In the process of hydraulic fracturing (fracking) for oil and gas extraction directly from black shale, large volumes of wastewater, that includes fracturing fluids and natural brines from the underground rock formation, return to the surface within weeks after fracking. The total volume of wastewater production in Pennsylvania alone has exceeded 40 million barrels per year since 2012, and the number has been increasing. The wastewaters are often highly saline and contain elevated concentrations of radium (Ra) (up to 7000 pCi/L) and barium (Ba) (up to 10000 ppm). Due to the adverse health effects of Ra and Ba, the Departments of Environmental Protection (DEP) in multiple states (including Pennsylvania, Ohio, etc.) determined that the wastewater sludge, which is disposed in landfills, must contain no more than 5 pCi/g of radium. The Environmental Protection Agency (EPA) also set an upper limit of barium in drinking water at 2 ppm. Therefore, proper removal of Ra and Ba from wastewaters is of urgent importance.
Co-precipitation of Ra and Ba with sulfate (SO42-) to form radio-barite has been accepted as a method to remove Ra from water due to the shared chemical properties between Ra and Ba. However, the method is not well established. Furthermore, our recent preliminary research by mixing wastewater containing varied concentrations of Ba, strontium (Sr) (another element that shares chemical properties with Ra and Ba), and Ra with sulfate-rich acid mine drainage (AMD) demonstrated that higher ratios of Sr/Ba enhanced Ra co-precipitation. This finding brings an opportunity to promote Ra removal more efficiently by using solutions with high Sr/Ba ratios. The aim of my research is to evaluate the influence of different Sr/Ba ratios on Ra co-precipitation as well as influences from different background solutions, and therefore, determine the most environmentally friendly as well as economically approachable method for Ra removal from wastewaters.
With support from the Dartmouth Alumni Fund, I conducted experiments to determine the levels of co-precipitation of these chemicals, and analyze the solids and liquids involved in the process.
Changes in Sr and Ba concentrations in solutions prior and after experiment were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) in the Earth Sciences Department. Ra levels in both liquids and solids were analyzed using gamma spectrometry in Dartmouth Short-lived Radionuclide Laboratory. Co-precipitated (Ba-Sr-Ra)SO4 solids were observed using scanning electron microscopy (SEM) in Earth Sciences Department and Dartmouth EM Lab.
The results demonstrated that the total Ra co-precipitated as solids decreased initially when Sr/Ba ratio increased from 0.1 to 1, followed by continuous increase when Sr/Ba ratio increased from 1 to 100 (Figure 1). Higher Sr/Ba ratio favors Ra co-precipitation, whereas the presence of Ca2+ adversely influences the growth of barite (BaSO4) crystals thus hinder Ra removal. This phenomenon was due to the formation of radio-barite ((Ba,Ra)SO4) at low Sr/Ba ratios and radio-celestine ((Sr,Ra)SO4) at high ratios. This study has implications on effective treatment of radioactive wastewater from fracking. Thanks to the funding received from the Alumni Fund, we were able to determine that increasing the Sr/Ba ratio and adding corresponding amount of SO42- will significantly improve the efficiency of Ra removal from wastewater.