Interaction of natural organic matter with mercury in coastal marine and lagoon environments

Raziskave so bile opravljene na več raziskovalnih postajah v Tržaškem zalivu in v lagunah Marano in Grado. Gre za okolja, v katerih vladajo različne fizikalne, kemijske in biološke razmere. Cilj doktorskega dela je bil preučiti povezave med organsko snovjo (OM) in živim srebrom (Hg) v omenjenih okoljih ter ugotoviti, kako različni okoljski dejavniki in različen izvor OM ter Hg vplivajo na morebitne povezave in njihov obseg. Tržaški zaliv je onesnažen s Hg iz rudnika Hg Idrija, laguni pa sta podvrženi vplivu dveh oblikHg iz dveh ločenih virov. En je prav tako idrijski rudnik, medtem ko je drugi nedolgo nazaj delujoča kloralkalna tovarna, locirana v zgornjem delu toka reke Aussa, ki se izliva v maranski sektor lagune. Delo se je osredotočalo na povezave med OM in Hg v vodnem stolpcu in sedimentih obeh sistemov. Izkazalo se je, da je organska snov v vodnem stolpcu Tržaškega zaliva pomemben vezalec Hg in tudi drugih kovin, povezav med njimi pa v sedimentih ni bilo mogoče zaznati. Podobno se je izkazalo tudi pri analizi vzorcev iz lagun. Razlike v povezavah med OM v vodnem stolpcu in sedimentih sopovezane z različno sestavo in strukturo OM, saj je v sedimentih večinoma mogoče najti močno rekalcitrantno OM, medtem ko je OM v vodnem stolpcu bolj biodostopna in vsebuje več funkcionalnih skupin, ki omogočajo vezavo kovin. V Tržaškem zalivu oz. celotnem severnem Jadranu občasno prihaja tudi do tvorbe makroagregatov, velikih skupkov OM. Raziskovanje mehanizmov njihove tvorbe in razgradnje ter sestave in strukture je izjemnega pomena, saj je s tem omogočentudi vpogled v dinamiko naravne OM v Tržaškem zalivu. Ob analizi netopne frakcije makroagregatov so bile zaznane visoke koncentracije nekaterihkovin, kar je kazalec vključitve mineralnih delcev v strukturo. Analiza topnega dela makroagregatov je pokazala, da ima raztopljena OM makroagregatov enake lastnosti in sposobnosti vezave Hg kot naravna OM v tem okolju. Združitev metode ultrafiltracije in uporabe radioaktivnih ali stabilnih izotopov bi v prihodnje pomembno pripomogla k še natančnejšemu poznavanju mehanizmov, ki vodijo do povezav med OM in kovinami v okolju. OM ima velik vpliv na biogeokemijsko kroženje Hg, saj je pomembna z vidika napajanja mikrobnega metabolizma, s tem pa tudi tistih mikrobov, ki vodijo pretvorbe Hg. Z izvedbo eksperimentov v okoljih, v katerih ima OM različen izvor, različen izvor ima pa tudi Hg, je bila potrjena hipoteza o pomembnosti biološke dostopnosti tako OM kot Hg pri tvorbi MeHg.Research was conducted in various experimental stations throughout Gulf of Trieste and Grado and Marano Lagoon. Ecosystems observed are governed by different physical, chemical and biological conditions. The main aim of this research was to investigate the existence and extent of interactions between organic matter (OM) and mercury (Hg). The influences of various environmental conditions were also studied. The most important factor in our study was the effect that the origin, thus structure and composition, of OM as well as Hg has on the existence and extent of interactions. Gulf of Trieste is contaminated with Hg from Idrija mercury mine whereas Hg found in Lagoon has two origins. One is Idrija mercury mine, while the other is chloralkali plant operating until recently and situated upstream of Aussa River. Aussa River is as an Aussa-Corno river system discharged into the Lagoon. During the researchdissolved and sedimentary OM and its interactions with Hg were investigated. It turned out that dissolved OM present in water column plays animportant role as Hg and other metal complexation agent, however no conclusive evidence was found that interactions between sedimentary OM and Hg exist. Similar results were obtained also on samples from the lagoon environment. It was concluded that the absence of interactions in sediments was a consequence of OM quality, meaning that its recalcitrance does not support Hg binding in the extents as water column OM does. Water column OM is apparently more bioavailable and contains functional groups more suitable for metal binding. Macroaggregates, large aggregates of OM, are relatively regularly formed in the waters of northern Adriatic, also in waters of Gulf ofTrieste. Knowledge on mechanisms of macroaggregate formation and degradationis necessary, since it offers an invaluable insight into dynamics of natural OM as well. Analyses on macroaggregate matrix showed high metal concentrations which suggested an incorporation of inorganic particles, while the results on interstitial soluble portion of macroaggregates revealed similar to identical properties in terms of structure and metal binding capacity as natural OM. In future the combined usage of ultrafiltration and radioactive or stable isotopes could prove to be invaluable tool for identifying the mechanisms behind interactions between OM and Hg. OM is one ofthe most important actors in the biogeochemical cycle of Hg, since it servesas a fuel for microbial metabolism. It supports also the microbes responsible for transformations of Hg. The experiments performed in different environments where OM and Hg have different origins allowed us to confirm the hypothesis on importance of bioavailability of OM and Hg in the formation of MeHg

Benthic fluxes of mercury during redox changes in pristine coastal marine sediments from the Gulf of Trieste (northern Adriatic Sea)

Abstract Purpose: The Gulf of Trieste (northern Adriatic Sea) is an example of a coastal environment contaminated with mercury (Hg). Contamination is a consequence of nearly 500 years of activity at the Idrija Mine (western Slovenia), which is the second largest Hg mine in the world. Oxygen depletion can be common in the Gulf of Trieste due to late summer stratification of the water column and accumulation of labile organic matter. Since changing redox conditions can have an impact on Hg transformations, we studied the effect of oxzgen depletion, in parallel with sulphide, iron (Fe), manganese (Mn), fluorescent dissolved organic matter (FDOM) and nitrogen (N) and phosphorus (P) availability, on total Hg and methylmercury (MeHg) fluxes from sediments. Materials and methodsČ Pore water concentrations and benthic fluxes of total dissolved Hg and MeHg were studied in situ and in microcosm laboratory experiments using flux chambers encompassing three different stagesČ oxic, anoxic and reoxidation. Results and discussion: Our experiments showed that inthe oxic stage there were small effluxes of MeHg to the water column, which increased in the anoxic stage and dropped rapidly in a subsequent reoxic stage, showing influx. Our results support the hzpothesis that MeHg desorptionfrom reduced metal hzdroxides under anoxic conditions, and co-precipitation with Fe-oxides and MeHg demethylation in the reoxidation stage, may play a major role in determining MeHg benthic fluxes. For Hg and MeHg, it appears that there is little relationship between their pore water distribution and flux and that of FDOM, i.e. humics. Conclusions: The results indicate that there was no significant difference in Hg and MeHg pore water levels and their benthic fluxes between the contaminated northern and central parts of the Gulf of Trieste and the pristine southern part. This suggests that shallow and stratified coastal marine environments, in general, representareas with a risk of high benthic release of toxic MeHg

Diagenesis and benthic fluxes of nutrients and metals during experimentally induced anoxia in the Gulf of Trieste (northern Adriatic Sea)

International audienceSequential nutrient regeneration and organic matter (OM) degradation in surface sediments of the Gulf of Trieste (northern Adriatic Sea) were examined using in situ benthic chambers under normoxic, anoxic and reoxic conditions. Intensive NH4+ and PO4− anoxic regeneration was subsequently slower in prolonged anoxia. NH4+ production was probably also a consequence of dissimilatory nitrate reduction to NH4+. The presence of anammox and laterally pumping of oxygenated water by benthic infauna explained the presence of NO3− in anoxia. Anoxic phases were characterized by enhanced dissolution of Sibiog, decreasing pore water Ca and Mg concentrations indicating carbonate precipitation and higher Fe and Mn concentrations as a result of reduction/respiration. Reoxygenation was characterized by enhanced bioturbation. Nitrification caused NH4+ decrease and P precipitated quickly as carbonate fluorapatite and FePO4. In addition adsorption of P onto Fe-hydroxides could also occur since Fe (and Mn) reoxidized quickly. Increased Ca levels suggested enhanced carbonate dissolution. Diffusive fluxes at the sediment–water interface (SWI), calculated from pore water modelling using diffusion-reaction model, revealed high anoxic NH4+ effluxes and Ca (and Mg) influxes. PO4− fluxes were very low and high NH4+/PO4− flux ratios in anoxic and reoxic phases suggested an excess of benthic inorganic N. Nutrient budgets at the SWI showed intensive anoxic recycling of inorganic N but low P and Si cycling in all redox phases.</p

Benthic biogeochemical cycling of mercury in two contaminated northern Adriatic coastal lagoons

Previous research recognized most of the Northern Adriatic coastal lagoon environment as contaminated by mercury (Hg) from multiple anthropogenic sources. Among them, Pialassa Baiona (P.B.) Lagoon, located near the city of Ravenna (Italy), received between 100 and 200 tons of Hg, generated by an acetaldehyde factory in the period 1957-1977. Further east, the Grado Lagoon has been mainly affected by a long-term Hg imput from the Idrija mine (western Slovenia) through the Isonzo River since the 16th century. Hg cyclingat the sediment-water interface (SWI) of the two lagoons was investigated and compared by means of an in situ benthic camber, estimating diffusive Hg and Methyl-Hg fluxes in the summer season..

The impact of induced redox transitions on nutrient diagenesis in coastal marine sediments (Gulf of Trieste, northern Adriatic Sea)

International audienc

BENTHIC FLUXES MEASUREMENT OF MERCURY AND METHYLMERCURY IN PIALASSA BAIONA (RAVENNA, ITALY)

Previous research on mercury (Hg) showed strong contamination of the Piallassa Baiona (P.B.) lagoon, near Ravenna. The lagoon received between 100 and 200 tons of Hg generated by an acetaldehyde factory in 1957-1977. In this study, the Hg cycling at the sediment-water interface in the P.B. lagoon was investigated by means of an in situ benthic chamber. The 8-h integrated flux of the methylated form was extremely low and estimated to be only 7% of the result obtained for a summer experiment performed in a similar Hg-contaminated environment (Grado lagoon). Conversely, the in situ flux of Hg accounted for a comparable amount to that observed in the Grado lagoon, although Hg contents in its sediments are almost 50% lower than in P.B. lagoon. Hg mobilization and sequestration in the system, limiting its bioavailability despite the high contents of metal buried in the bottom sediments, seem related to extremely anoxic conditions

Artificially induced migration of redox layers in a coastal sediment from the Northern Adriatic

Long-term experimental studies suggest that, under transient anoxic conditions, redox fronts within the sediment shift upwards, causing sequential rise and fall of benthic fluxes of reduced species (Mn(II), Fe(II) and S(-II)). Infaunal benthic organisms are associated with different redox fronts as micro-habitats and must be affected by such changes during natural hypoxia events. In order to document the geochemical evolution of the sediment during prolonged anoxia in the framework of an in situ experiment designed to mimic natural conditions, benthic chambers were deployed on the seafloor of the Northern Adriatic and sampled after 9, 30 and 315 days of incubation. Oxygen and sulfide were measured continuously in the early stages (9 days) of the experiment. High-resolution pore water profiles were sampled by DET probes and redox-sensitive species (S(VI), Mn(II)and Fe(II)) and alkalinity were measured. Starting oxygen saturation was about 80% within the chamber. After 7 days, anoxia was established in the bottom waters within the chambers. Mn(II) and Fe(II) started diffusing towards the anoxic water column until they reached the surficial sediment. Being reoxidized there, Mn and Fe reprecipitated, giving a rusty coloration to the seafloor. Infaunal species appeared at the sediment surface. After 20 days, all macro-organisms were dead. Decomposition of macroorganisms at the sediment–water interface generated S(-II) within the entire height of the chamber, leading to a downward flux of sulfides into the sediment, where they were quickly oxidized by metallic oxides or precipitated as FeS. S(-II) was below detection in the water column and pore waters at the end of the experiment. Our results suggest that S(-II) enrichment in the water column of coastal systems, which are episodically anoxic, is strongly controlled by the biomass of benthic macrofauna and its decay during anoxia, whereas its residence time in the water column is controlled by iron availability (as solid oxides or as dissolved reduced cations) within the sediment, even without water circulation