Radium Sorption to Iron (Hydr)oxides, Pyrite, and Montmorillonite: Implications for Mobility

Abstract

Radium (Ra) is a radioactive element commonly found within soils, sediments, and natural waters. Elevated Ra activities arising through natural and anthropogenic processes pose a threat to groundwater resources and human health, and Ra isotope ratios are used to decipher groundwater movement, estimate submarine discharge flux, and fingerprint contamination associated with hydraulic fracturing operations. Although adsorption to metal (hydr)oxides and certain clay minerals is well established as a dominant mechanism controlling Ra transport and retention, the extent of Ra sorption to other minerals and under variable environmental conditions (e.g., pH and salinity) is limited. Accordingly, we present results of sorption studies and surface complexation modeling (SCM) of Ra to ferrihydrite, goethite, montmorillonite, and pyrite, for a range of pH values and common background cations. Ra sorption to all substrates is observed under geochemical conditions considered, but varies according to mineral, solution pH and specific competing cations. Literature derived SCMs for Ra sorption were fitted to match either sorption impacts due to pH or different background cations, but were not able to predict the impacts of different geochemical conditions. Despite this, the use of SCMs provided a more mechanistic understanding of Ra sorption as compared to commonly used distribution coefficients.

Geochemical conditions conducive for retention of trace elements and radionuclides during shale–fluid interactions

“…For example, several studies have shown increased mobilization of 226 Ra from clays such as kaolinite and montmorillonite in the presence of a high concentration of NaCl. 45 Here, an increase in the ionic strength of the leaching uid had no pronounced effect on Ra activity in leachates, except for E1 treatment. Compared to low ionic strength treatment, high Ra activity was measured in leachates of E1-ox-Sal-pH7 ( Fig.…”

Change in the site density and surface acidity of clay minerals by acid or alkali spills and its effect on pH buffering capacity

“…The clay minerals were then collected via centrifugation and washed three times with 1 M NaNO 3 solution to change the clays to a Na + form. Although three washing cycles with NaNO 3 might not be enough to make clay minerals homoionic 47 , this pretreatment was chosen to allow for closer comparisons to previous studies, which determined clay minerals’ Ka and site density through surface complexation modeling 30,44,48 . Excess Na + was removed by washing five times with deionized water, and the final pH was in the range from 6 to 7.…”

Fate and transport of radioactive gypsum stack water entering the Floridan aquifer due to a sinkhole collapse

“…Overall, for the observed cases, the results suggest that there is a risk of drinking water contamination due to radionuclide transport. Although both the weak and strong binding ferrihydrite mineral performed similarly, the result can change depending on the competitive nature of the constituents in the soil, which can be seen in past analyses 22 . A previous study showed that radium levels in Tampa Bay are low.…”

“…The amount of abundant minerals in the aquifer can affect the overall remediation process; however it is complex to quantify the amount of potential adsorbents in the soil. From previous studies, a low pH and the presence of other elements and compounds in the solution may induce competition among them, which may then in turn affect the potential of adsorption occurring in the soils 21 , 22 .…”