Nous predisons la morphologie des figures de corrosion sur la face de base des cristaux de becquerelite, Ca(H 2 O) 8 [(UO 2 ) 6 O 4 (OH) 6 ], selon le deficit des valences de liaison des terminaisons anioniques des chaines de polyedres paralleles aux aretes. La morphologie predite est comparee aux observations faites lors des experiences de dissolution globale de monocristaux. Ces experiences ont ete effectuees dans des solutions de HCl a un pH de 2 et de 3.5, dans l'eau ultrapure, dans des solutions de Na 2 CO 3 a une concentration de 0.1 et 0.5 mol L -1 et a un pH de 10.5, dans des solutions de MCI a une concentration de 1.0 mol L -1 et un pH de 2 (M = Na, K), dans des solutions de 0.5 mol L -1 MCl 2 a un pH de 2 (M = Ba, Sr, Mg), et dans une solution de Pb(NO 3 ) 2 a une concentration de 0.5 mol L -1 a un pH de 2. Les figures de dissolution sur la face (001) des cristaux de becquerelite ont ete examinees en utilisant la microscopie a force atomique, la microscopie electronique a balayage et la microscopie optique. Les monticules de croissance sur la surface basale apparaissent a un pH dans l'intervalle 3.5-10.5. Les figures de corrosion apparaissent seulement dans les solutions ayant un pH de 2, indication que leur formation est favorisee par une forte activite de protons en solution. Les figures de corrosion formees dans une solution de HCI a un pH de 2 ne repondent pas aux exigences des elements de symetrie de la structure globale (groupe d'espace Pn2 1 a), peut-etre en raison du detachement non stoechiometrique des polyedres uranyles et a CaΦ n . La symetrie et l'allongement des figures de corrosion formees dans des solutions d'electrolytes de pH 2 varient selon le cation en solution. Les figures de corrosion allongees paralleles a [100] se forment dans des solutions aqueuses de BaCl 2 , KCl, NaCl et MgCl 2 , tandis que les figures de corrosion allongees paralleles a [010] se forment dans des solutions aqueuses de SrCl 2 et Pb(NO 3 ) 2 . Les figures de corrosion developpees sur des surfaces non traitees lors d'une synthese dans une solution porteuse de Ca sont allongees paralleles a [010]. Les figures de corrosion formees en presence de solutions contenant Ba, K, et Ca sont allongees paralleles aux rangees d'atomes de Ba, K et Ca dans les structures globales de billietite, compreignacite et becquerelite, et a l'allongement de leurs cristaux. Nous expliquons les differences dans l'allongement des figures de corrosion par un modele d'absorption impliquant les cations en solution et les sites specifiques sur la surface a la base. La stabilite de cette surface de base et de ses aretes en presence de divers cations en solution a ete determinee en utilisant les dimensions et le relief des figures de corrosion. La stabilite de la face de base diminue selon la sequence MgCl 2 > KCI > BaCl 2 > Pb(NO 3 ) 2 > NaCl > SrCl 2 en solution, et la stabilite des aretes diminue selon la sequence MgCl 2 > KCl > BaCl 2 > SrCl 2 > NaCl > Pb(NO 3 ) 2 en solution. Les deux sequences indiquent que les solutions contenant des cations de rayon ionique semblable a celui de Ca 2+ exercent la plus grande influence sur les processus de dissolution de monocristaux de becquerelite.
Molecular weight (MW) of humic materials is a key factor controlling proton and metal binding and organic pollutant partitioning. Several studies have suggested preferential adsorption of higher MW, more aromatic moieties to mineral surfaces; quantification of such processes is fundamental to development of predictive models. We used high pressure size exclusion chromatography (HPSEC) to quantify MW changes upon adsorption of a muck fulvic acid (MFA) extracted from a peat deposit to kaolinite and goethite, at pH 3.7, 6, and 8 at 22 °C, I = 0.01 (NaCl), 24-h reaction time. MFA adsorption affinity was greater for goethite than for kaolinite. At concentrations less than the adsorption maximum (Amax) for both adsorbents, the weight-average MW (Mw) of MFA remaining in solution decreased by as much as several hundred Daltons relative to control samples, indicating preferential adsorption of the higher MW components. At concentrations more than Amax, Mw of MFA in solution did not change appreciably. Although total adsorption decreased significantly as pH increased, fractionation as measured by change in Mw remained similar, perhaps indicating greater selectivity for higher MW components at higher pH. Absorptivities at λ = 280 nm normalized to mg C L−1 (ε) suggested preferential adsorption of more aromatic moieties to kaolinite. ε could not be used for goethite-reacted samples because high Fe concentrations in the aqueous phase brought about by goethite dissolution interfered with the spectroscopic analysis. Preliminary kinetic experiments suggested that smaller molecules adsorbed first and were replaced by larger molecules whose adsorption was thermodynamically favored.
This research investigated the potential role of siderophores in aerobic microbial Fe acquisition from natural organic matter (NOM; XAD-8 isolate and reverse osmosis concentrate pre- and post-Chelex® treatment) through the use of a siderophore-producing Pseudomonas mendocina wild type (WT) bacterium and an engineered mutant (Mt) that was incapable of siderophore production. NOM had complex effects on microbial growth under Fe-limited conditions as measured by optical density, most likely because of the presence of other toxic (trace) metals such as Al, NOM binding interference with additional trace metal nutrients, and/or biofilm development. However, a bioassay for cellular Fe status showed that both WT and Mt readily acquired Fe naturally associated with NOM. Thus, while siderophores may be useful for Fe acquisition from NOM by P. mendocina, they do not appear to be essential for this process.
Minerals are more complex than previously thought because of the discovery that their chemical properties vary as a function of particle size when smaller, in at least one dimension, than a few nanometers, to perhaps as much as several tens of nanometers. These variations are most likely due, at least in part, to differences in surface and near-surface atomic structure, as well as crystal shape and surface topography as a function of size in this smallest of size regimes. It has now been established that these variations may make a difference in important geochemical and biogeochemical reactions and kinetics. This recognition is broadening and enriching our view of how minerals influence the hydrosphere, pedosphere, biosphere, and atmosphere.
