Mercury mobilization and speciation linked to bacterial iron oxide and sulfate reduction: A column study to mimic reactive transfer in an anoxic aquifer.
Mercury (Hg) mobility and speciation in subsurface aquifers is directly linked to its surrounding geochemical and microbial environment. The role of bacteria on Hg speciation (i.e., methylation, demethylation and reduction) is well documented, however little data is available on their impact on Hg mobility. The aim of this study was to test if (i) Hg mobility is due to either direct iron oxide reduction by iron reducing bacteria (IRB) or indirect iron reduction by sulfide produced by sulfate reducing bacteria (SRB), and (ii) to investigate its subsequent fate and speciation. Experiments were carried out in an original column setup combining geochemical and microbiological approaches that mimic an aquifer including an interface of iron-rich and iron depleted zones. Two identical glass columns containing iron oxides spiked with Hg(II) were submitted to (i) direct iron reduction by IRB and (ii) to indirect iron reduction by sulfides produced by SRB. Results show that in both columns Hg was leached and methylated during the height of bacterial activity. In the column where IRB are dominant, Hg methylation and leaching from the column was directly correlated to bacterial iron reduction (i.e., Fe(II) release). In opposition, when SRB are dominant, produced sulfide induced indirect iron oxide reduction and rapid adsorption of leached Hg (or produced methylmercury) on neoformed iron sulfides (e.g., Mackinawite) or its precipitation as HgS. At the end of the SRB column experiment, when iron-oxide reduction was complete, filtered Hg and Fe concentrations increased at the outlet suggesting a leaching of Hg bound to FeS colloids that may be a dominant mechanism of Hg transport in aquifer environments. These experimental results highlight different biogeochemical mechanisms that can occur in stratified sub-surface aquifers where bacterial activities play a major role on Hg mobility and changes in speciation.
BRGM (D3E/BGE) 3 Av. Claude Guillemin, 45060, BP. 36009, Orléans Cedex 2, France. Electronic address: j.hellal@brgm.fr.
Jennifer
J
Hellal
University of Geneva, Institut F.-A. Forel, CP416, CH-1290 Versoix, Switzerland.
Stéphane
S
Guédron
University of Geneva, Institut F.-A. Forel, CP416, CH-1290 Versoix, Switzerland.
Lucie
L
Huguet
University of Bordeaux, UMR EPOC, Allée Geoffroy St. Hilaire, 33615 Pessac Cedex, France.
Jörg
J
Schäfer
BRGM (D3E/BGE) 3 Av. Claude Guillemin, 45060, BP. 36009, Orléans Cedex 2, France.
Valérie
V
Laperche
BRGM (D3E/BGE) 3 Av. Claude Guillemin, 45060, BP. 36009, Orléans Cedex 2, France.
Catherine
C
Joulian
University of Bordeaux, UMR EPOC, Allée Geoffroy St. Hilaire, 33615 Pessac Cedex, France.
Laurent
L
Lanceleur
BRGM (D3E/BGE) 3 Av. Claude Guillemin, 45060, BP. 36009, Orléans Cedex 2, France.
André
A
Burnol
BRGM (LAB/ENV), 3 Av. Claude Guillemin, 45060, BP. 36009, Orléans Cedex 2, France.
Jean-Philippe
JP
Ghestem
BRGM (D3E/BGE) 3 Av. Claude Guillemin, 45060, BP. 36009, Orléans Cedex 2, France.
Francis
F
Garrido
BRGM (D3E/BGE) 3 Av. Claude Guillemin, 45060, BP. 36009, Orléans Cedex 2, France.
Fabienne
F
Battaglia-Brunet
D001419:Bacteria / Q000378:metabolism; D003102:Colloids / Q000737:chemistry; D005290:Ferric Compounds / Q000737:chemistry* / Q000378:metabolism; D005296:Ferrous Compounds; D060587:Groundwater / Q000737:chemistry* / Q000382:microbiology*; D007501:Iron / Q000378:metabolism; D008628:Mercury / Q000032:analysis / Q000737:chemistry*; D008745:Methylation; D008767:Methylmercury Compounds / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D013431:Sulfates / Q000737:chemistry / Q000378:metabolism; D013440:Sulfides / Q000378:metabolism; D014874:Water Pollutants, Chemical / Q000032:analysis / Q000737:chemistry* / Q000378:metabolism
D003102:Colloids; D005290:Ferric Compounds; D005296:Ferrous Compounds; D008767:Methylmercury Compounds; D013431:Sulfates; D013440:Sulfides; D014874:Water Pollutants, Chemical; C000499:ferric oxide; D007501:Iron; D008628:Mercury; C022597:ferrous sulfide
(Methyl)mercury speciation; Groundwater; Iron oxides; Iron reducing bacteria; Mercury mobility; Sulfate reducing bacteria