Mechanisms And Influencing Factors Of Methylmercury Photodegradation In The Water Body Of Three Gorges Reservoir, China

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.