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

We have used solution uptake studies, X-ray absorption fine structure (XAFS ) spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy to s tudy the sorption of Cu(II) and 2,2'-bipyridine (bipy) on amorphous SiO2 (a m-SiO2). The specific goals of this study were to elucidate the composition of ternary surface complexes, to determine the mode of sorption at the mol ecular 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 ont o 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 inh ibition are more pronounced at higher bipy/Cu(II) ratios. The ratio of adso rbed 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) approximate to n when the predominant aqueous comple x is Cu(II)-bipy(n) for n = 1 or 2). EPR and XAFS results, together with FT IR results reported in a separate study, suggest that Cu(II) and bipy are m utually bonded. For Cu(II)-bipy(1), XAFS and EPR results suggest that the d ominant Cu(II) surface species is an inner s phere, mononuclear, monodentat e type A ternary surface complex [i.e., is bonded to the surface through th e Cu(II)]. XAFS results also show that the Cu(II)-surface bond length and i nferred Cu(II)-O-surface bond strength, in the presence of bipy, are compar able to those in the absence of bipy. For Cu(II)-bipy(2), spectroscopic res ults suggest that the predominant surface species is Cu(II)-bipy(2) bonded to the surface in inner sphere mode. Copyright (C) 1999 Elsevier Science Lt d.