Sulfate precipitation in produced water from Marcellus Shale for the control of naturally occurring radioactive...

Produced water (PW) generated during unconventional oil and gas extraction is characterized by very high total dissolved solids (TDS) that mainly consist of alkali and alkaline earth metals. Dominant PW management strategy (i.e., injection in Class II disposal wells) is scrutinized by regulatory agencies, and the public and PW treatment that enables high water and salt recovery (i.e., evaporation/crystallization) is being considered as an alternative. Produced water generated in the Marcellus Shale play also contains very high levels of Naturally Occurring Radioactive Material (NORM) in the form of Ra-226 and Ra-228, which is one of the key impediments for the recovery of high-quality salts. This study was designed to evaluate the efficiency of Ra-226 removal using co- and post-precipitation with barium sulfate to enable advanced PW treatment processes. High Sr/Ba molar ratios in PW lead to relatively low Ba²⁺ and Ra²⁺ removal, and Ba²⁺ concentration adjustment is necessary to achieve required treatment standards (i.e., [Ba²⁺] < 10 mg/L and [Ra²⁺] < 50 pCi/L). Seeding the reactor with barium sulfate enhanced Ba²⁺ and Ra²⁺ removal through induced heterogeneous precipitation of barite. However, it was necessary to simultaneously adjust the Sr/Ba ratio and barite level to achieve treatment requirements while maintaining reasonable detention time in the reactor (i.e., <30 min) and minimizing sludge production. Experimental and modeling results revealed that low Ba²⁺ and Ra²⁺ effluent concentrations, with minimized sludge production, can be achieved only when the barium sulfate saturation index was above 4.7, Sr/Ba molar ratio was below 2 and there was at least 25 g/L of barite "seed" in the system. This study provides useful guidelines for centralized wastewater treatment facilities in shale plays and serves to optimize pretreatment of produced water to enable recovery of valuable resources (i.e., clean water and usable salts).