Re-examination of cross-flow ultrafiltration for sampling aquatic colloids: evidence from molecular probes

Application of cross-flow ultrafiltration techniques to marine systems has greatly increased in recent years. However, the retention and permeation behavior of macromolecules and their associated trace elements during ultrafiltration is still controversial. In addition, the optimum concentration factor (CF) and the possibility for a breakthrough of high molecular weight (HMW) dissolved organic carbon (DOC) during ultrafiltration are still a matter of contention. The permeation and retention behavior of natural DOG, standard macromolecules and selected metals was examined on a 1 kDa Amicon S10N1 ultrafiltration cartridge, using molecular probes and radioactive metals spiked to natural seawater. Laboratory results from molecular probes show that significant fractions (> 40%) of low molecular weight (LMW) molecules (0.5 kDa rhodamine 6G and 0.6 kDa glutathione) are retained by a 1 kDa ultrafilter membrane, even under a CF of similar to 50. Therefore, the retention of LMW molecules can give rise to an overestimate of the colloidal fraction, especially under lower CFs. The percentage of HMW molecules passing through the I kDa membrane decreases rapidly with increasing size or MW. On average, similar to 15% of vitamin B-12 (1.3 kDa) and similar to 3% of a 3 kDa dextran pass through the 1 kDa membrane. However, permeation of a 10 kDa dextran through the 1 kDa membrane becomes negligible (< 0.6%). Thus, the permeation of HMW molecules is minimal during ultrafiltration, even under high CFs. The ultrafiltration behavior of natural DOC and LMW metals can be well predicted by a permeation model, and consistently shows an increasing concentration in the permeate with increasing CFs. Standard dextrans, rhodamine 6G and natural DOC are recovered at 92-95%, while losses of glutathione and vitamin B-12 to the membrane can be significant. Possible sorptive losses and overall mass balance of molecules are thus dominated by their physicochemical properties but not by their MWs. Most retained LMW DOC can be further removed during diafiltration whereas loss of HMW DOC during diafiltration is minimal. Since retention of LMW molecules is the main problem rather than the permeation of HMW molecules, a high CF (> 40) is recommended for isolating marine colloids by ultrafiltration. (C) 2000 Elsevier Science B.V. All rights reserved.