The importance of the suspended sediment for the uranium non-conservative behavior in the Changjiang Estuary

To study geochemical behavior of uranium in the Changjiang Estuary (CJE), surface water and surface sediment samples were collected along a salinity gradient during January 2014 and March 2013, respectively. The dissolved uranium concentration (DUC) and ²³⁴U/²³⁸U activity ratio (AR) were measured by the inductively coupled plasma mass spectrometer (ICP-MS). Sequential extraction experiments of surface sediments (collected from the freshwater end-member to the seaward boundary of the sandbar) were also conducted to analyze spatial distribution of U components and ²³⁴U/²³⁸U ARs in four fractions:desorption, calcium carbonate, Fe-Mn oxide and organic matter. The results showed that there is an extra source of the dissolved uranium into the CJE by other processes excepting U supplied from the ideal mixing process of river water and seawater. The excess DUC and suspended particle matter (SPM) showed a strong positive correlation (r²=0.96). Around 2 μmol U can be transferred from per kilogram particulate into aqueous phase through the desorption and calcium carbonate dissolution, but this phenomenon did not occur for the Fe-Mn oxides and particulate organic matter. At the low salinity region, the releasable particulate U with low ²³⁴U/²³⁸U ARs resulted in the measured ²³⁴U/²³⁸U ARs of the water column below the conservative mixing curve, although the DUC points fell on the conservative mixing line. However, flocculations of the iron hydroxide and fine particle may cause simultaneous depletion of the dissolved uranium. Consequently, when the U input and removal rates were approximately equal at the low salinity region, the dissolved uranium showed apparently conservative phenomenon. At the mid-high salinity region (with high SPM), excess dissolved uranium was observed in the water column, but the U exchange between the particulate and aqueous phases led to that the releasable ²³⁴U/²³⁸U ARs of the particles were close to the measured ²³⁴U/²³⁸U ARs in the water column.The excess dissolved uranium flux from the particle was estimated as (3.48±0.41)×10⁵ mol/a which occupied 19.3% particlulate U flux[(1.80±0.17)×10⁶ mol/a] into the CJE. The total flux (i.e., riverine flux plus additional flux) of the dissolved uranium from the CJE[about (2.68±0.13)×10⁶ mol/a] contributed about 11.7% of the DU input to the world ocean.