Colloidal Size and Redox State of Uranium Species in the Porewater of a Pristine Mountain Wetland

Uranium (U) speciation was investigated in anoxically preserved porewater samples of a natural mountain wetland in Gola di Lago, Ticino, Switzerland. U porewater concentrations ranged from less than 1 mu g/L to tens of mu g/L, challenging the available analytical approaches for U speciation in natural samples. Asymmetrical flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry allowed the characterization of colloid populations and the determination of the size distribution of U species in the porewater. Most of the U was associated with three fractions:

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Barbora Bártová, Rizlan Bernier-Latmani, Ashley Richards Brown, Gabrielle Dublet, Manon Frutschi, Giada Chantal Zünd
AMER CHEMICAL SOC, 2019
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https://infoscience.epfl.ch/record/270361?ln=en

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Uranium is a chemical element with symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 ar

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Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is mat

Species

In biology, a species (: species) is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by

Geochemical control on uranium(IV) mobility in a mining-impacted wetland

Alexandre Bagnoud, Rizlan Bernier-Latmani, Lydie Chesaux, Elena Suvorova Buffat, Yuheng Wang

Wetlands often act as sinks for uranium and other trace elements. Our previous work at a mining-impacted wetland in France showed that a labile noncrystalline U(IV) species consisting of U(IV) bound to Al-P-Fe-Si aggregates was predominant in the soil at locations exhibiting a U-containing clay-rich layer within the top 30 cm. Additionally, in the porewater, the association of U(IV) with Fe(II) and organic matter colloids significantly increased U(IV) mobility in the wetland. In the present study, within the same wetland, we further demonstrate that the speciation of U at a location not impacted by the clay-rich layer is a different noncrystalline U(IV) species, consisting of U(IV) bound to organic matter in soil. We also show that the clay-poor location includes an abundant sulfate supply and active microbial sulfate reduction that induce substantial pyrite (FeS2) precipitation. As a result, Fe(II) concentrations in the porewater are much lower than those at clay-impacted zones. U porewater concentrations (0.02-0.26 mu M) are also considerably lower than those at the clay-impacted locations (0.21-3.4 mu M) resulting in minimal U mobility. In both cases, soil-associated U represents more than 99% of U in the wetland. We conclude that the low U mobility reported at clay-poor locations is due to the limited association of Fe(II) with organic matter colloids in porewater and/or higher stability of the noncrystalline U(IV) species in soil at those locations.

2014

Thermodynamic and kinetic degradation reactions of organic substances in groundwater modelled with PHREEQC

Johann Jakob Holch

Aim of this diploma thesis is to approach to kinetic modelling of subsurface hydrochemistry with the program code phreeqc. The reaction considered is the degradation of organic mass and its concomitant reactions. Phreeqc is a program for thermodynamical based equilibrium calculations of geo- and hydrochemistry. Although the degradation of organic matter, and the associated reduction reactions due to the oxidation of organic matter, can be modelled with phreeqc, it does not account for kinetically controlled reactions, whose characteristic feature is the dependency of time. This study focuses on the state of the art of environmental modelling, the thermodynamical based calculation of the degradation of organic matter and on two mathematical approaches to model bacterial degradation of a contaminant, the Monod equation and the Michaelis-Menten approach. Sensitivity analyses of the respective models runs were done. As a result one can see, that the redox chemistry of a natural water depends strongly on the abundance of oxygen and nitrate and, as a matter of course, on the abundance of organic matter. Considering bacterial activity within the degradation process, two basic mathematical formulations have been stated. The Monod equation and the Michaelis-Menten formulation were implemented in a BASIC interpreter in PHREEQC. The sensitivity analyses show, that that a accurate determination of the parameters in laboratory or field is essential. The coupling of kinetically controlled reactions and thermodynamical based equilibrium calculations might lead to auspicious hydrochemical modelling capabilities.

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Biogeochemical plant-soil microbe feedback in response to climate warming in peatlands

Richard David Bardgett, Luca Bragazza, Alexandre Buttler, Julien Parisod

Peatlands act as global sinks of atmospheric carbon (C) through the accumulation of organic matter(1), primarily made up of decay-resistant litter of peat mosses(2). However, climate warming has been shown to promote vascular plant growth in peatlands, especially ericaceous shrubs(3). A change in vegetation cover is in turn expected to modify above-ground/below-ground interactions(4), but the biogeochemical mechanisms involved remain unknown. Here, by selecting peatlands at different altitudes to simulate a natural gradient of soil temperature, we show that the expansion of ericaceous shrubs with warming is associated with an increase of polyphenol content in both plant litter and pore water. In turn, this retards the release of nitrogen (N) from decomposing litter, increases the amount of dissolved organic N and reduces N immobilization by soil microbes. A decrease of soil water content with increasing temperature promotes the growth of fungi, which feeds back positively on ericaceous shrubs by facilitating the symbiotic acquisition of dissolved organic N. We also observed a higher release of labile C from vascular plant roots at higher soil temperatures, which promotes the microbial investment in C-degrading enzymes. Our data suggest that climate-induced changes in plant cover can reduce the productivity of peat mosses and potentially prime the decomposition of organic matter by affecting the stoichiometry of soil enzymatic activity.

Nature Publishing Group2013