The oxidation of volatile aqueous Hg(0) in aquatic systems may be important in reducing fluxes of Hg out of aquatic systems. Here we report the results of laboratory and field experiments designed to identify the parameters that control the photooxidation of Hg(0)(aq) and to assess the possible importance of this process in aquatic systems. The concentrations of elemental and total Hg were measured as a function of time in both artificial and natural waters irradiated with a UV-B lamp. No change in Hg speciation was observed in dark controls, while a significant decrease in Hg(0) was observed in UV-B irradiated artificial solutions containing both chloride ions and benzoquinone. Significant photooxidation rates were also measured in natural samples spiked with Hg(0)(aq); the photooxidation of Hg(0) then follows pseudo first-order kinetics (k = 0.6 h-1). These results indicate that the previously observed Hg(II) photoreduction rates in natural waters could represent a net balance between Hg(II) photoreduction and Hg(0) photooxidation. As calculated from Hg(0) photooxidation rates, the dominant Hg(0) sink is likely to be photooxidation rather than volatilization from the water column during summer days.
Biological nitrogen fixation constitutes the main input of fixed nitrogen to Earth's ecosystems, and its isotope effect is a key parameter in isotope-based interpretations of the N cycle. The nitrogen isotopic composition (δ(15)N) of newly fixed N is currently believed to be ∼-1‰, based on measurements of organic matter from diazotrophs using molybdenum (Mo)-nitrogenases. We show that the vanadium (V)- and iron (Fe)-only "alternative" nitrogenases produce fixed N with significantly lower δ(15)N (-6 to -7‰). An important contribution of alternative nitrogenases to N2 fixation provides a simple explanation for the anomalously low δ(15)N (<-2‰) in sediments from the Cretaceous Oceanic Anoxic Events and the Archean Eon. A significant role for the alternative nitrogenases over Mo-nitrogenase is also consistent with evidence of Mo scarcity during these geologic periods, suggesting an additional dimension to the coupling between the global cycles of trace elements and nitrogen.
The surface complexation model (SCM) has been used successfully to describe the adsorption properties of oxides and other solids, such as carbonates and sulphides. This model describes adsorption as a superposition of the electrostatic interactions at the solid-water interface with chemical reactions of solutes with reactive groups at the surface of the solid. The activities of the species at the interface depend on the electrostatic potential of the surface, and are calculated from a so-called 'coulombic term' whose exact form depends on the electrostatic description of the interface (e.g. double layer or triple layer). The surface complexation model has been used to model two kinds of experimental data: acid-base titration curves and metal or ligand sorption edges. It predicts that as the ionic strength, I, decreases, the absolute surface charge density also decreases at a given pH. At low ionic strength, the surface charge is less efficiently shielded and accumulation of charge on the surface is energetically more difficult. In the absence of any strongly binding ion, the SCM also predicts that the titration curves at various I intercept at the same pH, the zero proton condition (ZPC), where the concentrations of adsorbed H + and OHat the surface are equal. Sorption edges of a cation M n+ usually show a strong increase of adsorption with pH, as the competition of H + with the cation for the surface sites decreases. The slope of the [M ~+] adsorbed v s pH curve reflects the net number of H + ions displaced by the adsorption of M n+ which depends on the stoichiometry of the reaction and the variation of the surface potential with pH. In spite of the success of the SCM in describing sorption properties of oxides, its application to clays has not been straigthforward. In particular, some titration data on clays such as kaolinite and montmorillonite show an increase in the ZPC as ionic strength decreases; concomitantly, at low pH, the charge density is higher at low ionic strength, contrary to what is usually observed on oxides. Furthermore, fitting metal sorption on clays with the SCM has proven difficult (Bradbury and Baeyens, 1997). Here, we propose that the different sorption properties of clays compared to oxides may be explained by the porous character of clays, whose interlayers are permeable to water and electrolyte ions, together with their fixed internal charge resulting from isomorphic substitution. The electrostatics of a system consisting of a homogeneous porous solid bearing a fixed internal charge and immersed in a 1:1 electrolyte solution can be described by the Poisson-Boltzmann equation, leading to a generalization of the Gouy-Chapman theory. We thus obtain expressions for the internal and surface potential as a fimction of ionic strength, internal charge density and surface charge density.
Summary Biological nitrogen fixation, the main source of new nitrogen to the Earth's ecosystems, is catalysed by the enzyme nitrogenase. There are three nitrogenase isoenzymes: the Mo‐nitrogenase, the V‐nitrogenase and the Fe‐only nitrogenase. All three types require iron, and two of them also require Mo or V. Metal bioavailability has been shown to limit nitrogen fixation in natural and managed ecosystems. Here, we report the results of a study on the metal (Mo, V, Fe) requirements of Azotobacter vinelandii , a common model soil diazotroph. In the growth medium of A. vinelandii , metals are bound to strong complexing agents (metallophores) excreted by the bacterium. The uptake rates of the metallophore complexes are regulated to meet the bacterial metal requirement for diazotrophy. Under metal‐replete conditions Mo, but not V or Fe, is stored intracellularly. Under conditions of metal limitation, intracellular metals are used with remarkable efficiency, with essentially all the cellular Mo and V allocated to the nitrogenase enzymes. While the Mo‐nitrogenase, which is the most efficient, is used preferentially, all three nitrogenases contribute to N 2 fixation in the same culture under metal limitation. We conclude that A. vinelandii is well adapted to fix nitrogen in metal‐limited soil environments.
