] demonstrated that COOH-functionalized material has a higher affinity toward Cu (II) compared to Zn²⁺ due to its stronger electrostatic interaction. This result highlights the selective adsorption mechanism and the affinity of the functionalized adsorbent toward certain metal ions.

3.10. Effect of Coexisting Ions and Complex Compound on Cr (VI) and Cu (II) Adsorption

Generally, wastewater consists of various kinds of compounds and ions which can interfere with the effectiveness and practicability of the magnetite hydrogel beads for removing heavy metals. To achieve a better understanding, the adsorption ability of the CMC-CS-Fe₃O₄-Si hydrogel toward Cr (VI) and Cu (II) was evaluated in a multi-component solution containing coexisting ions or organic compounds such as Na⁺, SO₄²⁻, NO₃⁻, or citric acid. Based on the graph in Figure 20, the presence of 50 mg/L of coexisting ions or organic compounds demonstrated a notable effect on the adsorption capacity of Cr (VI) and Cu (II). The presence of anion (SO₄²⁻) resulted in a reduced adsorption capacity of 14.84% and 19.6% for Cr (VI) and Cu (II), respectively. The results demonstrate that SO₄²⁻ has an inhibitory effect on heavy metal adsorption, which is consistent with the findings by [61], who stated that SO₄²⁻ normally competes with HCrO₄⁻ ions for adsorption sites. Moreover, SO₄²⁻ has a stronger affinity for forming complexes with certain polymers, therefore limiting the adsorption sites for heavy metals [62].

Similarly, NO₃⁻ caused a comparable decrease in the adsorption capacity of Cu (II) by 23.37% (from 39.8 to 30.5 mg/g) but had a minimal impact on Cr (VI) adsorption (3.13% reduction). The significant decline in Cu (II) adsorption caused by NO₃⁻ was due to the increased competition between these ions, as reported in [63]. Previous studies have clearly described how coexisting anions interfere with the adsorption process by forming complexes with heavy metals ions or the adsorbent, causing a decreased in adsorption capacity [64,65,66]. In contrast, NO₃⁻ constitutes a weaker oxidizing agent than HCrO₄⁻ and has a low affinity for forming complexes with the CMC-CS-Fe₃O₄-Si surface, thus explaining its insignificant impact on Cr (VI) adsorption.

Correspondingly, the presence of Na⁺ had negligible effects, reducing the Cr (VI) and Cu (II) adsorption by only 2.50% and 3.89%, respectively. Interestingly, the presence of citric acid in the system had no effect on the q_e values, as the reduction percentages were less than 0.15%. Several studies suggest that citric acid exerts an insignificant effect on adsorption behavior and, in some cases, is able to enhance the metal adsorption condition by increasing the availability of the metal ions in the solution and facilitating their adsorption onto the adsorbent [63]. Generally, these magnetite hydrogel beads demonstrate a practical application with reasonable potential for heavy metal removal under the presence of coexisting ions and organic pollutants.

3.11. Potential Application for Methylene Blue Removal

In addition to heavy metals, this study also aimed to discover the potential of the CMC-CS-Fe₃O₄-Si adsorbent in removing pollutant dyes such as methylene blue (MB). Common pollutant dye such as MB is widely employed in the textile and paper industries; however, the discharge of this dye into water can endanger aquatic ecosystems and human health [67]. Even at low doses, MB can cause serious health problems, including respiratory problems, cancer, and neurological damage [68]. Therefore, the removal of this dye is crucial to protecting the environment. Based on the graph in Figure 21, the CMC-CS-Fe₃O₄-Si hydrogel beads demonstrated a substantial capacity for removing methylene blue dye, achieving 94.30% removal. This shows that this hydrogel offers a promising solution for reducing dye pollution in the environment.

3.12. The Comparison of Adsorption Capacity with Other Adsorbents

For comparison, the adsorption capacities of several adsorbents reported in the literature were referred to and are presented in

Table 4. Comparison of adsorption capacity with other adsorbents.

Adsorbents	Adsorbate	Adsorption Capacity	References
Magnetic magnetite nanoparticle	Chromium (VI) Copper (II)	8.67 mg/g 18.61 mg/g	[69]
Magnetic bentonite/carboxymethyl chitosan/sodium alginate hydrogel	Copper (II)	56.8 mg/g	[54]
Fe3O4@SiO-CS composite	Chromium (VI)	96.2 mg/g	[70]
Chitosan-manganese oxide nanocomposite	Chromium (VI)	61.56 mg/g	[71]
Chitosan/Montmorillonite-Fe3O4 microsphere	Chromium (VI)	58.8 mg/g	[72]
Nanobentonite/Nanocellulose/Chitosan aerogel	Chromium (VI)	98.9 mg/g	[73]
Cellulose nanocrystal/carboxymethylcellulose-sodium/polyvinyl alcohol hydrogel	Copper (II)	108 mg/g	[74]
Microcrystalline cellulose (MCC)	Copper (II)	0.44 mg/g	[75]
Carboxymethyl cellulose-graft-poly(acrylic acid)	Chromium (VI)	6.53 mg/g	[76]
Carboxymethyl cellulose/chitosan/silica cross-linked GPTMS	Chromium (VI)	16.08 mg/g	[28]
EDTA-modified chitosan-carboxymethyl cellulose	Copper (II)	142.95 mg/g	[77]
CMC-Cs-Fe3O4-Si	Chromium (VI) Copper (II)	53.00 mg/g 85.06 mg/g	This study

. Compared to other adsorbents, CMC-CS-Fe₃O₄-Si shows a reliable adsorption performance, suggesting its potential as a viable alternative for heavy metal treatment. However, further research should be initiated to elucidate deeper information on the interactions between silica and the polymer, particularly in enhancing adsorption capacity to develop a highly efficient adsorbent.

4. Conclusions

This study has successfully proven that the incorporation of silica extracted from bamboo culm provides structural integrity and mechanical stability to the magnetite hydrogel through good interfacial adhesion between the hydrogel network and the silica. Characterization by FTIR and EDS confirmed successful extraction of silica from bamboo. A notable improvement in the mechanical and thermal stability of this silica-containing hydrogel was validated by insignificant deformation and reduced weight loss under different pH and temperatures. Most importantly, silica incorporation managed to improve the hydrogel’s structural stability as well as its adsorption efficiency. Adsorption experiments have demonstrated high removal capacities for Cr (VI) (53.00 mg/g) and Cu (II) (85.06 mg/g), confirming the functionality of CMC-Cs-Fe₃O₄-Si for heavy metal adsorption. The presence of coexisting ions and complex compounds exhibited a slight effect on adsorption efficiency due to increased competition for binding sites. However, desorption and regeneration studies signified the hydrogel’s reusability for several cycles, highlighting its practical potential for pollutant treatment in water. Despite its great potential, future research is needed to explore the scalability of this method for large-scale wastewater treatment applications and investigate the long-term stability of silica-incorporated hydrogel beads in real-world conditions.