Release of Mercury from Intertidal Sediment to Atmosphere in Summer and Winter

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1 Chin. Geogra. Sci (2) DOI: /s Release of Mercury from Intertidal Sediment to Atmosphere in Summer and Winter LIU Ruhai 1, WANG Yan 1, SHAN Changqing 2, Ling Min 1, SHAN Hongxian 1 (1. Ministry of Education Key Laboratory of Marine Environment and Ecology, Ocean University of China, Qingdao , China; 2. Department of Urban and Environment, Binzhou Universtiy, Binzhou , China) Abstract: The release of mercury from intertidal sediment to atmosphere was studied based on the simulated experiment. The experiment samples were collected from the Haibo Estuary (S1) and the Licun Estuary (S2) of the Jiaozhou Bay in China, which are seriously polluted with mercury. The results show that the mercury in sediment releases rapidly to atmosphere under solar radiation. After 8 hours of solar radiation, mercury concentrations decrease from 5.62 µg/g and 2.92 µg/g to 2.34 µg/g and 1.39 µg/g in S1 and S2 sediments respectively in summer, and decrease from 5.62 µg/g and 2.92 µg/g to 4.58 µg/g and 2.13 µg/g respectively in winter. The mercury species in the sediment change markedly under solar radiation. The concentrations of mercury bound to organic matter decrease significantly from 2.73 µg/g to 0.31 µg/g in S1 and from 2.07 µg/g to 0.31 µg/g in S2, and the released mercury mainly comes from mercury bound to organic matter. Mercury flux shows distinguishing characteristic of diurnal change, and it increases rapidly in the morning with the rising of solar radiation intensity, but decreases in the afternoon. The mercury flux increases with sediment temperature and solar radiation intensity. The rapid release of mercury in intertidal sediment plays an important role in the regional mercury cycle. Keywords: mercury; intertidal sediment; mercury release; solar radiation; mercury species 1 Introduction Mercury is one of global pollutants, and the global mercury concentration was increasing more recently (Slemr and Langer, 1992). Among all the causes, the human activities such as the burning of fossil fuels (Wang et al., 2000), the incineration of waste (Horne and Williams, 1996) are important ones. In the meantime, volcano (Nriagu and Becker, 2003), weathering of rock and soil (Carpi and Lindberg, 1998; Bergan et al., 1999), water (Mason et al., 1994; Temme et al., 2003), vegetation (Poissant and Casimir, 1998), and the other natural sources could release mercury to the atmosphere. Metallic mercury has the highest vapor pressure among all metals. Mercury in the atmosphere could be transmitted to and deposit at other places by the atmospheric circulation, and the increase of mercury affects the health of human and the other wild life (UNEP, 2002). A variety of factors have been found to influence mercury emissions from soils and water surface, among which main factors are solar radiation, soil temperature, soil wetness, precipitation, air mercury level, atmospheric oxidants and perhaps atmospheric humidity (Kim et al., 1995; Poissant and Casimir, 1998; Gustin et al., 1999; 2002; Lindberg et al., 2002; Engle et al., 2006; Kuiken et al., 2008). Gas exchange of mercury between the water and the atmosphere is considered the major mechanisms driving mercury from the seawater to the air (Mason et al., 1994; Schroeder and Munthe, 1998). Mason et al. (2001) found that the photo induced processes in open-ocean surface waters could result in either a net oxidation or net reduction of mercury species. As a large amount of mercury are delivered to coastal water from the watershed, atmospheric deposition and urban sources, the biogeochemical process of mercury in coastal zones may lead to significant and enhanced production and mercury efflux to the atmosphere. Tideland is influenced by human activities greatly, and mer- Received date: ; accepted date: Foundation item: Under the auspices of National Natural Science Foundation of China (No ), the Program of the State Bureau of Oceanic Administration (No ) Corresponding author: LIU Ruhai. ruhai@ouc.edu.cn Science Press and Northeast Institute of Geography and Agroecology, CAS and Springer-Verlag Berlin Heidelberg 2010

2 100 LIU Ruhai, WANG Yan, SHAN Changqing et al. cury can accumulate in the region because of the discharge of pollutants derived from the coastal zones. The intertidal sediment are periodically inundated and exposed to the atmosphere, and mercury might be released to the air just like the soil (Gustin et al., 1999; Wang et al., 2004) and water (Costa and Liss, 2000; Gardfeldt et al., 2001). The tide, wave, and organism activity make the oxidation reaction of sediment change frequently, and the special environment might cause mercury release in intertidal sediment different from the soil and water surface. Canario and Vale (2004) reported that mercury was rapidly released from intertidal sediment when exposed to solar radiation. However, they did not analyze the effect of different mercury species in sediment. The objective of this study is to investigate the release of mercury in intertidal sediment under solar radiation in different seasons in order to calculate the mercury evasion flux. The species variance of mercury in sediment is determined in order to understand the effect of the mercury species. A further aim is to provide scientific evidence for preventing release of mercury from the intertidal sediment surface on a global scale. 2 Methodology 2.1 Sampling The sediments (0 20 cm) were collected at low ebb from the Haibo (S1) and Licun (S2) estuaries of the Jiaozhou Bay of China in August and December The two rivers are both municipal rivers of Qingdao City and the sediment there is heavily polluted. Samples were transported to the laboratory, air dried at room temperature and the coarse debris was removed prior to homogenizing. The samples used for experiment in winter were stored in pre-cleaned container at Experiments The simulated experiments were also performed in August (summer) and December (winter) Sediments were put into the glass dishes, and then tidal cycle was simulated. The sea water with the depth of 50 cm was added into the dishes. On the second day, the overlying water was drained, half of the sediments were exposed to the sun, and the other half were kept in dark (without solar radiation). Every hour a portion of samples was taken from the two kinds of sediment and was frozen immediately. The temperature of atmosphere and sediment, the water content of sediment and solar radiation intensity changed with time during the experiment were measured, respectively. Solar radiation intensity reached the highest value at about 12:00 o clock. The air temperature and the sediment temperature reached the highest values at 14:00 o clock, and then began to decrease. 2.3 Sediment analyses The samples were dried at 40 in oven, homogenized, and stored in pre-cleaned container for analysis. The sediments were digested at 145 by the method of V 2 O 5 -HNO 3 -H 2 SO 4 (Rasmussen et al., 1991) and mercury concentration was determined with cold vapor atomic fluorescence spectrophotometer (Beijing Titan Instruments Co. Ltd, AFS-920). NaBH 4 solution (0.5%) was used as a reducing agent and 2% HCl solution was used as a blank control. The bivalent mercury in the digested sample solution was reduced to element mercury by the 0.5% NaBH 4 solution. Then the element mercury was carried to the detector by argon. Measurements were made in parallels and soil samples being up to national standard (GBW07401, China) were added for quality control. The recovery rates ranged from 94.8% to 99.0%. Total mercury concentrations determined were 5.62 µg/g and 2.92 µg/g in S1 and S2 sediments, respectively. Mercury species in sediment were analyzed by the method of modified sequential extraction (Tessler et al., 1979; Pang et al., 1981). Organic matter was determined by the method of wet combustion (LY/T ). Sediment ph was measured with a ph meter (Radiometer Model phm 93, France). The organic matter contents were 9.8% and 11.3%, and ph were 7.8 and 8.0 in S1 and S2 sediments, respectively. 2.4 Flux calculation The calculation equation of release flux of mercury from sediment to atmosphere is as follows: F = m (C t C t+1 ) / A (1) where F is the release flux of mercury from time t to time t+1 (µg/(m 2 h)); m is the mass of sediment (g); C t, C t+1 are contents of mercury in sediment at time t and time t+1 (µg/g); A is the area of interface between sediment and atmosphere (m 2 ).

3 Release of Mercury from Intertidal Sediment to Atmosphere in Summer and Winter Results and Analyses 3.1 Variance of mercury concentration in sediment Variances of mercury concentrations in S1 and S2 sediments with time under solar radiation and in dark in different seasons are shown in Fig. 1. For the two sediments, there were similar variances in mercury concentrations whether in summer or in winter. Under solar radiation, mercury concentrations decreased slightly at first, and decreased sharply from 11:00 A. M. to 14:00 P. M., then decreased gradually from 15:00 P. M. to 16:00 P. M. However, in dark a gradual decrease in mercury concentrations was observed. In the summer experiment (August) mercury concentrations decreased from 5.62 µg/g and 2.92 µg/g to 2.34 µg/g and 1.39 µg/g in S1 and S2 sediments respectively, while they dropped to 4.58 µg/g and 2.13 µg/g in the winter experiment (December). About 58% and 52% of the initial mercury were lost in S1 and S2 sediments respectively in the summer experiment, while about 19% and 27% were lost in the winter experiment. The release rates of mercury in the summer were higher than those in the winter under solar radiation. However, mercury concentrations of sediment in dark changed more slightly in the summer, and almost had no change in the winter. The results indicated that mercury was released to atmosphere quickly under solar radiation. Fig. 1 Variance of total mercury in sediment under solar radiation and in dark in summer and winter 3.2 Variance of mercury species in sediment under solar radiation The mercury species at initial, 12:00 and 16:00 o clock in S1 and S2 sediments were determined for analyzing the contribution of different mercury species to the mercury release under solar radiation (Table 1). The concentrations of acid dissolved mercury, mercury bound to organic matter and residual mercury and total mercury all decreased, while that of water dissolved and exchangeable mercury increased. However, the amount of increased mercury was less than that of the total decreased mercury. Compared with the other species, the concentrations of mercury bound to organic matter decreased significantly from µg/g to µg/g in S1 and from µg/g to µg/g in S Mercury flux from sediment to atmosphere According to the Equation (1), mercury flux from the sediment to the atmosphere under solar radiation was calculated. Mercury flux showed distinguishing characteristic of diurnal change (Fig. 2). It increased in the morning, reaching the peak at 13:00 14:00 o clock in S1

4 102 LIU Ruhai, WANG Yan, SHAN Changqing et al. Table1 Variance of mercury species under solar radiation in the summer experiment (µg/g) Time (h) Ⅰ Ⅱ Ⅲ Ⅳ Total mercury S S Notes: Ⅰ: Water dissolved and exchangeable mercury; Ⅱ: Acid dissolved mercury; Ⅲ: mercury bound to organic matter; Ⅳ : residual mercury Fig. 2 Diurnal and seasonal variance of mercury flux sediment (2 830 µg/(m 2 h)) and at 12:00 13:00 o clock in S2 sediment (1 793 µg/(m 2 h)) in summer. In winter the time when the peak appeared was delayed, and the flux reached the peak at 12:00 13:00 o clock with µg/(m 2 h) in S1 and 943 µg/(m 2 h) in S2. The diurnal change characteristic was similar to the soil (Carpi and Lindberg, 1998) and wetland (Lindberg et al., 2002). The mercury flux from the sediment was much higher than that from soil (Gustin et al., 2002) and sea water (Slemr and Langer, 1992; Temme et al., 2003). 4 Discussion In this study, mercury released to atmosphere quickly under solar radiation, which was similar to the results of previous research in the naturally enriched substrates (Gustin, 2002). The concentrations of acid dissolved mercury, mercury bound to organic matter and residual mercury all decreased, while that of water dissolved and exchangeable mercury increased. The increased mercury might be transformed from other species, and the increased amount was less than the total decreased part. That is to say, the lost amount of mercury might be the part released to the atmosphere by Hg 0 under solar radiation. According to the change of mercury with time, the ratios of the lost amount of each species to the total were calculated. The ratio of mercury bound to organic matter in S1 was 73.8%, that of acid dissolved mercury was 13.9%, and that of other species was only 12.3%. Therefore, the released mercury mainly came from mercury bound to organic matter. Humics have an intrinsic capacity to reduce transition metals in natural waters (Alberts et al., 1974; Skogerboe and Wilson, 1981) and can enhance their reducing properties by transferring the absorbed energy to a suitable electron receptor (Nriagu, 1994) due to their ability to absorb light. The humic substances in fresh water (Xiao et al., 1995) and in sea water (Costa and Liss, 1999) were found to increase the production of Hg 0 for the photoreduction of Hg(II). Organic matter in sediment might show similar function. Results of previous researches showed that the transformation among different mercury species was complicated. Costa and Liss (2000) thought that Hg 2+ in seawater could be transformed to Hg 0 under solar radiation, and Canario and Vale (2004) found that mercury combined with OH, HS, Cl and mercury bound to organic matter all decreased after they adsorbed the ultraviolet radiation. Even the stable residual mercury and HgS could be photolyzed, and the mercury was released (Gustin et al., 2002). Compared with the mercury bound to organic matter, the stable residual mercury and

5 Release of Mercury from Intertidal Sediment to Atmosphere in Summer and Winter 103 HgS changed less under solar radiation. Temperature has great influence on the release of mercury, because it causes the change of chemistry reactive rate. In this study, there were significant relationships between mercury flux and sediment temperature (Fig. 3). The correlation coefficients were (P<0.01) for S1 sediment and (P<0.01) for S2 sediment respectively in summer, (P<0.01) and (P<0.01) respectively in winter. The increase of temperature caused the molecule to act quickly, which was prone to produce Hg 0. At the same time, the saturated vapor pressure of mercury was increased, which accelerated the mercury release. There were significant relationships between mercury flux and solar radiation intensity (Fig. 4) The correlation coefficients between the release flux of S1 and S2 sediments and solar radiation intensity were (P<0.05) and (P<0.01) respectively in summer, (P>0.05) and (P<0.05) respectively in winter. The ultraviolet with high energy might oxidate the organic matter and sulfide, it also deoxidated the high valency mercury to Hg 0. The stronger the radiation was, the faster the oxidation reduction reaction was. Previous researches presented that the solar radiation played an important role in mercury release from water surface (Costa and Liss, 2000), soil (Gustin et al., 2002) and the F denotes the flux of mercury (µg/(m 2 h)); T denotes the temperature (K) Fig. 3 Relationship between mercury flux and sediment temperature vegetation (Lindberg et al., 2002). The temperature of sediment in summer is much higher than that in winter. However, the release flux of mercury from sediment under solar radiation in winter was higher than that in dark in summer. Therefore, the temperature and solar radiation jointly played important roles on mercury release from sediment. Moreover, the other factors might also affect the release of mercury, such as the wind, relative humidity, etc. The environment in intertidal land is complicated. The tide, as well as current and benthic fauna often resuspends the surface sediment or makes the bottom anoxic sediment move to the surface. The difference between the simulated surrounding and the real state is so great that it is necessary to do further research in the field. 5 Conclusions The mercury in intertidal sediment rapidly releases to atmosphere under solar radiation both in summer and in

6 104 LIU Ruhai, WANG Yan, SHAN Changqing et al. F denotes the flux of mercury (µg/(m 2 h)) Fig. 4 Relationship between mercury flux and solar radiation intensity winter. The mercury species in the sediment change markedly under solar radiation. Mercury bound to organic matter loses 73.8% in the sediment of Haibo Estuary, and it may be the main source of released mercury. There are significant relationships between mercury flux and sediment temperature, solar radiation intensity, and the mercury flux increases with sediment temperature and solar radiation intensity. In the daytime, when the wind goes from sea to land, the released mercury may affect the air quality of cities on the land. The rapid release of mercury in intertidal sediment plays an important role in the regional mercury cycle. Acknowledgements The authors would like to thank Professor Sheng Lifang for the assistance with analysis of solar radiation. References Alberts J J, Schindler J E, Miller R W, Elemental mercury evolution mediated by humic acid. Science, 184(4139): DOI: /science Bergan T, Gallardo L, Rodhe H, Mercury in the global troposphere: A three-dimensional model study. Atmospheric Environment, 33(10): DOI: /S (98) Canario J, Vale C, Rapid release of mercury from intertidal sediment exposed to solar radiation: A field experiment. Environmental Science & Technology, 38(14): DOI: /es035429f Carpi A, Lindberg S E, Application of a teflon dynamic flux chamber for quantifying soil mercury flux: Tests and results over background soil. Atmospheric Environment, 32(5): DOI: /S (97) Costa M, Liss P S, Photoreduction of mercury in sea water and its possible implication for Hg 0 air-sea fluxes. Marine Chemistry, 68(1): DOI: /S (99) Costa M, Liss P S, Photoreduction and evolution of mercury from seawater. The Science of the Environment, 261(1): Engle M A, Gustin M S, Johnson D W et al., Mercury distribution in two Sierran forest and one desert sagebrush steppe ecosystem and the effects on fire. The Science of the Total Environment, 367(1): DOI: /j.scitotenv Gardfeldt K, Feng X, Sommar J et al., Total gaseous mercury exchange between air and water at river and sea surfaces in Swedish coastal regions. Atmospheric Environment, 35(17): DOI: /S (01)00560-X Gustin M S, Lindberg S, Marsik G et al., Nevada STORMS project: Measurement of mercury emissions from naturally en-

7 Release of Mercury from Intertidal Sediment to Atmosphere in Summer and Winter 105 riched surfaces. Journal of Geophysical Research, 104: Gustin M S, Biester H, Kim C S, Investigation of the light-enhanced emission of mercury from naturally enriched substrates. Atmospheric Environment, 36(20): DOI: /S (02) Horne P A, Williams P T, Sampling and analysis of mercury species in effluent gases derived from waste incineration. Waste Management, 16(7): DOI: /S X(96) Kim K, Lindberg S E, Meyers T P, Micrometeorological measurements of mercury vapor fluxes over background forest soils in eastern Tennessee. Atmospheric Environment, 29(2): DOI: / (94)00198-T Kuiken T, Zhang H, Gustin M et al., Mercury emission from terrestrial background surfaces in the eastern USA. Part I: Air/surface exchange of mercury within a southeastern deciduous forest (Tennessee) over one year. Applied Geochemistry, 23(3): DOI: /j.apgeochem Lindberg S E, Dong W, Meyers T, Transpiration of gaseous elemental mercury through vegetation in a subtropical wetland in Florida. Atmospheric Environment, 36(33): DOI: /S (02) Mason R P, Fitzgerald W F, Morel F M M, The biogeochemical cycling of elemental Mercury: Anthropogenic influences. Geochimica et Cosmochimica Acta, 58(15): Mason R P, Lawson N M, Sheu G R, Mercury in the Atlantic Ocean: Factors controlling air sea exchange of mercury and its distribution in the upper waters. Deep-Sea Research II, 48(13): Nriagu J O, Mechanistic steps in the photoreduction of mercury in natural waters. The Science of the Total Environment, 154(1): 1 8. DOI: / (94) Nriagu J, Becker C, Volcanic emissions of mercury to the atmospheric: Global and regional inventories. The Science of the Total Environment, 304(1): DOI: /j.scitotenv Pang SW, Qiu G K, Sun J F, Determine the species of mercury using the method of sequential extraction. Acta Scientiae Circumstantiae, 1(3): (in Chinese) Poissant L, Casimir A, Water-air and soil-air exchange rate of total gaseous mercury measured at background sites. Atmospheric Environment, 32(5): DOI: /S (97) Rasmussen P E, Mierle G, Nriagu J O, The analysis of vegetation for total Hg. Water, Air and Soil Pollution, 56: Schroeder W, Munthe J, Atmospheric mercury An overview. Atmospheric Environment, 32(5): DOI: /S (97) Skogerboe R K, Wilson S A, Reduction of ionic species by fulvic acid. Analytical Chemistry, 53: Slemr F, Langer E, Increase in global atmospheric concentrations of mercury inferred from measurements over the Atlantic Ocean. Nature, 355: DOI: / Temme C, Slemr F, Ebinghaus R et al., Distribution of mercury over the Atlantic Ocean in 1996 and Atmospheric Environment, 37(14): DOI: /- S (03) Tessler A, Campbell P G C, Bisson M, Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry, 51(7): United Nations Environment Programme (UNEP), Global Mercury Assessment. Geneva: UNEP Chemicals. Wang Q, Shen W, Ma Z et al., Estimation of mercury emission from coal combustion in China. Environmental Science & Technology, 34(13): DOI: /es j Wang S F, Feng X B, Qiu G L et al., Contrast of Hg flux of soil/ air in cold and warm season in Hongfeng Lake in Guizhou. Environmental Science, 25(1): (in Chinese) Xiao Z F, Stromberg D, Lindqvist O, Influence of humic substances on photolysis of divalent mercury in aqueous solution. Water, Air and Soil Pollution, 80: