【作者】 孙荣国；

【导师】 王定勇；

【作者基本信息】 西南大学 ， 农业环境保护， 2014， 博士

【摘要】 自上世纪50-70年代日本发生震惊世界的环境公害事件-“水俣病”以来,环境汞污染问题受到世界各国的高度重视。自此,汞的环境地球化学行为研究成为环境研究领域的热点与重点方向。研究证实新建水库具有“汞的活化效应”,是典型的“汞敏感生态系统”。其淹没的陆地(土壤和植被)向水体释放汞、底部厌氧环境生成的甲基汞(MMHg)是水库水体和鱼体内MMHg的主要来源。三峡水库是世界最大年调节型水库,水位涨落使库区周围形成垂直高度为30m、面积约350km2的消落带。约在每年的2月至6月,水库处于水位下降期,裸露的消落区将会被用作农耕地或长有大量草本植被；约从9月份起水位开始上涨,消落区被淹没,被淹没的土壤和植被会迅速向水体释放无机汞(IHg)和MMHg,且底部处于厌氧环境,有利于MMHg生成并向上覆水体释放。因此,这种周期性消落使得三峡水库年年具有新建水库的特征,可能存在“汞的活化效应”,有可能造成上覆水体和鱼体内MMHg浓度升高,属于典型的“汞敏感生态系统”。然而,目前三峡水库水体MMHg的生物地球化学行为还少见报道。表层水中的MMHg发生光化学降解反应在水体汞的生物地球化学循环过程中占有重要地位,是水环境中MMHg去甲基化反应的主要路径,同时也是水环境中MMHg浓度维持在较低水平的重要原因。在比较不同水域MMHg光降解的速率与对区域汞循环影响时发现,不同水环境间MMHg光降解速率、对区域汞循环的影响具有较大的差异。目前,实地水环境中的MMHg光降解研究主要集中在欧美水域,而在三峡水库这种典型的水环境系统中,其水文特征与已开展MMHg光降解研究水域的有较大的差异,且MMHg的光化学行为特征仍不明了。因此,三峡水库作为典型的水生生态系统,很有必要探究MMHg在其水域的光化学行为特征及机制。研究发现,水环境的MMHg光降解受多种因素的影响,如光照和波长是影响MMHg光降解的主要影响因素；·OH、1O2等自由基团参与或促进MMHg光降解反应过程;悬浮颗粒物、DOM、盐度等会降低MMHg的光降解反应速率；也有研究称MMHg光降解不受水体化学组成等因素的影响,仅受光照条件的影响。虽然目前关于实际水环境中的MMHg光降解行为已开展了相关的科学研究,但对于自然水体MMHg的光降解机制、路径等仍不明了,特别是氯盐、硝酸盐在MMHg光降解过程中的作用机制、对降解产物光化学行为的影响等仍不明确,且部分研究结果还存在有较大的争议。基于MMHg光降解在水体汞迁移、转化及循环过程中的重要地位,探究新建水库水体汞生物地球化学行为的必要性及重要性,以及目前三峡水库水体MMHg的光化学行为特征、影响因素、降解机制仍不明了,本研究以三峡水库为研究对象,选择典型消落区域的涪陵、忠县和奉节作为研究地点,探究三峡水库水体MMHg光降解的速率、通量、影响因素及降解机制,考察了Cl-、N03在MMHg光降解过程中的作用机制以及对降解产物光化学行为的影响。结果发现：1)三峡水库水体甲基汞光降解特征光照波长与强度对三峡库区水体MMHg光降解速率具有明显的影响作用。在水体表层,由UV-B引发的MMHg光降解速率最大,其次为UV-A引发的,可见光(PAR)引发的光降解的速率较小。UV-B、UV-A和PAR对对表层水体MMHg光降解的总速率的贡献分别为17.14%-21.30%、48.57%-61.54%和17.16%-34.29%。在水体表层,由于光照强度的作用,各波段引发的MMHg光降解速率具有明显的季节差异性,其中夏季的降解速率最高,其次为春季,冬季的光降解速率最低。在每个研究地点,各波段光照引发的MMHg光降解速率均随水深度的增加而呈现逐渐减小的趋势,其中UVB波段引发的光降解速率下降趋势最大,在水深10cm处基本降为0;其次为UV-A,由该波段引发的降解速率约在水深40cm处降为0,PAR波段光波穿透能力最强,可引发水深2.5m处的MMHg发生光降解反应。MMHg光降解通量存在较大的季节性差异(p<0.01)。夏季降解通量最大,为7.46-18.15ngm-2d-1,其次为春季(3.33-8.01ng m-2d-1),再次为秋季(1.02-2.71ng m-2d-1),冬季最小,为0.06-0.15ngm-2d-1。忠县段水域MMHg年降解通量最大,为2.92μg m-2y-1，其次为奉节段水域(2.42μgm-2y-1),最小的为涪陵段水域(1.13μgm-2y-1)。每一波段对整个水体MMHg降解通量的贡献有很大的差异,UV-B的贡献为7.47%-18.12%,UV-A的贡献为23.22%-50.58%,PAR的贡献为32.31%-69.13%。因此,对整个水体而言,UV-A和PAR对三峡水库水体MMHg光降解起关键作用。悬浮颗粒物、DOM、C1和NO3-对库区水体MMHg光降解具有重要影响,而Fe(Ⅲ).CU(Ⅱ)、 Mn(II)等重金属离子对对库区水体MMHg光降解的影响作用甚微。逐步回归分析结果显示,光照强度、悬浮颗粒物、DOM、 Cl和NO3-是影响三峡水库表层水体MMHg光降解的因素。光照强度对回归结果贡献最大,达到91.6%,其余变量贡献8.4%。通径分析结果表明,太是光照强度与NO3-具有很高的直接正向作用；SPM、DOM和C1-表现为直接负效应,。NO3具有最大的间接作用(0.65),其次为SPM (0.40)、DOM (0.34)。由光照强度本身引起了～67%的MMHg发生光降解反应。2)甲基汞光降解产物与反应历程的识别MMHg光降解反应呈一级动力学反应,主要降解产物为Hg0。光照波长和强度对MMHg光降解速率和产物有重要影响。在PAR、UV-A、UV-B、UV-C和PAR+UV-A+UV-B条件下在反应4h后,MMHg光降解速率分别为0.061h-1、0.562h-1、0.961h-1、1.221h-1和1.346h-1Hg0的释放速率分别是0.008ng min-1、0.222ng min-1、0.273ng min-1’0.220ng min-1和0.392ngmin-1。在反应器暴露于3、2、1UVA lamp条件下,MMHg光降解伪一级动力学速率常数分别为0.562、0.509、0.403h-1,Hg0的平均释放通量分别为0.222、0.207、0.166ngmin-1。黑暗条件下未发现MMHg光降解反应,也末检测到Hg0的生成。3)氯离子对甲基汞光降解、降解产物光化学行为的影响机制Cl-的存在对MMHg光降解反应具有明显的抑制作用。随Cl-浓度的增加(0-20mg L-1),MMHg光降解速率逐步减少。在浓度为0、0.02、0.2、2、20和200mg L-1时,MMHg的光降解速率分别为：1.35、1.00、0.92、0.45、0.34和0.37h-1。Cl-的存在情况下光照强度与波长对MMHg光降解有重要影响。在UV-C、UV-B、UV-A、自然光(NL)、PAR和黑暗条件下,MMHg光降解速率分别为0.96h-1、0.76h-1、0.29h-1、0.37h-1、0.04h-1和0；在UV-B紫外灯照射下,MMHg光降解速率随其强度的提高由0.14h-1增加至0.76h-1。Cl-不仅对MMHg光降解反应本身有抑制作用,对其降解产物的光化学行为也有重要影响。在NL条件下,随Cl-浓度的增加(0-20mgL-1),Hg0的释放通量比对照处理(CK)减少25%-75%。在不同波长和光强度处理下,MMHg光降解产物Hg0的释放通量也具有较大的差异。由PAR、NL、UV-A、UV-B和UV-C引发的MMHg光降解产生的Hg0的释放通量分别占加入总MMHg量的比例是(100ng)：1%、23%、2%、12%和4%;随UV-B强度的增加,Hg0的释放通量分别占加入总MMHg量的比例是4%、8%和13%。MMHg的形态是影响MMHg光降解反应的主要因素。Cl-的存在不仅影响MMHg的形态,而且影响MMHg光降解产物的形态,故Cl-表现为对MMHg光降解反应过程、降解产物的光化学行为均产生影响。在Cl-存在的情况下,Hg(Ⅱ)的光还原反应过程受到显著的影响。Hg(Ⅱ)的形态、光照条件是Cl-存在时影响Hg(Ⅱ)的光还原反应的重要因素。Cl-浓度与pH值是影响溶液中IHg形态的重要因子,因而两者的交互作用对Hg(Ⅱ)的光还原反应产生影响。除Cl-与Hg2+间的强结合力降低了溶液中活性的Hg2+的浓度以致反应速率下降外,Cl-的强氧化能力也是影响溶液中IHg氧化还原反应的重要因素,而且不同光照波段对这一过程的影响机制也有较大差异。4)硝酸根对甲基汞光降解、汞循环的影响在有紫外波段照射,添加NO3后MMHg光降解反应速率明显加快,Hg0的生成量极低,随NO3-浓度的升高,MMHg光降解的速率逐步增加；在PAR条件下,NO3-和BA均未对MMHg光降解速率、产物表现出影响作用。表明·OH具有促进MMHg光降解的作用,且会抑制MMHg降解产物Hg0的生成。NO3-对水中MMHg光降解及汞循环有重要影响。上覆水、底泥和间隙水中MMHg和溶解态汞(DHg)的浓度水平在白大均出现下降的趋势,而在夜间出现浓度上升的趋势,底泥中与间隙水中MMHg的浓度与上覆水中溶解氧的浓度间存在较强的负相关关系；活性汞(RHg)在间隙水和上覆水中的浓度呈现有光照时增高、黑夜时下降的变化趋势；在有NO3的情况下加剧了此变化规律。MMHg发生光降解反应是上覆水中MMHg浓度下降的主要原因,且NO3-的存在能促进上覆水中MMHg光降解速率。NO3光降解产生的·OH是促进MMHg光降解速率提高的主要基团。MMHg在白天和黑夜之间的扩散通量有显著的差异,夜间MMHg的扩散通量为6.04-6.92ng m-2d-1,是白天通量的1.6-2.4倍；各处理RHg的昼夜扩散速率没有显著的差异,扩散速率变化范围是3.25-3.43ngm-2d-1; DHg夜间的扩散速率为7.79-8.36ngm-2d-1,比白天的扩散速率大0.37-0.47倍。夜间MMHg是通过底泥-上覆水界面迁移的主要形态,白天RHg是主要形态,即底泥在白天期间是上覆水RHg的源,夜间是上覆水MMHg的源。本研究确定了光照强度、悬浮颗粒物、DOM、Cl和N03-是影响三峡水库表层水体MMHg光降解的因素,定量评估了各影响因素对MMHg光降解的作用方式；给出了MMHg光降解的反应历程,即首先降解生成无机汞,然后进一步发生还原反应生成Hg0；明确了Cl-在MMHg光降解及Hg(Ⅱ)光还原过程中的作用机制,发现MMHg形态是影响其降解的主要因素,Hg(Ⅱ)的形态是控制其还原反应速率的主要原因;确定了NO3-存在情况下水体汞的循环特征。研究结果对理解三峡水库运行以后汞的环境地球化学行为提供重要数据基础,也可为理解自然水体MMHg光降解机制、降解特征提供科学依据。更多还原

【Abstract】 Since methylmercury (MMHg) was found to be responsible for the Minamata disease in Japan in the middle of last century, mercury (Hg) pollution in the environment has been caused highly considered, then the research work of environmental geochemistry of Hg have became hotspots and priorities. New created reservoirs and flooding landscapes have an important environmental consequence of MMHg bioaccumulation because the decomposition of organic carbon in flooded soil and vegetation in reservoirs can improve the methylation rates of inorganic Hg to MMHg, which would result in the higher MMHg exposure of people depending on reservoir fisheries for food. Thus, young reservoirs are very sensitive to Hg, and hold a capacity of activating Hg. The recently completed Three Gorges Reservoir (TGR), the largest hydroelectric power plant in the world, floods a total area of630km2,350km2of which is a seasonally flooded water level fluctuating zone. Crop and/or herbaceous vegetation grow well here during during the dry period, from late February to early June. However, this zone is submerged during flooding periods, from mid August to early November. Organic matter, nitrate, and other chemicals from soil and vegetation transfer to the water column, resulting in significant changes to physical and chemical characteristics of the water column. Impoundment by the TGR dam has thus brought about many environmental concerns including eutrophication, and contamination by MMHg. So, TGR may be also very sensitive to Hg, and hold a capacity of activating Hg. However, we possess little mechanistic knowledge of biogeochemistry of MMHg in TGR.Photo-degradation (PD) is the dominant sink of MMHg in surface waters, resulting in low MMHg level in surface waters, and limiting bioaccumulation of Hg in aquatic organisms. Comparison of MMHg PD fluxes and its effects on Hg cycling for different ecosystems suggest that rates, fluxes, and influencing factors of MMHg PD in different water systems are varied significantly. At present, research works of MMHg PD in natural water systems were mainly carried out in western countries. The environmental conditions of TGR are significantly different from the ecosystems (e.g., Lake979, Arctic Alaskan Lake, and ELA) whose MMHg PD have been extensively studied. Intuitively, MMHg PD should be important for its cycling in TGR, and solar radiation and ambient factors induced by water level fluctuation should have important effects on MMHg PD processes. However, MMHg PD processes in TGR are not clearly understood, and the influencing factors for those processes are unknown.Previous studies have identified that many environmental factors, such as light condition, water component, etc., are involved in the PD process. MMHg PD can occur via a direct pathway by UV radiation and/or an indirect pathway mediated by·OH and1O2in surface water. PD processes could be inhibited by dissolved organic matter (DOM), increased salinity, and suspended particulate matter (SPM) through complexation of MMHg with DOM or Cl", or through influence of photo penetration. Although these studies have proposed some mechanisms of MMHg PD in surface waters, the entire suite of environmental variables affecting MMHg PD has not been fully elucidated, especially the effects of Cl-and NO3-on MMHg PD and PD products are still unclear.From the knowledge gaps outlined above, thus, the objectives of this study were to (1) identify PD rate constants, fluxes, spatial patterns, and influencing factors of MMHg in TGR,(2) investigate the mechanism of MMHg PD with the presence of Cl-and NO3-. The results showed that:1) Characteristics of MMHg PD in TGRLight intensity and wavelength ranges have significant effects on MMHg PD in TGR. In water surface, the highest PD rate constants were induced by UV-B radiation, followed by UV-A, and PAR. The contribution of PD rate constants were induced by UV-B, UV-A, and PAR were17.14%-21.30%,48.57%-61.54%, and17.16%-34.29%, respecitively. Rate constants of MMHg PD of each season varied significantly were due to dramatic variation of light intensities. The highest PD rates occurred in summer, followed by spring, autumn, and winter. All PD rate constants resulting from each wavelength range decreased rapidly with water depth. UV-B, UV-A, and PAR could induce MMHg PD in water column up to10,40, and250cm, respecitively.MMHg PD fluxes of each season varied significantly. The highest PD fluxes occurred in summer (7.46-18.15ng m-2d-1), followed by spring (3.33-8.01ng m-2d-1), autumn (1.02-2.71ng m-2d-1), and winter (0.06-0.15ng m-2d-1). MMHg PD fluxes were calculated as1.11μg m-2y-1for Fuling,2.82μg m-2y-1for Zhongxian,2.44μg m-2y-1for Fengjie. UV-B, UV-A, and PAR accounted for7.47%-18.12%,23.22%-50.58%, and32.31%-69.13%of MMHg PD in the entire water column, respectively, implying that both PAR and UV-A radiations were responsible for MMHg PD when integrated across the entire water column. SPM, DOM, Cl-, and NO3-play a key role in affecting MMHg PD in TGR, while some heavy metal ions, such as Fe(III), Cu(II), and Mn(II), have little effects on PD rate constants. Stepwise regression analysis showed that suspended particulate matter (SPM), DOM, DO, Cl-, and NO3-are involved in PD process. Light intensity was responsible for0.916of average MMHg PD rate constants in TGR. Path analysis indicated that light intensity and NO3-had highly positive direct effects (0.83and0.22), while SPM, DOM, and Cl-had negative direct effects (-0.13,-0.11, and-0.14) on PD rate constants. NO3-had greatest indirect effect (0.65) on PD rate constants, followed by SPM (0.40) and DOM (0.34). Solar radiation alone made a～67%direct contribution towards PD rate constants.2) MMHg PD products and reaction processesThe rate of MMHg PD is pseudo first-order with respect to MMHg concentration in the reactor, and Hg°is the end product. Light intensity and wavelength ranges have significant effects on MMHg PD processes. When the reactor exposed to PAR, UV-A, UV-B, UV-C, and PAR+UV-A+UV-B, MMHg PD rate constants were calculated to be0.061,0.562,0.961,1.221, and1.346h-1， respecitively, and the emission rates of Hg°were calculated to be0.008,0.222,0.273,0.220, and0.392ng min-1, respecitively. When the reactor exposed to3,2, and1UVA lamp(s), MMHg PD rate constants were calculated to be0.562,0.509, and0.403h-1, respecitively, and the emission rates of Hg°were calculated to be0.222,0.207, and0.166ng min-1, respecitively. While the experiments of dark, we did not observed MMHg PD and the end product of Hg°.3) Roles of Cl-in MMHg PD processesThe presence of Cl-can block MMHg PD processes. MMHg PD rate constants decreased with the increasing of Cl-concentrations. MMHg PD rate constants were calculated to be1.35,1.00,0.92,0.45,0.34, and0.37h-1when Cl-concentrations were0,0.02,0.2,2,20, and200mg L-1. Light intensity and wavelength ranges have significant effects on MMHg PD processes in the presence of Cl-. When the reactor exposed to UV-C, UV-B, UV-A, NL, PAR, and dark, MMHg PD rate constants were calculated to be0.96,0.76,0.29,0.37,0.04, and0h-1, respecitively. PD rate constants increased (0.14-0.76h-1) with UV-B radiation intensity.The presence of Cl-also has significant effects on the photochemical processes of the end products from MMHg PD. The emission fluxes of Hg°decreased25%-75%with the increasing of Cl-concentrations (0-20mg L-1) when the reactor exposed to NL. The emission fluxes of Hg°from MMHg PD varied significantly under different light intensity and wavelength ranges. The proportions of Hg°were1%,23%,2%,12%, and4%when the reactor exposed to PAR, NL, UV-A, UV-B, UV-C. Emission fluxes of Hg°increased (4%-13%) with UV-B radiation intensity.MMHg species is a key factor for controling PD rate constants. The presence of Cl-not only change the species of MMHg, but also affect the species of inorganic Hg generated from MMHg PD, thus, Cl could block MMHg PD and subsequently influence the end products from PD. The presence of Cl-can significantly affect photo-reduction rate constants of Hg(Ⅱ). Hg species and light conditions are important variables that involved in photochemical reactions of Hg(Ⅱ) with Cl-. The concentrations of Cl-and pH values have significant effects on Hg species and thus affect photo-reduction processes of Hg(Ⅱ). The decreased photo-reduction rate constants are not only caused by Cl-complexation, also the presence of Cl-improved photo-oxidation of Hg(0) is the key reason for the low photo-reduction rate constants of Hg(Ⅱ) with Cl-. Moreover, this effect is highly wavelength dependent.4) Effects of NO3-on MMHg PD and Hg cyclingUnder UV radiations, NO3-can significantly improve MMHg PD rate constants, and block the emission fluxes of Hg°. While under PAR radiations, NO3-did not show this effect. Those results demonstrate that·OH can elevate MMHg PD rate constants, and inhibit the emission fluxes of Hg°.The presence of NO3-has significant effects on MMHg PD proceses and Hg cycling in water systems. Both of concentrations of MMHg and THg in overlying water, sediment, and pore water decreased during daylight time, and increased during night. RHg concentraton increased during daylight time and decreased during dark time. The presence of NO3-increased those tendencies. Correlation anslysis shows that MMHg concentration in sediment and pore water have significant negative realtion to DO in overlying water. Photodegradation is the predominant sink of MMHg in overlying water. Photolysis of NO3-can generate·OH, which hold the capacity for impoving MMHg PD processes.Diffusive flux of MMHg during daylight time and dark time varied significantly. The diffusive flux of MMHg is6.04-6.92ng m-2d-1for night, which is1.6-2.4times greater than that for sunlight time. There were no differences of diffusive flux of RHg during daylight time and dark time, ranging from3.25ng m-2d-1to3.43ng m-2d-1. The diffusive flux of DHg is7.79-8.36ng m-2d-1for night, which is0.37-0.47times greater than that for sunlight time. Those suggest that MMHg is the predominant species that transfer across sediment-water surface during dark time, and RHg is the predominant species that transfer across sediment-water surface during sunlight time. Thus, sediment is source of MMHg to overlying water during dark time, and is the source of RHg to overlying water during sunlight time.This work found that light intensity, SPM, DOM, Cl-, and NO3-are the influencing factors involved in MMHg PD processes inTGR, estimated the contribution of those factors towards MMHg PD in TGR. Also, this work analysied the reacton processes of MMHg, identified the role of Cl-and NO3-in MMHg PD and Hg cycling. The results are of great importance for understanding Hg cycling characteristics in TGR after impounded. Also, the results are very important for underatanding the menchanism of MMHg PD in natural waters.更多还原