Size Distribution Characteristics And Influence Factors Of Trace Metals In Seawater Of Jiaozhou Bay

Dissolved organic matters(DOM)are of great significance in the biogeochemical cycles of trace metals in aquatic systems.The speciation,transformation and ecological functions of trace metals are closely related to their distribution and mutual transformation among DOM with different molecular size.It is likely that different sized components may have various sources,characteristics and behaviors.In this thesis,water samples from Jiaozhou Bay were collected to analyze the size partitioning characteristics and controlling mechanisms of dissolved(<0.45?m)Cu,Cr,Cd,Pb,Ni and Co.Besides,molecular size spectra,fluorescence spectra and ultraviolet-visible absorption spectra of DOM were measured to study the influence of compositions and structures of DOM to trace metal distribution characteristics.Influence of trace metal speciation and salinity were also investigated by comparing the size distribution characteristics of different metals at different salinities.The main results and conclusions obtained are as follows:1.Dissolved race metals in Jiaozhou Bay seawater were mostly partitioned in the<1 kDa fraction,with an average percentage of 70.1%and a decreased order of Cu,Cd,Cr,Ni,Co and Pb.DOM in Jiaozhou Bay seawater was also mainly distributed in the<1 kDa size range,mostly including free amino acids and fluvic acids with lower degree of humification and aromatization.About 47.5-86.6%(70.1%on average)of the trace metals in Jiaozhou Bay seawater were measured in the<1 kDa fraction,while the proportion of the 10 kDa-0.45?m fraction was only 5.4%on average.The proportion of low molecular weight fractions(<1 kDa)in the total dissolved trace metals gradually decreased according to the order of Cu,Cd,Cr,Ni,Co and Pb.The proportions of Cu(79.0%)and Cd(77.6%)distributed in small molecular size DOM were significantly higher than that of other trace metals,and then Cr(71.5%),Ni(67.3%)and Co(66.9%),and lastly Pb(58.2%).In the horizontal direction,seawater samples from northern Jiaozhou Bay possessed lower proportions of<1 kDa fraction(64.2%)than that from the mouth and outside of the bay(74.0%).In the vertical direction,surface seawater samples seemed to have lower proportions(68.1%)of<1 kDa fraction than bottom seawater samples(73.1%).The concentrations of dissolved organic carbon in Jiaozhou Bay seawater ranged from 0.58 mg/L to 1.36 mg/L,of which 62.1-86.0%were distributed below 1kDa and2.36-10.37%were distributed above 10 kDa,showing a considerable abundance of the smallest size fraction among the total size range.Seawater from the mouth and outside of the bay were characterized with higher proportions of smaller size components than that from northern area,while bottom seawater had higher proportions than surface seawater.The spatial distribution pattern of trace metals among 1-3 kDa,3-10 kDa and10 kDa-0.45?m was not clear.Fluorescent assay revealed that six types of fluorescence were detected according to the position of fluorescence peaks based on fluorescence of model compounds,involving three types of protein-like components(D,S,B)and three types of humic-like components(A,M,C).A pronounced decrease for fluorescent intensities of protein-like fractions appeared along with the ultrafiltration,while the intensities of humic-like fluorescence varied little with the decreased of membrane cut-offs,among which C peak possessed the gentlest trend.It was suggested that the high molecular size DOM in Jiaozhou Bay seawater was mainly composed of proteins,humic acids and marine humic substances,while the small molecular size DOM was comprised of fulvic acids and amino acids.The absorption spectral slope S_275-295 gradually increased with the decrease of the molecular size of DOM,suggesting that there were more in-suit generated organics from activities of phytoplankton and microorganisms in small molecular size fractions.It also indicated that the ratio of fulvic acids to humic acids was higher for small molecular size DOM.The effect of fulvic acids became more prominent along with the decrease of molecular size of DOM.What's more,the E₂:E₃ratio showed a rising trend as molecular size of DOM decreased,demonstrating a lower aromatization degree for small molecular DOM.2.The size distribution,chemical structure and complexing ability of DOM were the major factors controlling the size spectra of trace metals in Jiaozhou Bay seawater,while the trace metal speciation and salinity also made some difference.The low molecular size dominated distribution of trace metals was a result of the higher abundance and stronger metal binding capacity of low molecular size DOM.The size spectra of trace metals and dissolved organic carbon were basically the same,indicating that the molecular size distribution of DOM played a dominant role in shaping the size spectra of trace metals in the ocean.Besides,DOM in the<1 kDa size range was mainly composed of fulvic acids,which were abundant in carboxyl,phenolic hydroxyl,alcohol hydroxyl,ketone hydroxyl groups and had higher oxidation degrees and lower condensation degrees,thus had higher complexing abilities to trace metals.Proteins and highly humified humic acids in the>1 kDa fraction only played limited roles in the complexation of trace metals.The higher abundance and stronger metal binding capacity of low molecular size dissolved organic matters determined that the trace metals in Jiaozhou Bay seawater were mainly associated with the lower size range.The proportion of low molecular weight fractions(<1 kDa)in the total dissolved trace metals gradually decreased according to the order of Cu,Cd,Cr,Ni,Co and Pb,indicating the influence of metal species to trace metal distributions.The complexation forms and degrees were different for metals with different properties,thus resulted in various size distribution characteristics.In terms of complex mechanism,the complexation of Cu was multilayer interaction and could form stable internal complexes,while Cd,Ni and Co were more likely to react with the complexing sites on the surface of DOM and formed external complexes.As for complex strength,Cu had a higher affinity with the coordination sites of low molecular size DOM and the stability of complexes could be maintained without further connection and aggregation,while Pb possessed a higher competitiveness to the coordination sites of high molecular size DOM.Proportions of high molecular size components were 37.7-47.9%and 30.0-42.4%at the salinity of 1.1 and 27.5 respectively,indicating that increasing salinity was detrimental to the complexation between high molecular size DOM and trace metals.DOM with high molecular size were flocculated and precipitated due to the increasing ion strength and removed from the water in the end,while small molecular size fractions were less affected by the increasing salinity.The flocculation of DOM in seawater became insignificant when the salinity rose to a certain degree,and then salinity might indirectly affect the distribution processes of trace metals by influencing the upper adsorption p H value of them and interfering with biological activities.Furthermore,DOM derived from phytoplankton and microbial activities increased as salinity increased,which had a smaller size than DOM originating from terrestrial higher plants.