Redox oscillation affecting mercury mobility from highly contaminated coastal sediments: a mesocosm incubation experiment

A Emili,S Covelli,A Acquavita,L Carrasco,Nicola Pirrone

doi:10.1051/e3sconf/20130106003

A Emili, S Covelli + Show 3 more

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https://doi.org/10.1051/e3sconf/20130106003

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Abstract

Mercury (Hg) mobility at the sediment-water interface was investigated during a laboratory incubation experiment on highly contaminated sediments (up to 23 µg g -1 ) of the Gulf of Trieste. Undisturbed sediment was collected in front of the Isonzo River mouth, which inflows Hg-rich suspended material originating from the Idrija (NW Slovenia) mining district. Since hypoxic and anoxic conditions at the bottom are frequently observed, a redox oscillation was simulated in the laboratory at in situ temperature, using a dark flux chamber. Temporal variations of several parameters were monitored simultaneously: dissolved Hg and methylmercury (MeHg), O2, NH4 + , NO3 - +NO2 - , PO4 3- , H2S, dissolved Fe and Mn, dissolved inorganic and organic carbon (DIC and DOC). Benthic fluxes of Hg and MeHg were higher under anoxic conditions while re-oxygenation caused concentrations of MeHg and Hg to rapidly drop, probably due to re-adsorption onto Fe/Mn oxyhydroxides and enhanced demethylation. Hence, during anoxic events, sediments of the Gulf of Trieste may be considered as an important source of dissolved Hg species for the water column. However, re-oxygenation of the bottom compartment mitigates Hg and MeHg release from the sediment, thus acting as a natural defence from possible interaction between the metal and the aquatic organisms.

Highlights

The Gulf of Trieste has been affected by mercury (Hg) inputs from the Isonzo River, the largest contributor of this metal into the northern Adriatic Sea since the 16th century, due to its transport of cinnabar (HgS) rich tailings from the Idrija (Slovenia) mining district (Covelli et al, 2001)
During the oxic/anoxic transition, early diagenetic processes of organic matter consumption caused the remineralization of nutrients and the release of Fe/Mn and Hg species from the sediment
Hypoxic and anoxic conditions play a significant role in the remobilization of Hg from sediments at the Isonzo River mouth in the Gulf of Trieste

Summary

Introduction

The Gulf of Trieste (northern Adriatic Sea) has been affected by mercury (Hg) inputs from the Isonzo River, the largest contributor of this metal into the northern Adriatic Sea since the 16th century, due to its transport of cinnabar (HgS) rich tailings from the Idrija (Slovenia) mining district (Covelli et al, 2001). Hypoxic/anoxic conditions at the sediment–water interface (SWI) have been observed in the Gulf of Trieste, as a consequence of high loadings of nutrients and organic matter and strong late summer water stratification (Faganeli et al, 1991) Under such environmental conditions, sediments could act as a significant source of in situ Hg methylated species (Fitzgerald et al, 2007; Merritt and Amirbahman, 2009). The aim of this study was to evaluate Hg and MeHg cycling at the SWI in the Gulf of Trieste in case of anoxic events, simulating an oxic/anoxic transition in a dark incubated benthic chamber, at in situ temperature To this purpose, a volume of undisturbed sediment was sampled from an experimental site located in front of the Isonzo River mouth (Fig. 1), where an elevated Hg content was expected, on the basis of previous investigations (Covelli et al, 2001). In addition to Hg species, water samples were analyzed for H2S, nutrients (N and P), Fe and Mn, dissolved inorganic and organic C (DIC and DOC), to better understand the redox processes occurring in the incubated system

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