Forms Of Reduced Inorganic Sulfur (RIS) And Transformation Mechanism In Sediments Of The Dongtan, Chongming

Sulfur biogeochemistry is very sensitive to environmental conditions and has important influence on the global environmental change. The coastal wetland, as a typical transitional zone between land and ocean, is a multifunctional and complex ecosystem, as well as a significant responding and indicating area of global environmental changes and regional environmental problems. Therefore it is important to develop researches on the characteristics and behavior of sulfur in the sediments of coastal wetland. In this paper, Chongming Dongtan wetland was taken for an example. The study showed here involves the spatial and seasonal variability of reduced inorganic sulfur (RIS) species, the migration and transformation of RIS species and heavy metals species, and their influencing factors existed in tidal wetland.The results show that, the contents of acid-volatile sulfide (AVS) in surface sediments of Chongming Dongtan range from 0.844 to 3.531μmol/g, chromium(Ⅱ)-reducible sulfide (CRS) from 2.531 to 5.156μmol/g, elemental sulfur (ES) from 0.719 to 1.125μmol/g. CRS is the main RIS species in surface sediments, which occupies from 48.2% to 67.9%. The general content sequence of RIS is high tidal flat> middle tidal flat> low tidal flat. The content distribution of the RIS species in pillar sediments of Chongming Dongtan present irregular characteristic. The content of ES in high tidal flat gradually increases from 10 cm to 25 cm, while the content of AVS decreases. The content of humus in 5～10 cm middle tidal flat is high, which shows high rate of sulfate reduction and pyritization. So the contents of AVS and CRS are high. The palaeohydrodynamic condition in low tidal flat is strong, so the contents of AVS, CRS, ES in surface sediments (0～10 cm) are all limited.The total RIS content in middle tidal flat range from 7.175 to 7.660μmol/g, the contant in spring and winter is slightly higher than that in summer and autumn. The highest concentration of AVS and ES, and the lowest concentration of CRS were found in summer, and significantly differ from those in spring, autumn and winter. These phenomenon may due to the oxygen released by plant roots in summer, that a portion of pyrite was removed by secondary oxidation, and the conversion from AVS to CRS was restricted to the availability of active iron in summer.The total contents of RIS in flooding sediments and flooding-drought alternating sediments are both higher than that in drought sediments. There is good positive correlation between water content and AVS, Fe2+ content in the sediments, but the correlation between water content and CRS presents negative. The findings suggest that overlying water play an important role in the fate of reduced inorganic sulfur speciation in intertidal sediments. ES may be the limiting reactant that weakens the formation of CRS.With regard to salinity effects on the migration and transformation of reduced inorganic sulfur, there is no obvious rule for that. Compared to middle salinity (S=5‰), high salinity (S=15‰) or low salinity (S=1‰) is more conducive to the formation of RIS. Salinity or sulfate availability is not the direct control factors for RIS speciation in intertidal sediments.SEM-Zn/total-Zn, SEM-Pb/total-Pb and SEM-Cu/total-Cu present the highest value in flooding-drought alternating sediments, and SEM-Ni/total-Ni and SEM-Cr/total-Cr present the highest value in flooding sediments. In addition, Chongming semidiumal tital might encourage a portion of non-residual fraction of Cr, Ni, Pb, Zn to release into the water system and endanger aquatic organisms.SEM-Cr/total-Cr, SEM-Cu/total-Cu and SEM-Zn/total-Zn increased with flooding salinity, while SEM-Pb/total-Pb and SEM-Ni/total-Ni present the highest value in middle salinity sediments. Moreover, Middle salinity (S=5‰) could help to preserve non-residual fraction of Cr, Ni, Pb, Zn in flooding sediments, and reduce harm to aquatic organism.