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Stern J.C.,Florida State University | Stern J.C.,NASA | Foustoukos D.I.,Carnegie Institution of Washington | Sonke J.E.,CNRS Geosciences Environnement Toulouse | Salters V.J.M.,Florida State University
Chemical Geology | Year: 2014

The mobility of metals in soils and subsurface aquifers is strongly affected by sorption and complexation with dissolved organic matter, oxyhydroxides, clay minerals, and inorganic ligands. Humic substances (HS) are organic macromolecules with functional groups that have a strong affinity for binding metals, such as actinides. Thorium, often studied as an analog for tetravalent actinides, has also been shown to strongly associate with dissolved and colloidal HS in natural waters. The effects of HS on the mobilization dynamics of actinides are of particular interest in risk assessment of nuclear waste repositories.Here, we present conditional equilibrium binding constants (Kc,MHA) of thorium, hafnium, and zirconium-humic acid complexes from ligand competition experiments using capillary electrophoresis coupled with ICP-MS (CE-ICP-MS). Equilibrium dialysis ligand exchange (EDLE) experiments using size exclusion via a 1000Da membrane were also performed to validate the CE-ICP-MS analysis. Experiments were performed at pH3.5-7 with solutions containing one tetravalent metal (Th, Hf, or Zr), Elliot soil humic acid (EHA) or Pahokee peat humic acid (PHA), and EDTA. CE-ICP-MS and EDLE experiments yielded nearly identical binding constants for the metal-humic acid complexes, indicating that both methods are appropriate for examining metal speciation at conditions lower than neutral pH. We find that tetravalent metals form strong complexes with humic acids, with Kc,MHA several orders of magnitude above REE-humic complexes. Experiments were conducted at a range of dissolved HA concentrations to examine the effect of [HA]/[Th] molar ratio on Kc,MHA. At low metal loading conditions (i.e. elevated [HA]/[Th] ratios) the ThHA binding constant reached values that were not affected by the relative abundance of humic acid and thorium. The importance of [HA]/[Th] molar ratios on constraining the equilibrium of MHA complexation is apparent when our estimated Kc,MHA values attained at very low metal loading conditions are compared to existing literature data. Overall, experimental data suggest that the tetravalent transition metal/-actinide-humic acid complexation is important over a wide range of pH values, including mildly acidic conditions, and thus, these complexes should be included in speciation models. © 2013.

Beaulieu E.,CNRS Geosciences Environnement Toulouse | Godderis Y.,CNRS Geosciences Environnement Toulouse | Donnadieu Y.,CEA Saclay Nuclear Research Center | Labat D.,CNRS Geosciences Environnement Toulouse | And 2 more authors.
Nature Climate Change | Year: 2012

According to future anthropogenic emission scenarios, the atmospheric CO 2 concentration may double before the end of the twenty-first century. This increase is predicted to result in a global warming of more than 6°C in the worst case. The global temperature increase will promote changes in the hydrologic cycle through redistributions of rainfall patterns and continental vegetation cover. All of these changes will impact the chemical weathering of continental rocks. Long considered an inert CO 2 consumption flux at the century timescale, recent works have demonstrated its potential high sensitivity to the ongoing climate and land-use changes. Here we show that the CO 2 consumption flux related to weathering processes increases by more than 50% for an atmospheric CO 2 doubling for one of the most important Arctic watersheds: the Mackenzie River Basin. This result has been obtained using a process-based model of the chemical weathering of continental surfaces forced by models describing the atmospheric general circulation and the dynamic of the vegetation under increased atmospheric CO 2. Our study stresses the potential role that weathering may play in the evolution of the global carbon cycle over the next centuries. © 2012 Macmillan Publishers Limited. All rights reserved.

Jones M.T.,University of Iceland | Pearce C.R.,CNRS Geosciences Environnement Toulouse | Pearce C.R.,Open University Milton Keynes | Jeandel C.,University Paul Sabatier | And 5 more authors.
Earth and Planetary Science Letters | Year: 2012

The ratio of strontium isotopes, 87Sr/86Sr, in seawater is homogenous at any given time, yet varies considerably throughout the geological record. This variation is thought to stem from changes in the balance of predominantly radiogenic Sr entering the oceans via dissolved riverine transport, and unradiogenic Sr sourced from mid-ocean ridge hydrothermal activity. Recent evidence suggests, however, that hydrothermal exchange at mid-ocean ridges is a factor of 3 too low to balance Sr added to the oceans from dissolved continental riverine fluxes. Here we present evidence that the arrival and subsequent dissolution of riverine particulate material in seawater is a significant contributor of both radiogenic and unradiogenic Sr to the oceans. Batch experiments demonstrate that between 0.15% and 27.36% of Sr is liberated from riverine particulates to seawater within 6 months. The rates of release are dependent on surface area and particulate composition, with volcanic riverine material more reactive than continental riverine particulates. The observed rapid Sr release rate from riverine particulate material has important consequences for both chemical and isotopic mass balances in the ocean and the application of the 87Sr/86Sr weathering proxy to the geological record. The dissolution of riverine particulate material is likely, based on these findings, to at least partially account for the imbalance between Sr sources to the oceans. © 2012 Elsevier B.V.

Tipping E.,UK Center for Ecology and Hydrology | Lofts S.,UK Center for Ecology and Hydrology | Sonke J.E.,CNRS Geosciences Environnement Toulouse
Environmental Chemistry | Year: 2011

Humic Ion-Binding Model VII aims to predict the competitive reactions of protons and metals with natural organic matter in soils and waters, based on laboratory results with isolated humic and fulvic acids (HA and FA). Model VII is simpler in its postulated multidentate metal binding sites than the previous Model VI. Three model parameters were eliminated by using a formal relationship between monodentate binding to strong- and weak-acid oxygen-containing ligands, and removing factors that provide ranges of ligand binding strengths. Thus Model VII uses a single adjustable parameter, the equilibrium constant for monodentate binding to strong-acid (carboxylate) groups (KMA), for each metallic cation. Proton-binding parameters, and mean values of logK MA were derived by fitting 248 published datasets (28 for protons, 220 for cationic metals). Default values of logKMA for FA were obtained by combining the fitted values for FA, results for HA, and the relationship for different metals between logKMA and equilibrium constants for simple oxygen-containing ligands. The equivalent approach was used for HA. The parameterised model improves on Model VI by incorporating more metals (40), providing better descriptions of metal binding at higher pH, and through more internally consistent parameter values. © 2011 CSIRO.

Pokrovsky O.S.,CNRS Geosciences Environnement Toulouse | Pokrovsky O.S.,Russian Academy of Sciences | Shirokova L.S.,CNRS Geosciences Environnement Toulouse | Shirokova L.S.,Russian Academy of Sciences
Water Research | Year: 2013

This work describes variation of element concentration in surface water of a subarctic organic-rich lake during the diurnal cycle of photosynthesis. An unusually hot summer 2010 in European part of subarctic Russia produced elevated surface water temperature (28-30 °C) and caused massive cyanobacterial bloom. Diurnal variation of ∼40 dissolved macro and trace elements and organic carbon were recorded in the humic Lake Svyatoe in the White Sea drainage basin. Two days continuous measurements with 3 h sampling steps at the surface (0.5 m) allowed tracing cyanobacterial activity via pH and O2 measurement and revealed constant concentrations (within ±20-30%) of all major elements (Na, Mg, Cl, SO4, K, Ca), organic and inorganic carbon and most trace elements (Li, B, Sc, Ti, Ni, Cu, Ga, As, Rb, Sr, Y, Zr, Mo, Sb, medium and heavy REEs, Hf, Pb, Th, U). The concentration of Mn demonstrated a factor of 3 decrease during the day following Mn adsorption onto cyanobacterial cells due to ∼1 pH unit raise during the photosynthesis and Mn release during the night due to desorption from the cell surface. The role of Mn(II) photo-oxidation by reactive oxygen species could be also pronounced, although its contribution to Mn diurnal variation was much smaller than the adsorption at the cell surfaces. Similar pattern, but with much lesser variations (c.a., 10-20%), was recorded for Ba and Fe. On-site ultrafiltration technique allowed to distinguish between low molecular weight (LMW) complexes (<1 kDa) and high molecular weight (HMW) colloids (1 kDa-0.22 μm) and to assess their diurnal pattern. Colloidal Al and Fe were the highest during the night, when the contribution of HMW allochthonous colloids was maximal. Typical insoluble trivalent and tetravalent elements exhibited constant complexation (>80-90%) with HMW allochthonous organics, independent on the diel photosynthetic cycle. Finally, biologically-relevant metals (Cu, Co, Cr, V, and Ni) demonstrated significant variations of colloidal fractions (from 10 to 60%) not directly related to the photosynthesis. The majority of possible metal nutrients, being strongly associated with organic and organo-mineral colloids do not exhibit any measurable concentration variation during photosynthesis. The two types of element behavior during cyanobacterial bloom in the water column - constant concentration and sinusoidal variations - likely depend on element speciation in solution and their relative affinity to surfaces of aquatic microorganisms and complexation with authochthonous and allochthonous organic matter. © 2012 Elsevier Ltd.

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