Effects Of Phosphorus Speciation And Environmental Factors On Phosphorus Adsorption And Desorption In Surface Sediments From The Central Bohai Sea

Phosphorus（P）is a basic nutrient of life,which plays a key role in regulating primary productivity in Marine ecosystems.Phosphorus forms in sediments are different according to different binding forms,and its relative content can indicate the source and sedimentary environment of Phosphorus.It also determines the potential content of Bioavailable phosphorus（BAP）in sediments that can participate in sediment-water interface exchange.Phosphate exchange at sediment-water interface is an important process that affects phosphorus migration and transformation,bioavailability and phosphorus concentration in overlying water,and sediment adsorption and desorption of phosphorus is one of the main forms of this process.At present,studies on phosphorus forms and adsorption-desorption in sediments of Bohai Sea are mostly focused on small scale spatial range such as estuaries and bays,while there are few relevant studies on large spatial scale range such as the central sea area of Bohai Sea.Therefore,this study takes five regions in the central Bohai Sea area（Tangshan coast,Qinhuangdao coast,northeastern Bohai Sea,Bohai Strait,and southwestern Bohai Sea）as the research object.The occurrence forms of phosphorus in surface sediments of the study area were divided into Organic phosphorus（OP）and Inorganic phosphorus（IP）,among which Inorganic phosphorus included:Exchangeable phosphorus（Ex-P）,Fe and Al oxidation state（Fe/Al-P）,Occluded phosphorus（Oc-p）,Detrital phosphorus（De-P）and Calcium-bound phosphorus（Ca-P）were used to determine the composition and content of phosphorus in sediment samples from 23 sampling sites by fractional leaching method,and the potential BAP content was calculated.Its distribution characteristics and influencing factors were analyzed.In addition,the surface sediment samples from Bohai Strait were used as the research object,and the adsorption and desorption kinetics and thermodynamics experiments were carried out by changing the disturbance strength,particle size and temperature,to study the effects of different environmental factors on phosphorus adsorption and desorption by sediment.The phosphorus pollution of surface sediments in the central Bohai Sea was evaluated by different evaluation methods.The main research results are as follows:（1）Inorganic phosphorus（IP）is the main form of Total phosphorus（TP）in surface sediments of the central Bohai Sea.IP content in sediments of Tangshan coast,Qinhuangdao coast,northeast Bohai Sea,Bohai Strait and southwest Bohai Sea accounted for 78.39%,79.06%,71.46%,84.60%and 81.46%of TP content on average,respectively.（2）De-P is the main occurrence form of IP in the study area.Dep accounts for54.02%,52.12%,33.33%,69.41%and 57.28%of TP in sediments of Tangshan coast,Qinhuangdao coast,northeastern Bohai Sea,Bohai Strait and southwestern Bohai Sea,respectively.The contents of various phosphorus forms in IP were in the order of De-P>Oc-P>Fe/Al-P>Ca-P>Ex-P.According to correlation analysis,river input,contents of Al,Fe and Ca in sediments,grain size of sediments and sedimentary environment are the main factors affecting the contents and distribution of phosphorus in surface sediments of the central Bohai Sea.（3）According to the analysis of potential BAP content in sediments,the potential BAP content in surface sediments of Tangshan coast,Qinhuangdao coast,northeast Bohai Sea and southwest Bohai Sea accounts for 44.77%,46.94%,64.87%and 40.54%of TP,respectively,which has a strong potential to release phosphorus to water bodies.It provides the relevant information of sedimentation environment for the central bohai Sea area.（4）The sediment’s zero equilibrium phosphate concentration（EPC₀）of Bohai Strait is far less than the concentration of phosphate in the overlying water（EPC₀=0.36 mg/L<C_P=0.0047 mg/L）,showing the characteristics of phosphate released by the surface sediments to the water body and acting as the"Source".（5）According to the kinetics experiment,the phosphorus adsorption and desorption process of surface sediments in Bohai strait was rapid from 0 to 1h,and changed to slow after 1h,and tended to equilibrium after 18h.Lagergren equation and Exp Assoc equation have better fitting effect on the kinetics of phosphorus adsorption and desorption in surface sediments of Bohai Strait.Through thermodynamic model fitting,Henry model has better fitting effect on phosphorus adsorption and desorption of surface sediments in Bohai Strait under different disturbance and temperature conditions,while Langmuir model has the best fitting effect on phosphorus desorption of surface sediments under different particle size conditions.In a certain range,the increase of temperature,the increase of disturbance and the smaller the grain size of sediments will enhance the phosphorus adsorption and desorption capacity of surface sediments in Bohai Strait.It provides a theoretical reference for the change of circulating state of phosphorus solubilization between sediment and water body and the control of internal phosphorus pollution in sediment.（6）The content of TP in surface sediments in the study area ranged from 224.81mg/kg to 445.45mg/kg,with an average of 342.93mg/kg.According to the Sediment quality assessment guidelines of Ontario,Canada,the ecotoxicity of TP in the study area was at a safe level.Through statistical mathematical calculation and analysis,the environmental background value of TP in the study area ranged from 281.49 to 465.81mg/kg,and the average background value was 342.93 mg/kg.After single factor and comprehensive evaluation of TP pollution degree in sediments by using comprehensive pollution index method,it can be concluded that T2 and T4 of Tangshan coastal stations,Qinhuangdao coastal sites Q1,Q2,Q5,Q7,Bohai Strait sites Y1-Y4,and southwest Bohai sea sites Z1,Z4 are currently in the mild pollution stage,which should be paid attention to and treated.