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The interaction of V(IV) and V(V) with succinic acid was investigated by affinity capillary electrophore- sis (ACE) in aqueous acid solutions at pH values 1.5, 2.0 and 2.4, and different ligand concentra- tions. V(IV) and V(V) form protonated complexes with succinic acid ligand at this pH range. The loga- rithms of the stability constants, measured at 0.1 mol L-1 (NaClO4 /HClO4) ionic strength and 25 °C, are logβ111 =7.4 ± 0.2 and logβ122 =14.1 ± 0.5 for V(IV), and logβ111=7.3 ± 0.1 for V(V), respectively. The stability constant values, extrapolated to zero ionic strength with the Davies equation, are logβ°111=8.3 ± 0.2 and logβ°122 =15.6 ± 0.5 for V(IV) and logβ°111 =7.9 ± 0.1 for V(V). The application of ACE to the simultaneous equilibria of V(IV) and V(V) (injection of two analytes) was also attempted. When the results were compared with those obtained when introducing only one analyte in the capillary, using the traditional version of the method, similar stability constants and precision are obtained. The possibility of studying two analytes simultaneously decreases the time needed for the determination of the constants ; this feature is especially valuable when working with hazardous materials or when only small quantities of ligand are available.

The abundance and distribution of dissolved Re (DRe) were determined in the freshwater part of the Krka River (Croatia), which drains a karst landscape, and in the salinity gradient of its highly stratified estuary. Due to the low DRe concentration, a batch procedure consisting of a pre-concentration step using an anion exchange resin (Dowex) and analysis of DRe in 8 M HNO3 eluate using high-resolution inductively coupled plasma mass spectrometry (HR ICP-MS) was applied. Due to potentially inconsistent recoveries, which ranged from 60 to 87%, quantification was performed using the isotope dilution technique (ID). DRe concentrations in the Krka River increased downstream, from 6.2 pM at the spring site to 11.9 pM upstream of the estuary region. Weathering of the surrounding carbonate lithology is assumed to be the source of the natural Re. Two specific anomalies were registered: a strong increase in DRe concentration due to anthropogenic input near the town of Knin (27.5 pM) and a decrease at a downstream site caused by subsurface input of freshwater from the Zrmanja River, resulting in a relatively low DRe concentration (8.5 pM). In the estuarine region, a near- conservative behavior of DRe was found in the salinity gradient of the upper surface layer, with DRe concentrations ranging from 18 to 38 pM. Anthropogenic input was suspected within the estuarine segment near the urban area, causing a small positive deviation from the conservative line. In the bottom seawater layer, a minor decrease in DRe concentration in the most upstream estuarine regions was apparent, implying weak scavenging of Re

Combined chromatographic and spectral analysis was used to analyse V redox conversion in the aqueous solution in the presence of several biologicaly and environmentally important thiols: L-cysteine, thioacetic acid and ethanethiol. The L-cysteine proved to be most powerful reducens of V(V), followed with thioacetic acid and ethanethiol.

In the environment vanadium(V) is found in three oxidation states (III, IV and V) depending on physicho-chemical conditions (pH, Eh, ionic strength, its concentration, biology, organic matter content ect.) of certain environmental medium [1]. Knowledge of V species distribution is to this date very scarce due to its complex aquatic chemistry as well as modern analytical intrumentation constraints [2,3]. Especially, methods on accurate V(III) species determination in complex environmental matrices are poorly developed which makes estimation of V biogeochemical cycle uncomplete. It is known that V(III) is expected to be stable in strongly anoxic or euxinic conditions, such as sulphide-containing water samples or sediments. Furthermore, high affinity towards adsorption on various colloids found in natural aquatic systems as well as formation of strong complexes with various organic ligands is presumed. Possible formation of V(III) in certain aquatic environments can contribute to removal of V into sediments [2]. Therefore, determination of V(III) in natural environmental samples is highly needed for accurate estimation of V bioavailability, mobility and toxicitiy.Method developed by Yatirajam et al. (1979) was used in order to establish V(III) stability in various model solutions [4]. Procedure is based on the formation of selective complexes of V(III) and picnolic acid in respect to V(IV) and V(V) species present in samples. Upon complexation, V(III) species are then extracted into chloroform. Extracts were evaporated to dryness and the remaining content was dissolved in 2% HNO3. Samples were then measured using HR ICP-MS analytical intrumentation. For measurements using spectrophotometry, samples were measured immidiately upon extraction. Studies so far show good selectivity, reproducibility and accuracy which offers a promising method for V(III) determination in natural samples. Stated findings are planned to be further applied on natural anoxic sediment samples of Mljet and Rogoznica lakes (Croatia).Literature:Huang, J.H.; Huang, F.; Evans, L.; Glasauer, S. Vanadium: Global (bio)geochemistry. Chem. Geol. 2015, 417, 68–89, doi:10.1016/j.chemgeo.2015.09.019. Gustafsson, J.P. Vanadium geochemistry in the biogeosphere –speciation, solid-solution interactions, and ecotoxicity. Appl. Geochemistry 2019, 102, 1–25, doi:10.1016/j.apgeochem.2018.12.027. Costa Pessoa, J. Thirty years through vanadium chemistry. J. Inorg. Biochem. 2015, 147, 4–24, doi:10.1016/j.jinorgbio.2015.03.004. Yatirajam. V, Arya, S.P. EXTRACTION DETERMINATION AS VANADIUM ( II1 ) OF VANADIUM. Talanta 1978, 26, 60.

The reduction of vanadate (+V) in the presence of 3-mercaptopropionic acid was studied using a chromatographic method for the determination of vanadate (+V) versus vanadyl (+IV) species. Ion chromatography was combined with spectrophotometric investigation of the absorption properties of the solution. The chromatographic method for the separation of vanadate (+V) and vanadyl (+IV) was carried out with an anion exchange column. In the initial reaction mixture containing vanadate (+V) and 3-mercaptopropionic acid, ethylenediaminetetraacetic acid – EDTA was added in an excessive amount relative to the concentration of reactants in the solution. After the ligand exchange reaction, the added EDTA terminates the reduction, allowing redox speciation in the solutions. A strong pH dependence of the reduction rates in the investigated solution was observed. The vanadate reduction seems to proceed in 2 steps: 1) formation of the intermediate vanadate (+V)- thioester ; 2) reduction reaction and formation of the vanadyl (+IV)-thiol complex. The obtained results strongly suggest that the reaction of vanadate (+V)-thioester formation is proton catalyzed. It was observed that the overall reduction rates are pH dependent due to the complex vanadate (+V) solution speciation and changes in the ionic form of 3-mercaptopropionic acid.

The concentration and isotopic composition of uranium (δ238U, 234U/238U activity ratio) in combination with traditional isotopes (δ18O, δ13C) were examined as potential tracers of authigenic carbonate formation in a karst aquifer. The U concentration and 234U/238U activity ratios in the tufa-precipitating sections of two connected karst rivers (Krka and Zrmanja, Croatia) decreased downstream in water and in precipitated carbonate due to active self-purification processes, i.e. adsorption of isotopically lighter U(VI) on mineral particles, sedimentation and co-precipitation with carbonate. The isotopic composition of carbonate in tufa mostly resembled the 234U/238U activity ratio and the δ238U values of dissolved U in water but was also affected by the presence of detrital carbonate flushed into the river from soil and weathered bedrock. This interpretation was supported by the δ18O and δ13C values of tufa, which were shifted out of equilibrium with river water and dissolved in organic carbon and in their isotopic signature, which showed the presence of lithic carbonate. Large fluctuations of the δ238U values of water, leachable U (eluted in acetic acid buffered with Na-acetate) and residual U fraction could not be fully explained by available data due to the overlapping U isotopic signatures of leachable (mainly carbonate) and residual fractions of soil, bedrock and tufa. Therefore, a long-term, systematic, seasonal and event-based observation of the isotopic composition of dissolved and suspended particulate U in water is necessary. Nevertheless, the U isotopes were found to have the potential to be used as identifiers of authigenic carbonate and the storage of CO2 in terrestrial river sediments, to improve knowledge on fluxes within local and global biogeochemical carbon cycle.

The abundance and distribution of dissolved Re (DRe) has been determined in the freshwater part of the Krka River (Croatia), draining the karstic carbonate landscape, as well as in the salinity gradient of its highly stratified estuary. Due to low DRe concentration, the batch procedure consisting of pre-concentration step using an anion exchange resin (Dovex) and analysis of DRe in 8 M HNO 3 eluate using a high resolution inductively coupled plasma mass spectrometry (HR ICP-MS) was adopted. Due to the potential inconsistent recoveries, ranging from 60% to 87%, the quantification was performed by using the isotope dilution (ID) technique. The DRe concentration in the Krka River basically increased in the downstream direction, with the 6.2 pM at the spring site, reaching 11.9 pM before the estuarine region. The weathering of the surrounding carbonate lithology is assumed as the source of natural Re. The two specific anomalies were registered: a strong increase in DRe concentration due to the anthropogenic input near the town of Knin (27.5 pM) and a decrease at downstream site caused by the subterranean input of freshwater of the Zrmanja River leading to relatively low DRe concentration (8.5 pM). In the estuarine segment, a near-conservative behavior of DRe was found in the salinity gradient of the upper surface layer, with the DRe concentration ranging from 18 to 38 pM. An anthropogenic supply was suggested within the estuarine segment close to the urban area causing a small positive deviation from the conservative line. In the bottom seawater layer, a minor decrease of DRe concentration in the most upstream estuarine regions was evidenced, implying a potential weak scavenging of Re.

Precipitation of calcite from water fractionates strontium (Sr) isotopes because of preferential incorporation of light (86Sr) isotopes into the solid phase, making continental carbonates one of the most 88Sr depleted reservoirs. It was suggested that carbonate precipitation is the most likely process controlling δ88/86Sr composition of karst water. Therefore, the 88Sr enrichment of river water could be used for the estimation of Sr and carbonate precipitation at catchment scale.In the present study, we report on trace element partitioning and Sr isotope fractionation between tufa and water in the groundwater fed karst river Krka (Croatia). Water and tufa along with samples of bedrock and soil as the main contributors of dissolved and particulate Sr at seven main waterfalls and cascades along a 33 km section of the river were analysed for trace element and Sr isotope composition (δ88/86Sr).The highest δ88/86Sr values were measured in soils and in siliciclastic rocks, while in limestone, the δ88/86Sr values were similar to those of old tufa precipitated in the period between 96 and 141 ky BP. Recent tufa, however, was considerably depleted in 88Sr. The isotope fractionation between water and recent tufa varied a lot and was inversely correlated with Mg and Sr partitioning coefficients, while correlations with precipitation rates and temperature were rather weak. The δ88/86Sr of recent tufa was strongly correlated with the stable isotope composition of organic carbon, which indicates that apart from hydrochemical, hydraulic parameters and temperature, plants and microbial communities that knowingly stimulate the tufa formation also affect the isotope fractionation of Sr.

The molybdenum (Mo) and uranium (U) isotope compositions recorded in carbonate rich sediments are emerging as promising paleo-redox proxies. However, the effects of early diagenetic effects within the sediments on these isotope systems are not well constrained. We examined the Mo and U isotopic systematics in anoxic carbonate rich sediments in a semi enclosed karstic marine lake (Malo Jezero) of the Island of Mljet, Adriatic Sea.\ud Measurements of water column redox behavior in the lake since the 1950s, have shown a transition from anoxic-sulfidic conditions in the deeper water column to more oxic conditions and anoxia refined to the sediment and pore-waters. A 50 cm long sediment core from the deepest part of the lake, show a transition from moderate to high authigenic Mo and U accumulation with depth, consistent with the changing lake redox environment in the past. In the deep euxinic part of the core, the authigenic Mo and U are isotopically lighter and heavier, respectively, than seawater, following similar systematics as observed in other modern euxinic basins, with high, but non-quantitative, Mo and U uptake into the sediments.\ud Based on Bahamas bank carbonate sediments, it has been suggested that the 238U/235U ratio is ~+0.25‰ higher compared to seawater from the effects of early carbonate sediment diagenesis and this carbonate vs. seawater off-set is applicable to carbonate rich sediments across the geological past. The shallower part of lake sediment core was deposited under similar redox conditions as the Bahamas sediments, and these sediments show an average 238U/235U ratio + 0.31 ± 0.01‰ (2SE) higher than seawater. Although the average 238U/235U ratios for these two carbonate rich settings are similar, caution is necessary when inferring seawater 238U/235U compositions from such sediments, as they contain U from different sources (e.g. diagenetic uptake and carbonate-bound). The Mo isotope compositions within the same Malo Jezero sediments are variable but approaches the seawater composition at low pore-water H2S concentrations. This show the potential of using the Mo isotope composition from carbonate rich sediments to infer the seawater composition, however, further work is required to establish the link between the Mo isotope composition and the chemistry of the pore water environment.

Purpose We conducted this investigation in order to develop a simple method for the determination of bioavailable vanadium and its speciation within sediments. Materials and methods The developed method is based on the determination of acid-extractable vanadium, as this fraction is presumably bioavailable, and its subsequent speciation using IC–UV/Vis (ionic chromatography with an ultraviolet-visible detector). A published procedure for vanadium speciation in pore water was further optimized in order to separate V(IV) and V(V) species using column EDTA derivatization on sediment samples. The analytical approach was applied in order to assess acid-extractable vanadium and its speciation within estuary sediments of the Krka River at the Dalmatian coast of Croatia. Results and discussion The results imply that the majority of vanadium within estuary sediments is in the form of less soluble fraction while the acid-extractable fraction comprises approximately 40% of the total vanadium present. In contrast, open sea sediment (station K4) was completely dominated by the acid-extractable vanadium. The extracted vanadium is in the form of V (IV). Conclusions Ion-exchange chromatography with an UV/Vis detector is a promising analytical method for vanadium speciation, however, further optimization of the conditions (improvement of the sequential extraction procedure) is needed in order to assess speciation also within other vanadium sedimentary fraction

Chemical weathering of silicate rock is one of the major long-term sinks of atmospheric CO2. While several radiogenic isotope systems (e.g. Nd, Pb, Os) in the authigenic component of marine sediments show temporal patterns that could be interpreted in terms of fluctuations in chemical weathering rate over glacial cycles, recent work using beryllium isotopes suggests little such variation. However, our knowledge of the sources of 9Be to the oceans is incomplete. Here, we address some of the gaps in knowledge with new data on Be in Croatian and Scottish rivers and estuaries, as well as with mixing experiments between Amazon tributaries and ocean water. The new data, combined with data from the literature, confirm that riverine Be is only marginally affected by carbonate weathering, which only accounts for 0.4–1.5% of dissolved riverine Be fluxes. The behavior of Be in estuaries is highly variable, with some estuaries showing removal and others addition. Based on the available data, about 55% of dissolved riverine Be is removed in estuaries, with complexation by both organic vs. inorganic ligands appearing to exert a strong control on the specific behavior in a given estuary. Beryllium complexation with organic material facilitates transport across estuaries, whereas the presence of inorganic complexants results in loss. Presence of both ligand types may lead to stronger non-conservative behavior than the presence of only one. Combining different approaches for estimating the dissolved riverine Be flux and estuarine removal, we calculate that 13–30% (4–48% error range) of the marine 9Be input is derived from riverine dissolved loads, and 70–87% (53–96% error range) from other sources like desorption from particulate matter and release from shelf sediments. It seems likely that this 13–30% of the marine Be input would respond quickly to changes in continental weathering rates, instantaneously impacting marine Be isotopes. However, the exact response of marine Be isotopes to changes in continental weathering rates also depends on the timescale of Be release from the riverine particulate pool stored in sediments on continental margins.

An improved methodology was developed for V redox speciation in estuarine waters using a hyphenated technique consisting of ion chromatograph (IC) with an anion exchange column and a high-resolution inductively coupled plasma mass spectrometer (HR ICP-MS). This approach enables the direct determination of V(V), whereas reduced species (mainly V(IV)) are calculated by subtracting V(V) concentrations from the measured total V concentration. Based on the “on-column” V(V) chelation mechanism by EDTA, with the eluent composed of 40 mmol L−1 ammonium bicarbonate, 40 mmol L−1 ammonium sulphate, 8 mmol L−1 ethylenediaminetetraacetic acid and 3% acetonitrile, the method was successfully used for analyses of V redox speciation in samples taken in the vertical salinity gradient of the highly stratified Krka River estuary. Due to the matrix effects causing different sensitivities, a standard addition method was used for V(V) quantification purposes. The limit of detection (LOD) was also found to be matrix related: 101.68 ng L−1 in the seawater and 30.56 µg L−1 in the freshwater. Performed stability tests showed that V redox speciation is preserved at least 7 days in un-treated samples, possibly due to the stabilization of V-reduced species with natural organic matter (NOM). The dominant V form in the analysed samples was V(V) with the reduced V(IV) accounting for up to 26% of the total dissolved pool. The concentration of V(IV) was found to correlate negatively with the oxygen concentration. Significant removal of dissolved V was detected in oxygen depleted zones possibly related to the particle scavenging.

The increased amount of organic matter (OM) accompanied with the development of hypoxia/anoxia and appearance of the sulphur species are indicators of eutrophication in natural waters. To predict biogeochemical and ecological changes, specific OM components in the Lake Brljan (the karst Krka River, Croatia; station B and station B2 downstream) were studied during 2011 and in January 2012 using voltam¬metry. Surface‒active substances (eq. 0.018 to 0.150 mg dm–3 Triton‒X‒100) and Cu complexing capacity (54‒680 nmol dm–3) showed the highest values in surface water layer in May 2011 (B) and in November 2011 (B2), associated with the highest dissolved (0.505‒1.530 mg C dm–3) and particulate (0.051‒0.725 mg C dm–3) organic carbon and phytoplankton biomass. Autochthonously produced OM did not cause hypoxia/anoxia during the investigated period. Brljan's lake water could be characterised as oligotrophic with occasionally elevated OM concentration depending on season and Krka River water regime. This work is licensed under a Creative Commons Attribution 4.0 International License.

Rogoznica Lake is highly eutrophic marine system located on the Eastern Adriatic coast (43°32′N, 15°58′E). Because of the relatively small size (10,276 m 2 ) and depth (15 m) it experiences strong natural and indirect anthropogenic influences. Dynamics within the lake is characterized by the extreme and highly variable environmental conditions (seasonal variations in salinity and temperature, water stratification and mixing, redox and euxinic conditions, concentrations of nutrients) which significantly influence the biology inside the lake. Due to the high phytoplankton activity, the upper part of the water column is well oxygenated, while hypoxia/anoxia usually occurs in the bottom layers. Anoxic part of the water column is characterized with high concentrations of sulfide (up to 5 mM) and nutrients (NH 4 + up to 315 μM; PO 4 3− up to 53 μM; SiO 4 4− up to 680 μM) indicating the pronounced remineralization of the allochthonous organic matter, produced in the surface waters. The mixolimnion varies significantly within a season feeling effects of the Adriatic atmospheric and ocean dynamics (temperature, wind, heat fluxes, rainfall) which all affect the vertical stability and possibly induce vertical mixing and/or turnover. Seasonal vertical mixing usually occurs during the autumn/winter upon the breakdown of the stratification, injecting oxygen-rich water from the surface into the deeper layers. Depending on the intensity and duration of the vertical dynamics (slower diffusion and/or faster turnover of the water layers) anoxic conditions could developed within the whole water column. Extreme weather events such as abrupt change in the air temperature accompanied with a strong wind and consequently heat flux are found to be a key triggering mechanism for the fast turnover, introducing a large amount of nutrients and sulfur species from deeper parts to the surface. Increased concentration of nutrients, especially ammonium, phosphate, and silicates persisting for several months after the mixing event, together with anoxic stress conditions, additionally influence already stressed ecosystem, hence shifting the community structure and food/web interactions in this marine system.

Most stratified sulfidic holomictic lakes become oxygenated after annual turnover. In contrast, Lake Rogoznica, on the eastern Adriatic coast, has been observed to undergo a period of water column anoxia after water layer mixing and establishment of holomictic conditions. Although Lake Rogoznica's chemistry and hydrography have been studied extensively, it is unclear how the microbial communities typically inhabiting the oxic epilimnion and a sulfidic hypolimnion respond to such a drastic shift in redox conditions. We investigated the impact of anoxic holomixis on microbial diversity and microbially mediated sulfur cycling in Lake Rogoznica with an array of culture-independent microbiological methods. Our data suggest a tight coupling between the lake's chemistry and occurring microorganisms. During stratification, anoxygenic phototrophic sulfur bacteria were dominant at the chemocline and in the hypolimnion. After an anoxic mixing event, the anoxygenic phototrophic sulfur bacteria entirely disappeared, and the homogeneous, anoxic water column was dominated by a bloom of gammaproteobacterial sulfur oxidizers related to the GSO/SUP05 clade. This study is the first report of a community shift from phototrophic to chemotrophic sulfide oxidizers as a response to anoxic holomictic conditions in a seasonally stratified seawater lake.

Electroanalytical (voltammetric, polarographic, chronoamperometric) methods on an Hg electrode were applied for studying of different sulfur compounds in model and natural water systems (anoxic lakes, waste water, rain precipitation, sea-aerosols). In all investigated samples typical HgS reduction voltammetric peak, characteristic for many different reduced sulfur species (RSS: sulfide, elemental sulfur, polysulfide, labile metal sulfide and organosulfur species) was recorded at about -0.6 V vs. Ag/AgCl reference electrode. In addition, in anoxic waters which are enriched with sulfide and iron species, voltammetric peaks characteristic for the presence of free Fe(II) and FeS nanoparticles (NPs) were recorded at -1.4 V and around -0.45 V, respectively. Depending on the used electroanalytical method and experimental conditions (varying deposition potential, varying time of oxidative and/or reductive accumulation, sample pretreatment i.e. acidification followed by purging) it is possible to distinguish between different sulfur species. This work clearly shows a large potential of the electrochemistry as a powerful analytical technique for screening water quality regarding presence of different reduced sulfur species and their speciation between dissolved and colloidal/nanoparticle phases.

Molybdenum 98Mo/95Mo isotope ratios are a sediment paleo proxy for the redox state of the ancient ocean. Under sulfidic conditions, no fractionation between seawater and sediment should be observed if molybdate (MoO42–) is quantitatively transformed to tetrathiomolybdate (MoS42–) and precipitated. However, quantum mechanical calculations previously suggested that incomplete sulfidation could be associated with substantial fractionation. To experimentally confirm isotope fractionation in thiomolybdates, a new approach for determination of isotope ratios of individual thiomolybdate species was developed that uses chromatography (HPLC-UV) to separate individual thiomolybdates, collecting each peak and analyzing isotope ratios with multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Using commercially available MoO42– and MoS42– standards, the method was evaluated and excellent reproducibility and accuracy were obtained. For species with longer retention times, complete chromatographic peaks had to be collected to avoid isotope fractionation within peaks. Isotope fractionation during formation of thiomolybdates could be experimentally proven for the first time in the reaction of MoO42– with 20-fold or 50-fold excess of sulfide. The previously calculated isotope fractionation for MoS42– was confirmed, and the result for MoO2S22– was in the predicted range. Isotopic fractionation during MoS42– transformation with pressurized air was dominated by kinetic fractionation. Further optimization and online-coupling of the HPLC- MC-ICPMS approach for determination of low concentrations in natural samples will greatly help to obtain more accurate species-selective isotope information

Cyclic voltammetry on the Hg electrode was used to investigate the electrochemical behavior of NaCl/NaHCO3 electrolyte solutions supersaturated with respect to Zn sulfide phases. The voltammetric results clearly show how an Hg electrode, due to exchange between Hg2+ from an HgSadlayer and Zn2+ from solution, becomes the site for surface ZnSadlayer formation in the potential range −0.45 to −0.6 V. The exchange reaction is reversible, and the surface-formed ZnSadlayer 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.

Concentrations of sulfate, sulfide and intermediate sulfur species as well as quadruple sulfur isotope compositions of sulfate, sulfide and zero-valent sulfur (ZVS) were analyzed in the water column of Lake Rogoznica (Croatia), a stratified marine euxinic lake. The chemocline in the lake, which was located at 8.5–9.5 m depth, supports a dense population of purple phototrophic sulfide oxidizing bacteria from the genus Chromatium. The highest ZVS (5.42 μmol L−1) and sulfite (1.13 μmol L−1) concentrations were detected at the chemocline. Thiocyanate concentrations up to 288 nmol L−1 were detected near the bottom of the lake. The thiocyanate profile suggests that it diffuses up from the sediment, where it may be produced by the reaction of cyanide with sulfide oxidation intermediates. Multiple sulfur isotope fractionations between sulfate and sulfide were consistent with a model finding that disproportionation is not a dominant process below the chemocline. Microbial sulfide oxidation was found to be the dominant process of the reoxidative part of the sulfur cycle. Despite the absence of a clear signal for sulfur disproportionation in multiple sulfur isotope values, δ34S fractionations between sulfate and sulfide were in the range of 43.8–45.2‰, is relatively large in comparison to most laboratory culturing studies. Our results suggest that such fractionation is achieved by microbial sulfate reduction alone, which is in agreement with metabolic models and recent laboratory studies.

Multivariate statistical analyses were applied on the measured physico-chemical (Cd, Pb, Cu, Zn, Mg, Ca, O2, alkalinity, temperature, pH, SAS, DOC and DIC) and isotopic parameters (δ13C and δ18O) to estimate and distinguish anthropogenic from natural influences to the water system of the Krka River. Analyses were conducted on the data collected during six years from twelve sampling sites. On the basis of orientation, positioning and grouping of parameters arranged by biplots, four main hypotheses were defined and finally statistically confirmed. Thereof, two main and distinct processes occurring in the Krka River could be highlighted: (i) upstream pollution, caused by the inflow of untreated waste-waters of city of Knin and (ii) downstream selfpurification, caused by the sedimentation and/or co-precipitation of pollutants coupled by the inflow of clean subterranean water (groundwater recharge). Grouping of (i) hydrological and carbon cycle connected parameters, and (ii) anthropogenically influenced correlated parameters were proposed as a result of statistical analysis. Regarding the pH, it is shown that a stream section influenced by the subterranean inflow of Zrmanja River is statistically significantly different for all sampling campaigns during six years, being lower for about 0.5 pH unit.

A long-standing problem associated with voltammetric determination of iron and sulfide in reduced natural waters has been the nature of the presumed analyte responsible for a reduction peak at -1.1 V vs. Ag/AgCl. Cyclic voltammetry at the Hg electrode is used here to study solutions with different Fe(II) to sulfide ratios in chloride and acetate electrolytes (pH 6–7). The results indicate that the -1.1 V peak can be assigned to reduction of Fe2+ or its labile complexes on FeS layers that partially cover the Hg electrode. Hg electrodes covered with FeS act like FeS solid electrodes over a very wide potential range (-0.35 to -1.9 V). Two mechanisms for forming FeS layers on Hg are described. Over the broadest deposition potential range, the dominant mechanism involves attachment at the Hg surface of FeS nanoparticles, which are generated quickly in initially supersaturated mixtures of Fe(II) and S(–II). In a narrow deposition potential range, roughly -0.56 to -0.70, FeS layers are produced additionally by replacement of preformed HgS. Because Fe2+ is reduced at -1.1 V on FeS layers and at -1.4 V on bare Hg, it may be underdetermined when only the -1.4 V peak is measured.

International audience; The vertical distribution of reduced sulfur species (RSS including H2S/HS-, S-0, electroactive FeS) and dissolved Fe(II) was studied in the anoxic water column of meromictic Lake Pavin. Sulfide concentrations were determined by two different analytical techniques, i.e. spectophotometry (methylene blue technique) and voltammetry (HMDE electrode). Total sulfide concentrations determined with methylene blue method (Sigma H2SMBRS) were in the range from 0.6 mu M to 16.7 mu M and were substantially higher than total reduced sulfur species (RSSV) concentrations determined by voltammetry, which ranged from 0.1 to 5.6 mu M. The observed difference in the sulfide concentrations between the two methods can be assigned to the presence of FeS colloidal species.Dissolved Fe was high (>1000 mu M), whereas dissolved Mn was only 25 mu M, in the anoxic water column. This indicates that Fe is the dominant metal involved in sulfur redox cycling and precipitation. Consequently, in the anoxic deep layer of Lake Pavin, "free" sulfide, H2S/HS-, was low; and about 80% of total sulfide detected was in the electroactive FeS colloidal form. IAP calculations showed that the Lake Pavin water column is saturated with respect to FeSam phase. The upper part of monimolimnion layer is characterized by higher concentrations of S (0) (up to 3.4 mu M) in comparison to the bottom of the lake. This behavior is probably influenced by sulfide oxidation with Fe(III) oxyhydroxide species

Organic complexation in atmospheric precipitation is an important control mechanism of the biogeochemistry of metal ions. We have measured complexation of copper ions in bulk precipitation by the method of differential pulse anodic stripping voltammetry (DPASV), which introduces least perturbation in the samples. The precipitation samples were collected periodically during one year in the mainland area of Croatia (Zagreb) and at the Adriatic coast (Šibenik). The results have shown the complexation of copper ions in all the samples. The method of determination presumes the existence of 1:1 copper ion - ligand complexes. The complexing capacity of rainwater samples for copper ions ranged between 0.010 and 1.4 &micro ; M. The highest capacity was measured in April 2003 in rainwater sample from the Šibenik area, which could be associated to the contribution of biogenically driven organics to marine aerosol. Apparent stability constants determined were in the range of 6.1 -10. The sources of organic ligands in the samples could be multiple. The concentrations of dissolved organic carbon (DOC) as well as of surface active substances (SAS) were measured. The proportion of complexing ligands in these two organic matter fractions has also been discussed.

Molybdenum precipitates preferentially under reducing conditions ; therefore, its occurrence in sediment records is used as an indicator of paleoredox conditions. Although thiomolybdates (MoO4-xSx(2-) with x = 1-4) supposedly are necessary intermediates in the process of molybdenum precipitation under anoxic conditions, there is no information about their abundance in natural environments, because of a lack of element-specific methods with sufficiently low detection limits. Here, we optimized ion-pair chromatographic separation for coupling to an inductively coupled plasma-mass spectrometry detector (IPC-ICP-MS). 2-Propanol (10%-25% gradient) replaced the previously used acetonitrile (25%-75%) as the solvent, to reduce the carbon load into the plasma. In synthetic solutions, formation of thiomolybdates was found to occur spontaneously in the presence of excess sulfide and the degree of thiolation was highest at pH 7. Excess hydroxyl led to a transformation of thiomolybdates to molybdate. Under acidic to neutral conditions, precipitation of molybdenum and hydrolysis of tetrathiomolybdate were observed. Flash-freezing was found to be suitable to stabilize tetrathiomolybdate, with 2 mM) negatively affected the detection of molybdate, which eluted mainly in the dead volume, but had no negative effect on higher thiolated molybdates. Detection limits were ∼10 nM. With the newly developed IPC-ICP-MS method, thiomolybdates were found to form spontaneously in euxinic marine waters after adding a molybdate spike and occur naturally in sulfidic geothermal waters

Marine Rogoznica lake (RL), an eutrophic and euxinic environment situated at the eastern Adriatic coast was used as a natural laboratory to test the application of atomic force microscopy (AFM) in combination with the electrochemical and high temperature catalytic oxidation (HTCO) measurements for characterization of water column organic matter (OM) and reduced sulfur species (RSS) in relation to seasonal changes of environmental conditions. Water column of RL was explored at the nanoscale by the AFM during the anoxic holomictic event (S1, October 2011) and stratified winter (S2, January 2012) and spring (S3, May 2012) conditions in the oxic and anoxic water samples. Obtained results from the AFM uphold the electrochemical and HTCO measurements, indicating significant difference in the present type of OM during the holomictic, anoxic stress conditions in comparison to the samples collected during the stratification period. Differences in the OM type were discussed in line with the physical and biological processes that occurred in RL during sampling: mixing processes characterized by fast turnover of water layers and biological activity characterized by low (January 2012) and high (May 2012) primary production of diatoms and zooplankton grazing activities.

800x600 The Rogoznica Lake marine system is a small, karstic, naturally eutrophic and euxinic marine environment. Abundance and temporal distribution of phytoplankton was investigated in relation to environmental conditions in the period from 1998 to 2013. The 36 determined diatoms contributed 90% of the total phytoplankton abundance. The diatom composition is characterized by low species diversity and high single species abundance (up to 107 cells L–1). There were, on average, 2.6 diatom species per sample (maximum 14 diatom species per sample) reflecting extreme environmental conditions. Dominant diatoms Thalassionema nitzschioides , Cyclotella choctawhatcheeana , Dactyliosolen fragilissimus and Chaetoceros curvisetus occurred repeatedly and were alternately dominant in the lake during the whole research period. Some diatoms were dominant only in limited period, like Cyclotella choctawhatcheeana (evident since 2001), and Pseudo- nitzschia spp. (evident in the period 2002 to 2009). It appears that the interplay of environmental conditions such as variability in thermohaline and redox conditions, nutrient and reduced sulphur concentration influence the phytoplankton development and abundance in the lake. Normal 0 21 false false false HR X-NONE X-NONE MicrosoftInternetExplorer4

The characteristics outlines above (a well- studied hydrography-since 1994 ; a stable and constant oxic/anoxic interface ; and a stratified system that has persisted long enough to produce steady-state conditions relative to alkalinity and sulfide concentrations) make Rogoznica lake ideal sites to examine the processes that control the biogeochemistry of anoxic environments. The small size and physically stable nature of the lake allows researchers to examine small scale spatial and temporal variability as well as longer term processes. All our data show how particular sequences of meteorological events, some of which can be regarded as extreme, have affected the internal processes in the Lake. An integration of all results (since 1994 up to now) reveals an interesting possibility that this environment may well potentially serve as valuable sentinels of climate change

Mapping and monitoring of the seagrass Posidonia oceanica in the eastern (Croatian) part of the Adriatic Sea since 2004 indicates a significant decline in meadow density in an area impacted by inshore capture-based tuna aquaculture. The density and overall condition of P. oceanica meadows impacted by tuna farms near Fulija Islet was compared to two reference sites (Iž Island and Mrtovnjak Islet). The factors with the most significant influence on P. oceanica meadows were found to be the input of organic matter originating from the cages, as well as high epiphyte biomass caused by nutrient enrichment. Significant differences in nutrient concentrations were found between the sites impacted by tuna farms (Fulija Islet) and the control stations. Shoot density of the P. oceanica meadows decreased at the stations in close vicinity to the tuna farm, which suggests that the tuna farm activity strongly affected the surrounding meadows.

International audience; Recent publications have shown that the anodic reaction between FeS and Hg can be used for electrochemical detection of colloidal and particulate FeS in natural waters. Anodic waves that were recorded around -0.45 V (vs Ag/AgCl) in model solutions correspond to the electrochemical transformation of nanoparticulate FeS to HgS. Here, as a further step, the proposed approach is tested on anoxic, sulfidic, and iron-rich samples of a meromictic freshwater lake (Lake Pavin, France). Based on new and more comprehensive work on FeS electrochemistry in model and anoxic Lake Pavin samples, a new interpretation is given for previously recorded voltammetric signals in sulfide and iron rich environment, usually designated FeS(aq), and its role in controlling solubility of different FeS phases. A comparison of the depth profiles of S(-II) measured by voltammetry and the methylene blue method showed that the majority of S(-II) is in the form of FeS. In the monimolimnion layer, thermodynamic calculations based on total Fe(II) and S(-II) concentration, measured by ferrozine and the methylene blue method, predict precipitation of FeS with log K-s values between -3.6 and -3.8, very close to mackinawite's K-s value. In the upper part of the same layer, precipitation of greigite is predicted. It is shown that modification of a Hg electrode by surface-formed FeS has a significant influence on voltammetric Fe(II) determination, since reduction of Fe(II) under such conditions occurs both on bare (-1.4 V) and on FeS modified Hg surfaces (-1.1 V); Fe(II) may be underdetermined when only the -1.4 V peak is measured.

Environmental contextIn 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. AbstractThe 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 –1.9V v. Ag/AgCl). Faradaic impact transients were recorded only at the negative potentials (between –1.5 and –1.9V). 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 –1.9V 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 (~–1.5V) 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.

Eutrofikacija vodenih ekosustava vrlo je aktualna u jezerima europskog prostora. Obraštanje jezerskog dna i rubova jezera uz povećanu količinu otopljene organske tvari te kao rezultat razvoj hipoksično-anoksičnih uvjeta glavni su idikatori eutrofikacijskih procesa. Ovim projektom provodili smo sezonski monitoring osnovnih fizičko-kemijskih parametara poput temperature, vodljivosti, otopljenog kisika, zatim sadržaja i svojstava organske tvari, te sumpornih spojeva u vodenom stupcu jezera Brljan i nizvodno od jezera na lokacijama označenim kao B i B2 na Slici 1. Provedena istraživanja kao i postignuti preliminarni rezultati doprinjeli su boljem razumijevanju jezara Brljan kao važnog dijela sliva rijeke Krke te će se koristiti u budućoj kontroli kao i sprečavanju procesa koji bi doveli do eventualne eutrofikacije u navedenom području. Istraživanja/monitoring vodenog stupca jezera Brljan i nizvodno od jezera provodila su se ovisno o sezonskim uvjetima (vodni režim) kao i aktivnostima hidroelektrane Miljacka a vodeni stupac jezera Brljan i nizvodno od jezera ispitivan je s obzirom na: 1) Sadržaj otopljene i suspendirane organske tvari (DOC i POC) 2) Sadržaj i karakterizaciju organske tvari s obzirom na površinsku aktivnost 3) Kapacitet organske tvari za vezanje s ionima bakra 4) Sadržaj otopljenog kisika i sumpornih specija

Voltammetric and polarographic methods at the Hg electrode were used to study presence of sulfur species in different contrasting natural samples: stratified anoxic lakes, seawater lake Rogoznica Lake (Croatia) and crater lake Pavin Lake (France); rain precipitation and sea-aerosols. In all of these investigated samples typical HgS reduction voltammetric peak, characteristic for many different reduced sulfur species (RSS, sulfide, elemental sulfur, polysulfides, labile metal sulfide species, orgnosulfur compounds) was recorded at about -0.6 V vs. Ag/AgCl reference electrode. In addition to, in Pavin Lake anoxic samples voltammetric peaks characteristic for the presence of free Fe(II) and FeS nanoparticles were recorded at -1.4 V and around -0.45 V, respectively. Depending on experimental conditions (oxidative and/or reductive accumulation, different accumulation times and potentials or different stripping technique, acidification and purging) it is possible to make distinguish between different sulfur species. This work clearly shows a large potential of electrochemistry to be used as powerful analytical technique for determination and speciation of different reduced sulfur species in dissolved and colloidal phases in natural waters.

Rogoznica Lake, on the Adriatic coast of Croatia, undergoes vertical mixing in autumn, but quickly restratifies and becomes intensely sulfidic during the succeeding 11 months. Autumn's vertically homogeneous dissolved Mo concentrations quickly revert to a summer profile in which Mo concentrations are lower by an order of magnitude in the sulfidic layer than in the oxic layer. Other redox sensitive elements (As, U and V) are much less affected on this time scale. Below the depth where sulfidic waters become saturated with iron monosulfide, Mo concentrations become nearly constant. A similar pattern can be recognized in other euxinic basins and seems inconsistent with Mo removal by sorption on sinking particles. Precipitation of a previously unrecognized, probably nanoscale Fe(II)–Mo(VI) sulfide mineral is postulated to explain this phenomenon. By fitting the Rogoznica data, a provisional composition and solubility product constant for this material are derived. The mineral's formula approximates Fe5Mo3S14. Its Mo content is similar to that of a synthetic precipitate previously shown by X-ray spectroscopy to resemble molybdenum's host phase in black shales. Assuming saturation with this mineral, dissolved Mo in the deep water column of a number of other euxinic basins can be predicted in most cases to within a factor of three. The model has important implications for Mo behavior in euxinic basins. For example, the model attributes the near-total Mo removal from deep Black Sea waters to a serendipitous combination of pH and H2S concentration, implying that such removal would not be a general property of euxinic basins. This is a key point with regard to Mo isotope systematics. Contrary to previous views, the model assigns a critical role to pH in controlling the efficiency of Mo removal from euxinic basins.

The closed, isolated small systems, as the representatives of a “unique environmental feature”, are valuable natural laboratories for studying different biogeochemical processes. Saline Rogoznica Lake (“Dragon Eye”), situated on the Eastern Adriatic coast is such a system (10276 m2, 15 m deep) typical for many stratified, sulfide rich water bodies. The depth of mixolimnion changes seasonally and it is greatly influenced by meteorological conditions, i.e. temperature and rainfall. Vertical mixing usually occurs during winter when cold, oxygen-rich water from the surface sinks downwards. In 2009 we monitored seasonal distribution and variation of copper complexing capacity (LT), related apparent stability constants (Kapp), concentration of Cu2+ ions, surfactant activity (SAS), dissolved organic carbon (DOC) and reduced sulfur species (RSS). Our results have shown that LT is increasing with depth up to 8 m depth, while the concentrations of copper ions decrease with the depth due to the higher amount of RSS species in deeper layers. The values of log Kapp are also decreasing with depth as a consequence of a competition of organic ligands and sulfide ions for binding Cu. Below 8m depth the presence of high amounts of RSS (~1mM) influence the electrochemical measuring of copper ion and LT determination, contributing to the copper ion binding.

A multivariate statistical analysis was applied to the measured physico-chemical parameters to estimate anthropogenic and natural influences to water system of the Krka River. Analysis was conducted on the data from eight sampling sites collected during four sampling events. On the basis of data obtained by the biplot, four main hypotheses were defined and finally statistically confirmed. Thereof, two main and distinct processes occurring in the Krka River could be highlighted: (i) upstream pollution, caused by the inflow of untreated waste-waters from the town of Knin and (ii) downstream self-purification, caused by the settling down of pollutants and the inflow of clean subterranean waters.

Electrochemical quartz crystal microbalance (EQCM) measurements were employed for studying of the redox processes of FeS microparticles immobilized on Au electrode surface in contact with aqueous solutions of NaCl, NaCl-NaHCO3, and NaCl-Na2S. The objective was to shed light on the complexity of the iron sulfide electrochemistry. For the sake of comparison electrochemical behavior of FeS suspension was also investigated on the paraffin-impregnated graphite (PIGE) and hanging mercury drop (HMDE) electrodes. In order to understand the complex nature of the redox transformations of FeS, Mohr-salt was dissolved in NaCl and NaCl-NaHCO3 solutions, respectively, and the deposition-dissolution processes occurring in the course of the reduction of Fe(II) and reoxidation of Fe(0) were also monitored.

Electrochemical quartz crystal microbalance, combined with cyclic voltammetric, chronoamperometric, and potentiostatic measurements, was used to study electrodeposition/dissolution phenomena at a gold electrode in solutions containing Na2S. Spontaneous, open-circuit deposition processes as well as dissolution of the deposits in sulfide-free solutions have also been investigated. The potential range, scan rate, sulfide concentration, and pH have been varied. The results of the piezoelectric nanogravimetric studies are elucidated by a rather complex scheme involving underpotential deposition of sulfur at approximately −0.85 V vs. sodium calomel electrode, reductive dissolution of the deposited sulfur-containing layer at potentials more negative than approximately −0.9 V, and formation of a sulfur-containing multilayer at potentials more positive than −0.2 V. During the reduction of sulfur deposited on Au, a mass increase due to the formation of polysulfide species in the surface layer, accompanied by incorporation of Na+ counterions, can be observed that starts at approximately −0.4 V. This is a reversible process, i.e., during the reoxidation, counterions leave the surface layers. Frequency excursions during the electroreduction and reoxidation processes reveal existence of several competitive dissolution–deposition steps. Spontaneous interaction between Au and HS− species results in a surface mass increase at the open-circuit potential, and it also manifests itself in the substantial decrease of the open-circuit potential after addition of Na2S to the supporting electrolyte.

Dissolved organic carbon (DOC), surface active substances (SAS) and copper complexing capacity (CuCC) were studied in bulk precipitations collected periodically from 2003 to 2007 in the continental city of Croatia (Zagreb: n = 27) and in the city at the Adriatic coast (Sibenik: n = 38). DOC concentrations (Zagreb: 0.67–4.03 mgC/L with average concentration of 1.93 ± 0.76 mgC/L; Sibenik: 0.44–4.13 mgC/L with average concentration of 1.83 ± 0.83 mgC/L) are similar to those measured in other samples of continental rainwater in the northern hemisphere. The concentrations of SAS in samples from Zagreb ranged from 0.055 to 0.45 eq. Triton-X-100 mg/L with average concentration of (0.14 ± 0.06) eq. Triton-X-100 mg/L. SAS fractions were of a similar range in Sibenik (0.02–0.60 eq. Triton-X-100 mg/L) with an average concentration of 0.11 ± 0.06 eq. Triton-X-100 mg/L. However, the lowest values of SAS (between 0.02 and 0.04 eq. Triton-X-100 mg/L) were observed only in Sibenik (27%). We have estimated that the higher pH values were responsible for lower surface activity of organic matter in bulk samples from Sibenik. DOC may form complexes that control the transport and solubility of heavy metals in natural water. CuCC measured in Sibenik in the range 0.066–1.4 μM Cu 2+ was in general higher, compared to the one in Zagreb (0.010–0.586 μM Cu 2+ ) which is the result of biogenically driven organic contribution to the precipitation, especially in the warmer period of the year.

Mercury electrodes preconcentrate metal chalcogenide nanoparticles effectively, enabling their detection at submicromolar concentrations (as Sigma chalcogenide) by adsorptive cathodic stripping voltammetry. Understanding the unique behavior of nanoparticle analytes during preconcentration is critical for lowering detection limits and for quantification. A multistep mechanism is proposed on the basis of accumulation experiments with polydisperse copper sulfide (CuxS) nanoparticles. Particles first diffuse and adsorb at the Hg0 surface. When both the electrode and particles have negative surface potentials, this process resembles charge-impeded coagulation, obeying the Schulze-Hardy rule at various electrolyte strengths. Consequently, accumulation rates are surprisingly sensitive to electrolyte concentration. Choosing accumulation potentials where the electrode and particles have opposite surface potentials greatly improves collection efficiency, especially for the smallest particles. After adsorption, particles undergo transformations. One product is a more stable (harder to reduce) form of CuxS, interpreted to consist of adclusters or adlayers. A very significant (approximately 0.3 V) negative shift in reduction potential results from this transformation. Loss of analyte to at least one nonelectroactive product is also observed. Loss is greatest for the smallest particles and is sensitive to choice of accumulation potential. To improve accumulation efficiency, accumulation potentials more positive that the potential of zero charge of Hg electrodes are advantageous but care must be taken to remove dissolved chalcogenides under these conditions in order to avoid artifacts.

Voltammetric scans in sulfidic natural waters often reveal reduction peaks in the range, -0.9 to -1.35 V vs. Ag/AgCl. These peaks have been attributed to iron sulfide complexes or clusters. However, sols containing CuS nanoparticles now also are known to produce reduction peaks in this range. Here we investigate the voltammetric behavior of two additional metal sulfides at the Hg electrode in 0.55 M NaCl + 0.03 M NaHCO3 electrolyte, pH = 8.5. We show that Pb and Hg sulfides, either as suspended powders or as precipitated nanoparticles, also yield cathodic peaks between -0.9 and -1.35 V, similar to peaks obtained with CuS and FeS. For precipitated nanoparticles, the position and shape of these reduction peaks changes with ageing. Freshly formed nanoparticles produce less negative reduction peaks than aged nanoparticles. Peaks from aged nanoparticles often consist of two or more superimposed reduction peaks. When all other experimental parameters are held constant, the amount of nanoparticle analyte accumulated on the electrode increases with the amount of ageing (≤ 1 h). Addition of EDTA or acidification followed by purging can be used to distinguish PbS nanoparticles and Fe sulfide clusters from CuS and HgS nanoparticles or from colloidal S. This test was applied to interpret -0.9 to -1.35 V reduction peaks observed in two meromictic lakes. In conjunction with other evidence, this test suggests that FeS clusters are present in one case whereas colloidal S is present in the other. Interpreting -0.9 to -1.35 V voltammetric peaks observed in sulfidic natural waters requires caution, but these peaks are potentially rich sources of information about trace metal speciation.

Here was demonstrated for the first time a possible application of abrasive stripping voltammetry in the direct measurement of trace metals in anoxic, sulfidic marine sediments (peloid mud) from a small and shallow (0.2-1 m) marine lagoon in Central Dalmatia, Croatia. Trace amounts of sample compounds are transferred to the graphite electrode surface and electrochemical reduction or oxidation processes are followed by the cyclic voltammetry in seawater or 0.55 M NaCl as electrolyte. After a preelectrolysis at potentials ranging from -1.0 to -1.5 V (vs. SCE) a well-defined anodic stripping peak corresponding to the oxidation of metal deposits generated at deposition potentials is obtained around -0.74 V (vs. SCE). The same samples were studied by anodic stripping voltammetry at the Hg electrode and inductively coupled plasma-mass spectrometer (ICP-MS). ICP-MS showed higher concentrations of trace metals such as Al, Fe, Mo, Mn. A relatively high concentration of reduced sulfur species (RSS) (10-3 M) is determined electrochemically in porewater of the peloid mud, indicating that the magnitude of metal enrichment in the sediments is probably controlled by precipitation with sulfide.

Environmental context The electrochemical detection of many sulfur compounds in natural waters is based on the deposition of a HgS layer at the Hg electrode. In samples containing metal ions in excess of sulfide species, electrochemical exchange reactions between the HgS and the metal ion produce metal-sulfide voltammetric peaks. These peaks can easily be misinterpreted as dissolved sulfide species, and hence do not reflect the bulk state of the solution. Abstract Cyclic voltammetry on a Hg electrode was used to investigate the influence of metal ion (Zn, Cd, Cu, Fe, Pb, Co) on HgS deposition–dissolution in seawater conditions. Due to the exchange of electrons between Hg2+ from a HgS layer and free metal (M2+) from the solution (HgSlayer + M2+ + 2e– ↔ MSlayer + Hg0), the Hg electrode becomes the site for surface metal sulfide (MS) formation. The exchange reaction is reversible, and the surface-formed MS layer reduces at a more negative potential than HgS (MSlayer + 2e– + H+ → M0 + HS). The potentials of both electrode reactions, and the formation and reduction of the MS layer, are determined by the MS solubility product. In solutions containing excess of the free metal ions in comparison to the free sulfide, the exchange reaction produces MS voltammetric peaks, which can be misrepresented for the dissolved sulfide species. This research indirectly confirmed that the FeS electrochemical signal, usually recorded in an iron- and sulfide-rich environment at ~–1.1V v. Ag/AgCl, is not due to FeS reduction. The connection between the studied MS reduction peak potentials and the solubility products shows that the FeS layer formed by an electrochemical exchange reaction with HgS should be reduced at the Hg surface ~100mV more negative than free Fe2+.

Sedimentary molybdenum (Mo) and uranium (U) abundances, as well as their isotope systematics, are used to reconstruct the evolution of the oxygenation state of the surface Earth from the geological record. Their utility in this endeavour must be underpinned by a thorough understanding of their behaviour in modern settings. In this study, Mo-U concentrations and their isotope compositions were measured in the water column, sinking particles, sediments and pore waters of the marine euxinic Lake Rogoznica (Adriatic Sea, Croatia) over a two year period, with the aim of shedding light on the specific processes that control Mo-U accumulation and isotope fractionations in anoxic sediment. \ud \ud Lake Rogoznica is a 15 m deep stratified sea-lake that is anoxic and euxinic at depth. The deep euxinic part of the lake generally shows Mo depletions consistent with near-quantitative Mo removal and uptake into sediments, with Mo isotope compositions close to the oceanic composition. The data also, however, show evidence for periodic additions of isotopically light Mo to the lake waters, possibly released from authigenic precipitates formed in the upper oxic layer and subsequently processed through the euxinic layer. The data also show evidence for a small isotopic offset (~0.3‰ on 98Mo/95Mo) between particulate and dissolved Mo, even at highest sulfide concentrations, suggesting minor Mo isotope fractionation during uptake into euxinic sediments. Uranium concentrations decrease towards the bottom of the lake, where it also becomes isotopically lighter. The U systematics in the lake show clear evidence for a dominant U removal mechanism via diffusion into, and precipitation in, euxinic sediments, though the diffusion profile is mixed away under conditions of increased density stratification between an upper oxic and lower anoxic layer. The U diffusion-driven precipitation is best described with an effective 238U/235U fractionation of +0.6‰, in line with other studied euxinic basins. \ud \ud Combining the Mo and U systematics in Lake Rogoznica and other euxinic basins, it is apparent that the two different uptake mechanisms of U and Mo can lead to spatially and temporally variable Mo/U and Mo-U isotope systematics that depend on the rate of water renewal versus removal to sediment, the sulfide concentration, and the geometry of the basin. This study further emphasises the potential of combining multiple observations, from Mo-U enrichment and isotope systematics, for disentangling the various processes via which redox conditions control the chemistry of modern and ancient sediments.