Efficient removal of Pb(II) from aqueous solutions using chitosan-based hydrogels: PCG and PCC, adsorption performance and mechanism studies

doi:10.21203/rs.3.rs-4856911/v1

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Efficient removal of Pb(II) from aqueous solutions using chitosan-based hydrogels: PCG and PCC, adsorption performance and mechanism studies

https://doi.org/10.21203/rs.3.rs-4856911/v1

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Lead (Pb) is a highly toxic and persistent pollutant that poses a significant threat to human health, making the remediation of lead-contaminated water bodies an urgent priority. In this study, chitosan and acrylic acid were employed as base materials, with graphene oxide (GO) and carboxylated graphene (GC) acting as crosslinking agents to synthesize two novel chitosan-based hydrogels (PCG and PCC). Both PCG and PCC were characterized and subjected to systematic static adsorption experiments. Further investigations focused on PCC, examining the effects of coexisting ions and organic substances (humic acid and fulvic acid), adsorption-desorption cycles, and dynamic column experiments to assess its applicability in complex water environments. The results indicated that under the conditions of pH 4.72, temperature of 25°C, dosage of 0.2 g/L, and an initial concentration of 500 mg/L, PCG and PCC achieved maximum adsorption capacities of 323.83 mg/g and 446.09 mg/g, respectively. PCC exhibited excellent resistance to ion interference and demonstrated good reusability. Additionally, in dynamic column experiments with an influent flow rate of 2 ml/min and a hydrogel dosage of 200 mg, PCC effectively treated simulated wastewater with concentration of 50 mg/L for over 900 minutes. These findings indicate that the developed hydrogels exhibit great potential for large-scale application in the market.

chitosan

adsorption

PCG and PCC hydrogels

Lead (Pb) contamination is a significant global environmental issue. Pb is highly toxic and non-biodegradable, posing serious health risks to the human nervous and urinary systems due to its continuous migration and accumulation in ecosystems(Ni et al. 2019, Huang et al. 2020). The International Agency for Research on Cancer (IARC) classifies Pb as a Group 1 carcinogen, indicating a high carcinogenic risk to humans (Pearce et al. 2015). Excessive lead levels in water can harm humans through drinking, skin contact, or bioaccumulation in the food chain (Rahaman et al. 2021). Pb(II) exposure can lead to serious health issues, including anemia, kidney disease, neurological disorders, and even death (Josephson 2004). Consequently, the World Health Organization (WHO) recommends a maximum limit of 10 µg/L for Pb(II) in drinking water (Kinuthia et al. 2020). It is estimated that approximately 1 million people die from lead poisoning annually, with many more exposed to lower levels that cause lifelong health issues, including anemia, hypertension, and damage to the immune and reproductive systems. Therefore, the removal of lead from wastewater is an urgent priority (Rees and Fuller 2021). adsorption is a promising method for heavy metal removal due to its low cost, high efficiency, and ease of operation (Chen et al. 2022; Zhang et al. 2023).

Chitosan is a cost-effective, abundant, and renewable adsorbent characterized by strong chelating properties, which are primarily due to its numerous hydroxyl and amino groups. These functional groups enable chitosan to effectively adsorb heavy metal ions (Zhang et al. 2022, Ayub et al. 2022). It is extensively utilized as an adsorbent for removing lead ions from wastewater (Anush et al. 2020, Das et al. 2020). However, these functional groups also confer polarity and hydrophilicity to chitosan, leading to weak intermolecular interactions, especially in acidic environments. The increased interaction between the hydroxyl groups (-OH) in chitosan and water molecules under acidic conditions can make chitosan more susceptible to hydrolysis, thus limiting its effectiveness in water treatment applications. To mitigate these limitations, chitosan-based hydrogels are often physically and chemically crosslinked to enhance intermolecular bonding, thereby improving mechanical strength and resistance to hydrolysis (Lv et al. 2019). Furthermore, research (Paulino et al. 2011, Das et al. 2020) indicates that hydrogels based on chitosan can significantly enhance its adsorption capacity. These hydrogels offer controllable release properties and excellent reusability, making them as promising candidates for remediating heavy metal contamination in water bodies. However, commonly used crosslinking agents such as glutaraldehyde and epichlorohydrin are highly toxic, and residual traces within the hydrogel matrix can lead to secondary environmental pollution during water remediation, undermining ecological sustainability (Mishra and Tripathi, 2022). Additionally, excessive crosslinking can reduce the number of available adsorption sites, decreasing the adsorption efficiency of the hydrogel (Hu et al., 2020). For instance, crosslinking agents like glutaraldehyde can enhance the mechanical strength and stability of hydrogels, but they also reduce the number of available adsorption sites and may introduce potential toxicity, leading to secondary pollution (Zhao et al., 2021).

Graphene oxide (GO), an important `of graphene, consists of epoxy, hydroxyl, carboxyl, and carbonyl groups and boasts a large surface area of 2630 m²/g. It is considered a promising modification material due to its ability to enhance the strength and adsorption capacity of adsorbents (Anush et al. 2020). Research indicates that incorporating GO as a reinforcement in a biopolymer matrix can improve the thermal and mechanical stability of hydrogels. Additionally, GO reduces fouling tendencies during separation processes and enhances the hydrophilic properties of hybrid hydrogels (Zhao et al. 2019). Zhang et al. (2023) reported that graphene oxide and chitosan can spontaneously form composites via surface functional groups. The researchers developed CS/GO/ZnO composite materials using chitosan (CS) and zinc oxide (ZnO) as primary components, with graphene oxide (GO) serving as a crosslinking agent, this composite achieved a removal efficiency of 96.5% for Cr(VI) after120 min (Zhang et al., 2023). Further research by Chen et al. (2022) also highlighted the benefits of chitosan/graphene oxide composites, confirming their superior properties. Composites made with chitosan and graphene oxide (GO) improve the dispersibility of GO, preserve the adsorption sites on its surface, and, to some extent, enhance the mechanical strength of chitosan. Using GO as a crosslinking agent for chitosan-based hydrogels helps avoid environmental hazards associated with toxic crosslinking agents, potentially creating superior adsorption materials with enhanced capacity and stability. Furthermore, Yang and Cao (2022) demonstrated that the carboxyl functional groups on GO exhibit stronger electrostatic and chelation interactions with cations compared to hydroxyl and epoxy groups. Therefore, modifying GO to increase the carboxyl content on its surface can enhance its hydrophilicity and adsorption capacity. Carboxylated graphene (CG) can serve as a crosslinking agent for chitosan hydrogels, further enhancing their adsorption performance.

This study addresses the critical issue of lead pollution and the limitations of current chitosan-based hydrogel adsorbents, by incorporating graphene oxide (GO) and carboxylated graphene oxide(CG) into chitosan-based hydrogels, we have developed a novel hydrogel that is environmentally friendly, exhibits superior adsorption capabilities, and is recyclable and reusable. Adsorption experiments were conducted to investigate the adsorption and removal mechanisms of Pb(II) from water using the novel hydrogel, along with simulated wastewater treatment and adsorption dynamic column to evaluate its practical performance. This research offers theoretical and empirical insights into the use of biomass composite adsorbents for removing Pb(II) from water. It also lays a scientific foundation for developing Pb(II) removal technologies based on chitosan adsorbents and provides new perspectives on preventing and controlling lead contamination in groundwater.

The primary objectives of this research are: 1) to develop novel chitosan-based hydrogels, specifically PCG and PCC; 2) to conduct comprehensive static adsorption experiments to investigate the adsorption patterns and mechanisms of PCG and PCC; 3) to explore the adsorption mechanisms of PCC using adsorption models and characterization techniques; and 4) to evaluate the applicability of PCC through anti-ion interference experiments, simulated wastewater treatment, and dynamic column experiments. This study offers new perspective on the treatment of complex wastewater.

2.1. Chemicals, reagents, and instrumentation

All chemicals and reagents used in this study were of analytical grade unless otherwise specified. Chitosan (CS, > 85% deacetylation), graphite, KMnO4 (99.5%), H₂SO₄ (98%), H₂O₂ (30%), NaNO3, HCl (36%), NaOH (99%), HNO₃ (68%), MgCl₂, NaCl, CaCl₂, KCl, humic acid (HA), fulvic acid (FA), acetic acid, acrylic acid (AA), N,N'-methylenebisacrylamide (NMBA), and ammonium persulfate (APS) were utilized. A stock solution of Pb(II) (1000 mg/L) was prepared by dissolving the appropriate amount of PbCl₂ in deionized water (DIW). Solutions with varying concentrations were then prepared by serial dilution of this stock solution with DIW.The pH of the solutions was adjusted using analytical grade HCl and NaOH. Solutions were filtered through a 0.45 µm syringe filter. Analyses were performed in triplicate, and the average values were reported. The pH of the solutions was measured using a pH meter (HANNA pH-EC-TDS-°C, Italy). The concentrations of metal, including Pb²⁺, Ca²⁺, and Mg²⁺, were determined using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Agilent 5100, USA). The concentrations of anions, such as F⁻ and Cl⁻, were measured using an Ion Chromatography System (ICS-1100, Thermo Fisher Scientific Inc, USA).

2.2. Synthesis of GO and GC

Graphene oxide (GO) was synthesized using the improved Hummers method (Titelman et al. 2005), Carboxylated graphene oxide (CG) was prepared using the chloroacetic acid substitution method (Wang, 2020).

2.3. Synthesis of PCG and PCC

To prepare a chitosan solution, 2 g of chitosan (CS) was dissolved in 50 ml of 8% (v/v) acetic acid solution. Next, 40 ml of a 500 mg/L GC solution was poured into a beaker, followed by the addition of 10 ml of the chitosan solution. After stirring for 10 minutes, 0.12 g of APS, 0.15 g of NMBA, and 5 ml of acrylic acid (AA) were gradually added to the mixture. The mixture was thoroughly mixed, degassed using ultrasound, and the beaker was sealed with cling film, then the mixture was heated in an oven at 60°C for 3 hours, forming a polyacrylic acid/chitosan/carboxylated graphene oxide hydrogel (PCC). After cooling, the hydrogel was cut into 2 mm × 2 mm gel particles, which were soaked and rinsed with deionized water to remove any unreacted substances. The rinsed hydrogel was then freeze-dried for 72 hours to obtain a dry product. The preparation process is shown in Fig. 1.

Using the same procedure, GC was replaced with GO to prepare a polyacrylic acid/chitosan/graphene oxide hydrogel, referred to as PCG. The hydrogel without GO or GC is simply a polyacrylic acid/chitosan hydrogel, designated as PC.

2.4. Characterization of PCG and PCC hydrogels

The properly dried and pulverized PCC and PCG samples were characterized using a Fourier transform infrared spectrometer (NICOLET i10, Thermo Fisher Scientific, USA) for FTIR analysis and an X-ray diffractometer (Bruker AXSD8 Advance, Bruker, Germany) for functional group and crystalline phase analysis, respectively. The microstructure of the materials was analyzed using a scanning electron microscope (SU 8010, HITACHI, Japan). The BET characterization analysis of the materials was performed using a chemisorption analyzer (AutoChem 2950 HP, Micromeritics, USA), and the compressive performance of the hydrogel materials was tested using a micro-electromechanical universal testing machine (CMT4104, SNAS Materials Testing Co, Ltd., Shenzhen, China).

2.5. Swelling degree of PCG and PCG hydrogels

The swelling degree of the hydrogel was tested using NaOH and HCl solutions to prepare solutions with pH values ranging from 1.0 to 12.0 (± 0.05). Ten milligrams of hydrogel were placed in a 50 ml centrifuge tube and shaken in a constant temperature oscillator at 298 K and 180 rpm. After sufficient water absorption and swelling, surface moisture was removed using filter paper. The hydrogel was then weighed to calculate its swelling degree. Three parallel experiments were conducted for each group to ensure accuracy. The swelling degree (SD, g/g) was determined using Eq. (1) (Wang et al. 2023)

SD=$\:\frac{{m}_{\text{s}}-{m}_{\text{d}}}{{\text{m}}_{\text{d}}}$(1)

Where m_s (mg) and m_d (mg) are the masses of the swollen and dried hydrogels, respectively.

2.6. Adsorption equilibrium and kinetics experimental for Pb(II)

Adsorption equilibrium experiments for Pb(II) on PCG and PCC in aqueous solution were performed in batch mode. Dried PCG and PCC (10 mg) were added to Pb(II) solutions (50 mL) of varying concentrations (50–500 mg/L) and shaken (150 rpm) in a thermostat at 25 ℃. Adsorbed Pb(II) (Q_e, mg/g) was assessed using Eq. (2) (Wu et al. 2023); Percentage Pb(Ⅱ) removal was determined using Eq. (3), Where R (%) is the removal rate(Wu et al. 2023)

$$\:{Q}_{e}=\left({\text{C}}_{0}-{\text{C}}_{\text{e}}\right)\times\:\frac{v}{m}$$

$\:R=\frac{{C}_{0}-{C}_{e}}{{C}_{0}}\times\:$ 100(3)

Where Q_e (mg/g) is the amount of Pb(II) adsorbed on the surface at equilibrium, C₀ and C_e (mg/L) is the initial concentration and equilibrium concentration of Pb(II) respectively, v (mL) is the volume of solution, and m (mg) is the net weight of the dry hydrogel.

Kinetics experiments in batch mode were conducted using a fixed amount (10 mg) of adsorbent (dried PCG and PCC) in 50 mL Pb(II) solutions at temperatures of 298, 308, and 318 K. Samples were collected at predefined time intervals, and the concentration of Pb(II) was measured. Furthermore, experiments were conducted to investigate the effect of pH on Pb(II) sorption, using an initial pH range of 1–6 under conditions identical to those used in the adsorption kinetics experiments.

The data were further analyzed using the Langmuir and Freundlich models, which are the most commonly applied models for two-parameter isotherms in solid/liquid systems. The Langmuir model, which assumes monolayer adsorption on a homogeneous surface, is mathematically expressed as (Eq. (4)) (Eltaweil et al. 2020). The Freundlich model, which assumes adsorption occurs on a heterogeneous surface with varying binding energies, is mathematically expressed as (Eq. (5)) (Fan et al. 2013).

$$\:\frac{{C}_{e}}{{Q}_{e}}=\frac{{C}_{e}}{{Q}_{m}}+\frac{1}{{Q}_{m}\times\:{K}_{L}}$$

$$\:In\:{Q}_{e}=In\:{K}_{F}+\frac{1}{n}\:In\:{C}_{e}$$

where C_e (mg/L) and Q_e (mg/g) have the same meaning as Eq. (2), Q_m (mg/g) is the maximum adsorption capacity of Pb(II) on the adsorbent surface, and K_L (L/mg) is the Langmuir constant, K_F (mg/g) and n (adsorption strength) are Freundlich constants. The values of Q_m and K_L were calculated.

The kinetics of adsorption provide valuable information about the reaction mechanism. Therefore, the pseudo-first-order and pseudo-second-order kinetic models, as well as the intraparticle diffusion kinetic model, were used to investigate Pb(II) adsorption onto PCC and PCG hydrogels. The equation for the pseudo-first-order kinetic model can be expressed as (Eq. (6)) (Wu et al. 2023); the pseudo-second-order adsorption kinetics can be represented by (Eq. (7)) (Balci, 2004); and the intraparticle diffusion model can be described by (Eq. (8)) (Eltaweil et al. 2020).

$\:In（{Q}_{e}-{Q}_{t}）=In{Q}_{e}-{K}_{1}\times\:t$	(6)
$\:\frac{t}{{Q}_{t}}=\frac{1}{{K}_{2}}+\frac{t}{{Q}_{e}}$	(7)
$\:{Q}_{t}={K}_{id}+{t}^{0.5}+c$	(8)

where t (min) is the contact time between the adsorbent and the Pb(II) solution, Q_e(mg/g) is the amount of Pb(Ⅱ) adsorbed on the surface at equilibrium, Q_t (mg/g) is the adsorption capacity of Pb(II) in solution at t, and K₁ (min-1) is the first-order rate constant, K₂ (g/mg·min) is the second-order rate constant, K_id (mol/g·min_1/2) is the rate constant for the intraparticle diffusion relationship, and c is the constant for the diffusion effect of the boundary layer, which indicates the thickness of the boundary layer.

2.7. The influence of coexisting cations and coexisting organic matter

The effects of K⁺, Na⁺, Mg2⁺, and Ca²⁺ ions, as well as Humic Acid (HA) and Fulvic Acid (FA), on the hydrogel adsorption process were investigated. In the experiments, the concentration gradient for coexisting cations was set to 0-100 mg/L, while the concentration gradient for coexisting organic matter was set to 0–30 mg/L. After the reaction, the lead concentration in the solution was determined using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), and the adsorption capacity was calculated accordingly.

2.8. Regeneration studies

The regenerative capacity of the hydrogel adsorbent was evaluated through four consecutive adsorption-desorption cycles. For the adsorption experiment, 10 mg of the cleaned PCC hydrogel was added to a 50 mL solution with a concentration of 500 mg/L and stirred at 150 rpm for 12 hours at 298 K. In the desorption experiment, 10 mg of the adsorbed hydrogel was successively immersed in 1 M HCl and pure water, followed by thorough agitation for desorption and washing. The adsorption capacity was calculated using Eq. (2). The desorption rate (R_d) and desorption amount (Q_d) were calculated accordingly using Eq. (9) and Eq. (10).

$$\:{R}_{d}=\frac{{C}_{d}}{{{C}_{0}-C}_{e}}$$

$$\:{Q}_{d}={C}_{d}\times\:\frac{v}{m}$$

where R_d (%) represents the desorption rate of Pb(II) from the solution, C_d (mg/L) is the concentration of Pb(II) in the desorption solution at desorption equilibrium, C₀ (mg/L) and C_e (mg/g) have the same meaning as Eq. (2). Q_d(mg/g) is the amount of Pb(II) desorbed per unit mass of adsorbent, v (ml) represents the volume of the desorption solution, and m (mg) is the mass of the dried adsorbent being reused.

2.9 Simulated wastewater treatment and dynamic column experiment

Lake water was filtered, and the filtrate was used as the background solution to prepare a Pb(II) solution with a concentration of 50 mg/L, simulating wastewater. Adsorption experiments were conducted using this simulated wastewater. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and Ion Chromatography (IC) were employed to analyze the cations and anions in the samples. The adsorption capacity was then calculated based on these measurements.

The schematic diagram of the dynamic adsorption column experimental setup is illustrated in Fig. 2. The column has dimensions of 8 cm in height, 2 cm in inner diameter, 0.3 cm in thickness, and an effective volume of approximately 23 mL. As shown in Fig. 2, quartz sand and dry hydrogel are packed into the adsorption column. Quartz sand is filled at both the upper and lower ends of the column, while the middle section is layered with PCC hydrogel. To ensure optimal contact between the hydrogel and the injected solution, the column is filled with layers of dry hydrogel, each weighing approximately 200 mg. A layer of 300 mesh nylon gauze is placed at the inlet and outlet to cover the upper and lower sections of the filling. Additionally, an appropriate amount of degreased cotton is placed at the bottom and top of the column. Rubber pipes connect the ends of the column, with the lower end serving as the inlet and the upper end as the outlet. A peristaltic pump is used to maintain a stable inlet flow rate.

Dynamic column experiments were conducted by varying the dry hydrogel filling amounts in the sand column to 100, 200, and 300 mg, while maintaining flow rates at 2, 4, and 6 mL/min. The lead concentration in the effluent treated by the adsorption column was measured using ICP-OES, and breakthrough curves were plotted base on these measurements. The breakthrough point is defined as when the concentration of PbII(II) in the effluent reaches 5% of the influent concentration (C_t/C₀ = 5%). At this stage, the column is considered penetrated, and the corresponding time is termed the breakthrough time (min). The total effluent volume at this point is defined as the breakthrough volume (ml). The exhaustion point is defined when the concentration of Pb(II) in the effluent reaches 95% of the influent concentration (C_t/C₀ = 95%), indicating that the column has reached adsorption equilibrium. The corresponding time is termed the exhaustion time (min), and the total effluent volume at this stage is defined as the exhaustion volume (ml). Based on the experimental data, the total adsorption capacity (Q_total, mg) and the dynamic adsorption capacity (Q_E, mg/g) were calculated using the following formulas (Al-Zawahreh et al. 2022).

$$\:{Q}_{total}=\frac{U\:\times\:\:{C}_{0}}{1000}{\int\:}_{t=0}^{t={t}_{total}}{C}_{ad}dt$$

$$\:{Q}_{E}=\frac{{Q}_{total}}{m}$$

where Q_total (mg) is the total adsorption amount, U (mL/min) is the flow rate of the feed water, C₀ is the Pb concentration of the solution of the feed water (mg/L); and t_total is the total flow time (min), C_ad(mg/L) is the adsorption concentration, Q_E (mg/g) is the dynamic adsorption capacity, m (g) is the mass of filled PCC hydrogel.

3.1. Characterization

3.1.1 Results of SEM, FTIR and XRD and discussion.

Scanning electron microscopy (SEM) images revealed the three-dimensional porous structures of PCC, PCG, and PC (Fig. 3). As illustrated in Fig. 3a-3b, the surface of PC displays a tightly interconnected, spider-web-like structure within the pores. This structure facilitates water absorption and swelling in an appropriate pH environment, allowing the solution to permeate the gel interior and fully expose the adsorption sites (Rong et al. 2021). According to Fig. 3c-3d, PCG exhibits a uniform honeycomb-like porous structure. After the addition of GO, the filamentous network structure within the pores of PCG disappears. The continuously arranged honeycomb structure facilitates the distribution and support of external pressure. However, this closed-cell structure may restrict the movement of heavy metal ions within the hydrogel, which is unfavorable for adsorption (Zhao et al. 2021). Figure 3e-3f shows that PCC exhibits stronger connectivity in its pore structure and lacks the mesh-like structure seen in PC. However, compared to PCG, PCC features more small-volume pores, and the interiors and edges of the larger pores have more folds and crevices. This rougher surface structure is more conducive to the adsorption of heavy metal ions ions (Zhang et al. 2019). Furthermore, the microstructure of PCC after adsorption was examined. As depicted in Fig. 3h, at 500x magnification, PCC-Pb reveals that the hydrogel retains its three-dimensional network structure post-adsorption, with numerous internal pores. However, compared to the pre-adsorption network, the structure appears more irregular, with decreased uniformity and a denser composition. At 3000x magnification (Fig. 3i), SEM images of PCC-Pb show smaller-scale micropores with interstitial stacking between pore walls, forming structures resembling the accumulation of fine particles. This differs from the pre-adsorption surface microstructure and aligns with observations from various studies, suggesting the formation of new substances within PCC post-adsorption, leading to these structural change. (Kong et al. 2021).

The FTIR spectra of GO, GC, CS, PC, PCG,PCC and PCC-Pb are displayed in Fig. 4. As shown in Fig. 4a, the introduction of many oxygen-containing functional groups during the preparation of graphene oxide confirms the successful conversion of graphite flakes into GO using the modified Hummer’s method (Ye et al. 2015; Anush et al. 2020; Verma et al. 2022). In the spectrum of GC, the characteristic -OH peak at 3426.82 cm⁻¹ weakens, the peak at 1734.71 cm⁻¹ disappears, and new peaks emerge at 1600.69 cm⁻¹ and 1399.52 cm⁻¹, corresponding to the symmetric and asymmetric vibrations of -COO-, indicating the successful synthesis of GC (Modi and Bellare, 2019). For CS, the broad band at 3356.02 cm⁻¹ is attributed to the stretching vibrations of –NH₂ and –OH groups associated with chitosa (Anush et al. 2019; Das et al. 2020).

The FTIR spectra of synthesized PCC hydrogel are shown in Fig. 4b. The characteristic peak at 1558.30 cm − 1 corresponds to the amino group in the CS structure. The presence of characteristic peaks from both CS and GC in the PCC spectrum confirms the successful synthesis of the composite hydrogel. Comparative analysis of the FTIR spectra of PC, PCC, and PCG (Fig. 4c) shows no significant differences in the types of characteristic peaks among the three chitosan-based hydrogels, likely due to the complexity of PC's composition. However, noticeable differences in peak intensities, especially around 3566.73 cm⁻¹ and 1716.52 cm⁻¹, suggest an increased content of hydroxyl and carboxyl groups following the addition of GO and GC. This increase is conducive to the adsorption of heavy metal ions from water.

To elucidate the removal mechanism, FTIR spectra of PCC hydrogel before and after Pb(II) adsorption were measured (Fig. 4d). The intensities of vibrations corresponding to -OH and -COOH groups significantly decreased after Pb(II) adsorption, indicating the participation of these functional groups in the adsorption process (Luo et al. 2022). The adsorption mechanism can be described by the following equations:

2-COOH + Pb→-COO-Pb-OOC+2H

-OH + Pb²⁺→-O-Pb + H⁺

The X-ray diffraction patterns of the PCC, PCG, and PC hydrogels are shown in Fig. 4e. All three hydrogels exhibited a broad and blunt peak around 2θ = 20.06°, with no other distinct diffraction peaks. The intensity of the peak at 2θ = 20.06° varied slightly among the hydrogels, with PCG and PCC showing increased intensity compared to PC. This suggests that the presence of GO and GC reduces the disorderliness of the hydrogels, enhancing crystallinity and strengthening hydrogen bonding within the gel network (Liu et al., 2022).

The stress-strain curves of PCG and PCC are depicted in Fig. 4f. Both materials demonstrate excellent compressive properties, as shown in the figure. PCG and PCC experience brittle fracture at approximately 68.5% and 64% compressive strain, respectively, with compressive strengths of 97.5 kPa and 81.25 kPa. These high compressive strengths are advantageous for the practical application and recovery of hydrogel adsorbents, highlighting their good practical prospects.

3.1.2 Characterization of swelling degree and discussion.

The swelling degree refers to a polymer or gel material's capacity to absorb liquids or gases under specific conditions. For hydrogels, this property is crucial for their ability to adsorb heavy metal ions (Ren et al. 2023). Figure 6 illustrates the swelling degrees of PCG and PCC at various pH levels and contact times. Both hydrogels exhibited high swelling rates and degrees. According to Fig. 5a, PCG and PCC had the lowest swelling degrees (2.335 and 2.541 g/g) at pH 1, which increased to 54.649 and 64.676 g/g at pH 12. The hydrogels absorbed water rapidly, reaching equilibrium within 200 minutes, demonstrating excellent water absorption performance.

According to Fig. 5a, the solubility of PCG and PCC generally increases with rising pH, showing a rapid increase at pH levels above 8. However, within the pH range of 6 to 8, PCG's solubility temporarily decreases as pH increases. This phenomenon may be attributed to protonation and hydrogen bonding at lower pH levels, causing PCG to contract. As the pH rises, increased electrostatic interactions between carboxylate groups break hydrogen bonds, leading to greater molecular chain spacing and solubility. Furthermore, a higher pH enhances the solubility of Na⁺ ions. Beyond pH 6, the electrostatic shielding effect of Na⁺ ions reduces the hydrogel's swelling degree (Zhao et al. 2021). Deprotonation of some -NH₂ groups in the hydrogel network can cause the network to collapse, decreasing the swelling degree (Chen et al. 2021). As the pH continues to rise, electrostatic repulsion between carboxylate groups overcomes the shielding effect, causing the hydrogel molecular chains to unfold and the swelling degree to increase (Lv et al. 2019). Unlike PCG, the swelling degree of PCC consistently increases with rising pH, likely due to the higher carboxyl group content in PCC, where electrostatic repulsion remains stronger than the Na⁺ ion shielding effect as pH increases.

The BET parameters of PCG and PCC are provided in Table 1. Both hydrogels have large specific surface areas, facilitating the exposure of adsorption sites (Eltaweil et al. 2020). After Pb(II) adsorption, PCC showed a reduction in specific surface area and pore volume, likely due to the blocking of network structures by substances formed during the adsorption process. However, the average pore diameter of PCC-Pb increased to 4.8904 nm, possibly due to chemical reactions between Pb(II) and active sites on the adsorbent surface, forming complexes or precipitates that enlarge the pore diameter. These observations are consistent with changes in the microstructure before and after adsorption, confirming the generation of new substances during PCC's adsorption process, as noted in Mutabazi's research (Mutabazi et al. 2024).

Table 1

The BET analysis data for PCG and PCC
Material	BET Specific surface area(m²/g)	pore volume(cm³/g)	Average Pore diameter(nm)	Range of Pore diameter(nm)
PCG	1.2766	0.00183	4.243	1.82–53.39
PCC	3.9297	0.007869	4.659	1.85–51.36
PCC-Pb	2.4886	0.005435	4.890	1.83–52.88

3.2. Adsorption kinetics and modeling

The effect of contact time on Pb(II) adsorption by PCG and PCC, along with their corresponding kinetic fitting curves, is shown in Fig. 6a-6b. As observed, the adsorption capacities of both PCG and PCC for Pb(II) increase rapidly with contact time, eventually slowing down and reaching adsorption equilibrium. This equilibrium is achieved within 200 minutes for both hydrogels, indicating a relatively fast adsorption rate. To evaluate the kinetics of Pb(II) adsorption, the data were fitted to the pseudo-first-order and pseudo-second-order kinetic models. Figure 6a-6b shows that both hydrogels fit both models well within the temperature range of 298–318 K, with the pseudo-second-order model providing a better fit, as indicated by higher correlation coefficients in Table 2. The higher correlation coefficient for the pseudo-second-order kinetics suggests that the adsorption process of Pb(II) onto PCG is predominantly controlled by chemisorption, with the rate-limiting step likely being the chemical interactions between the hydrogel and the adsorbate (Xu et al. 2022). Similarly, PCC also fits the pseudo-second-order kinetic model better, indicating that chemisorption is the dominant mechanism.

Table 2

Kinetic Parameters for Pb(II) Adsorption by PCG and PCC
Material	Pseudo-first-order dynamic model			Pseudo-second-order dynamic model
Material	q_e (mg/g)	K₁(min^− 1)	R²	K₂(g/mg·min)	q_e (mg/g)	R²
PCG	93.316	0.0110	0.9491	1.76*10^− 4	101.7568	0. 9718
PCC	120.757	0.0127	0.9682	1.52*10^− 4	131.4501	0.9703

The Weber-Morris model was employed to better understand the adsorption mechanism and the rate-controlling steps affecting the kinetics (Qiu et al., 2009; Dhoble et al., 2011; Usman et al., 2022). Plotting q_t versus t^0.5 (Figs. 6c and 6d) revealed that the adsorption process for the hydrogels occurs in two stages. As depicted in Figs. 6c and 6d, both PCG and PCC fit well with the intraparticle diffusion model, indicating that the adsorption process can be divided into two distinct phases. The first phase is a rapid adsorption stage, characterized by the greatest osmotic pressure difference between the hydrogel and the aqueous system, leading to the quick influx of water into the hydrogel's network structure and a fast adsorption rate. The second phase is a gradual leveling-off phase, where adsorption approaches saturation and the adsorption capacity increases slowly until equilibrium is achieved (Hosain et al., 2020). The fits for both PCG and PCC are not purely linear and do not pass through the origin (c ≠ 0), suggesting the presence of boundary effects during the adsorption process. This indicates that intraparticle diffusion is not the sole rate-limiting step, and the adsorption of Pb(II) involves a complex process that includes both membrane diffusion and intraparticle diffusion (Qiu et al., 2009; Hosain et al., 2020).

3.3. Adsorption isotherm and modeling

The effect of the initial ion concentration on Pb(II) removal by hydrogels was investigated, as illustrated in Fig. 6e. As shown in Fig. 6e, the adsorption of lead ions by both types of hydrogels increased with the initial concentration until equilibrium was reached. The Langmuir and Freundlich adsorption isotherm models were fitted to the data, with their fits depicted in Fig. 6f and the corresponding correlation parameters listed in Table 3. Both models provided satisfactory fits for Pb(II) adsorption by PCG and PCC, exhibiting high correlation coefficients, which indicates their significant reference value. Specifically, the correlation coefficient for the Langmuir adsorption isotherm was slightly higher than that for the Freundlich model, suggesting that Pb(II) adsorption by PCG and PCC aligns more closely with the Langmuir model. This implies that the adsorption process is monolayer, homogeneous, independent, and reversible, characteristic of a monomolecular adsorption type. The adsorbent surface was uniform and consistent, with no interaction between adsorption sites (Shahraki et al., 2019; Shang et al., 2020). The value of 1/n for both hydrogels was less than 1 (Table 3), indicating the presence of van der Waals forces and electrostatic attraction, favorable for Pb(II) removal. The value of n in the Freundlich model also predicts the binding affinity between the sorbent and the metal ions. A smaller value indicates stronger interactions between sorbents and metal ions (Shahraki et al., 2019). According to the data, the 1/n value for the PCG hydrogel (0.278) was smaller than that for the PCC hydrogel (0.313), suggesting that the PCC hydrogel had a stronger affinity for lead ions compared to the PCG hydrogel. The K_F value, representing the adsorption per unit concentration, was 73.13 and 87.69 L/mg for PCG and PCC hydrogels, respectively, indicating excellent adsorption performance of these novel materials. Notably, the maximum removal capacity of Pb(II) by PCG and PCC hydrogels at 25°C was calculated to be 372.24 mg/g and 501.86 mg/g, respectively, according to the Langmuir model.

Table 3

Adsorption Isotherm Parameters for Pb(II) Adsorption by PCC
Material	Langmuir			Freundlich
Material	q_m (mg/g)	K_L(L/mg)	R²	K_F(L/mg)	1/n	R²
PCG	372.24	0.03277	0.9862	73.1291	0.278	0.8575
PCC	501.86	0.04465	0.9909	87.6936	0.313	0.9412

3.4. Thermodynamics of Pb(II) adsorption and model

The effect of temperature variation on the adsorption of Pb(II) by PCG and PCC was evaluated, and thermodynamic fitting was conducted for both materials. The calculated thermodynamic parameters for the adsorption of Pb(II) by PCG and PCC are listed in Table 4. According to Table 4, the correlation coefficients for PCG and PCC after fitting with the Gibbs equation are 0.9999 and 0.9926, respectively, indicating a good fit. The values of ΔG° for the adsorption processes of both hydrogels are negative and decrease continuously with increasing temperature (e.g., for PCG, ΔG° decreases from − 1.145 kJ/mol to -2.164 kJ/mol), indicating that the adsorption of Pb(II) by both materials is spontaneous and that higher temperatures facilitate the adsorption reaction (Wu et al., 2019). Temperature variations during the adsorption process can lead to changes in the system's entropy. If a decrease in temperature results in a decrease in the system's entropy, indicating molecules becoming more orderly on the solid surface, ΔS° will be negative. Conversely, if a temperature change increases the system's entropy, ΔS° will be positive (Hosain et al., 2020). In this study, the calculated values of ΔS° are positive (50.97 and 115.82 J/(mol·K) for PCG and PCC, respectively), indicating an increase in disorder during the adsorption process with increasing temperature, accompanied by increased randomness at the solid-solution interface, possibly leading to structural changes. The ΔH° values are positive (14.04 and 33.28 kJ/mol for PCG and PCC, respectively), indicating that the adsorption reaction of Pb(II) by the adsorbents is endothermic (Wu et al., 2019). Furthermore, the magnitude of ΔH° is closely related to the forces involved in the adsorption process. ΔH° values in the range of 2 to 40 kJ/mol are typically attributable to van der Waals forces, dipole bonding forces, hydrogen bonding, and/or ligand exchange (Zhang et al., 2022). The ΔH° values for PCG and PCC, 14.04 and 33.28 kJ/mol respectively, fall within this range, indicating that multiple reaction mechanisms, including van der Waals forces, dipole bonding forces, hydrogen bonding, and/or ligand exchange, may play significant roles in the adsorption process.

Table 4

Thermodynamic parameters of PCG and PCC
Material	T(K)	$\:\varDelta\:$G(kJ/mol)	$\:\varDelta\:$H⁰(kJ/mol)	$\:\varDelta\:$S⁰(J/mol·K)	R²
PCG	298	-1.145	14.0423	50.97	0.99999
	308	-1.656
	318	-2.164
PCC	298	-1.267	33.2847	115.82	0.99267
	308	-2.314
	318	-3.588

3.5. Effect of initial pH

The initial pH of the solution is a critical factor influencing the adsorption performance of hydrogels, as it affects the surface charge of the adsorbent and, consequently, the extent of lead ion adsorption (Parastar et al., 2021). Examining the effect of pH on hydrogel adsorption provides insights into their adsorption mechanisms and helps assess their environmental suitability. As illustrated in Fig. 5b, the adsorption capacity of PCG and PCC is significantly higher than that of CS and is highly sensitive to solution pH. The adsorption capacity of both hydrogels increases with rising pH, reaching a maximum at pH 5, and slightly decreases at pH 6. According to Wang et al. (Wang et al., 2022; Xu et al., 2022), high concentrations of H⁺ ions in the solution compete with Pb²⁺ for adsorption sites under acidic conditions, displacing some adsorbed Pb²⁺ ions and thereby reducing the adsorption capacity of the materials. Our study indicates that PCG and PCC, which are rich in carboxyl and hydroxyl groups, are affected by the competitive adsorption of H⁺ ions under acidic conditions. This competition impedes the binding of Pb²⁺ ions to the hydrogels, thus reducing their adsorption capacity at low pH. Additionally, under acidic conditions, the reduced swelling capacity may also impact the adsorption performance of hydrogels. Studies suggest that the degree of swelling significantly influences the ability of hydrogels to adsorb heavy metal ions (Han et al., 2024).

The swelling degree is a critical factor in determining the efficiency of hydrogels in adsorbing heavy metal ions, as it reflects their capacity to absorb solutions (Yin, 2022). As discussed in Section 3.1, acidic conditions negatively affect the water absorption and swelling of PCG and PCC hydrogels. Hydrogen bonding causes the hydrogel network structure to become more rigid, thereby limiting the exposure of adsorption sites to the solution and resulting in reduced Pb(II) adsorption (Liu et al., 2022). As the solution pH increases, deprotonation of functional groups in the hydrogels occurs, increasing the number of available adsorption sites and enhancing water absorption capacity. This expansion of the network structure enhances the contact area between adsorption sites and the solution, thereby improving adsorption efficiency. At pH 6, it is hypothesized that precipitation occurs between some Pb²⁺ ions and OH⁻ ions, which inhibits the complexation of Pb(II) with PCG and PCC, thus reducing adsorption capacity. Additionally, a comparison of the pH effect on Pb(II) adsorption reveals that PCC exhibits superior adsorption capacity across various pH levels, particularly at pH 1 (Fig. 5b), where PCC's adsorption capacity is approximately 5.5 times that of PCG, indicating PCC's excellent acid resistance.

4.1. Influence of coexisting cations and organic matter

The effects of coexisting cations and organic matter on Pb(II) adsorption by PCC are presented in Fig. 7a-7b. The presence of Na⁺, K⁺, Ca²⁺, and Mg²⁺ at low concentrations had minimal impact on PCC's adsorption performance, indicating that PCC exhibits some resistance to ion interference. Humic acid (HA) and fulvic acid (FA), the primary organic substances in natural water bodies, originate from the biodegradation of plants and microorganisms (Xie et al., 2020). Understanding the effects of these substances on adsorbent materials helps address complex water quality conditions. As shown in Fig. 7b, the adsorption capacity of PCC decreases progressively as the concentration of organic matter increases from 0 mg/L to 30 mg/L, suggesting an inhibitory effect of HA and FA on the hydrogel's adsorption. This inhibitory effect intensifies with increasing concentrations of HA and FA. This phenomenon is likely due to the competitive adsorption of HA and FA with Pb(II) in water, where excess organic matter occupies adsorption sites, as reported by Qu et al. (Qu et al., 2022).

4.2 Regeneration performance of PCC

The regeneration property of PCC hydrogel is a crucial factor in assessing its practicality and economic viability (Wu et al. 2019). A regeneration study was conducted over four successive cycles, as shown in Fig. 7c. The results indicate that the adsorption capacity for Pb(II) remained high at 389.9 mg/g after four cycles, demonstrating significant chemical stability. The desorption study revealed relatively low desorption rates for PCC, ranging from 42.5–65%, with a gradual decrease in these rates as the number of cycles increased. This decline in adsorption efficiency could be attributed to material loss and a reduction in available active sites after each regeneration, likely due to irreversible reactions between metal cations and anionic groups (Paulino et al. 2011; Lasheen et al. 2016; Hamza et al. 2019). As illustrated in Fig. 7d, the material exhibits rapid desorption capability, reaching desorption saturation within 240 minutes, suggesting that this novel hydrogel is an economically viable adsorbent for practical wastewater treatment (Hamza et al. 2019).

4.3 Simulated wastewater treatment and dynamic column experiment

To assess the effectiveness of PCC in removing Pb from simulated wastewater, adsorption experiments were conducted using simulated wastewater with lake water as the background matrix. As shown in Table 5, the simulated wastewater contained high concentrations of Pb²⁺, Ca²⁺, K+, Mg²⁺, Na⁺, Cl⁻, and SO₄²⁻. Figure 8a illustrates that even in wastewater with a complex composition, PCC demonstrates a high Pb removal rate, which increases with higher dosages. Effective Pb removal was observed when the PCC dosage was 0.8 g/L or more.

Table 5

Composition and Concentration of Substances in Simulated Wastewater
Substance	B	Ba	Ca	K	Mg	Na	Pb
concentration(mg/L)	0.217	0.108	60.0	4.29	14.1	23.6	42.1
Substance	Si	Sr	Zn	F	Cl⁻	NO₃⁻	SO₄²⁻
concentration(mg/L)	2.71	0.452	4.54	7.17	238	1.40	83.5

The adsorption breakthrough curves for PCC under various adsorbent fillings and flow rates are depicted in Fig. 8b-8c, with relevant parameters detailed in Table 6. Analysis Fig. 8b and the table reveals a gradual reduction in both breakthrough time and exhaustion time for PCC as the flow rate increases. This trend indicates that higher flow rates decrease the contact time between PCC and Pb(II) within the column. Additionally, an increase in flow rate leads to a reduction in both the total adsorption capacity and the dynamic adsorption capacity of the hydrogel. This phenomenon is consistent with the findings of Sami et al. (2022) and can be attributed to the reduced residence time of the solute within the simulated sand column (Al-Zawahreh et al. 2022). Consequently, the Pb solution entering the adsorption column does not have sufficient time to fully react and diffuse into the hydrogel network, resulting in the premature elution of the solute from the adsorption layer before equilibrium is reached.

Table 6

Parameters of PCC Dynamic Adsorption Column Breakthrough Curves Under Different Conditions
Flow rate (ml/min)	Amount of PCC (mg)	Breakthrough time (min)	Breakthrough volume (ml)	Exhaustion time (min)	Exhaustion volume (ml)	Total adsorption capacity (mg)	Dynamic adsorption capacity(mg/g)
2	200	900	1800	1110	2220	64.56	322.84
4	200	360	1440	600	2400	55.74	278.69
6	200	150	900	330	1980	42.91	214.56
2	400	1200	2400	1500	3000	92.41	231.02
2	600	1260	2520	1680	3360	101.76	169.61

As shown in Fig. 8c and Table 6, both the breakthrough time and exhaustion time increased with the amount of PCC filling in the adsorption column. This indicates that a greater quantity of adsorbent enhances the operational duration of the adsorption column. Additionally, the total adsorption capacity of Pb²⁺ from the solution consistently rose with the increased filling amount. This effect is likely due to the larger contact area and the availability of more active adsorption sites for Pb²⁺, similar to the observations by Pan et al. (Pan et al. 2019) in their study on the removal of antibiotics using alginate/graphene composite hydrogels in columns. However, a higher hydrogel filling amount resulted in a decrease in dynamic adsorption capacity, possibly due to the underutilization of adsorption sites within the hydrogel. In conclusion, the newly developed chitosan-based hydrogel demonstrated excellent adsorption performance in dynamic experiments, despite being affected by influent flow rate and adsorbent filling amount. It provided extended effective action time and showed potential for remediating lead contamination in water bodies. Moreover, the hydrogel maintained its integrity after the experiments and could be recycled and reused twice through washing. This characteristic suggests it could be an excellent filler for permeable reactive barriers in water body remediation, facilitating broader applications in addressing lead contamination.

This study successfully synthesized novel chitosan hydrogels, PCG and PCC, for the removal of lead ions from water. The adsorbents were comprehensively characterized, and systematic adsorption experiments were conducted. Further investigations on PCC included tests for resistance to ion interference and adsorption column evaluations to assess its practicality. The results indicate that PCG and PCC exhibit excellent adsorption capacities, achieving 323.83 mg/g and 446.09 mg/g, respectively. Additionally, PCC shows exceptional acid resistance and reusability. At pH 1, its lead ion adsorption capacity reaches 168.32 mg/g, which is 5.62 times greater than that of the PCG. Even after four cycles of use, PCC maintains satisfactory adsorption performance. Moreover, the presence of common coexisting cations and organic matter has minimal impact on PCC's performance. Simulated wastewater experiments and dynamic column tests confirm the practical applicability of PCC.

Contributions

Jingyu Sun: conceptualization; methodology; formal analysis; investigation; writing—original draft; writing—review and editing. Lili Liang:conceptualization; resources; writing—review and editing; supervision; project administration; funding acquisition; Huanying Pan: conceptualization, formal analysis, investigation, writing—review and editing; Zhenzhen Zheng: formal analysis, investigation, data curation, writing—review and editing. Xin Liu: investigation, writing—review and editing. Xinyu Cao: investigation, writing—review and editing.

Ethics Approval

There are no ethical issues with this study.

Consent to Participate

All authors agree to participate.

Consent to Publish

All authors agree to participate in the publication.

Competing interests

The authors declare no competing interests.

Data availability

The datasets used and analyzed in the current study are available from the corresponding author on reasonable request

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Efficient removal of Pb(II) from aqueous solutions using chitosan-based hydrogels: PCG and PCC, adsorption performance and mechanism studies

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