Molecularly engineering cellulose into a functional cellulose-based aerogel adsorbent for the recovery of precious metals from e-waste

Abstract

The demand for the recovery of precious metals has become increasingly prominent owing to their scarcity and wide application, while the facile preparation of sustainable absorption and separation materials via molecular engineering still face great challenges owing to the use of renewable polymer resources. Herein, a water-soluble cellulose levulinate ester (CLE) with ketocarbonyl groups was utilized as a 3D network scaffold to fabricate a cellulose-based aerogel adsorbent (CLE@PEI) by introducing polyethyleneimine (PEI) with multiple amine groups. The abundant functional groups and porous architecture enabled CLE@PEI to efficiently capture Au(III), Pt(IV) and Pd(II). The maximum adsorption capacity of CLE@50PEI reached a staggering 1752.0 mg g⁻¹ for Au(III) at 298 K, which was superior to those of Pt(IV) (1420.5 mg g⁻¹) and Pd(II) (495.1 mg g⁻¹). The adsorption process followed the pseudo-second-order kinetic model and Langmuir isotherm behavior. Additionally, the positively charged CLE@50PEI exhibited excellent selectivity for precious metals in multi-metal mixtures. The outstanding adsorption performance of CLE@50PEI may be attributed to the new “adsorption-in situ reduction” mechanism, in which the process primarily involved ion exchange coupled with electrostatic interaction and complexation as well as the in situ reduction of absorbed Au(III) to single crystalline flakes. Moreover, quantitative desorption of the CLE@50PEI-loaded metals was achieved in an acidic thiourea solution.

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Green foundation

1. This study uses cellulose levulinate ester (CLE), a renewable polymer, to create an efficient adsorbent for precious metals, thereby reducing reliance on non-renewable resources.

2. CLE@PEI aerogels have excellent adsorption capacities for precious metals and high selectivity from e-waste owing to their “adsorption-in situ reduction” mechanism. This enables efficient and precise precious metal recovery and maximizes resource utilization.

3. In future work, the presented cellulose-based aerogels could contribute to adsorbent design and help simplify research efforts, leading to more sustainable materials for precious metal recovery.

1. Introduction

Precious metals including gold, platinum, and palladium have driven civilizational advancement through their scarcity, enduring stability, and exceptional economic worth.^1–3 The exceptional electrical conductivity, corrosion resistance, and catalytic efficiency of precious metals endow them with extensive applications in various fields, including electronics, pharmaceuticals, catalysts and fuel cells.^4–7 The prolific utilization not only raises the demand for these precious resources to a higher level but also increases the production of e-waste. Au, Pt and Pd are included in the group of the most precious elements in e-waste. The effective recovery of these precious metals would ensure the sustainable availability of these limited resources. Conventional methods for separating precious metals from e-waste include chemical precipitation,⁸ solvent extraction⁹ and ion exchange,¹⁰ which suffer from several drawbacks such as high reagent consumption, long processing durations and the generation of hazardous sludge or other wastes that need to be disposed of. In response to these problems, biosorption is an efficient and eco-friendly alternative, which could significantly improve the sustainability of resource recovery because of its cost-effectiveness and easy regeneration.^11–13 The typical kinds of biomasses used for biosorption of precious metal ions include chitosan, cellulose, lignin and tannin derivatives and biopolymer alginate resins.¹⁴ For instance, Li et al. developed three kinds of polydopamine-, chitosan- and cellulose-based crosslinked biosorbents (PDA-TFN-A, chitosan-TFN-A and cellulose-TFN-A, respectively) for highly efficient adsorption and reduction of Au(III) via chemical crosslinking with tetrafluoroterephthalonitrile.¹⁵ At the optimum pH value of 2, the maximum adsorption capacity of PDA-TFN-A, chitosan-TFN-A and cellulose-TFN-A for Au(III) were 2771.8, 2680.0 and 1992.0 mg g⁻¹, respectively. Nevertheless, the multi-step synthesis processes, which involved the use of harsh organic solvents and extensive post-treatment procedures, represented the most significant deficiencies. Additionally, Zhao and coworkers fabricated magnetic chitosan-cystamine composites for selective recovery of Au(III) through a one-step method.¹⁶ Although the preparation process was simple, the maximum adsorption capacity of these composites was merely 478.47 mg g⁻¹.

The cellulose-based aerogel is regarded as a biosorbent with multifunctional substances due to its polyhydroxy structure and superior properties. As a sustainable natural polymer material, the cellulose-based aerogel skillfully introduces adsorption sites via careful chemical modification, and makes use of its abundant pore space to boost the adsorption process.^17–19 Despite these benefits, the current applications of cellulose-based aerogels are primarily focused on the adsorption of dyes,²⁰ carbon dioxide,²¹ heavy metals²² and oils,²³ rather than for the recovery of precious metals from e-waste. Moreover, although efforts have been made to develop cellulose-based aerogel adsorbents with enamine structures, these structures are prone to hydrolysis under strong acidic conditions, making them unsuitable for the recovery of precious metals (Fig. 1a).²⁴ In contrast, adsorbents containing imine structures have been found to be stable under strong acidic conditions, making them more suitable for this application,^25,26 as shown in Fig. 1b. However, it must be noted that these imine-structured adsorbents were synthesized by the crosslinking of bio-based materials (e.g., sodium alginate, nanocellulose) with polyethyleneimine (PEI) using glutaraldehyde as a crosslinking agent. Nevertheless, the potential risks associated with the utilization of irritants and toxic chemicals in the synthesis process remain unaddressed. As a result, ongoing research studies are focused on exploring novel cellulose derivatives with enhanced stability via molecular engineering, without the employment of harsh organic chemicals (Fig. 1c).

Fig. 1 Comparison of the structural stability of bio-based absorbents with an (a) enamine structure, (b) imine structure and (c) the novel structure presented in this work.

Cellulose levulinate (CLE) is a fully bio-based cellulose ester containing an acetyl propyl moiety in the levulinic acid chain with excellent water solubility.²⁷ It is easily fabricated by transesterification between α-angelica lactone and hydroxy groups of cellulose via the organic base/DMSO/CO₂ solvent system, as reported in our previous work.²⁷ Owing to the extensive chemical reactivity of the ketone moieties, CLE can be regarded as a highly precise and multifaceted building-block polymer. This enables the design and development of advanced materials through cutting-edge chemical transformations of the pendant acetyl propyl moiety. Therefore, taking advantage of the structural feature of ketocarbonyl groups and abundant hydrogen bonding sites of CLE, PEI with multiple amine groups is anticipated to be introduced into the cellulose matrix via a Schiff base reaction in the aqueous phase.^28,29

Herein, a facile and green strategy was developed to fabricate a cellulose-based aerogel adsorbent (CLE@PEI) for the selective recovery of precious metals from e-waste. The structural characteristic of CLE@PEI and the adsorption performances for Au(III), Pt(IV) and Pd(II) were evaluated systematically. Additionally, the adsorption mechanism under strong acidic conditions was examined in detail. The adsorption selectivity and reusability of CLE@PEI aerogel were also estimated. The findings demonstrated significant insights for the design and preparation of advanced functional adsorbent materials from natural polymers via molecular engineering.

2. Experimental section

2.1. Materials

Microcrystalline cellulose (MCC) (DP = 220) was supplied by Shanghai Aladdin Reagent Co., Ltd, which was dried under 80 °C for 24 h. 1,8-Diazabicyclo [5.4.0] undec-7-ene (DBU) dried by KOH, and dimethyl sulfoxide (DMSO, 99.9%), DMSO-d₆, as well as alpha-angelica lactone (α-AL), dried with 4A molecular sieves, were all acquired from Shanghai Aladdin Reagent Co., Ltd. Polyethyleneimine (PEI, M.W. 70 000, 50% water solution), NaAuCl₄·2H₂O, H₂PtCl₆·6H₂O, PdCl₂, FeCl₃·6H₂O, CuCl₂·2H₂O, NiCl₂·6H₂O and ZnCl₂·6H₂O were also obtained from Shanghai Aladdin Reagent Co., Ltd. CO₂ was supplied by Guiyang Sanhe Gas Co.

2.2. The preparation of CLE@PEI aerogels

CLE (DS = 1.3) was synthesized in accordance with our previous work,²⁷ as shown in Scheme 1. The ¹H NMR and ¹³C NMR spectra of CLE are shown in Fig. S1 and S2,† respectively. Firstly, CLE (0.5 g, 1.73 mmol) and 9.5 mL of deionized water were gradually added into a 25 mL beaker under magnetic stirring at 50 °C until completely dissolved, resulting in a 5 wt% CLE solution. Then, PEI (with 10 wt%, 30 wt%, 50 wt%, 70 wt% and 90 wt% mass ratios of CLE) was added into the mixture under stirring. A viscous liquid was formed by stirring for 1–5 min at room temperature, which then rested for 2–10 min after the stirrer was removed to form a hydrogel. Subsequently, it was immersed in deionized water (3 × 100 mL) to thoroughly eliminate any unreacted reagent. Lastly, the CLE@PEI aerogels were produced via freeze-drying procedure. A series of CLE@PEI aerogels were named according to the mass fraction of the PEI aqueous solution, as CLE@10PEI, CLE@30PEI, CLE@50PEI, CLE@70PEI and CLE@90PEI. For example, CLE@10PEI demonstrated that the content of PEI was 0.05 g in 0.5 g CLE solution.

Scheme 1 Preparation process of CLE (DS = 1.3).

The preparation of CLE is provided in the ESI.†

2.3. Characterization

Fourier transform infrared (FT-IR) spectroscopy was conducted using an iS50 FT-IR instrument with a frequency range of 550–4000 cm⁻¹. Nuclear magnetic resonance (NMR) spectra were recorded using a JEOL JNMECZ-400S/L1 spectrometer in DMSO-d₆. Scanning electron microscopy (SEM) images were obtained to analyze the surface morphology using a JEOL JSM-7500F microscope at an operating voltage of 10 kV. X-ray photoelectron spectroscopy (XPS) was conducted with a Thermo ESCALAB 250Xi analyzer utilizing Al Kα radiation for precise elemental and chemical state analysis. Thermal stability was assessed by thermogravimetric analysis (TGA) using a TGDTA/DSC apparatus under N₂, where 5–10 mg samples were heated from 30–800 °C at a heat rate of 10 °C min⁻¹. The pore structure was analyzed using a mercury intrusion porosimeter (AutoPore V 9600, Micromeritics, USA). Inductively coupled plasma optical emission spectroscopy (ICP-OES) was performed using an ICAP7400 plasma emission spectrometer to accurately quantify the metal ion concentrations. The UOP2000CSJ microscope was employed to capture images of metal particles on material surfaces.

2.4. Computational methods

To investigate the stability of the imine bond, density functional theory (DFT) calculations were performed to determine the bond dissociation energies (BDE) of the imine bond in Gaussian 16 software.³⁰ Geometry optimization and vibrational frequency calculations were carried out using the B3LYP functional with the 6-31G(d) basis set, along with the GD3BJ dispersion correction.^31–33 The solvent effects were accounted for using the SMD³⁴ solvation model with water as the solvent. For the single-point energy calculations, the M062X functional was employed with the def2-TZVP basis set,^35,36 also incorporating the SMD solvation model with water as the solvent.

2.5. Adsorption experiments

The adsorption experiments were conducted using the conventional batch method. Experiments for the precious adsorption capacity of the CLE@PEI aerogels with different PEI contents were analyzed by preparing 2000 mg L⁻¹ of Au(III), Pt(IV) and Pd(II) solutions in 0.1 M HCl medium. 10 mg CLE@PEI aerogels with different PEI content were respectively added into a 40 mL glass bottle containing 10 mL precious metal ions solution. Subsequently, the mixture was agitated with a thermostatic shaker for 24 h at 298 K. Then, the supernatant was filtered and diluted to 1–10 ppm prior to conducting ICP-OES analysis. All adsorption tests were performed under the same conditions, and the concentration of the adsorbent in each experiment was maintained at 1 g L⁻¹. The absorption capacity (q_e) of the CLE@PEI aerogels for precious metal ions were calculated according to eqn (1):where C₀ (mg L⁻¹) and C_e (mg L⁻¹) represent the initial and equilibrium concentrations of the precious metal ions, respectively. q_e (mg g⁻¹) is used to represent the equilibrium adsorption capacities of the CLE@PEI aerogel. M (g) and V (mL) represent the mass of the aerogel and the solution volume, respectively.

The effect of different hydrochloric acid (HCl) concentrations (0.01 M, 0.1 M, 0.5 M, 1 M, 2 M, 3 M and 4 M) on the adsorption efficiency at 298 K was investigated under 50 mg L⁻¹ of an initial precious metal ion concentration. The adsorption efficiency of the CLE@PEI aerogel was calculated according to eqn (2):

2.6. Adsorption–desorption evaluations

5 mg of the CLE@50PEI aerogel was separately added into a 40 mL glass bottle containing 5 mL of a 1000 mg L⁻¹ solution of Au(III), Pt(IV) and Pd(II). The mixtures were shaken, filtered and diluted before ICP-OES analysis. For the desorption batch experiments, the CLE@PEI aerogel with adsorbed metal ions was placed in a mixture solution containing 5 mL of 0.5 M thiourea in 0.1 M HCl for 5 h at 298 K. This adsorption–desorption experiment was repeated for five consecutive cycles.

Descriptions of the experiments for the adsorption kinetics, isotherms and selectivity are provided in the ESI.†

3. Results and discussion

3.1. Fabrication of CLE@PEI aerogels

Taking the biobased non-cytotoxic and water-soluble CLE as the building-block polymer, the Schiff base reaction between ketocarbonyl and PEI was conducted to construct the CLE@PEI aerogel with a 3D network.^27,37 The detailed preparation procedure for the CLE@PEI aerogel is illustrated in Fig. 2a. The water-soluble CLE (Fig. 2b) was employed as the main component of the 3D network scaffold. Subsequently, PEI was added into the aqueous solution as a chemical crosslinking agent to facilitate the formation of the 3D network. In this situation, with the rapid formation of imine bonds, CLE@PEI hydrogels can be efficiently prepared under mild conditions within 5 min, as depicted in Fig. 2c and d. Suffering from the freeze-drying process, the original 3D mesh structure of CLE@PEI could be maintained, as illustrated in Fig. 2e. Notably, compared to previously reported cellulose-based aerogel adsorbents containing enamine groups,²⁴ which were synthesized by reacting cellulose acetoacetate (CAA) with PEI, the CLE@PEI aerogels demonstrated superior structural integrity and retained their original 3D mesh structure even in strong acid solution, as illustrated in Fig. S3a–d.† This enhanced stability, arising from the Schiff base reaction between CLE and PEI, formed a stable imine-linked network. The stability of imine and enamine bonds was evaluated through bond dissociation energies (BDE) using DFT calculations, providing theoretical insights under various conditions. As shown in Fig. S4 and Table S1,† the results indicated that enamine bonds (–NH–CH=CH–) in CAA exhibited a lower BDE of 102.6 kcal mol⁻¹ in vacuum, which further decreased to 79.3 kcal mol⁻¹ under acidic conditions, suggesting reduced stability. In contrast, imine bonds (–C=N–) in CLE displayed a higher BDE of 149.7 kcal mol⁻¹ in vacuum and 148.1 kcal mol⁻¹ under acidic conditions, compared to enamine bonds, signifying greater stability. These findings were consistent with empirical results. As a result, the acid tolerance of imine bonds made them particularly advantageous for the adsorption of precious metals from e-waste.

Fig. 2 (a) Schematic of the synthesis process of CLE in the DBU/DMSO/CO₂ solvent system and CLE@PEI aerogel in aqueous solution. Digital photographs of (b) the CLE aqueous solution (5 wt%), (c) CLE and PEI (with 50 wt% mass ratios of CLE) mixture solution, (d) CLE@50PEI hydrogel, (e) CLE@50PEI aerogel on a dandelion.

3.2. Characterization of the CLE@PEI aerogels

FT-IR spectroscopy was employed to identify the structural feature of the CLE@PEI aerogel. In Fig. 3a, the stretching vibration of C=O at 1715 cm⁻¹ in CLE@PEI has completely disappeared. The characteristic C=N stretching peak at 1641 cm⁻¹ exhibited enhanced intensity and width due to the overlap with the C=C peak originating from the enol tautomerization of the ketone carbonyl group in CLE and the formation of an imine between CLE and PEI.^38,39 Furthermore, the XPS spectra confirmed the successful Schiff base reaction between PEI and CLE. In contrast to the XPS spectrum of CLE, the CLE@PEI aerogels exhibited a new peak at 398.70 eV (Fig. 3b), corresponding to the N 1s of amino groups and imine bonds. Additionally, the nitrogen content in CLE@PEI aerogels increased from 4.02% to 20.19% with the rise of the PEI content (Table S2†), indicating a significant enrichment of amine groups in the aerogel. The detailed deconvolution analysis of the C 1s peak for the CLE@50PEI aerogel exhibited four distinct peaks, namely O–C=O (288.07 eV), C=N (287.24 eV), C–O or C–N or O–C–O (285.99 eV) and C–C (284.8 eV) (Fig. S5a†),⁴⁰ indicating the successful preparation of the CLE@PEI aerogels. Moreover, TGA was employed to assess the thermal stability of the CLE@PEI aerogels with different PEI contents, as confirmed by Fig. S5b.† It was noteworthy that the maximum degradation temperatures of the CLE@PEI aerogels were 290.7–315.2 °C, higher than that of CLE (289.0 °C). All of these indicated the successful cross-linking and formation of a 3D network structure between CLE and PEI, which could enhance the structural stability of the aerogel.

Fig. 3 (a) FT-IR spectra of CLE, PEI and the CLE@50PEI aerogel. (b) XPS spectra of CLE and the CLE@PEI aerogels. (c) NH₂/C=O molar ratio and density of the CLE@PEI aerogels. (d) SEM image of the CLE@PEI aerogels with different PEI contents.

The NH₂/C=O molar ratio is of critical significance in determining the structure specificity of CLE@PEI aerogels during their preparation. Firstly, the NH₂/C=O molar ratio and the density of the CLE@PEI aerogels with different PEI contents are illustrated in Fig. 3c. The density ranges of the aerogels increased from 37.5 to 87.6 mg cm⁻³ as the molar ratio of NH₂/C=O increases, but it still remained at a rather lower density level. As shown in Fig. 2e, the CLE@50PEI aerogel exhibited no bending deformations on the dandelion fibers, thereby verifying its ultra-lightness. Moreover, the internal network structure of the CLE@PEI aerogels was observed by the SEM images. As shown in Fig. 3d, all of the aerogels with various PEI contents exhibited a uniform honeycomb-like porous structure with a pore size range of about 100–200 μm, which was beneficial for the adhesion of precious metal ions to the material. For the CLE@50PEI aerogel, the average diameter was approximately 144 μm (Fig. S6†). However, as the PEI content increases, the pore structure gradually became uneven, particularly in CLE@70PEI and CLE@90PEI, where the NH₂/C=O molar ratios were 1.27 and 1.68, respectively, indicating that the excess PEI in these adsorbents results in significant shrinkage and collapse.

3.3. Adsorption performance of CLE@PEI aerogels

The adsorption capacity of the CLE@10PEI-CLE@90PEI aerogels for Au(III), Pt(IV) and Pd(II) in 0.1 M HCl was investigated systematically, as depicted in Fig. 4a. It was distinctly observed that various CLE@PEI aerogels exhibited an increasing tendency with the improvement of the PEI content. The adsorption capacity for Au(III) increased from 424.7 mg g⁻¹ for CLE@10PEI to a maximum of 1585.0 mg g⁻¹ for CLE@90PEI. For Pt(VI) and Pd(II), the CLE@50PEI aerogels demonstrated the absorption equilibrium with 1119.8 and 490.2 mg g⁻¹, respectively. In addition, all CLE@PEI aerogels with different PEI contents indicated a significantly higher adsorption capacity for Au(III) compared to Pt(IV) and Pd(II), which was a common phenomenon that was presumably related to different adsorption mechanisms, as discussed later.⁴¹

Fig. 4 (a) Adsorption capacity of CLE@PEI aerogels with different PEI contents for Au(III), Pt(IV) and Pd(II) (C₀: 2000 mg L⁻¹, T: 298 K). (b) Effect of HCl concentration on the Au(III), Pt(IV) and Pd(II) adsorption by CLE@50PEI, and digital photographs of the Au(III) reduction at 0.01 mol L⁻¹ HCl (C₀: 50 mg L⁻¹, T: 298 K). (c) Adsorption capacity of the CLE@50PEI aerogel for Au(III), Pt(IV) and Pd(II) versus time (C₀: 2000 mg L⁻¹, T: 298 K). Corresponding adsorption kinetics fitting curve from the (d) pseudo-first-order model, (e) pseudo-second-order model, and (f) intraparticle diffusion model.

The HCl concentration represented a crucial environmental factor in the adsorption process, exerting a direct influence of the adsorbent's surface charge and the properties of the precious metal ions.⁴² According to the adsorption behavior shown in Fig. 4b, the adsorption efficiency of the CLE@50PEI aerogel for Pt(IV) and Pd(II) was observed to increase with the continuous rise in the HCl concentration from 0.01 to 0.1 M. Meanwhile, at 0.01 M HCl concentration, the unprotonated NH₂ groups in the CLE@50PEI aerogel catalyzed the reduction of Au(III), which caused the solution to rapidly turn purple-black (Fig. 4b). Subsequently, it was noticed that the CLE@50PEI aerogel exhibited the highest adsorption efficiency for Au(III), Pt(IV) and Pd(II) at 0.1 M HCl concentration, reaching 90%, 86% and 48%, respectively. Thereafter, the adsorption efficiency gradually decreased as the HCl concentration continued to increase. The adsorption behavior of the CLE@50PEI aerogel could be attributed to both the speciation of precious metal ions and the surface functional groups of CLE@50PEI at the investigated HCl concentration. As the concentration rose to 0.1 M HCl, predominant chemical forms of AuCl₄⁻, PtCl₆^2– and PdCl₄^2– demonstrated favorable electrostatic interactions with the CLE@50PEI cations.^43,44 Furthermore, when the HCl concentration exceeded 0.1 M, an excess of Cl⁻ competed with AuCl₄⁻, PtCl₆^2– and PdCl₄^2–, leading to co-ion effects that diminish the adsorption efficiency for Au(III), Pt(IV) and Pd(II).^45,46

The adsorption kinetics of the CLE@50PEI aerogel were investigated to understand the rate-controlling steps, which was crucial for predicting the adsorption rate and simulating the adsorption process. As seen in Fig. 4c, the CLE@50PEI aerogel exhibited a rapid adsorption rate for precious metal ions within the initial 15 min. This quick uptake was attributed to the strong electrostatic interactions between the numerous positively charged adsorption sites on the CLE@50PEI aerogel surface and the negatively charged precious metal complexes, leading to a continuous rise in the adsorption capacity. Subsequently, the adsorption rate reached a plateau with the equilibrium adsorption capacities of 999.0 and 467.7 mg g⁻¹ for Pt(IV) and Pd(II) at 30 minutes, respectively. In contrast, the equilibrium adsorption capacity for Au(III) was 1222.4 mg g⁻¹, which was achieved after 120 min. The experimental data for the adsorption kinetics were analyzed using pseudo-first-order^47,48 and pseudo-second-order kinetic models (Fig. 4d and e).^49,50 Based on the correlation coefficient values (R²) (Table S3†), the pseudo-second-order kinetic model fitted better, indicating that the chemical adsorption processes¹³ (such as ion exchange, electrostatic attraction and complexation) were the primary adsorption mechanism for CLE@50PEI.⁵¹

Furthermore, considering the physical diffusion of precious metal ions during the adsorption process, the diffusion mechanism was further revealed using an intra-particle diffusion model.⁵² As displayed in Fig. 4f and Table S4,† the curve was mainly divided into two linear segments, each representing a different rate constant k. The relatively higher k₁ value in the first stage indicated a rapid adsorption state attributed to surface adsorption, while the k₂ value in the second stage reflected a slower adsorption state associated with internal diffusion. This implied that intraparticle diffusion predominantly controls the overall adsorption rate, suggesting that the adsorption process primarily occurs within the internal structure of the CLE@PEI aerogel. Furthermore, the R² value for intraparticle diffusion was less than 1, and the fitted curve did not pass through the coordinate origin,⁵³ indicating the possibility of multiple adsorption mechanisms jointly controlling the diffusion process.

Isothermal experiments were conducted in the temperature range of 298 to 318 K using different initial concentrations of Au(III), Pt(IV) and Pd(II). The adsorption isotherm data followed an isothermal curve of increasing adsorption capacity with increasing initial concentration, demonstrating that the maximum adsorption capacity relied on the initial concentration of the adsorbent.⁵⁴Fig. 5a shows that the adsorption capacity increased rapidly with the initial concentration of precious metal ions (0–4000 mg L⁻¹) and then reached equilibrium at 298 K. The adsorption data were analyzed using the Langmuir^55,56 and Freundlich⁵⁷ isotherm models. Obviously, the Langmuir model provided higher correlation coefficients (R² ranging from 0.967 to 0.999) compared to the Freundlich model, indicating that the Langmuir isotherm model better describes the adsorption behavior (Fig. 5b–d, Fig. S7a–d† and Table 1). This suggests that the adsorption sites were uniformly distributed across the CLE@50PEI aerogel and the process followed monolayer chemisorption.⁵⁸ According to the Langmuir model, the maximum adsorption capacities of CLE@50PEI for Au(III), Pt(IV) and Pd(II) at 298 K were calculated to be 1752.0, 1420.5 and 495.1 mg g⁻¹, respectively (Fig. 5e), which were relatively consistent with the obtained experimental results (1620.0, 1297.0 and 473.7 mg g⁻¹, respectively). Furthermore, the dimensionless constant R_L of the Langmuir model was found to be between 0.024 and 0.962, while b_F < 1 indicated that CLE@50PEI has excellent potential for adsorption of Au(III), Pt(IV) and Pd(II) from e-waste solutions.⁵⁹

Fig. 5 (a) Adsorption capacity of the CLE@50PEI aerogel for Au(III), Pt(IV) and Pd(II) at different initial concentrations (T: 298 K). Corresponding adsorption isotherm fitting curves from the Langmuir model for (b) Au(III), (c) Pt(IV) and (d) Pd(II) adsorption on the CLE@50PEI aerogel. (e) Maximum adsorption capacity at different temperatures obtained from Langmuir isotherms. (f) Adsorption performance of the CLE@50PEI aerogel on Au(III), Pt(IV) and Pd(II) compared with other biomass adsorbents.

Table 1 Parameters derived from the Langmuir and Freundlich adsorption isotherms for the CLE@50PEI aerogel. Initial concentration of metal ions: 2000 mg L⁻¹

Samples	T (K)	Langmuir: Ce/qe = Ce/qm + 1/qmKL			Freundlich: ln qe = ln KF + bFln Ce
		q m (mg g^−1)	K L	R ^2	K F	b F	R ^2
Au(III)	298	1752.0	0.006	0.9993	43.760	0.507	0.8808
	308	1269.5	0.289	0.9669	56.007	0.459	0.8219
	318	925.9	0.480	0.9846	62.056	0.394	0.7652
Pt(IV)	298	1420.5	0.004	0.9953	38.719	0.473	0.9305
	308	1120.8	0.004	0.9841	45.460	0.418	0.869
	318	1056.0	0.005	0.9854	50.763	0.396	0.859
Pd(II)	298	495.1	0.012	0.9970	35.732	0.347	0.7232
	308	357.1	0.279	0.9940	46.646	0.280	0.5668
	318	342.5	0.564	0.9923	43.010	0.286	0.5654

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A quantitative comparison of the maximum adsorption capacity of the CLE@50PEI aerogel reported in the literature is summarized in Fig. 5f and Table S5.† Compared to biomass adsorbents such as protein amyloid nanofibrils,⁶⁰ chitosan derivatives,^59,61,62 cellulose derivatives,^15,26,63 tannin derivatives,^41,44,64,65 sodium alginate derivatives,²⁵ lignin derivatives^66,67 and waste paper,⁶⁸ the CLE@50PEI aerogel in this work exhibited excellent adsorption capacities for Au(III), Pt(IV) and Pd(II), with maximum adsorption capacities of 1752.0, 1420.5 and 495.1 mg g⁻¹, respectively. The remarkable adsorption capacities of the CLE@50PEI aerogel were due to its amine-functionalized surface and multi-pore structure, which enhanced the adsorption sites for precious metal ions, enabling efficient recovery. Unlike most modified biobased adsorbents, which were typically powdered, this cellulose-based aerogel was environmentally friendly, easily recyclable and provides a sustainable option for precious metal recovery.

3.4. Mechanism study

XRD analysis was utilized to examine the crystallinity of the aerogel after the adsorption of precious metals. In the XRD patterns, the reduction of Au(III) to Au(0) was confirmed by the observation of Au(0) peaks at 37.95°, 44.38°, 64.52°, 77.54° and 81.72° in the spectra of the Au-loaded CLE@50PEI (Fig. 6a).⁶⁹ Conversely, the XRD patterns for the Pt-loaded CLE@50PEI and Pd-loaded CLE@50PEI did not show any characteristic peaks for elemental platinum and palladium (Fig. 6a). To gain further understanding of the adsorption mechanisms, XPS measurements of the CLE@PEI aerogel after metal adsorption were carried out to analyze the elements of the aerogel, as well as the atomic valence states of Au, Pt and Pd after adsorption.^69–71 As shown in Fig. 6b, the new peaks at 86, 76, 338 and 197 eV were assigned to Au 4f, Pt 4f, Pd 3d and Cl 2p, respectively, after loading the precious metals. The high-resolution spectra of Au 4f, Pt 4f, Pd 3d are presented at Fig. 6c–e. Two pairs of Au 4f_7/2 and Au 4f_5/2 corresponding to Au⁰ and Au(III) were observed simultaneously at 83.13/86.87 and 85.82/89.48 eV (Fig. 6c), respectively, indicating that the adsorption–reduction process of CLE@PEI adsorbents favors the overall adsorption of Au.⁷²

Fig. 6 (a) XRD pattern and (b) XPS analysis of CLE@50PEI, Au load-CLE@50PEI, Pt load-CLE@50PEI and Pd load-CLE@50PEI. XPS spectrum of the CLE@50PEI aerogel after adsorption of (c) Au(III), (d) Pt(IV), (e) Pd(II). (f) Digital micrograph image of the CLE@50PEI after adsorption of Au(III). (g) TEM micrograph of an Au-platelet. (h) Color map of the EDS spectrum image of the particle in (g), showing the color overlay of Au. (i) HR-TEM image of the lattice fringes of the Au (111) plane.

The gold nanoparticles and elemental single crystalline flakes are also presented in Fig. 6f and g, confirming the reduction process. Moreover, Fig. 6h presents the color of the Au signal of the particle and confirmed the hexagonal nanosheets of the Au-platelets (purple). The high-resolution TEM (Fig. 6i) of the corresponding selected area displays the lattice fringes with the interplanar spacings of ∼0.235 nm assigned to the Au (111) plane.

Meanwhile, two pairs of Pt 4f_7/2 and Pt 4f_5/2 corresponding to Pt(II) (PtCl₄^2–) and Pt(IV) (PtCl₆^2–) were observed at 72.88/76.19 and 75.09/78.46 eV (Fig. 6d),^70,73 respectively, indicating the partial reduction of Pt(IV). Moreover, two pairs of Pd 3d_5/2 and Pd 3d_3/2 corresponding to Pd(II) (PdCl₄^2–) and Pd(II) (Pd(H₂O)Cl₃⁻) were also detected at 338.22/343.42 and 336.31/341.21 eV, respectively (Fig. 6e).^73,74 Benefiting from the high reduction potentials (ORP: Au(III) = 1.00 V, Pt(IV) = 0.73 V), the reduction of Au(III) and Pt(IV) was easily achieved.⁷⁵ In contrast, Pd could not be reduced due to its low reduction potential (0.62 V). These results were in agreement with the XRD results shown in Fig. 6a. In addition, the metal morphology of CLE@50PEI after adsorption of Pt(IV) and Pd(II) could be observed in Fig. S8a and b.† It was not surprising that the CLE@PEI aerogels exhibited an exceptionally high uptake capacity for Au(III) compared to Pd(II) and Pt(IV), which might have been attributed to the subsequent reduction and release of elemental gold, freeing up more adsorption sites for Au(III).

The adsorption mechanisms are determined by the adsorbent's surface properties, particularly the functional groups anchored to its surface. As shown in Fig. 7a, the N 1s spectrum of the CLE@50PEI aerogel before metal adsorption was deconvoluted into three peaks representing the –NH₂ (401.19 eV), –NH– (400.10 eV) and –N < (398.76 eV) groups, respectively, which reflected the functional properties of PEI.^70,76 Following the absorption of precious metal ions, these characteristic amide groups were protonated in acidic chloride medium, leading to the creation of several positive centers (Fig. 7b–d). For Au load-CLE@50PEI, the nitrate and protonated amine groups are illustrated at 402.09 eV (–NO₂), 401.29 eV (–NH₃⁺) and 400.53 eV (–NH₂⁺), as well as 399.79 eV (–NH⁺), respectively (Fig. 7b). The protonated forms of these amine groups in the CLE@PEI adsorbents may lead to the adsorption of anionic precious metal chlorine complexes through ion exchange and subsequent electrostatic attraction.^70,77 In addition, the appearance of the –NO₂ peak substantiated the reduction of Au(III) by the CLE@PEI adsorbents. The reduction process of Au(III) is illustrated with formula (3), as shown in below.²⁵ The majority of functional groups containing N atoms are capable of acting as mono-, bis- and polydentate ligands, facilitating the formation of complexes with precious metals.⁴¹ The FTIR spectra of CLE@50PEI and CLE@50PEI adsorbed with precious metals were further analyzed to elucidate the complexation mechanism. As seen in Fig. 7e, significant changes are observed in the spectra of Au, Pt and Pd load-CLE@50PEI. The –NH₂ stretching vibration peak, initially observed at 3433 cm⁻¹ shifted to 3396 cm⁻¹ and 3422 cm⁻¹, respectively, suggesting that an additional complexation mechanism was involved in CLE@50PEI adsorption,⁷⁸ as illustrated in Fig. 7f.

Fig. 7 (a) N 1s peaks of the CLE@50PEI aerogel before adsorption. N 1s peaks of the CLE@50PEI aerogel after adsorption of (b) Au(III), (c) Pt(IV) and (d) Pd(II). (e) FT-IR spectra of the CLE@50PEI and Au-, Pt- and Pd-loaded-CLE@50PEI. (f) Possible adsorption mechanism of the CLE@50PEI aerogel for Au(III), Pt(IV) and Pd(II).

Reduction of Au(III):

In summary, the amine groups of the CLE@PEI aerogels underwent electrostatic interactions via ion exchange and complexation interactions. Simultaneously, the reductions of Au(III) and Pt(IV) were also proceeding, all of which facilitated the adsorption of precious metals.

3.5. Adsorption selectivity and recyclability of the CLE@50PEI aerogel

The adsorption selectivity of precious metal ions by aerogels was investigated to determine the potential interference of co-existing ions. The acidic leach solution composition of the discarded mobile phone printed circuit boards was simulated for industrial applications of the developed adsorbents, and the adsorption selectivity towards precious metals from solutions containing Cu(II) (23.8 mg L⁻¹), Zn(II) (5.6 mg L⁻¹), Ni(II) (3.0 mg L⁻¹), Fe(III) (667.0 mg L⁻¹), Au(III) (331.0 mg L⁻¹), Pd(II) (121.0 mg L⁻¹) and Pt(IV) (743.0 mg L⁻¹) was explored.^41,79,80 As displayed in Fig. 8a–c, the CLE@50PEI aerogel possessed an outstanding selectivity for Au(III), Pt(IV) and Pd(II), with the adsorption rates of 75.7%, 67.5% and 97.1%, respectively. On the contrary, the adsorption capacity for Zn(II), Fe(III), Cu(II) and Ni(II) was maintained at a fairly low level. This phenomenon mainly occurred due to the fact that at 0.1 M HCl concentration, the stable AuCl₄⁻, PtCl₆^2–/PtCl₄^2– and PdCl₄^2– complexes could be formed and attained charge balance with protonated amine groups anchored in the CLE@50PEI aerogel.⁴⁴ This tight absorption performance of the ionized precious metals is disadvantageous for the absorption of other base metals.

Fig. 8 (a) Concentration of the metal ion mixture before and after treatment with the CLE@50PEI aerogel. (b) Adsorption capacity of the CLE@50PEI aerogel for different metal ions. (c) Adsorption efficiency of the CLE@50PEI aerogel for different metal ions. (d) Reusability of the CLE@50PEI aerogel (C₀: 1000 mg L⁻¹, T: 298 K).

The reusability of adsorbent materials is another crucial factor in determining their practical feasibility. In the process of regeneration, acidic thiourea is commonly employed to desorb precious metals from loaded adsorbents by forming metal complexes.^25,81 Building on this approach, in the present study, a mixture of 0.5 M thiourea in 0.1 M HCl media served as the eluent to regenerate the adsorbent CLE@50PEI aerogel. As displayed in Fig. 8d, the adsorption capacity of CLE@50PEI for Au(III) decreased from 900 to 212 mg g⁻¹ as the number of adsorption–desorption cycles increased, while the desorption rate still remained above 90%. The decline in the Au(III) adsorption capacity during regeneration might be due to the reduction of Au(III) to gold particles, in which some amine groups were reduced to nitro groups, accompanied by a slight collapse of the pore structure, as observed in the SEM images (Fig. S9a and b†). TGA analysis showed a significant drop in the maximum decomposition temperature of CLE@50PEI, from 303.7 °C before adsorption to 189.7 °C after adsorption, further indicating structural changes and pore collapse, as shown in Fig. S10.† The adsorption capacity of CLE@50PEI for Pt(IV) and Pd(II) also remained almost unchanged with the increasing number of adsorption–desorption cycles, while the desorption rate decreased to 56.5% and 67.0%, respectively. The above situation might be due to the strong affinity of the special sites for Pt(IV) and Pd(II). The results confirmed that the desorption of the loaded precious metal ions was achieved using the mixture of 0.1 M HCl and 0.5 M thiourea solution. The selectivity and recyclability of the CLE@50PEI aerogel were compared with those of other recently reported adsorbents, including biobased adsorbents,^60,82 covalent organic frameworks (COF)⁸³ and polymeric-based adsorbents,^84–88 as shown in Table S6.† The CLE@50PEI aerogel demonstrated superior selectivity for Pd(II) (97.1%), although its selectivity for Au(III) and Pt(IV) was relatively lower. In terms of recyclability, it achieved five adsorption–desorption cycles, aligning with most studies. This made it a promising candidate for practical applications.

4. Conclusion

In this work, fully biobased non-cytotoxic, water-soluble CLE can be used as a robust precursor to construct a series of cellulose-based aerogel adsorbents via the Schiff base reaction between CLE and PEI in water. A sustainable and facile strategy was developed to fabricate the CLE@PEI aerogel for selectively recovering precious metals from e-waste. Benefiting from the steady imine architecture and abundant amine groups, the CLE@50PEI aerogel exhibited outstanding adsorption performance for Au(III) (1752.0 mg g⁻¹), Pt(IV) (1420.5 mg g⁻¹) and Pd(II) (495.1 mg g⁻¹). The adsorption process followed the pseudo-second-order kinetic model and Langmuir isotherm behavior, indicating a monolayer chemisorption. A new “adsorption in situ reduction” mechanism was proposed, which involved the ion exchange coupled with electrostatic interaction and complexation, accompanied by the in situ reduction of Au(III) to gold nanoparticles and single crystalline flakes. In addition, the protonated amine groups allowed the aerogels to selectively capture the chloride anion complexes of precious metals in mixed solutions of e-waste. Moreover, the CLE@50PEI aerogel exhibited reusability during the 5 adsorption–desorption cycles. In summary, the prepared environmentally sustainable cellulose-based adsorbent possesses excellent adsorption capacity, high selectivity adsorption and stable performance under strong acid conditions, holding significant potential in practical applications for the recovery of precious metals from e-waste.

Data availability

All the data supporting this article have been included in the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (22275041, 22173094), Introduced Talent Research Project of Guizhou University ([2022]16), Basic Research Project of Guizhou University ([2023]01), Science and Technology Department of Guizhou Province ([2024]087, Z2024021), Qiankehe-Platform JSZX(2025)004 and the Qiankehe-Central Government Guided Local Development Funds (2025)032.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5gc00100e

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