| Abstract
| - Thiol-functionalized mesostructured silica with anhydrous compositions of (SiO2)1-x(LSiO1.5)x,where L is a mercaptopropyl group and x is the fraction of functionalized framework silicon centers, areeffective trapping agents for the removal of mercuric(II) ions from water. In the present work, we investigatethe mercury-binding mechanism for representative thiol-functionalized mesostructures by atomic pairdistribution function (PDF) analysis of synchrotron X-ray powder diffraction data and by Raman spectroscopy.The mesostructures with wormhole framework structures and compositions corresponding to x = 0.30 and0.50 were prepared by direct assembly methods in the presence of a structure-directing amine porogen.PDF analyses of five mercury-loaded compositions with Hg/S ratios of 0.50−1.30 provided evidence forthe bridging of thiolate sulfur atoms to two metal ion centers and the formation of chain structures on thepore surfaces. We find no evidence for Hg−O bonds and can rule out oxygen coordination of the mercuryat greater than the 10% level. The relative intensities of the PDF peaks corresponding to Hg−S and Hg−Hg atomic pairs indicate that the mercury centers cluster on the functionalized surfaces by virtue of thiolatebridging, regardless of the overall mercury loading. However, the Raman results indicate that thecomplexation of mercury centers by thiolate depends on the mercury loading. At low mercury loadings(Hg/S ≤ 0.5), the dominant species is an electrically neutral complex in which mercury most likely istetrahedrally coordinated to bridging thiolate ligands, as in Hg(SBut)2. At higher loadings (Hg/S 1.0−1.3),mercury complex cations predominate, as evidenced by the presence of charge-balancing anions (nitrate)on the surface. This cationic form of bound mercury is assigned a linear coordination to two bridging thiolateligands.
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