| 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. |
PDF Full Text Request