Phosphorylated Cellulose-Based Porous Monoliths for Efficient and Eco-Friendly Heavy Metal Ion Adsorption

Abstract

Heavy metal ion contamination in aquatic systems presents substantial environmental and public health challenges, demanding innovative remediation solutions. This study reports the synthesis of phosphorylated cellulose (PC) monoliths via thermally induced phase separation (TIPS) as a sustainable solution for heavy metal removal. Through systematic optimization of phosphorylation degree (ranging from 16.3% to 49.5%) and pore architecture, we developed materials with exceptional adsorption capacity and flow characteristics. Comprehensive characterization (SEM, FTIR, and elemental analysis) confirmed the successful incorporation of the phosphate group while revealing tunable three-dimensional porous structures controlled by cellulose acetate concentration (80–120 mg mL⁻¹) and phosphorylation parameters. Optimal Cu²⁺ adsorption occurs at 43.9% phosphorylation, coupled with stable permeability, under continuous flow conditions. The monolith effectively removes heavy metal ions (e.g., Cu²⁺, Ni²⁺, Cd²⁺) across a wide pH range, driven by electrostatic and hard-soft acid-base interactions. Additionally, the material maintains an absorption capacity of over 90% after multiple regeneration cycles. These findings highlight the potential of PC monoliths as cost-effective, scalable, and environmentally friendly adsorbents for heavy metal ion removal in water treatment applications.

1. Introduction

The discharge of untreated industrial wastewater into natural ecosystems has led to severe heavy metal ion contamination, posing significant threats to environmental sustainability and human health. Heavy metal ions, such as lead (Pb²⁺), cadmium (Cd²⁺), mercury (Hg²⁺), and chromium (Cr⁶⁺), are non-biodegradable and accumulate in biological systems, leading to long-term ecological and health risks [1,2]. Consequently, the development of efficient and sustainable remediation technologies for heavy metal-contaminated wastewater has become a crucial research focus [3].

Various purification technologies have been developed to address heavy metal contamination, including chemical precipitation, ion exchange, electrochemical reduction, adsorption, liquid-liquid extraction, and membrane filtration [4,5,6,7,8]. Among these, chemical precipitation is the most widely applied due to its simplicity and cost-effectiveness. This technique involves adjusting the pH of wastewater to convert soluble metal ions into insoluble forms. However, while widely applied, chemical precipitation generates large volumes of metal-containing sludge, complicating disposal and increasing operational costs [9,10]. Ion exchange is another commonly used method, offering advantages such as metal selectivity, reusability, and relatively low environmental impact. However, its high initial cost, limited pH tolerance, and maintenance challenges hinder widespread adoption [11,12].

In recent years, adsorption has emerged as a preferred purification technology for heavy metal ion removal due to its versatility, high efficiency, and minimal secondary pollution. The performance of adsorption can be significantly enhanced through chemical modification and the selection of suitable adsorbents [13]. Bio-based adsorbents have attracted considerable attention due to their availability and low raw material costs. Cellulose, a naturally occurring biopolymer, has gained particular interest owing to its renewability, biodegradability, non-toxicity, biocompatibility, and chemical modifiability, expanding its application potential. Functionalized cellulose derivatives containing anionic groups, such as carboxyl, amino, and phosphate groups, exhibit excellent chelating properties, making them effective adsorbents for the removal of metal ions, organic compounds, and cationic dyes from wastewater [3,14,15,16,17,18].

Currently, cellulose-based heavy metal adsorbents are primarily available in the form of microspheres, particles, fibers, or porous aerogels [19,20,21]. While particle-based adsorbents provide a high specific surface area and numerous adsorption sites, they may cause clogging in flow systems, necessitating higher background pressures to maintain fluid movement. Modified cellulose fiber-based adsorbents offer good mechanical properties; however, their limited surface area restricts heavy metal adsorption capacity. In contrast, porous aerogels, with their highly porous structure, large surface area, numerous adsorption sites, and excellent permeability, have been recognized as promising heavy metal adsorbents [22]. Nevertheless, the production of conventional cellulose-based aerogel is costly, time-consuming, and results in poor mechanical strength [23,24,25,26]. These structural limitations hinder their application in dynamic water treatment systems, as their porous framework may collapse under pressure, impeding water flow and reducing overall treatment efficiency.

Thermally induced phase separation (TIPS) has been proposed as a rapid and cost-effective approach to address these limitations for fabricating rigid, aerogel-like monolithic porous materials [27]. Temperature-induced liquid-liquid phase separation (L-L TIPS) enables the formation of porous materials with a dual-network, three-dimensional interconnected pore structure, making them highly suitable for water treatment applications [28,29]. Compared to traditional particulate adsorbents such as activated carbon and ion-exchange resins, these materials offer a higher specific surface area, high-density active sites, excellent permeability, low back pressure, and superior mechanical stability [30,31,32]. Additionally, their rigid and stable porous framework prevents structural collapse, enabling effective fixation and long-term operation in continuous flow environments. Therefore, fabricating monolithic porous cellulose materials via the TIPS method, followed by functional group grafting, presents a promising strategy for developing cellulose-based porous heavy metal adsorbents suitable for flow-through systems.

Based on the aforementioned background, this study aims to develop a phosphorylated cellulose-based porous adsorbent optimized for dynamic flow environments to enhance heavy metal ion removal from wastewater. A thermally induced phase separation method was employed to fabricate monolithic cellulose-based porous materials with a high specific surface area and a dual-continuous, three-dimensional interconnected network structure. The material was further modified through phosphorylation to introduce negatively charged phosphate groups, improving its complexation capability with cationic metal ions (e.g., Cu²⁺, Cd²⁺). The adsorption performance of the phosphorylated porous materials was evaluated under simulated continuous flow conditions using a peristaltic pump. Additionally, its adsorption mechanism and capacity were systematically analyzed by examining internal structure, chemical composition, and permeability of the monolithic adsorbent. This study aims to provide a scalable and adaptable solution for heavy metal ion removal, contributing to the advancement of water purification technologies.

2. Materials and Methods

2.1. Chemicals and Materials

Commercial cellulose acetate (CA) (Mn = 5.0 × 10⁴, 39.7 wt% acetyl content), N,N-dimethylformamide (DMF), sodium hydroxide (NaOH), ethanol (EtOH), and 1-hexanol were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China. Sulfuric acid (H₂SO₄), urea (CH₄N₂O), sodium dihydrogen phosphate dihydrate (NaH₂PO₄·2H₂O), disodium hydrogen phosphate dodecahydrate (Na₂HPO₄·12H₂O), and sodium chloride (NaCl) were obtained from Shanghai HuShi Reagent Co., Ltd., Shanghai, China. Heavy metal salts, including copper(II) chloride dihydrate (CuCl₂·2H₂O), nickel(II) chloride hexahydrate (NiCl₂·6H₂O), and cadmium chloride dihydrate (CdCl₂·2.5H₂O), were supplied by Nanjing Chemical Reagent Co., Ltd., Nanjing, China. All chemicals were of analytical grade and used without further purification.

2.2. Preparation of Monoliths

2.2.1. Preparation of Cellulose Monolith

In this study, a previous preparation method [33] was used to prepare cellulose monoliths by deacetylating cellulose acetate (CA) monoliths, which were prepared using the thermally induced phase separation (TIPS) method. Briefly, 2 g of CA powder was dissolved in 10 mL of DMF under gentle stirring at 85 °C for 3 h. Subsequently, 15 mL of 1-hexanol was added dropwise to the solution, maintaining a DMF-to-1-hexanol volume ratio of 2:3. The mixture was stirred until it became uniformly transparent. The solution was then transferred into a glass cylindrical mold and cooled to 5 °C for 1 h to induce phase separation. After gelation, the monolith prototype was removed from the mold and washed thoroughly with methanol and ethanol to remove unreacted solvents. The CA monolith was hydrolyzed in a 0.5 M NaOH methanol solution at room temperature for 3 h to obtain the cellulose monolith. Finally, the monolith was washed repeatedly with deionized water and methanol, followed by vacuum drying at room temperature.

2.2.2. Preparation of Phosphorylated Cellulose Monoliths

Phosphorylated cellulose (PC) monoliths were prepared by immersing the cellulose monoliths in 6 mL of a phosphorylation reaction solution containing urea, NaH₂PO₄·2H₂O (0.56 g), and Na₂HPO₄·12H₂O (0.48 g). The mixture was stirred for 1 h to ensure uniform distribution of the phosphorylation reagents. The monoliths were then dried at 140 °C for 4 h to facilitate the reaction between phosphate groups and cellulose hydroxyl groups. After phosphorylation, the PC monoliths were washed multiple times with a sodium chloride aqueous solution to remove unreacted phosphate salts, followed by rinsing with deionized water. Finally, the PC monoliths were vacuum-dried at room temperature. The synthesis mechanism of the cellulose monolith and PC monolith is illustrated in Figure 1. The abbreviations of the prepared monoliths are listed in

Table 1. Abbreviation for monoliths.

Monoliths	Urea (g)	Abbreviation
Cellulose acetate monolith	/	CA monolith
Cellulose monolith	/	/
Phosphorylated cellulose monolith	0.2	PC-0.2
Phosphorylated cellulose monolith	0.5	PC-0.5
Phosphorylated cellulose monolith	1.0	PC-1.0
Phosphorylated cellulose monolith	1.5	PC-1.5
Phosphorylated cellulose monolith	1.5	PC-1.5

.

2.3. Characterization

The morphology and microstructure of all samples were analyzed using a Gemini SEM 300 field emission scanning electron microscope (SEM, Carl Zeiss AG, Jena, Germany) with an accelerating voltage of 10 kV. The samples were sputter-coated with gold prior to imaging. Fourier-transform infrared (FTIR) spectroscopy (Thermo Nicolet IS10, Waltham, MA, USA), inductively coupled plasma atomic emission spectroscopy (ICP-AES) (NexION-350X, Waltham, MA, USA), and X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, K-Alpha X, Waltham, MA, USA) were employed to analyze the chemical composition and functional groups of the monoliths. The specific surface area was determined by nitrogen adsorption-desorption measurements using a surface area analyzer (Micromeritics ASAP 2460, Norcross, GA, USA).

2.4. Permeability Testing

Permeability is a critical property of porous materials, indicating their ability to conduct fluids, which is closely related to the material’s structure and morphology. The porous monoliths were tightly fitted within heat-shrink tubing to assess permeability. The loaded monoliths were connected to a BT100J-1A peristaltic pump (Beijing Huining Weiye Instrument Co., Ltd., Beijing, China) and a KDM30 digital pressure gauge (Krone, Japan) via polypropylene tubing, ensuring a constant liquid flow rate. Prior to each measurement, the monoliths were rinsed with water to stabilize the flow rate. The permeability coefficient was determined using Darcy’s law,

$$B_0={\displaystyle \frac{L\times v\times \mu }{∆P}}$$

(1)

$$v={\displaystyle \frac{q}{A}}$$

(2)

In which L (cm) is the length of monolith, v (cm s⁻¹) is the linear velocity of a given fluid in flow system, μ (Pa∙s) is the viscosity of fluid, B₀ is the permeability of the porous media, ∆P is the pressure drop of the fluid through the monoliths (Pa), q is flow rate (mL s⁻¹), A is cross sectional area to flow (cm²), and Darcy (1 Darcy = 1 × 10⁻⁸ cm²).

Furthermore, the Thomas kinetic model was applied to fit the data and determine the maximum adsorption capacity. The corresponding formula is presented below:

$$\ln \left({\displaystyle \frac{C_t}{C_0}}-1\right)=k_{TH}\left({\displaystyle \frac{m{q}_{ad}}{Q}}-C_{0}t\right)$$

(3)

where C_t is the concentration of the metal ion solution at the outlet at time t, C₀ is the initial concentration of metal ion solution, k_TH is the flow rate coefficient, m is the mass of the adsorbent, q_ad is the maximum adsorption capacity, Q is the flow rate (mL min⁻¹), and t is the time (min).

2.5. Determination of the Degree of Phosphorylation

For the quantitative analysis of phosphate incorporation, PC monoliths were digested in H₂SO₄ (90 °C, 24 h) to hydrolyze the phosphate esters. The liberated phosphate ions were subsequently measured using ICP-AES, and the degree of phosphorylation was calculated according to Formula (4).

$$\mathrm{phosphorylation}\ast \left(\%\right)={\displaystyle \frac{{10}^{-3}CV/M{r}_{\left(P\right)}}{\mathrm{m}/M{r}_{\left(cellulose\right)}}}\times 100\%$$

(4)

where C is the concentration of phosphorus measured by ICP-AES, V is the volume of the H₂SO₄ solution containing phosphorus, Mr_(P) is the relative molecular weight of phosphorus, Mr_(cellulose) is the relative molecular weight of the repeating unit of cellulose, and m is the mass of cellulose used for phosphorylation.

2.6. Dynamic Adsorption Experiments and Recyclability

Continuous-flow adsorption experiments were conducted using a fixed-bed column system to evaluate the dynamic adsorption performance of the phosphorylated cellulose (PC) monoliths. A PC monolith with a length of 1 cm was packed into a heat-shrink tubing (inner diameter: 10 mm). Solutions containing heavy metal ions at a concentration of 500 ppm were passed through the monolith at a constant flow rate of 1 mL min⁻¹ using a peristaltic pump. The effluent was collected at regular intervals, and the metal ion concentrations were analyzed using a UV/Vis spectrophotometer (Infinite M200, Tecan) at wavelengths of 562 nm for copper, 530 nm for nickel, and 520 nm for cadmium. Prior to the experiment, the monolith system was equilibrated with deionized water to ensure stable baseline conditions. The breakthrough curve was plotted as the ratio of effluent concentration to influent concentration (C/C₀) versus time.

Copper (II), nickel (II), and cadmium (II) ions were selected as target pollutants due to their widespread presence in industrial wastewater and their significant environmental and health risks. The pH of the metal ion solutions was measured and adjusted using standard procedures to ensure consistency across experiments. Continuous-flow adsorption offers several advantages over batch adsorption, including operational simplicity, the ability to handle large influent volumes, high throughput, and scalability. Additionally, this method ensures maximum utilization of the adsorbent’s active sites, as the dynamic flow conditions promote efficient contact between the metal ions and the adsorbent.

Desorption experiments were conducted immediately after the adsorption trials to assess the regenerability and reusability of the PC monoliths. The metal-loaded monoliths were regenerated by passing an appropriate desorption agent (e.g., 0.1 M HCl) through the column at a flow rate of 3 mL min⁻¹ for 10 min. This process was repeated for 10 consecutive adsorption-desorption cycles to evaluate the stability and performance of the monolith under repeated use. After each desorption step, the monolith was rinsed with deionized water to remove residual desorption agents and prepare it for the next cycle. All experiments were performed at room temperature (25 ± 1 °C).

3. Results and Discussion

3.1. Morphological and Chemical Characterization of Monolithic Structures

SEM analysis revealed significant morphological variations among cellulose monoliths fabricated with different CA concentrations (Figure 2a–c). All monoliths exhibited a uniform, three-dimensional porous network with well-interconnected pores. Notably, increasing the CA concentration from 80 to 120 mg mL⁻¹ resulted in a systematic reduction in average pore size. This structural evolution arises from phase separation dynamics, where higher CA concentrations promote the formation of a denser polymer matrix that constrains pore expansion during gelation.

The permeability of the monolith is a critical parameter for its application in continuous flow systems. Permeability coefficients were derived from pressure loss measurements at flow rates of 1, 3, and 5 mL min⁻¹, a method that minimized background pressure fluctuations and ensured reliable assessments (Figure 2d). Monoliths with higher CA concentrations exhibited significantly lower permeability coefficients, attributable to the increased skeletal junction density during thermally induced phase separation (TIPS), which reduced pore dimensions and consequently diminished permeability. Based on this analysis, the 80 mg mL⁻¹ CA concentration was selected to optimize water permeation in phosphorylated cellulose (PC) monoliths. As shown in Figure 2e,f, phosphorylation treatment induced slight pore enlargement. Subsequent permeability measurements revealed that the PC-1.0 monolith exhibits a permeability coefficient of 5.47 × 10⁻¹³ cm², 6.7-fold higher than the unmodified cellulose monolith (8.19 × 10⁻¹⁴ cm²) (Figure 2g). This improvement is attributed to the enlarged pore structure after phosphorylation. A 100 g weight was applied to both CA and PC monoliths to evaluate mechanical stability. The phosphorylated variant retained its structural integrity (Figure 2h), demonstrating that the modification preserved mechanical strength without pore collapse, ensuring suitability for dynamic flow experiments.

Figure 3a shows the pore size analysis of the cellulose monolith and phosphorylated cellulose (PC) monolith, measured using ImageJ software (V1.8.0.112). The results indicate that the PC monoliths exhibit larger pore sizes compared to the pristine cellulose monoliths, consistent with the permeability measurements. The introduction of urea during phosphorylation plays multiple roles: it prevents cellulose degradation, catalyzes the reaction, and enhances the swelling of the cellulose framework, thereby improving the penetration of the phosphorylation reagent. This enhanced swelling ultimately leads to an increase in pore size following phosphorylation, as illustrated in Figure 3b.

In addition, BET surface area analyses were conducted. As shown in Figure 3c, the N₂ adsorption–desorption isotherms of both the CA monolith and the cellulose monolith exhibited Type IV curves with H2 hysteresis loops, characteristic of mesoporous structures with nanoscale dimensions. A broad pore size distribution centered around 10 nm was observed. In contrast, the PC-1.0 sample displayed a Type IV isotherm with an H3 hysteresis loop (Figure 3d), suggesting that the internal pore structures became more open after modification, although some finer pore structures were partially disrupted. Further analysis of the specific surface area and average pore size (Figure 3e) revealed that both parameters decreased after phosphorylation. These changes indicate that nanoscale pore structures either collapsed or disappeared during the reaction, resulting in an overall increase in larger-scale pore sizes. This finding is consistent with the pore size measurements presented in Figure 3a.

Figure 4a–d present SEM images of phosphorylated cellulose (PC) monoliths with varying phosphorylation levels, all prepared at a fixed cellulose acetate (CA) concentration of 80 mg mL⁻¹. The samples are designated as PC-X, where X corresponds to the urea content (0.2, 0.5, 1.0, or 1.5 g) in the 6 mL phosphorylation reaction solution, listed in Table 1. SEM analysis reveals a clear correlation between urea content and pore architecture. At urea quantities ≤ 1.0 g (≤2.8 M), both pore size and structural dimensions exhibit a positive dependence on urea concentration. Conversely, when the urea content exceeds 1.0 g, a reduction in pore size becomes evident.

Katsuura and Mizuno [34] demonstrated that increasing urea content elevates charge density, which, in turn, weakens hydrogen bonding between cellulose molecular chains. Beyond a critical threshold (1.5 g urea in our system), further disrupting inter-chain hydrogen bonding. This explains the observed pore structure shrinkage in PC-1.5 monoliths, where excessive urea compromised the cellulose framework’s hydrogen bonding network during drying. These results underscore the need to carefully optimize urea concentration to balance phosphorylation efficiency with structural integrity in cellulose-based porous materials. Precise control of urea content is therefore essential for developing stable, high-performance monoliths with superior adsorption capacity.

The chemical structural transformations from cellulose acetate (CA) to cellulose and subsequently to phosphorylated cellulose (PC) were systematically characterized using FTIR spectroscopy, XPS analysis, and EDS mapping. As shown in Figure 4e, the FTIR spectrum of CA exhibits a very weak peak at 3500 cm⁻¹, corresponding to residual hydroxyl (-OH) groups stretching vibration after acetylation. The peak at 2940 cm⁻¹ is attributed to the C-H stretching vibration of CH₂ and acetyl-CH₃ groups. The absorption at 1735 cm⁻¹ owing to the C=O stretching vibration of the acetyl group, while the peak at 1367 cm⁻¹ is caused by acetyl C-H bending vibrations. A band at 1215 cm⁻¹ is associated with acetyl C-O stretching. The characteristic pyranose ring vibration and glycosidic C-O-C bond stretching appear at 1032 cm⁻¹ [35,36,37]. Following NaOH treatment, the complete disappearance of acetyl-related peaks (1735, 1367, and 1215 cm⁻¹) coupled with the emergence of a strong hydroxyl band at 3500 cm⁻¹ confirms the full conversion of CA to cellulose monoliths.

In contrast to the cellulose monolith, the PC monoliths exhibit new peaks at 1230 cm⁻¹, 930 cm⁻¹, and 832 cm⁻¹, attributed to P=O, P-OH, and P-O-C structures, respectively, indicating the successful phosphorylation of the cellulose monolith [38]. The progressive intensification of these bands correlates with increasing phosphorylation degree, clearly demonstrating urea concentration-dependent reaction efficiency. Notably, the introduced phosphate groups confer negative charges to the cellulose matrix, significantly enhancing both material hydrophilicity and metal ion affinity through improved electrostatic interactions.

As presented in Figure 4f, the phosphate content increased linearly from 0.96 mmol g⁻¹ (PC-0.2, 16.3% phosphorylation) to 2.91 mmol g⁻¹ (PC-1.5, 49.5% phosphorylation), establishing a direct positive correlation between urea concentration and phosphorylation efficiency.

XPS survey spectra (Figure 4g) revealed distinct phosphorus signals in PC monoliths that were completely absent in the unmodified cellulose monolith. High-resolution XPS analysis further elucidated the chemical environment of incorporated phosphate groups: (1) The O1s spectrum (Figure 4h) displayed two resolved components at 531.2 eV (P-O) and 532.7 eV (C-OH), while (2) the P2p spectrum (Figure 4i) showed characteristic doublet peaks at 133.5 eV (PO₄³⁻ 2p_3/2) and 134.4 eV (PO₄³⁻ 2p_1/2). These results unambiguously demonstrate the formation of covalent P-O-C bonds through phosphorylation, in excellent agreement with the FTIR spectroscopic data [39]. The uniform distribution of phosphorus throughout the phosphorylated monoliths was confirmed by EDS elemental mapping (Figure 4j).

3.2. Continuous Flow Adsorption and Regeneration Studies

The adsorption behavior of metal ions on phosphorylated cellulose (PC) monoliths was evaluated using a peristaltic pump-based continuous flow system. As illustrated in Figure 5, the phosphate groups in the monolith facilitated metal ion adsorption primarily through electrostatic interactions, with adsorption efficiency being modulated by solution pH, ionic strength, and metal-phosphate affinity.

Breakthrough curve analysis (Figure 6a) revealed a direct correlation between phosphorylation degree and Cu²⁺ adsorption capacity, with higher phosphorylation levels yielding stronger metal binding. Among the tested samples, PC-1.0 (43.9% phosphorylation) demonstrated optimal performance, combining enhanced adsorption capacity resulting from its high phosphate content, structural stability confirmed by SEM imaging, and a uniform microarchitecture suitable for continuous flow operation. This balanced combination of properties established PC-1.0 as the most promising candidate for practical water treatment applications.

To demonstrate the superior Cu²⁺ adsorption capacity of phosphorylated cellulose monoliths, we conducted comparative adsorption experiments using both unmodified cellulose and PC-1.0 monoliths in a simulated flow system. After adsorption, the PC-1.0 monolith exhibited a distinct color transition to characteristic Cu²⁺-complex green, showing the outstanding adsorption ability for Cu²⁺, while the cellulose monolith retained its original white coloration, indicating negligible Cu²⁺ uptake (Figure 6b). Furthermore, the treated Cu²⁺ solution showed significant decolorization after passing through the PC-1.0 monolith. These qualitative observations, combined with quantitative adsorption data, unequivocally demonstrate the exceptional Cu²⁺ sequestration capability of phosphorylated cellulose monoliths.

The adsorption behavior of metal ions on PC monoliths exhibited significant variations depending on their chemical properties and interactions with phosphate groups. Comprehensive evaluation of divalent metal ions (Cu²⁺, Ni²⁺, and Cd²⁺) across different pH conditions (Figure 6c) revealed distinct adsorption patterns consistent with the Hard-Soft Acid-Base (HSAB) theory, which describes the affinity between metal ions (acids) and ligands (bases) based on their electronic properties. Hard acids, such as Ni²⁺, typically consist of small, highly charged cations with low polarizability. They exhibit strong interactions with hard bases, such as oxygen-containing ligands. Soft acids, such as Cd²⁺, are characterized by larger, more polarizable cations. They exhibit strong interactions with soft bases, such as phosphate groups. The PC-1.0 monolith exhibited the highest adsorption capacity for Cu²⁺ ions among the three tested metal ions. This can be attributed to the additional stability imparted by the octahedral structure of the Cu (II)-phosphate complex, which is susceptible to the Jahn-Teller effect. Furthermore, phosphate groups, acting as soft bases, exhibit a strong affinity for soft acids like Cd²⁺, resulting in a higher adsorption capacity for cadmium ions compared to nickel ions.

The adsorption performance of PC-1.0 exhibited strong pH dependence, as illustrated in Figure 6c. Under acidic conditions (pH < 4), the protonation of phosphate groups decreased their negative charge density, weakening electrostatic interactions with metal cations, and elevated H⁺ concentrations directly competed with metal ions for binding sites, leading to a decrease in adsorption capacity. This pH-sensitive behavior indicates that optimal PC-1.0 performance requires neutral to mildly acidic conditions, while strongly acidic environments (pH < 3) significantly compromise adsorption capacity.

According to the Thomas kinetic model (Formula (3)), the experimental data were fitted as shown in Figure 6d. A 90% confidence band was established, within which nearly all data points from both the early and later stages of dynamic adsorption were contained. The fitting yielded a correlation coefficient (R²) of 0.97971, indicating a good agreement between the experimental data and the model. These results demonstrate that the Thomas model is suitable for describing the flow system. Moreover, the intercept and slope (b) were obtained from the fitting results, and the maximum adsorption capacity (q_ad) was calculated to be 174.18 mg/g using Formula (3). Compared with similar adsorbents, PC-1.0 exhibited outstanding performance, as shown in

Table 2. The adsorption property of Cu²⁺ compared with other adsorbents.

Adsorbent	Adsorption Property (mg/g)	Efficiency (%) and Cycle Times (x)	Ref.
cellulose acetate/chitosan membranes	46.7	91.5% (6)	[40]
Cellulose nanofibril-PVA-AA aerogel	30.0	89% (5)	[41]
Aminosilylated nanocellulose aerogels	99.0	/	[42]
Bamboo Pulp Aerogels	72.73	<80% (3)	[43]
Covalent Crosslinking Cellulose/G Aerogels	80.12	90.5% (5)	[44]
PC-1.0	172	91.7% (10)	This work

.

The PC monoliths exhibited excellent regeneration capability through acid treatment, with 1 M HCl effectively desorbing >90% of the adsorbed metal ions while preserving the material’s structural integrity. As shown in Figure 6e, the adsorption capacity remained stable (>90% of the initial value) through 10 consecutive adsorption-desorption cycles, demonstrating exceptional reusability. This regeneration performance, combined with their robust mechanical stability, establishes PC monoliths as practical adsorbents for continuous-flow water treatment systems.

The PC monoliths exhibit outstanding performance for heavy metal ion removal in continuous-flow systems, with adsorption efficiency governed by the degree of phosphorylation, hierarchical pore architecture, and metal-phosphate affinity constants. Among the synthesized materials, the PC-1.0 monolith, with a balanced phosphorylation level and stable pore structure, emerges as the optimal candidate for efficient metal ion adsorption. The ability to regenerate the monoliths with high recovery efficiency further enhances their suitability for practical applications in water treatment and metal ion recovery.

4. Conclusions

This study provides a scalable and cost-effective method for synthesizing phosphorylated cellulose (PC) monoliths with tailored porous structures and high adsorption capacities for heavy metal ions. It offers deep insights into the interplay between material structure, phosphorylation degree, and adsorption performance. Increasing urea content enhances the phosphorylation level, which in turn improves the adsorption capacity of the monoliths. However, excessive urea can lead to structural instability, highlighting the importance of balancing phosphorylation degree with mechanical integrity. The PC monoliths demonstrate exceptional performance in continuous-flow systems, effectively removing heavy metal ions, such as Cu²⁺, Ni²⁺, and Cd²⁺, across a broad pH range. Furthermore, the PC monoliths exhibit excellent reusability, with recovery efficiencies consistently above 90% even after multiple regeneration cycles. These findings underscore their potential as sustainable and environmentally friendly materials for heavy metal ion remediation in water treatment applications.