研究生知识库

锌是一个比较活泼的过渡金属元素，主要以Zn2+形式存在于大气圈、水圈、生物圈和固体地球中，并广泛参与到各种地质作用中。锌在工业、生物医学、食品业等人类活动中被大量使用，预计2019年人类对精锌的需求量将达到1512万吨。同时锌是生命活动必需的一种微量元素，是生物体内多种酶或辅酶的组成成分，对生物的生命活动起着重要作用。虽然锌是生物生命必需的微量元素，但在高浓度时却成为有毒元素。据估计每年有超过300万吨锌被人为释放到环境中，势必改变自然环境锌的生物地球化学行为和循环，甚至会造成锌污染。锌同位素已成为认清锌在自然界中的生物地球化学循环的新手段，这得益于锌同位素测试的技术手段的成熟和提高，主要是多接收电感耦合等离子体质谱仪（MC-ICP-MS）测试技术的应用。大量学者为了解不同储库中锌同位素变化，对不同样品的锌同位素组成进行了测定。而不同的锌同位素组成，反应了不同的地质作用和机理，对掌握锌在自然界中的生物地球化学循环意义重大。随着不同储库中锌同位素变化的发现，锌同位素在成矿预测与行星演化、全球气候变化示踪、污染源及人为活动影响示踪和生命医学等研究领域被大量应用。河流在元素的全球生物地球化学循环中起着重要作用，更与人类活动息息相关。在地球表层复杂的“关键带”中，岩石或土壤风化的产物会最终汇集到河流中并被运移到海洋。在全球物质循环中，河流承载了从陆地到海洋近90%的物质。同时，在人口集中和工农业活动密集的地区，河流不但提供了水源，更是众多人类活动排放的废弃物的载体，因此，很容易受到污染。目前，河流锌同位素研究主要集中在欧洲和美洲地区，很多过程机制还未被完全认知，特别是这些研究主要针对的是背景区河流或人为污染的小型河流，大型人为干扰河流研究几乎仍是空白，更没有对大型人为干扰河流中不同时间（季节）和空间（上游到下游）上锌同位素的系统研究。珠江年径流量在我国仅次于长江，具有特殊的地理位置和经济地位，然而对于流域内重金属锌的来源、迁移转化、演变机制以及潜在的环境风险等方面的研究几乎还是空白。本研究在洪水期（2015年5月）和枯水期（2015年12月）分别采集了珠江流域内干流和主要支流的样品。同时为了获取不同源中的锌同位素组成谱，本研究还收集了流域内典型的基岩、土壤、地下水、生活污染、工业废水和矿山废水等样品。通过分析以上样品中的水化学参数、主微量元素浓度、有机质含量（碳、氮和磷），结合锌同位素数据，取得了以下结论：（1）珠江流域水体中HCO3-和Ca2+分别是最主要的阴、阳离子；珠江悬浮颗粒物的化学组成含量由高到低依次为：Al3+＞Fe3+＞K+＞Ca2+＞Mg2+＞Ti4+＞Na+＞Mn2+。其中易溶元素Mg2+、Ca2+和Na+相对上地壳亏损，浓度低于世界平均值，表明流域风化强度高。其他元素浓度和世界河流中的平均值接近，受人为活动影响不大；（2）珠江溶解态锌浓度的变化范围为0.1-9.7μg/L，平均浓度为1.2μg/L，略高于全球河流中的锌浓度0.6μg/L；珠江及其支流悬浮颗粒物中锌浓度范围为91.3-908.6μg/g，平均值为279.2μg/g，和世界河流中的平均值接近，相对上地壳富集；（3）珠江中溶解态锌同位素δ66Zn组成范围-0.26-1.19‰，总体高于全球其他河流中的δ66Zn值；悬浮颗粒物中锌同位素δ66Zn变化范围为0.19-1.33‰，平均值为0.77‰；（4）内部过程（生物吸收和颗粒物吸附）不是造成珠江锌同位素变化的主要原因；（5） 珠江溶解态锌主要来自不同源的混合，其中碳酸盐风化为主要贡献源，人为源次之，“湖库水”不容忽视。结合端元混合模型计算出碳酸盐的贡献比例范围为30-80％，自然源作为珠江溶解态锌的主要源平均贡献超过60％。人为源贡献比重从上游到下游逐渐增加，贡献比例不超过50％，流域内平均值贡献为25％。“水库效应”形成的“湖库水”的贡献平均值为15％；（6）珠江悬浮颗粒物中锌主要来自不同的自然和人为源的混合，其中以硅酸盐和碳酸盐为主，从上游到下游人为源贡献逐渐增加。来自硅酸盐的贡献平均值为40％左右，采样点M10中来自硅酸盐的贡献是最大的，接近90%；人为源贡献比例平均值20％左右。碳酸盐的贡献平均值为40％左右，且向珠江下游逐渐递减；（7）“水库效应”明显影响珠江中锌及其同位素地球化学，具体表现为：流域碳酸盐基岩风化后进入河流，受到上游河流大坝拦截，“水库效应”致使含碳酸盐矿物多的悬浮颗粒物吸附更多溶解态中的锌而沉淀到库底，结果使得溶解态锌浓度降低，锌同位素偏轻。悬浮颗粒物中最终具有低锌、高钙浓度和偏重的锌同位素（比碳酸盐偏重0.30‰左右）的特征；（8）珠江溶解态锌同位素δ66Zn值高于已报道的全球其他河流的值，主要是由于碳酸盐基岩对珠江中锌的贡献。表明前人的研究可能低估了流域基岩以碳酸盐岩为主的入海河流中的δ66Zn的值。尽管这个开放性问题需要进一步开展工作，珠江流域锌同位素研究为平衡海洋中的锌同位素提供了新视野。

Zinc (Zn) is a relatively active transition metal, which mainly exists in the form of Zn2+ in the atmosphere, hydrosphere, biosphere and solid earth, and is widely involved in various geological processes. Zn is widely used in human activities such as industry, biomedicine, and food. It is expected that the demand for refined Zn will reach 15.12 million tons in 2019. At the same time, zinc is a trace element necessary for life. It is a component of various enzymes or coenzymes in the organisms and plays an important role in the life activities such as metabolisms. Although zinc is an essential trace element in biological life, it becomes a toxic element at high concentration. It is estimated that more than 3 million tons of zinc are artificially released into the environment each year, which is bound to cause zinc pollution of the natural environment and affect the biogeochemical cycle of zinc. Thanks to the maturity and improvement of the purification and analysis techniques, especially the application of new multi-receiving inductively coupled plasma mass spectrometry, Zn isotopes have become a new apporach for studying the biogeochemical cycle of zinc in nature. A large number of papers have reported the zinc isotope compositions of different samples in order to understand the changes of zinc isotope signatures in between different reservoirs. Different zinc isotopic compositions reflect different geological processes and mechanisms, and are of great significance for moitoring the biogeochemical cycle of zinc in nature. Zinc isotopes have been widely used in studies such as mineralization prediction, planetary evolution, global climate change tracing, pollution source deciphering and metabolism tracer and even life health research fields.Rivers play an important role in the global biogeochemical cycle of elements. In the complex “Critical Zone” of the Earth's surface, the products of rock or soil weathering will eventually be brought into the river and ultimately transported to the ocean. as a result, rivers transport nearly 90% of the msterials from land to the ocean. On the other side, rivers are closely related to human activities. In areas where population and industrial and agricultural activities are concentrated, rivers not only provide water resource, but also act as carriers of wastes emitted by human activities, and are therefore highly susceptible to pollution. The previous studies of zinc isotope in rivers mainly exists in Europe and America, and mainly focus on rivers in pristine areas or small human-impacted rivers, many mechanisms and/or processes controlling the geochemistry of Zn and its isotopes in river system remain still unclear, especially for the large anthropogenically-impacted rivers such as the Pearl River in China. The Pearl River is the second largest river in the annual runoff in China, behind the Yangtze River. The Pearl River Basin has a special geographical position and economic statue in China. However, up to now, little research has been performed on Zn in the Pearl River, and the sources, transportation, transformation mechanisms and the potential environmental risks of this heavy metal zinc in the whole basin still remain unclear.In the study, samples were collected from both main tributaries and mainstream of the Pearl River Basin during the flood period (May 2015) and the dry season (December 2015), and the water and suspended particulate matters (SPM) were carefully analyzed. In order to obtain a global spectrum of zinc isotope compositions, the potential sources, such as the typical bedrocks, soils, groundwaters, domestic pollution, industrial wastewaters and mine wastewaters were also systematically collected and analysized. By combining zinc isotope data with other water geochemical parameters, the concentration of major and trace elements, and the organic matter content (carbon, nitrogen, and phosphorus), we obtained the following main conclusions: (1) HCO3- and Ca2+ are the most abundant cations and cations in the waters of the Pearl River Basin respectively, indicating that the geochemistry of the dissolved load is mainly controlled by the weathering of carbonates. The chemical concentrations for typical metals of suspended particles in the Pearl River show a decrease in the order of Al3+>Fe3+>K+>Ca2+>Mg2+>Ti4+>Na+>Mn2+. Among them, the solubale elements Mg2+, Ca2+ and Na+ show depletion relative to the upper crust, with concentrations lower than the world average. The concentration of other elements is close to the average values of the world’s rivers, implying a limited impact of human activities;(2) The concentration of dissolved zinc in the Pearl River ranges from 0.1 to 9.7μg/L, with an average concentration of 1.2μg/L, slightly higher than the mean concentration of 0.6μg/L in the global rivers; the concentration of zinc in the suspended particles show a range of 91.3-908.6μg/g, with an average of 279.2μg/g, which is close to the average value in the world's rivers, and is relatively enriched compared to the UCC;(3) The δ66Zn values of dissolved zinc in the Pearl River is from -0.26‰ to -1.19‰, relatively higher than the δ66Zn value in other rivers worldwide; the variation of δ66Zn in suspended particles is 0.19-1.33‰, with an average of 0.77‰;(4) Internal processes in the river (biosorption and interaction between particle and water) are likely not the main cause of zinc isotope changes in the Pearl River;(5) The dissolved zinc in the Pearl River mainly comes from the mixing of different sources, among which the weathering of carbonate dominates, even much more important than the contribution of human activities, the contribution of “Reservoir-Lake” water cannot be ignored. Based on the calculation from the end-member hybrid model, the contribution of natural source ranges from 30% to 80%, with an average of up to 60%. The contribution of anthropogenic sources has gradually increased towards the lower reaches of the Pearl River, with a contribution ratio of no more than 50% and an average of 25% for the whole basin. The average contribution of the “Reservoir-Lake” water is about 15%; (6) The zinc in the suspended particles mainly derives from the mixing of different sources as well, with natural allumino-silicates and carbonate as dominant sources, and the contribution of anthropogenic sources is minor, which is gradually increasing downstream. The average contributing proportions for the three main sources are about 40%, 40% and 20%, respectively. The sample M10 has the the largest contribution of natural source, close to 90%;(7) Our study demonstrates clearly the “Reservoir effect” on zinc and its isotope systematics in the Pearl River. In fact, when the materials derived from the basin (carbonate) weathering enters the river, the hydrodynamic changes introduced by dams or riverine lakes wouls lead to long-term interaction of dissolved load with particulate matters, favoring for example the adsorption of dissolved zinc with heavy isotopes, resulting in low Zn concentration and higher δ66Zn in water, while higher δ66Zn and high calcium concentration in suspended particles (up to 0.30‰ than the carbonate); (8) Interestingly, the δ66Zn value of dissolved load in the Pearl River is much higher than all values reported for other rivers in the world. It is speculated that the carbonate bedrock is widely distributed in the basin, and the unique subtropical climate conditions are conducive to the intense weathering, which release a huge amount of dissolved Zn with relatively high δ66Zn values, being a main contributor of dissolved Zn. The input of this dissolved Zn with high δ66Zn from the Pearl River would be an alternative explanation for the high δ66Zn in the ocean.