XAFS spectroscopy study of Cu(II) sorption on amorphous SiO2 and gamma-Al2O3: Effect of substrate and time on sorption complexes

We have used X-ray absorption fine structure (XAFS) spectroscopy to investi gate Cu(II)sorption complexes on high surface area amorphous silica (am-SiO 2) and gamma-Al2O3. For Cu(II) on gamma-Al2O3, analysis of XAFS data collec ted after a solution-solid total contact time of 80-170 h showed that monom eric Cu(II) species predominate at surface coverages of 0.007 and 0.05 mu m ol m(-2). Cu(II) on the gamma-Al2O3 surface has aluminum second neighbors a t about 2.8 a. Geometrical considerations indicate that this distance is co nsistent with models of Cu(II) binding to the gamma-Al2O3 surface in inner- sphere bidentate or monodentate modes on Al(O,OH)(6) octahedra. For Cu(II) sorbed on am-SiO2, analysis of XAFS data collected after a solution-solid t otal contact time of 80-110 h showed that a dimeric Cu(II)-surface complex predominates, along with a minority monomeric Cu(II) surface species, at 0. 03 and 0.05 mu mol m(-2). The XAFS-derived Cu-Si distance is in the range o f 2.98 to 3.05 Angstrom, suggesting that Cu(II) binds to am-SiO2 in an inne r-sphere, monodentate fashion. XAFS spectra of Cu(II) sorbed on am-SiO2 col lected after 20-30 h total contact time are quantitatively different from t hose collected after 80-90 h contact time. Analyses of these spectra indica te that the ratio of dimeric to monomeric Cu(II) surface complexes has: inc reased with contact time. A discussion of the processes responsible for the different Cu(II) complexes on am-SiO2 and gamma-Al2O3 is presented. (C) 19 98 Academic Press.