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Environmental Processes
Radium Sorption to Iron (hydr)oxides, Pyrite, and Montmorillonite: Implications for Ra Mobility Michael A Chen, and Benjamin D. Kocar Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05443 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018
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Environmental Science & Technology
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Radium Sorption to Iron (hydr)oxides, Pyrite, and Montmorillonite: Implications for Mobility
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Michael A. Chen and Benjamin D. Kocar*
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Dept. of Civil and Environmental Engineering,
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Massachusetts Institute of Technology
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*Corresponding author,
[email protected] 6
Abstract
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Radium (Ra) is a radioactive element commonly found within soils, sediments and natural waters.
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Elevated Ra activities arising through natural and anthropogenic processes pose a threat to
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groundwater resources and human health, and Ra isotope ratios are used to decipher groundwater
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movement, estimate submarine discharge flux, and fingerprint contamination associated with
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hydraulic fracturing operations. Although adsorption to metal (hydr)oxides and certain clay
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minerals is well established as a dominant mechanism controlling Ra transport and retention, the
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extent of Ra sorption to other minerals and under variable environmental conditions (e.g. pH and
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salinity) is limited. Accordingly, we present results of sorption studies and surface complexation
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modeling (SCM) of Ra to ferrihydrite, goethite, montmorillonite, and pyrite, for a range of pH
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values and common background cations. Ra sorption to all substrates is observed under
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geochemical conditions considered, but varies according to mineral, solution pH and specific
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competing cations. Literature derived SCMs for Ra sorption were fitted to match either sorption
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impacts due to pH or different background cations, but were not able to predict the impacts of
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different geochemical conditions. Despite this, the use of SCMs provided a more mechanistic
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understanding of Ra sorption as compared to commonly used distribution coefficients.
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Table of Contents (TOC) /Abstract Art
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Introduction
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Chronic ingestion and inhalation of radioactive materials, including radium (Ra) and radon (Rd),
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represents an ongoing threat to human health worldwide.1 Of these, Ra is ubiquitous in soils,
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aquifers, and natural waters owing to the radioactive decay of primordial 235U, 238U, and 232Th,
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and often accounts for the dominant fraction of total radiation found in groundwater. All isotopes
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of Ra are unstable, and four (223Ra, 224Ra, 226Ra, and 228Ra) possess half-lives sufficient to persist
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within environmental systems and present a risk for human exposure (11.4 days, 3.6 days, 1600
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years, and 5.75 years, respectively). Moreover, 226Ra is the parent radionuclide of 222Rn; chronic
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inhalation of 222Rn increases risk of lung cancer.2 Hence, geochemical controls on Ra mobility
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are directly tied to the mobility and accumulation of Rn within soil-sedimentary systems.
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Several geochemical processes impart overarching controls on Ra within soils and
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aquifers. Alpha-recoil, the ejection of daughter radionuclides from soil and sedimentary minerals
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into adjacent porewater, is a primary process sourcing Ra to groundwater. Ongoing alpha recoil
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progressively elevates porewater Ra activities until hydrologic flushing removes the equilibrating
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solution, or Ra achieves secular equilibrium with its parent and daughter nuclides. Most aquifer
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systems contain low (e.g. U, Th,
