274061 Fate of Radium in Marcellus Shale Flowback Water

Tuesday, October 30, 2012: 8:30 AM
331 (Convention Center )
Tieyuan Zhang, Civil & Environmental Engineering, University of Pittsburgh, Pittsburgh, PA, Elise Barbot, Laboratoire Mécanique, Modélisation et Procédés Propres, Aix en Provence, France and Radisav Vidic, Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, PA

Radium is naturally occurring radioactive element that is found in shale gas wastewater (flowback and produced water). Ra226 is the main isotope that is often found in Marcellus Shale flowback water at activities exceeding 10,000 pCi/L. In comparison, the EPA maximum contaminant level (MCL) for drinking water is 5pCi/L and the maximum radioactivity for disposal in RCRA-D non-hazardous landfill is 25 pCi/g. This study is designed to investigate the fate of Radium in flowback/produced water during storage in surface impoundments.

When flowback/produced water is placed in an impoundment, dissolved Ra would react with suspended solids and bottom sediments. Adsorption and lattice replacement (or co-precipitation) reactions are the controlling mechanisms for Ra uptake by the solids that may be present in impoundments. The importance of the difference between these two mechanisms lies in the difference in Radium release from these solids due to changes in ambient conditions, which may produce differences in environmental impacts.
Pure Ra solids (e.g., RaSO4) tend not to precipitate because Ra2+ concentrations are too low to reach the saturation limit despite a very low solubility product for species like RaSO4 (Ksp = 5.5e-11). However, since Ra is a member of alkaline-earth metals and has properties similar to barium, strontium and calcium, it is common to find Radium as co-precipitate in solids (sulfates and carbonates) of these carrier metals. Ra-BaSO4 lattice replacement reaction, which is often termed as co-precipitation, is the dominant pathway for the removal of 226Ra in the presence of sulfate ions. Co-precipitate of Radium depends on the Barite content in solid and free Ba2+ in solution.
In the absence of sulfate and carbonate ions to form insoluble precipitate, Ra adsorption would be a dominant pathway for Ra removal from solution. Alkaline earth sulfate, clay minerals, organic matter, iron/manganese oxide and iron sulfide are known Ra adsorbents. Their adsorption capacity is related to the overall negative charge, which creates the cation exchange capacity (CEC). Ionic strength and pH of the solution and competition from alkaline earth metals can be used as a control factor for Ra removal in impoundments and natural system. High ionic strength (often as high as 3M) and presence of alkaline earth cations (e.g., 6mmol/L Ba2+) would weaken the adsorption capacity and impede lattice replacement, respectively.
In this study, actual suspended solids and bottom sediments collected from flowback/produced water impoundments were studied for their ability to adsorb Ra under relevant environmental conditions. Composition of suspended solids and bottom sediments was determined by ICP-MS and X-ray diffraction. Radium activity in both solid and liquid phase was analyzed by Liquid Scintillation Counting (LSC) to investigate the significance of Ra removal in impoundment. Sequential extraction procedure (SEP) was applied to analyze Ra distribution in different fractions of the solids: (a) exchangeable cations, (b) Fe-Mn oxides (c) Organic matter, and (d) the residual (mainly quartz, clay and Barite). The influence of pH, ionic strength, and alkaline earth metal ions was tested in an effort to determine if the removal mechanism can be attributed to adsorption or co-precipitation. Toxicity Characteristic Leaching Procedure (TCLP) was conducted to determine disposal options for Radium enriched solids created in the impoundments. The results of these experimental investigations will be presented at the conference.

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