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Synthesis and Characterization of Gallic Acid Resin and Its Interaction with Palladium(II), Rhodium(III) Chloro Complexes Mustafa Can,*,† Emrah Bulut,‡ and Mahmut Ö zacar‡ †

Institute of Sciences and Technology and ‡Department of Chemistry, Sakarya University, 54187 Sakarya, Turkey ABSTRACT: Gallic acid (GA) and formaldehyde condensation reaction was carried out under alkaline condition to prepare the gallic acid−formaldehyde resol resin (GAR). Obtained resin was used as adsorbent for recovery of palladium(II) and rhodium(III) ions from chloride-containing solutions. This kind of recovery was very simple and useful for generating little secondary wastes. Interaction of adsorption also was investigated: Chloropalladium(II) species were reduced to Pd(0), while hydroxyl groups of GAR were oxidized during the adsorption. Proposed adsorption of the aquachlororhodium(III) species mostly takes place via ligand exchange mechanism. GAR, Rh-adsorbed GAR, and Pd-adsorbed GAR were characterized by scanning electron microscopy, energy-dispersive spectrometry, X-ray diffraction spectroscopy, and Fourier transform infrared− attenuated total reflection spectroscopy.

1. INTRODUCTION Because of lower ore reserve and a wide range of industrial applications, an important part of the platinum group metals (PGMs) placed on the market provided by recycling processes. PGMs with the exception of the automotive sector have a low amount of use. PGMs are present in the form of chlorocomplexes with complicated solution chemistry. The species composition is dependent on factors such as chloride concentration, pH, ionic strength, temperature, and the age of the solution. The formation of metal complexes by PGMs is related to the solution composition. This in turn may affect the adsorption mechanism involved, i.e., chelation rather than ion exchange, and the affinity of the metal species for sorption sites on the adsorbents. Solution chemistry of PGMs are generally very different from that of base metals.1 Active carbon,2 ion-exchange resins,3−7 and low-cost adsorbents8−10 are suitable for adsorption. Large volumes of published data exist regarding the recovery or removal of base metals from aqueous solutions, but the same cannot be said for precious metal recovery. Published precious metal biosorption data have focused on gold recovery,9,11 but over the past 15−20 years, interest in the recovery of strategically valuable metals such as platinum and palladium has increased.3,12,13 Tannins, whose major role in plants is mainly defensive, represent secondary metabolites widely distributed in various sectors of the higher plant kingdom.14,15 Gallotannins, a subclass of hydrolyzable plant tannins, are receiving increasing attention as an excellent resource for replacing petroleumderived phonolic compounds.16 Hydrolysis of hydrolyzable tannins produces gallic acid to a large extent. Other phenolic acids are also present, but in low proportions.17 Gallic acid consists of an aromatic ring bearing a carboxyl group and three adjacent hydroxyl groups.18 Conventional syntheses of polyphenol resins are classified into chemical and enzymatic methods.18 In the chemical methods, phenol reacts with formaldehyde in acidic or alkaline media, forming novolac or resol resins, respectively.11 Adam and Holmes19 first demonstrated the capacity of this type of © 2012 American Chemical Society

resin to exchange cations on the very weakly dissociating phenolic groups.20,21 These can be used for isolating and separating rare alkali metals,22 actinides,23−27 heavy metals,28−33 and precious metals.34−36 The increasing demand for the PGMs for production of catalysts and in related industries, combined with the limited resources available, has led to increasing interest in the recovery of these strategic elements.37 Only a few studies have been carried out on the separation of PGMs using tannin derivatives.13,38,39,35 Recently, studies have begun to focus significantly on the mechanisms involved in the binding of PGM to tannin resin. Initially, this took the form of conjecture, but now spectroscopic methods such as variants of X-ray and Fourier transform infrared (FTIR) spectroscopies for the elucidation of binding mechanisms are widely used. These methods confirmed that no chemical change to the adsorbent took place after metal loading, suggesting that the acidic conditions merely favored electrostatic interaction between PGM and polyphenolics.36 FTIR spectroscopy is a powerful analytical method for monitoring the resin formation and interaction mechanism with PGM. It offers a unique capability in terms of its quantitative measurement of the conversion of a specific functional group. In particular, the appearance of the hydroxyl group, methylol group, and dimethylene ether bridges can be easily monitored.16,40 In addition to using X-ray diffraction (XRD) patterns, redaction of PGM can be easily realized from sharp peaks.41,42 Scanning electron microscopy (SEM) has been used to show the particle size and morphology of these resins. Energy-dispersive X-ray (EDX) analysis was performed to determine elemental distribution for scanning surfaces in samples.43 Received: Revised: Accepted: Published: 6052

February 20, 2012 April 3, 2012 April 7, 2012 April 8, 2012 dx.doi.org/10.1021/ie300437u | Ind. Eng. Chem. Res. 2012, 51, 6052−6063

Industrial & Engineering Chemistry Research

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2.4. Adsorption Studies. The palladium(II) and rhodium(III) solutions were prepared by dissolving RhCl3.·3H2O and PdCl2, respectively, in different acid media at 50 mg/L concentrations. The palladium ions adsorption capacity decreases when the concentration of chlorine increases. In the case of rhodium ion adsorption, high chloride complexes are less difficult to obtain than aqua-containing rhodium complexes. Thus, different acid media are used to prepare stocks. This issue will be examined in future papers dealing with the subjects of kinetic and isotherm calculations. The 50 mL metal ion solutions prepared were stirred after 200 mg of GAR particles were added. The stirring rate was the same for all. All adsorption experiments were carried out in a standard and strictly adhered to batchwise system at 20 °C for 1 h. The initial pH of the solutions was controlled by adding a small amount of HCl, HNO3, NaOH, and HClO4. At the end of the adsorption period, 15 mL samples were centrifuged and the solutions were filtered through 0.45 μm Milipore filter paper to avoid any solid particle in the aqueous phase. Samples were measured using AAS. All of the adsorption tests were performed at least twice so as to avoid wrong interpretation due to any experimental errors. FAAS was calibrated using 0, 4, 12, and 20 ppm standard solution for Rh(III) and 0, 2, 6, and 10 ppm standard solution in 1 M HCl. Samples were diluted to the measurement limits of the AAS equipment for precise results. The amounts of the adsorbed metal ions were calculated from the concentrations in solutions before and after the adsorption process. Results were taken from the average of three scans for each sample.

This study focused on two main areas. First, investigation of gallic acid−formaldehyde resol resin (GAR) formation using several techniques such as FTIR, SEM, energy-dispersive spectrometry (EDS), and XRD. Second, interaction characterization of palladium(II) and rhodium(III) ions in chloridecontaining aqueous solutions onto obtained GAR particles.

2. MATERIALS AND METHODS 2.1. Materials. Gallic acid (molecular weight = 170) was purchased from Alfa Aesar GmbH & Co. RhCl3·3H2O, PdCl2, NH3, HCOH, HNO3, HCl, NaCl, and, NaOH were purchased from Merck Co. AAS standard solutions for determination of PGM were purchased from UltraScientific Co. All other reagents were analytical grade. The aqueous Pd(II) stock solution was prepared from solid PdCl2 in 1.0 M HNO3. The aqueous Rh(III) stock solution was prepared from solid RhCl3·3H2O in 1.0 M HCl. The studied solutions of palladium(II) were obtained by dilution with NaOH or HNO3 to adjust the H+ concentration to the desired value. Moreover, a suitable chloride concentration was obtained. Both of the working stock solutions were prepared to contain 200 mg/dm3 Pd(II) and Rh(III). 2.2. Instruments. The contents of Rh(III) and Pd(II) in solutions were analyzed by flame atomic absorption spectrometry (FAAS; Shimadzu 6701F). FTIR spectra analysis of resins was performed by Shimadzu IR Prestige-21 at 1 cm−1 resolution. Samples were analyzed using a ceramic light source, KBr/Ge beam splitter, and a deuterated L-alanine triglycine sulfide (DLATGS) detector. The FTIR spectra analyses of GA, GAR, Rh-adsorbed GAR, and Pd-adsorbed GAR were carried out. The spectra were recorded from 700 to 1800 cm−1 (accumulating 25 scans) on samples at room temperature. To eliminate moisture and CO2 interference, background spectra were recorded before analysis of the samples. Later on, it was corrected by applying IRsolution software’s Kubelka−Munk function attenuated total reflection (ATR)-correction function. SEM and EDS experiments were carried out on JEOL JSM6060LV scanning electron microscope operated at 20 kV. The morphology and size of GAR were investigated using SEM. To show the presence of Rh and Pd on the GAR, EDS analysis was performed. To clarify the interaction mechanism, GAR was analyzed by X-ray diffraction measured with RIGAKU Dmax = 2200 at a Cu anode producing Kα radiation (40 kV, 30 mA). The specific surface area of the resin was determined with Brunauer−Emmett−Teller (BET) nitrogen adsorption using a Micromeritics Flow Sorb 2300. 2.3. Preparation of GAR. A 500 mL aliquot of of deionized water was heated at 60 °C in a boiling flask. An 85 g (0.5 mol) amount of GA was dissolved in deionized water. A 372.5 mL aliquot of an aqueous solution containing 37% formaldehyde was added to bring the molar ratio of formaldehyde to gallic acid to 2. This was followed by stirring for 5 min to adjust the temperature to 85 °C. The appropriate amount of 13.3 N NH3 was added immediately to adjust the pH to 8. The solution reacted for 3 h at 85 °C under continued mixing.17 At the end of the production of GAR, the amount of formaldehyde was determined iodometrically as 1.26 per thousand. This value does not exceed the appropriate limit of 1%.18 Approximately 1000 mL of yellow-brown precipitate had the solution’s pH adjusted to pH 2 with HNO3. Subsequently, the acidcontaining solution was filtered and washed with distilled water. The mixture was allowed to stand after the filtering was done. Finally, the GAR resin was cooled and freeze-dried.17

3. RESULTS AND DISCUSSION 3.1. Preparation of GAR. Alkaline-catalyzed (resol) GARs are prepared similar to phenol−formaldehyde resins. The resol Table 1. Effect of F/GA Molar Ratio on Pd2+ Capacity (CoGAR = 49.9 mg/L, 0.2 g of Dry-Basis GAR, 20 °C, pCl = 3, v = 50 mL, and t = 60 min) resin no.

F/GA ratio

Pd2+ capacity (mg/g)

% removal

GAR1 GAR2 GAR3 GAR4 GAR5 GAR6 GAR7 GAR8 GAR9

1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00

3.78 4.91 6.23 8.96 11.61 11.12 10.34 9.29 7.23

30.24 39.28 49.84 71.68 92.88 88.96 82.72 74.32 57.84

resins formaldehyde/phenol (F/P) molar ratio is generally higher than 1.18,44 In this study, GA is instable in relatively high acidic or basic conditions because of rearrangement to its enonic form.45 The low reactivity of GA may be attributed to the steric hindrance caused by hydroxyl aromatic groups.17 To avoid this undesirable situation, to minimize steric hindrance, resol formation pH 8 was selected to provide mild alkaline conditions (preexperiments not shown here). In addition, to determine the most appropriate F/GA ratio for adsorption of PGMs, molar ratios ranging from 1 to 3 produced different GAR resins. For these resins, adsorption preexperiment results are shown in Table 1. It can be observed that the optimal F/P is equal to 2. This ratio has been used for subsequent works. 6053

dx.doi.org/10.1021/ie300437u | Ind. Eng. Chem. Res. 2012, 51, 6052−6063


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Figure 3. FTIR spectra of GA, GAR, and Pd- and Rh-adsorbed GAR.

There are two steps leading to formation of GAR: methylolation and condensation reactions as shown in Figure Table 2. Assignment of Absorption Bands of the GA wave no. (cm−1)

Figure 1. Reaction of GA with formaldehyde: (a) methylation reaction of GA; (b) formation of methylene bridges in GAR; (c) formation of dimethyl ether bridges in GAR; (d) transformation of dimethyl ether bridges to methylene bridges.

Figure 2. Structure of GAR.

GA is soluble in water; the solubility of GAR was measured to achieve resistance under adsorption conditions. Different curing times have been carried out. Preexperiments showed that 3 h was enough time for resistance. The out of plane deformation vibrations of the C−H bond in the benzene rings give absorption bands in the 729−961 cm−1 range (Figure 3, GA). These bands can be used to monitor the degree of crosslinking.46 Disappearance of these peaks in GAR spectra (Figure 3, GAR) indicates insoluble cross-linking has occurred. In this manner, the formation of insoluble cross-linked structure is indicated. Gallic acid consists of an aromatic ring bearing a carboxyl group and three adjacent hydroxyl groups. These carboxyl groups, besides deactivating the molecule toward formaldehyde, limit the amount of methylol formations to two per molecule, which leads to linear molecules, and reaction takes place slowly.17

assignmenta

1695

ν(CO), ν(CC)

1651 1614

ν(CC) ν(CC), dip(C−H), dip(ring)

1539

ν(CC), dip(C−H), ν(C−O), dip(O−H)

1506 1470 1437

ν(CC) ν(CC) ν(CC), dip(C−H), ν(C−O)

1373

ν(CC), dip(C−H), ν(C−O), dip(ring)

1337

dip(O−H), ν(CC), dip(C−H), ν(C−O)

1306

ν(C−O), dip(C−H), dip(O−H), dip(ring), ν(CC)

1240

dip(C−H), ν(C−O), dip(O−H)

1202 1101

dip(O−H) dip(O−H), ν(CC), dip(O−H)

1045

v(CC), v(C−O)

1018

ν(C−O), ν(CC), ν(C−O)

961 866 791 762

dip(ring), ν(C−O) t(CH) t(CH) ν(CC), ν(C−O), dip(ring)

729

t(C−O), dop(C−H), t(CO)

nature carboxylic acid, benzene ring benzene ring benzene ring, benzene ring, benzene benzene ring, benzene ring, phenolic, phenolic benzene ring benzene ring benzene ring, benzene ring, phenolic benzene ring, benzene ring, phenolic, benzene phenolic, benzene ring, benzene ring, phenolic phenolic, benzene ring, phenolic, benzene ring, benzene benzene ring, phenolic, phenolic carboxylic acid phenolic, benzene ring, carboxylic acid benzene ring, carboxylic acid phenolic, benzene ring, carboxylic acid benzene, phenolic benzene ring benzene ring benzene ring, carboxylic acid, benzene carboxylic acid, benzene ring, carboxylic acid

ν = stretching, d = deformation, ip = in plane, op = out of plane, and t = torsion. The contributions to IR intensities and frequencies were written highest to lowest.

a

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Table 3. FTIR Spectral Properties of GAR before and after Sorption Pd(II) and Rh(III) absorption bands (cm−1) IR peak

before sorption

after Pd sorption

after Rh sorption

assignmentsa

1 2 3 4 5 6 7 8 9 10 11 12 13

1607 1557 1514 1505 1454 1416 1298 1217 1109 1024 976 930 874

1613 disappear 1516 disappear 1452 disappear 1304 1217 1109 1045 977 disappear 824

1609 disappear 1514 1505 1454 1418 1296 1217 1109 1024 976 925 833

ν(CC), dip(C−H) benzene ring ν(CC), dip(C−H) benzene ring ν(CO) phenolic ν(CC) benzene ring ν(CC) benzene ring, ν(CO) phenolic ν(CC), dip(C−H) benzene ring, ν(CO) phenolic ν(CO), dip(O−H) phenolic, ν(CC) benzene ring dip(O−H) carboxylic acid rther, dip(O−H) phenolic, dip(O−H) carboxylic acid ν(CO) phenolic, carboxylic acid, and ether dop(C−H) benzene ring ν(CO) carboxylic acid, ν(CO) phenolic t(C−H) benzene ring

a ν = stretching, d = deformation, ip = in plane, op = out of plane, and t = torsion. The contributions to IR intensities and frequencies were written highest to lowest.

Table 4. Influence of pH on Palladium(II) Adsorption and Its Relationship between the Amount of Generated Hydrogen Ions (CoGAR = 49.9 mg/L, 0.2 g of GAR, 20 °C, pCl = 3, V = 50 mL, t = 60 min) pH0

pHlast

adsorplanan Pd(II) (mg/g)

[H]0 (mmol)

[H]final (mmol)

Δ[H] (mmol)

1.98 2.48 3.07 3.55 4.08 4.47 5.00

1.97 2.46 2.98 3.31 3.52 3.60 3.66

8.85 9.86 10.67 11.13 11.48 11.67 11.15

10.47 3.31 0.85 0.28 0.08 0.03 0.01

10.64 3.50 1.05 0.49 0.30 0.25 0.22

0.17 0.19 0.20 0.21 0.22 0.22 0.21

Figure 5. Plots of adsorption capacity vs initial metal concentration (1 g of GAR, 20 °C, pHPd = 4.5, [H+]Rh = 1, pClPd = 3, [Cl−]Rh = 1 M, V = 1000 mL).

confirming GAR’s FTIR spectra. These are mono- or polynuclear hydroxymethyl phenols which are stable at room temperature but are transformed into three-dimensional, crosslinked, insoluble and infusible polymers by the application of heat.44 The GAR structure can be seen from Figure 2. Although we want a linear polymer, if polymerization takes place only at ortho positions, there is a small amount of dimethyl ether bridges that still remain, and this three-dimensional structure will emerge. Although polymerization occurred in an acidic environment, all dimethyl ether bridges transformed to methylene bridges; in this way a linear polymer can be said to be built.18 Due to the insoluble cross-linked structure of GAR formed by the reaction of GA with formaldehyde, a perfect adsorbent property is seen for the adsorption of PGMs and unwanted compounds from aqueous solutions.37,48 3.2. FTIR Spectroscopy Studies. Figure 3 represents the FTIR-ATR normalized spectra of GA, GAR and Pd- and Rhadsorbed GAR. The main bands and their assignments in GA are as follows: stretching vibrations of the aromatic ring ν(C− C)/ν(CC) at 1695, 1651, 1614, 1539, 1506,1470, 1437, 1373, 1045, and 762 cm−1,49 stretching vibrations of the carboxylic acid ν(C−C) at 1695, 1045, 1018, and 762 cm−1,16,50 in plane deformation vibrations of the carboxylic acid dip(O−H) at 1202 and 1101 cm−1,50 torsional vibrations of the carboxylic acid t(C−O), t(CO) at 729 cm−1,40,51 stretching vibrations of the phenolic group ν(C−OH) at 1539, 1437, 1337, 1306, 1240, 1018, and 961 cm−1,40 in plane

Figure 4. Speciation of Pd(II) as a function of pCl (pH = 4.5).

1. The first step, methylolation, is an electrophilic aromatic substitution reaction and, consequently, the products obtained will be substituted in ortho positions (Figure 1a). The second step is a condensation reaction. In this moment, two mechanisms involve a hydroxymethyl group with either a GA forming methylene linkage (Figure 1b) or a hydroxymethyl group forming dimethyl ether linkage (Figure 1c), releasing a water molecule. Although the dimethyl ether linkage transforms the methylene linkage by application of heat (Figure 1d), a little dimethyl ether bridge still remains in the resin because the condensation reaction was deliberately stopped.47 This will be 6055



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polymerization and shifted to 1556 cm−1.53 A peak reflecting ν(C−C)/ν(CC) and dip(C−H) of the benzene at the 1506 cm−1 region disappears, and two small new peaks are composed. The 1470, and 1437 cm−1 peaks at GA are combined to create peaks at 1454 cm−1. A new small peak in the spectra of GAR at 1416 cm−1 mostly represents benzene ring’s ν(CC) and dip(C−H) vibrations. Complete disappearance of the bands at 1373, 1337, 1306, 1240, and 1202 cm−1 peaks at GA are combined to create broad peaks at 1217−1298 cm−1. For GA, a peak reflecting the deformation of the carboxylic acid dip(O−H) group is present at 1101 cm−1, as shown in Figure 3. It can be clearly seen that the intensity of this peak gradually increases and expands, while the peak is slightly shifted to 1109 cm−1 in the GAR resin. The 1045 and 1018 cm−1 peaks at GA are combined to create the peak at 1109 cm−1. This peak mostly represents ν(CO) and dip(O−H) of carboxylic acid groups.53 The 1101 and 1045 cm−1 peaks at GA are combined to create the peak at 1109 cm−1. This peak mostly represents ν(CO) of carboxylic acid and dip(O−H) of phenolic groups.44 The out of plane deformation vibrations of the C−H bond in benzene rings at 961, 866, and 791 cm−1 completely disappeared due to polymerization.53 For the same reason vibrations at 762 and 729 cm−1 disappeared too. The 1539, 1337, 1306, 1240, 1101, and 1018 cm−1 peaks mostly represent dip(O−H) of the phenolic groups.44,53,54 Some of them completely disappear; some of them still remain but combine with other peaks in GAR spectra. Despite the use of phenolic OH groups during the formation of resin, 1454, 1298, and 1109 cm−1 peaks indicate that GAR contains OH groups. These groups can interact with PGM ions, and this is clearly seen in the GAR-Pd and GAR-Rh spectra (Figure 3). The 1217 cm−1 peak mostly represents dip(O−H) of carboxylic acid.52 Also, the formation of dimethyl ether bridges (-CH2−O−CH2-) appeared as a new peak at 1109 cm−1. Since the most characteristic absorption of aliphatic ethers is a strong band in the 1150−1085 cm−1 region due to asymmetrical C− O−C stretching, this band usually occurs near 1125 cm−1.16,40,44,50 Peaks 1045, 1018, and 762 cm−1 in GAR spectra combined to 1024 cm−1 in GAR spectra. This corresponded to single-bond (C−O) stretching vibrations of the -COOH group.50,55 The peak intensities at 1483, 1362, 1287, 1186, and 1156 cm−1 are reduced and shifted to 1505 and 1454 cm−1, creating broad peaks due to the formation of methylene bridges (-CH2-).16,44,51 The out of plane deformation vibrations of the C−H bond in the benzene rings give absorption bands in the 729−961 cm−1 range. This group disappears during the process of polymerization; therefore, these behaviors are attributed to the difficulty of phenolic ring deformation when it is highly crosslinked.46,54,56 The functional groups before and after adsorption on GAR and the corresponding infrared absorption bands are shown in Table 3. The spectra display various absorption peaks, confirming the structure of GAR. These band shifts indicated that the bonded -OH groups play a major role in Pd(II) and Rh(III) sorption on GAR. The changes in 1454, 1298, 1024, and 930 cm−1 wave numbers with the realization of adsorption provide us evidence. As shown in Figure 3 and Table 3, disappearing peaks 1454 and 1298 cm−1 and shifting peaks at 1454, 1298, 1024 to 1452, 1304, and 1045 in the spectra for GAR-Pd are evidence of oxidizing hydroxyl groups of GAR to quinone carbonyl groups.57−59 On the other hand, the 1613 cm−1 wavenumber in the GAR-Pd spectra

Figure 6. SEM micrograph of Pd(II)-adsorbed (a), Rh(III)-adsorbed (b), and natural (c) GAR particles (1000× magnification, 20 kV).

deformation vibrations of the phenolic group dip(O−H) at 1539, 1337, 1306, and 1101 cm−1, and torsional vibrations of benzene t(CH) at 866, and 791 cm−1.40,52 Assigned spectra peaks are presented in Table 2, and the contributions to IR intensities and frequencies were written highest to lowest. Formation of GAR leads to obvious changes in FTIR spectra: The new structure consists of a large molecule that is attenuated by many of the peaks. Due to participate chemical reaction during formation of GAR, the stretching vibrations of benzene group’s carbons corresponding to 1695, 1651, and 1614 cm−1 regions showed decrease and shifted to 1607 cm−1. Note that 1695 cm−1 corresponds to the CO stretching vibration.50 The in plane deformation vibration of the C−H bond in benzene rings at 1539 cm−1 is greatly decreased with 6056



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Figure 7. Energy-dispersive spectroscopic analysis (EDS) of pattern of Pd(II)-adsorbed (a) and Rh(III)-adsorbed (b) GAR (20 kV; scattering angle, 35°; measuring time, 10 s).

the degree of adsorption is caused by reduction of the degree of freedom.16,40 The 1217 and 1109 cm−1 bands in the FTIR spectra of GARPd and GAR-Rh remained after adsorption. Due to the appearance of carboxylic acid’s O−H bond in plane deformation at 1217 cm−1, the band shows that carboxylic acid in the structure did not interact with the PGM metals. From the FTIR results of GAR, the band at 1109 cm−1 was also evidence for the existence of dimethyl ether bridge (−CH2− O−CH2−) moieties.16,40,44 3.3. Adsorption Studies. All Pd(II) and Rh(III) species in Cl− ion containing aqueous solutions do not interact with GAR particles. /The adsorption capacity and adsorption rate of palladium onto the GAR particles depend on the distribution of

represents quinone CO asymmetric and benzene CC symmetric vibrations.60 Figure 8 shows GAR resin Pd(II) interaction. When GAR-Rh spectra are compared to GAR-Pd, band intensities are higher because the Rh(III) ions adsorbed less than Pd(II) ions. Even if shifted, the presence of bands 3, 5, 6, 7, 9, 10, and 12 in Table 2 is also evidence that phenolic structure moieties on GAR still remain.49,53,56 Possible interaction between rhodium chloro aqua complexes and GAR may be surface complex formation, represented in Figure 9. When the spectra of Rh- and Pd-adsorbed GAR were compared with the spectrum of the GAR (Figure 3), in particular to GAR-Pd spectra, many small peak series disappeared. In general it may be argued, proportionally, that 6057



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Figure 8. Adsorption mechanism steps of Pd(II): (a) surface complex formation and (b) oxidation−reduction reactions between PdX2 and adjacent hydroxyl groups of GAR.

Figure 9. Adsorption mechanism of Rh(III) species: (a) [RhCl6]3− and (b) [RhCl5H2O]2− surface complex formation with protonated π carbons of GAR. (c) A side reaction in the bulk solution occurs between Cl− and H2O to form OH− ions. The H+ ion adsorbs competitively.

palladium; the Pd cationic species (PdCl+, Pd2+) and nonionic form (PdCl2) are more favorable than anionic species (PdCl42−, PdCl3−),1,12,59,61 while a low chloride concentration (10−3 M) was especially chosen since the favorable species may occur in this condition.61,62 Table 4 shows the influence of pH on Pd(II) adsorption under the condition of [Cl]total = 0.001 mol/ L. The pH = 4.5 which is the highest adsorption efficiency determined experimentally. Under these conditions, to determine the distribution of Pd species, computer software Hydra and Medusa63 are used. Figure 4 shows that the Cl− concentration is a key parameter for the distribution of Pd species. When [Cl]total = 0.001 mol/dm3 or greater, the

Pd(II) and Rh(III) species because the adsorbabilities of species differ from each other due to their different chemical structure.11 From adsorption studies with condensed tannin and nitrogen functional group containing resins, the optimum pH with the highest adsorption efficiency is around pH 2.1,12,59,61 In another study, Wang et al.35 discussed Pd(II) sorption on hydrolyzed tannin in hydrochloric acid solutions. The optimum pH for the adsorption capacity is pH 5.6, they reported. For two studies made with hydrolyzed tannin by Ma et al.13 and Kim and Nakano,59 this value is pH 3. It was reported that the sorption amount and mechanisms are influenced by the speciation of 6058



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fact that adsorption has peaked out. The adsorption capacity of GAR particles with respect to Pd(II) and Rh(III) were determined as 99.45 and 25.12 mg/g, respectively. Langmiur single-layer adsorption capacity of GAR was found to be 99.62 mg of Pd(II)/g and 69.43 mg of Rh(III)/g (calculations will be in a subsequent paper). This result shows GAR particles can be used efficiently for removal of Pd(II) and Rh(III) when it is compared to those in literature.2,35,61,69 3.4. Instrumental Measurements. A typical SEM microimages of Pd(II)- and Rh(III)-adsorbed powder samples of GAR are shown in Figure 6. These micrographs reveal the homogeneous nature of the surface morphology of the GAR powders which also shows that GAR particles possess a porous structure (Figure 6a). With this porous structure, Pd(III) ions adsorbed to the surface; later on they can move into the interior of the pore. This also increases adsorption capacity. The porous structure of GAR is partially damaged during Rh(III) adsorption in 1 M HCl solution (Figure 6b). This is contrary to the previous situation, decreasing the adsorption capacity. The EDS patterns of Pd(II)- and Rh(III)-adsorbed GAR are given in Figure 7. EDS analysis did not provide us precise information about the metal binding mechanism on the surface, but it also provides us with the ability to make supportive comparisons. However, when two EDS analyses were considered, the weight percentage of Pd was greater than Rh. As the expected result, Rh(III)-adsorption capacity on GAR particles showed less than Pd(II) ions. Another important point to note here is the suggestion to us about the nature of the adsorption mechanism. In the EDS pattern of Pd at Figure 7b, palladium and chlorine occur on the surface at 11 and 0.65%, respectively. This chlorine may only exist as a complex with Pd on the surface of the GAR particles. Therefore, at the end of the adsorption period, only small amounts of palladium exist on the surface as the surface complex formed. Probably, a large amount of palladium reduced to metallic form with the GAR particles. This situation can be proved from the XRD pattern in Figure 11a.12,35,58,59 Furthermore, Na+ ions enter competitively with Pd(II) ions into the adsorption; that can be said. When compared to Pd(II) and Rh(III) EDS spectra (Figure 7), the percentages are respectively 11 and 0.85% of the surface. The percentage of Cl− ions is 1.94 and 0.65%, respectively, too. Despite the adsorption conditions being different, the presence of a very small amount of rhodium against chlorine is explained by Rh(III) complexed with Cl− ions on the GAR surface. This shows the Rh(III)-adsorption stage is surface complex formation. XRD pattern results (Figure 11) will help us to make a final decision on this issue. The specific surface area (BET) of GAR particles was determined as 30.63 m2/g. Measurements were carried out before the adsorption experiments. The preliminary investigation of the resin particles by the N2 sorption isotherm data obtained at the temperature of liquid nitrogen using an automated physisorption instrument (Micromeritics Flow Sorb 2300, automatic analyzer) showed that particles may resolutely refer to macroporous material with a macropore diameter of 100 nm. 3.5. Adsorpstion Mechanism. The schematic diagrams shown in Figures 8 and 9 illustrate the adsorption mechanisms of Pd(II) and Rh(III), respectively. According to Figure 8, the adsorption of Pd(II) follows two steps; first, surface complex formation takes place between the GAR particles and chloro palladium species (Figure 8a). Second, while palladium

Figure 10. Diagram illustrating the interaction of [RhCl6]3− (or [RhCl5(H2O)]2−) with oxygen functional groups of GAR. A side reaction in the bulk solution occurs between Cl− and H2O to form OH− ions. The H+ ion adsorbs competitively.

predominant Pd species are PdCl2 and Pd(OH)2. Both of these occupy 45% of [Pd]total, which means that nonionic Pd species predominate in the aqueous solution rather than anionic species. It was difficult to estimate the accurate adsorption amount only by GAR particles with initial pH 4.5, because some of the Pd species precipitates remained in aqueous solution and not on GAR particles. To reduce this, 1 M HNO3 containing palladium stock solution was set to pH 4.5 as the experimental condition. Adsorbent was added immediately. Thus, before the deposition of insoluble palladium species is aggregated, adsorption begins. It is evident that protons formed during the adsorption will lower the pH of the medium again. However, decreasing the amount of palladium in the solution must be considered. When all of these are considered, insoluble palladium species will begin to transform into soluble species. The hydrolysis of palladium was observed experimentally at the initial pH 4.5, by blank test performed without GAR particles. The results suggest that the adsorption mechanism is not homogeneous in this initial pH. It was also observed that the pH of the solution was reduced during the adsorption of Pd(II) onto GAR particles (Table 4). This change is attributed to the hydrogen ions released from the poly(hydroxyphenol) groups of GAR. When Rh(III) ions are concerned, the situation is a little more complicated. Water molecules participate to the complex structure over time, and this becomes more than palladium complexes.64 And with relatively large rhodium aqua rhodade complexes, adsorption is made harder. To avoid this negative influence, the experiments can be made at the higher pH.2,65,66 In addition, to eliminate the water molecule’s effect, Cl− ions should be used. In this way, relatively small complexes should be involved. Consider, at the higher HCl concentrations, GAR particles beginning to dissolve. Moreover, too high HCl concentrations are changing GAR crystallinity and thus reduce the accessibility because the acid changes the shape of the adsorption internal sites.67 Given all of these, the most appropriate media for Rh(III) adsorption is aqueous solution containing 1 M HCl. Adsorption experiments were performed in 1 M HCl containing solutions. In these conditions, Rh(III) exists under [RhCl6]3−/[RhCl5H2O]2− > 1 concentration ratio.66,68 Consequently, Rh(III) is the most difficult adsorbent in platinum group metals.2,64 Figure 5 shows adsorption capacities of Pd(II) and Rh(III) ions at various initial metal concentrations. In all cases the plots form a plateau at higher metal concentrations, suggestive of the 6059



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Figure 11. XRD pattern of (a) palladium- and (b) rhodium-adsorbed GAR particles. Vertical lines represent the XRD peaks of metallic palladium (Pd0).

complex reduces to the metallic form (Pd0), hydroxyl groups of GAR oxidized to quinone carbonyl groups (Figure 8b). The generation of Pd0 on the GAR after the adsorption was elucidated by XRD diffractogram shown in Figure 11a. The five peaks at 2θ = 40.3°, 46.7°, 68.3°, 82.1°, and 86.6°, which definitely belong to elemental crystalline palladium, verify the reduction of Pd(II) to Pd0. This suggests Pd(II) underwent subsequent reduction after being adsorbed onto the GAR surface.12,35,58,59 As mentioned above, with disappearing peaks 1454 and 1298 cm−1 and shifting peaks at 1454, 1298, 1024− 1452, 1304, and 1045 in Figure 3 GAR-Pd spectra may also explain the oxidation hydroxyl groups of GAR to quinone

carbonyl groups. Note that, under this study's conditions, the predominant palladium species are PdCl2 and Pd(OH)2. As shown in Table 4, pH decreases due to the hydrogen ions released from GAR. The amount of generated hydrogen ions was plotted against the adsorbed amount of palladium between the initial pH 2 and pH 5, as shown in Figure 12. The slope elucidates that the release of two hydrogen ions results in the reduction of one Pd(II) through two electron transfer from GAR to Pd species. The concentration of proton having a gradient slope from the outer to the inner has been true in general. Due to the entire surface not being covered with 6060



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the XRD pattern in Figure 11 shows an amorphous structure which proves Rh(III) complexes have not reduced to its metallic form.12,35,58,59

4. CONCLUSIONS GA−formaldehyde resol type condensation has been studied. GAR characterization was done by using FTIR and SEM instruments. GAR particles, which are obtained by the condensation reaction between formaldehyde and PG, do not dissolve in water, whereas GA dissolves in water. To represent PGM, two metals bearing different chemical properties, Pd(II) and Rh(III), were used. The adsorption capacity of GAR particles with respect to Pd(II) and Rh(III) were determined as 99.45 and 25.12 mg/g, respectively. For the recovery of PGM, the results obtained herein show that GAR particles can be used as an effective adsorbent from aqueous solution. The adsorption of Pd(II) and Rh(III) species onto GAR can occur via different steps and mechanisms. It was found that Pd(II) was adsorbed onto the GAR particles as a reduced metallic Pd through redox reaction mechanism, chloropalladium(II) species were reduced to Pd(0), while hydroxyl groups of GAR were oxidized during the adsorption. The adsorption process can be divided into two steps: fast adsorption by the ligand substitution at the initial stage and slow adsorption by the subsequent redox reaction after the ligand substitution reaches an equilibrium state, with different adsorption rates between the Pd(II) ionic species (PdCl2 (or Pd(OH)2) > PdCl3− > PdCl42−). GAR particles were used as adsorbent for the removal of Rh(III) ions from 1 M HCl solutions. Rh(III) species [RhCl5H2O]2− and [RhCl2(H2O)4]− are adsorbed onto a polyphenol resin surface. Two-site surface exchange and complexation as possible adsorption mechanisms were proposed and discussed; meanwhile reduction of Rh(III) species to metallic form did not occur under these conditions. Obtained GAR was characterized by FTIR spectroscopy before and after adsorption of rhodium metal ions. Ideas are supported by the EDX, XRD, and FTIR spectra and sith stoichiometric evidence. Adsorption kinetic, equilibrium isotherm, thermodynamics, and system design can be examined in subsequent studies. The results suggest that GAR particles are very useful as an adsorbent in a novel recovery system for PGMs. For the recovery of Pd(II) and Rh(III), GAR adsorbent is quite an effective way to achieve a series of unit operations (extraction, reduction, adsorption, and solid−liquid separation) simultaneously onto the GAR, and there is little secondary waste (nonadditives for reduction and precipitation). Therefore, this system shows good promise as a recovery method for PGMs. It is expected that not only the efficient Pd(II) and Rh(III) adsorption but also the separation from the mixture of metal ions will be possible using GAR particles by controlling the solution composition (pH, pCl, ionic strength, and distribution of PGM species).

Figure 12. Relationship between the amount of released hydrogen ions from GAR particles and the amount of Pd(II) adsorbed onto GAR (CoGAR = 49.9 mg/L, 0.2 g of resin, 20 °C, pH = 4.5, pCl = 3, V = 50 mL, t = 60 min).

palladium at high concentrations, it can be presumed that release of a proton gradient was not observed. At the initial stage, although a contributory effect of the COOH group of GAR to adsorption, the main reason for the adsorption capacity is reduction of palladium(II) with the polyphenolic structure of GAR. When forming quinone, protons separated from GAR into solution as a strong acid. However, the COOH group of GAR is in a weak acidic nature. These mentioned results support the proposed mechanism according to Figure 8. As shown in Figure 9, the adsorption of Rh(III) follows the ionization and adsorption model. When there is an absence of the phenolic and acidic groups, the π sites in the carbons of the phenol play an important role in [RhCl6]3− ions (or [RhCl5H2O]2−) adsorption on the GAR surface. According to this model, when the solution was used at a higher pH, a large amount of H3O+ was first adsorbed in the π sites.69 These sites could be the π sites, which are capable of acting as the electron donors (Lewis-type acid) to form a coordination bond with anionic aqua chloro complexes of Rh(III) in which they act as a Lewis-type base. Subsequently, there is a slow intramolecular transformation of the latter to an inner sphere (chelate) complex. Tthe slow intramolecular conversion reaction (the postulated rate-limiting step) apparently occurs away from the interface.71 At formation of the GAR stage, carboxyl groups in GA molecules limit the amount of methylol formations as two per molecule, which leads to linear resin molecules.17 Due to carbons of the phenol ring oxygen, bonded groups to carboxyl, dimethyl ether, and methylene, a relatively small amount of free π site carbons located at the GAR molecules,72 the metal complex attached decreasingly to the free π site carbons of the surface through an electrostatically held proton. Considering this, as another mechanism, rhodium species may be coordinates with two OH units in PG units of PGNR to form quinons.73,74 In aqueous solution, the hydroxyl groups behave as the amphoteric species being dissociated or protonated depending on the pH value of the solution.75 According to Figure 10, with high acid concentration such as 1 M HCl, protonated hydroxyl groups of GAR molecules interact with anionic rhodium species.70 This suggests that adsorption can be engaging these positive sites. Use of a two-site model for the surface has indicated that this approach does provide efficient capacity and fast kinetics on the adsorption of anionic Rh(III) species by GAR particles. After the realized adsorption,

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*Tel.: 0264 295 60 41. Mobile phone: 0505 253 21 00. Fax: 0264 295 59 50. E-mail: [email protected]. Notes

The authors declare the following competing financial interest(s):This study was supported by the Scientific Research 6061



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Projects Commission of Sakarya University (Project number: 2009 50 02 005).

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ACKNOWLEDGMENTS This study was supported by the Scientific Research Projects Commission of Sakarya University (Project No. 2009 50 02 005).

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