【作者】 赵健；

【导师】 郑祥民；

【作者基本信息】 华东师范大学 ， 第四纪地质学， 2011， 博士

【摘要】 河口滨岸潮滩是海陆相互作用的重要地带,是全球生产力最高的生态系统之一。进入河口水环境中的Hg,会通过各种迁移转化过程在水体不同介质中重新分配。沉积物是Hg发生累积和富集的主要汇库,累积于沉积物中的Hg会通过甲基化过程生成甲基汞(MeHg),具有一定的生态风险,也可以通过一系列的物理、化学和生物过程释放出来,造成的“二次污染”,对人类健康带来潜在危害。因此,有关河口潮滩中Hg生物地球化学行为的研究一直是环境科学研究的热点问题之一。本文在国家自然科学基金“饮用水源地底泥重金属再悬浮释放与水源地水质安全研究”(批准号：40701164),和上海市优秀学科带头人项目“长江口滨岸湿地汞的地球化学循环及其生态危害”(批准号：07XD14010)等项目的支持下,主要选取了长江口滨岸潮滩地区,研究了不同环境介质中Hg的含量水平、时空分布、赋存形态、生物可利用性及其生态风险,探讨了不同盐沼植被芦苇、海三棱藨草和互花米草对沉积物中Hg累积的影响及其机制,分析了短期厌氧条件下沉积物中Hg的动力学过程和机制,揭示了不同环境条件下沉积物中Hg的再悬浮迁移转化过程与机制及其水环境效应。获得的主要研究成果如下：(1)长江口近岸水体中溶解态Hg(HgD)的含量在35-421 ng/L之间变化,有53.3%的水样中HgD的含量超过了我国地表水质标准(GB3838-2002)中的Ⅲ类水限值,含量高值主要出现在浒浦-白茆岸段和罗泾-顾路岸段。沉积物中总汞(THg)含量在0.058～0.541μg/g之间变化,位于EC-TEL和EC-PEL之间,对当地水生生物偶尔会有负面效应,含量高值主要出现在浒浦-浏河、罗泾-顾路和芦潮港岸段。柱样沉积物中Hg的含量高值出现在5 cm深度左右,孔隙水中HgD的含量高值出现在表层0.5 cm深度。粘粒和有机质含量是影响沉积物中Hg空间分布的主要因子。沉积物中甲基汞(MeHg)含量在0.05～0.37 ng/g之间变化,在THg中仅占到0.012～0.196%,与THg含量之间不存在相关性,与有机质含量呈正相关(r=0.488),与沉积物平均粒径呈负相关(r=-0.579)。(2)元素Hg (Hg-e)和硫化汞(Hg-s)是长江口潮滩沉积物中Hg的主要赋存形态,分别占到THg含量的9.0%-50.3%和37.7%-85.3%。其次是有机螯合态Hg(Hg-o)含量,水溶态(Hg-w) Hg和“人类胃酸”酸溶态(Hg-h) Hg含量最低。用Hg-w、Hg-h和Hg-o三种赋存形态之和表征Hg的生物可利用性,浮桥低潮滩沉积物中Hg的生物可利用性最高,其次是浏河和浦东机场低潮滩,与Hg的总量之间并不存在相关性。地累积指数评价结果显示,利用Hg的形态含量计算的∑Igeo值更能反映沉积物的实际污染水平及其对水生生物的危害大小,沉积物没有受到污染。根据THg的Eri值与Hg赋存形态的∑Eri值评价的沉积物潜在生态风险等级一致,但由生物可利用态Hg带来的生态风险并不依赖于沉积物中THg含量的变化,长江口潮滩沉积物都属于低生态风险等级。(3)芦苇带、海三棱藨草带与互花米草带表层沉积物中Hg的含量范围分别为0.009～0.365μg/g、0.020～0.771 gg/g和0.078～0.186μg/g,均明显高于非植被带(p<0.05)。植被的促淤作用可导致沉积物中Hg含量的增加。在沉积物柱样中,三种盐沼植被带在不同深度形成峰值含量。芦苇带沉积物中Hg的含量受根际氧化条件和植被根系周期性生长和死亡影响。互花米草和海三棱藨草带沉积物中Hg的含量主要受植被促淤固沙能力的影响,其次才受到植被根系周期性生理活动的影响。主成分分析和Pearson相关分析发现,在盐沼植被根系参与下,沉积物中的有机质含量、<16μm的细颗粒含量、以及沉积物温度、含水率和电导率等理化指标共同作用,一起影响着互花米草带和海三棱藨草带沉积物中Hg的含量和分布。(4)芦苇与互花米草根系中Hg的含量远高于茎叶,Hg的茎叶：根系比值分别为0.32和0.55。海三棱蔗草茎叶中Hg的含量明显高于根系,茎叶：根系比值达到了2.88。芦苇根系吸收的Hg要明显高于互花米草和海三棱藨草,而海三棱藨草茎叶中累积的Hg最高。三种盐沼植被地上与地下组织中Hg的富集系数都在0.55以下。在崇明东滩盐沼带中,三种盐沼植物根系中Hg的储量在0.02～0.11 mg Hg/m2之间,其中芦苇根系中Hg的储量最大,平均值为0.07 mg Hg/m2,其次是互花米草,海三棱藨草最低。三种盐沼植物茎叶中Hg的储量在0.01～0.05 mg Hg/m2之间,海三棱藨草茎叶中Hg的储量最大,平均值为0.03 mg Hg/m2,其次是互花米草,芦苇茎叶中Hg的储量最低。(5)崇明东滩夏季盐沼带沉积物柱样中酸同时可提取态Hg (SEM-Hg)和酸可挥发硫(AVS)的平均含量分别在0.66～1.03μmol/kg和0.848～4.381 mmol/kg之间变化,3个植被带之间差异均不显著(p>0.05)。柱样中SEM-Hg含量与AVS含量之间不存在相关性。3个植被带沉积物中SEM/AVS比值在ND～0.066之间,Hg的生物可利用性较低,与利用MeHg的评价结果一致,在柱样中均在2～4 cm深度出现最高值,说明表层沉积物中Hg的生物可利用性相对较高。从空间分布来看,崇明东滩表层沉积物中MeHg在THg中的比例较高,生物可利用性相对较强,沿长江口南岸而下直到浦东机场,Hg的生物可利用性逐渐增强。(6)在短期厌氧动力学试验中,沉积物中SEM-Hg的含量占到了THg含量的20.1%～51.9%,在淹水过程中,沉积物中部分Hg被释放出来,但进入孔隙水的HgD的通量值较小,仅有-0.004～0.011μmol/kg·d。在淹水前10天期间,沉积物固相中SEM-Hg的降幅最大,但此时孔隙水中HgD的含量增加并不多,硫酸盐还原释放出的S2-可能与沉积物中释放出来的Hg2+化合形成HgS沉淀固定在沉积物中。沉积物中乙酸钠提取态Hg的含量仅占到THg含量的0.8%～18.5%,在试验中含量均明显降低。在淹水10天以后,沉积物中的Fe、Mn硫化物和有机质也可以吸附部分Hg。淹水后期添加有机质的沉积物孔隙水中HgD的含量比密闭无有机质添加试验低了43%,有机质降解使TOC逐渐增多,对Hg的吸附量增加,降低了系统中Hg的毒性。(7)在动力再悬浮试验中,水体中表现为以颗粒态Hg(HgP)的吸附为主,水动力条件是控制水体中Hg分配行为的关键因素。在盐度和盐度×动力耦合再悬浮试验中,上覆水中Hg的log10Kd分别为2.8～3.8和2.7～4.5。在盐度较低的环境条件下,动力条件是影响上覆水中Hg行为的主要因子。当盐度在1‰以上时,上覆水中的HgP出现解吸行为,当动力条件达到250 r/min以上时,解吸作用增强,上覆水中HgD出现正通量,表现为向上覆水的释放。pH和pH×盐度耦合再悬浮试验上覆水中Hg的log10 Kd值分别为2.8～3.9和1.4～4.0。当水体盐度较低而pH值发生变化时,上覆水中有大量HgD被Al、Fe的氢氧化物胶体所吸附。在水体盐度较高的情况下,水体中以OH-离子与Hg2+的反应为主,并生成了HgO发生沉淀。(8)中、低潮滩上覆水中Hg的分配系数分别为2.2～4.5和1.8～4.3,中潮滩表层颗粒物被扰动起来以后,对上覆水中Hg的吸附作用要弱于低潮滩,这与中潮滩沉积物相对较粗的颗粒粒径以及较少的有机质含量有关。较高的盐度条件和较大的动力条件耦合在一起,可以促进低潮滩沉积物中的Hg的释放及其在水相中的滞留,上覆水中HgD的总通量与盐度值之间呈非常好的正相关关系(0.990),当盐度值>11.5%o时,上覆水中以HgD的正通量为主,Hg的生物可利用和毒性增强。中潮滩在再悬浮过程中,上覆水中主要以悬浮颗粒对Hg的吸附作用为主,上覆水中HgD的总通量与盐度值之间并不存在线性关系。本文的研究特色与创新之处包括以下几点：(1)揭示了不同盐沼植被对沉积物中Hg累积特征的差异性影响及其机制；(2)明确了短期厌氧条件下沉积物中Hg的动力学过程,弄清了Fe、Mn等金属硫化物和沉积物TOC是影响厌氧条件下Hg迁移转化的主要因子；(3)弄清了水体动力条件是影响Hg再悬浮行为的关键因子,当盐度在1‰以上时,较高的动力条件促进了上覆水中HgP的解吸,在水体盐度较高的情况下,pH值增加使水体中的OH-离子与Hg2+反应生成了HgO沉淀。沉积物自身的性质也是决定再悬浮过程中Hg迁移转化行为的重要因素。更多还原

【Abstract】 Estuarine intertidal flat is an important interaction area between land and sea, and is one of the most productive ecosystems in the world. After being transported into the estuarine water environment, Hg is re-allocated among different media by migration and transformation processes. Sediments are the main storage of Hg in estuary and coastal systems, which may be transformed into methylmercury, a highly bio-accumulative form for organisms and humans, can also be released to the overlying water through a series of physical, chemical and biological processes, causing "secondary pollution" of the water environment and potential hazard to human health. Therefore, the biogeochemical behavior of Hg in estuarine intertidal flat has been a hot topic of scientific research.With the support of the National Natural Science Foundation " Heavy metal release during sediment resuspension and the security of water quality in drinking water source " (No.40701164), and the Outstanding Academic Leader Project of Shanghai " Mercury geochemical cycle and its ecological hazard in coastal wetlands of the Yangtze Estuary"(No.07XD14010) and other projects, research work of this paper was carried out in the intertidal flat of the Yangtze estuary. The concentrations, spatial and temporal distribution characteristics, species, bioavailability and ecological risks of Hg in different environmental media were studied, and the effects of different salt marsh vegetation, such as Phragmites australis, Scirpus mariqueter and Spartina alterniflora, on the accumulation characteristics of Hg in sediments and its mechanisms were discussed. Then the dynamics of Hg in sediments under short-term anaerobic conditions and its mechanisms were simulated. Finally, the transport and transformation processes of Hg and its mechanisms during sediment resuspension under different environmental conditions and its effects on water environment were revealed. The main research results obtained are as follows:(1) The concentrations of dissolved Hg (HgD) in coastal water of the Yangtze Estuary varied between 35～421 ng/L,53.3% of water samples beyond the classⅢlimits of surface water quality standards in China (GB3838-2002). High Hg values were found in Xupu-Baimao coasts and Luojing-Gulu coasts. The concentrations of total Hg (THg) in sediments varied between 0.058～0.541μg/g, which lies in the limits of EC-TEL and EC-PEL, would occasionally take a negative effect on the local aquatic life. High Hg values were found in Xupu-Liuhe, Luojing-Gulu and Luchaogang coasts. In sediment cores, high levels of Hg were mainly distributed in about 5 cm depth in sediments and in pore water of the top 0.5 cm depth. Clay and organic matter contents were the main factors affecting the spatial distribution of Hg in sediments. The concentrations of methylmercury (MeHg) in sediments varied between 0.05～0.37 ng/g, only accounting to 0.012～0.196% in THg, and has no correlation with THg concentrations, but positively correlated with organic matter content (r=0.488) and negatively correlated with the average sediment grain size (r=-0.579).(2) Elemental Hg (Hg-e) and mercuric sulfide (Hg-s) were the main species of Hg in intertidal sediments of the Yangtze estuary, accounting for 9.0%～50.3% and 37.7%～85.3% of the THg concentrations respectively, followed by organo-chelated Hg (Hg-o), and water soluble Hg (Hg-w) and "human stomach acid" soluble (Hg-h) was the lowest. With the total concentration of Hg-w, Hg-h and Hg-o to represent the bioavailability of Hg, the highest bioavailability of Hg in low intertidal sediments was found in Fuqiao coast, followed by Liuhe and Pudong Airport coasts. Concentrations of the above bioavailable Hg were not correlated with THg. Geoacumulation Index evaluation showed that the∑Igeo values calculated based on the species of Hg better reflected the actual pollution levels of sediments and its hazard to aquatic organisms, and the sediments were not contaminated by Hg. According to Eri values of THg and∑Eri values of Hg species, the sediment potential ecological risk levels were consistent, but the ecological risks of bioavailable Hg does not depend on the THg levels in sediments. The intertidal sediments of the Yangtze estuary were at low ecological risk levels evaluated by bioavailable Hg.(3) The concentrations of Hg in surface sediments of the P. australis, S. mariqueter and S. alterniflora zone varied between 0.009～0.365μg/g,0.020～0.771μg/g and 0.078～0.186μg/g, respectively, all significantly higher than the values in the non-vegetation zone (p<0.05). Vegetation can lead to sediment deposition and Hg accumulation in sediments. In sediment cores, peak values occurred at different depths in three kinds of salt marsh vegetation zones. The Hg concentrations in sediments of the P. australis zone were influenced by rhizosphere oxidation conditions and by periodic growth and mortality of plant roots. While the Hg concentrations in sediments of the S. mariqueter and S. alterniflora zones were mainly influenced by the sediment retard ability, followed by the vegetation root periodic physiological activities. Principal Component Analysis and Pearson Correlation Analysis results showed that organic matter content,<16μm fine particle content, sediment temperature, water content, electrical conductivity and other physical and chemical indicators together influenced Hg concentrations and distribution in the sediments of S. mariqueter and S. alterniflora zones. (4) The concentrations of Hg in roots of P. australis and S. alterniflora were much higher than in stems and leaves, with the leaf:root ratios of 0.32 and 0.55 respectively. While the concentrations of Hg in stems and leaves of S. mariqueter were significantly higher than in roots, with the leaf:root ratio reaching 2.88. The highest concentrations of Hg in roots were found in P. australis and in stems and leaves were found in S. mariqueter. Enrichment factors of Hg in the ground and underground tissues of the three kinds of vegetation were all below 0.55. At Chongming salt marsh zone, the reserves of Hg in three plant roots varied between 0.02～0.11 mg Hg/m2, of which the largest reserves of Hg were in P. australis (0.07 mg Hg/m2), followed by S. alterniflora. the reserves of Hg in three plant stems and leaves varied between 0.01～0.05 mg Hg/m2, of which the largest reserves of Hg were in S. mariqueter (0.03 mg Hg/m2), followed by S. alterniflora.(5) The average concentrations of simultaneously ectracted metals (SEM-Hg) and acid volatile sulfide (AVS) varied between 0.66～1.03μmol/kg and 0.848～4.381 mmol/kg in sediment cores of Chongming Dongtan in summer, respectively, both of which had no significant difference among the three vegetation zones (p>0.05). In sediment cores, SEM-Hg concentrations were not correlated with AVS contents. The SEM/AVS ratios in three vegetated sediments varied between ND～0.066, suggesting low bioavailability of Hg, which was consistent with the evaluation results of %MeHg. The highest values of SEM/AVS ratios were in 2～4 cm depths of the sediment cores, showing that the relatively high bioavailability of Hg occurred in surface sediments. From the spatial distribution, the percentages of MeHg in THg in surface sediments were relatively higher in Chongming Dongtan and increased along the south bank of the Yangtze estuary until the Pudong Airport coast, suggesting the increasing bioavailability of Hg in sediments of Chongming Dongtan and Pudong Airport coasts.(6) In the short-term anaerobic dynamics tests, the concentrations of SEM-Hg in sediments accounted for 20.1%～51.9% of the THg concentrations. During the flooding process, part of Hg in sediments was released, but the fluxes of HgD into the pore water were only -0.004～0.011μmol/kg·d. During the first 10 days of the flooding, the SEM-Hg concentrations decreased significantly, but HgD concentrations in the pore water only increase little. S2- released from the sulfate reduction may be formed HgS compounds with the released Hg2+ and deposited in sediments. The concentrations of sodium acetate-extractable Hg in sediments only accounted for 0.8%～18.5% of THg, and was significantly reduced during the flooding. After 10 days of the flooding, Fe, Mn sulfides and organic matter can also absorb part of the Hg in sediments. In post-flooding, the concentrations of HgD in pore water of sediment added organic matter were lower by 43% to the sediment with no organic matter under air-closed conditions. The degradation of organic matter resulted in the increase of TOC, and further promoted the adsorption of Hg in sediments, which would reduce the toxicity of Hg in the system.(7) In the hydrodynamic resuspension test, the adsorption of Hg to suspended particles was the main process. Hydrodynamic condition was the key factor to control the partition behavior of Hg in water. In salinity and salinityxdynamic coupling resuspension tests, the partition coefficients of Hg (log10Kd) in the overlying water were 2.8-3.8 and 2.7～4.5, respectively. Under the lower salinity conditions, hydrodynamic was the main factor affecting the behavior of Hg in the overlying water. When salinity more than 1‰, desorption of particulate Hg (HgD) occurred in the overlying water. When the dynamic conditions reached to 250 r/min or more, desorption enhanced, and positive fluxes of HgD to the overlying water were found, suggesting that Hg was released or desorbed from the sediments. In pH and pH×salinity coupling resuspension tests, the log10Kd values of Hg in the overlying water were 2.8-3.9 and 1.4～4.0, respectively. When the water pH values varied under lower salinity conditions, a large number of HgD combined with Al, Fe hydroxide colloids by adsorption in the overlying water. In the case of high salinity water, the OH- ion was compounded with Hg2+, and generated the HgO precipitation.(8) The log10Kd values of Hg in overlying water of the middle and low intertidal sediments were 2.2～4.5 and 1.8～4.3, respectively. Particles resuspended from the middle intertidal sediments had relatively weaker adsorption of Hg in overlying water than from low intertidal sediments, which was related to the relatively coarse particle size and less organic matter content in the middle intertidal sediments. Under conditions of high salinity coupled with large power, the release of Hg from low intertidal sediments increased and Hg retention in the aqueous phase extended. Total fluxes of HgD to overlying water positively correlated with salinity (0.990). When salinity values more than 11.5%o, positive fluxes of Hg to the overlying water occurred, suggesting the increased bioavailability and toxicity of Hg. During the resuspension of middle intertidal sediments, adsorption of Hg on suspended particles was the main process, and no linear relationship was found between total fluxes of HgD and salinity.Innovations of this PhD dissertation are mainly:(1) The different effects of three salt marsh vegetations on Hg accumulation characteristics in sediments and its mechanisms were revealed.(2) The dynamics of Hg in sediments under short-term anaerobic conditions were determined, and Fe, Mn sulfides and TOC were clarified to be the main factors that affected Hg transport and transformation in sediments under anaerobic conditions.(3) Hydrodynamic condition was revealed to be the key factor that affected the resuspension behavior of Hg in water. When salinity more than 1‰, high dynamic conditions promoted desorption of Hgp in the overlying water. In the case of high salinity water, increased pH values resulted in the conformation of HgO precipitation between OH" ion and Hg2+. The character of sediments was also an important factor determining the transport and transformation behavior of Hg during resuspension process.更多还原