Importance of acid polysaccharides for Th-234 complexation to marine organic matter
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In order to study the role of polysaccharides (PS) in the colloidal organic matter (COM) pool for complexing 234Th, controlled laboratory experiments were carried out to determine the chemical nature of the strong Th(IV) binding to macromolecular organic ligands (> 1 kDa). The partition coefficient of 234Th between marine COM and solution, Kc, is higher than that for any known marine mineral sorbent. PS-enriched fractions of COM had the highest partition coefficient (Kc) of any sorbent for 234Th. Kc of 234Th and other metals, including Fe, Mn, Zn, Pb, and Pu, were up to an order of magnitude higher than that for bulk COM. Most importantly, log Kc values correlated linearly with the fraction of PS-enriched carbon (fPS) of marine COM, as Kc = Kc(O) 102.2fPS. The log Kc value of 7.9 of the pure PS end-member fraction was very similar to the highest values obtained for model acid PS (log Kc 8). A value for the conditional stability constant for Th binding to the pure PS end member of 107.8 could be determined from a concentration of strong acid binding sites (with a pKa 3) of 1.3 mmol g-1 COM. Through the novel use of gradient gel electrophoresis (including two-dimensional polyacrylamide gel electrophoresis), the strong Th(IV) binding ligand was shown to be 13 kDa in size and to have strong acidic functional groups. We propose that the observed variability of OC: 234Th ratios in suspended, and sinking matter in the ocean might be caused by the variability of PS content.
