Sorption irreversibility and coagulation behavior of 234Th with marine organic matter

Abstract

Abstract The partitioning of 234 Th to natural organic matter (NOM) in the colloidal size range (1 kDa–0.1 μm) was evaluated in order to examine the sorption and coagulation behavior of marine colloidal organic matter. Colloids were isolated using large volume cross-flow ultrafiltration and the partitioning of 234 Th was quantified using stirred cell ultrafiltration and radioactive assay. The uptake of 234 Th by NOM is irreversible over a period of 5 days, implying that over the mean life of 234 Th, very little release of 234 Th would occur after binding to NOM. Furthermore, the Th–NOM complex is stronger than the Th–EDTA complex, as EDTA was unable to displace the 234 Th from its association with NOM. The extent of the initial partitioning of 234 Th to suspended matter and colloids is similar and independent of pH in the range from 3 to 9. Coagulation experiments show that 234 Th complexed with low molecular weight (1–10 kDa) colloids is transferred to larger (>0.1 μm) filter retained fractions. However, 234 Th is transferred to a greater extent than is organic matter and this results in greater partitioning coefficients for 234 Th onto particle phases with time. The final equilibrium between 0.1 μm filter-retained and filter-passing 234 Th activity is the same regardless of whether the Th was tagged initially to colloidal or suspended matter fractions. The coagulation of colloidal organic matter, COM, consists of both fast and slow steps, with kinetic rate constants on the order of 0.02–0.03 and 0.003–0.007 h −1 , respectively. The stickiness (or collision efficiency) factor, α , for COM was experimentally determined to be 0.7(−) for seawater conditions. Using the colloidal pumping model of Honeyman and Santschi [J. Mar. Res. 47 (1989): 951], the ‘predicted’ “fast-phase” coagulation rate coefficient is 0.03 h −1 in our coagulation experiments when the measured α value and the experimental conditions are used for model inputs. These experiments demonstrate that coagulation is the dominant step in the transport of 234 Th to the particulate phase.