Palladium(II) Chloride Complex Ion Recovery from Aqueous Solutions

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Palladium(II) Chloride Complex Ion Recovery from Aqueous Solutions Using Adsorption on Activated Carbon Marek Wojnicki,*,† Robert P. Socha,‡ Zbigniew Pędzich,§ Krzysztof Mech,∥ Tomasz Tokarski,∥ and Krzysztof Fitzner† †

AGH University of Science and Technology, Faculty of Non-Ferrous Metals, Mickiewicz Av. 30, 30-059 Krakow, Poland Institute of Catalysis and Surface Chemistry, Polish Academy of Science, 8 Niezapominajek Str., 30-239 Krakow, Poland § AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Mickiewicza 30 Av., 30-059 Krakow, Poland ∥ AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, A. Mickiewicza 30 Av., 30-059 Krakow, Poland ‡

S Supporting Information *

ABSTRACT: The process of palladium(II) chloride complex ion adsorption on activated carbon was investigated. The influence of variable parameters such as the initial concentration of the Pd(II) complex (from 0.04 to 2.25 mmol·kg−1) and temperature (from 294 to 348 K) was determined. Experiments were conducted in the solution with pH equal to 0 and 1. It was found that the Freundlich adsorption isotherm describes the process better than the Langmuir model. The observed sorption capacity as high as q294K = 42.4 mg·g−1 and q323K = 67 mg·g−1 for activated carbon was found at 294 and 323 K, and it is increasing with temperature increase. The obtained results showed that the palladium(II) complex may react with functional groups present on the surface of activated carbon. The thermodynamic parameters such as the equilibrium constant and heat and entropy of adsorption were derived. The enthalpy value is equal to −6.7 kJ·mol−1, and the entropy is equal to −23 J·mol−1·K−1. It is suggested that, during chemisorption of palladium(II) chloride complex ions, the palladium dichloro-oxide or PdCl2 is formed on the surface of activated carbon. solutions, e.g., palladium has a 2+ oxidation state, which is very stable and can be easily adsorbed by activated carbon. In the case of acidic solutions containing chloride ions, palladium forms chloride complexes. The stability constant as well as redox potential for palladium(II) chloride complexes can be found in the literature24,25,27 and can be used for the calculation of a speciation diagram. Results of these calculations are shown in the Supporting Information (Table S1 and Figure S1). It is clear that Pd(II) chloride complexes are less stable in comparison with cyanide complexes. There are several reports which indicate that, during the adsorption process on activated carbon, palladium(II) is reduced to the metallic form.15,16,26,28 Consequently, we studied the conditions under which such a reduction reaction was possible. The aim of this work is to investigate and describe the conditions of Pd(II) ion recovery using adsorption on activated carbon as the sorbent, and to find out if this carbon is also a reductant of Pd(II) complexes. The reason for this search is the fact that KGHM Polska Miedź is the only platinum metal group producer in Poland. In this company, the existing technology is based on high pressure leaching of slime (obtained during electrorefining of copper),29

1. INTRODUCTION It is well-known that activated carbon is able to adsorb heavy metals from aqueous solutions.1−5 As an example, the removal of Cd(II),6−8 Hg(II),9−14 and Pb(II)6 ions by adsorption on activated carbon, from the wastewater, can be given. However, there are also precious metals such as platinum,1 palladium,15,16 and gold,17,18 which can be extracted in this way, even if their concentration in the solution is very low. Thanks to that, it is possible to recover those metals from diluted aqueous solutions which are usually produced during the recycling processes. Of course, other methods of recovery of those metals are known, e.g., reduction,19 ion exchange,20−23 and electrodeposition.24,25 Wołowicz and Hubicki showed that the adsorption process of Pd(II) onto strongly basic ion exchange resins in the chloride HCl−NaCl solutions can be successfully carried out and Lewatit MP-500 is a promising resin to carry out Pd(II) complex adsorption.26 The same authors23 demonstrated that some of the weakly basic ion exchange resins exhibit high sorption capacity. They found that this process can be fast and can be described by the Langmuir isotherm. However, those methods are appropriate rather for the systems where the recovered metal concentration is relatively high. The mechanism of the adsorption process depends on initial conditions such as the pH of the solution, the temperature, and the form of adsorbed metal ions. In the case of cyanide © XXXX American Chemical Society

Received: October 10, 2017 Accepted: January 24, 2018

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DOI: 10.1021/acs.jced.7b00885 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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total volume of the solution was equal to 200 cm3. Depending on the Pd(II) concentration, the sample of about 0.8−3.5 cm3 of the solution was withdrawn using an automatic pipet and was analyzed spectrophotometrically (Shimadzu model PC 2501, Japan) to detect changes of palladium(II) chloride ion concentration. The volume of the sample depends on the size (optical path length) of the quartz cuvette. Before the UV− vis analysis, the sample was introduced into a thermostat which is a part of the spectrophotometer to attain a constant temperature of 298 K. After the analysis, the sample was returned to the cyclic reactor to maintain a constant volume of the reagents. Experiments were carried out in two variants with and without mixing, and in the temperature range from 294 to 348 K. The XPS studies were carried out in the ultrahigh vacuum (UHV) (3 × 10−10 mbar) system equipped with a hemispherical analyzer (SES R4000, Gammadata Scienta, Sweden). The Mg Kα source of incident energy of 1256.6 eV was applied to generate core excitation. The spectrometer was calibrated according to ISO 15472:2001. The energy resolution of the system, measured at the full width at half-maximum (fwhm) for the Ag 3d5/2 excitation line, was 0.9 eV. Before the analysis, samples of activated carbon were powdered using an agate mortar and pestle. Then, the obtained powder samples were pressed into indium foil. The analysis area of such a prepared sample was about 3 mm2. No gas release and no change in the sample composition were observed during the measurements. The CasaXPS 2.3.12 software was applied for the analysis of the XPS spectra. No charging was observed for the studied catalysts; therefore, no additional calibration of the spectra energy scale was applied. In the spectra, the background was approximated by a Shirley profile. The spectra deconvolution into a minimum number of components was done by the application of the Voigt-type line shapes (70:30 Gaussian/ Lorentzian product). The analytic depth of the XPS method was estimated as 10.2 nm. The calculations were performed with QUASES-IMFPTPP2M Ver 2.2 software according to the algorithm proposed by Tanuma et al.30 This estimation takes into account 95% of photoelectrons escaping from the surface. The experimental error of the XPS analysis is approximately ±3%. SEM analyses were carried out with the use of a Hitachi SU70 (Thermo Scientific) microscope (equipped with EDS, WDS, and EBSD add-on device). The ash content was determined by burning the carbon sample. For this purpose, ceramic crucibles were used. In the first step, the ceramic crucible was washed and dried for 12 h at a temperature of 393 K and then the temperature was elevated to 1093 K for the next 6 h for the crucible to obtain its constant weight. After that, the ceramic crucibles were cooled down in the desiccator to prevent moisture adsorption. The AC samples of ca. 2 g were burned at a temperature of 1093 K for 12 h. After this time, the samples were cooled down in the desiccator. It was found that the ash content is equal to 8.39 ± 0.09% of the total mass. The specific surface area of the carbon was determined using the BET method and using nitrogen gas (Quantachrome, Nova 1200). It was assumed that a single nitrogen molecule can cover 0.16 nm2 of the substrate area. The distribution of pore size as well as the surface area of the material was measured using a mercury porosimeter (Quantachrome, model P60).

with the use of hydrochloric acid and chlorine gas. In the next step of this process, palladium is recovered from the solution using sodium formate as a reducing agent. Nevertheless, some metal losses in the total balance of the process are observed. The proposed adsorption process is a potential method enabling reduction of metal losses. It should also be mentioned that activated carbon is much cheaper than ion exchangers and due to this fact it can be more attractive for the recovery process. During the studies, the influence of temperature and initial concentration of Pd(II) were investigated. For this purpose, the activated carbon from Norit, type GF40, was used. This choice was dictated by the physicochemical properties of this activated carbon as well as its price. First of all, this carbon is commercially available in the form of pellets, making it much easier to separate from the solution. To produce this activated carbon, coconut shells are used. Furthermore, the pellets have high mechanical resistance and expanded surface area (above 1431 m2·g−1). Finally, thanks to chemical treatment during the manufacturing process (activation using steam and ortophosphoric acid), this activated carbon has a significant surface concentration of functional groups (about 18 mmol·g−1)1 which can act like a reductant or ion exchanger.

2. EXPERIMENTAL SECTION In all experiments, commercially available activated carbon (GF40 from Norit) in nonmodified form was used. Palladium(II) chloride complex was obtained by dissolution of palladium(II) chloride (Avantor Performance Materials Poland, analytical purity) in hydrochloric acid (Avantor Performance Materials Poland, 37%, analytical purity). The amount of hydrochloric acid required to dissolve palladium(II) chloride and palladium(II) chloride complex ion formation was calculated according to the following reaction: PdCl 2 + 2HCl ⇄ H 2PdCl4

(1)

Palladium(II) chloride itself is insoluble in water. For that reason, an excess of hydrochloric acid was used. Its addition also adjusted the proper pH in the system. As was mentioned in the Introduction, the Pd(II) complexes may hydrolyze. However, in the conditions studied, only one form of the Pd(II) complexes, i.e., PdCl42−, is stable (see Supporting Information Figure S1). Finally, the stock solution was obtained with the concentration of Pd(II) equal to 0.1127 mol·kg−1 and pH 1. All other solutions used in the experiments were obtained by dissolving the stock solution. To maintain a constant pH, the 0.1 mol·kg−1 hydrochloric acid was used. To investigate the stripping process, additional standard solutions were prepared. For this purpose, 1 g of palladium(II) chloride (99.999%, Alfa Aesar) was dissolved in 1 dm3 of 1 mol· kg−1 HClO4 (70%, A.P., Avantor Performance Poland). The finally obtained solution was stored protected against the light. The measurements of the changes of palladium(II) chloride complex ion concentration due to the adsorption onto activated carbon were carried out in the thermostated cyclic glassy reactor which was immersed in the water bath kept at constant temperatures (±0.2 °C). The glass reactor was sealed to prevent solution evaporation. In this system, the temperature can be controlled from room temperature up to 95 °C. After the constant temperature in the system was reached, a suitable amount of activated carbon was introduced into the aqueous solution containing palladium(II) chloride complex ions. The B



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or 223 nm) were recorded. According to Lambert−Beer’s law, these changes are proportional to the concentration of Pd(II) chloride complex ions. Absorption coefficients ε determined for the Pd(II) complex were found to be 208.2 ± 5.2, 8113.4 ± 59.1, and 24338.7 ± 764.8 cm−1·mol−1·kg for the wavelengths 447, 279, and 223 nm, respectively (see the Supporting Information). Next, the influence of the initial concentration of Pd(II) chloride complex ions on the equilibrium concentration in the solution after activated carbon addition was investigated. According to Lambert−Beer’s law, the decrease of the solution absorbance at these wavelengths is proportional to the decrease of Pd(II) concentration in the solution. We assumed that this decrease corresponds to the amount of palladium adsorbed on the surface of AC. From the calculated change of concentration of Pd(II) with time, kinetic curves for the deposition process were derived. Registered kinetic curves, [PdCl42−] vs time, are shown in Figure 2. It can be observed that the concentration of PdCl42− in the solution is approaching the constant value, which is different from zero. This observation is the most important in this case, since it indicates the equilibrium distribution of the Pd(II) chloride complex between the solution and the activated carbon. The existence of such an equilibrium in the system may provide the information about the thermodynamics of the adsorption process. The experiments with six different initial concentrations of palladium(II) chloride complex ions were carried out at two different temperatures: 294 and 323 K. Those experiments were repeated twice. However, due to time-consuming experiments, at 323 K, only initial and final concentrations were determined. Moreover, to increase the rate of the process, constant mixing of the solution was applied. This mixing should shorten the time required to obtain equilibrium, but it should not affect the equilibrium concentration itself. Then, from the obtained results, Freundlich as well as Langmuir isotherms were derived. In our case, the Freundlich isotherm can be expressed by the following equation

XRD analysis was performed using a Rigaku Miniflex II desctop X-ray diffractometer. Registered XRD data were analyzed using PDXL software v. 1.7.0.0 (Rigaku). Each sample taken for XRD analysis was dried at 60 °C for 24 h. Then, the pellets of activated carbon were triturated using an agate mortar and pestle. Finally, the obtained powder was analyzed using a rotating sample holder.

3. RESULTS 3.1. Characterization of Activated Carbon. At first, physical as well as chemical properties of the activated carbon were analyzed. The presence of the surface functional groups on it was analyzed using the Boehem method. The experimental data as well as the methodology can be found in our previous paper.1 It was shown that activated carbon contains a significant number of acidic functional groups (about 18 mmol·g−1), while about 78% of them are the carbonylic functional groups. It is known that the adsorption ability of activated carbon is related to its surface as well as to its porosity.31 In the case of the studied activated carbon, two different methods were applied to investigate the material’s structure. The first one is commonly called the BET method after the names of this method’s authors (Brunauer−Emmett−Teller). In this method, the isotherm of nitrogen adsorption is determined. From the obtained parameters of this isotherm and assumed surface area occupied by the nitrogen molecule equal to 0.162 nm2,31 the specific surface area is calculated. In the studied case, it is equal to 1431 m2·g−1. The second one is based on mercury porosimetry (Quantachrome, USA, model P60). Figure 1

V x = = βcr1/ p Vm m

(2)

where cr is the equilibrium concentration of Pd(II), c0 is the initial concentration of adsorbed substance, x is the amount of m Pd(II) adsorbed on the absorbent, x can be calculated as x = c0 − cr, m is the mass of the adsorbent, and β are p are constants dependent on temperature as well as adsorbed component. Next, it is possible to transform eq 2 to the logarithmic form.

Figure 1. Pore volume distribution as a function of pore diameter in Norit GF 40.

( mx ) vs log(c ), it is possible

Thanks to that, from the graph log

r

to determine the Freundlich adsorption capacity as well as the “n” parameter.

shows the obtained pore size distribution diagram with the use of this technique. The obtained results indicate that the material can be qualified as microporous. The main fraction of the pore diameter was in the range 1−10 μm, but a wide distribution of pore diameter up to 4 nm was detected. The porosity of carbon was found to be equal to 31 vol %. These results suggest that the studied activated carbon may exhibit excellent adsorption properties. It is well-known that the pore structure, as well as the surface area, has a significant influence on adsorption efficiency.32,33 3.2. Influence of Pd(II) Initial Concentration on the Equilibrium in the System. In the next part of our work, the changes of the absorbance of the highest intensity (either 279

⎛x⎞ 1 log⎜ ⎟ = log(β) + log(cr ) ⎝m⎠ p

(3)

Contrary to the Freundlich isotherm, the Langmuir isotherm is based on the theoretical model. Langmuir assumed34 that there is an equilibrium between active sites on the adsorbent surface and the adsorbed substance, and the maximum adsorption capacity corresponds to the monolayer of adsorbed substance. We found that, in our case, the Langmuir isotherm does not fit to experimental data well. C



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Figure 2. Influence of the initial concentration of Pd(II) chloride complex ion initial concentration on equilibrium concentration. Experimental conditions: pH 1, Cl− = 0.1 mol·kg−1.

Consequently, eq 3 was used to describe our experimental data (Figure 3). The Freundlich isotherm describes the analyzed case very well. The determined Freundlich isotherm parameters for two different temperatures are gathered in Table 1. It can be seen that in our case the temperature has a slight influence on Freundlich adsorption (β) and “1/p” parameters. In Freundlich’s original work, the 1/p parameter is defined as a constant dependent on the temperature and type of adsorbed substance. From the obtained experimental data, the distribution of [PdCl42−] between the solution and the carbon surface can be derived. The adsorption capacity can be defined as follows26

qT =

(Co − Cr ) ·V m

(4)

where V is the volume of the system in this case equal to 300 mL and qT denotes the adsorption capacity at constant temperature. In our case, q294K = 42.4 mg·g−1 and q323K = 67 mg·g−1. 3.3. XPS Studies. The full XPS spectrum of the material after and before the adsorption process is shown in Figure 4. Taking into account the observed decrease of Pd(II) concentration in the solution, the Pd load was estimated to be equal to 0.7% (m/m). It can be seen that the peak coming from palladium appears. Deeper analysis of this peak may give us information about the D



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Figure 5. XPS analysis of the Pd3d signal.

Figure 3. Freundlich isotherm determined for two different temperatures.

Table 2. Results of XPS Analysis of Pd3d Table 1. Freundlich Isotherm Parameters temperature (K)

β

1/p

R2

symbol

294 323

0.035 ± 0.033 0.062 ± 0.02

0.49 ± 0.03 0.53 ± 0.01

0.98 0.99

A B

form of Pd species PdO or Pd nano on strongly electrophilic substrate PdO3 or PdCl2

area under the peak

fraction of the form (%)

16.4

71

6.6

29

XRD analyses were performed in the angle range from 10 to 90°. It can be suggested that the registered XRD pattern shown in Figure 6 corresponds to PdCl2 and perhaps another form of Pd

Figure 4. Spectra of AC before and after adsorption of Pd(II), [PdCl42−]0 = 5 × 10−4 mol·kg−1, [C] = 1.67 g·dm−3, pH 1, T = 323 K.

electronic structure of Pd. The obtained results are shown in Figure 5 and Table 2. Two peaks are observed in the energy range from 334 to 348 eV. This suggests that palladium, after adsorption, exists in at least two different forms. It cannot be excluded that some part of Pd(II) is reduced to the metallic form. Small amounts of palladium can exist in the form of halogens and oxides. Unfortunately, in the case of palladium, several chemical compounds have a similar binding energy. For that reason, it is difficult to identify them unambiguously. To explain this uncertainty, further investigations and analysis of the obtained material were performed. In the case of the existence of PdCl2 on the surface of activated carbon, there should be a simple method for palladium(II) stripping according to eq 1. 3.4. XRD Analysis of the Sample. The sample after adsorption was dried at a temperature of 60 °C to prevent thermal decomposition of the product. After that, the pellets of AC were pulverized using an agate mortar and pestle. Then, the obtained dust was introduced into the rotating sample holder.

Figure 6. XRD analysis of activated carbon Norit GF40, after (black) and before (red) the adsorption process.

such as Pd2OCl2. The amount of the metallic form of Pd seems to be extremely low and in XRD analysis is not observed. This observation seems to be in good agreement with Simonov et al.28 They have shown that the metallic phase content at the surface of activated carbon is up to 1.13%, where for some experimental conditions this amount was down to 0.09%. One may assume that metallic Pd does not exist on the carbon surface. 3.5. SEM Analysis. Using a scanning microscope (SU-70), analyses of the sample were performed. The sample of activated carbon after an adsorption trial was dried out at 60 °C. Then, the pellet was divided into two and the obtained face surface was polished using sand paper (granulation 1000) until the surface was flat. After that, the EDS analyses were performed. E



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Figure 7. (A) SEM analysis of activated carbon Norit GF 40, (B) magnification of selected area, (C and D) EDS cross section analysis after adsorption of palladium(II) chloride complex ions.

Figure 8. Linear analysis of the cross section of the pellet after the adsorption process.

total surface of the activated carbon determined by the BET method. In its final form, it can be in the form of either Pd2OCl2 or PdCl2 species, which can occur simultaneously on the surface. However, from our analysis, it is clear that the metallic phase of Pd is absent. Moreover, due to the similarity of the position of PdCl2 and Pd2OCl2 peaks on XRD, it is very difficult to make sure which compound really exists on the surface. It can only be concluded that it is not pure Pd but its compound which is formed on the carbon surface. 3.6. Palladium(II) Stripping from AC Surface. In the case of the existence of PdCl2 on the surface of AC, it should be possible to desorb it according to eq 1. Consequently, desorption tests were performed at room temperature (295 K), using 100 cm3 of hydrochloric acid (concentration 1 mol· kg−1) or 100 cm3 of perchlorate acid (concentration 1 mol· kg−1). It has to be mentioned that adsorption experiments were conducted at pH equal to 1, while the stripping process was conducted at pH equal to 0. The sample of loaded AC by Pd(II) was taken from a previous experiment of adsorption.

The acceleration voltage was equal to 20 kV. The obtained results are shown in Figure 7. The color intensity is proportional to the analyzed element concentration (see Figure 7C and D). In Figure 8, the plot of element concentration vs radius of a pellet is shown. It should be noted that the origin of the coordinates corresponds to the middle of the cross section of the pellet. It is demonstrated that the concentration of palladium is similar in the whole range of the cross section. Moreover, it can be observed that, in the case of some fluctuations of palladium concentration, the same fluctuations can be seen for the chloride concentration dependence. This in turn suggests that those two elements are in some way connected to each other. This observation seems to support the results obtained from XRD. Unfortunately, in the case of EDS analysis, the intensity is not directly proportional to element concentration, so the conclusion related to the stoichiometry cannot be drawn. Taking this into account, it can only be stated that the adsorption process results in palladium(II) adsorption on the F



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The preloaded AC was separated from the solution by filtration, and then, it was dried at 333 K. Next, dry preloaded AC was mixed with hydrochloric acid solution. A desorption test was carried out for about 10 months. During the first 2 days, fast increase of the Pd(II) concentration in the solution was observed, to finally obtain constant values after 12 days. These results are shown in Figure 9 as black squares. To make sure

Figure 10. Palladium desorption using 1 mol·kg−1 HClO4.

carbon matrix, the second one to strong interactions, and the third one to very strong interactions (irreversible adsorption). From this experiment, it is clear that, as the result of adsorption, a soluble palladium(II) compound is formed on the surface, while metallic palladium is absent. The results of the elution experiment suggest that at least a part of the deposit is PdCl2, since PdO is insoluble in acid. For the same reason, the other part of the deposit can be Pd2OCl2; however, this compound is not included in the XPS database, and it is impossible to be verified. The only existing data is the XRD database. Summarizing, the palladium can be removed from the AC simply by changes of pH of the solution; an additional experiment was performed, where the stripping process was performed using 100 cm3 of 1 mol·kg−1 HClO4. As can be seen, there is a desorption process; however, the UV−vis spectrum of the stripped species is far away from the spectrum of PdCl42− as well as for species synthesized as the reference (see Supporting Information Figure S2C). This suggests that the stripping process is not an opposite reaction to eq 8 but a new reaction path. 3.7. Thermodynamics of Adsorption. Since it is known that, in the solution, PdCl42− ions are present, the process of adsorption must be initiated by the contact of these ions with the carbon surface. Therefore, taking into account the results of the analysis given above, we suggest that the adsorption process takes place as follows:

Figure 9. Efficiency of the desorption process.

that this is really the final equilibrium, the samples were left in the solution for the next 10 months. The values obtained after this time are shown as red dots. The excess of hydrochloric acid is significant, and it should be sufficient to dissolve PdCl2. The results demonstrate that the state of equilibrium is not affected by time, suggesting that the observed state is the final one. It seems that during this experiment only about 50−65% of Pd(II) species can be eluted from the AC surface. As was mentioned above, an excess of hydrochloric acid was applied. Therefore, on the one hand, incomplete elution can be explained by the formation of Pd2OCl2 or another Pd(II) compound on the surface of activated carbon. On the other hand, it may be simply related to the occurrence of a new equilibrium state different from the initial one. To cut off speculations, a simple second desorption test was performed, in which carbon from the first desorption test was used. The experimental conditions and procedure were the same as those during the first desorption test. It was observed that elution of Pd(II) is taking place again. After the first 24 h, up to 17% of the remaining Pd(II) was desorbed, while, after 2 months, the overall desorption efficiency reached a level of 98.4%. An additional experiment was also performed where, to remove adsorbed Pd from the surface of AC, 1 M HClO4 was used. This choice is related to the fact that it is strong acid. Moreover, it is known that perchlorate generally is not forming complexes with transition metals. The obtained results have shown that it is possible to remove adsorbed palladium also using this acid. However, due to insufficiency of chloride ions, aqua complexes of Pd(II) are formed. The registered spectrum as well as standard solutions are shown in Figure 10. This in turn suggests that there is an ion exchanging mechanism. Wieckowski et al.35 have widely described and discussed the interaction between carbon and PdCl2. They have divided types of interactions of PdCl2 with carbon into three groups. The first one is related to weak interactions between Pd(II) and the

PdCl4 2 − + Corg ⇄ PdCl4 2 −@Corg → products

(5)

In this reaction scheme, the first step corresponds to adsorption of complex species on the surface, while the second one, to the irreversible transformation of the adsorbed complex to the solid products. Since Corg corresponding to active functional groups at the surface exists on both sides of the adsorption equilibrium, reaction 5 can be simplified to the final form PdCl4 2 − ⇄ PdCl4 2 −ads → products

(6)

For the first step of reaction 6, the equilibrium concentration (see Supporting Information eq S6) was used to calculate the apparent equilibrium constant, K*. Details of these calculations are given in the Supporting Information (section “Thermodynamics of Adsorption”). The apparent equilibrium constant was defined in this study as the ratio G


Journal of Chemical & Engineering Data K* =

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[PdCl4 2 −]ads 2−

[PdCl4 ]soln

temperature. However, it should be noted that this result corresponds only to the first step of reaction 6, which we called the adsorption step. If the second step, which corresponds to the transformation of the adsorbed species into a stable product, exhibits a negative Gibbs free energy change, the overall ΔG0,* change for reaction 6 may be negative, and then, the overall process is spontaneous. It may also mean that with increasing temperature fewer Pd(II) chloride complexes remain on the carbon surface in the adsorbed form, while a more stable product in the PbCl2 form appears. From the results obtained by fitting eq 9 to the experimental data shown in Figure 11, the ΔS0,* as well as ΔH0,* values were determined. The enthalpy is equal to −6.7 kJ·mol−1 and the entropy is equal to −23 J·mol−1·K−1, respectively. The negative value of the ΔH0,* indicates that the studied process is exothermic and the product is energetically stable.5 The negative value of ΔS0,* can be related to the decrease in the degrees of freedom of the system due to the immobilization of the adsorbed species. With this in mind, it may be suggested that the positive Gibbs free energy for the first step is related to the formation of the intermediate state, which can only be observed thanks to the slow rate of the whole process.

(7)

[PdCl42−]soln

where is the concentration of palladium(II) chloride complex ions in the solution determined spectrophotometricaly, after achieving an equilibrium state in the system, and [PdCl42−]ads is calculated form the mass balance. Consequently, the Gibbs free energy change ΔG0,* for reaction 6 was calculated as ΔG 0, * = −R ·T ·ln(K *) = ΔH *,0 − T ·ΔS*,0

(8)

and after rearrangement of eq 8, the linear plot ln(K *) = −

ΔH *,0 ΔS*,0 + RT R

(9)

shown in Figure 11 can be obtained. The data used for calculations, with corresponding K* and ΔG0,* values, are gathered in Table 3.

4. DISCUSSION The process of PdCl42− complex ion adsorption on activated carbon (Norit GF40) was investigated in the range of temperature from 298 to 348 K and for Pd(II) chloride complex concentrations from 0.042 to 2.2 mmol·kg−1. The capacity of the carbon depends on the experimental conditions and may reach the level of 42 mg·g−1 at the temperature 294 K. This load increases slightly with temperature up to 67 mg·g−1 at a temperature of 323 K. It may mean that the process of transformation of adsorbed Pd(II) complexes takes over the process of their adsorption on the carbon surface. The results of adsorption are about 4 times higher as compared with commercially available anion exchange resins like Lewatit Mp-50026 or Lewatit Mp-500A.26 Also, it is very important that the price of activated carbon is much lower in comparison with anion exchange resins. Thanks to that, it seems that activated carbon can be applied in the industry for Pd recovery from dilute aqueous solutions. The results of adsorption were described with the Freundlich adsorption isotherm. It was found that the Freundlich isotherm describes the results in a satisfactory manner. It is not surprising, since the Freundlich isotherm is often applied to adsorption of the solute from liquid solutions onto solids with good results. However, this isotherm being an empirical relationship, lacks mechanistic background. Thus, it is difficult to assign physical meaning to the parameters derived from these model parameters. The Langmuir isotherm is based on the model which was developed to describe gas−solid adsorption. Several assump-

Figure 11. Graphical determination of the enthalpy and the entropy of the adsorption process.

The Gibbs free energy change with temperature takes the form ΔG 0, * = −6700 + 23.0·T [J·mol−1]

(10)

It is difficult to explain the meaning of positive Gibbs free energy change. It can only be presumed that this process is not spontaneous in the investigated range of temperature. A similar effect was observed by Zulfikar, during his study on adsorption of humic acid onto pyrophyllite.36 Observation of positive Gibbs free energy changes during the adsorption processes was investigated in depth by Chattoraj et al.37 They suggested that the ΔG0,* for the adsorption process is positive due to the spontaneous excess hydration of the interfacial phase in the presence of inorganic salt. In our case, the calculated values of Gibbs energy changes are very small, and are getting more positive with increasing Table 3. Results of Gibbs Free Energy Calculationsa T (K) 298 323 348 a

[PdCl42−]0 (mol·kg−1) −4

8.4 × 10 3.48 × 10−4 8.2 × 10−4

[PdCl42−]soln (mol·kg−1) −4

4.4 × 10 1.99 × 10−4 5.07 × 10−4

[PdCl42−]ads from eq 9 −4

3.99 × 10 1.49 × 10−4 3.17 × 10−4

K* from eq 7

ln(K*)

ΔG0,* (J·mol−1)

0.91 0.74 0.62

−0.0923 −0.2964 −0.4823

229.9 795.9 1395.4

K* - apparent equilibrium constant. H



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tions were made like a finite number of identical sites on the surface, equal affinity of these sites for the adsorbate, and formation of the monolayer on the surface. Apparently, in our case, this model is not able to describe the experimental data. The most probable reason is the competition between the solvent and the solute in reaching the empty site on the surface. Analysis of this isotherm performed by Harter,38 who used this model in an ion adsorption system, showed that applicability of the Langmuir equation is strongly limited, especially for maximum adsorption determination. Harter has shown that the quality of the fit depends on two parameters: the range of concentrations measured and the fitting procedure. The fitting error may reach even 50%. Despite a good fit, the Freundlich adsorption isotherm does not give insight into the thermodynamics of the process. Simonov et al.28 have investigated the phenomena of palladium(II) chloride complex ion adsorption on graphitelike carbon materials. They have shown that, in such a system, two processes occur simultaneously. The first one is related to the formation of π-complexes of PdCl2 with fragments of the carbon matrix, and the second one is related to the reduction reaction of Pd(II) to Pd(0). Our results suggest that, during chemisorption of palladium(II) chloride complex ion, either the dipalladium dichloro-oxide or PdCl2 is formed on the surface of activated carbon, and the metallic palladium is absent from the carbon surface. This conclusion is supported by the elution experiments which have shown that Pd(II) can be stripped from the AC surface using concentrated hydrochloric acid. These results suggest that the final product of the adsorption process is soluble in the solution of hydrochloric acid. Thus, it is most likely PdCl2, since palladium oxide is insoluble in this solution. Consequently, to get insight into the thermodynamic process, we suggested a two-step reaction (6). From our experimental data, we were able to derive thermodynamic parameters of the first step of the adsorption process. The enthalpy change is equal to −6.7 kJ, and the entropy change is equal to −23.0 J·mol−1 K−1. The negative value of ΔH0,* indicates that the process is exothermic and may be related to strong electrostatic interaction of the highly electrophilic surface of activated carbon and Pd(II) chloride complex. The negative value of the entropy change, ΔS0,*, can be related to the decrease in the number of degrees of freedom of the system connected with the adsorption process. From the point of view of possible industrial application, it is interesting to note that the lower the temperature, the more efficient the adsorption of Pd(II) complex species seems to be.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00885. Table with the stability constant of selected Pd(II) complexes; stability diagram calculated for the studied case; methodology of molar absorption coefficient determination as well as UV−vis spectra of analyzed solutions; methodology of apparent equilibrium constant calculation; and Langmuir isotherm fitting to experimental data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48126174126. Fax: +4812633-23-16. E-mail: [email protected]. ORCID

Marek Wojnicki: 0000-0002-1387-0000 Funding

This work was supported by the National Science Center of Poland under grant number 2016/23/D/ST8/00668 Sontata 12. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mrs. Ewa Zalecka (Brenntag Polska) for kind supply of activated carbon samples. REFERENCES

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5. CONCLUSIONS Our results suggest that, during chemisorption of palladium(II) chloride complex ion, either the dipalladium dichloro-oxide or PdCl2 is formed on the surface of activated carbon, and the metallic palladium is absent from the carbon surface. The capacity of the carbon depends on the experimental conditions and may reach a level of 42 mg·g−1 at the temperature 294 K. This load increases slightly with temperature up to 67 mg·g−1 at a temperature of 323 K. The process of palladium adsorption should be carried out at temperatures as low as possible. I



Article

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Palladium(II) Chloride Complex Ion Recovery from Aqueous Solutions

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