Determination Of Cosmogenic ³²P And ³³P By Liquid Scintillation Spectrometer And Its Environmental Implications

Phosphorus is one of important biogenic elements in marine ecosystems.Cosmogenic nuclides ³²P and ³³P are ideal tracers for studying the dynamics processes of marine P-cycle process.Liquid scintillation counting is widely used because of its advantages,determining ³²P and ³³P simultaneously in environmental samples.Given that atmospheric input is the main way for ³²P and ³³P to enter the ocean,studies of estuaries and adjacent areas using ³²P and ³³P should initially determine the atmospheric deposition fluxes of ³²P and ³³P.At the same time,due to the particulate activity of P,resuspension affects the atmospheric deposition flux of P isotopes.The behavior of radioactive P in offshore marine waters is influenced by the settling and resuspension of particulate matter.What’s more,to use radioactive P for tracer studies,its geochemical behavior in the aqueous environment also needs to be studied.In addition,clay minerals,an important part of suspended particulate matter and sediments,have an important effect on P sorption.In this thesis,a methodology for the simultaneous determination of ³²P and ³³P in environmental samples was developed based on liquid scintillation counting by optimizing the measuring conditions and the choice of the nuclide energy interval.Also,analytical method for the determination of ³²P and ³³P in environmental samples by Quantulus GCT 6220 was established.By this method,atmospheric deposition observations were selected from the Shanghai area near the East China Sea,and the variation of atmospheric deposition of nuclides and the influencing factors were investigated by analysis of ³²P and ³³P in single rainfall events.Then the sorption process of ³²P by typical clay minerals（illite）was studied.The main results are as follows:（1）The Quantulus GCT 6220 measurement method was developed and the measuring conditions were optimized firstly:The results showed that the counting efficiency was high when the volume ratio of cocktail to sample solution was greater than 2,the volume of mixed solution was greater than 10 m L,the light avoidance time was greater than 6 h,and pH of the solution was 6～7.While using Quantulus GCT 6220 to determine ³²P and ³³P,the counting range of ³²P was selected to be 150～1710 ke V and that of ³³P(replaced by ¹⁴C)to be 0～150 ke V.The interference between simultaneous measurements of ³²P and ³³P could be eliminated by subtracting the contribution of ³²P at 150～0 Ke V from the single nuclide measurements,with measurement efficiencies of64.6±3.7%(³²P)and 54.8±8.7%(³³P),respectively.The experimental method in this thesis could be applied to analyze ³²P and ³³P in environmental samples.Based on this method,the lower limits of detection were obtained.（2）Based on the above established determining method,observations of ³²P and ³³P in atmospheric deposition in Shanghai during 2021 were carried out.The results showed that volume-weighted activities of ³²P and ³³P varied widely from 0.25±0.06 to1.10±0.06 dpm/L and 0.05±0.04 to 0.87±0.06 dpm/L during single rainfall events,while it was observed that activity concentrations of ³²P and ³³P were lower in August with more rainfall,than those in November with less rainfall.However,³³P/³²P activity ratio varied smaller in comparison with those of activity concentrations of ³²P and ³³P,fluctuating between 0.43±0.02 and 1.15±0.06,where higher ³³P/³²P activity ratio could be observed under the influence of special weather conditions such as typhoons.A decrease in nuclide activity was observed during continuous rainfall,suggesting that rain has a diluting effect on ³²P and ³³P in atmosphere.Comparison of the different methods for calculating monthly depositional fluxes,despite the differences in the results,it was feasible to estimate the monthly fluxes within a certain error from a limited number of samples in a month,when it was considering the short half-life of³²P.The derived monthly depositional fluxes for ³²P and ³³P in Shanghai were 49.4dpm/m²/month and 52.8 dpm/m²/month,respectively,which are similar to those in Mumbai,India.（3）As the most abundant clay mineral in Yangtze estuary,the sorption of P by illite is not negligible.Simulating the marine environment,laboratory sorption experiments of³²P were carried out.The results showed that the sorption kinetic process of ³²P could be divided into three stages:rapid sorption,slow sorption and sorption equilibrium,which were in accordance with the quasi-secondary kinetic equation.As the temperature increased,the time to reach sorption equilibrium was shortened.Freundlich model could better describe the isothermal sorption of ³²P by illite.The sorption process of ³²P on illite was observed as a spontaneous,heat-absorbing,entropy-increasing process.Results under different environmental conditions:The sorption percentage increased continuously with increasing concentration of particulate matter,and the sorption rate showed a trend of fast and then slow,finally reaching the sorption equilibrium.Low pH and low salinity was favorable to the sorption of ³²P on illite.Phosphate and humic acid inhibit the sorption of ³²P on illite.In summary,based on the establishment of a method for the determination of ³²P and³³P in environmental samples by Quantulus GCT 6220,this thesis observed the atmospheric depositional fluxes of ³²P and ³³P in single rainfall events in Shanghai in2021,analyzing the variation characteristics of the deposition fluxes and their influencing factors.In addition,different calculating methods for monthly depositional fluxes were compared,and monthly depositional fluxes in Shanghai were extrapolated,confirming the feasibility of the method,which would help to provide new ideas for subsequent analytical studies of atmospheric depositional fluxes of the two nuclides.In addition,the study of the mechanism of ³²P sorption by illite and different environmental factors also provided indispensable basic data for the study of marine P-cycle processes using isotope tracing techniques.