Department of Civil and Environmental Engineering, University of Pittsburgh , Pittsburgh, Pennsylvania 15261, United States.
Environ Sci Technol. 2014 Apr 15;48(8):4596-603. doi: 10.1021/es405168b. Epub 2014 Apr 7.
Radium occurs in flowback and produced waters from hydraulic fracturing for unconventional gas extraction along with high concentrations of barium and strontium and elevated salinity. Radium is often removed from this wastewater by co-precipitation with barium or other alkaline earth metals. The distribution equation for Ra in the precipitate is derived from the equilibrium of the lattice replacement reaction (inclusion) between the Ra(2+) ion and the carrier ions (e.g., Ba(2+) and Sr(2+)) in aqueous and solid phases and is often applied to describe the fate of radium in these systems. Although the theoretical distribution coefficient for Ra-SrSO4 (Kd = 237) is much larger than that for Ra-BaSO4 (Kd = 1.54), previous studies have focused on Ra-BaSO4 equilibrium. This study evaluates the equilibria and kinetics of co-precipitation reactions in Ra-Ba-SO4 and Ra-Sr-SO4 binary systems and the Ra-Ba-Sr-SO4 ternary system under varying ionic strength (IS) conditions that are representative of brines generated during unconventional gas extraction. Results show that radium removal generally follows the theoretical distribution law in binary systems and is enhanced in the Ra-Ba-SO4 system and restrained in the Ra-Sr-SO4 system by high IS. However, the experimental distribution coefficient (Kd') varies widely and cannot be accurately described by the distribution equation, which depends on IS, kinetics of carrier precipitation and does not account for radium removal by adsorption. Radium removal in the ternary system is controlled by the co-precipitation of Ra-Ba-SO4, which is attributed to the rapid BaSO4 nucleation rate and closer ionic radii of Ra(2+) with Ba(2+) than with Sr(2+). Carrier (i.e., barite) recycling during water treatment was shown to be effective in enhancing radium removal even after co-precipitation was completed. Calculations based on experimental results show that Ra levels in the precipitate generated in centralized waste treatment facilities far exceed regulatory limits for disposal in municipal sanitary landfills and require careful monitoring of allowed source term loading (ASTL) for technically enhanced naturally occurring materials (TENORM) in these landfills. Several alternatives for sustainable management of TENORM are discussed.
镭在非常规天然气开采水力压裂的返排和产出水中与高浓度的钡和锶以及高盐度一起存在。镭通常通过与钡或其他碱土金属共沉淀来从这种废水中去除。沉淀物中镭的分布方程是从晶格取代反应（包括）的平衡推导出来的，该反应在水溶液和固相中的镭（2+）离子和载体离子（例如钡（2+）和锶（2+））之间进行，并且通常用于描述这些系统中镭的命运。尽管镭-锶硫酸盐（Kd = 237）的理论分配系数比镭-钡硫酸盐（Kd = 1.54）大得多，但以前的研究集中在镭-钡硫酸盐平衡上。本研究评估了在变化的离子强度（IS）条件下，镭-钡-硫酸盐和镭-锶-硫酸盐二元系统以及镭-钡-锶-硫酸盐三元系统中共沉淀反应的平衡和动力学，这些条件代表了非常规天然气开采过程中产生的盐水。结果表明，镭的去除通常遵循二元系统中的理论分布规律，在高 IS 下，镭在钡-硫酸盐系统中的去除得到增强，而在锶-硫酸盐系统中受到抑制。然而，实验分配系数（Kd'）变化很大，无法通过分布方程准确描述，这取决于 IS、载体沉淀的动力学，并且不考虑通过吸附去除镭。三元系统中镭的去除受钡-硫酸盐共沉淀的控制，这归因于 BaSO4 成核速率快，以及 Ra（2+）与 Ba（2+）的离子半径比与 Sr（2+）的离子半径更接近。即使在共沉淀完成后，水疗过程中载体（即重晶石）的回收在增强镭的去除方面也被证明是有效的。基于实验结果的计算表明，集中式废物处理设施中产生的沉淀物中的镭水平远远超过在城市卫生垃圾填埋场中处置的监管限值，需要仔细监测这些垃圾填埋场中技术增强的天然存在物质（TENORM）允许的源项加载（ASTL）。讨论了几种 TENORM 可持续管理的替代方案。
