Role of Suspended Particulate Matter in Regulating the Behavior of Dissolved Uranium in the Yellow River Estuary

In order to examine the mixing behavior of dissolved uranium (U) in estuaries under different suspended particulate matter (SPM) regimes, three laboratory-based experiments were conducted by mixing seawater with river water containing different concentrations of SPM. Comparing this study with other field and laboratory-based experiments, dissolved U behaved differently depending upon the concentration of SPM. When SPM concentrations are > 0.8 g/L in the Yellow River, desorption/dissolution of U from SPM becomes predominant and dissolved U is enriched relative to the theoretical mixing line. However, when SPM concentrations are < 0.8 g/L, dissolved U behaves conservatively with some extent of removal during estuarine mixing. 234U/238U activity ratios were somewhat constant showing no measurable isotopic fractionation during physical mixing and U sorption/desorption to/from particles. Addition of dissolved 238U desorbed/dissolved from SPM during the annual Yellow River water-sediment regulation scheme (Jun 30th–Jul 14th, 2014) was estimated at 6.4 × 1011 dpm, about 9% of the total riverine flux of dissolved 238U during that same period. This study represents a contribution to studies of dissolved U in muddy rivers and estuaries throughout the world. Results reported here provide not only a perspective to better estimate U flux from rivers to the ocean but also new insights into better understanding its estuarine mixing behavior and controlling factors.