Pore‐scale evaluation of uranyl phosphate precipitation in a model groundwater system

Abstract

[1] The abiotic precipitation of uranium (U(VI)) was evaluated in a microfluidic pore network (i.e., micromodel) to assess the efficacy of using a phosphate amendment to immobilize uranium in groundwater. U(VI) was mixed transverse to the direction of flow with hydrogen phosphate (HPO 4 2À ), in the presence or absence of calcium (Ca 2þ ) or sulfate (SO 4 2À ), in order to identify precipitation rates, morphology and types of minerals formed, and effects of mineral precipitates on pore blockage. Precipitation occurred over the time scale of hours to days. Relative to when only U(VI) and HPO 4 2À were present, precipitation rates were 2.3 times slower when SO 4 2À was present, and 1.4 times faster when Ca 2þ was present; larger crystals formed in the presence of SO 4 2À. Raman backscattering spectroscopy and micro-X-ray diffraction results both showed that the only mineral precipitated was chernikovite, UO 2 HPO 4 Á 4H 2 O; energy dispersive X-ray spectroscopy results indicate that Ca and S are not incorporated into the chernikovite lattice. A pore-scale model was developed, and simulation results of saturation ratio (SR ¼ Q/K sp ) suggest that chernikovite is the least thermodynamically favored mineral to precipitate (0 < SR < 1) compared to uranyl hydrogen phosphate and Na-autunite (13 < SR < 40), and uranyl orthophosphate and Ca-autunite (when Ca 2þ is present; SR > 10 5 ). Fluorescent tracer studies and laser confocal microscopy images showed that densely aggregated precipitates blocked pores and reduced permeability. The results suggest that uranium precipitation with phosphate as chernikovite is rapid on the time scale of remediation for the conditions considered and can block pores, alter fluid flow paths, and potentially limit mixing and precipitation.

Pore-Scale Process Coupling and Effective Surface Reaction Rates in Heterogeneous Subsurface Materials

“…For example, the reaction rate at the molecular scale can be heterogeneous, even on a single mineral phase (Piana et al 2006;Spagnoli et al 2006;Boily and Rosso 2011;Lüttge et al 2013); the rates of reactions occurring in nano-pores can be infl uenced by water ordering, surface curvature, and overlapping double layers Liu 2009, 2012;Argyris et al 2010;Bourg and Steefel 2012); and mineral surface reactivity can be modifi ed as reactions proceed (Alekseyev et al 1997;Benner et al 2002;White and Brantley 2003;Bernard 2005;Noiriel et al 2005;Luquot and Gouze 2009). The pore geometry and connectivity can also be changed by mineral precipitation (Steefel and Lichtner 1994;Davis et al 2006;Zhang et al 2010a;Yoon et al 2012;Fanizza et al 2013;Boyd et al 2014) and dissolution (Noiriel et al 2004(Noiriel et al , 2005(Noiriel et al , 2009Bernard 2005;Flukiger and Bernard 2009;Luquot and Gouze 2009;Kang et al 2010), microbial growth (Taylor and Jaffe 1990;Nambi et al 2003;Thullner et al 2004Thullner et al , 2007Knutson et al 2005;Engesgaard 2007, 2012;Brovelli et al 2009;Zhang et al 2010b;Kirk et al 2012;Tang et al 2013) and biomineralization (Zhang and Klapper 2010;…”

Precipitation in Pores: A Geochemical Frontier

“…Already a method that takes advantage of the low solubility of Ba-Ra-Sr sulfate minerals has been proposed as an aboveground treatment strategy for hydraulic fracturing wastewater (Zhang et al 2014), but it may be possible to utilize this for subsurface applications in porous media. Another example is that of uranium, which potentially could be remediated using hydroxyapatite which dissolves and causes a uranium phosphate precipitate to form (Arey et al 1999;Fanizza et al 2013). Instead of using abiotic hydroxapatite directly, less expensive bone meal has been considered (Naftz et al 1998).…”

Lattice Boltzmann-Based Approaches for Pore-Scale Reactive Transport

“…In particular, a coupled pore-scale reactive transport model was rigorously validated against well-controlled microfl uidic experimental results to demonstrate the importance of realistic pore confi gurations, fl ow and transport physics and geochemistry, and to predict how mineral precipitation alters fl ow paths by pore plugging for different chemical conditions Fanizza et al 2013;Boyd et al 2014;Oostrom et al 2014). The coupled model consists of LBM for fl uid fl ow and FVM for reactive transport with mineral precipitation and dissolution.…”

“…Heterogeneous reactions were solved using the boundary condition given by Equations (10-13). Pore-scale experiments led by Werth and co-workers (Zhang et al 2010a;Fanizza et al 2013;Boyd et al 2014) were used. The micromodel consisted of a 2 × 1 cm pore network containing a pattern of cylindrical posts (300 m in diameter) with 180-m pore spaces and 40-m pore throats, a depth of ~20-40 m, and a porosity of ~0.39.…”