Partition coefficients of the radionuclide into colloidal organic matter from seawater of the west Pacific Ocean and the northern Gulf of Mexico

<p>Partition coefficients of the radionuclide into colloidal organic matter.&nbsp;To study the binding mechanisms of radionuclides to organic moieties in colloidal organic matter (COM),marine colloids (1 kDa–0.2 μm) were isolated by cross-flow ultrafiltration from seawater of the west Pacific Ocean and the northern Gulf of Mexico. For the same purpose, exopolymeric substances (EPS) produced by laboratory cultured diatoms were collected as well. In our study areas, colloidal organic carbon (COC) concentrations ranged from 6.5 to 202 μg-C/L in the Pacific Ocean, and were 808 μg-C/L in the Gulf of Mexico. The COM compositions (organic carbon, organic nitrogen, proteins, total hydrolysable amino acids, total polysaccharides, uronic acids, hydroxamate siderophores, hydroquinone) were quantified to examine the relationships between partition coefficients (Kc) of five different radionuclides, 234Th, 233Pa, 210Pb, 210Po and 7Be, and concentration ratios to COC of individual chelating biomolecules that could potentially act as a chelating moiety. The range of partition coefficients (Kc, reported as logKc) of radionuclides between water and the different colloidal materials was 5.12 to 5.85 for 234Th, 5.19 to 6.01 for 233Pa, 4.21 to 4.85 for 210Pb, 4.87 to 5.68 for 210Po, and 4.49 to 4.92 for 7Be, similar to values previously reported for lab and field determinations under different particle concentrations. While any relationship obtained between Kc and abundance of specific moieties could not be taken as proving the existence of colloidal organic binding ligands for the different radionuclides, it could suggest possible organic moieties involved in the scavenging of these natural radionuclides. Together with results from isoelectric focusing of radiolabeled COM, we conclude that binding to different biomolecules is nuclide-specific, with colloidal hydroxamate siderophoric moieties being important for the binding of Th and Pa radionuclides. Hydroquinones/ quinone (HQ/Q) facilitated redox and chelation reactions seem to be involved in the binding of Pa and Be. However, the actual mechanisms are not clear. Individual amino acids, proteins, total polysaccharides and uronic acids did not yield significant relationships with logKc values of the different radionuclides. Nonetheless, our results provide new insights into the relative importance of different potential ligand moieties in COM in the binding and possible scavenging of specific radionuclides in the ocean.</p>放射性核素在胶体有机质中的分配系数。为探究放射性核素与胶体有机质（colloidal organic matter, COM）中有机官能团的结合机制，本研究通过错流超滤法从西太平洋及墨西哥湾北部的海水中分离得到了粒径范围为1 kDa–0.2 μm的海洋胶体。为达成同一研究目标，本研究同时收集了实验室培养硅藻所产生的胞外聚合物（exopolymeric substances, EPS）。本研究的采样区域中，太平洋海域的胶体有机碳（colloidal organic carbon, COC）浓度范围为6.5~202 μg-C/L，墨西哥湾海域的COC浓度则为808 μg-C/L。本研究对COM的组成成分（有机碳、有机氮、蛋白质、总水解氨基酸、总多糖、糖醛酸、异羟肟酸铁载体、对苯二酚）进行了定量分析，以探究5种放射性核素——234Th、233Pa、210Pb、210Po及7Be的分配系数（Kc）与潜在可作为螯合官能团的单一螯合生物分子相对于COC的浓度比值之间的关联。各放射性核素在水相与不同胶体基质间的分配系数（以logKc计）范围如下：234Th为5.12~5.85，233Pa为5.19~6.01，210Pb为4.21~4.85，210Po为4.87~5.68，7Be为4.49~4.92，该范围与此前在不同颗粒物浓度下通过实验室及现场测定得到的结果相近。尽管通过Kc与特定官能团丰度之间的关联无法直接证明不同放射性核素存在胶体有机结合配体，但该关联可以提示参与这些天然放射性核素清除过程的潜在有机官能团。结合放射性标记COM的等电聚焦实验结果，本研究得出结论：放射性核素与不同生物分子的结合具有核素特异性，其中胶体异羟肟酸铁载体官能团对Th与Pa类放射性核素的结合过程至关重要。对苯二酚/醌（HQ/Q）介导的氧化还原与螯合反应似乎参与了Pa与Be的结合过程，但具体的作用机制仍未明确。单一氨基酸、蛋白质、总多糖及糖醛酸与各放射性核素的logKc值之间均未呈现显著关联。尽管如此，本研究的结果仍为探究海洋中不同潜在配体官能团在COM内参与特定放射性核素结合及潜在清除过程的相对重要性提供了新的见解。