Selective Recovery of Palladium (II) from Acidic Solutions Using Dithio- and Benzimidazolylthio-Functionalized Resins

Abstract

Adsorbents derived from Merrifield’s resin and a reaction with three functionalizing ligands namely 1,2-ethanedithiol (M-EDT), 1,2-benzenedithiol (M-BDT), and 2-benzimidazolylmethylthio acetic acid (M-BITAA) were synthesized for the recovery and separation of PGMs from simulated solutions. M-EDT, M-BDT and M-BITAA resins were characterized by the FTIR, UV-Vis, TGA, CHNS and SEM techniques, which confirmed significant structural modifications in these resins. A batch adsorption study revealed that M-BITAA exhibited the highest capacity for Pd(II), with about 0.244 mmol·g⁻¹, while that of both M-EDT and M-BDT resins was below 0.094 mmol·g⁻¹. The adsorbents obeyed the Langmuir isotherm in 0.8 M HCl solution. Batch adsorption further showed, in a competitive study, that M-BITAA was not selective for Pd(II) but an attractive sorbent for other PGMs such as Pt(IV), which may be advantageous for solutions containing these PGMs.

1. Introduction

The increasing demand for platinum group metals, coupled with their limited availability in nature, has made their recovery a priority, especially considering the high cost of mining and refining [1]. Efficient and cost-effective methods of recovery from secondary sources such as spent catalytic converters, electronic waste and industrial by-products are needed [2,3]. Traditional recovery techniques, such as solvent extraction, precipitation, and electrochemical techniques, often fail to provide sufficient selectivity, leading to poor recovery rates or loss of other valuable metals. Furthermore, these methods can be environmentally damaging due to the use of toxic chemicals and the high energy costs associated with the processes [4]. Therefore, the need for more sustainable and efficient recovery technologies is evident. Recent research has highlighted the potential of adsorption-based techniques using functionalized resins [5]. These resins, when modified with specific functional groups, can selectively bind PGMs while excluding other metals, making them ideal for the targeted extraction. One such resin is Merrifield resin, originally developed for peptide synthesis but has proven to be an effective material for adsorption when functionalized [6,7]. The incorporation of various functional groups onto Merrifield resin has shown promise in improving the selectivity and efficiency of capturing heavy metal ions from acidic solution [8]. One study investigated the use of Merrifield resin to immobilize quercetin by functionalizing the resin with oxi-(alkyl)n-OH spacers containing 6, 7, and 10 methylene units. This resulted in three variants of quercetin-immobilized materials, which were tested for Pb(II) adsorption under dynamic conditions at pH 5. The optimal adsorption capacities ranged from 0.64 to 1.21 mg·g⁻¹ (equivalent to 0.0031 to 0.0058 mmol·g⁻¹) [9]. In another application, Merrifield resin was modified with ethylenediamine, methylamine, and dimethylamine to form primary, secondary, and tertiary amine-functionalized resins, respectively. These were evaluated for SO₂ adsorption using a packed bed column in a temperature-controlled oven. SO₂ gas was introduced at 100 mL·min⁻¹ at 25 °C, with nitrogen as the purging gas, and adsorption was monitored via the effluent stream using a gas analyzer. The tertiary amine-functionalized resin showed the best SO₂ removal, achieving 99% efficiency at 25 °C, which declined to 78.95% at 75 °C [10].

The use of functionalized Merrifield resin in PGMs recovery represents an innovative approach that can address many of the challenges faced by traditional recovery methods. Functionalization involves the modification of a substrate, such as a resin or polymer, with specific functional groups that enhance its ability to selectively bind target metal ions [11,12]. Various functional groups have been explored for their ability to selectively bind Pd(II) ions, with thiol, amine, and imidazole [13,14] groups being among the studied. These functional groups can form strong bonds with Pd(II) generally through their S, N or O atom centers on the molecular structure, enhancing the resin’s ability to adsorb the metal from solution [15,16]. Studies have demonstrated the effectiveness of functionalized Merrifield resins in selectively adsorbing Pd(II) from complex mixtures of metals [17]. Some of these functionalized Merrifield resin adsorbents exhibit high adsorption capacities, exceeding the average efficiencies for Pd recovery [17,18,19]. Furthermore, these resins can be regenerated and reused multiple times without significant loss of adsorption capacity, making them cost-effective and sustainable for large-scale applications. The development of these advanced functionalized adsorbents is a significant step forward in the field of PGMs recovery, offering a more efficient and environmentally friendly alternative to traditional methods. This study focuses above all on the functionalization of Merrifield resin with dithiols and benzimidazolylthio functional groups for improved recovery of Pd(II).

2. Materials and Methods

2.1. Materials

Specialized materials including Merrifield Resin (50–100 mesh, 1% cross-linked, 2.5–4.0 mmol Cl⁻·g⁻¹, Sigma-Aldrich, Co, St Louis, MO, USA), Pd(II) standard (1000 ± 3 mg·L⁻¹, Teknolab, Drøbak, Norway), 1,2-benzenedithiol (≥96%, Sigma-Aldrich, Co, St Louis, MO, USA), 1,2-ethanedithiol (≥98%, Fluka analytical, Fluka Chemie AG, Buchs SG, Switzerland) and 2-(benzimidazolylmethylthio) acetic acid (Aldrich, Milwaukee, WI, USA) were used as obtained from storage or pre-conditioned accordingly. Millipore water was prepared and used in all water-involving instances. The comprehensive list of the materials is presented in the Supplementary Materials (Table S1).

2.2. Characterization Techniques

Fourier-Transform Infrared spectroscopy (Bruker Tensor 27) equipped with an attenuated total reflectance (ATR) (Bruker Optics GmbH & Co. KG, Ettlingen, Germany) was used to analyze the molecular composition and structural bonding of pristine and functionalized resins between 4000 and 400 cm⁻¹. Scanning electron microscopy (SEM) was employed with a Joel SEM JSM IT100 (JEOL Ltd, Akishima, Japan) operated at 10 kV or 20 kV at a 7 or a 16 mm working distance. To improve the conductivity, all samples were coated prior to imaging by sputter coating using gold. The diameter of the materials was measured from micrographs with ImageJ software (v1.54J). Thermogravimetric analysis (TGA, SDT Q600, TA Instruments—Waters LLC, New Castle, DE, USA) was measured by heating the samples at a rate of 10 °C·min⁻¹ from 100–650 °C. The data obtained were analyzed through a TA instrument © analysis software v5.5.24. UV-Vis spectra (Shimadzu UV-3100 spectrophotometer, Shimadzu Corporation, Kyoto, Japan) were employed for solid state detection of aromatic benzene and any other conjugated system and samples were scanned with photons in the range of 200–800 nm. The spectrometer was digitally operated using UV Probe v2.42 software. CHNS elemental analysis (carbon, hydrogen, nitrogen and sulfur) was performed on a Thermofisher Scientific organic (Elemental analyzer Flash 2000, ThermoFisher Scientific Inc., Waltham, MA, USA) after calibration with Cysteine, BBOT, Methionine and Sulfanilamide standards. The elemental composition of metal ions was determined on a Perkin Elmer Avio 200 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, PerkinElmer, Inc., Waltham, MA, USA).

2.3. Synthesis Methods

Prior to functionalization, the swelling capacity of Merrifield resin was gravimetrically examined by immersion (10.80 mg/10 mL THF). The resulting suspension was thoroughly shaken to allow for swelling at room temperature (25 °C) for 24 h and then surface-dried and quickly weighed in accordance with the approach of Krakovský et al. [20]. The swelling ratio was calculated according to Equation (1).

$$SR={\displaystyle \frac{{mass}_{swollen}}{{mass}_{dry}}}$$

(1)

2.3.1. Functionalization of Merrifield Resin with 1,2-Ethanedithiol and 1,2-Benzenedithiol

The functionalized Merrifield resin, M-BDT (using 180.18 mg of resin) and M-EDT (using 358.44 mg of resin) were obtained using 1 mmol (84 µL) of 1,2-ethanedithiol (EDT) or 0.51 mmol (72 mg) of 1,2-benzenedithiol (BDT) in suitable amounts of triethyl amine (TEA) under refluxing at 60 °C for 12 h. The resulting solid was then filtered and extensively washed with tetrahydrofuran (THF), a tetrahydrofuran–water (THF–H₂O) (1:1) mixture, H₂O, acetone, toluene, and acetone prior to characterization. When functionalizing the resin with EDT or BDT, dimethylformamide (DMF) was used as the solvent, which is a polar aprotic solvent that can effectively dissolve both the dithiols and facilitate nucleophilic substitution. TEA was employed in assistive amounts to deprotonate the dithiol, enhancing its nucleophilicity for substitution reactions with the benzyl chloride groups on the resin. This combination of solvent and base ensures effective functionalization of the resin, forming thiol-based functional groups on the polymer.

2.3.2. Functionalization of Merrifield Resin with 2-Benzimidazolylthio Acetic Acid

The functionalization of Merrifield resin with 2-benzimidazolylthio acetic acid (BITAA) was carried out using an esterification method modified from the work of Matsumoto et al. [21]. Then, 348.66 mg of Merrifield resin was swelled by suspension in THF overnight, while about 1 mmol (206.58 mg) of BITAA was weighed and mixed with an equimolar amount of potassium carbonate (K₂CO₃) and tetrabutylammonium iodide (N(n-Bu)₄I), and the mixture was stirred at 90 °C until dissolved and colorless in THF. The resulting solution was then introduced to the swelled resin suspension and refluxed at 60 °C for 12 h. After esterification, the brown solid was filtered and extensively washed with THF, the H₂O–THF (1:1) mixture, H₂O, acetone, toluene, acetone, and then dried prior to characterization.

The schematic for functionalization of Merrifield resin with EDT, BDT and BITAA ligands and the proposed chelating bidental for the complexes is shown in Figure 1. Disparities in adsorption ability are anticipated due to differences in the ligand environment and steric effects. M-BITAA, with its bulkier benzimidazolyl group, may form stronger complexes due to pi back bonding but with limited accessibility to metal ions. M-BDT and M-EDT feature thiol groups, with M-BDT having more steric hindrance due to its aromatic structure, while M-EDT is more flexible. The structural variations in these resins influence the stability, accessibility, and efficiency of metal ions.

2.3.3. Acid Stability

Functionalized derivatives, M-BITAA, M-EDT, and M-BDT, were evaluated for acid stability. Approximately 5–8 mg of each resin was weighed and immersed in 10 mL of hydrochloric acid (HCl) solutions at different concentrations (0.5, 0.8, and 1.0 M). The resins were incubated at room temperature for 24 h. Following acid exposure, the resins were filtered, thoroughly washed with deionized water, and dried. The mass of each sample was measured before and after acid treatment.

2.4. Batch Adsorption

Batch experiments were conducted at a fixed dosage of 10 mg of adsorbent in 10 mL of HCl (0.01–1.5 M) containing 5 mg·L⁻¹ of metal ion. The optimum conditions were used to obtain the adsorption efficiency (E%) and capacity (q_e, mmol·g⁻¹) according to Equations (2) and (3), respectively, where C_o and C_e are, respectively, the concentrations (mmol·L⁻¹) of metal ions before and after the adsorption. V and W denote the volume (mL) of solution containing metal ion and the weight of adsorbent (mg), respectively. The isotherms were obtained by the Langmuir, Freundlich and Temkin models using Equations (4)–(6), respectively. For the Freundlich model, q_max (mmol·g⁻¹) denotes the maximum adsorption capacity, q_e (mmol·g⁻¹) at the equilibrium. K_L is the Langmuir coefficient (L.mmol⁻¹). K_F is a constant (Freundlich coefficient) related to the adsorption capacity, while n is the Freundlich constant. K_T and b, respectively, denote the Temkin equilibrium binding coefficient and the Temkin constant. R denotes the gas constant, and T denotes the system temperature. The influence of temperature was investigated at 25, 35, 45, 55, and 65 °C and the thermodynamic analysis was determined using the Van’t Hoff equation (Equation (7)) and the Gibbs free energy equation (Equation (8)). The Van’t Hoff equation consists of variables, including K_eq, the dimensionless equilibrium constant, enthalpy (ΔH) and entropy (ΔS).

Pseudo-first order (PFO) kinetics, pseudo-second order (PSO) kinetics, the Elovich equation, and intra-particle diffusion (IPD). The rate equation of PFO and its linearized form are given in Equations (9) and (10), where q_t and q_e are values of the amount adsorbed at t and equilibrium time, respectively, and k₁ is the PFO rate constant. For PSO, the linearized form described in Equation (11) was used, where the q values are the same as explained for PFO and k₂ is the PSO rate constant.

$$\mathrm{E}\%={\displaystyle \frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{o}}}}\ast 100\mathrm{ }\%$$

(2)

$$\mathrm{E}\%={\displaystyle \frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{o}}}}\ast 100\mathrm{ }\%$$

(3)

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{\mathrm{W}}}\ast \mathrm{V}$$

(4)

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}}$$

(5)

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{({\displaystyle \frac{1}{\mathrm{n}}})}$$

(6)

$$\ln {\mathrm{K}}_{\mathrm{e}\mathrm{q}}=-{\displaystyle \frac{\Delta \mathrm{H}}{\mathrm{R}\mathrm{T}}}+{\displaystyle \frac{\Delta \mathrm{S}}{\mathrm{R}}}$$

(7)

$$\Delta \mathrm{G}=\Delta \mathrm{H}-\mathrm{T}\Delta \mathrm{S}$$

(8)

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}(1-{\mathrm{e}}^{(-{\mathrm{k}}_1\mathrm{t})})$$

(9)

$$\ln \left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=\mathrm{l}\mathrm{n}{\mathrm{q}}_{\mathrm{e}}-{\mathrm{k}}_1\mathrm{t}$$

(10)

$${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}}={\displaystyle \frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}}+{\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}}$$

(11)

Selectivity Study

Competitive studies were performed using solutions containing Pd(II), Pt(IV), and Ir(III) metal ions at different concentration ratios, as shown in

Table 1. Concentration ratios of metal ions.

	Pd (mmol·L−1)	Pt (mmol·L−1)	Ir (mmol·L−1)
Mixture 1	0.047	0.047	0.047
Mixture 2	0.094	0.047	0.047
Mixture 3	0.047	0.094	0.094

. These experiments were carried out at room temperature, 60 min, 150 rpm and optimum [Cl⁻] (0.8 M). The selectivity factor (R) of the adsorbent towards metal ions was calculated according to Equation (12), shere q_M and q_{competing metal} (mmol·g⁻¹) are the adsorption capacity of the metal ion of interest and that of the competing metal ion, respectively.

$${\mathrm{R}}_{\mathrm{w}\mathrm{r}\mathrm{t}}\mathrm{C}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{M}}}{{\mathrm{q}}_{\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}}}$$

(12)

3. Results

3.1. Characterization of Materials

Figure 2 shows the swelling behavior of Merrifield resins in tetrahydrofuran (THF) after 24 h of immersion. The resin exhibited noticeable expansion and softening, forming a gel-like phase, indicating successful swelling. The swelling ratio (SR) was found to be SR = 8.97 ± 0.68, confirming that THF is an efficient swelling agent for these resins. However, this value was slightly higher compared to those obtained by Santini et al. [22], who reported swelling of 7.7 and 3.1 at a shorter contact time. Figure 3a shows the IR spectrum of pristine Merrifield (M) and functionalized Merrifield (M-BDT, M-EDT and M-BITAA) resins. The strong peak at about 684 cm⁻¹ was attributed to ν (Benzyl-Cl), while the peak at 770 cm⁻¹ is likely due to out-of-plane C-H bending vibrations of the benzene ring. This region is characteristic of aromatic substitutions, and its presence suggests that the benzyl structure remains intact after functionalization. These observations were consistent with peaks reported by Majavu et al. [23] and Dardouri et al. [24]. The peak located at 1268 cm⁻¹ is characteristic of M resins and was attributed to CH₂-Cl bending. This peak exhibited reduced intensities in M-BDT-, M-EDT- and M-BITAA-functionalized resins compared to pristine M resin. The decreased intensity of this peak might indicate a decrease in the CH₂-Cl population. Furthermore, all the resins displayed characteristic peaks at 1591, 1533, 1462, 2999, 2885, and 2883 cm⁻¹, consistent with benzene ring vibrations and -CH stretches, aligning with findings from Liu et al. [25] and Boruah et al. [26]. The characteristic vibrations mentioned appear slightly shifted towards high wavenumbers (1600–1200 cm⁻¹) or low wavenumbers (3000–2800 cm⁻¹). The appearance of wavenumber shifting between unfunctionalized Merrifield resin and a derivative has been previously reported by Chen [27]. The slight extent of these shifts could be indicative of preservation of the bulk polymer structure after functionalization. Furthermore, these shifts in M-BITAA could possibly be due to the introduction of S-, N- or O-bearing functionalities, causing an electron-withdrawing effect on the aromatic ring associated with these materials, and consequently its bond-shortening or stiffening and arising conjugation differences, as pointed out for benzene ring vibrations. Similarly to M-BITAA, shifts observed in M-BDT and M-EDT could be due to the reduction in -C-Cl bonds relative to -CH bonds and Cl substitution by N and S after the functionalization. Figure 3b shows the UV-Vis spectra of pristine resin stacked alongside functionalized resins. These results revealed notable shifts in electronic features. The unmodified M resin exhibited an absorption peak at 279 nm, corresponding to the polystyrene structure and aromatic groups [28,29]. Similarly to the IR results, slight shifts were observed in the functionalized resins (M-BDT, M-EDT, and M-BITAA), suggesting changes in the electronic environment, more so for the dithiols, where there are relatively higher absorbance values than for M-BITAA and the pristine resin. These shifts are consistent with previous studies [30,31]. The functionalization led to increased electronic conjugation, confirming the successful modification of the resin surface.

The thermal stability of the materials, as shown in Figure 3c, further revealed distinct degradation for pristine and functionalized resins. Pristine M resin showed a two-step degradation process at 237 °C and 400 °C, with respective weight loss of 15% and 60%, and this was found to be comparable to the TGA pattern observed by Pisk et al. [32], whereby such weight loss pattern was attributed to the decomposition of Merrifield resin bulk structure. Similarly to the pristine resin, the decomposition of functionalized M-BITAA also largely occurred as a multistep process at 50, 245 and 420 °C, corresponding to a weight loss of 1.90, 15.56 and 55%, respectively. This is also associated with decomposition of the bulk structure of Merrifield resin [33]. In contrast to pristine M and M-BITAA resins, the functionalized M-EDT decomposed mainly at 63 (0.62%), 263 (7.21%), 313 (7.08%) and 400 °C (48.24%), and M-BDT decomposed mainly 180 (5.35%), 275 (5.96%), 390 (54.84%), and 545 °C (9.54%), respectively.

Overall, the maximum degradation temperature was highest for the pristine M resin compared to its functionalized derivatives, contrasting with the findings of Boruah and Das. [34], who studied Schiff base-functionalized resins. This discrepancy highlighted that sometimes Merrifield resin becomes a little resistant to thermal degradation after functionalization, while it may also become less thermally resilient sometimes as a function of factors like functionalization conditions, such as the solvent, temperature and functionalizing agent’s concentration [35]. The differences in decomposition events emphasize the effects of functionalization on the resin’s thermal stability.

Figure 4 shows the SEM images of pristine M and functionalized M-BDT, M-EDT and M-BITAA resins. These micrographs revealed that the smoothness of the pristine M resin’s outer surface was altered, indicating the effect of the functionalization process. In contrast, the functionalized M-EDT, M-BDT and M-BITAA resins exhibited minor surface agglomerations, which were finer in M-BITAA and coarser for M-EDT resins, as also reported for dendrimer-functionalized Merrifield resins [36,37]. The occurrence of a functionalizing agent on the surface of Merrifield resin beads was attributed to the solvent effect, reaction conditions or ligand reactivity [38]. The results further revealed that the functionalization of pristine M had a direct impact on its diameter depending on the additive. In fact, it was found that M-BITAA functionalization increased the diameter of pristine M (211.2 µm), while M-EDT and M-BDT tended to decrease it. The diameters of the functionalized resins were, respectively, 193 µm, 207 µm and 224 µm for M-BDT, M-EDT and M-BITAA. These changes in the diameter of the resins align with findings from Lapinte et al. [39] and Kappert et al. [40], who attributed size variations to solvent effects and ligand properties. The observed effects are consistent with other studies on functionalized resins [41].

CHNS analysis for the elemental composition of resin before and after the functionalization was also carried out to support the SEM results. As can be seen in

Table 2. The elemental analysis of the pristine resin resins with a loading capacity of 2.5–4.0 mmol Cl⁻·g⁻¹ and the functionalized resins.

Resins	C	H	N	S	Ligand Loading (mmol·g−1)
M (Pristine)	79.00	5.00	0.50	0.00	N/A
M-EDT	79.00	6.00	0.70	0.03	0.0022
M-BDT	78.00	6.00	0.80	0.01	0.0018
M-BITAA	76.00	7.00	4.00	0.04	0.020

, it was found that pristine M resin contains carbon (79.00%), hydrogen (7.00%) and nitrogen (0.50%) with no sulfur. The degree of functionalization varied notably among the modified resins. M-EDT and M-BDT exhibited poor functionalization, as reflected in their very low sulfur content (0.03% and 0.01%, respectively). This indicates that the incorporation of dithiol groups was limited, leading to reduced sulfur percentages and lower functionalization ligand loading (0.0022 mmol·g⁻¹) for M-EDT and only 0.0018 mmol·g⁻¹ for M-BDT). In contrast, M-BITAA demonstrated a significantly higher degree of functionalization, as indicated by its substantial increase in nitrogen content (4.00%) and the highest functionalization ligand loading at 0.020 mmol·g⁻¹. This suggests a more successful incorporation of benzimidazolyl functional groups. The observed changes in carbon and hydrogen percentages further support these trends, with M-BITAA showing the most pronounced variations due to the additional functional groups introduced. M-BITAA exhibited a significant decrease in carbon (76.47%), and an increase in hydrogen (7.44%) and nitrogen (4.46%), with sulfur at 0.040%. These changes are consistent with the functionalization processes, where specific ligands alter the resin’s elemental composition, as suggested in the literature [42]. The calculation for ligand loading was carried out as shown in the Supplementary Materials (Calculation S1).

Figure 5 shows the results obtained from the acid stability of functionalized Merrifield resins. (N/A: not applicable)

These results demonstrate an increasing mass loss with higher acid concentration for all functionalized resins, with M-EDT and M-BDT showing the highest susceptibility to acid-induced degradation, with average mass losses approaching 0.8% at the highest acid concentration. M-BITAA showed intermediate stability, and the lowest % mass loss compared to M-EDT and M-BDT. The observed trend suggests that functionalization of the Merrifield resin, particularly with EDT and BDT groups, increases vulnerability to acid hydrolysis. These findings highlight the importance of both resin structure and functional group chemistry in determining acid resistance.

3.2. Adsorption Studies

Figure 6 and Figure 7 show the results obtained at various acidic conditions and contact time using the M-BITAA absorbent, which had a high degree of functionalization, as previously discussed. It was observed that the shortest time to reach equilibrium and the maximum E% at each [HCl] value were in the order 0.01 M < 0.05 M < 0.1 M < 1.5 M < 0.8 M ≈ 0.5 M. The highest capacity was observed at 0.8 M HCl, which might be attributed to high adsorption sites on the functionalized resins. As the Pd(II) concentration increased, the adsorption efficiency decreased, likely due to saturation of active sites on the adsorbents [43]. Conversely, the adsorption capacity increases with concentration, reflecting a greater uptake of Pd(II) at higher concentrations [44]. Figure 8a,b shows the effect of the initial Pd(II) concentration of 0.8 M on the adsorption efficiency (E%) and capacity (q_e) for functionalized M-BDT, M-EDT, and M-BITAA resins. Among other functionalized resins, M-BITAA exhibits superior performance, achieving the highest q_e values. The high M-BITAA q_max (>0.235 mmol·g⁻¹) reflects strong interaction via the N and S center [45,46,47]. The adsorption capacities for Pd(II) follow the order of M-BITAA > M-EDT > M-BDT, which corresponds to the order of the observed ligand loading (Table 2).

The M-BITAA moiety features N, S coordination sites [48,49] and pi back-bonding character, significantly boosting its adsorption capacity. M-EDT and M-BDT, with two S atoms, can form bidentate complexes and the lesser-performing 1,2-benzenedithiol might be due to the rigid aromatic structure restricting its ability to form efficient chelate complexes [50,51] or may simply be attributed to the ligand loading. With M-BDT, q_e begins to plateau in the range of 0.517–0.564 mmol·L⁻¹, suggesting this as the optimum concentration. M-EDT shows a similar trend, with q_e also levelling off around 0.517–0.564 mmol·L⁻¹. In contrast, M-BITAA exhibits increasing q_e, reaching a plateau around 0.893–0.940 mmol·L⁻¹. These results highlight the differing performance and capacities of the functionalized resins.

Table 3. The established values of adsorption capacity for the adsorbents of this study compared to other studies in batch adsorptions.

Adsorbent Name	qe (mmol·g−1)	Adsorbent Dosage (mg)	[M] (mmol·L−1)	pH/ [Acid]	Refs
M-BITAA	0.247	10	0.0467–1.316	0.8 M	This work
M-EDT	0.0674	10	0.0467–1.316	0.8 M	This work
M-BDT	0.0319	10	0.0467–1.316	0.8 M	This work
Triisobutyl phosphine sulfide functionalized resins	0.117	20	4.70	1.0 M	[52]
Poly (m-aminobenzoic acid) chelating polymer	0.227	300	0.282–0.705	pH 2.0	[18]
Moss (Racomitrium lanuginosum) biomass	0.886	100	0.235–2.819	pH 5	[19]
Polyamine/polystyrene-based beads and nanofibers	0.040 nanofibers 0.00188 microbeads	150	0.0235–0.0584	1.0 M	[22]

compares the adsorption capacity obtained in this study and those reported in the literature on similar modified resins. The capacity (q_e) of the best-performing M-BITAA resin obtained in the current study was slightly higher compared to that obtained in the study by Sanchez et al. [52] and Mildan and Gülfen [18] but lower compared to Sari et al. [19]. This discrepancy is obviously due to different experimental conditions, such as pH and the nature of the modified resins. Kinetic studies were carried out at a 0.8 M concentration for M-BITAA, M-BDT, and M-EDT, and the results are shown in Figures S2–S4, while Table S2 summarizes the key parameters obtained from these fitting models. It was found that functionalized M-BITAA resin best fitted to Elovich, IPD and PSO with an R², over 0.9, while the M-EDT and M-BDT adsorbents best obeyed the PSO model, with an R² value over 0.9.

These experimental results were also fitted into selected isothermal models to obtain mechanistic indications of the adsorption activity, and the results are shown in Figure 9. The results revealed that Langmuir isotherm model compliance occurred for all the adsorbents, more so than for the other tested models. This might indicate that chemisorption is the main interaction occurring between metal ion and the functionalized Merrifield resins. Figure 10 further shows the effect of temperature on the adsorption capacity of functionalized M-EDT, M-BDT and M-BITAA resins across 60 min. These results indicate that the q_e values of all the modified resins increased with increasing contact time and were higher at the lowest temperatures (room temperature). The decrease in adsorption capacity with increasing temperature suggests that the adsorption process for Pd(II) onto the functionalized resin was exothermic. While complexation-based adsorption is often endothermic due to the formation of coordinate bonds [52], certain exothermic behaviors can occur, particularly when physical interactions or pre-organized ligand environments dominate the adsorption process [53]. Additionally, increased temperature may reduce the stability of the adsorbent-Pd(II) complex or promote desorption, especially in systems where hydrogen bonding or weaker van der Waals forces are involved alongside complexation.

The evolution of q_e with time conformed with a cubic polynomial function and all the modified resins exhibited exothermic interaction with Pd(II) throughout both the contact time and temperature [54]. The adsorption equilibrium data obtained were used to evaluate the dimensionless constant K_eq based on the concentration of target ion adsorbed with respect to equilibrium concentration and suitably plotted according to the Van’t Hoff equation (Equation (7)) and shown in Figure 11. It can be seen from this plot that all the materials exhibited positive slopes at all contact times where plateaus were observed around 45 min. The thermodynamic parameters were obtained from the data of the plots using the Gibbs free energy equation, Equation (8), and are presented in Table S3. The enthalpy values were negative, confirming exothermicity of process and were about 50 to 250 times the entropy values, which implied that the process is enthalpy driven [55]. The Gibbs free energy (ΔG) values increased with temperature at all contact times until the 45th minute, signifying a reduction in spontaneity with a temperature increase, with the values of M-BITAA being comparable to the free energy levels associated with chemisorption [56]. M-BDT at the 15th minute had the highest ΔG values, while M-BITAA, from the 45th to the 60th minute, had the smallest ΔG values; thus, these were, respectively, the situations and moments of the adsorption process where the least spontaneous and the most spontaneous adsorption happened. All the adsorbents displayed an increase in spontaneity with a contact time increase. Subsequent to the above experiment, functionalized M-BITAA resin was selected and used for further study on the competition of Pd(II) with Pt(IV) and Ir(III). The obtained results are shown in Figure 12 and the selectivity factors in

Table 4. The selectivity factors of M-BITAA towards Pd(II) in competition with Pt(IV) and Ir(III).

	Rwrt Pt(IV)	Rwrt Ir(III)
Mixture 1	0.81	3.58
Mixture 2	2.04	28.92
Mixture 3	0.32	1.91

.

M-BITAA demonstrated preferential adsorption for Pt(IV) over Pd(II) and Ir(III) with the selectivity factors of Pd(II) with respect to Pt(IV) (R_wrt Pt(IV) being 0.81, 2.04 and 0.32, respectively, for mixtures 1, 2 and 3, while the R_wrt Ir(III) values were 3.58, 28.92 and 1.91, respectively, for the three different competition solutions mixtures 1, 2 and 3). The selectivity factors obtained highlighted the potential of the functionalized resins for simultaneous recovery of PGMs in complex mixtures [57,58,59,60,61,62]. This trend can be linked to the unique coordination environment provided by the benzimidazole moiety, which is conducive to the hexacoordination typically required by Pt(IV).

4. Conclusions

This study investigated the adsorptive extraction of Pd(II) using Merrifield resin functionalized with dithiol (M-BDT, M-EDT) and benzimidazolyl-thioacetic acid (M-BITAA). Pristine and functionalized resins were characterized by the FTIR, UV-Vis, TGA, CHNS and SEM techniques. The functionalization process enhanced the adsorption performance by introducing specific functional groups. The optimum Pd(II) uptake was achieved at 0.8 M HCl, with M-BITAA exhibiting the highest capacity. All the functionalized resins demonstrated exothermic adsorption behavior. The adsorption data followed the Langmuir isotherm model, indicating that the chemisorption process was involved, while the thermodynamic studies revealed that the adsorption process was enthalpy-driven. This study further assessed the selectivity of the resins, and M-BITAA showed a promising separation factor between Pd(II) and Ir(III), rather than Pt(IV). This confirmed that the prepared M-BITAA resin is a potential and promising candidate for PGMs’ recovery and separation from aqueous acidic solutions of wastewater streams.