Structure and composition of copper(II)-2,2′-bipyridine sorption complexes on amorphous SiO₂

Abstract

We have used solution uptake studies, X-ray absorption fine structure (XAFS) spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy to study the sorption of Cu(II) and 2,2′-bipyridine (bipy) on amorphous SiO₂ (am-SiO₂). The specific goals of this study were to elucidate the composition of ternary surface complexes, to determine the mode of sorption at the molecular level, and to quantify the effect of the bipy ligand on the sorption behavior of Cu(II) at macroscopic scale. Uptake measurements as a function of pH and bipy to Cu(II) ratio show that bipy enhances Cu(II) sorption onto am-SiO₂ at the lowest total Cu concentration studied, Cu_tot = 0.0001 M, but inhibits Cu(II) sorption at Cu_tot = 0.0016 M. Both enhancement and inhibition are more pronounced at higher bipy/Cu(II) ratios. The ratio of adsorbed bipy (bipy_ads) to adsorbed Cu(II) (Cu(II)_ads) is very close to the ratio of bipy to Cu(II) in the predominant aqueous solution complex (i.e., bipy_ads/Cu(II)_ads ≈ n when the predominant aqueous complex is Cu(II)-bipy_n for n = 1 or 2). EPR and XAFS results, together with FTIR results reported in a separate study, suggest that Cu(II) and bipy are mutually bonded. For Cu(II)-bipy₁, XAFS and EPR results suggest that the dominant Cu(II) surface species is an inner sphere, mononuclear, monodentate type A ternary surface complex [i.e., is bonded to the surface through the Cu(II)]. XAFS results also show that the Cu(II)-surface bond length and inferred Cu(II)-O_surface bond strength, in the presence of bipy, are comparable to those in the absence of bipy. For Cu(II)-bipy₂, spectroscopic results suggest that the predominant surface species is Cu(II)-bipy₂ bonded to the surface in inner sphere mode.

Introduction

This study examines the effect of a specific organic ligand, 2,2′-bipyridine (hereafter referred to as bipy) (Fig. 1 ) on the sorption behavior of aqueous Cu(II) [Cu(H₂O)₆²⁺] on amorphous silica (am-SiO₂) using a combination of macroscopic uptake measurements and spectroscopic techniques. This particular system serves as a simplified example of more complex natural aquatic and soil systems in which metal ions are often complexed or bound by organic matter or mineral surfaces, or both. Such complexation affects the distribution of metal ions between solid and aqueous phases and their speciation in each. This, in turn, affects the mobility, transport, cycling, bioavailability, and toxicity (Babich and Stotzky, 1983) of these ions. Similarly, the thermodynamic stability and the degradation rates of contaminant organic compounds depend on the composition and structure of aqueous and surface complexes in which they are present.

We chose to examine the effects of bipy on the sorption of Cu(II) on am-SiO₂ because bipy is a simple, yet important, organic molecule that has been well studied and exhibits interesting effects on the adsorption of metal ions on am-SiO₂ and other oxides Bourg et al 1979, Park et al 1993, Ludwig and Schindler 1995. Bipy belongs to a class of organic compounds referred to as nitrogen-containing polycyclic aromatic compounds. Some compounds in this class, which are acutely toxic and have teratogenic activities, have been introduced into the environment. Bipy is the raw starting material for the synthesis of diquat and paraquat (Reglone® and Gramoxone®), which are important herbicides Summers 1980, Matolcsy et al 1988. Related monocyclic compounds are also potentially dangerous environmental contaminants. Pyridine and its derivatives, for example, are present naturally in the environment in trace amounts. They have also been introduced in localized areas at toxic concentrations through agricultural and industrial activities; in particular, the production of synthetic fossil fuel (Ronen et al., 1994). The interaction of these ligands with metal cations can affect the fate and transport of both the ligands and the cations. Among these ligands, bipy has been most studied in the context of coadsorption of metal cations and organic compounds on various metal oxides.

A previous study of the sorption of Cu(II)-bipy complexes on the surface of Aerosil-300 (an amorphous silica made by Degussa) (Ettlinger et al., 1991) revealed that bipy dramatically shifts the pH of Cu(II) sorption on am-silica surfaces to lower values; that is, bipy increases the sorption of Cu(II) on am-silica (Bourg et al., 1979) while retaining the “cation-like” pH dependence. Assuming the formation of type A ternary complexes (complexes where the metal cation bonds directly to the oxygen or hydroxyl group on the mineral surface while retaining its bonding to an organic ligand; Schindler, 1990) and negligible sorption of free bipy, Bourg et al. (1979) were able to fit the uptake data. They and other researchers suggested that increased affinity of Cu(II) for the am-SiO₂ surface in the presence of bipy might result from a change in the electronic structure of Cu(II) induced by the coordinated bipy molecules Bourg et al 1979, von Zelewsky and Bemtgen 1982, Schindler 1990 or a more negative free energy of sorption caused by additional interaction of the ligand with the solid surface (Schindler and Stumm, 1987).

Bourg et al. (1979) suggested that Cu(II)-bipy may have a higher affinity for surface functional groups with oxygen donors than free Cu(II) because π-acceptor ligands, like bipy, can draw electron density from the d-orbitals of the metal ion, and therefore, enhance the positive charge of the Cu(II) center Bourg et al 1979, Schindler 1990. This hypothesis is based on the observation that for the analogous reactions in solution, Cu(II)-bipy₁ has higher affinity than free Cu(II) for oxalate and phthalate, which are ligands with oxygen donor functional groups (Sigel, 1975). However, the hypothesis is not supported by extended Hückel MO calculations (Schindler, 1990).

von Zelewsky and Bemtgen (1982) studied the influence of a number of organic ligands on Cu(II) sorption on Aerosil 300 using electron paramagnetic resonance (EPR) spectroscopy. Uptake of Cu(II) as a function of pH was also estimated based on the amplitudes of the EPR spectra. Their results confirm that bipy increases sorption of Cu(II) on Aerosil 300. Furthermore, they interpreted the EPR results as indicating that when the ratio of total bipy (bipy_tot) to total Cu(II) (Cu(II)_tot) is 1 or 2 the surface complexes formed are (≡SiO−)Cubipy₁ and (≡SiO−)Cubipy₂, respectively. The surface complex is (≡SiO−)Cubipy₂ when bipy_tot/Cu(II)_tot increases to ≥2. von Zelewsky and Bemtgen (1982) also estimated the stability of the surface complexes to be in the order: (≡SiO−)Cubipy₁ ≥ (≡SiO−)Cubipy₂ ≫ (≡SiO−)Cu. However, these stability estimates are not sufficient to distinguish among different modes of surface attachment of Cu(II) and Cu(II)-bipy complexes. In summary, EPR spectra suggest that the sorbed Cu(II) and bipy are bonded to each other and that the resulting surface complex is relatively immobile, thus adding weight to the hypothesis of an inner sphere ternary complex with surface SiO⁻ groups.

An alternative explanation for the increased sorption of Cu(II)-bipy, involving hydrophobic interaction of bipy with the surface, was first suggested by Schindler and Stumm (1987). Park et al. (1993) report uptake data that generally support bipy-induced enhancement of Cu(II) sorption. They find weak pH dependence at low pH and increasing enhancement of Cu(II) sorption at higher bipy_tot/Cu(II)_tot ratios, and interpret these observations as evidence that hydrophobic expulsion or bonding contributes to the stability of the ternary surface complex.

The predominant hypothesis adopted by previous investigators suggests that Cu(II) sorbs onto am-SiO₂ from aqueous solutions dominated by Cu(II)-bipy₁ or Cu(II)-bipy₂ as one or more ternary type A complexes bonded to the surface through the Cu(II) ion, perhaps augmented by hydrophobic bonding. In recent work on the system Cu(II)/bipy/TiO₂(anatase), Ludwig and Schindler (1995) have also considered the possibility of type B sorption complexes (complexes where the organic ligand bonds directly to the mineral surface and the metal cation is bonded to the organic ligand but not to the mineral surface; Schindler, 1990). Here we test these possibilities for the Cu(II)/bipy/am-SiO₂ system in several ways, including:

• verifying that Cu(II) sorption is enhanced by bipy;

• determining the stoichiometry of adsorbed bipy to adsorbed Cu(II) on the surface;

• determining that Cu(II) and bipy are associated in the sorption complex;

• determining whether any Cu(OH)₂ or polynuclear Cu complexes form on the surface in the presence of bipy;

• determining the mode of sorption, in particular, whether the complex is inner sphere or outer sphere; and

• assuming the sorption complex is inner sphere, determining whether it is type A (bound to the surface through Cu(II) ions) or type B (bound to the surface through the bipy ligand).

We have used a combination of macroscopic sorption measurements, X-ray absorption fine structure (XAFS), and EPR spectroscopies to determine whether Cu(II) and bipy adsorb independently or are bonded together as a Cu(II)-bipy surface complex. Direct measurement of the sorption densities of both Cu(II) and bipy permit qualitative definition of the influence of bipy on Cu(II) sorption and quantitative calculation of the average bipy/Cu(II) ratio on the surface, and hence help to define the average composition of sorption complexes. The pH dependence of Cu(II) uptake will help to determine whether the sorbed Cu(II) or Cu(II)-bipy complex behaves more like a cation, a neutral complex, or an anion Benjamin and Leckie 1981, Schindler 1990. The infrared (IR) spectra of free aqueous bipy and bipy coordinated to Cu(II) are different Cheah 1997, Cheah et al 1999b, thus IR spectra of the sorption complex can be used to determine whether the majority of sorbed bipy is free or bonded to Cu(II). Similarly, because the EPR parameters of “free” aqueous Cu(II) ion are different from those of Cu(II)-bipy Noack and Gordon 1968, Walker and Sigel 1971, EPR spectroscopy can help determine whether the sorbed copper and bipy are bonded or associated in a complex. This technique can also provide information about the coordination geometry of sorbed Cu(II) (Hathaway and Billing, 1970), and hence about the structure of the sorption complex, because EPR spectra are sensitive to the different stereochemistries of Cu(II) complexes.

X-ray absorption fine structure spectroscopy can provide valuable information on the local structural environment of the Cu(II) center in Cu(II)/bipy/am-SiO₂ surface complexes. Most important in this context, quantitative interpretation of Cu-XAFS spectra may permit identification of silicon among second neighbor atoms in the coordination environment of sorbed Cu(II)—and the Cu–Si interatomic distance, R_Cu–Si. If R_Cu–Si is so short that neither bipy nor water of hydration can be present between Cu(II) and the Si atom, then we can conclude that Cu(II) forms a direct chemical bond with oxygen on the am-SiO₂ surface. Furthermore, any statistically significant difference in the copper to first neighbor (oxygen and possibly nitrogen) distance between the ligand-present and the ligand-absent system is also important information. For instance, a shorter distance in the presence of the ligand would be consistent with the hypothesis that bipy draws electron density from Cu(II) and thus, increases the affinity of Cu(II) for oxygen donor groups on the am-SiO₂ surface.