Co-Precipitation of Radium with Barium and Strontium Sulfate and Its Impact on the Fate of Radium during Treatment of Produced Water from Unconventional Gas Extraction
TIEYUAN ZHANG, KELVIN GREGORY, RICHARD W. HAMMACK, AND RADISAV D. VIDIC
Radium-226/228 is formed by natural decay of uranium-238 and thorium-232 and occurs in natural gas brines brought to the surface following hydraulic fracturing.(1) Because radium is relatively soluble over a wide range of pH and redox conditions, it is the dominant naturally occurring radioactive material (NORM) and an important proxy for radioactivity of waste streams produced during unconventional gas extraction.(2, 3) Radium is a member of alkaline-earth group metals and has properties similar to calcium, strontium, and barium. Oral radium uptake can lead to substitution of calcium in bones and ultimately long-term health risks. Radium-226 activity in Marcellus-Shale-produced water ranges from hundreds to thousands picocuries per liter (pCi/L), with a median of 5350
pCi/L.(1) The total radium limit for drinking water and industrial effluents is 5 and 60 pCi/L, respectively.(4)
Radium activity in fl owback water from the Marcellus Shale play shows positive correlation with total dissolved solids (TDS) and barium content, despite the differences in reservoir lithologies.(1, 5) This fi nding is consistent with the fact that the radium/barium ratio is often constant in unconfi ned aquifers, implying that the radium co-precipitation into barite controls the activity of radium.(6) The high TDS (680-345 000 mg/L)(7) in produced water from Marcellus Shale gas wells is one of the main considerations when choosing a proper radium treatment technology. While there are several treatment options for radium removal, none is as costeffective in high TDS brines as sulfate precipitation.(8) Despite a very low solubility product for RaSO4 (Ksp,RaSO4 = 10
–10.38),(9) it is not likely to observe pure RaSO4 precipitate because of very low radium concentrations in the produced water. However, radium may co-precipitate with other carrier metals.