Preliminary Research On Temporal And Spatial Distribution Of Arsenic In Chongmin Dongtan Wetland And Its Influencing Factors

It has been known that high content of arsenic in the environment has bad effect on human bodies and creatures. So it is necessary to study the temporal and spatial distribution of arsenic and its migration and transformation law in nature so as to be more targeted in prevention and treatment.Tidal beach is subjected to periodic tidal inundation. It is in alternately changing environment of oxidation-reduction and strongly active in water and sediment exchangement between the materials.So it is an ideal place for the study of migration and transformation of arsenic in the natural environment. The paper selected Chongming Dongtan Nature Reserve as a typical area to study sediment, tidal creek water, pore water and three typical tidal vegetation (Phragmites, Spartina, Bulrush). We used physical and chemical analysis methods to reveal the temporal and spatial distribution of total arsenic in sediments and water; the law of three typical intertidal vegetation accumulation of arsenic. We used step extraction method to study the different forms of arsenic in the surface sediments and to explore the vertical variation of different forms of arsenic in the sediments under Spartina grass and bulrush vegetation. We studied the characteristics of arsenic in the pore water and explored the effect of pH, Eh, salinity, grain size, organic matter, sulfides, Fe-Al-Mn oxide and hydroxide on the migration and transformation of arsenic.Based on the study, some conclusions can be drawn.1.The total arsenic concentration in the surface sediments of Chongming Dongtan National Nature Reserve is within the requirement. Compared to other countries, it is at relatively low level.The content of arsenic in the same sampling point shows that spring and winter> summer and autumn. There is high content of total arsenic in the surface(0-5cm)sediment under two typical vegetation, but the vertical trends are quite different. The trend of total arsenic and<16μm particle size fractions are in good agreement.2.The content of total arsenic in water of tidal creeks does not have significant seasonal changes and it is within the requirement. Compared to the estuary of Yangtze River and the river of other countries, the content of dissolved arsenic is higher than the others.3.There are differences in the distribution of total arsenic in the three typical vegetations.The concentrations of total arsenic in roots and stems and leaves of reed are all low. But the capillary roots of Spartina and bulrush contain much higher arsenic than the stems and leaves.4. The concentration of five different states of arsenic in the intertidal sediments are in the order of residual>amorphous bound>crystalline bound>strongly exchangeable>weakly exchangeable. The vertical variartion of different states of arsenic in the sediments are complex. The contents of high exchangeable, amorphous/crystal forms of arsenic are decreasing with the depth and the content of residual arsenic is gradually increased with the depth. The arsenic in the sediments may exist the transformations from well-crystallized oxides of Fe and Al to amorphous and poorly-crystalline hydrous oxides of Fe and Al and then to the state of exchangeable.5.The concentrations of arsenic in the pore water under two kinds of vegetation are above 21cm.The release of arsenic is associated with the reduction of iron oxides and hydroxides.6.The concentration of arsenic in the intertidal sediment is controlled by grain size. They have a positive correlation between themselves. Redox revironment, leading to the reduction of Fe-Al-Mn oxides and hydroxides is the second largest factor to control the migration of arsenic. The contents of weak exchangeable bound and amorphous arsenic increase with the rise of pH in 6.0-7.0. The degradation of organic matter, water content of sediment, temperature and salinity also affect the distribution of arsenic in sediment by changing the redox environment and the role of competitive adsorption with arsenic directly.7.The migration of arsenic in sediment is mainly through the sediment-pore water interface. Iron and sulfur cycle control this cycle. In the weak reducing environment arsenic is mainly adsorbed on the surface of iron oxides and hydroxides or binding with iron, aluminum, manganese, calcium to generate some insoluble salts. At this time sulfur mainly exists in the form of sulfate and it does not play a major role in the migration of arsenic. As the enchancement of reducing environment, arsenic is released to the pore water with the reduction of iron, aluminum, manganese oxides and hydroxides.Arsenic move in the direction of lower concentration in the pore water.Upward migration of arsenic will be adsorbed on oxide surface with the enchancement of re-oxidation. Downward migration of arsenic is mainly affected by sulfur.They will generate arsenic sulfide or metal sulfide and be re-deposited on the surface of sediment. As the role of plant roots of oxygen, when the reduced environment becomes weak, the sulfide will be oxidized, which combined with the subsequently release of arsenic.