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Abstract

Copper(II) and palladium(II) adsorption from single-component, Cu(NO₃)₂ or K₂PdCl₄, and two-component Cu(NO₃)₂-K₂PdCl₄ aqueous solutions by bentonites provided by two different Ukrainian deposits was investigated in this study. As was found, types of the obtained adsorption isotherms depended on the bentonite origin, the nature of the adsorbed metal ion and the component quantity in the aqueous solutions under study. The most complicated adsorption isotherms were obtained for Pd(II) adsorption from the single-component Pd(II) and two-component Pd(II)-Cu(II) solutions. The adsorption of each metal ion decreased in the presence of the second one being evidence of competitive adsorption and confirming the nonhomogenity of adsorption sites.

Keywords

Bentonite adsorption isotherms degree of adsorption palladium(II)copper(II)

Introduction

In our earlier works, we used Ukrainian natural bentonites as supports for anchoring Pd(II) and Cu(II) complexes. Depending on the bentonite origin, such supported metal complexes have different catalytic activity in the reaction of carbon monoxide oxidation with air oxygen (Rakitskaya et al., 2013). Among many factors influencing the catalytic activity of these surface complexes, the mechanism of their formation and the strength of bonds formed between each metal ion with the support surface depending on its affinity to the adsorbent are most essential (Rakitskaya et al., 2015). The nature of metal ions, their sizes and charges determine their affinities to the same adsorbent, and each metal ion has its specific affinities to different adsorbents. In the case of natural sorbents, i.e. bentonites, the difference in affinity to them of the same metal ion can be explained by available quantities of montmorillonite, as a main phase, and quartz, calcite, field spars, as impurities in a particular bentonite. This is confirmed by selectivity series of metal ions obtained for bentonites of different origin (Abollino et al., 2008; Bereket et al., 1997; Olu-Owolabi et al., 2016; Reddy et al., 2007; Shimizu et al., 2008; Taha et al., 2016; Vezentsev and Volovicheva, 2007).

Fe³⁺ > Yb³⁺ > Zn²⁺ > Рb²⁺ > Ni²⁺ > Na⁺ (Shimizu et al., 2008); Al³⁺ > Fe³⁺ > Cr³⁺ > Zn²⁺ > Ni²⁺ > Cu²⁺ > Na⁺ (Reddy et al., 2007); Pb²⁺ = Cd²⁺ < Cu²⁺ < Zn²⁺ < Mn²⁺ < Ni²⁺ (Abollino et al., 2008); Pb²⁺ > Cd²⁺ > Ni²⁺ (Taha et al., 2016); Pb²⁺ > Cu²⁺ > Cd²⁺ (Olu-Owolabi et al., 2016); Pb²⁺ > Cd²⁺ > Cu²⁺ (Bereket et al., 1997); Fe³⁺ > Рb²⁺ > Cu²⁺ (Vezentsev and Volovicheva, 2007).

The Langmuir constant, K, also can be used as an affinity rating. For example, its value for Cu²⁺ is less than for Cd²⁺ (Karapinar and Donat, 2009) and more than for Ni²⁺ (Liu and Zhou, 2010).

The data obtained for Cu²⁺/Ni²⁺ (Liu and Zhou, 2010), Cu²⁺/Cr³⁺ (Chen et al., 2016) and Cu²⁺/Pb²⁺ (Zhu et al., 2011) two-component solutions were of particular interest for us because they indicate the fact of competitive adsorption and the different inhibitory effects of the metal ions.

The literature data (Chen et al., 2016; Karapinar and Donat, 2009; Liu and Zhou, 2010; Malamisa and Katsou, 2013; Vezentsev and Volovicheva, 2007; Zhu et al., 2011) include no information about affinity of palladium(II) to bentonites. Palladium(II) concentrating by its adsorption was investigated only for clinoptilolite, mordenite and their H-forms (Korkuna and Vrublevs’ka, 2002; Korkuna et al., 2004, 2006; Vrublevs’ka and Korkuna, 2002, 2003; Vrublevs’ka et al., 1999). It may be caused by precipitation of palladium(II) hydroxides on the bentonites due to high pH values of their surface.

The aim of the work was to study the mutual influence of copper(II) and palladium(II) in the course of their adsorption on bentonites provided from two Ukrainian deposits.

Experimental

Bentonites from Gorbskoye (Trans-Carpathian region) and Dashukovskoye (Cherkassy region) deposits, denoted as N-Bent(G) and N-Bent(D), with chemical compositions converting to main oxides (wt. %): SiO₂ (50.0); Al₂O₃ (18.5); Fe₂O₃ (7.6) and SiO₂ (49.6); Al₂O₃ (13.5); Fe₂O₃ (7.2), respectively, were used as adsorbents.

They were investigated by X-ray diffraction, infrared (IR) spectroscopy and pH metry (Rakitskaya et al., 2013).

The main phase of montmorillonite was identified in the bentonite samples at the following values of angles of reflection, 2θ, and interplanar spacing (d, Å): 5.930° (14.90)—19.820° (4.48)—34.925° (2.56)—54.905° (1.67) for N-Bent(D) and 6.395° (13.82)—20.015° (4.44)—35.075° (2.56) for N-Bent(G). Both samples contained α-quartz (2θ = 26,630°, d = 3.34 Å). Additionally, N-Bent(D) contained calcite (2θ = 29,405°, d = 3.035 Å) while N-Bent(G) contained kaolinite (2θ = 12,470°, d = 7.09 Å, and 25,080°, d = 3.55 Å).

The polyphase composition of these natural bentonites was confirmed by IR spectral data.

The pH values for aqueous suspensions of N-Bent(D) and N-Bent(G) are 8.75 and 3.96, respectively.

An initial solution of palladium(II) (as K₂PdCl₄) was prepared as follows: 0.71 g of palladium(II) chloride and 0.6 g of potassium chloride, preliminarily dried at 200℃ for 1 h, were dissolved in 50 mL of bidistilled water. Then, 15 mL of concentrated hydrochloric acid were added, and then, the more water was added up to 200 mL. The Pd(II) concentration in the solution was 2 × 10⁻² M (2.128 mg/mL), and its pH was 2.1.

An initial solution of copper(II) (as Cu(NO₃)₂) was prepared as follows: 2.42 g of Cu(NO₃)₂ × 3H₂O were dissolved in 50 mL of bidistilled water, 10 mL of 1 M HNO₃ were added, and then, the more water was added up to 500 mL. The Cu(II) concentration in the solution was 2 × 10⁻² M (1.271 mg/mL), and its pH was 3.6.

Adsorption of the metals by bentonites was carried out under static conditions as follows: 1 g of each natural bentonite with a grain size of 0.5–1.0 mm was added to 100 mL of the single-component or two-component aqueous solution under study placed in a conical glass flask. The contents of the flasks were continuously shaken with the help of an electrical shaker at 25℃ for 2 h.

The Cu(II) and Pd(II) contents were determined by flame version (propane–butane–air) of atomic absorption spectrophotometry using an AAS–1 N atomic absorption spectrophotometer (Carl Zeiss, Jena). The interfering matrix effect of the solution was solved by the method of standard addition with background correction. In particular, the wavelength of resonance radiation in the palladium(II) concentration measuring was 247.6 nm with a background correction at λ = 246.7 nm; the copper(II) concentration was determined at λ = 324.8 nm with a background correction at λ = 323.1 nm.

The degree of metal adsorption was estimated based on the difference between Pd(II) and Cu(II) concentrations presenting in initial and equilibrium solutions.

Results and discussion

In order to understand how the adsorption of copper(II) and palladium(II) on N-Bent(G) и N-Bent(D) depends on their mutual influence, it was carried out from both single-component, Cu(NO₃)₂ or К₂PdCl₄, and two-component Cu(ІІ)-Pd(ІІ) solutions at the constant palladium(II) concentration of 2.8 × 10⁻⁴mol/L when copper(II) adsorption was investigated and at the constant copper(II) concentration of 3.0 × 10⁻⁴mol/L when palladium(II) adsorption was studied.

Figure 1 shows the isotherms of copper(II) adsorption from the Cu(NO₃)₂ single-component solutions (curve 1) and from the Cu(ІІ)-Рd(ІІ) two-component solutions (curve 2) for N-Bent(G). Figure 2 shows the similar adsorption isotherms in the case of N-Bent(D).

Figure 1.

Isotherms of Cu(II) adsorption on N-Bent(G) from the single-component Cu(II) (1) and two-component Cu(II)-Pd(II) (C_Pd(II) = 2.8 × 10⁻⁴mol/L) (2) aqueous solutions.

Figure 2.

Isotherms of Cu(II) adsorption on N-Bent(D) from the single-component Cu(II) (1) and two-component Cu(II)-Pd(II) (C_Pd(II) = 2.8 × 10⁻⁴mol/L) (2) aqueous solutions.

From Figures 1 and 2, it can be deduced that the nature of bentonite has a significant effect on the copper(II) adsorption. According to the classification proposed by C.H. Giles (Parfitt and Rochester, 1983), the adsorption isotherms for the single-component Cu(NO₃)₂ solution are of L type for N-Bent(G) and H type for N-Bent(D). The adsorption isotherms for the two-component Cu(II)-Pd(II) solutions are of L type both for N-Bent(G) (L1) and N-Bent(D) (L4). The degree of copper(II) adsorption from the Cu(NO₃)₂ solutions by N-Bent(G) decreases from 95% to 30%, and it decreases from 100% to 40% for N-Bent(D). As can be seen from Figures 1 and 2, palladium(II) suppresses copper(II) adsorption. The degree of copper(II) removal from the two-component solutions by N-Bent(G) decreases from 69% to 23%, and it reduces from 61% to 31% when N-Bent(D) is used.

As can be seen from Figure 3, for N-Bent(G), in the case of palladium(II) adsorption, the isotherms are concave downward.

Figure 3.

Isotherms of Pd(II) adsorption on N-Bent(G) from the single-component Pd(II) (1) and two-component Pd(II)-Cu(II) (C_Cu(II) = 3.0 × 10⁻⁴mol/L) (2) aqueous solutions.

Such a form is characteristic of S type isotherms: S3 type isotherm is observed for the single-component solutions and S1 type is observed for the two-component solutions. The increase in Pd(II) concentration in the single-component solution from 1 × 10⁻⁴ to 20 × 10⁻⁴mol/L caused the decrease in the degree of palladium(II) removal from 30% to 21%. The presence of copper(II) lowered the degree of palladium(II) removal by 8% to 10%. For N-Bent(D) (Figure 4), the profiles of palladium(II) adsorption isotherms are much more complicated and cannot not be attributed to any known isotherm type: in the absence of Cu(II), at first, the degree of palladium(II) adsorption decreases and then increases and amounts to 38% while, in the two-component solution, the degree of palladium(II) adsorption declines even greater (down to 3%) and then rises to 36%. Such a phenomena may be caused by high pH value of N-Bent(D) influencing the composition of Pd(II) complexes (Baes Jr and Mesmer, 1976). It should be noted that the minimum values of Pd(II) adsorption coincide with the saturation of the N-Bent(D) surface with copper(II) ions (Figure 2). Thus, the competition between Pd(II) and Cu(II) in order to occupy adsorption sites on N-Bent(D) surface is confirmed. However, the copper(II) ions on N-Bent(D) have a greater inhibitory effect than in the case of palladium(II) adsorption on N-Bent(G).

Figure 4.

Isotherms of Pd(II) adsorption on N-Bent(D) from the single-component Pd(II) (1) and two-component Pd(II)-Cu(II) (C_Cu(II) = 3.0 × 10⁻⁴mol/L) (2) aqueous solutions.

Some isotherms were analyzed using a Langmuir equation $\frac{C_{eq}}{Γ_{{Me}^{2 +}}} = \frac{1}{Γ_{\infty} K} + \frac{1}{Γ_{\infty}} C_{eq}$ where C_eq is an equilibrium concentration of Mе²⁺ (Cu²⁺ or Pd²⁺), mol/L; Г_Me2⁺ is a specific adsorption at the equilibrium Mе²⁺concentration, mol/g; Г_∞ is a limiting value of the specific adsorption corresponding to the monolayer filling of the adsorbent surface by Mе²⁺ ions, mol/g; K is a constant indicating the affinity of Mе²⁺ to the adsorbent surface L/mol.

The Langmuir equation parameters and correlation coefficients, R², estimated for the systems under study are presented in Table 1. They show that К_(Cu) and К_(Pd) values are not substantially dissimilar explaining the mutual inhabitation of the adsorption of these metal ions at their compresence.

Table 1.

Characteristic Langmuir equation parameters (Г∞, K) and correlation coefficients (R²).

Adsorbent	System	Langmuir equation constants		R²
Adsorbent	System	Г_∞ × 10⁵ (mol/g)	K × 10⁻⁴ (L/mol)	R²
N-Bent(G)	Cu(II)	3.20	3.66	0.99
N-Bent(G)	Cu(II)-Pd(II)	1.74	2.25	0.99
N-Bent(G)	Pd(II)	0.53	2.53	0.99
N-Bent(G)	Pd(II)-Cu(II)	0.34	1.52	0.99

Conclusion

Adsorption properties of bentonites from two Ukrainian deposits were studied. Both they contain montmorillonite as the main phase and substantially differ by their pH values: 8.75 for N-Bent(D) and 3.96 for N-Bent(G). As was found, the types of adsorption isotherms depended on the origin of bentonites and the nature of metal ions adsorbed. The L type isotherms of Cu(II) and Pd(II) adsorption from single-component Cu(NO₃)₂ and К₂PdCl₄ solutions were observed for N-Bent(G). In the case of N-Bent(D), the H type isotherm was found for Cu(II) adsorption and the very complicated isotherm for Pd(II) adsorption. This was explained by the high pH value of N-Bent(D) aqueous suspension influencing the composition of Pd(II) and Cu(II) complexes. As for adsorption from two-component solutions, irrespective of the nature of bentonites, the adsorption of one ion significantly decreased in the presence of another one indicating the competitive adsorption and nonhomogenity of adsorption sites. In the case of adsorption on N-Bent(G), the values of Langmuir constants, K, i.e. 3.66 × 10⁴L/mol for Cu(II) and 2.53 × 10⁴L/mol for Pd(II), were not substantially dissimilar explaining the mutual inhibition of the adsorption of these metal ions at their compresence.

Footnotes

Acknowledgements

Work “Some features of Pd(II) and Cu(II) adsorption with bentonites and activity of bentonite based catalysts in the reaction of carbon monoxide oxidation” first presented at the “15th Ukrainian–Polish Symposium on theoretical and Experimental Studies of Interfacial Phenomena and their Technological Applications,Lviv,Ukraine,12–15 September 2016.”

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: The study was carried out with the support of the Ministry of Education and Science of Ukraine.

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