| The flocculation of fine-grained sediments is one of the main reasons to cause the estuarine deposition. With the combining effect of salt water and fresh water, fine-grained sediments in estuaries have drastic changes: flocculation, settlement, resuspension, deflocculation, diffusion and aggradation. Flocculation, in particular, affects the transport and the fate of not only the sediments but also the metallic and the organic materials. Therefore, the study on the flocculation process of fine-grained sediments by the combining effect of salinity and humus in the high-turbid system is of great help in understanding the mechanism of the formation of TM (the turbidity maximum) and illustrating the deposition dynamics of sediments (Shen et al., 1983; Shen et al., 1982; Milliman et al., 1983). As one of the basic research of the 6th key plan (LOICZ) of IGBP, the study of flocculation of fine grained sediments has been paid more attention. Up till now, studies on flocculation have mainly shown the effect of the dynamic condition (Zhang, 1992; Jiang et al., 2002; Shi, 2000) and of the exterior environment (Ruan, 1991; Partheniades, 1965; Matz, 1983; Van, 1988), while less research has been focused on its intrinsic factors. Much research on the flocculation mechanism has reflected the importance of salinity (Guan et al., 1992; Chen, 1987; Jiang et al., 1995); however, it is also held that the effect of organic matters is stronger than that of salinity (Dyer, 1988; Eisma, 1991; Berhane, 1997). The flocculation of fine grained sediments is effected by many parameters in the estuary, so the flocculation mechanism is not very clear for the limitation of the related flocculation theory and in-situ research technology.The Changjiang Estuary is one of the key estuaries in the world, and it is listed in the LOICZ plan (LOICZ No.72).As we know, the sediments with organic matters take up 60 to 75 per cent of the total sediments in quantity in the Changjiang Estuary (Li et al., 2001), and the salinity increases in the course of the Changjiang river's flowing into the sea. The abundance of organic matters and the variation of the salinity in Changjiang estuary are two important factors that affect the sediment flocculation. So it is highly necessary to put the two factors together in the scientific study of the flocculation mechanism.This study is supported by the National Natural Foundation of China (50579021) and the National Key Basic Research Program of China (No. 2002CB412405). By using more advanced in-situ equipments and analyzing methods, the survey was carried out in Feb. , 2006 (dry season) and Aug. , 2006 (flood season) to study the current speed, current direction, water temperature, salinity, sediment concentration, pH, floc size and the specific weight of fluid mud in the Changjiang estuary. And the water samples and sediment samples were collected in order to analyze the cations in water, the metal in sediment, dissolve organic carbon (DOC) and particle organic carbon (POC). All the original data is helpful for the study of flocculation mechanism. At the same time, the room experiments have been done to study the factors of flocculation. The effects of cations and organic matters on the fine-grained sediments have been analyzed through the synthetic study of the aspects of flocculation/coagulation power (F), diameter (D) and Zeta potential (Z). And themicrocosmic configuration of the flocs has been analyzed by using the Scan Electron Microscope and the Fourier Transform Infrared Spectrometry. Based on the combining flocculation mechanism of C-P-OM of fine grained sediments in the Changjiang estuary, the distribution of the particle metal contamination was also studied.(1) The fine grained sediments flocculate easily in the Changjiang estuary. In this survey, the floc size is about 63.2μm, ten times bigger than the single particle. And it ranged from 27.4μm to 107um. In the Xuliujing area, it is about 42.5-60.4 μm, smaller than it is in the Turbidity Maximum, about 57.3-79.2μm. The gap is mainly caused by the spatial distribution of salinity in the Changjiang estuary. The averare floc size is about 60.4-79.2um in the flood season, bigger than it in the dry season, about 42.5-66.6μm. The current speed affects the floc size greatly, and the biggest floc size often occurred in the middling speed, about 0.4-0.7 m/s.(2) Total cations concentration (TCC) increases from Xuliujing to the sea. And the vertical distribution is obvious in the South Passage and the North Passage, while the same phenomenon does not exist in the Nan Channel. In the Turbidity Maximum, TCC is about 40mmol/L-110mmol/L in the flood season, while it is 150mmol/L-260mmol/L in the dry season. The distribution of all kinds of cations is closely related to the salinity. As same as the variation of TCC, the distribution of flocculation ability (FA) occurs an increased trend from Xuliujing to the sea. There is obvious phenomenon in the vertical distribution. Because the salt water effects seriously in the South Passage and North Passage in the dry season, FA is higher than it in the flood season.(3) The concentration of dissolve organic carbon (DOC) decreases from Xuliujing area to the sea. And there is not obvious phenomenon in vertical distribution. In the Turbidity Maximum, DOC is higher in the dry season than it in the flood season. The concentration of particle organic carbon (POC) increases from Xuliujing to the sea. And there is an increased trend of the concentration from the bottom to the surface. In the dry season, POC is lower than in the flood season, and in the flood tidal, it is higher than in the ebb tidal. POC% occurs an obvious increased trend in the vertical distribution, and it is the lowest in the bed sediment. The study of relativity of organic matters with S and TSS shows that DOC has little relation with S and TSS, while POC has a positive relation with S and TSS, and POC% has a negative relation with S and TSS. The relativity of POC with TSS in the dry season is better than in the flood season.(4) With the increase of cations, F and D became greater and Z became smaller. Aluminum ion has the great effect on the coagulation of fine-grained sediments for the sweep flocculation of Al(OH)3 precipitates, while magnesium ions and sodium ions cause flocculation mainly by compressing the electrical double layer; while with the increase of humus, F became smaller. D and Z became greater. It is related to the organic matters can connect with fine-grained sediments through functional groups. The flocculation mechanism of combining flocculation is different from those of cations or organic flocculants alone. As the main influencing factors on flocculation, humus and cations have different mechanism in effect. Under the condition of combining effect, humus, through its functional groups, tends to pack on the surface of the fine-grained particles and connects with other flocs to form relatively big andloose flocs; while the function of cations lie in their supplying electrolyte and condensing the electric double layer to form dense flocs and the high-concentration cations can attract many negative electric flocs. Then the large flocs are to be formed. For the great amount of organic compounds and the variation of salinity in the Changjiang Estuary, the combining flocculation model can successfully demonstrate the mechanism of the formation of the fine-grained sediments in the high-turbid area.(5) The concentration of particle metal of the suspended sediments has an decreased trend from Xuliujing area to the sea in the spatial distribution, but the highest value occurs in the Turbidity Maximum. In a tidal cycle, the concentration of particle metal changes with the current speed. The same trend occurs in the distribution of particle metal in the bed sediment. In the vertical distribution, the concentration of particle metal decreases from the surface to the bottom. The analysis of relativity shows that the particle metal in the suspended sediment has a positive relation with POC% and has a negative relation with S; most of all kinds of particle metal has a negative relation with TSS and has a positive relation with the percent of fine grained sediment (<32um) . The transportation of metal is controlled by Fe-Mn oxide and organic matter in the suspended sediment. In the Turbidity Maximum of the Changjiang Estuary, the flocculation often occurs for the high concentration of fine grained sediment in the flood season. Meanwhile, the metal absorbed by the fine sediment settles down on the bed load. So the particle metal is higher in the bed sediment, especially in the fluid mud in the flood season. In the dry season, the bed sediment resuspention often occurs caused that the metal will be released into the water or changed into the particle metal of the suspended sediment. Because the coarsening bed sediment decreases the absorption to the metal, the particle metal in the bed sediment decreases in the dry season.
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