Geochemical Behavior And Release Mechanism Of Vanadium In Sediments Of Jiulong River Estuary

Vanadium is one of the essential trace elements for humans and animals,but if consumed in large amounts,it can lead to vanadium poisoning,which in severe cases can lead to death.With the development of industrialization and urbanization,a large number of pollutants from industry,cities and agriculture are pumped into the sea.As a transition zone between land and sea,estuaries and coastal areas are subjected to enormous pollution,much of which is deposited along with coastal sediments.Vanadium in water will be absorbed by suspended particles and quickly accumulate in sediments after particle precipitation.Meanwhile,vanadium in sediments will also be released into water under certain conditions,resulting in vanadium pollution.Therefore,sediments are both sources and sinks of vanadium pollution.In recent years,with the intensification of pollution control from external sources such as industrial and agricultural wastewater,the contribution of sediments as endogenous pollution sources to water pollution is gradually increasing.Most of these pollutants are transported and transformed in the microenvironment at the sediment-water interface.Therefore,the study of many environmental pollution processes has also developed from traditional scale to micro scale(the smallest nano-scale).This paper selected the Jiulong river estuary,the second big river of Fujian,pore water and sediment in Jiulong river estuary area as the research object,using two kinds of high resolution in situ sampling technology-Diffusive Gradients in Thin-films(DGT)technique and High resolution dialysis devices(HR-Peeper)technology,in the spring,summer,autumn and winter four quarters sample respectively.The distribution characteristics of vanadium in surface sediments and pore water of Jiulong River Estuary and the geochemical behavior of vanadium migration and transformation at sediment-water interface were discussed under spatiotemporal changes.And the mechanism of vanadium release and resupply between sediment and pore water was analyzed by DIFS model.The main results obtained in this paper are as follows:(1)The results of sediment physicochemical parameter analysis show that the total organic matter content is higher in summer,which is due to the high productivity in summer.The unsteady form of vanadium is mainly reducible state.From the perspective of spatial distribution,the highest content of bioavailable forms was found in JE1,which indicated that the pollution potential of vanadium in sediments in the mangrove wetland was greater.From the perspective of seasonal variation,the bioavailable content and pollution potential are the highest in summer,and the pollution potential is the lowest in winter.(2)The seasonal comparison results of dissolved vanadium and DGT-Labile vanadium concentration showed that in July,summer,the highest temperature,intense biological activities,high biological growth and metabolism,making the surface sediments with high temperature,high oxygen content and high organic matter content,resulting in the highest content of dissolved vanadium and DGT-Labile vanadium.However,in January of winter,the contents of dissolved vanadium and DGT-Labile vanadium were the lowest due to low temperature,low oxygen and low organic matter.(3)The vertical distribution of DGT-Labile vanadium shows that the distribution characteristics of DGT-Labile vanadium are generally high in the upper part and low in the lower part.The vertical distribution characteristics of dissolved vanadium indicate that the distribution characteristics and migration transformation of vanadium in the study area can be divided into four types.The first one is that the upper part is high in the vertical direction,and the concentration gradually decreases with the increase of depth.This is because under the conditions of low temperature,low organic matter content and low biological activity,the vanadium in the pore water mainly comes from the overlying water.The second is that the concentration in the vertical direction remains low at the upper part,and gradually increases with the increase of depth to the lower part.There is a significant positive correlation between dissolved vanadium and dissolved manganese and iron at these points,indicating that the release of vanadium into pore water is due to the reduction and dissolution of adsorbed phases(such as manganese and iron(hydrogen)oxides)and low oxygen conditions.The third is a crest at the surface,followed by a lower concentration.This is because the surface sediments are rich in a large amount of active organic matter.Under the action of aerobic bacteria,the organic matter is decomposed and a large amount of vanadium is rapidly released,which is quickly adsorbed by iron and manganese oxides in the process of upper and lower diffusion.The fourth is that the concentration increases continuously from the top with the increase of depth and decreases at the bottom,which is caused by the release of vanadium by the upper layer controlled by aerobic bacteria and the reduction of Fe and Mn,and vanadium is reduced out at the bottom.(4)The study on the exchange flux between sediment and overlying water shows that,in different seasons,vanadium is mainly difused from overlying water to pore water in January,and the diffusion flux is the lowest.In July,the migration flux of vanadium from sediment to overlying water is the largest.From the influencing factors of flux,reduction of iron and manganese will lead to the reactivation of vanadium,but the oxidation zone on the surface will inhibit the reduction of iron and manganese,and control the migration of vanadium upward in the overlaying water.Under the influence of the properties of vanadium itself,vanadium can be oxidized into a high-priced state,which improves its solubility.If the adsorption rate of iron and manganese oxide is exceeded,the oxidation conditions will promote the migration of vanadium upward to the overflowing water.When aerobic bacteria decompose active organic matter,a large amount of dissolved vanadium will be released,resulting in a higher vanadium content in the surface layer and promoting the release of vanadium from the sediment to the overlying water.(5)The simulation results of the DIFS model show that in January,the desorption rate of solid phase active vanadium in sediments is low and the risk of release is small,but the gradual accumulation of a large number of solid phase active vanadium will bring a great risk of vanadium release.In April,a large amount of solid phase active vanadium was released into pore water,and the ability of sediment to replenishing pore water was significantly enhanced,and the potential of vanadium release was the highest.In July,due to the large consumption of solid phase active vanadium in April,the replenishing capacity of the sediment to the pore water vanadium decreased.In October,the solid phase active vanadium in sediments began to accumulate slowly again,and the replenishing capacity of vanadium in pore water of sediments increased to some extent.