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, Cutot = 0.0001 M, but inhibits Cu(II) sorption at Cutot = 0.0016 M. Both enhancement and inhibition are more pronounced at higher bipy/Cu(II) ratios. The ratio of adsorbed bipy (bipyads) 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., bipyads/Cu(II)ads ≈ n when the predominant aqueous complex is Cu(II)-bipyn 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)-Osurface 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.