The Research To Humic Acid And Heavy Metal Geochemistry In Sediments Of Eastern Chongming Tidal Flat

The migmatization of salt and fresh water and the ocean tidal power in the estuary lead to the deposition of a large number of sediment in the estuary region and the formation of a typical estuarine wetlands.Tidal flat sediments are mainly controlled by the hydrodynamic force(wave, tidal current),the physical and chemical processes(source,flocculation,sedimentation and redox reaction)and biological processes(excrement,bioturbation).In the nature,the interface of the oxidizing and oxygen-free environment have great geochemical significance,and it may become the enrichment place of many elements.During early diagenetic process of the sediment,many heavy metals often remigrate with Fe,Mn which are sensitive to redox condition of the environment and formed enrichment in the redox boundary layer through redox cycle of Fe,Mn. The distribution of heavy metals in sediment is controlled by the the circulation of iron and manganese.While in the early diagenesis process of the sediment,the mineralization of organic matter is the most important driving force.The mineralization of organic matter will change the physical and chemical conditions of the depositional environment and make all kinds of materials transformate,relocation and redistribution,and resulted in the exchange of materials between sediments and overlying water.The humification is a very important process in the course of the mineralization of organic matter,and the humic humus is an important product of the process. Humic acid is a very important component of the humus.Humic acid can combine the heavy metals in the environment strongly and affect the migration and transformation of heavy metals. Humic acid itself has a special effect of oxidation and reduction.The effect of humic acid on the migration and transformation of heavy metals in the sediments is of great significance.The migration of all kinds of metal ions in the environment and its accumulation and diagenesis in the sediments are relative to of complexation and absorption effects that humic acids to metal ions.In the estuary sediments,most chelate of the metal ions is with humic acids.The interaction of humic acid and metal ions have an important influence on the biogeochemical behavior of metal ions.And it plays an important regulating role to the conservation nutrients of the sediment,the formation of well-structure and the regulation of acidity-basicity,the water and soil conservation,the degradation or removal of the toxic and harmful substances in the sediment and water environment,the nutritional balance of plant elements and the structure and function of the whole sediments and aquatic ecosystem structure.The dissertation was supported by science & technology project:"The nutrient transfer and circle in wetland ecosystem in Chongming Island." In January,April,July and September 2007,we collected respectively the columnar sediments in high,middle and low tidal flat of spring,summer, autumn and winter,and test the parameters of grain size,the total organic matter,humic acids(including humic acid and fulvic acid),the total content of metal element(including Fe,Mn,Cu,Zn,Pb,Rb,Al,Zr,Cr),the total content of S and Si and heavy metals bound to iron-manganese oxides(Cu,Zn,Cr,Fe,Mn).We analysed the influencing factors that affect the migration and transformation of heavy metals at different topography positions of the tidal flat,the distribution and accumulation characteristics of humic acids and its various components in the sediment and its impac on the migration and transformation of heavy metals in the sediments.In addition,we analysed the feasibility that Zr as a geochemical marker to reflect changes of erosion and deposition,of the of the tidal flats.The main conclusions can be summed up as follows:1 The leading factors that affect the distribution and migration of heavy metals in sediments of low,middle and high tidal flat:the main factors that affect the distribution and migration of heavy metals in the low tidal flat are grain size,the sorce of matters and the role of hydrodynamic force.While compared with the low tidal flat,the function of organic matters(mainly humic acid) enhanced in the middle and high tidal flat.The distribution of heavy metals in sediment of the middle and high tidal flat is affected by the combined action of grain size of sediment,the sorce of matters,the re-migration after the deposition during the early diagenesis and the humic acids.2 The enrichment depth of Zr in the sediments can be used as geochemical markers to reflect the alluvial and eroding changes in tidal flats.Winter and spring give priority to the avlluvial fanction,and summer and autumn give priority to erosion in eastern Chongming.The seasonal cut-and-fill depth is about 10cm.3 The temporal and spatial distribution of bioavailable heavy metals:The distribution in the direction vertical to shore is the middle tidal flat＞high tidal flat＞low tidal flat.The bioavailable heavy metals show the consistent vertical distribution and is comparatively consistent with that of the total heavy metals.4 The temporal and spatial distribution of the organic mattes:The maximum content of organic matter(including total humic acids,humic acid and fulvic acid)in the surface sediments of tidal flat appears in spring or winter,and that of the minimum in summer.And the maximum in the horizontal direction is in the middle tidal flat.The seasonal changes of the humic acid and fulvic acid in the surface sediment of the middle tidal flat and high tidal flat shows the opposite distribution trend.In the low tidal flat,organic matter,including the total organic matter,the total humic acid(including humic acid and fulvic acid)decrease with depth increases on the whole,and that in the high tidal flat increase with depth increases on the whole.The vertical distribution of humic acid in the middle tidal flat shows strong consistency with that of total organic matter.The vertical distribution of spring and summer is contrary to that of autumn and winter.The distribution of humic acid in the middle tidal flat is affected by the biological surface aggregation,therefore the content in the surface layer is comparatively higher.While the maximum content of fulvic acid in the profile is not at the surface layer basically because of its strong ability to migrate,but at some depth below the surface.5 The maximum interaction between humic acid and heavy metals appears in the middle tidal flat. The correlation between humic acid and bioavailable heavy metals is low in spring,and high in summer,autumn and winter.6 The effect of humic acid and fulvic acids,different degradation products of humic acid,on the total heavy metals in sediment:Fulvic acid will promote the dissolution and migration of heavy metals in sediment because of its strong ability to move,thus reduce the heavy metal content in a certain depth.Humic acid has strong ability to complex metal ions and form a stable complex with metal ions,thus it will promote the enrichment of heavy metals at a certain depth.7 In the middle tidal flat and high tidal flat,the vertical distribution of heavy metals is affected by the combination of the cycle of iron and manganese,humic acid and fulvic acid impact.In the middle tidal flat fulvic acid plays a leading role on the distribution of heavy metals in the sediment.The peak content of humic acid and fulvic acid in the sediments of a certain depth is usually corresponding to the low content of the bioavallable heavy metals,and suppress the accumulation of the bioavailable heavy metals in the sediment.While the high content of fulvic acid at a certain depth in the high tidal flat would reduce the availability of heavy metals,And the effect of humic acid to bioavailable heavy metals is mainly to form a composite colloid with iron-manganese oxide,thereby promote the absorption of iron-manganese oxide to heavy metals and enhance the bioavalability of heavy metals.Humic acid has a comparatively stronyer effect on heavy metals in the sediment of the high tidal flat on the whole.