Environmental Geochemistry Characteristics Of Se, Te, As, Sb, Bi And Dissolved Sulifde In The Yangtze River Estuary And Its Adjacent Waters

As integral component of global ocean, marginal seas is the major blue resourcestock and has even bright exploration potential since high primary productivity. Tracemetals play crucial roles in ecosystem function and their supply therefore controls thestructure, and possibly the productivity, of marine ecosystems. Thus Understandingthe environmental geochemical cycling of these micronutrients as well as theirgeochemical characteristic facilitates and furthers marine ecology study from boththeory and practice. Environmental geochemistry of trace metals (Se, Te, As, Sb, Bi)is one of the emerging topics in marine science and becomes a focal point with theimplementation of international scientific program GEOTRACES and IMBER. Sulfuris reliable indicator of redox characteristic as carrier of electron transfer underanaerobic environment. Some sulfur-bearing compounds attract great concern as toxicpollutants. However, offshore ecosystem is degraded and threatened by growinganthropogenic activities, therefore environmental geochemistry characteristics of suchmicronutrients needs to be further investigated so as to contribute to environmentaldiagnose then restoration.Yangtze River Estuary and its adjacent waters are the typical areas underintensive influence of human activities with high primary productivity. On the basisof the field investigation during2009to2011, Se, Te, As, Sb, Bi and sulfide weredetermined to examine the content level and distribution profile, to distinguish theirsource and behavior and to identify, among the many potential factors, thosehydrographic, chemical and microbial processes that can explain the observeddistributions intermediates in this unique environment. A series of results andviewpoints were presented as follows: 1Concentrations and species of Se, Te, As, Sb, Bi were investigated in theYangtzr River Estuary and its adjacent waters.Influenced by the amount of runoff, averages of Se, Te, As, Sb, Bi and sulfide insummer were higher than those in spring. Compared with those reported in othercoastal regions and the open oceans of the world, concentrations in the Yangtze RiverEstuary and its adjacent waters were in the medium level, which did not exceed thegiven threshold. However, the essential nature and toxicity of them in natural watersdepends on not only the content but also the species. The chemical form andbioaccumulation would increase the ecological risk even if the trace concentration andmay result in the abundance exceeding toxicity thresholds at higher trophic levels.In spring, due to the high affinity of low valence and the good solubility of highvalence, Se(VI), Te(VI) and Sb(V) are the dominant species for Se, Te and Sb,respectively. In summer, species is slightly different for As and Sb controlled by theenvironmental factors and biological processes. As(III) and As(V) plays the equallyimportant role in the surface but As(V) predominates in the bottom water. It is theother way round for Sb.2Key processes controlling the distribution of Se, Te, As, Sb, Bi wereelucidated in the Yangtze River Estuary and its adjacent waters.Concentrations of Se, Te, As, Sb, Bi and sulfide presented typical regionalvariation and obvious seasonal distribution, reflecting the effect of water mass mixing,solution-sedimentation and adsorption-desorption processes. There was a horizontalgradient in the investigated area, with high values appearing along the west coastinfluenced by the terrestrial runoff from the Yangtze River, Qiantang River andZhengjiang Coastal Current. With the moving of shelf front towards the open sea,values decreased gradually and then increased and stratified in the west side of thecontinental shelf where the Kuroshio Current and Taiwan Warm Current reached.Behaviors in estuary were estimated by the relationships of Se, Te, As, Sb, Biand sulfide to salinity. Results showed that Se(IV) and Te(IV) behaved conservativelyin the whole investigated area. Se(VI) and Te(VI) had conservative behavior whensalinity is lower than30. Yet, due to the incursion of Taiwan Warm Current, the deposition of organic material and the release of sediment, there was obvious addingfor Se(VI) and Te(VI) in the high salinity waters. As(III) and As(V) exhibitednon-conservative behavior but oppositely Sb(III) and Sb(V) behaved conservativelyin the estuary. Bi behaved conservatively in spring, and showed conservative behaviorin summer within the salinity range of20-30. For dissolved sulfide, it behavedseasonally, with conservative behavior in spring and summer but non-conservativebehavior in autumn and winter.Processes of adsorption-desorption of suspended particulate material (SPM) havegreat effect on the distribution profile. It is found that Se, Te, Sb and sulfide,especially their low valence, can be desorbed from the SPM and released in the watercolumn. Further study pointed out that conditions of18-25oC and pH7.9-8.0,>27oCand pH7.9-8.0,25oC and pH<8.15, high temperature are the optimum environmentfor releasing Se, Te, Sb and S, respectively. Differently, As performed adsorbabilityto SPM and moreover the higher temperature and pH value do good to be adsorbed.Bi was adsorbed on SPM under conditions with low temperature and pH as well ashigh oxidation-reduction potential but was desorbed from SPM within the temperatureof23-25oC and the pH of7.9-8.0.3Internal relationships among Se, Te, As, Sb, Bi and sulfide and their coupledrelationship with eco-environmet were preliminarily discussed to illustratetheir ecological functions in the Yangtze River Estuary and its adjacentwaters.In spring of surface water, Te(IV) behaved similarly to Bi and were togethercontrolled by the redox condition, pH, riverine input and adsorption-desorptionprocesses. In bottom water, Se(IV), Sb(III) and Sb(V) were dependent on water massmigration, pH and solution-adsorption-desorption equilibrium. Meanwhile, they weresimilar to the property of P. By contrast, Te(VI) was controlled by the temperatureand oxidation-reduction condition. In summer of surface water, Sb(III), Sb(V) andSe(IV) originated from continent input from adjacent waters, water mass mixing andrelease of SPM and sediment. Se(VI) were influenced by dissolved sulfide and pH. Inbottom water, Bi and Te(IV) related closely, controlled by the water mass input, physical mixing, oxidation-reduction condition, as well as the solution-sedimentationand adsorption-desorption processes. Additionally, As(III) and Se(IV) varied indifferent pH condition.As essential elements, Se and As certainly related to P in our study, manifestingthe competition in being ingested by phytoplankton and the involvement in thebiological cycle. Further research pointed out the bioavailability of arsenic is the bestunder13-15oC. As non-essential elements, rather surprising, Te, Sb and Bi beenexceptionally displayed the nutrition effect as well, indicating great biologicalpotency and potential. Particularly, Te related closely to P and Bi competed to N.However, Sb behaved like both N and P.