|Other Abstract||The interaction between sulfur and iron is the main reason for the formation of black and odorous in coastal water. The coupling process of sulfur and different forms of iron in coastal sediments can directly or indirectly affect the quality evolution of water and sediments. Coastal zone as the most intense interaction between land and sea, is characterized by complex water quality and hydrological conditions, mixed alternation of salty and freshwater, high pollution level, and accumulation and deposition of a large amount of suspended particulate matters. The migration and transformation of different sulfur forms and and its coupling mechanism with iron have been one of the research highlights in the same field. There are numerous coastal rivers in Jiaodong Peninsula. The exogenous pollution of most rivers has been effectively controlled, but some rivers and coastal water are still suffered from black and odorous, which may be closely related to the coupling of sulfur and iron in sediments. However, the understanding of sulfur and iron cycle and their coupling mechanisms in coatal sediments have been still unclear, causing that the evolution of coastal water quality cannot be deeply analyzed and accurately controlled. Therefore, guided by the above problems, this paper focused on the sulfur cycle in sediments of " river basins - estuarine - offshore" system, and explored the migration and transformation of sulfur and its coupling mechanism with iron in coastal sediments, which aiming to provide a scientific basis for the identification of the environmental quality evolution process of coastal zones, and also to supply technical supports for the prevention and control of coastal water and sediment pollution. The major findings were as follows:
(1) Vertical distribution and diagenesis mechanism of sulfur and iron in a sediment core from coastal areas: The vertical sedimentation characteristics and diagenesis mechanism of reduced inorganic sulfur and ractive iron in an offshore sediment core (about 4 m) were systematically studied using the improved cold diffusion method. Results showed that the grain-size was predominantly composed of silt (73.6 %) in sediments, and reduced inorganic sulfur was primarily occupied by CRS (65 %) in sediments. The AVS content presented a narrow range with depth increase, whereas the contents of CRS and ES were higher at the top and bottom layer than that at the middle layer. RFe was dominated by RFe(II) (80 %), and its concent increased with sediment depth. Microbial decomposition of organic matter mainly occured on the shallow surface sediments (0 to -30 cm), which facilitated the conversion of AVS into CRS. With the depth increase (-160 to -370 cm), the enhaced sulfate reduction resulted in a greater accumulation rate of AVS overing the conversion rate. For the deep sediments (-160 to -370 cm), the chemical reduction of iron was the main process, which generated ES and promoted the transformation of AVS to CRS. The low values of DOP (0.01 - 0.24) and DOS (0.03 - 0.25) indicated that pyritization and sulfidation were not limited by the RFe, but by the reactive organic matter contents.
(2) Mechanism of the lateral migration and transformation of sulfur and iron in different sedimentary environments of coastal rivers: The spatial distribution, occurrence forms, migration and transformation characteristics of sulfur and iron in surface sediments of Jiaolai River with potential industrial pollution, Jiahe River as a drinking water source and Guangdang River with river bed hardening were studied by using cold diffusion extraction method, chemical extraction method and XPS. Results showed that the forms of sulfur and iron in surface sediments of the three rivers are predominantly composed of organic sulfur (89 %、85 % and 77 %) and labile Fe(II) (88 %、71 % and 84 %), respectively. The low values of DOP （< 0.52）and high AVS/CRS ( > 0.3) in the Jiaolai River, the middle and lower reaches of the Jiehe River and the Guangdang River indicated a low degree of pyritization and large accumulation of AVS in sediments. The CRS formation was limited by the reactive organic matter contents. The high DOP (up to 0.73) and low AVS/CRS (< 0.3) in the middle and upper reaches of Jiahe River indicated that large accumulation of CRS was limited by CRS formation. Both results of C/N ratios and XPS analysis showed that exogenous organic matter input promoted sulfate dissimilatory reduction and the enrichment of fulvic acid sulfur (FAS, 76 % and 75 %) and AVS in surface sediments of Jiaolai River (FAS, 76 %; AVS, 65 %) and Guangdang River (FAS, 75 %; AVS, 70 %). Neverthless, endogenous biological input promoted assimilative sulfate reduction and the
enrichment of humic acid sulfur (HAS) and CRS in sediments of Jiahe River (FAS, 74 %; AVS, 70 %).
(3) Coupling mechanism of sulfur and iron in surface sediments of coastal watershed under high phosphate and salinity stress: Taking two coastal rivers with high phosphate and high salinity in the " basins - estuarine - offshore " system of Jiaodong Peninsula as the research area, the coupling mechanism of sulfur, iron and phosphorus in surface sediments was investigated. Results showed that the high salinity promoted the accumulation of the CRS (56 %) and the fixation of HCl-P (71 %), and induced the activation of NaOH-P in surface sediments, thus increasing the risk of water eutrophication. Correlation analysis showed that CRS formation was limited by the TOC. And ES could be used as an indicator of RIS and TOC storage loss in sediments under high salinity stress. High phosphorus stress boosted the dissimilatory reduction of iron oxides and the NaOH-P (58 %) fixation, inhibiting the reduced inorganic sulfur enrichment in sediments. AVS was oxidated to iron hydroxides in sediments, which could serve as potential sources of NaOH-P formation when sediments were anthropogenically disturbed. Correlation analysis showed that the transformation of AVS to CRS was limited by the ES contents in sediments under high salinity stress.
(4) Effects of salinity on vertical migration and biological interactions of sulfur and iron in estuarine-bay sediments: Taking “Wulong River Estuary-Dingzi Bay- Yellow Sea” as the study area, the biogeochemical characteristics of sulfur and iron influenced by along an estuarine salinity gradient were investigated. Results showed that the combined effect of salinity and humic acid facilitated the flocculation of fine-grained sediments (50.44 to 76.54 %), resulting in decreasing the particle size from the estuary to the ocean, thereby enhacing the adsorption of TOC (0.32 to 0.67 %). The results of microbiome and three-dimensional fluorescence parallel factor analysis showed that salinity directly promoted conformational change of DOM or indirect photodegradation, which made DOM more biodegradable. Furthermore, salinity increased the richness and diversity of sediment bacterial community, and then enhanced the degradation of humic-like in surface sediments, which was not conducive to the enrichment of organic sulfur in surface sediments (0 to -14 cm). Reduced inorganic sulfur and organic sulfur contents increased with the increase of the salinity gradient, among which reduced inorganic sulfur increased with depth. Organic sulfur content at the middle layer (-14 to -22 cm) was higher than that of the top layer. Reduced inorganic sulfur and organic sulfur were dominated by CRS (51 - 69 %) and FAS (63 - 82 %), respectively. Correlation analysis showed that the formation of CRS and FAS was restricted by reactive organic matter and ES contents, respectively. The RFe in sediments was manily occupied by RFe(III) (94 %), which was decreased with depth. Salinity improved the microbial activity and promoted the dissimilatory reduction of sulfate and chemical reduction of iron, thereby enhancing the enrichment of reduced inorganic sulfur and organic sulfur in subsurface and bottom sediments.
(5) Simulation of environmental effects of tidal processes on the coupling mechanism of sulfur and iron in sediments: Typical malodorous sediments were selected and used to carry out outdoor tidal simulation experiment. The response mechanism of dry-wet alternation to the biogeochmical coupling of sulfur and iron in high (long-term dry-wet), medium (short-term dry-wet) and low (flooded) tidal flat sediments was investigated. Results showed that the dry-wet alternation inhibited microbial activity and TOC mineralization, and promoted RFe(III) enrichment in high (TOC, 7.86 %; RFe(III), 85 %) and middle tide flats (TOC, 10.13 %; RFe(III), 77 %). There was a positive correlation between moisture and AVS, indicating that the flooding condition was favorable for AVS formation through sulfate reduction. Reduced inorganic sulfur content was ordered as follows: low tide beach > mid tide beach > high tide beach, decreasing by 46.82 μmol/g, 63.05 μmol/g, 156.56 μmol/g, respectively. CRS and RFe(III) were the main species of reduced inorganic sulfur and RFe (CRS, 48 %; RFe(III), 55 %), respectively, and ES was an important driving factor for CRS formation in high tidal flat based on correlation analysis. Reduced inorganic sulfur and RFe in middle tidal flat and low tidal flat were composed mainly of AVS (69 % and 71 %) and RFe(II) (54 % and 55 %), respectively. Correlation analysis showed that the conversion from AVS to CRS was restricted to the availability of sulfide in both middle tidal flat and low tidal flat.|