Porous g-C₃N₄ modified with phenanthroline diamide for efficient and ultrafast adsorption of palladium from simulated high level liquid waste

Abstract

The efficient recovery of palladium from high level liquid waste (HLLW) is of growing importance for achieving sustainable development and resolving challenges in radioactive waste treatment. Nonetheless, the hunt for strong materials for the selective adsorption of palladium ion (Pd(II)) at high acidity remains a formidable obstacle. In this work, hierarchically porous g-C₃N₄ with high specific surface area was synthesized as a matrix of adsorbent that provides a rapid transport channel for Pd(II). Subsequently, N,N′-diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline (DAPhen) was modified for integration into g-C₃N₄ to create CN-DAPhen for obtaining an adsorbent with excellent selectivity and efficient adsorption for Pd(II). The adsorption of Pd(II) in HNO₃ medium was examined using batch experiments. CN-DAPhen displayed quick adsorption kinetics, and equilibrium was reached within 5 min. By fitting the data using the Langmuir model, the maximum adsorption capacity of CN-DAPhen was calculated to be 390.63 mg g⁻¹. In practice, the functionalized nanomaterial was employed to recover Pd(II) from simulated HLLW containing competing metal ions. The adsorption selectivity of CN-DAPhen for Pd and Ru was also explored by combining adsorption experiments and DFT calculation. On this basis, a continuous process was developed for separating Pd and Ru from HLLW. Furthermore, the excellent radiation resistance and reusability of CN-DAPhen endowed it with great practical potential for the treatment of HLLW. This study not only introduces a novel species with superior adsorbent capability in selectively recovering palladium but also reveals a potential strategy for the development of robust nanomaterials for nuclear waste cleanup.

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Environmental significance

The presence of vast volumes of high level liquid waste (HLLW) poses an unprecedented challenge to environmental protection due to the long-term storage and management of HLLW. As the representative of platinum group metals, Pd must be extracted from spent radioactive fuel before disposal to prevent its high melting point from inhibiting the vitrification of HLLW. Adsorption is one of the most effective strategies for the removal of Pd from HLLW. The porous g-C₃N₄ nanomaterials modified with phenanthroline diamide have high resistance to strong acid and radiation degradation, and can also efficiently and rapidly adsorb Pd in HLLW. This approach of functional group modified nanomaterials gives more opportunities for the removal of metal ions in aqueous solutions.

1. Introduction

Palladium (Pd), one of the platinum group metals (PGMs), is suffering from severe depletion in silicate earth, although the demand for Pd has increased significantly due to the exceptional expansion of catalytic systems and cars.^1–3 As a result, research into the efficient separation and recovery of Pd is critical. In addition to the main sources such as ores containing PGMs, secondary sources such as waste electrical and electronic equipment, and inactivated catalysts^4,5 have gained prominence as Pd recycling sources. Extensive studies are now being conducted on the possibility that high level liquid waste (HLLW) from spent nuclear fuel represents a secondary source of Pd.⁶

According to prior research studies, approximately 11 kg fission-generated palladium is expected to be present in each metric ton of the spent fuel from a fast reactor, which is substantially more than the content of Pd in the Earth's crust.^7,8 Furthermore, the recovery of palladium from a stream of radioactive waste would be economically advantageous, since the amount of HLLW accumulating throughout the globe continues to grow. Nonetheless, it is important to note that significant challenges remain in palladium separation owing to the complexity of the HLLW situation and similar features of palladium with various interfering metals.⁹ Particularly noteworthy is that ¹⁰⁷Pd, among all palladium isotopes, is the radioactive nuclide with a half-life of 6.5 × 10⁶ years. Even though ¹⁰⁷Pd may generate soft β-emitters with 35 keV, this amount of radioactivity is suitable for industrial use.¹⁰ HLLW has to be vitrified before final geological disposal, which is generally operated at temperatures below 1200 °C.¹¹ Otherwise, the corrosion of equipment and volatilization of radioactive elements would be severe. Pd(II) in HLLW would hinder the vitrification of HLLW due to their high melting points.¹² Furthermore, because of the low solubility of Pd(II) in borosilicate glass, with their accumulation, the formed separate phases and gradual sedimentation of the dispersed phases in the absence of convective mixing during the vitrification process will cause a short circuit and temperature gradients, deteriorating the stability of the glasses.¹³ Therefore, Pd must be extracted from spent radioactive fuel prior to disposal. More importantly, the high concentration of nitric acid and significant long-term radioactivity in HLLW call for the adoption of more effective and durable materials that are highly resistant to radiation degradation and severe acidity.

To remove Pd from HLLW, many approaches have been suggested, including electrolysis,¹⁴ solvent extraction,¹⁵ adsorption,¹⁶ precipitation,¹⁷ and photocatalysis.¹⁸ Due to its cheap cost, simplicity of operation, and adaptability, adsorption is the most efficient process of these technologies.¹⁹ Carbon nanotubes,^20–22 clay minerals,²³ zeolites,²⁴ natural and modified bentonite,²⁵ and graphene^26,27 have all been widely researched to remove Pd from wastewater during the last several decades. Meanwhile, inexhaustible possibilities for new materials are constantly emerging. In recent years, carbon nitrogen compounds have garnered considerable attention because of their desirable surface area, nontoxicity, dependable stability, and inexpensive cost. On the basis of theoretical calculations, carbon nitride is crystallized in five different forms in nature. Graphite structure carbon nitride (g-C₃N₄) has sparked the greatest attention among the five crystalline structures because of its better stability.²⁸ Specifically, the numerous amine groups (–NH₂, –NH–, and =N–) on g-C₃N₄ materials are the consequence of the presence of triazine and tri-s-triazine/heptazine as the fundamental tectonic units of g-C₃N₄.²⁹ These amine groups would also provide plentiful effective sites for functional modification of g-C₃N₄ to improve the adsorption capacity. Increased adsorptive selectivity for Zn(II), Pd(II), and Cd(II) was seen in Fe₃O₄-doped g-C₃N₄ because of the formation of conjugation between Fe₃O₄/g-C₃N₄ and heavy metal ions.³⁰ Using the synergy of ion exchange, reduction, and complexation, a ternary Fe₃O₄/g-C₃N₄/carbon composite could efficiently adsorb Cr(VI). It has an adsorption capability of 50.09 mg g⁻¹.³¹ Consequently, it is of the highest importance to devise an efficient modification process for producing g-C₃N₄ with high adsorption capacities.

Nevertheless, the majority of techniques still have issues with insufficient efficiency and/or inadequate selectivity. Rarely have selective adsorbents for Pd(II) recovery in an HLLW environment with multiple interferences been described, much less those for Pd(II) recovery.³² The combination of adsorbent materials with highly selective adsorption functional groups has recently facilitated research into new adsorbents for selective metal separation.^33–35 The insertion of functional groups such as crown ethers, cuproaromatics, and cyclodextrins into particular substrates can improve the selectivity for metal ions.^36–42N,N′-Diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline (DAPhen) is a relatively novel N-heterocyclic dicarboxamide ligand that was discovered in recent years to exhibit excellent affinity and selectivity for actinides.^43–47 Early literature has demonstrated that multivalent chelating phenanthroline-based ligands may generate extremely stable complexations with palladium ions.^48–55 For instance, in 0.1 mol L⁻¹ NaClO₄ and at 25 °C, the complex stability constant lg K(phen) values for Pd(II) is over 20, whereas it is below 10 for other metal ions such as Fe(III), Fe(II), Co(II), Ni(II), Cu(II), Pb(II), Am(III), and La(III), indicating much stronger affinity and higher selectivity for Pd(II) compared to the other metal ions.^58,59 Given that phenanthroline demonstrated capacity for the absorption of Pd(II), we speculated that DAPhen would function as a substitute adsorbent for Pd enrichment by making use of the N donor of the phenanthroline moiety and O donor of amide moieties capable of coordinating to Pd(II). As a result, the functionalization of g-C₃N₄ with DAPhen groups is thus a promising method for the selective adsorption and separation of Pd(II) from HLLW.

Herein, a series of amino-rich porous g-C₃N₄ was prepared via calcination, and the g-C₃N₄ with the largest specific surface area was selected as the adsorbent matrix. Subsequently, the g-C₃N₄ was modified with N,N′-diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline (DAPhen) groups and the adsorbent (CN-DAPhen) was obtained. The introduction of DAPhen groups equipped CN-DAPhen with great adsorption ability for Pd(II) in high concentration of HNO₃ media. Furthermore, the adsorption mechanism of Pd(II) was investigated via adsorption kinetics combined with isotherms. Moreover, the adsorption selectivity for Pd(II) on CN-DAPhen was explored in detail by combining experiment and DFT calculation. Based on the excellent adsorption selectivity, a separation procedure for recycling Pd(II) as well as Ru(III) from simulated high level liquid waste (HLLW) was designed, and the target ions were separated from simulated HLLW successfully by the procedure. In addition, CN-DAPhen displayed outstanding reusability and radiation stability, which endowed CN-DAPhen with great practical potential for the recovery of Pd from HLLW.

2. Experimental section

2.1 Chemicals and materials

Pd(II), Cs(I), Fe(III), Ni(II), Ru(III), Sr(II), Cd(II), Cr(III), and Nd(III) were obtained as their nitrate salts for use in adsorption experiments (Northern Weiye Metrology Technology Co., Ltd., 99%). Nitric acid (HNO₃, 65.0–68.0%), dichloromethane (CH₂Cl₂, 99.5%), thionyl chloride (SOCl₂, analytical reagent), oxalic acid (C₂H₂O₄·2H₂O, 99.5%), and sodium hydroxide (NaOH, 96.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Melamine (C₃H₆N₆, 99%), triethylamine (Et₃N, 99.5%), and N-ethyl-p-toluidine (98%) were purchased from Energy Chemical Co., Ltd. The analytical reagent 1,10-phenanthroline-2,9-dicarboxylic acid was purchased from Alfa Aesar Chemical Co., Ltd. The Nanjing Shanjing Industrial Gas Factory supplied the high-purity nitrogen (N₂, 99.999%). Ultrapure water (ρ = 18.25 MΩ cm) used in the experiment was generated by a Kertone Lab Vip ultrapure water system.

2.2 Synthesis

2.2.1 Synthesis of g-C₃N₄. The simple oxalic acid-induced supramolecular assembly method was used to create the amino group-rich porous g-C₃N₄. The oxalic acid solution was typically made by dissolving 1.0 g of oxalic acid into 30 mL of ultrapure H₂O. To make the melamine–oxalic acid supramolecular precursor, 2 g of melamine was added to the oxalic acid solution and the mixture was stirred for 2 hours. Then, to synthesize amino group-rich porous g-C₃N₄, the obtained supramolecular precursor was transferred without drying to a crucible and calcined at 550 °C for 4 hours (3 °C min⁻¹) to prevent drying. After free cooling, porous g-C₃N₄ rich in amino groups was produced and classified as CN-x. To test the effect of oxalic acid dosage on the regulation of g-C₃N₄ tri-s-triazine structure, various doses of oxalic acid were employed to build amino group-rich porous g-C₃N₄ denoted as CN-x (x = 0, 0.5, 1, 2, and 3, where x represents the mass of oxalic acid).

2.2.2 Synthesis of CN-DAPhen. 150 mg 1,10-phenanthroline-2,9-dicarboxylic acid and 15 mL thionyl chloride at 90 °C were refluxed for 3 hours in a flask under a nitrogen environment with a flow rate of 200 mL min⁻¹. The dispersion was then cooled to ambient temperature, and the remaining SOCl₂ solvent was eliminated by distillation under reduced pressure. The deep yellow solid was dissolved in 10 mL of CH₂Cl₂ without additional purification, and then a combination of 10 mL of CH₂Cl₂, 0.6 mL of Et₃N, and 300 mg of g-C₃N₄ (CN-3) was added. The reaction dispersion was afterwards heated to 40 °C and shielded by N₂ for three hours. Next, 0.45 mL of N-ethyl-p-toluidine was added to the mixture, which was stirred at 40 °C for an additional three hours. The product was separated by centrifugation and washed five times with CH₂Cl₂. The CN-DAPhen product was then dried to a constant weight at 60 °C in a drying oven.

2.3 Characterization of materials

Scanning electron microscopy (SEM, Hitachi SU8220) and high-resolution transmission electron microscopy (HR-TEM, Hitachi H-7650) were used to examine the morphologies of g-C₃N₄ (CN-x) and CN-DAPhen. A fully automated gas adsorption analyser (Quantachrome Autosorb iQ) at 77 K was used to measure the N₂ adsorption–desorption isotherms of g-C₃N₄ (CN-x) and CN-DAPhen, and multipoint Barrett–Joyner–Halenda (BJH) and multipoint Brunauer–Emmett–Teller (BET) techniques were used to quantify the pore size distributions and specific surface areas. A UV-vis-NIR spectrometer (Shimadzu Solidspec 3700, Japan) was applied to obtain the UV-vis spectra. An X-ray diffractometer (XRD, Panalytical, X'Pert PRO MPD) was applied to detect the crystal structure of the prepared samples. The elemental compositions of CN-x were analyzed by an Elementar Vario EL cube element analyzer. A Bruker AVANCE AV400 (Bruker BioSpin GmbH) spectrometer was used to scan the solid-state ¹H and ¹³C cross polarization magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra. X-ray photoelectron spectroscopy (XPS) was performed with monochromatic Al Kα X-ray radiation (1486.6 eV) on a Thermo-VG Scientific ESCALAB 250. A pH meter (PHSJ-3F, Shanghai REX Instrument Factory) was used to test the pH levels. In order to determine the concentrations of Pd(II) and other metal ions in the solution, inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkinElmer Optima 7300DV) was used.

2.4 Palladium adsorption experiments

The stock solution of Pd(II) (1000 mg L⁻¹) was produced in aqueous HNO₃ solutions, and batch tests were conducted to determine Pd(II) adsorption. In a typical experiment, 0.8 mg of CN-DAPhen (0.4 g L⁻¹) was combined with Pd(II) stock solution and ultrapure water to achieve an initial Pd(II) concentration of 100 mg L⁻¹. All adsorption studies were previously pH-adjusted using insignificant amounts of HNO₃ and NaOH solutions. After 3 hours of shaking at 298 K at 80 rpm, a mixed cellulose ester membrane with a pore size of 0.22 μm was used to separate the solid phase from the solution. The equilibrium adsorption amount (q_e) of Pd(II) was computed by the following eqn (1),

q_e = (c₀ − c_e)V/m (1)

where c₀ (mg L⁻¹) and c_e (mg L⁻¹) are the palladium concentrations before and after adsorption, respectively. m (g) and V (L) denote the volume of the aqueous phase and the quantity of the adsorbent, respectively.

The thermodynamic equilibrium constant K_d (mL g⁻¹) is calculated from eqn (2), and the selectivity coefficient (S) is defined in eqn (3). S, where K_d,Pd and K_d,M are the thermodynamic equilibrium constants of Pd(II) and competing metal ions, respectively, was used to measure the capacity of Pd(II) to separate from other metal ions.

K_d = 1000(q_e/c_e) (2)

S_Pd/M = K_d,Pd/K_d,M (3)

A selective adsorption test was conducted in a solution containing 100 mg L⁻¹ of Pd(II), Cs(I), Fe(III), Ni(II), Ru(III), Sr(II), Cd(II), Cr(III), and Nd(III). With the exception of Cs(I), which was identified by atomic absorption spectroscopy, the concentration of Pd(II) and other metal ions in the filtrate was evaluated by ICP-OES. An irradiated adsorption experiment was used to test the adsorbent's radiation stability. The adsorption system (100 mg L⁻¹ Pd(II) concentration with 0.4 g L⁻¹ CN-DAPhen dosage) was exposed to a ⁶⁰Co radiation field (12 kCi, situated at the University of Science and Technology of China) to obtain an absorbed dose in the range of 0–100 kGy. The concentration of Pd(II) following adsorption was determined in the same way as previously stated. Pd(II) was desorbed by adding a solution of 6 mol L⁻¹ HNO₃ to the used adsorbent and stirring it for 6 hours. After centrifugation (10 000 rpm, 5 min), the solid phase was separated. The regenerated adsorbent was then utilized for adsorption after being rinsed with ultrapure water and dried under vacuum. The adsorption–desorption performance of CN-DAPhen was cycled five times under similar circumstances.

2.5 Theoretical calculation

All theoretical calculations were carried out using Gaussian 16W and GaussView 6.0. To optimize the geometries of palladium or other metal species and their complexes with the adsorbent in the liquid phase, B3LYP-SDD/6-311G(d,p) was used. At the M062X-SDD/6-311G(d,p) level, higher precision single-point energy calculations in the liquid phase were conducted. The SMD model was used to account for the solvent impact. Enthalpy changes (ΔH), Gibbs free energy changes (ΔG), and binding energy (ΔE) were calculated using frequency calculations with thermal adjustments at the M062X-SDD/6-311G(d,p) level of theory.

3. Results and discussion

3.1 Materials characterization

The CN-DAPhen adsorbent was prepared via a sequential amidation method, as shown in Scheme 1. Firstly, g-C₃N₄ (CN-x) was synthesized utilizing a straightforward oxalic acid-induced supramolecular assembly method. Then, consecutive amination processes were conducted using 1,10-phenanthroline-2,9-dicarbonyl chloride and N-ethyl-p-toluidine to synthesize CN-DAPhen. Solid-state ¹H, ¹³C NMR, XPS, XRD, UV-vis, N₂ adsorption–desorption, SEM, and TEM investigations were performed to confirm and characterize the successful synthesis and structure of CN-DAPhen.

Scheme 1 Graphical illustration for the synthesis of amino group-rich porous g-C₃N₄ (CN-x) and the g-C₃N₄-DAPhen (CN-DAP) adsorbent.

All CN-x exhibited type II N₂ adsorption–desorption isotherms, the sharp increase in adsorption quantity of N₂ at P/P₀ > 0.8 indicated the existence of large mesopores (Fig. S1a†). The distributions of pore sizes indicated that all CN-x exhibited hierarchically porous structures with pores ranging in size from macropores to micropores (Fig. S1b†). Meanwhile, all CN-x exhibited a similar mean pore size of around 16 nm (Table S1†). As shown in Table 1, the CN-0 sample exhibited a relatively low specific surface area (11.36 m² g⁻¹) and pore volume (0.052 cm³ g⁻¹) due to the severe aggregation of the tri-s-triazine structure.^56,57 It could be seen from the pore size distribution that the introduction of oxalic acid resulted in an increasing number of small mesopores (2–3 nm), as well as large mesopores (∼30 nm) of the generated CN-x with increasing oxalic acid. The SEM pictures of CN-0 and CN-3 also demonstrate that the materials have a stacked nanosheet form without the addition of oxalic acid, while the addition of oxalic acid obviously enhances the pore structure (Fig. S4†). The specific surface areas and pore volumes of the samples gradually increased with increasing amounts of oxalic acid. The specific surface area of CN-3 reached 23.16 m² g⁻¹, and the pore volume reached 0.089 cm³ g⁻¹.

Table 1 Specific surface areas and pore volumes of CN-x

Sample	S BET (m^2 g^−1)	V total (cm^3 g^−1)
CN-0	11.36	0.052
CN-0.5	13.86	0.066
CN-1	19.35	0.077
CN-2	21.43	0.084
CN-3	23.16	0.089

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XPS was used to examine the elemental composition and chemical states of the CN-x samples. It was discovered that the produced CN-x samples include the components C, N, and O (Fig. 1a). According to the XPS results, as the oxalic acid content increases, the C/N ratio gradually decreased and the C/N of CN-3 is 0.96, which is smaller than that of the CN-0 sample (1.06). The elemental chemical states of C and N may be more accurately determined using the high-resolution XPS spectra. The N 1s high-resolution XPS spectrum is shown in Fig. 1b, which can be deconvolved into three peaks. The tri-s-triazine rings, sp²-hybridized N (C=N–C), as well as tertiary N (N–(C)₃) provide characteristic signals at 398.6 and 399.3 eV, respectively.^60,61 Specifically, the peak at 401.01 eV can be assigned to the N atoms of amino groups,^62,63 and peaks for N atoms of amino groups were observed in all samples. Compared to the CN-0 sample, the amino groups are much more prominent in CN-x, suggesting that more amino groups are incorporated into the resulting CN-x through the oxalic acid-induced supramolecular production process. To be precise, the computed N atoms of N–H contents for the CN-0, CN-0.5, CN-1, CN-2 and CN-3 are 9.69, 12.98, 14.07, 18.62 and 23.19%, respectively, indicating that the porous g-C₃N₄ prepared employing the oxalic acid-induced supramolecular production process contains a greater amount of amino groups. Elemental analysis was also carried out to evaluate the elemental content of CN-x. As detailed in Table S2,† all of the samples were composed mostly of C, N, and O, with relatively low concentrations of H, in agreement with previous reports.⁶⁴ With the increase of oxalic acid content, the content of N atoms increased, which agreed well with result of XPS. The atomic ratios of C to N (C/N) of CN-0, CN-0.5, CN-1, CN-2, CN-3 prepared by adding oxalic acid are were 0.72, 0.69, 0.66, 0.65, and 0.62, respectively, which were lower than that of the theoretical C/N value of g-C₃N₄ (0.75). The lower C/N ratios of the prepared CN-x points to more uncondensed terminated amino groups.^65,66 Moreover, decreasing C/N from CN-0 to CN-3 confirmed the conclusion that the amino groups content increased as the oxalic acid increased. Therefore, in addition to the XPS data, the conclusion that the addition of oxalic acid increases the amino content of g-C₃N₄ is corroborated by the data of elemental analysis.

Fig. 1 Low resolution XPS spectra (a) and XPS N 1s spectra (b) of CN-x. UV-vis absorption spectra (c) and powder XRD patterns (d) of CN-x.

Fig. 1c shows the UV-vis absorption spectra of the CN-x, which is used to analyze the surface functional groups of various g-C₃N₄ samples. The CN-0 sample had an absorption edge of about 460 nm, which correlated well with that of typical bulk g-C₃N₄.^67,68 After the introduction of oxalic acid, the CN-0.5 displays the increased visible-light absorbance (450–800 nm), which is commonly ascribed to the n–π* transitions of –NH₂ involving the lone pairs of the edge N atoms of the tri-s-triazine structure.^69,70 The visible light absorption capacity of CN-x at 450–800 nm also gradually increased when the proportion of oxalic acid was increased, and CN-3 exhibited the best visible light-absorption capacity. Moreover, with the growth of oxalic acid, the matching hue of CN-x progressively deepens, which is consistent with the results of the UV-vis absorption spectra. Powder XRD characterizations were performed in order to study the layered-stacking mechanism of the as-synthesized products. As seen in Fig. 1d, all of the samples exhibited two separate peaks with the same peak position and varying strength. The in-plane repeating structure of tri-s-triazine is shown by the (100) peak at 12.98° in the CN-0 sample, while the interlayer stacking of the conjugated aromatic systems is shown by the (002) peak at 27.54°.^71,72 The results from XRD measurements indicate that the amorphous texture of CN-x was not significantly changed with the addition of oxalic acid, which is also observed in other g-C₃N₄-based porous materials.⁷³ Therefore, more amino groups would favor the amidation modification, and CN-3 was thus chosen as a representative of g-C₃N₄.

The ¹³C CP-MAS SSNMR spectra revealed that g-C₃N₄ (CN-3) was successfully modified with DAPhen (Fig. 2a). The triazine carbon atoms on g-C₃N₄ only produced one clear peak at δ = 156 ppm, which was the only peak that could be identified. After modification with DAPhen groups, a peak appeared at 172.4 ppm that corresponds to the carbonyl carbon atoms in amide (g) and carbon atoms in phenanthroline (e). A sequence of peaks with chemical shifts between 132 and 155 ppm matched the aromatic carbon atoms of the N-ethyl-p-toluidine (j–m) and phenanthroline (a–f) moieties. The methylene carbon atoms were allocated to the 54.2 ppm peak (h). At the lowest chemical shifts of 14.5 ppm, the methyl group signal in ethyl (i) and tolyl (n) was visible. In addition, after introducing the DAPhen group, CN-DAPhen exhibits a slightly weakened visible light response (450–800 nm), which is usually attributed to the reduction of the –NH₂ structure in g-C₃N₄ during the amidation reaction (Fig. S2†).

Fig. 2 ¹³C CP/MAS NMR of CN-3 and CN-DAPhen (a). XPS N 1s spectra of CN-3 (b) and CN-DAPhen (c).

The XPS results further confirmed the successful modification of DAPhen on CN-3. It is possible to deconvolute the C 1s spectra of CN-3 and CN-DAPhen into two peaks (Fig. 2b and c), which were attributed to the carbon atoms C–C/C=C (284.8 eV) and N=C–(N)₂ (288.2 eV), respectively. Due to the introduction of the phenanthroline skeleton in the DAPhen group during the modification, the relative peak areas of the carbon of C–C/C=C grew and that of N=C–(N)₂ noticeably decreased. This further proved that DAPhen had been introduced. Meanwhile, the change in C–C/C=C and N=C–(N)₂ could be used to assess the DAPhen group's modification density on CN-3. The modification density of the DAPhen groups was calculated to be 1.655 mmol g⁻¹ based on the molar content for the carbon atoms of C–C/C=C in the total carbon atoms of g-C₃N₄ (12.85%) and CN-DAPhen (70.74%). The abundant amino groups on CN-3 can provide abundant reaction sites, which contributed to the modification density of DAPhen on CN-3.

The surface morphology and structure of the CN-3 and CN-DAPhen are depicted in Fig. 3a–d. The modified CN-DAPhen displays the same type II N₂ adsorption–desorption isotherms as pristine CN-3 (Fig. 3e). The sharp increase in the adsorption quantity of N₂ at P/P₀ > 0.8 indicated the existence of large mesopores. It was clear from the pore size distributions that the CN-3 and CN-DAPhen had a hierarchical porous structure that included pores of varying sizes, from macropores to micropores (Fig. 3f). Additionally, it seems that the specific surface area and the total pore volume of CN-3 both were not dramatically affected by the modification. However, in terms of average pore size, the modification slightly increased the mean pore size of the adsorbent from 15.38 (CN-3) to 17.89 (CN-DAPhen) (Table S1†). It can be inferred that the hierarchical pore structure of CN-3 remains after modification. The macroporous structure in CN-DAPhen can enhance mass transfer, while the microporous structure with huge surface area could offer extra binding sites for the adsorbate, so the hierarchical pore structure of CN-DAPhen is favorable for adsorption. Moreover, despite the introduction of the DAPhen group in the synthesis of CN-DAPhen, XRD patterns show identical peaks of g-C₃N₄ and CN-DAPhen at 12.98° and 27.54°, which confirms that the intrinsic layer graphite-like structure is preserved (Fig. S3†).

Fig. 3 TEM images of CN-3 (a) and CN-DAPhen (b). SEM images of CN-3 (c) and CN-DAPhen (d). N₂ adsorption–desorption isotherms (e) and the corresponding pore size distributions (f) of CN-3 and CN-DAPhen.

3.2 Adsorption performance of Pd(II) on the CN-DAPhen

To explore the palladium adsorption ability of CN-DAPhen, tests were conducted at various pH levels (Fig. 4a). Due to the lack of adsorption groups, pristine g-C₃N₄ (CN-3) was incapable of adsorbing Pd(II) at low pH (pH = 1.0–3.0), while CN-DAPhen exhibited significantly increased adsorption amounts at pH = 1–3 compared to CN-3. When the pH was more than 3, the increased q_e was caused mostly by the hydrolysis and precipitation of Pd(II).⁷⁴ Although q_e of CN-DAPhen still rose with pH at pH > 3, interference from hydrolysis and precipitation of Pd(II) was too substantial to be ignored. However, the Pd(II) adsorption amount remained at 115.2 mg g⁻¹ at pH = 1. Furthermore, the difference in adsorption capacity at pH = 1.0–3.0 is not significant. Hence, additional adsorption studies were conducted at pH = 1 to prevent Pd(II) precipitation. Due to the significant Pd adsorption quantity at pH = 1, we hypothesized that CN-DAPhen may efficiently adsorb palladium in acidic surroundings. Thus, we set out to study its adsorption behavior in more acidic environments. When the concentration of HNO₃ rises from 1 mol L⁻¹ to 5 mol L⁻¹, the adsorption capabilities of CN-DAPhen fall from 85.9 mg g⁻¹ to 47.2 mg g⁻¹, as shown in Fig. 4b. The capacity of CN-DAPhen for Pd adsorption decreases with increasing HNO₃ concentration. This is because of an increase in the protonation of N atoms in the phenanthroline moiety, leading to a decrease in the coordination capacity with Pd.⁷⁵

Fig. 4 Adsorption capacities of Pd(II) on g-C₃N₄ and CN-DAPhen at different pH values (a). Adsorption capacities of Pd(II) on g-C₃N₄ and CN-DAPhen under high HNO₃ concentrations (b) (adsorbent dosage = 0.4 mg mL⁻¹, [Pd]₀ = 100 mg L⁻¹, T = 298 ± 1 K, t = 6 h).

Fig. 5a displays the impact of the contact time on the Pd(II) adsorption. It is evident that the adsorption capacity increases rapidly upon contact, and equilibrium with a q_e value of about 124.1 mg g⁻¹ was reached within 5 min. Increasing the contact duration does not result in greater Pd(II) absorption, and this ultrafast kinetics will aid in the very effective adsorption of metals. Pseudo-first-order⁷⁶ and pseudo-second-order⁷⁷ models were used to fit the experimental results. A strong connection between the kinetic data may aid in the comprehension of the kinetic mechanism behind the adsorption process. The following equation exemplifies the pseudo-first-order model (eqn (4)) and pseudo-second-order (eqn (5)):

ln(q_e − q_t) = ln q_e − k₁t (4)

t/q_t = 1/k₂q_e² + t/q_e (5)

where q_e (mg g⁻¹) and q_t (mg g⁻¹) correspond to the adsorption amounts of Pd(II) at equilibrium and at contact time t (min). k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹) are the adsorption rate constants of pseudo-first-order and pseudo-second-order models, respectively.

Fig. 5 Adsorption kinetics (a) and pseudo-second-order model fitting curves (b) of the CN-DAPhen adsorption capacities for Pd(II). Adsorption capacities of Pd(II) on CN-DAPhen (c) at different temperatures and the fitting plots of ln K_dversus 1/T for CN-DAPhen (d) (adsorbent dosage = 0.4 mg mL⁻¹, [Pd]₀ = 100 mg L⁻¹, T = 298 ± 1 K, pH = 1.0 ± 0.1, t = 6 h).

The fitted kinetic parameters are summarized in Table 1. The best fitting results of CN-DAPhen were obtained using the pseudo-second-order model with the correlation coefficient (R²) being 0.9998. The estimated value of q_e by the pseudo-second-order model is significantly more similar to the experimental result, confirming the correctness of this model. Fig. 5b and Table 2 depict the fitted curve of the linear pseudo-second-order model. The findings revealed that Pd was chemically adsorbed on CN-DAPhen.⁷⁸

Table 2 Kinetic parameters of Pd(II) adsorption on CN-DAPhen

Pseudo-first-order			Pseudo-second-order
q e,1 (mg g^−1)	k 1 (min^−1)	R ^2	q e,cal (mg g^−1)	k 2 (g mg^−1 min^−1)	R ^2
4.126	0.0057	0.0075	124.1	0.0807	0.9998

---

The thermodynamic parameters for Pd(II) adsorption on CN-DAPhen were derived using temperature-dependent adsorption isotherms, as shown in Fig. 5c. The fact that the Pd(II) adsorption capability increased with temperature indicates that the adsorption process is endothermic. Subsequently, additional thermodynamic parameters (ΔH⁰, ΔS⁰, and ΔG⁰) for the adsorption process were determined. The standard enthalpy change (ΔH⁰) and entropy change (ΔS⁰) were derived from the linear fit of ln K_d to 1/T (Fig. 5d) using the van't Hoff eqn (6). Additionally, the Gibbs free energy change (ΔG⁰) was calculated by eqn (7),

ΔG⁰ = ΔH⁰ − TΔS⁰ (6)

ln K_d = −ΔH⁰/RT + ΔS⁰/R (7)

where K_d (mL g⁻¹) is the thermodynamic equilibrium constant derived by eqn (2), R (8.314 J mol⁻¹ K⁻¹) is the universal gas constant for gasses and T (K) is the absolute temperature.

Table 3 contains all of the determined thermodynamic parameters. For the adsorption of Pd(II) on CN-DAPhen, similar thermodynamic values were found. A positive ΔH⁰ value indicated that the adsorption process was endothermic. The negative value of ΔG⁰ revealed the natural occurrence of spontaneous adsorption. Since ΔG⁰ decreases with increasing temperature, a greater temperature was selected for the adsorption process. In addition, the positive value of ΔS⁰ indicated a spontaneous adsorption behavior with high affinity and an enhancement in randomization on the surface of CN-DAPhen.

Table 3 The thermodynamic parameters of U(VI) adsorption on CN-DAPhen

ΔH^0	ΔS^0	ΔG^0 (kJ mol^−1)						R ^2
(kJ mol^−1)	(J mol^−1)	298 K	308 K	318 K	328 K	338 K	348 K
10.17	41.44	−2.178	−2.593	−3.007	−3.421	−3.836	−4.250	0.9896

---

The effect of the starting Pd(II) concentration on the adsorption capacities was investigated to have a better understanding of the CN-DAPhen adsorption behavior. At ambient temperature, metal ion solutions ranging from 10–800 mg L⁻¹ were used in batches comprising 0.4 mg mL⁻¹ adsorbent. The adsorbents demonstrate increased adsorption capacities with increasing Pd(II) concentration. When c_e exceeds around 400 mg L⁻¹, the curves flatten and go toward equilibrium, indicating that the adsorption has reached its maximal capacity. For a comparative examination of the adsorption behavior, Langmuir and Freundlich isotherm models were fitted to the adsorption equilibrium data (Fig. 6a). Nonlinear adsorption is depicted by the Langmuir model, which suggests that metals are absorbed on a homogeneous surface by monolayer adsorption without interactivity between adsorbed species.⁷⁹ The model may be described by eqn (8),

c_e/q_e = 1/(K_L × q_m) + c_e/q_m (8)

where q_e (mg g⁻¹) is the equilibrium Pd(II) concentration on the adsorbent, c_e (mg L⁻¹) is the equilibrium Pd(II) concentration in the solution, q_m (mg g⁻¹) is the maximum monolayer adsorption capacity, and K_L (L mg⁻¹) is the Langmuir adsorption constant. Nonetheless, the Freundlich model represents adsorption on a heterogeneous (multiple layer) surface with uniform energy, as shown in the following eqn (9):

ln q_e = ln K_F + (1/n) × ln c_e (9)

where K_F and n are the Freundlich constants for multilayer adsorption capacity and adsorption intensity, respectively.⁸⁰Table 4 displays the fitted parameters of the adsorption isotherm. Compared to other models, the adsorption behavior of Pd(II) was better described by the Langmuir model (Fig. 6b, R² = 0.9879). The projected maximal adsorption capacity of 390.6 mg g⁻¹ was quite similar to the experimental values. The R² value of the Freundlich model (0.9008) suggested that the model (Fig. 6c) could not fully capture the adsorption behavior seen in our experiment. It is clear that CN-DAPhen has a substantially higher adsorption capacity in acidic conditions when compared to other remarkable porous adsorbents utilized for the same application (Table S3†).

Fig. 6 The adsorption isotherms of Pd(II) on CN-DAPhen (a), linear fitting of the isotherm by Langmuir model (b) and Freundlich model (c) (adsorbent dosage = 0.4 mg mL⁻¹, [Pd]₀ = 100 mg L⁻¹, T = 298 ± 1 K, pH = 1.0 ± 0.1, t = 6 h).

Table 4 Isotherm fitted parameters of Langmuir and Freundlich models for Pd(II) adsorption of CN-DAPhen

Langmuir model			Freundlich model
k L (L mg^−1)	q m (mg g^−1)	R ^2	K F (L mg^−1)	n	R ^2
0.0117	390.6	0.9879	12.87	1.819	0.9008

---

To explain the observed exceptional performance, research into the adsorption process is essential. It also helps to build better materials using both empirical and logical methods. The surface chemical makeup and bonding environment of the adsorbents are expected to be altered when palladium is adsorbed onto the CN-DAPhen. In order to investigate the adsorption process, XPS studies with CN-DAPhen were carried out both before and after adsorption at pH = 1. Compared with the XPS survey spectrum of CN-DAPhen before adsorption, Pd 3d appeared in the spectrum of CN-DAPhen–Pd after adsorption (Fig. 7a), confirming that the Pd species were successfully adsorbed on CN-DAPhen. The new peaks at 338.3 eV and 343.6 eV corresponding to the 3d_3/2 and 3d_5/2 of palladium implied that the oxidized state of the adsorbed palladium is Pd(II) (Fig. 7b).⁸¹ To further corroborate the binding behavior of CN-DAPhen with Pd, the O 1s high-resolution spectra of the adsorbents after adsorption were analyzed. The results from the spectrum (Fig. 7c) suggested that there were two types of O atoms (the O atoms in O–Pd at 534.4 eV and the O atoms in O=C/O–N at 532.0 eV), and the ratio of the total O atoms to O atoms in O–Pd was around 4.96. Meanwhile, the element content of O to Pd was 11.2 in CN-DAPhen after adsorption, indicating that the ratio of O atoms that corresponded with Pd atoms was 2.25. As a result, a Pd corresponded with one DAPhen group that possessed two C=O moieties, which agreed well with the result of a 1 : 1 complex (DAPhen–Pd) obtained from previous literature reports.⁸² Therefore, in the subsequent DFT simulation calculation, the combination of one DAPhen with one Pd(II) was also used. The mechanism for the adsorption of Pd(II) by CN-DAPhen on XPS is also included.

Fig. 7 Low resolution XPS spectra of CN-DAPhen before and after adsorption (a), XPS Pd 3d spectra (b) and O 1s spectra (c) of CN-DAPhen–Pd (after adsorption).

The presence of more than 40 components and 400 nuclides in HLLW, and the coexistence of different metal ions may lead to competitive adsorption, so the selectivity of palladium adsorption has been regarded as one of the most crucial factors when evaluating an adsorbent.⁷ Selecting the metal ions with high content in the actual HLLW as a representative, the selective adsorption performance on Pd(II) of CN-DAPhen in the simulated nuclear waste includes Cs(I), Fe(III), Ni(II), Ru(III), Sr(II), Cd(II), Cr(III), and Nd(III). The results are highly encouraging as CN-DAPhen exhibited adsorption selectivity for Pd(II) to other metal ions coexisting in solution (Fig. 8a). In the solution containing different concentrations of HNO₃, CN-DAPhen exhibited high S over 24.8 for competitive metal ions (Fig. 8b). The excellent selectivity of CN-DAPhen for Pd is further explained by theoretical calculations. The geometric structures of the functional group for CN-DAPhen and the complexes of CN-DAPhen–M were optimized at the M062X-SDD/6-311G(d,p) level of theory, as shown in Fig. S6.† The bond length of the CN-DAPhen–Pd complex is shorter than that of other CN-DAPhen–M complexes (Table S4†), confirming that the CN-DAPhen–Pd complex is more stable. The complexation ability was also investigated by analyzing the charge distribution of the complexes, including the charge on the metal ions and nitrogen atoms.^56,83,84 The natural charges of the nitrogen atoms in CN-DAPhen–Pd were more negative than those in other CN-DAPhen–M, which meant that CN-DAPhen–Pd was more likely to give electrons to coordinate with Pd(II). Moreover, according to DFT calculation, the different structures of CN-DAPhen–M have different reaction Gibbs free energies (Table S5†). All of the ΔG values are negative and the Gibbs free energy of CN-DAPhen–Pd possessed smaller ΔG of −546.65 kJ mol⁻¹ than that of other CN-DAPhen–M. The high ΔG value (more negative) suggests that CN-DAPhen favors Pd(II) more than other metal ions in water media, which is quite consistent with the experimental results.

Fig. 8 Adsorption amounts of Pd(II) and the other 8 interfering metals in simulated HLLW by the CN-DAPhen at different HNO₃ concentrations (a) and the corresponding S values (b). Recovery rate of Pd and Ru in simulated HLLW by the separation process (c) (adsorbent dosage = 0.4 mg mL⁻¹, [Pd]₀ = [Cs]₀ = [Fe]₀ = [Ni]₀ = [Ru]₀ = [Sr]₀ = [Cd]₀ = [Cr]₀ = [Nd]₀ = 50 mg L⁻¹, T = 298 ± 1 K, pH = 1.0 ± 0.1, t = 6 h).

As the concentration of nitric acid increases, the adsorption capacity of CN-DAPhen for both Pd(II) and other metal ions gradually decreases, but the separation coefficient gradually increases. When the nitric acid concentration reached 5 mol L⁻¹, the adsorption capacity of CN-DAPhen for Pd(II) was still 50.9 mg g⁻¹ (Fig. 8a) and CN-DAPhen displayed extraordinarily high S values over 64.4 towards all of the tested metal ions (Fig. 8b). The protonation degree of the DAPhen groups increased with the increase of nitric acid concentration, causing the depressed adsorption ability of CN-DAPhen for metal ions, which was why the adsorption capacities of CN-DAPhen for the metal ions decreased as the concentration of nitric acid increased.⁸⁵ On the other hand, Pd(II) occupied more unprotonated DAPhen groups at a higher concentration of nitric acid owing to the strongest affinity, causing the increased S under the high concentration of nitric acid.

Ru, another platinum group metals element of interest in HLLW, also needs to be removed. In 1 mol L⁻¹ of nitric acid, CN-DAPhen exhibited considerable adsorption selectivity of Ru to other competitive ions with S over 1.97, indicating the selective adsorption ability. According to our DFT calculation results, CN-DAPhen–Ru possessed ΔG of −475.07 kJ mol⁻¹, which was only larger than that of CN-DAPhen–Pd. The stronger affinity between Ru and the DAPhen groups compared to other metal ions (except Pd) was the reason why CN-DAPhen exhibited adsorption selectivity for Ru. As a result, it is possible to separate Pd and Ru separately from HLLW by adjusting the concentration of nitric acid. Based on this conclusion, a simple procedure for the separation of Pd and Ru from simulated HLLW using CN-DAPhen is proposed (Scheme 2) based on the selective adsorption properties of CN-DAPhen for different metal ions in HLLW. After three adsorptions in a 5 mol L⁻¹ HNO₃ system and desorption, 91.6% of Pd was recycled. The acidity of the system was then adjusted to 1 mol L⁻¹. After three more adsorptions, 53.7% of Ru was recycled (Fig. 8c). Overall, it is anticipated that the CN-DAPhen adsorbent has the ability to selectively recover palladium and ruthenium from radioactive streams.

Scheme 2 Process for the separation of Pd and Ru from simulated HLLW based on CN-DAPhen.

Given the extremely radioactive nature of HLLW, radiation resistance is a crucial factor to consider when evaluating adsorbents. The adsorption was processed under a particular dosage to determine the radiation-induced change in the CN-DAPhen adsorption performance. As demonstrated in Fig. 9a, the adsorption amount of Pd(II) gradually decreased with the increase of the absorbed dose. During the adsorption procedure, the radiolysis of the DAPhen group by ionizing radiation decreases the adsorption quantity as the absorbed dosage increases. However, when the absorbed dosage was as high as 100 kGy, the CN-DAPhen adsorption amount for Pd(II) remained at 113.5 mg g⁻¹, which was 88.8% of the original adsorption quantity. This revealed that CN-DAPhen was resistant to radiation. It is deemed vital to regenerate the adsorbents by releasing palladium in order to significantly minimize material usage. As seen in Fig. 9b, CN-DAPhen exhibited good reusability. The adsorption of Pd(II) on regenerated CN-DAPhen decreased slightly and remained at 90.9% after 5 successive adsorption–desorption cycles, indicating that CN-DAPhen is highly recyclable.

Fig. 9 Radiation stability of CN-DAPhen (a) (adsorbent dosage = 0.4 mg mL⁻¹, [Pd]₀ = 100 mg L⁻¹, T = 298 ± 1 K, pH = 1.0 ± 0.1, t = 6 h). Reusability of CN-DAPhen (b) (adsorbent dosage = 4 mg mL⁻¹, [Pd]₀ = 100 mg L⁻¹, T = 298 ± 1 K, pH = 1.0 ± 0.1, t = 6 h).

4. Conclusions

In this work, the amino group-rich porous g-C₃N₄ was fabricated through the facile oxalic acid-induced supramolecular assembly approach. We have shown the utilization of a novel adsorbent, CN-DAPhen, synthesized by amidation of g-C₃N₄ and DAPhen, for the efficient and selective recovery of palladium from the simulated high level liquid waste (HLLW). In 0.1 mol L⁻¹ HNO₃, CN-DAPhen performed well, with a maximum palladium capacity of 390.63 mg g⁻¹. An abundant porous structure equipped CN-DAPhen with ultrafast adsorption kinetics for Pd and the adsorption reached equilibrium within 5 min. The excellent fit of the pseudo-second-order kinetic model also suggested chemical adsorption. In addition, our thermodynamic study indicated the exothermal and spontaneous adsorption. CN-DAPhen also exhibited several other features that benefited the removal of Pd(II) from HLLW, including outstanding radiation resistance and good reusability. In the simulated HLLW, CN-DAPhen exhibited an extremely high selectivity for palladium over 9 coexisting cations, which may be attributed to a unique adsorption mechanism. Furthermore, a continuous process was developed based on the adsorption selectivity for the separation of Pd and Ru from simulated HLLW. The results of this study are promising palladium recovery nanomaterials with high radiation resistance, capacity, and selectivity. We hope that these materials will help us design and develop reliable palladium recovery devices.

Author contributions

Yizhi Chen: data curation, validation, conceptualization, writing – original draft. Peng Zhang: investigation, formal analysis. Yu Yang: conceptualization, writing – review & editing. Qi Cao: writing – review & editing. Qiqi Guo: DFT calculation, formal analysis. Yusen Liu: writing – review & editing. Hanbao Chong: formal analysis. Mingzhang Lin: supervision, project administration, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22276180).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2en00868h

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