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

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 ₄^2-), in the presence or absence of calcium (Ca ²⁺) or sulfate (SO₄^2-), 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 HPO₄ ^2- were present, precipitation rates were 2.3 times slower when SO₄^2- was present, and 1.4 times faster when Ca ²⁺ was present; larger crystals formed in the presence of SO ₄^2-. Raman backscattering spectroscopy and micro-X-ray diffraction results both showed that the only mineral precipitated was chernikovite, UO₂HPO₄ · 4H₂O; 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 = 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 Ca²⁺ is present; SR > 105). 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.