Importance of acid polysaccharides for 234 Th complexation to marine organic matter

In order to study the role of polysaccharides (PS) in the colloidal organic matter (COM) pool for complexing 234 Th, 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 234 Th between marine COM and solution, K c , is higher than that for any known marine mineral sorbent. PS‐enriched fractions of COM had the highest partition coefficient ( K c ) of any sorbent for 234 Th. K c of 234 Th 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 K c values correlated linearly with the fraction of PS‐enriched carbon (ƒ PS ) of marine COM, as K c 5 K c (0) × 10 2.2ƒPS The log K c value of ~7.9 of the pure PS end‐member fraction was very similar to the highest values obtained for model acid PS (log K c ~ 8). A value for the conditional stability constant for Th binding to the pure PS end member of 10 7.8 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: 234 Th ratios in suspended, and sinking matter in the ocean might be caused by the variability of PS content.