INTRODUCTION

Sorption is currently one of the most promising and popular methods for isolating and separating ions of palladium and other platinum metals from chloride solutions. Unfortunately, it is not possible to take into account and foresee all the factors affecting the sorption of various metal ions (solution composition, salt background, sorbent pore size, etc.) in advance. Therefore, there is a need to conduct numerous experiments, which, in turn, generates a stream of studies reported, the number of which has increased immeasurably in the last 5–10 years, especially in the field of analytical chemistry in order to prepare samples of complex composition, such as ores, rocks, and refining products. This circumstance is largely facilitated by the undeniable advantages of sorption over other methods, in particular, the high rate of reaching equilibrium and selectivity [1–4]. Among the works on the sorption of noble metal ions, the vast majority refers to the sorption of Pd(II) ions from hydrochloric acid or aqueous chloride solutions. Nitrogen-containing and sulfur-containing sorbents of various types prevail as sorbents providing a relatively selective extraction of this ion [5–8]. It should be emphasized that, as a rule, synthetic organic polymers serve as the sorbent matrix, although there are also natural polymeric sorbents, for example, chitosan, which is a linear polysaccharide containing amino groups. The presence of functional hydroxyl and amino groups provides ample opportunities for modifying chitosan and obtaining sorbents with the required mechanical and structural characteristics [9, 10]. In [11], the results of sorption of platinum metal ions on biomasses derived from algae modified with various polyethyleneimines are discussed. It was indicated that desorption of Pd(II) is possible with thiourea solutions. No fundamental differences from sorption on analogous sorbents based on synthetic polymers were found [9–11]. The possibilities of polymer thioethers in selective sorption preconcentration for analytical purposes, as well as the acceleration of sorption due to the introduction of nitrogen-containing groups into thioethers, are considered [12]. The dependence of the selectivity of these sorbents on the nature of the substituent at the nitrogen atom was revealed. Polymeric fibrous porous sorbents based on MOFs organometallic frameworks functionalized with quaternary ammonium bases are effective for extracting palladium(II) ions from strongly acidic and/or cyanide solutions [13, 14]. Note that in many of the works analyzed by us, the emphasis is shifted towards the synthesis of various polymers and copolymers, including organosilicon ones, which have a sorption capacity for non-ferrous, heavy, and noble metal ions, predominantly for Pd(II) ions [8, 15, 16]. However, the availability of these sorbents, their selectivity, and the possibility of implementing desorption processes are not discussed.

Among sorbents for separating palladium(II) ions from chloride solutions, a special place is occupied by chemically modified silicas, which, despite their relatively low sorption capacity compared to organopolymer sorbents, favorably differ from them by a high rate of establishment of sorption–desorption equilibrium. Back in the 1980s, silica with grafted monoamine groups SiO2–O–Si(CH2)3NH2, obtained using the commercially available modifier γ-aminopropyltriethoxysilane (C2H5O)3Si(CH2)3NH2, was successfully used to extract platinum metal ions, first of all for Pd(II) [1]. However, silica modified with sulfur-containing molecules is of much more interest, especially in recent years. Silica gel coated with graphene oxide and modified with mercapto groups SiO2@GO–SH is successfully used in the processing of secondary raw materials. During the chemisorption of palladium(II) ions, they are reduced to metal [17].

Silicas modified with mercaptopropyl, mercaptophenyl, dipropyl disulfide, thiadiazolthiol, dithiocarbamate, and thiourea derivatives have been synthesized, which quantitatively extract Pd(II) ions from 0.5–4 M hydrochloric acid solutions [18]. In this case, they are separated from ions of non-ferrous, alkaline earth and alkali metals, as well as from other platinum metals. It was shown [18] that the selectivity of the target ion extraction increases upon passing from thionic to thiol sulfur. It has been established that the predominant mechanism of sorption of palladium(II) is complexation with functional groups of platinum(IV) sorbents, i.e., ion metathesis [11, 19, 20]. An increase in the degree of sulfoethylation of the aminopolymer matrix promotes a decrease in the sorption of platinum(IV) and, as a consequence, an increase in the selectivity of sorption of palladium(II) with respect to this ion. This effect is most significant for sorbents based on polyallylamine [20]. The conclusion about the mechanism of sorption was made in the works listed above without involving any research methods and, rather, out of habit, without expecting a different result.

In this work, data on the sorption of Pd(II) from hydrochloric acid chloride solutions by selective silicas with grafted sulfur-containing molecules are summarized, and its purpose is to reveal the features of palladium(II) complexation during sorption.

EXPERIMENTAL

The reagents were PdCl2 synthesized according to the procedure reported [21]; HCl, NaCl, NH3·H2O (all chemically pure grades, Russian State Standard) (Khimmed); 3-glycidyloxypropyltrimethoxysilane, diisopropylethylamine, mercaptoethanol, mercaptoacetic, and thiosalicylic acids (Sigma Aldrich). The sorbent was synthesized using silica gel Davisil Grade 62 (W.R. Grace&Co., USA), fraction 0.07–0.2 mm (60–200 mesh), specific surface 327 m2/g, pore size 13.7 nm. Organic solvents (toluene, ethanol, and acetonitrile, TD Khimmed) were used without preliminary distillation.

Synthesis of sulfur-containing sorbents. A solution of equimolar amounts of an organosulfur compound (mercaptoethanol, mercaptoacetic acid, thiosalicylic acid), 3-glycidyloxypropyltrimethoxysilane and diisopropylethylamine in 100 mL of toluene was stirred at room temperature for 2 h. Then 20 g of silica was added to the mixture and the resulting suspension was stirred with a mechanical stirrer while heating in a boiling water bath for 8 h. Next, toluene was slowly removed in a boiling water bath in an air flow until the sorbent was free flowing. The sorbent was transferred to the filter, washed with ethanol until the characteristic smell of the organic sulfur compound disappeared, and dried on the filter. The synthesis of a sorbent with grafted mercaptoethanol was also described [22], and the synthesis of silica modified with thiosalicylic acid was patented [23]. The characteristics of the synthesized sorbents are given in Table 1.

Table 1. Characteristics of the synthesized sorbents
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Experimental technique. Sorption of Pd(II) was carried out under static conditions from hydrochloric acid solutions (0.1–6 M HCl; \(C_{{{\text{Cl}}}}^{-}\) = 10–100 g/L) at room temperature. The volume of solutions in each experiment was 10–15 mL, the weight of the sorbent was 30–50 mg, СPd = 5 × 10–3 mol/L. Pd(II) sorption isotherms were plotted for solutions with initial metal concentrations from 1 × 10–4 to 1 × 10–2 mol/L in solution, sorbent weight 50 ± 0.3 mg, solution volume 10 mL. The solutions after sorption were separated from the sorbent by filtration and centrifugation (using a medical laboratory centrifuge TsLn-16 with a rotor, Russia) and the content of palladium in the solution was determined. The amount of sorbed metal was found from the difference between its content in the solution before and after sorption. Samples for investigation by spectral methods (ESP, IR, and XPS) were prepared by treating sorbents with palladium(II) solutions in 1 M HCl, metal concentration in the solution \(C_{{{\text{Pd}}}}^{{{\text{initial}}}}\) = 1 × 10–2 mol/L, which, according to sorption isotherms, corresponds to the maximum saturation of sorbents with ions metal.

Methods. The structural characteristics of the sorbents were determined by low-temperature nitrogen adsorption on an ASAP 2020 automatic analyzer (Micromeritics, USA). The elemental analysis of the sorbents for the content of carbon, hydrogen, and nitrogen was carried out on a CHN-2400 Perkin Elmer analyzer. The content of metal ions in the solution was determined by inductively coupled plasma mass spectrometry on an Agilent 7900 mass spectrometer (Agilent Technologies, USA). The analysis results were processed using the Agilent Mass Hunter software (Agilent Technologies, USA). The IR spectra of the samples were obtained on a Thermo Scientific Nicolet IR200 IR Fourier spectrometer by the method of attenuated total reflection on an attachment with a ZnSe crystal. The spectra were recorded in the range 4000–650 cm–1 (transparency region of crystalline ZnSe). Electronic absorption spectra of solutions were recorded on a Specord UV-Visible Heλios α spectrophotometer (USA) in the wavelength range 200–800 nm in quartz cuvettes with an absorbing layer thickness of 1 cm or on a quartz substrate, in the case of sorbents saturated with metal ions prepared in the form of suspensions in Vaseline oil. Diffuse reflectance spectra of solid samples were recorded on a Specord M-40 spectrometer. The diffuse reflectance spectra were reduced to the form corresponding to the absorption spectra using the Gurevich–Kubelka–Munk function in the coordinates F(R) = (1 – R2)/(2R) [24]. The composition of the sorbent surface was studied by X-ray photoelectron spectroscopy on a Theta Probe spectrometer from Thermo Fisher Scientific (Great Britain) using monochromatic AlKα radiation. Photoelectron spectra were recorded in the mode of constant absolute energy resolution with a step of 0.1 eV. The measurements were performed at a pressure in the analysis chamber of ~5 × 10–7 Pa. The analyzed spectra were approximated by a Gaussian profile or their sum, and the background due to secondary electrons and photoelectrons that experienced energy losses was approximated by a straight line. The energy scale of the spectrometer was calibrated according to the standard procedure using the following binding energies: Cu 2p3/2, 932.7 eV; Ag 3d5/2, 368.3 eV; Au 4f7/2, 84.0 eV. Quantitative analysis was carried out on the basis of elemental sensitivity coefficients, the values of which are included in the software for the device. Surface charging was taken into account by the shape of the peak in the C1s spectrum (284.8 eV). The charge was compensated using a low energy electron gun. The electron energy was ~0.5 eV. The electronic and geometric structures of a number of sulfur-containing sorbents and their complexes with platinum group metals were studied with the DFT M06 quantum-chemical method [25], which is focused on calculating the enthalpies of reactions involving transition metals, using the def2szvp and def2tzvp [26] basis sets with the GAUSSIAN’16 software package [27]. Note that in the software package [27] they are calculated using the following relations:

$$\Delta {{G}^{0}} = \Delta {{H}^{0}}-T\Delta {{S}^{0}}\left( {T = 298{\text{ K}}} \right),$$
(1)
$$\Delta {{H}^{0}} = \Delta {{E}_{{{\text{tot}}}}} + \Delta ZPE + \Delta H',$$
(2)

where Etot are total energies of systems; ZPE is the zero-point vibration energies;

$$\Delta H' = {{E}_{{{\text{vibr}}}}} + {{E}_{{{\text{rot}}}}} + {{E}_{{{\text{trans}}}}} + RT.$$
(3)

The vibrational contribution to the thermodynamic functions (including ZPE) is obtained from calculations of the normal vibration frequencies of the compounds in the harmonic approximation. The solvation energies of the compounds were calculated within the polarizable continuum model [28, 29]. Visualization of molecular structures and normal vibrations was provided by the ChemСraft graphic program [30].

RESULTS AND DISCUSSION

We chose three starting organosulfur compounds, namely mercaptoethanol (ME) HSCH2CH2OH, mercaptoacetic acid (MAA) HSCH2COOH, also known as thioglycolic acid, and thiosalicylic acid (TSA) o-HSC6H4COOH, which is a representative of the class of sulfur-containing reagents, thiophenol derivatives with electron-withdrawing substituents in the aromatic nucleus of the general formula HSC6H4X. Mercaptoethanol is an accessible and cheap reagent, the interest in which is due to the fact that when it is grafted to the surface of silica according to the method described above, structures similar to thiacrown ethers are formed on the surface [22]. Mercaptoacetic acid is an affordable and cheap product that is produced industrially by the reaction of chloroacetic acid with sodium hydrosulfide. It differs from mercaptoethanol in the presence of an electron-withdrawing carboxyl group, which should increase the selectivity of binding of palladium(II) ions.

For the synthesis of sulfur-containing sorbents, the immobilization method with preliminary synthesis of a sulfur-containing organosilicon modifier was used, similar to that used in some patents reported by R. Izatt [31, 32], the scheme of which is given below:

At the first stage of the research, the dependences of the sorption of Pd(II) ions on the synthesized chemically modified silicas on time, the concentration of acid, chloride ion, and proton were obtained, and sorption isotherms were plotted. It was established that the time to reach constant values of sorption (sorption equilibrium) does not exceed 2 min, which is typical for sorbents based on modified mesoporous silicas. The sorption time is practically independent of the nature of the functional group. In all further experiments, the phase contact time was 5 min.

Figure 1 shows the dependences of the sorption of Pd(II) ions on the synthesized sorbents, on the basis of which it can be concluded that an increase in the concentration of acid in solutions leads to a decrease in the sorption of palladium(II) ions for all sorbents, regardless of the nature of the grafted group. The data of Fig. 1 reflect the overall effect of a simultaneous increase in the concentration of H+ and Cl– ions. If, however, we separate the effects and consider the dependence of sorption on the concentration of chloride ions at a constant acidity of the solution and the dependence of sorption on the concentration of H+ at a constant salt background, then it turns out that it depends only on the proton concentration, and the influence of the salt background is insignificant. Sorption of palladium(II) on ME and TSA sorbents is almost independent of changes in the concentration of H+ ions in the range of 0.01–2 mol/L and the concentration of Cl– ions in the range of 10–100 g/L. This means that the degree of extraction of palladium on these sorbents is resistant to possible changes in the composition of the solution, which is of great practical importance.

Fig. 1.
figure 1

Dependence of Pd(II) sorption on sulfur-containing sorbents on HCl concentration in solutions: СPd = 5 × 10–3 mol/L; msorb = 0.05 g; Vsol = 10 mL; phase contact time 5 min.

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Figure 2 shows the sorption isotherms of the Pd(II) chloride complex on sulfur-containing chemically modified silicas. Obviously, in the case of the ME sorbent, the value of the sorption capacity for palladium (0.61 mmol/g) is approximately two times lower than the amount of grafted ligands (1.16 mmol/g). Therefore, one palladium(II) ion reacts with two grafted sulfur-containing ligands. A similar result was also obtained for the MAA sorbent, although its sorption capacity was approximately three times lower (Table 1).

Fig. 2.
figure 2

Sorption isotherms of Pd(II) ions on sulfur-containing sorbents.

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Of course, one cannot draw a conclusion about the mechanism of the formation of complexes in the sorbent phase only on the basis of the stoichiometry of the interaction of the palladium(II) ion with the grafted groups of sorbents. Therefore, to elucidate the question of the mechanism of complex formation, we used spectral methods of investigation, including infrared (IR), electron (UV-Vis), and X-ray photoelectron (XPS) spectroscopies.

The IR spectra of all samples contain intense bands belonging to the matrix, namely 1106 cm–1 (asymmetric Si–O–Si stretching vibrations), 802 cm–1 (symmetric Si–O–Si stretching vibrations), and 959 cm–1 (stretching vibrations of silanol groups Si–O–H and Si–O–). The modification of the SiO2 surface leads to the appearance of new bands in the spectra of the samples, indicating a change in the chemical composition of the surface. However, their low intensity is most likely due to the fact that the content of the grafted modifier is relatively low compared to the content of SiO2. The intense absorption of the silanol groups of silica practically screens the absorption due to vibrations of the C–S–C bonds, which is characterized by a weak intensity. The most informative were the IR spectra of the MAA sorbent before and after the sorption of the Pd(II) ion (Table 2).

Table 2. Assignment of bands in the IR spectra of the MAA-D sorbent before and after the sorption of palladium(II)
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The binding of the modifier to the SiO2 surface can be judged on the basis of the presence of stretching vibrations of the C–H, C=O, and COO– bonds in the IR spectra of the studied samples. The shift of the band of asymmetric stretching vibrations νas(COO–) in the sample after sorption indicates the formation of a bond between the carboxyl group and the metal ion, and the difference between the frequencies of asymmetric and symmetric stretching vibrations of the coordinated carboxyl group Δ = νas(COO–) – νs(COO–) = 225 cm–1 corresponds to the monodentate bonding of the carboxyl group with the metal ion [33]. With this character of complex formation, a noticeable increase in the intensity of the ν(C=O) vibration band at 1726 cm–1 becomes understandable. The possible involvement of the carboxyl group of the grafted mercaptoacetic acid (along with the sulfur atom) in the binding of palladium explains the sharper decrease in sorption on the MAA sorbent with an increase in the acidity of the solution compared to the ME sorbent.

Let us consider the electronic absorption spectra of sorbents saturated with palladium(II) ions and colored orange in different shades (Fig. 3). Thus, in the spectrum of the ME + Pd(II) sorbent phase, absorption bands are observed at 244 (D = 2.5), 303 (D = 2.4), 353 (D = 2.3), and 399 nm (D = 2.1) (for comparison: in the electronic absorption spectra of the initial sorbent, there are absorption bands at 208 (D = 1.8) and 224 nm (D = 1.7), a bend at 247 nm (D = 1.6), and there is no absorption starting from 290 nm. In the electronic absorption spectra of the MAA + Pd(II) sorbent phase, i.e. saturated with Pd(II) ions, absorption bands are observed at 206 (D = 2.4), 244 (D = 2.1), and 306 nm (D = 2.0), 327 sh (D = 1.9), 360–370 sh (D = 1.6–1.5), 430–440 sh nm (D = 1.2–1.1) (for comparison, only a weak absorption band at 204 nm (D = 0.972) can be distinguished in the electronic absorption spectrum of the initial sorbent. In the case of the TSA + Pd(II) phase, the spectrum contains bands at 209 (D = 2.3), 226 (D = 2.2), 260 nm (D = 1.9), 328 nm sh (D = 1.7–1.6) and 443 sh nm (D = 0.9) (for comparison: in the spectrum of the TSA sorbent, absorption bands are observed at 264 (D = 2.1) and 314 nm (D = 1.8), and there is no absorption starting from 370 nm). Thus, new absorption bands appear in the electronic absorption spectra of the sorbents after sorption compared to the spectra of the initial sorbents. In all three spectra of sorbents, after sorption, absorption bands at 279 and 473 nm, characteristic of the tetrachloropalladate(II) ion [34], are not observed, which is associated with the formation of palladium(II) complexes with functional groups of sorbents. The fact that the spectra obtained for the MAA + Pd(II) and TSA + Pd(II) samples are similar indicates that the structure of the adsorbed palladium(II) complexes is similar.

Fig. 3.
figure 3

Electronic absorption spectra of sorbent samples saturated with Pd(II) ions in Vaseline oil: ME (1); MAA (2); TSA (3).

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This conclusion is also confirmed by the diffuse reflectance spectra obtained for the same samples. As an example, Fig. 4 shows the diffuse reflectance spectra of the ME sorbent saturated with palladium(II) ions in the coordinates F(R) = (1 – R2)/(2R). The band at 425 nm present in this spectrum indicates a change in the ligand environment compared to the initial complex [PdCl4]2– and complexation with the functional group of the grafted sorbent molecule. Note also that the diffuse reflectance spectra for other sulfur-containing sorbents saturated with palladium(II) practically do not differ from those shown in Fig. 4.

Fig. 4.
figure 4

Diffuse reflectance spectrum of the ME sorbent sample saturated with Pd(II) ions.

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Even more information is provided by X-ray photoelectron spectra. They make it possible to judge not only the degree of oxidation of palladium but also the nature of its ligand environment. The binding energies for palladium generally correspond to its +2 oxidation state [35, 36], and the presence of two components indicates the presence of two forms of palladium complexes with different ligand environments (Fig. 5, Table 3) [37, 38].

Fig. 5.
figure 5

X-ray photoelectron spectra of Pd3d samples obtained by sorption of the [PdCl4]2– complex on (a) ME and (b) TSA sorbents.

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Table 3. Binding energies S2p, Pd3d for modified silicas saturated with palladium(II) ions
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The XPS data confirm the presence of sulfide sulfur in the samples, which is characterized by the S2p3/2–S2p1/2 spin–orbit doublet with a binding energy of 164.0–165.5 eV. For the Pd + ME sample, the contents of palladium, chlorine, and sulfur are 0.64, 0.96, and 0.48 at % respectively. We believe that this indicates the palladium(II) complexation with the sulfur atoms of the sorbent with the displacement of two chloride ions on average from the inner coordination sphere of the central atom. In the Pd + TSA sample, the XPS method did not detect the presence of chlorine at all. It is possible that a chelate complex is formed on the surface with the participation of the sulfur atom and the oxygen atom of the carboxyl group of the grafted thiosalicylic acid.

It seemed appropriate to substantiate the above assumptions about the nature of the binding of the sorbed Pd(II) ion to the sulfur-containing molecule grafted to the silica surface by quantum-chemical calculations of the electronic and geometric structures of the studied ligands and complexes with these ligands, as well as the corresponding thermodynamic characteristics of complexation reactions. If we turn to the formulas of sulfur-containing compounds L, which were used for the synthesis of sorbents (Table 1), it becomes obvious that they can be represented in general form as R’SR, where R' is a group directly attached to the SiO2 surface, and R is a changing radical at the sulfur atom (L1, L2, L3). The values of negative charges on the donor atom S obtained using the atomic basis set def2tzvp, which can be used to easily estimate the donor ability of free ligands L (they are presented in Table 4), showed that both in the gas phase (g) and in an aqueous solution (s) they decrease only slightly in the series q(S)L1 > q(S)L2 > q(S)L3.

Table 4. Distribution of electron density in ligands L and Pd(II) complexes (def2tzvp basis, g is gas, s is solution): q are charges on atoms (according to Mulliken), W(A–B) are the Wiberg bond multiplicities [39], ∑W(A) are the total values of the multiplicity of bonds of an atom in a molecule
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According to the NBO analysis for [Pd(L)2Сl2] coordination compounds, the calculated multiplicities of metal–sulfur bonds W(Pd–S) determined by the Wiberg indices [39] were also found to be quite close and, on average, equal to 0.5–0.6 units of multiplicity. Such small differences in the considered characteristics of the R’SR ligands at very different R groups are explained by the electronegativity equalization principle formulated by Sanderson [40]: during the formation of M–S bonds, the transfer of electron density occurs not only from the donor sulfur atom but also includes its pumping of the entire ligand system, including the unchanged group R'. The total value of the Wiberg indices ∑W for an atom in the compound is associated with its valence activity. The value of ∑W(S) in these compounds also levels off to 2.9–3.0 units (Table 4).

A simple analysis of the redistribution of electron density during complexation, even in the extended atomic basis def2tzvp, did not allow us to establish differences in the donor ability of ligands L. For [Pt(L)2Cl2] compounds, this approach gave similar results in terms of the characteristics W(M–S) and ∑W(S), and they turned out to be very close to the above described characteristics and interatomic distances in complexes [Pd(L)2Cl2]. Perhaps, for this reason, the process of separation of Pd(II) and Pt(II) complexes in the joint presence is difficult.

As is known [41], substitution reactions in square planar Pd(II) and Pt(II) complexes proceed through the formation of five-coordination intermediates, and it was concluded that activation barriers in nucleophilic substitution reactions for Pd(II) complexes are significantly lower than for analogous compounds Pt(II). However, the search for intermediates with an estimate of the relative activation energies in the reactions of nucleophilic substitution of coordinated chloride ions for large and branched sulfur-containing ligands is a very difficult task. Therefore, in this work, we evaluated only the thermodynamic characteristics, i.e., the relative changes in the Gibbs energy, which take into account the initial reagents and products in complexation reactions. Considering the possibility of chemical reactions occurring in solution, we used the fundamental requirement that the thermodynamic Gibbs potential ∆G0 decreases in the reacting system. The energy characteristics of the compounds for subsequent calculations of the ∆G0 values of the reactions obtained in this work are given in Table 5. Complexation reactions and the corresponding values of the change in ∆G0 in the gas phase (g) and with allowance for the solvent (s) are given in Table 6.

Table 5.   Energy characteristics of compounds calculated for reactions (def2tzvp and * def2svp bases): Gibbs energies in the gas phase and taking into account the solvent \(G_{{{\text{gas}}}}^{0},\) \(G_{{{\text{solv}}}}^{0}\) (a.u.), solvation energy of compounds \(\Delta G_{{{\text{solv}}}}^{0}\) (kJ/mol)
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Table 6.   Complexation reactions and the corresponding calculated values of the change in the Gibbs energy ∆G0 in the gas phase (g) and taking into account the solvent (s)
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The interaction of [PdCl4]2– reagents and sulfur-containing ligands L according to reactions 1, 4–6, 11, 12 (Table 6) could lead to the formation of the following products: [Pd(L1)2Сl2]0, [Pd(L2)2Сl2]0, [Pd(bi-L2)Сl2]0, [Pd(bi-L2)2]2+, [Pd(L3)2Сl2]0, and [Pd(bi-L3)2]2+. Here, the ligands denoted as bi-L2 and bi-L3 contain a carboxyl group and can exhibit bidentate character during complex formation due to sulfur and oxygen atoms (Figs. 6–8).

Fig. 6.
figure 6

Optimized structure of complex [Pd(bi-L2)Cl2]0.

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Fig. 7.
figure 7

Optimized structure of complex [Pd(bi-L2)2]2+.

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Fig. 8.
figure 8

Optimized structure of complex [Pd(bi-L3)2]2+.

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However, for these reactions, large positive values of the Gibbs energy were also obtained both in the gas phase \(\left( {\Delta G_{{{\text{gas}}}}^{0}} \right)\) and taking into account the solvent \(\left( {\Delta G_{{{\text{solv}}}}^{0}} \right),\) testifying to the impossibility of their occurrence (except for process 11, where \(\Delta G_{{{\text{gas}}}}^{0}\) < 0, but \(\Delta G_{{{\text{solv}}}}^{0}\) > 0). The effects leading to these results can be explained as follows: in the initial compound [PdCl4]2– and the above products of the putative reactions, the Pd–Cl bonds are stronger than the Pd–S and Pd–О bonds (for bidentate S- and O-coordination) of ligands L2 or L3. Based on the NBO analysis, in the systems [Pd(L)2Cl2]0 and [Pd(bi-L2)Cl2]0 the average values of the Wiberg indices for ligands L1, L2, L3 are close to the following: W(Pd–Cl) = 0.65, W(Pd–S) = 0.55, W(Pd–О) = 0.39 multiplicity units (Table 4). The increase in the value of \(\Delta G_{{{\text{gas}}}}^{0}\) is associated with the reagent, the [PdCl4]2– ion, with four stronger Pd–Cl bonds, which ensures the total increase in the enthalpy \(\Delta H_{{{\text{gas}}}}^{0},\) which makes the main contribution to the value of \(\Delta G_{{{\text{gas}}}}^{0}\) in these reactions.

A solvent, especially a polar one, lowers the Gibbs energy of the compounds \(G_{{{\text{solv}}}}^{0}\) relative to this value in the gas phase \(G_{{{\text{gas}}}}^{0}.\) The calculated relative changes in the values of \(G_{{{\text{solv}}}}^{0}\) can already be qualitatively compared with the experimental ones. It should be kept in mind that the solvation of the doubly charged [PdCl4]2– anion (\(G_{{{\text{solv}}}}^{0}\) = –786.55 kJ/mol) is more significant than the solvation of two singly charged Cl– anions (\(2G_{{{\text{solv}}}}^{0}\)= –627.26 kJ/mol). The solvation of all neutral ligands L and chloride complexes is relatively low: the value of \(G_{{{\text{solv}}}}^{0}\) is only 24–45 and 82–100 kJ/mol for L and complexes of the [Pd(L1)2Cl2]0 type, respectively (Table 5).

In solutions with a low concentration of hydrochloric acid, complexes [Pd(H2O)2Cl2]0 may exist. For processes 2, 7, 8, 14 with initial [Pd(H2O)2Cl2]0 and ligands L, various possible products were also considered. In accordance with the criterion for the occurrence of reactions in solution \(G_{{{\text{solv}}}}^{0}\) < 0, the following compounds can be formed in these reactions: [Pd(L1)2Сl2]0, [Pd(L2)2Сl2]0, [Pd(bi-L2)Сl2]0, and [Pd(L3)2Сl2]0; for the former and the latter in the case of the reagent [Pd(H2O)2Cl2]0, the calculated criteria ∆G0 are the most optimal. However, for the reaction 14 with ligand L3, the formation of the product [Pd(L3)2Cl2]0 is not confirmed by XPS experimental data, which indicate that this reaction system does not contain products with coordinated Cl– anions. The absence of chloride ligands, in general, could indicate the formation of a stable [Pd(L3)4]2+ compound with monodentate S-coordination of ligands L3 or a [Pd(bi-L3)2]2+ complex in which the sulfur-containing ligand exhibits bidentate properties. However, in the first case, the formation of a Pd(II) complex with four S-coordinated L3 ligands will be hindered by the steric features of the bulky ligand groups. For the product [Pd(bi-L3)2]2+ with two Pd–O bonds, which are even weaker than Pd–Cl or Pd–S (see W(A–B) in Table 4), in reaction 13 the thermodynamic stability of the products must be less than that of the reactants. In the presence of fully aquated [Pd(H2O)4]2+ complexes in the solution, because of weak metal–ligand bonds and, as a result, low values of \(G_{{{\text{gas}}}}^{0}\) and \(G_{{{\text{solv}}}}^{0}\), the total values of \(G_{{{\text{gas}}}}^{0}\) and \(G_{{{\text{solv}}}}^{0}\) in reactions 3, 9–10, and 15 have large negative values, which indicates the possibility of reactions with the initial aquacomplex. It is possible that the thermodynamically favorable formation of a complex of a metal ion with ligand L3 that does not contain coordinated chloride ions ([Pd(bi-L3)2]2+) is associated precisely with the presence of Pd(II) aqua complexes in the system.

Thus, the Pd(II) complexation with all ligands L under consideration are thermodynamically unfavorable at high HCl concentrations, when the fraction of [PdCl4]2– ions is the largest. With a decrease in the concentration of HCl and the presence of [Pd(H2O)2Cl2]0 complexes in solutions, it is permissible to replace the H2O ligands in them with sulfur-containing ligands L1 and L2 with the formation of compounds of the type [Pd(L1)2Cl2]0, [Pd(L2)2Cl2]0, and [Pd(bi-L2)Cl2]0. In weakly acidic solutions, which may contain initial aqua complexes [Pd(H2O)4]2+, the processes of substitution of coordinated H2O molecules for sulfur-containing ligands with the formation of complexes [Pd(L1)2Cl2]0, [Pd(bi-L2)1(H2O)2]2+, [Pd(bi-L2)2]2+, and [Pd(bi-L3)2]2+ are thermodynamically favorable.

CONCLUSIONS

The sorption of Pd(II) ions from hydrochloric acid solutions by silica chemically modified with sulfur-containing organic compounds was studied. Dependences of sorption from chloride solutions under static conditions on time, HCl concentration, and Cl– ions were obtained; sorption isotherms were constructed. A combination of spectral methods (IR, UV-Vis, X-ray photoelectron spectroscopies) proved that sorption proceeds according to the coordination mechanism. Using the DFT M06 quantum-chemical method in the def2tzvp basis in the gas phase and taking into account the H2O solvent, the structure of sulfur-containing sorbents and their complexes with palladium was simulated, which made it possible to substantiate the nature of the dependence of sorption on the acid concentration and confirm the possibility of the formation of palladium(II) complexes with bidentate coordination of ligands in the silica phase modified with thiosalicylic and mercaptoacetic acids.