Particulate/solution analysis of 226Ra, 230Th and 210Pb in sea water sampled by in-situ large volume filtration and sorption by manganese oxyhydroxide

A method is described for the determination of the natural series isotopes 230Th, 226Ra and 210Pb in sea water. Large volume in-situ pumps were used first to collect the particulate fraction on 1-μm membrane filters, and second to sorb the dissolved fraction from the filtered stream on to manganese oxyhydroxide-impregnated filter cartridges. The values thus obtained are compared with other data from the same oceanographic area obtained by alternative sampling and analytical methods. The dissolved Th and Pb fractions are essentially quantitatively extracted by the first of the pair of Mn-impregnated cartridges used and good data are obtained. Lower and more variable efficiencies are observed for Ra which cannot be determined so reliably. This behaviour contrasts dramatically with that reported recently in laboratory experiments at near-neutral pH, where Ra was found to behave ideally towards Mn oxyhydroxide sorption while readily hydrolyzed elements such as Th do not. This difference in marine and laboratory behaviour at neutral pH is ascribed to surface availability for hydrolyzed species in homogeneous and heterogeneous systems, respectively