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Elzinga, E. J. and D. L. Sparks. 2002. X-ray absorption spectroscopy study of the effects of pH and ionic strength on Pb(II) sorption to amorphous silica . Environ. Sci. Technol., 36: 4352 -4357.

Environ. Sci. Technol., 36 (20), 4352 -4357, 2002. 10.1021/es0158509 S0013-936X(01)05850-3

Web Release Date: September 14, 2002

Copyright © 2002 American Chemical Society

X-ray Absorption Spectroscopy Study of the Effects of pH and Ionic Strength on Pb(II) Sorption to Amorphous Silica

Evert J. Elzinga* and Donald L. Sparks

Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19717-1303

Abstract:

Pb(II) sorption to hydrous amorphous SiO2 was studied as a function of pH and ionic strength using XAS to characterize the sorption products formed. Pb sorption increased with increasing pH and decreasing ionic strength. The XAS data indicated that the mechanism of Pb(II) sorption to the SiO2 surface was pH-dependent. At pH < 4.5, a mononuclear inner-sphere Pb sorption complex with ionic character dominated the Pb surface speciation. Between pH 4.5 and pH 5.6, sorption increasingly occurred via the formation of surface-attached covalent polynuclear Pb species, possibly Pb-Pb dimers, and these were the dominant Pb complexes at pH 6.3. Decreasing ionic strength from I = 0.1 to I = 0.005 M NaClO4 significantly increased Pb sorption but did not strongly influence the average local coordination environment of sorbed Pb at given pH, suggesting that the formation of mononuclear and polynuclear Pb complexes at the surface were coupled; possibly, Pb monomers control the formation of Pb polynuclear species by diffusion along the surface, or they act as nucleation centers for additional Pb uptake from solution. This study shows that the effectiveness of SiO2 in retaining Pb(II) is strongly dependent on solution conditions. At low pH, Pb(II) may be effectively remobilized by competition with other cations, whereas sorbed Pb is expected to become less susceptible to desorption with increasing pH. However, unlike for Ni(II) and Co(II), no lead phyllosilicates are formed at these higher pH values; therefore, SiO2 is expected to be a less effective sink for Pb immobilization than for these other metals.

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