Geochemical Characteristics Of Sulfur And Its Coupling Mechanism With Iron And Phosphorus In Estuary Sediments

Inorganic sulfide is one of the most active sulfur forms in sediments. The geochemical cycling of inorganic sulfide is closed related to the geochemical behavior of iron, phosphorus and heavy metals in sediments. Its transformation processes such as the oxidation-reduction, have directly effects on the form, activity and environmental behavior of iron and phosphorus in sediments. Estuary is the last barrier to prevent land-based pollutants from being washed into the sea and is typical ecologically sensitive region and environmental fragile zone. Therefore, studies on the geochemical characteristics of sulfide and its coupling mechanism with iron and phosphorus cycles in estuarine sediments is very significant for understanding the mechanism of sulfur, iron and phosphorus cycles on coastal and global scale.In this dissertation, we chose different types of sediments in the Yuniao River estuary as study objects characterized by heavy pollution in the northern China. The spatial and temporal variations of distributions of dissolved S^2-, Fe²⁺ and dissolved active phosphates（DRP） and their dynamic diffusion process were systematically studied by the diffusive gradients in thin-films technique（DGT）. The results indicated that the interactional and promoting mechanisms of S-Fe-P were remarkable. In this study, the geochemical characteristics of sulfide and its coupling mechanism with iron and phosphorus cycles in heavily polluted coastal sediments were deeply investigated by combining the data of date of the grain sizes, organic matter, solid inorganic sulfide, active iron, phosphorus forms and the correlations among them for further understanding the coupling mechanism of S-Fe-P cycles. A simulation experiment of tidal process was conducted for understanding the immersion process of seawater and fresh water in natural tide, and for quantifing the influence of tide on the geochemical cycles of sulfur, iron and phosphorus in heavily polluted estuarine sediments. Major results are as follows:（1） The spatial and temporal distributions of sulfide and its dynamic diffusion process were studied in estuarine sediment of the Yuniao River. The results showed vi that a large amount of organic matters existing in the sediments were controlled by fine particles. The content of organic matter in the muddy sediments was obviously higher than that in sandy sediments. Sulfate reduction was controlled by the organic matters which concentrations were much higher in the mud sediments than those in sandy sediments. The rate of sulfate reduction was relatively higher in the surface and subsurface sediments and gradually decreased with depth. Due to the available sulfate and active organic matter sediments were rich in the intertidal sediments, the rate of sulfate reduction was relatively high. Because of the lower content of available sulfate, sulfate reduction rate was relatively low in freshwater sediments and the dissolved S^2-maintained a relatively stable equilibrium.（2） The spatial and temporal distributions of inorganic sulfide and its transfer mechanism were studied in the estuarine sediments of the Yuniao River. We found that the sulfate reduction rate was relatively high in the sediment. Reducing inorganic sulfide（RIS） acid existed as volatile sulfide（AVS） in the surface and subsurface sediments, while the RIS was pyrite（CRS） in the deep sediment. Significant correlation between dissolved S^2- and AVS was found, indicating that AVS was dominated by dissolved S^2-, and the activity and biological toxicity of inorganic sulfide was very high in the sediment of the Yuniao River. Low degree of pyritization（DOP） and high ratio of AVS/CRS suggested that the active iron oxide was the major control factor of conversion from H₂S to FeS, while the elemental sulfide（ES） was the major control factor of conversion from AVS into CRS. The geochemical cycling of inorganic sulfide was significantly influenced by seasonal cycle. The accumulation of dissolved S^2- and AVS increased in summer and autumn and decreased in winter and spring caused by temperature related sulfate reduction. The value of DOP and content of ES in summer were obviously higher than those in winter, and the multi-sulfofication was the main pathway of the pyritization.（3） The spatial and temporal distributions of active iron and dissolved Fe²⁺ were investigated in the estuarine sediments of the Yuniao River. The results showed that the concentrations of active iron and dissolved Fe²⁺ were low in estuarine sediment of the Yuniao River. The high degree of sulfidization（DOS） indicated that Fe S and FeS₂ were the dominant iron form in the sediments and the degree of sulfidization of iron was extremely high. The activity of iron oxide became the main factor controlling conversion of H₂S to Fe S. Although the dissimilatory iron reduction and the chemical iron reduction existed simultaneously, the process of iron reduction was dominated by chemical iron reduction.（4） The spatial and temporal distributions of dissolved active phosphate（DRP） and phosphorus speciation were studied in the estuarine sediments of the Yuniao River. Our results showed that the iron phosphorus（Fe-P） and organic phosphorus（OP） were the main speciation of phosphorus, accounting for more than 80% of total phosphrus, while the contents of exchangeable phosphorus（Ex-P） and calcium bound phosphorus（Ca-P） were relatively low. The contents of Fe-P and OP in winter were significantly higher than those in summer. The seasonal variations of Fe-P were obviously influenced by sulfate reduction and iron reduction while the microbial activity played an important role in seasonal variations of OP.（5） The results of geochemical cycle of inorganic sulfide and the coupling mechanism of S-Fe-P cycles in estuarine sediments of the Yuniao River showed that the heavy polluted estuarine sediments were rich in available sulfate and organic matter, and the sulfate reduction rate in it was very high. The H₂S, generated by sulfate reduction, could react with iron oxide to form Fe S and FeS₂ and which limit the activity of iron and inorganic sulfide. Phosphate was released from absorption and fixed by iron oxide. Due to the salinity difference during the tidal alternation, the exchange diffusion fluxes of dissolved S^2-, Fe²⁺ and DRP increased between the overlying water and sediment, resulting in promotting the migration and diffusion of phosphate from sediment to the overlying water and the biological availability of phosphorus, indicating that the risk of eutrophication increased in the coastal area.