Optimization Of Low-activity Radium Isotope Determination In Seawater

In the open ocean,radium isotopes are useful tracers of water mixing.However,the low activity of radium in the open ocean makes it difficult to measure precisely.In the common method,making evenly MnO2-coated cartridges to enrich radium was difficult.Besides,lab safety associated with the long ashing time and ash loss during transfer were issues under concern.Therefore,in this study,we optimized the method to determine activities of radium in the open ocean.Firstly,we replaced the commonly used acrylic cartridge with a cheaper polypropylene cartridge,which took much shorter time,6 hours,to be ashed,compared with the usual 48 hours.Secondly,the MnO2coated cartridge was prepared in a matching holder where a hot acidic KMnO4 solution was circulated.The radium extraction efficiency of this MnO2-coated cartridge was 1236%higher than that prepared with the cartridge directly immersed in the same KMnO4 solution.Lastly,instead of directly transferring the ash to a measurement vial,a mixture of hydroxylamine hydrochloride and hydrochloric acid was used to completely leach the ash of the MnO2-coated cartridge for 226Ra and 228Ra,followed by coprecipitation by BaSO4.Thus,loss of ash during transferring can be avoided.In addition,the variation in the measurement efficiency of high purity germanium gamma spectrometer due to the variation in the ash weight would be avoidable.We also determine measurement efficiencies and extraction efficiencies of the optimized method and factors that affect the efficiencies.The measurement efficiency of Radium Delayed Coincidence Counter(RaDeCC)for 224Ra on MnO2-coated cartridges was 0.257-0.331,and the measurement efficiency was not influenced by the moisture content of MnO2-coated cartridges.We also found that the radium activity of the seawater can only cause 4-14%variation in the radium extraction efficiency of the MnO2-coated cartridge.And the flow rate,4-7 L min-1,employed in radium enrichment on the MnO2-coated cartridge,contributed to 3-4%variation in the radium extraction efficiency of the MnO2-coated cartridge.Futhermore,we applied the optimized method to determine radium activities in the South China Sea basin,and found that the Ra extraction efficiency of MnO2-coated cartridge in the field was lower that in the preliminary experiment conducted in the lab.However,we still got perfect long-lived radium profiles after efficiency correction.At stations SS1(14°N,116°E)and WXS(16.7763°N,110.6087°E),the activity of 226Ra was 8.7-32 dpm 100 L-1,low in the surface and increasing with depth until 1500 m at Station SS1 and 800 m at Station WXS,then stable with depth.224Ra and 228Ra were 0.34-5.9 dpm 100 L-1 and 0-19 dpm 100 L-1,respectively.224Ra and 228Ra showed similar pattern vertically,i.e.,they were high in the surface and decreased with depth until 200 m.We estimated the nitrate fluxes to be 0-6.1 mmol N m-2 d-1 at Station SS1 and 0-3.9 mmol N m-2 d-1 at Station WXS via an exponential relationship between 228Ra and nitrate.At both stations,the maximal nitrate flux appeared at the bottom of the mixing layers.Corresponding maximal organic carbol flux was 40 mmol C m-2 d-1 at Station SS1 and 26 mmol C m-2 d-1 at Station WXS.