title: Uranium incorporation into amorphous silica.
abstract: High concentrations of uranium are commonly observed in naturally occurring amorphous silica (including opal) deposits, suggesting that incorporation of U into amorphous silica may represent a natural attenuation mechanism and promising strategy for U remediation. However, the stability of uranium in opaline silicates, determined in part by the binding mechanism for U, is an important factor in its long-term fate. U may bind directly to the opaline silicate matrix, or to materials such as iron (hydr)oxides that are subsequently occluded within the opal. Here, we examine the coordination environment of U within opaline silica to elucidate incorporation mechanisms. Precipitates (with and without ferrihydrite inclusions) were synthesized from U-bearing sodium metasilicate solutions, buffered at pH ∼ 5.6. Natural and synthetic solids were analyzed with X-ray absorption spectroscopy and a suite of other techniques. In synthetic amorphous silica, U was coordinated by silicate in a double corner-sharing coordination geometry (Si at ∼ 3.8-3.9 Å) and a small amount of uranyl and silicate in a bidentate, mononuclear (edge-sharing) coordination (Si at ∼ 3.1-3.2 Å, U at ∼ 3.8-3.9 Å). In iron-bearing synthetic solids, U was adsorbed to iron (hydr)oxide, but the coordination environment also contained silicate in both edge-sharing and corner-sharing coordination. Uranium local coordination in synthetic solids is similar to that of natural U-bearing opals that retain U for millions of years. The stability and extent of U incorporation into opaline and amorphous silica represents a long-term repository for U that may provide an alternative strategy for remediation of U contamination.
authors.affiliation: Department of Earth and Environmental Sciences, California State University, East Bay , Hayward, California 94542, United States.
authors.forename: Michael S
authors.initials: MS
authors.lastname: Massey
authors.forename: Juan S
authors.initials: JS
authors.lastname: Lezama-Pacheco
authors.forename: Joey M
authors.initials: JM
authors.lastname: Nelson
authors.forename: Scott
authors.initials: S
authors.lastname: Fendorf
authors.forename: Kate
authors.initials: K
authors.lastname: Maher
mesh_terms: D000327:Adsorption; D005290:Ferric Compounds; D007501:Iron / Q000737:chemistry; D010087:Oxides / Q000737:chemistry; D017640:Silicates; D012822:Silicon Dioxide / Q000737:chemistry*; D014501:Uranium / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D005290:Ferric Compounds; D010087:Oxides; D017640:Silicates; C029019:opoka; C025349:sodium metasilicate; D014501:Uranium; D012822:Silicon Dioxide; C092844:ferric oxyhydroxide; D007501:Iron