A New Impregnated Adsorbent for Noble Metal Ion Sorption

Noble metals (NM) such as gold, platinum, palladium, and rhodium are widely applied in the electronics and automotive industries. Thus, the search for cheap and selective sorbents for noble metals is economically justified. Nitrolite does not sorb noble metal ions. A new impregnated sorbent was prepared. The natural sorbent Nitrolite was impregnated with Aliquat 336 using a new warm impregnation method. After the impregnation process, Nitrolite adsorbed platinum(IV), palladium(II), and gold(III) ions from the chloride solutions. The values of the sorption capacity for palladium(II) and platinum(IV) ions were 47.63 mg/g and 51.39 mg/g, respectively, from the 0.1 M HCl model solution. The sorption capacity for gold(III) ions was estimated to be 73.43 mg/g from the 0.1 M HCl model solution. An exhausted catalytic converter was leached, and platinum(IV), palladium(II), and rhodium(III) were transferred to the chloride solution. The impregnated sorbent Nitrolite–Aliquat 336 was used in the investigations of the platinum(IV), palladium(II), and rhodium(III) ions’ sorption from a real solution. The impregnated sorbent Nitrolite–Aliquat 336 proved to be suitable for the recovery of platinum(IV) and palladium(II) ions, whereas rhodium ions were not sorbed from the leached solutions. Notably, 1 M thiourea in the 1 M HCl solution desorbed platinum(IV), palladium(II), and gold(III) above 94%.


Introduction
Noble metals such as platinum, palladium, rhodium, and gold are thermally and chemically resistant. Owing to their unique properties, they are used for the production of automotive exhaust gas converters [1]. Enantioselective domino reactions are promoted by the chiral catalysts derived from noble metals [2]. Precious metals are also a capital investment. During the ongoing war in Eastern Europe, the prices of precious metals are very high [3]. There are many ways to isolate precious metals through the use of a leaching solution. Platinum can be removed via the precipitation of ammonium hexachloroplatinate [4]. Similar precipitation has been used for platinum removal from a chloride solution [5]. Noble metals have a positive redox potential that can be reduced by negative redox potential metals. The cementation process can be used for the removal of noble metals from solutions. Gold thiocyanate complexes were removed from a thiocyanate solution via cementation with iron powder [6]. Gold can be reduced by zinc or organic acids such as oxalic acid. Aside from precipitation, solvent extraction is used to separate precious metals. Cyanex 272 (bis (2,4,4-trimethylpentyl)phosphinic acid) was used for the separation of gold(III) ions from platinum(IV), palladium(II), rhodium(III), and iridium(III) in hydrochloric acid solutions (0.5-9 M) [7]. Palladium(II) ions were separated from platinum(II), rhodium(III), and iridium(III) in a concentrated 6M hydrochloric acid solution using LIX63 (5,8-diethyl-7-hydroxydodecane-6-oxime) [8]. TBP (tri-n-butylphosphate) and TOPO (tri-n-octylphosphine oxide) were tested for the separation of platinum(IV) from rhodium(III) ions in hydrochloric acid solutions. From an economical point of view, considering the separation process, this indicates that TBP is better than TOPO for the separation solvent impregnation of Purolite MN202 using Cyanex 272, 0.105 g of Cyanex272/g Purolite MN202 was found [16]. This comparison demonstrates the effectiveness of the new impregnation method. Aside from titration, FTIR-ATR spectra were recorded to confirm the presence of Aliquat 336 in Nitrolite.
The FTIR spectra of Aliquat 336, Nitrolite, and Nitrolite impregnated with Aliquat 336 are presented in Figure 1. All three spectra were compared with each other. The symmetric and asymmetric stretching vibrations of C-H occur at 2924 cm −1 and 2855 cm −1 , which is evidenced in Aliquat 336 spectrum [21]. The same bands were observed in the Nitrolite-Aliquat 336 FTIR spectrum. The band at 1624 cm −1 is due to the stretching vibrations of the hydroxyl groups from H2O contained in the Nitrolite-Aliquat 336 sorbent. In the spectra of pure Aliquat 336 and Aliquat 336-impregnated Nitrolite, a characteristic peak for quaternary ammonium salts was observed (1465 cm −1 ) [22]. Asymmetric stretching vibrations of SiO4 and AlO4, which were found in Nitrolite, corresponded to a narrow peak at 1015 cm −1 . The rocking vibrations of (-CH2-)n from Aliquat 336 were detected at 729 cm −1 . The band at 594 cm −1 is attributed to the pseudo-lattice vibrations of Nitrolite [23]. The stretching vibrations of O-H might be caused by the incidence of a wide band at 3396 cm −1 . The SEM images of Nitrolite are presented in Figure 2a,b. As can be seen, Nitrolite is a porous material. The SEM images of Nitrolite-Aliquat 336 are presented in Figure 2c,d. The surface of the impregnated sorbent is smooth, and it can be seen that pores are filled with Aliquat 336. The SEM results in Figure 2 confirm that Aliquat 336 was present on the Nitrolite surface, and the impregnation process was efficient. The SEM images of Nitrolite are presented in Figure 2a,b. As can be seen, Nitrolite is a porous material. The SEM images of Nitrolite-Aliquat 336 are presented in Figure 2c,d. The surface of the impregnated sorbent is smooth, and it can be seen that pores are filled with Aliquat 336. The SEM results in Figure 2 confirm that Aliquat 336 was present on the Nitrolite surface, and the impregnation process was efficient.
Molecules 2023, 28, x FOR PEER REVIEW 3 of 13 solvent impregnation of Purolite MN202 using Cyanex 272, 0.105 g of Cyanex272/g Purolite MN202 was found [16]. This comparison demonstrates the effectiveness of the new impregnation method. Aside from titration, FTIR-ATR spectra were recorded to confirm the presence of Aliquat 336 in Nitrolite. The FTIR spectra of Aliquat 336, Nitrolite, and Nitrolite impregnated with Aliquat 336 are presented in Figure 1. All three spectra were compared with each other. The symmetric and asymmetric stretching vibrations of C-H occur at 2924 cm −1 and 2855 cm −1 , which is evidenced in Aliquat 336 spectrum [21]. The same bands were observed in the Nitrolite-Aliquat 336 FTIR spectrum. The band at 1624 cm −1 is due to the stretching vibrations of the hydroxyl groups from H2O contained in the Nitrolite-Aliquat 336 sorbent. In the spectra of pure Aliquat 336 and Aliquat 336-impregnated Nitrolite, a characteristic peak for quaternary ammonium salts was observed (1465 cm −1 ) [22]. Asymmetric stretching vibrations of SiO4 and AlO4, which were found in Nitrolite, corresponded to a narrow peak at 1015 cm −1 . The rocking vibrations of (-CH2-)n from Aliquat 336 were detected at 729 cm −1 . The band at 594 cm −1 is attributed to the pseudo-lattice vibrations of Nitrolite [23]. The stretching vibrations of O-H might be caused by the incidence of a wide band at 3396 cm −1 . The SEM images of Nitrolite are presented in Figure 2a,b. As can be seen, Nitrolite is a porous material. The SEM images of Nitrolite-Aliquat 336 are presented in Figure 2c,d. The surface of the impregnated sorbent is smooth, and it can be seen that pores are filled with Aliquat 336. The SEM results in Figure 2 confirm that Aliquat 336 was present on the Nitrolite surface, and the impregnation process was efficient.

Kinetics of Noble Metal Ion Sorption on the Impregnated Sorbent Nitrolite-Aliquat 336
The kinetics of noble metal ion sorption on the impregnated sorbent Nitrolite-Aliquat 336 was investigated. Noble metal ions' concentrations were determined using the ICP-OES method. The influence of the removal percentage (R%) on the contact time was determined in the HCl concentration range of 0.1-6 M. The recovery percentage (R%) of gold(III) was calculated as follows: where C is the concentration of the adsorbed noble metal ions calculated from the difference in the solution concentration before and after the sorption process, and C0 is the initial concentration of noble metal ions. The influence of R% on the phase contact time is presented in Figure 3. The results in Figure 3 were obtained from gold(III), palladium(II), platinum(IV), and rhodium(III) mixtures. As can be seen, rhodium ions were not sorbed by the impregnated sorbent Nitrolite-Aliquat 336. This provides evidence of strong competition among noble metal ions in terms of functional groups present on the sorbent surface.

Kinetics of Noble Metal Ion Sorption on the Impregnated Sorbent Nitrolite-Aliquat 336
The kinetics of noble metal ion sorption on the impregnated sorbent Nitrolite-Aliquat 336 was investigated. Noble metal ions' concentrations were determined using the ICP-OES method. The influence of the removal percentage (R%) on the contact time was determined in the HCl concentration range of 0.1-6 M. The recovery percentage (R%) of gold(III) was calculated as follows: where C is the concentration of the adsorbed noble metal ions calculated from the difference in the solution concentration before and after the sorption process, and C 0 is the initial concentration of noble metal ions. The influence of R% on the phase contact time is presented in Figure 3. The results in Figure 3 were obtained from gold(III), palladium(II), platinum(IV), and rhodium(III) mixtures. As can be seen, rhodium ions were not sorbed by the im-   Four kinetic models, namely pseudo-first-order, pseudo-second-order, Elovich, and intra-particle models, were used to analyze gold(III), platinum(IV), and palladium(II) sorption on the impregnated sorbent Nitrolite-Aliquat 336. The linear form of these models can be described using the following equations: .
(Intra-particle diffusion) On the basis of the above equations, the values of kinetic parameters were calculated. The kinetic parameters are presented in Table 1.
For the investigated noble metal ions, small values of the determination coefficient were obtained based on the intra-particle diffusion model. This indicates that the sorption of platinum(IV), palladium(II), and gold(III) chloride complexes is not limited by intra-particle diffusion. The values of coefficients of determination for the Elovich model are higher than for PFO. The best values of coefficients of determination were obtained for the pseudo-second model. This model best described the kinetics of platinum(IV), palladium(II), and gold(III) ions' sorption on the impregnated sorbent Nitrolite-Aliquat 336. A similar PSO model correlated the data well during Pt(IV) ions' sorption on SIRs (XAD7-DB30C10) [25]. Four kinetic models, namely pseudo-first-order, pseudo-second-order, Elovich, and intra-particle models, were used to analyze gold(III), platinum(IV), and palladium(II) sorption on the impregnated sorbent Nitrolite-Aliquat 336. The linear form of these models can be described using the following equations: On the basis of the above equations, the values of kinetic parameters were calculated. The kinetic parameters are presented in Table 1.
For the investigated noble metal ions, small values of the determination coefficient were obtained based on the intra-particle diffusion model. This indicates that the sorption of platinum(IV), palladium(II), and gold(III) chloride complexes is not limited by intra-particle diffusion. The values of coefficients of determination for the Elovich model are higher than for PFO. The best values of coefficients of determination were obtained for the pseudosecond model. This model best described the kinetics of platinum(IV), palladium(II), and gold(III) ions' sorption on the impregnated sorbent Nitrolite-Aliquat 336. A similar PSO model correlated the data well during Pt(IV) ions' sorption on SIRs (XAD7-DB30C10) [25].

Isotherms of Noble Metal Ion Sorption on the Impregnated Sorbent Nitrolite-Aliquat 336
It is very important to determine the sorption capacity of platinum(IV), palladium(II), and gold(III) ions on the impregnated sorbent Nitrolite-Aliquat 336. Sorption capacity allowed for a comparison of the sorption properties of Nitrolite-Aliquat 336 with those of other sorbents. In addition, differences were observed in the sorption capacity for different ions. The isotherms of Au(III), Pd(II), and Pt(IV) ions' sorption in 0.1 M HCl on the impregnated sorbent Nitrolite-Aliquat 336 in 0.1 M HCl are presented in Figure 4. The isotherms of platinum(IV), palladium(II), and gold(III) sorption were determined for single ions. As shown in Figure 4, the maximum sorption capacity was obtained for gold(III) ions. The sorption capacity for gold(III) ions was estimated to be 73.43 mg/g. The sorption capacities for palladium(II) and platinum(IV) ions were lower than for gold(III) ions. The values of sorption capacity for palladium(II) and platinum(IV) ions were 47.63 mg/g and 51.39 mg/g, respectively. These high values of sorption capacity were obtained because the sorbent Nitrolite did not sorb platinum(IV), palladium(II), and gold(III) ions before impregnation. Two Langmuir and Freundlich isotherm models were used for the determination of constants for Au(III), Pd(II), and Pt(IV). The parameters were calculated from the following equations: where qe is the concentration of noble metal ions in the sorbent (mg/g); Ce is the equilibrium concentration (mg/L); b is the Langmuir isotherm constant (L/mg); and Q0 is the maximum monolayer coverage capacity (mg/g). (Freundlich) where qe is the concentration of noble metal ions in the sorbent (mg/g); KF is the characteristic constant related to the adsorption capacity (L/g); n is the adsorption intensity; and Ce is the equilibrium concentration (mg/L). The calculated parameters of the isotherm models for the Au(III), Pd(II), and Pt(IV) sorption on the impregnated Nitrolite-Aliquat 336 are presented in Table 2. The smallest values of coefficients of determination were obtained for the Freundlich model. This indicates that the sorption of Au(III), Pd(II), and Pt(IV) on the impregnated Nitrolite-Aliquat 336 does not comply with the Freundlich model.
The highest values of coefficients of determination were obtained for the Langmuir model. This isotherm model best described platinum(IV), palladium(II), and gold(III) ions' sorption on the impregnated Nitrolite-Aliquat 336. The Langmuir model assumes the formation of a monolayer and allows for the determination of sorption capacity. The sorption capacity Q0 calculated from the Langmuir model is close to the value found in Figure 4. For example, the capacity calculated for gold(III) ions was 73.49 mg/g, while the determined value was 73.43 mg/g. The sorption capacity for Pt(IV) (51.59 mg/g) was higher than for SIR (XAD7-DB30C10), which was 15.03 mg/g as reported in the literature [25].
Mesoporous silica impregnated using (3-(3-(methoxycarbonyl)benzylidene)hydrazinyl)benzoic acid (MBHB) ligand exhibited a larger sorption capacity for palladium(II) ions (184.5 mg/g) than Nitrolite-Aliquat 336 [26]. Moreover, a high sorption capacity of palladium(II) ions (213.67 mg/g) was obtained Two Langmuir and Freundlich isotherm models were used for the determination of constants for Au(III), Pd(II), and Pt(IV). The parameters were calculated from the following equations: where q e is the concentration of noble metal ions in the sorbent (mg/g); C e is the equilibrium concentration (mg/L); b is the Langmuir isotherm constant (L/mg); and Q 0 is the maximum monolayer coverage capacity (mg/g).
where q e is the concentration of noble metal ions in the sorbent (mg/g); K F is the characteristic constant related to the adsorption capacity (L/g); n is the adsorption intensity; and C e is the equilibrium concentration (mg/L). The calculated parameters of the isotherm models for the Au(III), Pd(II), and Pt(IV) sorption on the impregnated Nitrolite-Aliquat 336 are presented in Table 2. The smallest values of coefficients of determination were obtained for the Freundlich model. This indicates that the sorption of Au(III), Pd(II), and Pt(IV) on the impregnated Nitrolite-Aliquat 336 does not comply with the Freundlich model.
The highest values of coefficients of determination were obtained for the Langmuir model. This isotherm model best described platinum(IV), palladium(II), and gold(III) ions' sorption on the impregnated Nitrolite-Aliquat 336. The Langmuir model assumes the formation of a monolayer and allows for the determination of sorption capacity. The sorption capacity Q 0 calculated from the Langmuir model is close to the value found in Figure 4. For example, the capacity calculated for gold(III) ions was 73.49 mg/g, while the determined value was 73.43 mg/g. The sorption capacity for Pt(IV) (51.59 mg/g) was higher than for SIR (XAD7-DB30C10), which was 15.03 mg/g as reported in the literature [25]. Mesoporous silica impregnated using (3-(3-(methoxycarbonyl)benzylidene)hydrazinyl)benzoic acid (MBHB) ligand exhibited a larger sorption capacity for palladium(II) ions (184.5 mg/g) than Nitrolite-Aliquat 336 [26]. Moreover, a high sorption capacity of palladium(II) ions (213.67 mg/g) was obtained for a nanoconjugate adsorbent (NCA) [27]. The conjugate adsorbent achieved a high sorption capacity for gold(III) ions, 183.42 mg/g [28]. The maximal capacity of an amine-based functionalized mesoporous silica adsorbent was 94.92 mg Pd/g in a nitrate(V) solution [29]. Palladium ions were effectively sorbed by silica-based IsoBu-BTP/SiO 2 -P adsorbent, obtaining a sorption capacity of 0.71 mmol/g [30]. As can be seen, the impregnated Nitrolite-Aliquat 336 acquired smaller values of sorption capacity than the mentioned sorbents, but its advantage is being a simple method of preparation.

Mechanism of Noble Metal Ion Sorption
The impregnation process allows for the incorporation of quaternary ammonium groups from Aliquat 336 into the Nitrolite sorbent. The sorption process occurs according to the ion exchange mechanism. Ion exchange can be presented using the following equations: The ion exchange mechanism is consistent with the kinetics results. An increase in the acid concentration reduced the sorption capacity, and the reaction equilibrium shifted to the right.

Desorption
Desorption is very important as a sorbent can be used several times. Our research results reveal that the anionic complexes [AuCl 4 ] − , [PdCl 4 ] 2− , and [PtCl 6 ] 2− are sorbed by positive functional groups such as quaternary ammonium. Nitrogen has a constant positive charge, and in order to desorb noble metal ions, its charge must be changed to positive. Thiourea allows for the exchange of chloride ligands for thiourea ligands. Therefore, it was used as the desorption solution: 1 M thiourea in 1 M HCl.
As can be seen in Figure 5 noble metal ions were desorbed effectively. Palladium(II) ions were desorbed the best, D%-97.3%. Platinum(IV) (95,4%) and gold(III) (94.1%) ions were equally well desorbed. Similar to our investigations, thiourea in HCl solution was used for the desorption of platinum(IV) and palladium(II) from di-hexylamine solventimpregnated resin DHA-SIR and di-hexyl sulfide solvent-impregnated resin DHS-SIR, respectively [31]. for a nanoconjugate adsorbent (NCA) [27]. The conjugate adsorbent achieved a high sorption capacity for gold(III) ions, 183.42 mg/g [28]. The maximal capacity of an amine-based functionalized mesoporous silica adsorbent was 94.92 mg Pd/g in a nitrate(V) solution [29]. Palladium ions were effectively sorbed by silica-based Iso-Bu-BTP/SiO2-P adsorbent, obtaining a sorption capacity of 0.71 mmol/g [30]. As can be seen, the impregnated Nitrolite-Aliquat 336 acquired smaller values of sorption capacity than the mentioned sorbents, but its advantage is being a simple method of preparation.

Mechanism of Noble Metal Ion Sorption
The impregnation process allows for the incorporation of quaternary ammonium groups from Aliquat 336 into the Nitrolite sorbent. The sorption process occurs according to the ion exchange mechanism. Ion exchange can be presented using the following equations: The ion exchange mechanism is consistent with the kinetics results. An increase in the acid concentration reduced the sorption capacity, and the reaction equilibrium shifted to the right.

Desorption
Desorption is very important as a sorbent can be used several times. Our research results reveal that the anionic complexes [AuCl4] − , [PdCl4] 2− , and [PtCl6] 2− are sorbed by positive functional groups such as quaternary ammonium. Nitrogen has a constant positive charge, and in order to desorb noble metal ions, its charge must be changed to positive. Thiourea allows for the exchange of chloride ligands for thiourea ligands. Therefore, it was used as the desorption solution: 1 M thiourea in 1 M HCl.
As can be seen in Figure 5 noble metal ions were desorbed effectively. Palladium(II) ions were desorbed the best, D%-97.3%. Platinum(IV) (95,4%) and gold(III) (94.1%) ions were equally well desorbed. Similar to our investigations, thiourea in HCl solution was used for the desorption of platinum(IV) and palladium(II) from di-hexylamine solvent-impregnated resin DHA-SIR and di-hexyl sulfide solvent-impregnated resin DHS-SIR, respectively [31].

Recovery of Noble Metals from the Exhausted Catalytic Converter
Exhausted catalytic converters are a valuable source of precious metals [32]. After leaching, a solution contains three platinum metals: platinum(IV), palladium(II), and

Recovery of Noble Metals from the Exhausted Catalytic Converter
Exhausted catalytic converters are a valuable source of precious metals [32]. After leaching, a solution contains three platinum metals: platinum(IV), palladium(II), and Molecules 2023, 28, 6040 9 of 13 rhodium(III). The concentrations of these ions were 26.45 mg/L for Pt(IV), 38.85 mg/L for Pd(II), and 7.25 mg/L for Rh(III). As sorbent tests proved, it removes Pt(VI) and Pd(II) ions from the mixture, while Rh(III) ions are not sorbed.
As shown in Figure 6, the impregnated Nitrolite-Aliquat 336 is selective and removes ions of Pt(IV) (97.94%) and Pd(II) (94.3%) from the mixture after leaching. The impregnated sorbent is not suitable for the sorption of rhodium(III) ions. Our future research will focus on obtaining a selective sorbent for rhodium(III) ions.
Molecules 2023, 28, x FOR PEER REVIEW 9 of 13 rhodium(III). The concentrations of these ions were 26.45 mg/L for Pt(IV), 38.85 mg/L for Pd(II), and 7.25 mg/L for Rh(III). As sorbent tests proved, it removes Pt(VI) and Pd(II) ions from the mixture, while Rh(III) ions are not sorbed. As shown in Figure 6, the impregnated Nitrolite-Aliquat 336 is selective and removes ions of Pt(IV) (97.94%) and Pd(II) (94.3%) from the mixture after leaching. The impregnated sorbent is not suitable for the sorption of rhodium(III) ions. Our future research will focus on obtaining a selective sorbent for rhodium(III) ions.

Materials
The natural sorbent Nitrolite is an aluminosilicate with the following formula: (K,Na,1/2Ca)2‧Al2O3‧10SiO2‧H2O. The total capacity of Nitrolite is 0.7 eq/L (NH4 + ions), and the bed size is in the range of 0.63-1.4 mm. Nitrolite was washed prior to experiments in order to remove surface dust.

Materials
The natural sorbent Nitrolite is an aluminosilicate with the following formula: (K,Na,1/2Ca) 2 ·Al 2 O 3 ·10SiO 2 ·H 2 O. The total capacity of Nitrolite is 0.7 eq/L (NH 4 + ions), and the bed size is in the range of 0.63-1.4 mm. Nitrolite was washed prior to experiments in order to remove surface dust.
Commercial-grade extractant Aliquat 336 was used for the impregnation of Nitrolite. The chemical structure of Aliquat 336 is shown in Figure 7. Aliquat 336 is methyltrioctylammonium chloride, F.W. 404.17 (d = 0.884 g/mL), and it was supplied by Aldrich, Steinheim, Germany. As shown in Figure 6, the impregnated Nitrolite-Aliquat 336 is selective and removes ions of Pt(IV) (97.94%) and Pd(II) (94.3%) from the mixture after leaching. The impregnated sorbent is not suitable for the sorption of rhodium(III) ions. Our future research will focus on obtaining a selective sorbent for rhodium(III) ions.

Materials
The natural sorbent Nitrolite is an aluminosilicate with the following formula: (K,Na,1/2Ca)2‧Al2O3‧10SiO2‧H2O. The total capacity of Nitrolite is 0.7 eq/L (NH4 + ions), and the bed size is in the range of 0.63-1.4 mm. Nitrolite was washed prior to experiments in order to remove surface dust.

Instrumentation
The concentration of noble metal ions was determined via ICP-OES (inductively coupled plasma-optical emission spectrometry) method using a Varian 720ES spectrometer. The values of ICP-OES operating parameters are presented in Table 3. The impregnated sorbent samples were shaken using a laboratory shaker Elpin+ type 358, Lubawa, Poland, at an amplitude of 8 and a speed of 160 c.p.m.
An FTIR-ATR (Fourier transform infrared spectroscopy-attenuated total reflectance) spectrometer Agilent Technologies Cary 630 was used for sample investigations.
A conductivity value was measured using an electrode (type PS-2Z) connected to the Elmetron CP-401 multifunction meter.
Scanning electron microscopy (SEM) measurements were conducted by means of a Quanta 3D FEG scanning electron microscope produced by FEI Company (Oregon, OH, USA).

Nitrolite Impregnation Procedure
Briefly, 1 g of Aliquat 336 was heated to 333 K. A 2 g sample of Nitrolite was mixed with 1 g of hot Aliquat 336 (ratio of Aliquat 336 g:Nitrolite g-1:2) and stirred for 2 h. The mixture of the impregnated sorbent and solution was washed with distilled water to remove the impregnation solution and then dried at 298 K. The sorbent prepared in this way was used in the research.

Determination of Aliquat 336 Concentration in Nitrolite
Aliquat 336 concentration in Nitrolite was determined via the titration of 0.1957 g Aliquat 336 in an acetone-water medium (12.5 mL acetone + 12.5 mL water) using a 0.1 M AgNO 3 solution. During titration, the conductivity of the solution was measured. A 0.4069 g sample of Nitrolite impregnated with Aliquat 336 in the acetone-water medium (12.5 mL acetone + 12.5 mL water) was titrated with a 0.1 M AgNO 3 solution. The titration endpoint was determined from a graph plotting conductivity vs. titrant volume.

Catalytic Converter Digestion and Sorption Platinum Metals
An exhausted catalytic converter was digested in hydrochloric acid and peroxide solution. To this end, 20 g of the catalytic converter was crushed and milled. The milled catalytic converter was contacted with 100 mL of water and 100 mL of 36% HCl. The catalyst was mixed with the solution in a flask, and hydrogen peroxide (100 mL 30% H 2 O 2 ) was added in portions. The contents of the flask were maintained at 333 K for 2 h. After cooling, the solution was filtered off from the catalyst residue. It was analyzed via ICP-OES to determine concentrations of precious metals. Before sorption, the solution was diluted 10 times.

Desorption Studies
For desorption studies, 0.1 g of sorbent was loaded with gold(III), palladium(II), and platinum(IV) using 10 mL of the 50 mg/L solution in 0.1 M HCl, and the agitation period of 24 h was applied. In addition, 1 M thiourea in 1 M HCl solution was used as a desorption agent. The solution was analyzed using ICP-OES to determine the concentration of precious metals.

Conclusions
This research involves an investigation of the possibility of preparing a cheap impregnated sorbent Nitrolite-Aliquat 336 for the sorption of noble metal ions such as gold(III), palladium(II), and platinum(IV). The proposed warm impregnation procedure makes it possible to obtain an impregnated sorbent without the use of organic solvents. This method of impregnation enabled the preparation of an impregnated sorbent with a large content of the impregnating agent, i.e., 0.25 g of Aliquat 336/gNitrolite. In the 0.1 M HCl solution, platinum(IV), gold(III), and palladium(II) ions were sorbed effectively, and the values of R% were 99.2% for Au(III), 99.3% for Pd(II), and 97.5% for Pt(IV). An increase in the HCl concentration caused a decrease in the value of R%. The maximum sorption capacity was estimated to be 73.43 mg/g for the sorption of gold(III) ions, 51.39 mg/g for platinum(IV) ions, and 47.63 mg/g for palladium(II) ions from the 0.1 M HCl model solution. The Langmuir isotherm and the PSO kinetic models described the sorption of noble metals well. The prepared impregnated sorbent enabled the selective recovery of palladium(II) and platinum(IV) ions from the solution after leaching of the spent catalytic converter. In the experiment using the real leaching solution, the values of R% were high for Pt(IV) (97.94%) and Pd(II) (94.3%). This sorbent is not suitable for rhodium(III) ions' sorption. The test results confirmed that Au(III), Pt(IV), and Pd(II) ions can be desorbed from impregnated Nitrolite-Aliquat 336 using 1 M thiourea in the 1 M HCl solution. The values of D% were 97.3%for Pd(II), 95.4% for Pt(IV), and 94.1% for Au(III).
Author Contributions: Methodology, investigation, writing-review and editing, supervision, G.W.; conceptualization, investigation, data curation, writing-original draft preparation, supervision, K.Z.; conceptualization, methodology, writing-review and editing, supervision, Z.H. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: Not applicable.