Monitoring iron mineralogy and contaminant mobility using geoelectrical approaches
Supervisors: Jared West, Caroline Peacock (University of Leeds) and Sam Shaw (University of Manchester)
Toxic and radioactive chemicals are present into the subsurface environment in both natural conditions
(e.g. As contaminated groundwater aquifers in Bangladesh) and industrially contaminated sites including
nuclear ‘Legacy’ sites (e.g. Sellafield in Cumbria, and the Rifle nuclear site, Colorado, USA).
Understanding subsurface geochemistry is vital in order to predict contaminant movement, and decide on
groundwater abstraction strategies or remedial actions. Understanding the geochemistry of toxic
elements, and their interaction with natural minerals will often determine the best approach. One important
group of minerals are natural iron minerals such as ferrihydrite, goethite and magnetite; these are
ubiquitous in sedimentary environments (Figure 1a, b, c) and play a crucial role in the environmental
mobility of the toxic species concerned (e.g. uranyl, technetium, chromate, arsenic). The geochemical
conditions within subsurface sediments are controlled by a complex interplay of reactions involving these
minerals, microbes and dissolved ions. The reductive dissolution of Fe(III) (oxyhydr)oxide minerals
(e.g.ferrihydrite and goethite) by iron reducing organisms (e.g. Geobacter and Shewanella species)
produces soluble Fe(II) that plays a key role in controlling contaminant mobility, leading to either dramatic
decrease (e.g. uranium, technicium) or increase (e.g. arsenic in Bangladesh groundwater) in contaminant
solubility/bioavailability. Remediation of radioactively contaminated sites has been attempted via injection
of electron donors such as lactate and acetate (these are readily available waste products of other
industries) into the subsurface in order to immobilize the toxic or radioactive species concerned via
bioreduction, which produces reduced iron species (i.e. ferrous iron) in solution and in the solid phase (e.g.
magnetite/pyrite). Iron bearing phases (e.g. zero-valent iron and green rust, Figure 1a) have also been
introduced into the subsurface for remediation, in order to reductively immobilize a range of contaminants.
Figure 1a. Iron mineral (green rust) nanoparticles
Figure 1b. Iron oxides in natural soil
Figure 1c. Iron oxide precipitates in mine drainage
Spectral Induced Polarisation (SIP) is the measurement of the response of a material to an applied
sinusoidal electrical field; the variation of response with frequency, the electrical spectra, depends strongly
on the mineral-water interface, which also controls the interaction between minerals and contaminants.
Therefore, SIP measurements may be a good way to detect both geochemical conditions and contaminant
mobility. Field measurement of the SIP response of rocks and soils has been used to detect metallic ore
bodies for over 50 years. The SIP response will show a peak at a frequency controlled by electrical
polarization phenomena; semiconductive minerals (iron oxides and sulphides) give large SIP responses
because the particles themselves can polarize. Hence, large SIP responses are expected where semiconductive minerals such as sulfides and magnetite precipitate in reducing conditions; indeed, such
changes in SIP response of natural sediments according to redox conditions have been reported from field
studies during bioremediation by acetate injection at the Rifle site, Colorado. Such SIP responses may
indicate insitu formation of iron minerals such as magnetite and/or pyrite which are themselves indicative
of the reducing geochemical conditions that immbolise contaminants; however, this remains unconfirmed.
In order to eventually understand field SIP data generated at contaminated sites, we propose a systematic
study of the electrical spectra of sediments containing iron minerals, beginning with ‘model’ systems based
on idealized mineralogy. Initial experimental work at the University of Leeds Cohen Biogeochemistry
Laboratories
(http://www.see.leeds.ac.uk/business-and-consultation/facilities/cohen-laboratory/),
will
investigate the effect of pH, metal ions with differing adsorption behaviour, and redox-activity on the SIP
signature of model soils consisting of magnetite/haemtite/pyrite grains dispersed in quartz sand. These
measurements will be complemented by potentiometric titrations of the component minerals and model
soils to determine their surface sorption properties, and synchrotron-based X-ray Absorption Spectroscopy
(XAS) analysis (carried out at the Diamond Light Source Synchrotron at Harwell, Oxfordshire
(http://www.diamond.ac.uk/) to investigate the oxidation state and molecular adsorption mechanisms. A
second phase of laboratory work will investigate the origins of the large SIP responses measured in the
field at the Rifle nuclear site, CO (http://ifcrifle.pnnl.gov/); to identify whether these originate from
authigenic semiconductive minerals (magnetite or sulphides) formed during bioremediation efforts (acetate
injections). The experiments will inform improved mechanistic understanding of the processes which
produce the observed SIP responses for semiconductive minerals, and whether these are diagnostic for
contaminant mobility. This will establish the extent to which field Spectral Induced Polarisation
measurements can yield useful data on geochemical conditions and the resulting environmental mobility of
radionuclides, and the success of any remediation approaches.
Undertaking this project will provide you with training in a range of state of the art low temperature
geochemical techniques, including field sampling, mineral synthesis and wet-chemical analysis, as well as
SIP measurements, potentiometric titrations and synchrotron-based X-ray Absorption Spectroscopy (XAS)
analysis. It will provide a thorough grounding in the field of environmental transport and fate of
radionuclides, and environmental remediation approaches. Opportunities will be provided to present your
work at National and International conferences and meetings. The project will equip you for a future career
in academia, environmental regulation, or environmental consultancy.
Supervisor References
Ahmed, IAM; Benning, LG; Kakonyi, G; Sumoondur, AD; Terrill, NJ; Shaw, S (2010) Formation of Green Rust Sulfate: A Combined in Situ
Time-Resolved X-ray Scattering and Electrochemical Study, LANGMUIR, 26, 6593-6603. doi:10.1021/la003035j
Hubbard CG; West LJ; Shaw S; Morris K; Brookshaw D; Lloyd JR; Kulessa B (2011) In search of experimental evidence for the
biogeobattery, Journal of Geophysical Research G: Biogeosciences, 116, . doi: 10.1029/2011JG001713
Hubbard CG; West LJ; Rodriguez-Blanco JD; Shaw S (2012) Spectral induced polarization of ferrous iron interactions with magnetite,
Geophysical Research Letters (in review).
Peacock, C. L. and Sherman, D. M. (2004). Copper(II) sorption onto goethite, hematite and lepidocrocite: A surface complexation model
based on ab initio molecular geometries and EXAFS spectroscopy. Geochimica et Cosmochimica Acta 68, 2623-2637.
doi:10.1016/j.gca.2003.11.030
Peacock, C. L. and Sherman, D. M. (2004). Vanadium(V) adsorption onto goethite (a-FeOOH) at pH 1.5-12: A surface complexation model
based on ab initio molecular geometries and EXAFS spectroscopy. Geochimica et Cosmochimica Acta 68, 1723-1733.
doi:10.1016/j.gca.2003.10.018
Sherman, D.M., Peacock, C.L. and Hubbard, C.G. (2008). Surface complexation of U(VI) on goethite (a-FeOOH). Geochimica et
Cosmochimica Acta 72, 298-310. doi:10.1016/j.gca.2007.10.023
West, LJ; Handley, K; Huang, Y; Pokar, M (2003) Radar frequency dielectric dispersion in sandstone: Implications for determination of
moisture and clay content, WATER RESOUR RES, 39, . doi:10.1029/2001WR000923
Yee, N; Shaw, S; Benning, LG; Nguyen, TH (2006) The rate of ferrihydrite transformation to goethite via the Fe(II) pathway, AM MINERAL,
91, 92-96. doi:10.2138/am.2006.1860