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The influence of aqueous silica on the hydrolysis of iron(III) nitrate and chloride salts in dilute aqueous solutions (m Fe ϳ 0.01 mol/kg) was studied at ambient temperature using X-ray absorption fine structure (XAFS) spectroscopy at the Fe K-edge. Results show that in Si-free iron nitrate and chloride solutions at acid pH (pH Ͻ 2.5), Fe is hexa-coordinated with 6 oxygens of H 2 O-and/or OH-groups in the first coordination sphere of the metal, at an Fe-O distance of 2.00 Ϯ 0.01 Å. With increasing pH (2.7 Ͻ pH Ͻ 13), these groups are rapidly replaced by bridging hydroxyls (-OH-) or oxygens (-O-), and polymerized Fe hydroxide complexes form via Fe-(O/OH)-Fe bonds. In these polymers, the first atomic shell of iron represents a distorted octahedron with six O/OH groups and Fe-O distances ranging from 1.92 to 2.07 Å. The Fe octahedra are linked together by their edges (Fe-Fe distance 2.92-3.12 Å) and corners (Fe-Fe distance ϳ3.47 Ϯ 0.03 Å). The Fe-Fe coordination numbers (N edge ϭ 1-2; N corner ϭ 0.5-0.7) are consistent with the dominant presence of iron dimers, trimers and tetramers at pH 2.5 to 2.9, and of higher-polymerized species at pH Ͼ 3. At pH Ͼ 2.5 in the presence of aqueous silica, important changes in Fe(III) hydrolysis are detected. In 0.05-m Si solutions (pH ϳ 2.7-3.0), the corner linkages between Fe octahedra in the polymeric complexes disappear, and the Fe-Fe distances corresponding to the edge linkages slightly increase (Fe-Fe edge ϳ 3.12-3.14 Å). The presence of 1 to 2 silicons at 3.18 Ϯ 0.03 Å is detected in the second atomic shell around iron. At basic pH (ϳ12.7), similar structural changes are observed for the iron second shell. The Fe-Si and Fe-Fe distances and coordination numbers derived in this study are consistent with (1) Fe-Si complex stoichiometries Fe 2 Si 1-2 and Fe 3 Si 2-3 at pH Ͻ 3; (2) structures composed of Fe-Fe dimers and trimers sharing one or two edges of FeO 6-octahedra; and (3) silicon tetrahedra linked to two neighboring Fe octahedra via corners. At higher Si concentration (0.16 m, polymerized silica solution) and pH ϳ 3, the signal of the Fe second shell vanishes indicating the destruction of the Fe-Fe bonds and the formation of different Fe-Si linkages. Moreover, ϳ20 mol.% of Fe is found to be tetrahedrally coordinated with oxygens in the first coordination shell (R Fe-O ϭ 1.84 Å). This new finding implies that Fe may partially substitute for Si in the tetrahedral network of the silica polymers in Si-rich solutions. The results of this study demonstrate that aqueous silica can significantly inhibit iron polymerization and solid-phase formation, and thus increase the stability and mobility of Fe(III) in natural waters. The silica "poisoning" of the free corner sites of iron-hydroxide colloids should reduce the adsorption and incorporation of trace elements by these colloids in Si-rich natural waters.