Natural seawater from the Gulf of Mexico with a salinity of 35.0 was filtered and ultrafiltered sequentially through a 0.4 \u00b5m cartridge (GE-Osmonics) and a 1 kDa ultrafiltration membrane (Millipore-Amicon), to remove suspended particles and colloidal matter. 210Po and 210Pb (in equilibrium) in 1 N HNO3-2 N HCl were purchased from Eckert & Ziegler Isotopes Products. 7Be tracer was produced at the Paul Scherrer Institute, Switzerland and preserved in 0.5 N HCl. All experiments were conducted in a constant temperature laboratory to avoid the influence of changing temperature.\u00a0
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For sorption experiments, 49 ml of the <1 kDa ultrafiltrate of seawater in a stirred-cell unit was added with 1 ml of non-complexing Tris-HCl buffer solution to maintain a constant pH of 8.1 and to prevent formation of pseudo-colloids during the addition of spikes. For the single-sorbent experiments, 2 mg of inorganic particles was added into the solution while stirring, corresponding to a particle concentration of about 40 mg/l. Successively, a total of 0.2 ml of 210Pb, 210Pb and 7Be spikes, with the activities of 100 Bq, 100 Bq and 150 Bq respectively, were added drop by drop while stirring.\u00a0 The pH value was checked after adding spikes and found to be 8.1. For the single MOC experiments, procedures were the same as described above. The final concentration of HA, APS or BSA was at 1 mg/l.\u00a0
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An equilibrium time of 2 h was consistently used in all experiments. After 2 h, the particulate or colloidal phases were separated through ultrafiltration using a 1 kDa membrane (Amicon YM1). The >1 kDa retentate (final volume of ~4 ml) and aliquot of the <1 kDa ultrafiltrate or permeate solutions (10 ml) were transferred into a counting vial for \u03b3-counting.
\nFor the binary-sorbent experiments, macromolecular organic matter was added after the inorganic nanoparticles. The experimental solution was then stirred for 30 min to allow inorganic particles to interact with MOCs. After that, spikes were added. Other experimental procedures were the same as the single-sorbent experiments.
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Retentate and permeate samples were measured for 210Pb and 7Be by non-destructive gamma counting using a Canberra ultra-high purity germanium well detector. The counting efficiencies at different sample geometries (1-10 ml) at 46.5 keV for 210Pb and 477.6 keV for 7Be were calibrated using tracers with certified activities.\u00a0
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The activities of 210Po in samples were measured using \u03b1-spectrometry. For the permeate samples, 5 ml of permeate was acidified to pH <1.5 with HCl, then\u00a0 4 Bq 209Po was added, followed by ascorbic acid powder, 1 ml of hydroxylamine hydrochloride (20%) and 1 ml of sodium citrate (25%).\u00a0 Finally, the mixed solution was adjusted to a pH of 1.5. Polonium isotopes, including 209Po and 210Po, were plated on a silver disc at 90\u00baC for 4 h under stirring. Procedures for the retentate samples were the same, except that the retentate was digested with mixed acids. Then, the solution was dried to near dryness.\u00a0
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\u00a0Activities of 209Po and 210Po on silver disc were counted by a Canberra alpha analyst system. The procedural efficiencies of Po isotopes were determined by the measured 209Po and initially added 209Po activities.
Improved applications of 210Po, 210Pb and 7Be as geochemical proxies require more detailed understanding
\nof their interactions with particles. Here, laboratory sorption experiments were carried out to examine
\nthe adsorption of 210Po, 210Pb and 7Be and their fractionation on inorganic nanoparticles, including SiO2,
\nCaCO3, Al2O3, TiO2 and Fe2O3, in the presence or absence of macromolecular organic compounds (MOCs)
\nthat include humic acids (HA), acid polysaccharides (APS) and proteins (BSA), in natural seawater. Results
\nshowed that nanoparticle sorption was not greatly enhanced over that of microparticles as would
\nbe expected from their much higher specific surface areas, likely indicating their aggregation in seawater.
\nIt was found that synergistic interactions between inorganic nanoparticles, MOCs, and radionuclides
\ndetermined the sorption, although their adsorption was particle composition-dependent. MOCs enhanced
\nthe sorption of selected nuclides on most nanoparticles. On average, in the presence of MOCs, partition
\ncoefficients (Kc ) of 210Po, 210Pb, and 7Be on nanoparticles increased 2.9-, 5.0- and 5.9-fold, respectively.
\nThe effect of MOCs could be explained for 210Po and 210Pb from their different log Kc values on inorganic
\nnanoparticles. In addition, fractionation effects between 210Po and 210Pb (or between 210Pb and 7Be)
\ncould be quantified from their relative log Kc values on end-member sorbent components. Applications
\nof both 210Po\u2013210Pb and 7Be\u2013210Pb pairs as particle dynamics tracers could be more quantitative when
\nthe nature of the organic coatings is taken into account.
Activities of nuclides were calculated by the counts and counting time. The recoveries were quantified based on the total measured activity of each nuclide (including permeate and retentate phases) and initially added spike activity. The partition coefficient was calculated using the activity of each nuclide in retentate divided by that in permeate and the content of total nanoparticle.
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BCO-DMO Processing Notes:
\n*\u00a0added conventional header with dataset name, PI name, version date
\n* modified parameter names to conform with BCO-DMO naming conventions
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