应用稳定同位素技术研究海水脱硫燃煤电厂所排放汞的归宿和影响

Since mercury (Hg) exists in raw coal, the waste seawater discharged from coal-fired power plant equipped with seawater de-sulphurization system contains large amount of Hg. Our previous study shows that part of Hg in seawater may transform to be elemental Hg and transport into air, while part of Hg exists as dissolved Hg and particulate Hg and transfers onto sediment. The complicate environmental conditions in sea area, such as natural events of water current, wind, biological processes and human activities of navigation and dredge, affect fates of the discharged Hg. The traditional method of concentration comparison cannot meet the requirement of further study for Hg pollution, but stable isotope technique opens a way out. The proposed study is going to establish various analytical techniques for Hg isotopes in different sample matrix, and analyze the samples collected from air, seawater and sediment in the study sea area near a typical coal-fired power plant equipped with seawater de-sulphurization system in Xiamen, for Hg species and stable isotopes. The isotope fractions of raw coal, waste de-sulphurized seawater, seawater, air (as particulate) and sediment samples will be compared; the sources and fates of Hg will be identified; effecting range, regularity of the pollution and its effecting factors will be investigated; and finally, a quantification protocol including analysis methods and mathematic model will be established. The study results can provide the measures to other similar plants for assessing and predicting Hg pollution in sea area.

由于煤里含汞，配备海水脱硫工艺的燃煤电厂排放的脱硫海水中含有大量汞。本课题组前期研究表明：脱硫海水中汞会发生形态变化，部份成单质汞挥发进入大气；部份以溶解态/悬浮颗粒态存在，向沉积物迁移。由于海区环境复杂，海流、海风、生物等自然活动和船舶行使、航道疏浚等人为活动，都影响着海水脱硫燃煤电厂排放的汞的归宿。常规总汞分布比较法已不能满足汞污染研究的需要，稳定同位素技术将大显身手。本课题拟在建立各介质中汞稳定同位素分析方法的基础上，利用厦门海域燃煤电厂海水脱硫项目提供的现场采集大气、海水和沉积物样品，分析其中的汞形态及其稳定同位素，比较煤样、脱硫海水、海区海水、大气颗粒态沉降和沉积物样品中汞的同位素组成，识别海区汞的来源和归宿，探讨海水脱硫燃煤电厂排放汞的影响范围、动态规律及其影响因素，建立汞对海域环境影响的定量分析技术和数值模型。研究成果可推广用于其他海区同类海水脱硫项目的海域汞污染评估和预测。

利用稳定同位素技术，以一座配备典型海水脱硫系统的燃煤电厂及其周边为研究区域，探索海水脱硫和排放过程中汞的迁移转化，及煤中汞在燃烧、排放及扩散过程中的汞同位素分馏，研究脱硫海水中汞对所排放海域环境的影响，识别海区汞的来源，评估汞排放对周边大气环境的影响。研究成果可为评价海水脱硫工艺给近岸海域带来的汞污染提供科学数据，为同类燃煤电厂的汞污染及其控制提供评估和预测技术。主要成果有：.1..建立并完善了各类环境样品（海水、大气和固态样品）中各形态汞的采集富集与预处理方法，着重研发了吹扫-捕集系统及热解-吸收方法。优化了汞同位素分析系统。在精度、准确度不低于原有系统的基础上，改进型汞同位素分析系统的汞信号强度是原有的两倍。.2..研究了脱硫海水及其排放海域中各形态汞（颗粒态、溶解态、总汞）的浓度、同位素质量分馏（MDF）和非质量分馏（MIF）特征。发现脱硫海水及其悬浮颗粒物易富集重的汞同位素。脱硫海水在曝气处理中，颗粒态Hg的δ202Hg未改变，但溶解态和总汞的δ202Hg有所下降。脱硫海水排海后，颗粒态和总汞的δ202Hg随排污口距离的增加而变负，逐渐趋近当地的本底值。估算了脱硫海水中汞对排放海域的影响范围。利用海水汞同位素数据、二元混合模型对海域中汞的来源进行了解析。.3..研究了燃煤、烟气净化及海水脱硫各工艺中汞的浓度分布及汞同位素特征。煤相关样品具有由磁同位素效应引起的汞同位素MIF。煤中Hg2+的还原、烟气中Hg0的氧化、烟气Hg2+转移至飞灰形成颗粒态Hg以及曝气过程脱硫海水中汞赋存形态变化的产物，均富集轻质量数的汞同位素。发现海水脱硫时海水富集烟气中的重质量数汞同位素，烟气富集轻质量数汞同位素。.4..探讨了单质汞由液相向气相扩散迁移过程的同位素分馏机制。溶解气态汞具有比气态单质汞偏正的δ202Hg值及偏负的Δ199Hg。单质汞由表层海水向大气扩散时，发生核体积效应导致的汞同位素MIF。确定曝气有利于单质汞由液相向气相扩散，尤其促进199Hg、200Hg与201Hg向大气的扩散。.5..考察了电厂周边大气颗粒物及湿沉降的汞同位素特征，发现大气颗粒物及湿沉降均有偏负的MDF及略微偏正的偶数汞同位素MIF，大气颗粒态汞与雨水颗粒态汞具有偏负的奇数汞同位素MIF，湿沉降与雨水溶解态汞具有偏正的奇数汞同位素MIF。结合气象数据对大气颗粒物及湿沉降中汞的来源进行了分析。