Structure of heavy metal sorbed birnessite. Part III: Results from powder and polarized extended X-ray absorption fine structure spectroscopy

Binding mechanism of arsenate on rutile (1 1 0) and (0 0 1) planes studied using grazing-incidence EXAFS measurement and DFT calculation

“…It was also reported that a zero CN pol value of Co-O bond occurred in an on-top complex of Co(II) adsorbed on a-Al 2 O 3 (0 0 0 1) surface when the bond was absolutely normal to the E-vector (Shirai et al, 1992(Shirai et al, , 1994. The difference of CN As-Ti between both polarized scans can be used to (Manceau et al, 2002;Waychunas et al, 2005). Based on the two equations, the polar angles (/) of BB complex obtained from GI-EXAFS fitting were 31 ± 8°on (1 1 0) surface and 43 ± 5°on (0 0 1) surface, while the experimental / of MM complex on (1 1 0) and (0 0 1) surfaces were 15 ± 6°and 18 ± 9°, respectively.…”

Zinc surface complexes on birnessite: A density functional theory study

“…Among the trace metals of major interest, Zn appears to be influenced strongly in its biogeochemical cycling by sorption on birnessite minerals found in soils, aquifers, streams, and wetlands . Detailed molecular-scale studies of sorbed Zn 2+ species have been published for chemically-and microbially-synthesized birnessite (Manceau et al, 2002;Toner et al, 2006), natural soil and marine birnessite (Marcus et al, 2004;Manceau et al, 2007), and Zn-Mn coprecipitates on plant roots (Lanson et al, 2008). These studies reveal that Zn 2+ binds to Mn(IV) vacancy sites in hexagonal birnessite, forming triple-corner-sharing (TCS) inner-sphere surface complexes with the three surface O surrounding a vacancy.…”

“…However, the local coordination environment of Zn sorbed by birnessite does not merely replicate that in chalcophanite. Manceau et al (2002) have shown that, in addition to octahedrally-coordinated Zn (Zn VI -TCS), tetrahedrally-coordinated Zn (Zn IV -TCS) bound to three surface O and one H 2 O can exist in Zn-birnessite, preferentially at low surface coverage of Zn. Structural parameters reported for their sample ZnBi8, i.e., nearest-neighbor distances, d(Zn-O) = 1.97 Å and d(ZnMn 1st ) = 3.35 Å, and a Zn-O coordination number near 4, are representative of the Zn IV -TCS species (Table 1).…”

“…The synthetic birnessite for which Manceau et al (2002) first reported the presence of Zn IV -TCS contained about 13 mol % Mn(III) substituted for Mn(IV) in the octahedral sheet (Lanson et al, 2002), and the formation of Zn IV -TCS was accordingly conjectured to result from the presence of this structural Mn(III) (Manceau et al, 2002). Similarly, the natural marine Zn-birnessite sample investigated by Marcus et al (2004), whose octahedrally-coordinated Zn content is at most 7 mol %, was reported to have about 10 mol % Mn(III) substituted in the octahedral sheet.…”

“…Lastly, doubly-occupied Zn-TCS complexes (i.e., Zn IV -TCS + Zn IV -TCS; Zn VI -TCS + Zn VI -TCS; Zn IV -TCS + Zn VI -TCS) were considered by analogy with the double Zn occupancy found in chalcophanite. Structural distortion of a Mn(IV) vacancy site was also investigated to understand why interatomic distances determined by X-ray absorption spectroscopy often are not compatible with those determined by X-ray diffraction, with reconciliation between the two then requiring ad hoc displacement of the surface O positions around a vacancy (Manceau et al, 2002;Villalobos et al, 2005;Lanson et al, 2008).…”