Efficient and Selective Removal of Palladium from Simulated High-Level Liquid Waste Using a Silica-Based Adsorbent NTAamide(C8)/SiO2-P

In order to realize the effective separation of palladium from high-level liquid waste (HLLW), a ligand-supported adsorbent (NTAamide(C8)/SiO2-P) was prepared by the impregnation method in a vacuum. The SiO2-P carrier was synthesized by in situ polymerization of divinylbenzene and styrene monomers on a macroporous silica skeleton. The NTAamide(C8)/SiO2-P adsorbent was fabricated by impregnating an NTAamide(C8) ligand into the pore of a SiO2-P carrier under a vacuum condition. The adsorption performance of NTAamide(C8)/SiO2-P in nitric acid medium has been systematically studied. In a solution of 0.2 M HNO3, the distribution coefficient of Pd on NTAamide(C8)/SiO2-P was 1848 mL/g with an adsorption percentage of 90.24%. With the concentration of nitric acid increasing, the adsorption capacity of NTAamide(C8)/SiO2-P decreases. Compared to the other 10 potential interfering ions in fission products, NTAamide(C8)/SiO2-P exhibited excellent adsorption selectivity for Pd(II). The separation factor (SFPd/other metals > 77.8) is significantly higher than that of similar materials. The interference of NaNO3 had a negligible effect on the adsorption performance of NTAamide(C8)/SiO2-P, which maintained above 90%. The adsorption kinetics of Pd(II) adsorption on NTAamide(C8)/SiO2-P fits well with the pseudo-second order model. The Sips model is more suitable than the Langmuir and Freundlich model for describing the adsorption behavior. Thermodynamic analysis showed that the adsorption of Pd(II) on NTAamide(C8)/SiO2-P was a spontaneous, endothermic, and rapid process. NTAamide(C8)/SiO2-P also demonstrated good reusability and economic feasibility.


Introduction
Nuclear energy, as a clean and reliable energy source, is favored by many countries due to its unique advantages.High-level liquid waste (HLLW) generated from fuel treatment contains more than 40 elements, such as Ru, Rh, Pd, Tb, Tc, Cs, Sr, Mo, I, etc. [1,2] Palladium is a platinum-group metal with a silver luster.The palladium in the HLLW includes stable isotopes 104 Pd (17 wt.%), 105 Pd (29 wt.%), 106 Pd (21 wt.%), 108 Pd (12 wt.%), and 110 Pd (4 wt.%), and radioactive isotope 107 Pd (17 wt.%).The half-life of 107 Pd is 6.5 × 10 6 years.Compared with other elements in HLLW (such as Rh and Ru), the radioactivity is very weak and considered safe to recover [3][4][5].During the subsequent treatment of HLLW through vitrification, palladium tends to deposit at the bottom of the furnace because of its high melting point and low solubility in the high-level radioactive nuclear waste glass, which greatly reduces the stability of the glass body and increases the difficulty of vitrification.To enhance the vitrification efficiency, palladium needs to be extracted from HLLW in advance.On the other hand, due to the excellent catalytic properties, thermal stability, corrosion resistance, and other characteristics of palladium, it is an important strategic material with economic and industrial value [6,7] and has been widely used in catalysis [8], actor manufacturing [9], aerospace [10], automobile [11], biomedicine [12], and other aspects.Palladium, like other platinum group metals, has a scarce reserve, and its supply always requires secondary resources [13].Comprehensive treatment of secondary resources and efficient recycling of Pd are of great significance to increase the supply of Pd and promote the sustainable development of multiple industries [14][15][16][17].Therefore, recovering palladium from HLLW not only facilitates the subsequent treatment of HLLW through vitrification, but also makes use of the secondary resources to generate Pd.
Solvent extraction is a widely used method for recovering palladium from HLLW [1,2,18,19].It is well known that Pd(II) belongs to a soft Lewis acid, and can easily complex with the ligand containing a soft-donor N or S atom [20].N,N,N ′ ,N ′ ,N ′′ ,N ′′hexaoctyl-nitrilotriacetamide (NTAamide(C8)) has received increasing attention, as a new soft non-heterocyclic N-donor extractant.The structure of NTAamide(C8) is shown in Figure 1.Sasaki et al. [21] used NTAamide(C8) to recover Pd(II) through extraction electrodeposition, and the extraction rate could reach above 91% in all cycles.Wang et al. [22] studied the extraction ability of NTAamide(n-Oct) for Pd(II) and confirmed its good selectivity, extraction efficiency, and loading capacity.Sasaki et al. [23] studied the extraction behavior of NTAamide(C8) for 70 metals and found that the extraction rate for Pd(II) was the highest, indicating that NTAamide(C8) had good selectivity for Pd(II) and could be used to separate Pd(II) from multi-element mixed solutions, such as HLLW.
Nanomaterials 2024, 14, x FOR PEER REVIEW 3 of 19 Considering the advantages of NTAamide(C8) extractant and SiO2-P carrier, this study aims to combine them to prepare a new adsorption material, which is expected to achieve high selectivity and adsorption percentage simultaneously.The adsorption performance and mechanism of recovery of palladium ion from simulated HLLW were investigated.

Chemicals
Nitric acid was supplied by Sinopharm Chemical Reagent Co., Ltd.(Beijing, China).Rhodium nitrate was provided by Kunming Bosen Precious Metal Materials Processing Co., Ltd.(Kunming, China).The standard solutions of Gd, Ho, Dy, Ce, Sn, Re, Sm, Pr, Tb, Mo, and Sr were obtained from Guobiao Testing & Certification Co., Ltd.(Beijing, China).All chemicals were of analytical grade.Deionized water was obtained through a Smart-Q15 system (Hitech Instruments Co., Ltd.Shanghai, China) in the lab.Palladium nitrate dihydrate, ruthenium nitrosyl nitrate, and other chemicals were purchased from Shanghai Macklin Biochemical Co., Ltd.(Shanghai, China).All chemicals were analytical grade.

Preparation of SiO2-P
The inorganic organic composite carrier SiO2-P was prepared by an in situ polymerization method [35,36].The preparatory flow sheet of the SiO2-P is illustrated in Figure 2. First, the stabilizers in styrene and divinylbenzene were removed by vacuum distillation and washed with a dilute alkali solution, respectively.Porous SiO2 was added to the round-bottom flask of a rotary evaporator, and the flask was rotated at 40 rpm.The air in the flask was displaced by first vacuuming and then purging with inert gases.The air pressure was reduced to 3 kPa again.Acetophenone and diethyl phthalate were used as diluents and placed in a beaker.Then, the monomers (divinylbenzene and styrene) were added to the beaker and mixed thoroughly, followed by adding initiator AIBN and V-40.Then, the mixture was poured into the round-bottom flask to fully mix with SiO2.Inert gas was introduced to ensure that the device is in an oxygen-free environment.The copolymerization reaction started between styrene and divinylbenzene upon heating.After the reaction was completed, the product SiO2-P was washed and dried before use.Despite the high recovery efficiency and the potential to scale up, solvent extraction methods suffer from emulsification and prominent three-phase problems.The extraction process also consumes a large volume of solvents and generates toxic organic waste.In addition, solvent extraction has high requirements for subsequent treatment and economic investment.Due to these drawbacks, some alternative separation methods have been developed [24][25][26][27][28].The chemical adsorption method [25,28,29] is widely used due to its high efficiency and low cost, especially for recovery metals at low concentrations.At present, the adsorbents mainly include inorganic material adsorbents, nanomaterial adsorbents, ion exchange resin adsorbents, and biomaterial adsorbents.Awual et al. [30] prepared a highly selective and efficient adsorbent by immobilizing the synthesized N,N-bis(salicylidene)1,2bis(2-aminophenylthio)ethane ligand onto mesoporous silica monoliths, and its maximum sorption capacity for Pd(II) ions was 163.13 mg/g.Mincke et al. [31] synthesized three chitosan derivatives and used them to recover Pd(II) and Pt(IV) from acidic solutions.These absorbents showed high adsorption rates, reaching equilibrium within 30 min.Their maximum adsorption capacities for Pd(II) were 262.6 mg/g, 154.7 mg/g, and 340.3 mg/g, respectively.However, due to the coexistence of platinum group metals in HLLW and their similar chemical properties, adsorbents tend to adsorb them simultaneously, making it impossible to selectively adsorb Pd.This needs further in-depth study.
SiO 2 -P is a composite carrier composed of porous silica and organic copolymers.The adsorbents based on SiO 2 -P demonstrate a high adsorption rate, good reusability, and environmental friendliness.Yu et al. [32] prepared a macroporous silica polymer (TRPO/SiO 2 -P) to recover scandium from sulfuric acid solutions.The adsorption was selective, reaching an equilibrium within 120 min with a saturated adsorption of approximately 13.3 mg/g.Ning et al. [33] used silica-based isoBu-BTP/SiO 2 -P adsorbent to separate Ru, Rh, and Pd from nitrate solution, and their saturated adsorption capacities reached 0.37 mmol/g, 0.34 mmol/g, and 0.71 mmol/g, respectively.Su et al. [34] prepared a silica-based polymer adsorbent (isopentyl-BTBP/SiO 2 -P) by a maceration method in a vacuum for efficient and continuous separation of Pd from complex systems.In a HNO 3 solution of 3 M, the distribution coefficient of Pd was 5226 mL/g, and the maximum adsorption capacity was 31.7 mg/g.
Considering the advantages of NTAamide(C8) extractant and SiO 2 -P carrier, this study aims to combine them to prepare a new adsorption material, which is expected to achieve high selectivity and adsorption percentage simultaneously.The adsorption performance and mechanism of recovery of palladium ion from simulated HLLW were investigated.The inorganic organic composite carrier SiO 2 -P was prepared by an in situ polymerization method [35,36].The preparatory flow sheet of the SiO 2 -P is illustrated in Figure 2. First, the stabilizers in styrene and divinylbenzene were removed by vacuum distillation and washed with a dilute alkali solution, respectively.Porous SiO 2 was added to the round-bottom flask of a rotary evaporator, and the flask was rotated at 40 rpm.The air in the flask was displaced by first vacuuming and then purging with inert gases.The air pressure was reduced to 3 kPa again.Acetophenone and diethyl phthalate were used as diluents and placed in a beaker.Then, the monomers (divinylbenzene and styrene) were added to the beaker and mixed thoroughly, followed by adding initiator AIBN and V-40.Then, the mixture was poured into the round-bottom flask to fully mix with SiO 2 .Inert gas was introduced to ensure that the device is in an oxygen-free environment.The co-polymerization reaction started between styrene and divinylbenzene upon heating.After the reaction was completed, the product SiO 2 -P was washed and dried before use.2.2.2.Preparation of NTAamide(C8)/SiO2-P NTAamide(C8) ligand was loaded onto the SiO2-P carrier by the impregnation method in a vacuum [37][38][39].The preparatory flow sheet of the NTAamide(C8)/SiO2-P is illustrated in Figure 3. First, an appropriate amount of methanol was added to SiO2-P and 2.2.2.Preparation of NTAamide(C8)/SiO 2 -P NTAamide(C8) ligand was loaded onto the SiO 2 -P carrier by the impregnation method in a vacuum [37][38][39].The preparatory flow sheet of the NTAamide(C8)/SiO 2 -P is illustrated in Figure 3. First, an appropriate amount of methanol was added to SiO 2 -P and mixed thoroughly, followed by solid-liquid separation.The above steps were repeated three times, and then the obtained SiO 2 -P was dried.SiO 2 -P (10 g) was placed in a round-bottom flask.Then, 5 g of NTAamide(C8) was dissolved in an appropriate amount of dichloromethane and transferred to the round-bottom flask.The flask was rotated at 100 rpm on a rotary evaporator at room temperature and pressure for 1 h, and then left standing for 1 h.The flask was subsequently heated in a water bath to 310 K while maintaining the rotation speed at 100 rpm for 1.5 h before turning on the cooling water circulation device.After the above steps, the vacuum pump was turned on to slowly depressurize the extractant into the channel of SiO 2 -P until the diluent dichloromethane was completely volatilized.After drying, the NTAamide(C8)/SiO 2 -P was obtained.

Preparation of NTAamide(C8)/SiO2-P
NTAamide(C8) ligand was loaded onto the SiO2-P carrier by the impregnation method in a vacuum [37][38][39].The preparatory flow sheet of the NTAamide(C8)/SiO2-P is illustrated in Figure 3. First, an appropriate amount of methanol was added to SiO2-P and mixed thoroughly, followed by solid-liquid separation.The above steps were repeated three times, and then the obtained SiO2-P was dried.SiO2-P (10g) was placed in a roundbottom flask.Then, 5 g of NTAamide(C8) was dissolved in an appropriate amount of dichloromethane and transferred to the round-bottom flask.The flask was rotated at 100 rpm on a rotary evaporator at room temperature and pressure for 1 h, and then left standing for 1 h.The flask was subsequently heated in a water bath to 310 K while maintaining the rotation speed at 100 rpm for 1.5 h before turning on the cooling water circulation device.After the above steps, the vacuum pump was turned on to slowly depressurize the extractant into the channel of SiO2-P until the diluent dichloromethane was completely volatilized.After drying, the NTAamide(C8)/SiO2-P was obtained.

Adsorption Experiments
A certain amount of the adsorbent was mixed with the metal ion solutions (0.01-1 M) in a centrifuge tube, which was placed in a constant-temperature water bath and oscillated at 280 rpm.After the adsorption reached equilibrium, the mixture was centrifuged and the supernatant was used to analyze the concentration of metal ions by ICP-OES.The acidity of the solution, the dosage of the adsorbent, the concentration of metal ions, the temperature, and the coexisting ions were adjusted to evaluate the static adsorption performance of the adsorption material.Several important physical parameters such as adsorption capacity (Q, mg/g), adsorption percentage (E), distribution coefficient (K d , mL/g), and separation factor (SF) are defined as follows: where C 0 and C (mg/L) represent the initial and equilibrium metal ion concentrations, respectively, m (g) is the mass of the adsorbent, and V (mL) is the volume of the aqueous phase.A and B represent two different components in the same aqueous phase.

Characterization
To determine the morphology and elemental mapping of the adsorbent, SEM characterization was performed.The SEM results of NTAamide(C8)/SiO 2 -P and the SEM-EDS results of Pd-loaded NTAamide(C8)/SiO 2 -P are shown in Figure 4.The surface of the NTAamide(C8)/SiO 2 -P sphere is relatively rough, which indicates that NTAamide(C8) entered the inner pores of SiO 2 -P.The uniform distribution of N elements indicates that NTAamide(C8) was uniformly loaded onto SiO 2 -P.The results after adsorption show that Pd elements are evenly distributed on the surface of the sphere, confirming the adsorption of Pd.The SEM results of SiO2-P are shown in Figure S1.In addition, the organic content of NTAamide(C8) in NTAamide(C8)/SiO2-P was determined to be 35.63%through a TG thermal analyzer (Figure S2).By analyzing N2-adsorption-desorption isotherm and pore diameter distribution, it was calculated that the BET surface area is 26.4 m 2 /g, pore volume is 0.17 cm 3 /g, and the average pore diameter is 29.4 nm (Figure S4 and Table S1).The SEM results of SiO 2 -P are shown in Figure S1.In addition, the organic content of NTAamide(C8) in NTAamide(C8)/SiO 2 -P was determined to be 35.63%through a TG thermal analyzer (Figure S2).By analyzing N 2 -adsorption-desorption isotherm and pore diameter distribution, it was calculated that the BET surface area is 26.4 m 2 /g, pore volume is 0.17 cm 3 /g, and the average pore diameter is 29.4 nm (Figure S4 and Table S1).

Effect of NTAamide(C8)/SiO2-P Dosage
As shown in Figure 6, with the increasing dosages of NTAamide(C8)/SiO2-P, the adsorption percentage of Pd increases.When 80 mg of NTAamide(C8)/SiO2-P is added, the adsorption percentage reaches 98.41% (Equation ( 2)).More adsorbents provide more adsorption sites for Pd(II), resulting in a lower equilibrium concentration and higher adsorption percentage.

Effect of NTAamide(C8)/SiO 2 -P Dosage
As shown in Figure 6, with the increasing dosages of NTAamide(C8)/SiO 2 -P, the adsorption percentage of Pd increases.When 80 mg of NTAamide(C8)/SiO 2 -P is added, the adsorption percentage reaches 98.41% (Equation ( 2)).More adsorbents provide more adsorption sites for Pd(II), resulting in a lower equilibrium concentration and higher adsorption percentage.

Selectivity of NTAamide(C8)/SiO 2 -P
The adsorption selectivity of NTAamide(C8)/SiO 2 -P was studied (Figure 7).In 0.2 M nitric acid, NTAamide(C8)/SiO 2 -P has no obvious adsorption effect on 10 metal ions.Comparatively, the distribution coefficient of Pd (Equation (3)) is significantly higher, suggesting that NTAamide(C8)/SiO 2 -P has good adsorption selectivity (SF Pd/other metals > 77.8 (Equation ( 4)) and can be used to separate Pd from other metal ions.NTAamide(C8)/SiO 2 -P was compared with two similar materials.Me 2 -CA-BTP/SiO 2 -P [37] is a novel silicabased adsorbent, which is prepared to separate minor actinides from fission products in HLLW.The K d of Pd is below 100 mL/g and SF Am/Pd value is 17 in 0.1 M HNO 3 .IsoBu-BTP/SiO 2 -P [33] has good adsorption capacity for Pd, but also adsorbs a large amount of Ru and Rh.It is unable to selectively separate Pd.Compared with these adsorptions, NTAamide(C8)/SiO 2 -P has superiority in selectivity.NTAamide(C8) has nitrogen donor atoms at the center of the backbone.In the HSAB (Hard-Soft-Acid-Base) framework, NTAamide(C8) is classified as a soft base.Among these metal ions, Pd is the only soft acid with good electron acceptance ability.Pd(II) can coordinate with the ligands through nitrogen donor atoms on the central frame and carbonyl groups.

Effect of NTAamide(C8)/SiO2-P Dosage
As shown in Figure 6, with the increasing dosages of NTAamide(C8)/SiO2-P, the adsorption percentage of Pd increases.When 80 mg of NTAamide(C8)/SiO2-P is added, the adsorption percentage reaches 98.41% (Equation ( 2)).More adsorbents provide more adsorption sites for Pd(II), resulting in a lower equilibrium concentration and higher adsorption percentage.

Selectivity of NTAamide(C8)/SiO2-P
The adsorption selectivity of NTAamide(C8)/SiO2-P was studied (Figure 7).In 0.2 M nitric acid, NTAamide(C8)/SiO2-P has no obvious adsorption effect on 10 metal ions.Comparatively, the distribution coefficient of Pd (Equation (3)) is significantly higher, suggesting that NTAamide(C8)/SiO2-P has good adsorption selectivity (SFPd/other metals > 77.8 (Equation ( 4)) and can be used to separate Pd from other metal ions.NTAamide(C8)/SiO2-P was compared with two similar materials.Me2-CA-BTP/SiO2-P [37] is a novel silicabased adsorbent, which is prepared to separate minor actinides from fission products in HLLW.The Kd of Pd is below 100 mL/g and SFAm/Pd value is 17 in 0.1 M HNO3.IsoBu-BTP/SiO2-P [33] has good adsorption capacity for Pd, but also adsorbs a large amount of Ru and Rh.It is unable to selectively separate Pd.Compared with these adsorptions, NTAamide(C8)/SiO2-P has superiority in selectivity.NTAamide(C8) has nitrogen donor atoms at the center of the backbone.In the HSAB (Hard-Soft-Acid-Base) framework, NTAamide(C8) is classified as a soft base.Among these metal ions, Pd is the only soft acid with good electron acceptance ability.Pd(II) can coordinate with the ligands through nitrogen donor atoms on the central frame and carbonyl groups.

Effect of NaNO3 Concentration on Adsorption Performance
During the adsorption process, the N donor is protonated, requiring nitrate ions to maintain the charge balance.Therefore, the effect of sodium nitrate on the adsorption was investigated.The concentration of NaNO3 varies from 0 to 5 M, and the adsorption of Pd(II) by NTAamide(C8)/SiO2-P was studied in 0.2 M nitric acid (Figure 8).The system without NaNO3 can achieve a high adsorption percentage.The presence of NaNO3 slightly improves the adsorption percentage of NTAamide(C8)/SiO2-P.The highest adsorption percentage (92.4%) is achieved when the NaNO3 concentration is 2 M, which is about 2.2% higher than the system without NaNO3.Considering that NaNO3 has a negligible effect on adsorption, the system without NaNO3 is used for subsequent experiments.

Effect of NaNO 3 Concentration on Adsorption Performance
During the adsorption process, the N donor is protonated, requiring nitrate ions to maintain the charge balance.Therefore, the effect of sodium nitrate on the adsorption was investigated.The concentration of NaNO 3 varies from 0 to 5 M, and the adsorption of Pd(II) by NTAamide(C8)/SiO 2 -P was studied in 0.2 M nitric acid (Figure 8).The system without NaNO 3 can achieve a high adsorption percentage.The presence of NaNO 3 slightly improves the adsorption percentage of NTAamide(C8)/SiO 2 -P.The highest adsorption percentage (92.4%) is achieved when the NaNO 3 concentration is 2 M, which is about 2.2% higher than the system without NaNO 3 .Considering that NaNO 3 has a negligible effect on adsorption, the system without NaNO 3 is used for subsequent experiments.

Adsorption Kinetics
The kinetics of Pd(II) adsorption on NTAamide(C8)/SiO2-P was studied at 298 K. Pseudo-first order kinetics (Equation ( 5)) and pseudo-second order kinetics (Equation ( 6)) were used to fit the curves of adsorption capacity vs. time.
where Qe and Qt (mg/g) (Equation ( 1)) represent the equilibrium adsorption capacity and adsorption capacity at time t(min), respectively; k1 (min −1 ) and k2 (mg/(g•min)) represent the adsorption rate constants of pseudo-first and pseudo-second order kinetics, respectively.
Their non-linear form is described as Equations ( 7) and ( 8), respectively: ( ) As shown in Figure 9a and Table 1, the pseudo-second order model has a higher correlation coefficient (R 2 ), and its theoretical equilibrium adsorption capacity (Qe) is closer to the experimental value (Qe.exp), suggesting it is more suitable to describe the adsorption process.The correlation coefficient was fitted through the formula.It indicates the degree of agreement between the experimental data and the model.

Adsorption Kinetics
The kinetics Pd(II) adsorption on NTAamide(C8)/SiO 2 -P was studied at 298 K. Pseudo-first order kinetics (Equation ( 5)) and pseudo-second order kinetics (Equation ( 6)) were used to fit the curves of adsorption capacity vs. time.(6) where Q e and Q t (mg/g) (Equation ( 1)) represent the equilibrium adsorption capacity and adsorption capacity at time t(min), respectively; k 1 (min −1 ) and k 2 (mg/(g•min)) represent the adsorption rate constants of pseudo-first and pseudo-second order kinetics, respectively.
Their non-linear form is described as Equations ( 7) and ( 8), respectively: As shown in Figure 9a and Table 1, the pseudo-second order model has a higher correlation coefficient (R 2 ), and its theoretical equilibrium adsorption capacity (Q e ) is closer to the experimental value (Q e.exp ), suggesting it is more suitable to describe the adsorption process.The correlation coefficient was fitted through the formula.It indicates the degree of agreement between the experimental data and the model.In order to explain the diffusion mechanism, the intraparticle diffusion model (Equation ( 9)) was applied to the experimental data [40]: where kid (mg/(g•min 0.5 )) represents the intraparticle diffusion rate constant and the value of C is affected by boundary layer thickness.
As shown in Figure 9b and Table 1, the plots present two steps.The first step was surface diffusion stage and the second step was intraparticle diffusion stage.The plot did not pass through the origin, indicating that intraparticle diffusion is not the only ratedetermining step.The decrease in slope indicates that the adsorption sites were occupied, and the adsorption rate decreased.The adsorption gradually reached equilibrium.In order to explain the diffusion mechanism, the intraparticle diffusion model (Equation ( 9)) was applied to the experimental data [40]: where k id (mg/(g•min 0.5 )) represents the intraparticle diffusion rate constant and the value of C is affected by boundary layer thickness.
As shown in Figure 9b and Table 1, the plots present two steps.The first step was surface diffusion stage and the second step was intraparticle diffusion stage.The plot did not pass through the origin, indicating that intraparticle diffusion is not the only ratedetermining step.The decrease in slope indicates that the adsorption sites were occupied, and the adsorption rate decreased.The adsorption gradually reached equilibrium.

Adsorption Isotherm
By changing the initial concentration of Pd in the solution, the adsorption isotherm at 298 K, 303 K, 308 K, 313 K, and 318 K were measured.Five adsorption isotherm models (Langmuir, Freundlich, Temkin, Dubinin-Radushkevich, and Sips (Equations ( 10)-( 14))) were used to fit the experimental data to understand the relationship between adsorption capacity (Q) and Pd equilibrium concentration (Figure 10 and Table 2).From Table 2, the R 2 of the Sips isotherm model is higher than those of the other four isotherm models.Therefore, the Sips isotherm model is more suitable for describing the adsorption behavior of Pd(II) on NTAamide(C8)/SiO 2 -P.The Sips isotherm is a combination of the Langmuir and Freundlich isotherm models, and it is based on the Freundlich equation assumption [40].It is speculated that the adsorption of palladium is through a multi-molecular layer chemical process.1/n is the Sips isotherm exponent, which can be used to indicate the non-uniformity of adsorption sites.If 1/n < 1, the adsorption sites on the adsorbent surface are not uniformly distributed.The Sips isotherm model also indicates a finite limit on adsorption capacity.The theoretical saturated adsorption capacity of NTAamide(C8)/SiO 2 -P is higher than that of a similar material IONP@SiO 2 (6.5 mg/g) [41].

Adsorption Thermodynamics
To understand the thermodynamics of NTAamide(C8)/SiO 2 -P adsorption and clarify the relationship between the adsorption and temperature, the adsorption isotherm experiments were conducted at 298-318 K, and the thermodynamic calculation was carried out by the Van't Hoff equation (Equations ( 15) and ( 16)).
where K F is the model constant of the Freundlich adsorption isotherm model.∆G θ , ∆H θ , and ∆S θ are the changes in Gibbs free energy (J/mol), enthalpy (J/mol), and entropy (J/(K•mol)), respectively.R is the universal gas constant (8.314J/(K•mol)).
Although the R 2 of the Sips isotherm model is higher, the deviation of Ks is relatively large.The Sips isotherm model is based on the Freundlich equation assumption, and the R 2 value of the Freundlich isotherm model is close to that of Sips.Therefore, K F of the Freundlich adsorption isotherm model was used for the thermodynamic calculation.
The calculation results are shown in Figure 11 and Table 3.A positive ∆H value indicates that the adsorption of Pd(II) is an endothermic reaction, which is consistent with the adsorption isotherm experiments (the equilibrium adsorption capacity increases with the temperature).The ∆G value remains negative at different temperatures, indicating that the adsorption of NTAamide(C8)/SiO 2 -P is a spontaneous process.The ∆S value is 44.5 J/K•mol for the adsorption process, suggesting that the adsorption is driven by entropy.

Adsorption Mechanism
The interaction mechanism of NTAamide(C8)/SiO2-P was studied using XPS and FT-IR.
The XPS spectra of NTAamide(C8)/SiO2-P and Pd-loaded NTAamide(C8)/SiO2-P are shown in Figure 12a.The presence of C1s, N1s, and O1s peaks and the two Auger peaks of C and O in the spectrum is consistent with the composition of NTAamide(C8)/SiO2-P.The N1s peak emerges at 400 eV, indicating that NTAamide(C8) has been loaded onto SiO2-P.A characteristic peak of Pd3d is found at 338 eV after adsorption, indicating that Pd has been adsorbed by NTAamide(C8)/SiO2-P.The XPS spectra of N1s is shown in Fig-

Adsorption Mechanism
The interaction mechanism of NTAamide(C8)/SiO 2 -P was studied using XPS and FT-IR.The XPS spectra of NTAamide(C8)/SiO 2 -P and Pd-loaded NTAamide(C8)/SiO 2 -P are shown in Figure 12a.The presence of C1s, N1s, and O1s peaks and the two Auger peaks of C and O in the spectrum is consistent with the composition of NTAamide(C8)/SiO 2 -P.The N1s peak emerges at 400 eV, indicating that NTAamide(C8) has been loaded onto SiO 2 -P.A characteristic peak of Pd3d is found at 338 eV after adsorption, indicating that Pd has been adsorbed by NTAamide(C8)/SiO 2 -P.The XPS spectra of N1s is shown in Figure 12b.For NTAamide(C8)/SiO 2 -P and Pd-loaded NTAamide(C8)/SiO 2 -P, the binding energy of C 3 N are 399.76eV and 400.02 eV, respectively.The change in binding energy of C 3 N implies that it may play an important role in the adsorption behavior.The specific mechanism will be further studied through FT-IR.The FT-IR spectra of NTAamide(C8)/SiO 2 -P and Pd-loaded NTAamide(C8)/SiO 2 -P are shown in Figure 13.The characteristic peak at 798cm −1 corresponds to the stretching vibration of Si-O, while the peaks at 470 and 1108 cm −1 are associated with the stretching vibrations of Si-O-Si.The out-of-plane deformation vibration of the aromatic hydrocarbon (=C-H) is observed at 700 and 1462 cm −1 .The characteristic peaks at 1380 cm −1 and 1650 cm −1 correspond to the deformation vibration of the methyl group (CH 3 ) and the stretching vibration of C=O in NTAamide(C8).The stretching vibration peaks of methylene (C-H) in NTAamide(C8) appear at 2856 and 2927 cm −1 .The wide peak at 3450 cm −1 is due to the vibration of H-O-H in water that has not been completely removed.NTAamide(C8) contains a tertiary amine structure (R 3 N), which exhibits double peaks at 1230~1030 cm −1 .However, due to the stronger characteristic peak of Si-O-Si at 1108 cm −1 , the double peaks become a wider peak.After adsorption, the characteristic peaks are slenderer, indicating that the tertiary amine structure may coordinate with Pd(II), which is consistent with the composition of NTAamide(C8)/SiO 2 -P.According to the relative literature [22,42], the 2:1 ligand/Pd 2+ complexes were formed in a low concentration of HNO 3 solution.Two NTAamide(C8) ligand molecules coordinated with one four-coordinated Pd 2+ via one of the three carbonyl groups and the central N atom, forming a complex cation.There are two NO 3 − ions anion as the counterpart ions for charge balance, and the NO 3− ions only have a salting-out effect on the ligands under this condition [22].The single peak originally located at 1650 cm −1 was split into two peaks at 1650 and 1595 cm −1 , which is also consistent with the adsorption mechanism.contains a tertiary amine structure (R3N), which exhibits double peaks at 1230~1030 cm −1 .However, due to the stronger characteristic peak of Si-O-Si at 1108 cm −1 , the double peaks become a wider peak.After adsorption, the characteristic peaks are slenderer, indicating that the tertiary amine structure may coordinate with Pd(II), which is consistent with the composition of NTAamide(C8)/SiO2-P.According to the relative literature [22,42], the 2:1 ligand/Pd 2+ complexes were formed in a low concentration of HNO3 solution.Two NTAamide(C8) ligand molecules coordinated with one four-coordinated Pd 2+ via one of the three carbonyl groups and the central N atom, forming a complex cation.There are two NO 3 − ions anion as the counterpart ions for charge balance, and the NO 3− ions only have a salting-out effect on the ligands under this condition [22].The single peak originally located at 1650 cm −1 was split into two peaks at 1650 and 1595 cm −1 , which is also consistent with the adsorption mechanism.NTAamide(C8) has nitrogen donor atoms at the center of the backbone.In the HSAB (Hard-Soft-Acid-Base) framework, NTAamide(C8) is classified as a soft base.Pd(II) can coordinate with the extractant through nitrogen donor atoms on the central frame and carbonyl groups.NO3 -is required to participate in the reaction to maintain charge balance.

Reusability
Reusability is an important factor in evaluating the performance of adsorbents.The reusability of NTAamide(C8)/SiO2-P was studied through six cycles of adsorption and desorption.First, 50 mg of the adsorbent was added to the Pd solution to reach adsorption equilibrium under certain conditions, and then the solution was removed.After drying, the used adsorbent was desorbed in a mixed solution of thiourea (0.01 M) and nitric acid (0.1 M) at 298 K while shaking at 280 rpm.As shown in Figure 14, the adsorption percentage after six adsorption-desorption cycles was 82.92% (a decrease of approximately 7.3% from the original value).The solid-liquid separation operation during the experiment may result in a slight loss of the adsorbent, which can lead to a decrease in adsorption percentage.Therefore, the NTAamide(C8)/SiO2-P adsorbent has good reusability, making   − is required to participate in the reaction to maintain charge balance.

Reusability
Reusability is an important factor in evaluating the performance of adsorbents.The reusability of NTAamide(C8)/SiO 2 -P was studied through six cycles of adsorption and desorption.First, 50 mg of the adsorbent was added to the Pd solution to reach adsorption equilibrium under certain conditions, and then the solution was removed.After drying, the used adsorbent was desorbed in a mixed solution of thiourea (0.01 M) and nitric acid (0.1 M) at 298 K while shaking at 280 rpm.As shown in Figure 14, the adsorption

Figure 12 .
Figure 12.(a) XPS spectra of NTAamide(C8)/SiO2-P and Pd-loaded NTAamide(C8)/SiO2-P, (b) high resolution scans of N 1s spectrum before and after adsorption.The FT-IR spectra of NTAamide(C8)/SiO2-P and Pd-loaded NTAamide(C8)/SiO2-P are shown in Figure 13.The characteristic peak at 798cm −1 corresponds to the stretching vibration of Si-O, while the peaks at 470 and 1108 cm −1 are associated with the stretching vibrations of Si-O-Si.The out-of-plane deformation vibration of the aromatic hydrocarbon (=C-H) is observed at 700 and 1462 cm −1 .The characteristic peaks at 1380 cm −1 and 1650 cm −1 correspond to the deformation vibration of the methyl group (CH3) and the stretching

Figure 13 .
Figure 13.FT-IR spectra of NTAamide(C8)/SiO 2 -P and Pd-loaded NTAamide(C8)/SiO 2 -P.NTAamide(C8) has nitrogen donor atoms at the center of the backbone.In the HSAB (Hard-Soft-Acid-Base) framework, NTAamide(C8) is classified as a soft base.Pd(II) can coordinate with the extractant through nitrogen donor atoms on the central frame and carbonyl groups.NO 3 − is required to participate in the reaction to maintain charge balance. NO