【作者】 江雪艳；

【导师】 于志刚； 康兴伦；

【作者基本信息】 中国海洋大学 ， 海洋化学， 2008， 博士

【摘要】 铀系核素因其在海洋学过程中被广泛用作示踪剂和测年手段而被关注,铀在海洋中的分布及其源和汇的研究一直是很多研究者感兴趣的问题,而河流作为海洋中铀的主要输入源,其河水中铀的浓度对于研究海洋中铀的收支和平衡具有重要意义。河口区由于盐度、悬浮泥沙浓度等因素的时空变化频繁,加之吸附解吸等过程控制着化学元素在颗粒固相和溶液相之间的分配,因此对铀元素向海洋输送通量的影响很大。另外河口和滨海的潮间带及盐沼也是对海洋中铀的收支平衡有重要影响的区域,被认为是海洋中铀的重要归宿之一。黄河以其高含沙量而闻名,已有研究发现黄河水中的溶解铀含量不但远远超出了世界河流的平均值,而且也高于海水中铀的浓度,因此对黄河及其河口铀的浓度、来源及其行为进行研究具有区域特殊性意义。与黄河口邻近的莱州湾,其南岸具有广阔的潮间带及盐沼,是研究盐滩沉积物铀的输出及演变的良好场所。本研究采集了黄河干流和黄河口的水样及沉积物、莱州湾钻孔的地下卤水和沉积物样品,主要对其中的铀同位素进行了研究,并进行了实验室模拟,取得了如下研究结果:1、黄河干流溶解铀的浓度介于2.04 -7.83μg/l之间,平均值4.38μg/l,远高于世界河流平均值,也超过了海水的溶解铀浓度;溶解铀浓度沿黄河干流呈上升趋势。黄河水中234U/238U活度比介于1.36-1.67之间,平均值1.4。较高的234U/238U活度比说明黄河水中的溶解铀受到了硅酸盐风化及地下水输入的影响。2、黄河中游及其支流流经黄土高原,干旱半干旱的气候使得风化产物在颗粒表面富集,强烈的物理侵蚀又使大量黄土颗粒及风化壳进入黄河,黄土颗粒表面及风化壳中的铀极易溶出造成黄河水中铀浓度升高;另一方面,黄土中的铀还可以通过被淋滤进入地表径流和地下水,最终以溶解铀的形式被带入黄河。根据黄土悬浮平衡和淋滤模拟实验结果以及黄河径流和泥沙数据,利用箱式模型对黄河中游溶解铀来源的估算表明,黄河中游溶解铀2/3来自悬浮颗粒物释放,1/3来自地下水及径流输入。3、黄河口溶解铀呈现非保守行为,盐度低于20时有外来的溶解铀进入水体。每年有大约3.5×104 kg的溶解铀从悬浮颗粒物解吸或从沉积物间隙水中释放进入河口水体中。加上河流输入的溶解铀,每年河口水体输入的总溶解铀量为9.8×104 kg。这个数值占全球河流铀通量的1%左右。4、现场调查和模拟实验均显示,黄河口混合带溶解铀和磷酸盐随盐度增加具有同步变化的趋势。磷酸盐的增加主要来自于底沉积物间隙水向上覆水体的扩散,而增加的溶解铀主要来自水体中悬浮颗粒物的解吸。5、莱州湾南岸地下卤水溶解铀的浓度高于相同浓缩倍数的莱州湾海水的铀浓度,且程度随浓缩倍数的增大而增大,说明卤水中的溶解铀还有其他来源。该地区大面积的盐沼为海水中的铀在盐沼的输出提供了有利条件,盐沼沉积物中由于胶体絮凝或与铁锰氧化物共沉淀而从水体中沉淀下来的铀,可能在其被埋藏后经历的漫长的地质变迁过程中通过地下水的淋滤而进入卤水体系,或由于有机物的降解而导致铁锰氧化物被还原从而使部分铀元素被释放而重新进入地下卤水中,这是卤水中溶解铀的另一个重要来源。而地下卤水中较高浓度的HCO3-和PO43-可以和卤水中的溶解铀形成稳定的配合物从而使高浓度的铀得以保持。更多还原

【Abstract】 U-series nuclides have been used extensively as tracers and chronometers of many oceanographic processes. Thus, the distribution of U in the oceans and its source and sink terms are of particular interest. The uranium concentration of river runoff, the dominant transport pathways which supply significant amounts of uranium to the ocean, is important to the evaluation of uranium budget and mass balance in the global ocean. The salinities and concentrations of suspended sediments in the estuarine mixing zone vary with time and space, which alter the element partitioning between solid phase and solution via adsorption and desorption and change the riverine input of uranium significantly. The intertidal salt marshes are also considered to be strong sinks of uranium at all salinities which will exert influences on the global uranium budget in the ocean.Yellow River is famous for its high suspended sediments. It has been found that the dissolved uranium in the Yellow River is not only higher than the average concentration of the world rivers but also exceed that of the average seawater. Therefore, it is of particularly interest in studying the concentrations, sources and behavior of uranium in the Yellow River and its estuary. The broad intertidal salt marshs along the southern coast of Laizhou Bay, which is adjacent to the Yellow River estuary, can serve as excellent sites to study the removal and evolution of uranium in the salt marsh sediments. In this study, the waters and sediments of the Yellow River and its estuary as well as the underground brine along the southern coast of Laizhou Bay were sampled. The uranium isotopes and other relevant elements of the samples were measured. Based on the data obtained from the field investigation and the laboratory simulation, the main results can be summarized below. 1. The concentrations of dissolved uranium in the main channel of Yellow River range between 2.04 -7.83μg/l, averaging of 4.38μg/l, which is not only much higher than the global average river water uranium concentration, but also higher than the average concentration for seawater of salinity 35. The uranium concentrations generally increase downstream. The 234U/238U activity ratios of the Yellow River water vary from 1.36- 1.67 with a majority of values of 1.4. The relatively high 234U/238U activity ratios indicate the contribution of silicate weathering as well as the groundwater input.2. The Chinese Loess Plateau covers most of the middle reaches of the Yellow River drainage basin. The arid-semiarid climate make the uranium accumulated in grain surface and weathering crusts of loess deposits and the severe physical erosion results in these loess grain and weathering crusts entering into the Yellow River and the uranium can be released or dissolved easily from these materials which will enhance the concentrations of uranium in the Yellow River. On the other hand, the uranium can be leached to the surface runoff and ground water and then enter into the Yellow River ultimately as dissolved uranium. Based on the data of equilibration experiments and leaching experiments as well as the water and sediments discharge of Yellow River, the sources of dissolved uranium in the middle reaches of Yellow River is estimated by a box model. Which give a result that the suspended sediments contribute two thirds while the ground water and runoff contribute one third of dissolved uranium in the middle reaches of Yellow River.3. The dissolved U behaves non-conservatively at salinities <20 in the Yellow River estuary. There is addition of U dissolved from suspended sediments and/or diffused from interstitial water of bottom sediment as about 3.5×104 kg/yr to the estuarine waters. In addition to the dissolved uranium inputs by the river, the inventory of uranium input from the estuary of Yellow River to the ocean should be 9.8×104 kg/ yr, which account for about 1% of the global riverine uranium flux. 4. Both field investigation and laboratory experiments indicate that in the Yellow River plume mixing zone, the variation in dissolved U with salinity assumes a similar pattern as that of PO43-. The diffusion of interstitial water from bottom sediment influences more the enrichment of PO43-, whereas desorption from the suspended sediments seems more responsible for the elevated dissolved U concentrations.5. The dissolved uranium concentration of underground brine along the southern coast of Laizhou Bay are higher than the concentrated seawater of Laizhou Bay under the same concentration factor and the discrepancy increase with the concentration factor, indicating sources of dissolved uranium other than the concentrated seawater. The wide intertidal salt marshes serve the condition of the removal of uranium from the seawater. The uranium deposited with the flocculation of colloid and/or coprecipitated along with the iron/ manganese oxides at the surface sediments may be leached by the groundwater during the geological evolution after being buried, or be released to the underground brine owing to the reduction of iron/ manganese oxides along with the degradation of organic material. The relatively high concentrations of HCO3- and PO43- in the underground brine can stabilize the high level of dissolved uranium by forming uranyl complex.更多还原