Sorption irreversibility and coagulation behavior of Th-234 with marine organic matter
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The partitioning of 234Th to natural organic matter (NOM) in the colloidal size range (1 kDa-0.1 m) 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 234Th was quantified using stirred cell ultrafiltration and radioactive assay. The uptake of 234Th by NOM is irreversible over a period of 5 days, implying that over the mean life of 234Th, very little release of 234Th 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 234Th from its association with NOM. The extent of the initial partitioning of 234Th to suspended matter and colloids is similar and independent of pH in the range from 3 to 9. Coagulation experiments show that 234Th complexed with low molecular weight (1-10 kDa) colloids is transferred to larger (> 0.1 m) filter retained fractions. However, 234Th is transferred to a greater extent than is organic matter and this results in greater partitioning coefficients for 234Th onto particle phases with time. The final equilibrium between 0.1 m filter-retained and filter-passing 234Th 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, 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 value and the experimental conditions are used for model inputs. These experiments demonstrate that coagulation is the dominant step in the transport of 234Th to the particulate phase. 2001 Elsevier Science B.V. All rights reserved.
