2018. Coupled Mo-U abundances and isotopes in a small marine euxinic basin: constraints on processes in euxinic basins. Geochimica et Cosmochimica Acta 222 , pp.Abstract 19 20 Sedimentary molybdenum (Mo) and uranium (U) abundances, as well as their isotope 21 systematics, are used to reconstruct the evolution of the oxygenation state of the surface Earth 22 from the geological record. Their utility in this endeavour must be underpinned by a thorough 23 understanding of their behaviour in modern settings. In this study, Mo-U concentrations and 24 their isotope compositions were measured in the water column, sinking particles, sediments 25 and pore waters of the marine euxinic Lake Rogoznica (Adriatic Sea, Croatia) over a two year 26 period, with the aim of shedding light on the specific processes that control Mo-U 27 accumulation and isotope fractionations in anoxic sediment. 28 Lake Rogoznica is a 15 m deep stratified sea-lake that is anoxic and euxinic at depth. The 29 deep euxinic part of the lake generally shows Mo depletions consistent with near-quantitative 30Mo removal and uptake into sediments, with Mo isotope compositions close to the oceanic 31 composition. The data also, however, show evidence for periodic additions of isotopically 32 light Mo to the lake waters, possibly released from authigenic precipitates formed in the upper 33 oxic layer and subsequently processed through the euxinic layer. The data also show evidence 34 for a small isotopic offset (~0.3‰ on 98 Mo/ 95 Mo) between particulate and dissolved Mo, even 35 at highest sulfide concentrations, suggesting minor Mo isotope fractionation during uptake 36 into euxinic sediments. Uranium concentrations decrease towards the bottom of the lake, 37where it also becomes isotopically lighter. The U systematics in the lake show clear evidence 38 for a dominant U removal mechanism via diffusion into, and precipitation in, euxinic 39 sediments, though the diffusion profile is mixed away under conditions of increased density 40 stratification between an upper oxic and lower anoxic layer. The U diffusion-driven 41 precipitation is best described with an effective 238 U/ 235 U fractionation of +0.6‰, in line with 42 other studied euxinic basins. 43 3 Combining the Mo and U systematics in Lake Rogoznica and other euxinic basins, it is 44 apparent that the two different uptake mechanisms of U and Mo can lead to spatially and 45 temporally variable Mo/U and Mo-U isotope systematics that depend on the rate of water 46 renewal versus removal to sediment, the sulfide concentration, and the geometry of the basin. 47

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Cyclic voltammetry on the Hg electrode was used to investigate the electrochemical behavior of NaCl/NaHCO 3 electrolyte solutions supersaturated with respect to Zn sulfide phases. The voltammetric results clearly show how an Hg electrode, due to exchange between Hg 2+ from an HgS adlayer and Zn 2+ from solution, becomes the site for surface ZnS adlayer formation in the potential range −0.45 to −0.6 V. The exchange reaction is reversible, and the surface-formed ZnS adlayer persists at the Hg electrode surface until −1.3 V during cathodic scans. Near −1.3 V, it is reduced. In the same solution, evidence for reduction of bulk Zn sulfide species including nanoparticles was not obtained. The approach emphasized here can be readily extended to any other system consisting of metal electrode and chalcogenide anions, pointing to the importance of choosing experimental conditions (i.e., deposition potential, stirring, and accumulation times) to avoid artifacts and wrong interpretation of data due to surface formation of metal sulfide species.

Environmental context. In anoxic environments FeS is both an important mediator in the Fe and S biogeochemical cycles and plays a vital role in controlling the scavenging and availability of many trace metals. Electrochemical detection of colloidal and particulate FeS in natural waters can be done by voltammetric measurements. The recorded anodic waves, however, are rather qualitative and lack information on the FeS concentration and size distribution.Abstract. The interactions of FeS nanoparticles (NPs) with a hanging mercury drop electrode in NaCl solutions were monitored by chronoamperometric measurements. Collisions of FeS NPs with the mercury surface were studied over a wide range of electrode potentials (between 0 and À1.9 V v. Ag/AgCl). Faradaic impact transients were recorded only at the negative potentials (between À1.5 and À1.9 V). It was shown that the mercury electrode surface modified with a FeS adlayer catalyses sodium reduction by shifting the potentials of this process to more positive values. This catalytic process together with possible hydrogen evolution is assumed to be the physicochemical basis for the determination of FeS NPs. Chronoamperometric measurements at the electrode potential of À1.9 V showed that the reduction processes of sodium and hydrogen on FeS NPs upon collision are the main cause of sharp reduction current transients. At sufficiently positive electrode potentials (,À1.5 V) the colliding FeS NPs would not be immediately repelled; instead they remained adhered to the mercury surface, causing 'staircase-like' chronoamperometric signals. It appears that recorded reduction current transients are carrying FeS NPs' size information, which is consistent with parallel dynamic light scattering (DLS) measurements.

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BackgroundElemental sulfur (S) persists in natural aquatic environment in a variety of forms with different size distributions from dissolved to particulate. Determination of S speciation mainly consists of the application of chromatographic and electrochemical techniques while its size determination is limited only to the application of microscopic and light scattering techniques. S biological and geochemical importance together with recent increases of S industrial applications requires the development of different analytical tools for S sizing and quantification. In recent years the use of electrochemical measurements as a direct, fast, and inexpensive technique for the different nanoparticles (NPs) characterization (Ag, Au, Pt) is increasing. In this work, electrochemical i.e. chronoamperometric measurements at the Hg electrode are performed for determination of the size distribution of the S NPs.ResultsS NPs were synthesized in aqueous medium by sodium polysulphide acidic hydrolysis. Chronoamperometric measurements reveal the polydisperse nature of the formed suspension of S NPs. The electrochemical results were compared with dynamic light scattering measurements parallel run in the same S NPs suspensions. The two methods show fairly good agreement, both suggesting a log-normal size distribution of the S NPs sizes characterized by similar parameters.ConclusionsThe preliminary results highlight the amperometric measurements as a promising tool for the size determination of the S NPs in the water environment.

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