The reactivity of iron oxyhydroxide nanoparticles in low pH and high ionic strength solutions was quantified to assess abiotic contributions to oxidation–reduction chemistry in acidic brine environments, such as mine groundwater seepage, lakes in Western Australia, and acid mine drainage settings, which are of global interest for their environmental impacts and unique geomicrobiology. Factors expected to influence accessible and reactive surface area, including Fe(II) adsorption and aggregate size, were measured as a function of pH and CaCl2 concentration and related to the kinetics of redox reactions in aqueous suspensions of synthetic goethite (α-FeOOH), akaganeite (β-FeOOH), and ferrihydrite (Fe10O14(OH)2) nanoparticles. Aqueous conditions and iron oxyhydroxides were chosen based on characterization of natural iron-rich mine microbial mats located in Soudan Underground Mine State Park, Minnesota, USA. Quinone species were used as redox sensors because they are well-defined probes and are present in natural organic matter. Fe(II) adsorption to the iron oxyhydroxide mineral surfaces from aqueous solution was measurable only at pH values above 4 and either decreased or was not affected by CaCl2 concentration. Concentrations at or above 0.020 M CaCl2 in acetate buffer (pH 4.5) induced particle aggregation. Assessment of Fe(II) adsorption and particle aggregation in acidic brine suggested that accessible reactive surface area may be limited in acidic brines. This was supported by observations of decreasing benzoquinone reduction rate by adsorbed Fe(II) at high CaCl2 concentration. In contrast, the hydroquinone oxidation rate increased at high CaCl2 concentrations, which may be due to suppressed adsorption of Fe(II) generated by the reaction. Results suggest that iron geochemical cycling in acidic brine environments will be substantially different than for iron oxyhydroxides in low-saline waters with circumneutral pH. These findings have implications for acidic brine lakes and acid mine drainage locations that contain precipitated iron oxyhydroxides.
Bibliographical noteFunding Information:
The authors wish to thank Calvin Alexander and Scott Alexander (University of Minnesota) for providing geochemistry data for Soudan Mine groundwater seeps; Lindsey Briscoe and Ryan Lesneiwski (University of Minnesota) for assistance with field work; Jeffry Sorensen (University of Minnesota) and Mahaling Balassubramanian and Dale Brewe (Advanced Photon Source) for assistance with beamtime; Clara Chan and Colleen Hansel for providing iron mineral standards for EXAFS; Benjamin Wilson for nitrogen sorption analyses, and Kairat Sabyrov for akaganeite synthesis procedure. This work was supported by the John Wertz Fellowship , as part of the Department of Chemistry Fellowships for Excellence in Graduate Studies at the University of Minnesota (J.H.S.), NSF grants ECS-1012193 and CHE-1507496 (R.L.P. and W.A.A.), and Minnesota Environment and Natural Resources Trust Fund as recommended by the LCCMR (B.M.T.). Parts of this work were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network ( www.mrfn.org ) via the MRSEC program. The iron EXAFS research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357 .
© 2016 Elsevier Ltd
- Abiotic iron oxidation
- Abiotic iron reduction
- Acidic brine
- Fe(II) adsorption
- Iron oxide nanoparticles