Abstract
PurposeAs the formation of toxic and bioaccumulative methylmercury (MeHg) in Hg-contaminated sediments is of great concern worldwide, suitable remediation options are needed. Activated carbon (AC) amendment is a contested alternative due to uncertainties surrounding sorption efficiency and its potential role in aiding MeHg formation. The purpose of this study was therefore to demonstrate AC performance under favourable conditions for Hg-methylation and to further understand the role AC plays in the methylation process.Materials and methodsMercury-contaminated sediment (57.1 mg kg−1) was sampled from the Gunneklev fjord, a site known as the most heavily contaminated fjord in Norway. In a laboratory experiment, lignite AC (A-AC, 5%) or activated biochar (A-BC, 5%) along with dried algae biomass, serving as an excess source of easily degradable organic matter (OM) and sulphate, were added to sediment samples that were kept anoxic and dark over a period of 12 months.Results and discussionThe amount of MeHg in sediment and porewater of the amended samples were measured at 0, 1, 3, 6, and 12 months and compared to an unamended control. A net increase of MeHg in the sediment was observed in both control and amended samples, but contrary to expectations, sediment MeHg was 5 and 3 times higher in the A-AC and A-BC treatments, respectively, relative to the control after 12 months. As the stimulation of Hg-methylation could not be attributed to the sorbents supplying more available OM or sulphate for dissimilatory sulphate reduction, it is speculated that the sorbents rather aid this process through shuttling of electrons between the substrates involved. Meanwhile, the A-AC and A-BC amendments strongly reduced the available MeHg-concentration in porewater (by 87% for A-AC and by 93% for A-BC after 12 months), confirming that AC sorbents can be used to effectively limit the transport of MeHg from sediments.ConclusionWhen considering remediation of OM-rich Hg-contaminated sediments with AC, caution is thus warranted, as the overall effect of reducing MeHg-transport out of the sediment could partly be offset by an increased fraction of MeHg in the sediment. Thin-layer capping with AC might therefore be preferable to complete mixing of AC and sediment.
Highlights
Mercury (Hg) pollution is a widespread contamination issue throughout the world, in industrial as well as remote areas (Fernandez-Luqueno et al 2013)
The MeHg concentration is in the lower range of what has previously been reported for the Gunneklev fjord (Olsen et al 2018), and could have been affected by of aerobic demethylation during sample preparation (Ullrich et al 2001)
Conditions proved favourable for Hg-methylation under the regime of the experimental setup, as a sharp increase in MeHg porewater concentration was observed in the control during the first month of the trial, increasing from the initial 9 ± 2 to 393 ± 79 ng L−1
Summary
Mercury (Hg) pollution is a widespread contamination issue throughout the world, in industrial as well as remote areas (Fernandez-Luqueno et al 2013). The top millimetres to centimetres, contaminated coastal marine sediments are often anoxic (Dell’Anno et al 2009), and conducive to the formation of MeHg, especially in the presence of fresh, degraded organic matter (OM) that can serve as electron donor (Ndungu et al 2016). The placement of sand or clay caps as remediation measures can lead to an upward extension of anoxic conditions in initially oxic sediments and subsequently result in increased MeHg formation (Johnson et al 2010). Ndungu et al (2016) demonstrated that a thin clay capping layer increased MeHg formation, OM added as algae was the limiting factor in this process and the cap succeeded in reducing the release of said MeHg. Due to the contentious effect of other remediation approaches, addition of actively sorbing materials such as biochar or activated carbon (AC) has been suggested as a methodology to reduce total Hg (THg) and MeHg porewater concentrations, and risk (Gomez-Eyles et al 2013)
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