悬浮泥沙在长江口铀非保守行为中的重要作用

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

利用高精度的电感耦合等离子体质谱仪对2014年1月长江口表层水中溶解铀浓度及其^(234)/^(238)U比值、2013年3月长江口表层沉积物中各矿物组分的铀含量及其^(234)/^(238)U比值进行了测定,研究了其空间分布特征和影响因素。结果表明:除了长江径流和海水之外,长江口还有其他的溶解铀来源。水体中过剩铀与悬浮颗粒物浓度呈现显著相关性(r2=0.96)。对长江口表层沉积物进行的序列提取实验进一步表明,水体中悬浮颗粒物或沉积物中可解吸态和碳酸钙结合态铀可以在河口区域释放进入水体,而铁锰氧化物和有机物结合铀比较稳定,不受河口区混合过程的影响。每千克颗粒物或沉积物能够释放约2μmol颗粒态铀,使其转化为溶解态。然而,铁氢氧化物和细颗粒物的絮凝吸附作用也可使溶解铀同时从河口水体中清除。在低盐度区,铀的清除和添加过程速率相近,使溶解铀呈现暂时的"伪保守"现象:颗粒态释放的铀具有明显低的^(234)/^(238)U比值,导致水体的^(234)/^(238)U低于保守混合值。在中高盐度区域,溶解铀呈现明显的富集现象。但是由于水相和颗粒相中的铀交换,可释放颗粒态铀的^(234)/^(238)U接近溶解铀的^(234)/^(238)U比值,从而导致水体的^(234)/^(238)U比值呈现出保守性。长江口颗粒物的铀释放通量为(3.48±0.41)×105 mol/a,约占输入的总颗粒态铀通量(1.80±0.17)×106 mol/a的19.3%。长江口输入东海的溶解铀总通量(河流溶解态铀与河口添加铀之和)为(2.68±0.13)×106 mol/a,约为世界河流入海铀通量的11.7%。

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 234 U/238 U activity ratio （AR）were measured by the inductively cou-pled 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 distri-bution of U components and 234 U/238 U ARs in four fractions：desorption,calcium carbonate,Fe-Mn oxide and or-ganic 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 trans-ferred 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 234 U/238 U ARs resulted in the measured 234 U/238 U 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 dis-solved 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 aque-ous phases led to that the releasable 234 U/238 U ARs of the particles were close to the measured 234 U/238 U 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.,river-ine 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.