Highly Stable Covalent Organic Framework for Palladium Removal from Nuclear Wastewater

Abstract

The effective management of High-Level Liquid Waste (HLLW) is critical for environmental and human health protection. The presence of platinum group metals (PGMs) in HLLW, particularly their refractory nature due to their high melting points, complicates vitrification processes. This study presents a targeted adsorption strategy using COF-42 for Pd²⁺ sequestration in HLLW systems. The comprehensive characterization of COF-42 and its Pd-loaded counterpart (Pd@COF-42) via PXRD, FT-IR, TGA, XPS, and SEM confirms structural robustness and successful Pd²⁺ incorporation. Kinetic and thermodynamic analyses reveal pseudo-second-order adsorption behavior with a maximum capacity of 170.6 mg/g, highlighting the exceptional Pd²⁺ affinity. Systematic optimization identifies HNO₃ concentration (≤3 M) and adsorbent dosage (≤30 mg) as critical parameters governing adsorption efficiency through protonation–deprotonation equilibria. Furthermore, COF-42 exhibits superior selectivity for Pd²⁺ over 13 competing metal ions and maintains ~80% adsorption efficiency after four regeneration cycles. These performance metrics originate from the synergistic interplay of (1) framework flexibility enabling adaptive Pd²⁺ coordination, (2) hierarchical porosity facilitating ion diffusion, and (3) dense –NH/–NH₂ groups acting as electron-rich chelation sites.

1. Introduction

High-Level Liquid Waste (HLLW) is produced during nuclear power generation and must undergo strict treatment and disposal to prevent environmental and human health risks due to its high radioactivity and toxicity. The current common method for treating HLLW involves glass curing and then placing it underground to isolate it from the environment [1,2]. However, the temperature during the glass curing process is below 1200 °C, which is insufficient to melt platinum group metals (PGMs), causing them to accumulate at the bottom of the melting pool and disrupt the furnace’s normal operation. Therefore, it is crucial to separate PGMs from HLLW before the glass curing process to guarantee the treatment procedure is conducted successfully.

Furthermore, separating PGMs like Pd²⁺ from HLLW allows for recycling and reuse, offering significant industrial value due to their unique properties and wide-ranging applications in catalytic converters, electronics, pharmaceuticals, and jewelry [3,4,5,6,7,8,9,10,11,12]. The demand for PGMs is expected to remain robust in the future. However, the separation of PGMs is a crucial process across various industries, with different methods proposed and utilized, including chemical and physical separation, each with its own set of advantages [13,14]. By contrast, chemical methods are commonly employed in mining and refining operations to extract PGMs from ores or other materials. Advanced techniques like ion exchange and adsorption are also utilized for precise and efficient PGM separation. Notably, adsorption has emerged as a prominent method due to its simple process, exceptional efficiency (achieving up to 99.9% removal capacity), and low energy consumption compared to other methods [15]. Various materials such as activated carbon, mesoporous silica, chitosan, and 2D materials have been employed as adsorbents [16,17,18,19,20]. Ideally, an adsorbent should possess a high surface area, excellent stability, and high porosity with specific adsorption sites [21].

Here, covalent organic framework (COF) materials have emerged as a promising option in the realm of adsorption. These materials are recognized as a novel category of crystalline porous substances, with their organic components consisting entirely of light atoms connected by covalent bonds like C-C, C-N, and C-O [22,23,24]. COFs offer numerous advantages due to their exceptional crystallinity, high porosity, adjustable pore size, ease of processing, and low density [25,26,27]. These distinctive characteristics empower COFs to efficiently adsorb a variety of molecules and ions. The remarkably low density of COFs stems from their well-organized porous frameworks and light atoms, facilitating their broad utilization in areas such as photoelectricity [28,29], electronic sensing [30,31,32], catalysis [33], drug delivery [34,35], supercapacitors [36], and adsorption [37,38,39,40]. Furthermore, researchers have compiled information on the synthesis, properties, and applications of COFs [41]. Substantial investigations have been conducted on the adsorption and recovery of metals and organic pollutants, including heavy metal ions like Au, Cu, Fe, Pb, Hg, and Co; pesticides; and industrial by-products, shedding light on the adsorption mechanisms involved [21,32,42,43,44,45].

Using COF materials for adsorption offers a significant advantage from their ability to selectively capture specific molecules and ions due to their specific affinity within the binding sites. Through precise structural design, researchers can customize COF material properties to target substances, presenting promising potential for separating molecules and ions under various conditions. Apart from their selectivity, COF materials also demonstrate high adsorption capacities. The extensive surface area of COF materials allows ample space for adsorption, leading to high capacities. This characteristic renders COF materials suitable for diverse applications, ranging from environmental cleanup to gas storage. Additionally, COF materials are recognized for their stability and durability. In contrast to conventional adsorbents that may deteriorate over time, COF materials retain their structure and adsorption capabilities even after numerous adsorption and desorption cycles. This longevity positions COF materials as a cost-efficient and sustainable choice for adsorption purposes.

In this study, a crystalline mesoporous COF material called COF-42 is synthesized via a hydrothermal condensation reaction involving 1,3,5-triformylbenzene (TB) and 2,5-diethoxyterephthalohydrazide (DTH). The resulting COF-42 exhibits a significant specific surface area and excellent physic-chemical stability, presenting the first application for adsorbing Pd²⁺ in simulated HLLW. Compared to the previous work [46], this study employs S-free and N-rich groups (-N-NH₂), demonstrating their robust Pd²⁺ adsorption capability under harsh acidic environments. Although the N content in this study is lower than that of Xie’s work [47], it still maintains a favorable Pd²⁺ adsorption performance, further highlighting the critical role of N in Pd²⁺ adsorption. In summary, this work systematically investigates the Pd²⁺ adsorption process using an N-rich group of functionalized COFs in simulated HLLW, elucidates the adsorption mechanism, and provides fundamental research support for the removal and recovery of Pd²⁺ from HLLW.

2. Materials and Methods

2.1. Materials

Diethyl 2,5-dihydroxy-terephthalate (96%), 1,3,5-Benzenetricarboxaldehyde (97%), and 1,4-Dioxane (≥99.5%) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). All other regents, such as potassium carbonate, potassium iodide, acetone, ethyl iodide, methanol, methylene chloride, ethanol, and hydrazine hydrate were acquired from Sinopharm Chemical Reagent Co., Ltd., also located in Shanghai, China. and were procured as analytical grade materials. These solvents were employed as received without additional purification steps.

2.2. Preparation of COF-42

The synthesis of DTH followed the established protocol outlined in reference [48]. In brief, 0.5 g diethyl 2,5-dihydroxy-terephthalate was combined with 2 g potassium carbonate and 40 mg potassium iodide in 40 mL acetone. To this mixture, 400 mL ethyl iodide was introduced, and the reaction system was heated under reflux with continuous stirring for 48 h. The resultant mixture underwent filtration, during which the solid residue was washed thoroughly with heated acetone to yield a yellow solution. The subsequent removal of solvent via rotary evaporation produced a residue that was suspended in water and subjected to extraction. Organic phases were pooled, washed with brine solution, desiccated, and concentrated using rotary evaporation. Recrystallization in methanol afforded the purified solid. This material was then dissolved in a mixture of 45 mL ethanol and 6 mL hydrazine hydrate, followed by refluxing for 40 h. White crystalline precipitates formed upon cooling, which were isolated through filtration and subsequently washed with both water and ethanol.

Synthesis of COF-42: A pale-yellow powder was synthesized by dissolving 5 mg of TB and 18 mg of DHT in 250 μL of anhydrous dioxane and 750 μL of mesitylene. The mixture underwent sonication for 15 min in an ultrasonic bath, followed by the addition of 100 μL of 6 M aqueous acetic acid. The reaction system was flash-frozen at 77 K, evacuated to 150 mTorr, and thermally treated at 120 °C for 72 h. A pale-yellow solid precipitated at the tube base, which was collected via filtration or centrifugation. The product was sequentially rinsed with anhydrous dioxane and acetone, soaked in acetone for 24 h, and dried under ambient conditions at 10⁻² mTorr for 12 h to yield the final powder (Figure 1 and Figure S1) [49].

2.3. Adsorption Performance of COF-42 Towards Pd²⁺

A 10 mmol/L metal ion solution, 5 mol/L of HNO₃, and a 5 mol/L NaNO₃ working solution were prepared. The solutions’ pH were adjusted using HNO₃. A measured quantity of adsorbent was mixed with the metal ion solution in a glass vessel and agitated in a temperature-controlled water bath at 280 rpm. Solid–liquid separation was performed using a 0.45 μm syringe filter, and the filtrate was diluted for analysis. The Pd²⁺ concentration in the supernatant was quantified via ICP-OES. The partition coefficient (K_d, mL/g) and equilibrium adsorption capacity (q_e, mg/g) were calculated using Equations (1) and (2).

$$K_d={\displaystyle \frac{C_i-C_e}{C_e}}\times {\displaystyle \frac{V}{m}}$$

(1)

$$q_e={\displaystyle \frac{(C_i-C_e)\times V}{m}}$$

(2)

where V represents the total solution volume (mL), m is the adsorbent mass (g), and C_i and C_e are the initial and equilibrium concentrations of Pd²⁺, respectively.

2.4. Characterizations

Structural Characterization Protocol, Crystallographic Analysis: Phase identification was performed on a Rigaku SmartLab 3KW high-resolution X-ray diffractometer (Cu Kα radiation, λ = 1.5406 Å) operating in the range of 3° to 30° (2θ), with a continuous scanning rate of 5°·min⁻¹. Thermal Behavior: Thermal analysis was conducted using Mettler Toledo TGA/DSC3+ instrumentation under a dynamic N₂ purge. Samples underwent controlled heating (10 K·min⁻¹) from 298 K to 1073 K in α-Al₂O₃ crucibles. Microstructural Imaging: Surface topography and elemental mapping were acquired via field-emission SEM (Hitachi Regulus 8100, 5 kV acceleration voltage, Hitachi Ltd., Tokyo, Japan) coupled with energy-dispersive X-ray spectroscopy (EDX). Porosity Profiling: N₂ physisorption at 77 K (Micromeritics 3Flex, Mettler Toledo, Zurich, Switzerland) determined the specific surface area (BET model) and pore size distribution (NLDFT method). Spectroscopic Studies: FTIR (Thermo Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA) was recorded in transmission mode (4000–400 cm⁻¹, 4 cm⁻¹ resolution) using KBr pellets. XPS (Thermo ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) was conducted with a monochromatic Al Kα source, charge neutralization was applied, and the spectra calibrated to C 1s. Elemental Quantification: ICP-OES (Horiba JY2000-2, HORIBA, Paris, France) measurements followed microwave-assisted acid digestion (HNO3/HCl 3:1 v/v, 453 K).

2.5. Calculations

Periodic density functional theory (DFT) calculations were performed using the Vienna Ab Initio Simulation Package (VASP) [50]. The Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) functional was employed to describe electron exchange–correlation interactions [51]. Core–electron interactions were represented by projector-augmented wave (PAW) pseudopotentials with a plane-wave basis set (kinetic energy cutoff: 450 eV), explicitly treating valence electrons [52]. Convergence criteria were as follows: (1) Electronic iterations were deemed converged when total energy fluctuations fell below 10⁻⁵ eV; (2) structural relaxation ceased when Hellmann–Feynman forces were reduced below 0.05 eV/Å; (3) a Γ-centered 1 × 1 × 1 k-point mesh was utilized for slab model sampling; and (4) Fermi surface broadening was applied via a Gaussian smearing scheme (σ = 0.05 eV). Long-range van der Waals interactions were accounted for using the Grimme DFT-D3 method with Becke–Johnson damping to enhance physisorption energy precision [53]. A periodic boundary condition framework was implemented with a 15 Å vacuum layer along the z-direction to suppress inter-slab coupling. The adsorption energy (Eads) of the adsorbate molecule was determined as follows:

E_ads = E_mol/surf − E_surf − E_mol

(3)

where E_mol/surf, E_surf, and E_mol correspond to the energy of the adsorbate molecule adsorbed on the surface, the energy of clean surface, and the energy of molecule, respectively. All energies were referenced to the fully relaxed ground states.

3. Results and Discussion

3.1. The Characterization of COF-42

Surface and Porosity Analysis: The surface area and pore structure of COF-42 are evaluated through nitrogen adsorption isotherm measurements at 77 K. The isotherm exhibits pronounced adsorption activity at a low relative pressure (P/P° < 0.04), followed by a distinct stepwise increase between P/P° = 0.04–0.18. A narrow hysteresis loop in the adsorption–desorption curve confirms a type-IV mesoporous material (Figure 2a). The specific surface area is calculated to be 614.8 m²/g, with an average pore diameter of 2.6 nm determined by Barrett–Joyner–Halenda (BJH) analysis (Figure 2b).

Crystallinity Characterization: Powder X-ray diffraction (PXRD) analysis of COF-42 reveals characteristic peaks at 2θ = 3.2° (d = 27.59 Å, (100)), 7.3° (d = 12.10 Å, (200)), and 9.1° (d = 9.71 Å, (210)), consistent with its crystalline framework (Figure 2c) [32,46]. Bragg’s law-derived lattice spacings confirm the structural integrity and high crystallinity of the material, critical for its adsorption efficiency.

Thermal Stability Evaluation: Thermogravimetric analysis (TGA) demonstrates COF-42’s thermal resilience (Figure 2d). Minimal mass loss (~6.5%) below 320 °C corresponds to residual solvent evaporation [54]. Structural stability persists up to 320 °C, with major decomposition (~43.1% mass loss) occurring at 320–550 °C due to framework functional group degradation. Above 550 °C, progressive carbon backbone pyrolysis dominates.

3.2. The Sorption Performance of COF-42 Towards Pd²⁺

After confirming the COF-42 structure, the dynamics of COF-42 towards Pd²⁺ adsorption are further studied. Firstly, the adsorption isotherm is determined using 5 mg of COF-42 at room temperature in a 3 M HNO₃ solution. The adsorption isotherm represents the relationship between adsorption capacity and the equilibrium concentration of metal ions by altering the concentration of single metal ions at a specific temperature. Typically, there are two main models used to fit the adsorption isotherm curves. The Langmuir model describes adsorption sites that are uniformly distributed with equal capacity, following a monolayer adsorption mechanism. On the other hand, the Freundlich model suggests surfaces with heterogeneous adsorption, featuring varying adsorption energies [55]. The Langmuir and Freundlich models are expressed by specific equations.

$$\mathrm{Langmuir}\mathrm{model}:{\displaystyle \frac{C_{\mathrm{e}}}{q_e}}={\displaystyle \frac{1}{q_{\max }{K}_L}}+{\displaystyle \frac{C_e}{q_{\max }}}$$

(4)

$$\mathrm{Freundlich}\mathrm{model}:\log q_e=\log K_F+{\displaystyle \frac{1}{n}}\log C_e$$

(5)

where q_e and q_m stand for the equilibrium and theoretical saturation adsorption capacities of the adsorbent in millimoles per gram (mg/g), respectively; C_e stands for the equilibrium adsorption concentration of metal ions in millimoles per liter (mg/L); K_L (L/mg) and K_F (mg/g) are the Langmuir and Freundlich model constants, respectively; and n indicates the adsorption strength. Thus, the fitted results are completed using the Langmuir and Freundlich models, as shown in Figure 3a and Table S1. The Freundlich model demonstrates a higher correlation coefficient in isotherm fitting at elevated temperatures, while the correlation coefficients of both models become comparable (>0.9) at lower temperatures (e.g., 298 K), indicating the coexistence of multiple adsorption models [56]. Here, COF-42 exhibits a high saturation adsorption capacity of approximately 170.6 mg/g at 298 K, attributed to the abundant accessible Pd²⁺ adsorption sites distributed across the channel of COF-42.

The adsorption capacity of COF-42 for Pd²⁺ in a 3 M HNO₃ solution is evaluated through the analysis of experimental data using quasi-first-order and quasi-second-order kinetic models, as depicted in Figure 3b. The equations are represented as follows:

$$\ln (q_e-q_t)=\ln q_t-k_{1}t$$

(6)

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

(7)

Here, k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹) denote the pseudo-first-order rate constant and pseudo-second-order rate constant of adsorption, respectively. q_t (mg g⁻¹) indicates the amount of Pd²⁺ adsorbed at time t (min), while q_e (mg g⁻¹) represents the equilibrium amount of Pd²⁺ adsorbed [57].

The fitting results of the adsorption are shown in Table S2, indicating that the adsorption kinetics align well with the pseudo-second-order kinetic model due to the coefficient of determination R² being close to 1. Furthermore, the calculated equilibrium adsorption capacity closely approximates the experimental results compared to the quasi-first-order kinetic calculations. Therefore, the chemical reaction is the controlling step in the adsorption process. The initial adsorption rate is represented by k₂q_e², where 92.36% of the adsorption capacity can be attained within 300 min, leading to equilibrium. The relatively fast kinetics found in COF-42 materials can be due to the very high concentration of -NH/-NH₂ species and a significant number of ordered pore channels that enhance the diffusion of Pd²⁺ ions. After a contact time of as long as 10 h, the Pd²⁺ can be hard to detect due to its concentration falling below the ICP-OES detection limit. These results emphasize the high effectiveness of using COFs as promising candidates for the removal of Pd²⁺ from HNO₃ conditions.

The effect of nitric acid concentration on COF-42’s Pd²⁺ adsorption behavior is systematically studied by modulating solution acidity. As shown in Figure 3c, the adsorption capacity demonstrates a volcano-shaped trend, peaking at 1 M HNO₃. Remarkably, even under strongly acidic conditions (3 M HNO₃), COF-42 retains 93.8% of its maximum adsorption performance. Comparative analysis reveals COF-42’s superior acid tolerance among COFs, with less than 10% fluctuation in adsorption efficiency across the tested pH range—a critical indicator of structural robustness under harsh acidic environments.

Moreover, as depicted in Figure 3d, the adsorption capacity changes along with the solid-to-liquid ratio, and the experiment is conducted at room temperature under 3 M acidity. With the COF-42 dosage increasing from 2mg to 20mg, the Pd²⁺ adsorption rate rises from 21.71% to 97.62%. When the COF-42 dosage increases from 20mg to 30mg, the adsorption rate increases from 97.62% to 99.07%. This indicates that the adsorption of Pd²⁺ tends towards saturation, with the adsorption rate nearing 100%.

Selective Pd²⁺ recovery from multi-metal ion systems is a critical determinant for practical extraction efficiency. To evaluate COF-42’s application potential, competitive adsorption experiments are conducted in 3 M HNO₃ at ambient conditions, revealing exceptional selectivity for Pd²⁺ over 13 coexisting ions (Rh, Ru, Gd, Ho, Dy, Ce, Sn, Re, Sm, Pr, Tb, Mo, Sr), as quantified in Figure 3e. The material demonstrates superior Pd²⁺ uptake with negligible affinity for competing ions. This selectivity originates from (1) –NH/–NH₂ chelation sites aligned within the framework’s pores, (2) ligand flexibility enabling optimal Pd²⁺ coordination geometry, and (3) synergistic pore confinement effects that concentrate binding motifs. The abundance of accessible nitrogenous binding sites within COF-42’s channels establishes its unique capability for targeted Pd²⁺ sequestration, highlighting its viability in complex acidic waste streams.

Further experiments are conducted on COF-42 under conditions of 3 M acidity and a metal ion concentration of 0.05 g/L for repeated use. Initially, 5 mg of COF-42 is added to a 10 mL centrifuge tube, then incubated at 25 °C in a shaker for 20 h. The supernatant is obtained by centrifugation, and the metal ion concentration is measured by ICP-OES. The remaining COF-42 solid is added to thiourea, placed in a constant temperature shaking incubator, and shaken at 25 °C for 24 h. After centrifugation and washing with distilled water, a second adsorption test is conducted. This process is repeated four times. Although there is a slight decrease in the adsorption and desorption capacity of Pd²⁺ after four repetitions (~80% retention), the difference is not significant. Moreover, the use of only 5 mg of adsorbent and there being some loss during the adsorption and desorption steps have a considerable impact. Consequently, COF-42 materials with –NH/–NH₂ exhibit a high potential for Pd²⁺ adsorption capacity and selectivity, coupled with their excellent recyclability, making them valuable for a variety of applications.

COF-42 is recognized as a superior Pd²⁺ adsorption material due to its mesoporous channels and high crystallinity. Additionally, the designed –NH/–NH₂ species in this study are noted for their remarkable binding interactions with Pd²⁺, enhancing the adsorption performance towards Pd²⁺. To elucidate the adsorption mechanism, FTIR, XPS, and SEM spectra are measured and analyzed before and after Pd²⁺ adsorption in the subsequent investigation.

3.3. The Sorption Mechanism of COF-42

To analyze the adsorption mechanism of COF-42 towards Pd²⁺ in more depth, the FT-IR, XPS, and SEM images and elemental distributions of COF-42 before and after Pd²⁺ adsorption are comprehensively studied. The results confirm the successful copolymerization of the raw materials and Pd²⁺ binding interactions. The FT-IR spectra in Figure 4a show peaks in the range of 4000–480 cm⁻¹. Specific peaks at 1614 cm⁻¹ and 1215 cm⁻¹ correspond to v_C=N moiety stretching modes [58]. The appearance of these new peaks indicates the successful formation of imine bonds [43].

Furthermore, the XPS spectra of COF-42 and Pd@COF-42 (Figure 4b and Figure 5) reveal distinct O1s, N1s, and C1s signals in the pristine material. Post-adsorption, the emergence of Pd3d doublet peaks (Figure 4b and Figure 5e) confirms effective Pd²⁺ chemisorption through strong coordination interactions. Critical nitrogen speciation changes are observed in Pd@COF-42 (Figure 5a,c): A new N1s component at 405.8 eV is assigned to nitrate coordination. A binding energy shift to 401.4 eV is indicative of N–Pd covalent bonding. Comparing the fits before and after adsorption, it can be assumed that the -NH/-NH₂ group contributes to the adsorption of palladium, especially the -NH group. Particularly, the dominant N1s-Pd²⁺ interaction (vs. the attenuated O1s-Pd²⁺ response) under 3 M HNO₃ confirms nitrogen-centered chelation. This coordination involves charge compensation through nitrate counterions while maintaining Pd²⁺’s octahedral geometry. The persistent crystallinity observed in XPS aligns with COF-42’s structural stability during metal coordination.

The SEM images show that COF-42 is composed of microparticles (Figure 4c). Following Pd²⁺ adsorption (Figure 4d), a noticeably large structure is observed, likely due to a significant amount of Pd²⁺ being adsorbed within the building channels of COF-42. EDX analysis indicates a uniform distribution of the main elements (C, N, O) in the COF-42 structure both before and after Pd²⁺ adsorption, suggesting successful copolymerization and structural stability during the adsorption process. Additionally, the even distribution of Pd²⁺ on the surface of Pd@COF-42 demonstrates effective Pd²⁺ adsorption on the COF-42 structure, with no visible agglomeration occurring.

To investigate the interaction mechanism between Pd²⁺ and COF-42, density functional theory (DFT) simulations are employed to quantify the binding energetics of Pd²⁺ with COF-42 (Figure 6). This computational approach provides the atomic-level resolution of electron density distributions and interaction thermodynamics. Key findings include the following: Stronger N–Pd bonding (−1.75 eV) compared to O–Pd interactions (−1.67 eV). Negative binding energies confirming spontaneous chemisorption. The energy differential (0.08 eV) explicitly demonstrates preferential Pd²⁺ coordination to nitrogen sites within the framework. These substantial binding energies (−1.75 to −1.67 eV) surpass typical physisorption thresholds, validating the dominant covalent character of Pd–N/O bonding. The computational results align with experimental XPS data, establishing a coherent mechanism for Pd²⁺ sequestration via nitrogen-directed chemical adsorption.

Integrating the results from characterizations and DFT analysis, it is evident that COF-42 exhibits a strong and stable adsorption mechanism. This can be primarily attributed to the abundance of active nitrogen (N) sites distributed across its surface. These active N sites serve as key coordination centers, facilitating strong interactions with Pd²⁺ through chemical bonding. The coordination interaction not only enhances the adsorption strength but also imparts high selectivity towards Pd²⁺, ensuring that the material preferentially adsorbs the target ions even in the presence of competing species. This selectivity is critical for applications in complex environments, such as wastewater treatment or resource recovery. In addition to its chemical affinity for Pd²⁺, the structural properties of COF-42 play a vital role in its adsorption performance. The material’s high specific surface area provides ample space for ion access, while its large porosity promotes efficient mass transfer and the diffusion of Pd²⁺ into the interior adsorption sites. The interconnected porous network ensures that ions can be rapidly transported to active sites, improving adsorption kinetics.

4. Conclusions

To summarize, this study successfully constructs well-defined 2D porous COF-42 and applies it to effectively adsorb Pd²⁺. The structure and morphology of COF-42 are measured before and after Pd²⁺ adsorption, indicating high integrity and stability. The analysis of adsorption dynamics and thermodynamics shows that the process follows a pseudo-second-order kinetic model. The maximum adsorption capacity at 3 M HNO₃ solution is 170.6 mg/g, and the adsorption equilibrium could be reached within 300 min. The exceptional adsorption performance is attributed to the synergistic effect of flexible ligand, framework porosity, and the concentrated -NH/-NH₂ groups of COF-42, which exhibit a strong affinity for Pd²⁺. Therefore, COF-42 demonstrates an excellent capture efficiency towards Pd²⁺ in a strong acid environment, indicating its potential application in Pd²⁺ adsorption and recycling.