Formation of Zn–Ca phyllomanganate nanoparticles in grass roots

Abstract

International audienceIt is now well established that a number of terrestrial and aquatic microorganisms have the capacity to oxidize and precipitate Mn as phyllomanganate. However, this biomineralization has never been shown to occur in plant tissues, nor has the structure of a natural Mn(IV) biooxide been characterized in detail. We show that the graminaceous plant Festuca rubra (red fescue) produces a Zn-rich phyllomanganate with constant Zn:Mn and Ca:Mn atomic ratios (0.46 and 0.38, respectively) when grown on a contaminated sediment. This new phase is so far the Zn-richest manganate known to form in nature (chalcophanite has a Zn:Mn ratio of 0.33) and has no synthetic equivalent. Visual examination of root fragments under a microscope shows black precipitates about ten to several tens of microns in size, and their imaging with backscattered and secondary electrons demonstrates that they are located in the root epidermis. In situ measurements by Mn and Zn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray diffraction (XRD) with a micro-focused beam can be quantitatively described by a single-phase model consisting of Mn(IV) octahedral layers with 22% vacant sites capped with tetrahedral and octahedral Zn in proportions of 3:1. The layer charge deficit is also partly balanced by interlayer Mn and Ca. Diffracting crystallites have a domain radius of 33 Å in the ab plane and contain only 1.2 layers (not, vert, similar8.6 Å) on average. Since this biogenic Mn oxide consists mostly of isolated layers, basal 00l reflections are essentially absent despite its lamellar structure. Individual Mn layers are probably held together in the Mn–Zn precipitates by stabilizing organic molecules. Zinc biomineralization by plants likely is a defense mechanism against toxicity induced by excess concentrations of this metal in the rhizosphere

Surface complexation of Pb(II) by hexagonal birnessite nanoparticles

“…The two nanoparticle diameters we considered are smaller than the 3-to 6-nm coherent scattering domain in the a-b plane that has been deduced from X-ray diffraction data for biogenic and chemically-synthesized hexagonal birnessites (Villalobos et al, 2006;Grangeon et al, 2008;Lanson et al, 2008) because they represent a compromise between realistic particle size and computational cost, allowing to investigate details (Fig. 3).…”

Zinc surface complexes on birnessite: A density functional theory study

“…f Zn-Mn coprecipitate on plant roots (Lanson et al, 2008), based on spectra of 6KR e , chalcophanite, and Zn δ-MnO 2 d . Fig.…”

“…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, other birnessite samples shown to contain the Zn IV -TCS species comprise sheets of octahedra with only Mn(IV) present [sediment birnessite, Isaure et al (2005); bacteriogenic birnessite and δ-MnO 2 , Toner et al (2006); quartz-coating birnessite, Manceau et al (2007); plant-root Zn-Mn coprecipitate, Lanson et al (2008)]. Therefore, it does not seem necessary to connect the existence of Zn IV -TCS uniquely to the presence of Mn(III) substituted in the octahedral sheet and the fundamental basis for the occurrence of the four-coordinate surface species remains unclear.…”

“…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).…”