Spills of petroleum and refined products commonly result in plumes where biogenic Fe(II) accumulates in sediment; delaying recovery to oxic conditions after source removal. We are characterizing the chemical forms of sediment-bound Fe(II) in contaminated groundwater related to a crude oil pipeline failure near Bemidji MN, USA. On a per volume groundwater basis, the concentrations of sediment-bound Fe(II) are much larger than the aqueous Fe(II) concentrations (up to 600 µM). Possible sediment-bound forms of Fe(II) include phases such as ferroan calcite and siderite; biogenic magnetite and green rust; layered double hydroxides (LDH, e.g. Fe2Al(OH)6(CO3)0.5H2O); and Fe(II) sorbed via ion-exchange and surface complexation. Applying X-ray absorption spectroscopy to identify chemical forms of sediment-bound biogenic Fe(II) was confounded by high background Fe concentrations. We applied standard chemical extractions (0.5M HCl, 1M MgCl2, 1M Actot pH 4.6), and introduced other extraction solutions (5mM EDTA; 20mM FerroZine; both in 0.1M NH4Cl/10mM PIPES pH 7; and 11.6 mM CaCO3 at 1 atm PCO2 at pH 6 with and without FerroZine) to quantify forms of solid-bound Fe(II). Results suggest the dominant form of sediment-bound Fe(II) resembles an LDH rather than ferroan calcite or siderite. Three-day 0.5M HCl treatments extracted 18 µmol/m2 Fe2+ (64% of total extractable Fe) from a &lt;2mm fraction (BET surface area 1.2 m2/g and monolayer coverage ~4µmol/m2) of a core from a Fe-reducing zone. A series of 5 acetate extractions yielded 14 µmol/m2 Fe2+ of 18 µmol/m2 total Fe. While the previous solutions removed all calcite, EDTA extracted almost as much total Fe as acetate but removed only approximately half of the calcite. Five consecutive FerroZine extractions at pH 7 removed 12µmol/m2 Fe2+ but only approximately 1/4 of the calcite. Net calcite dissolution was suppressed by pre-equilibrating the solutions with calcite at pH 6 and 1 atm PCO2, while still extracting 14-15 µmol/m2 Fe2+ with FerroZine, and 4 µmol/m2 when FerroZine was absent. Sorbed Fe, which was assayed in isotope exchange experiments with 57Fe conducted in artificial groundwater at pH 7.5-7.6 and in solutions pre-equilibrated with calcite at pH 6 and 1 atm PCO2,, comprised only a fraction of total sediment-bound Fe(II) based on HCl extractions.

Chemical forms of sediment-bound Fe(II) in sediments from an aquifer contaminated by a petroleum spill

255th National Meeting (2018)

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