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 be havior of macromolecules and their associated trace elements during ultrafi ltration is still controversial. In addition, the optimum concentration fac tor (CF) and the possibility for a "breakthrough" of high molecular weight (HMW) dissolved organic carbon (DOC) during ultrafiltration are still a mat ter of contention. The permeation and retention behavior of natural DOG, st andard macromolecules and selected metals was examined on a 1 kDa Amicon S1 0N1 ultrafiltration cartridge, using molecular probes and radioactive metal s 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 ult rafilter membrane, even under a CF of similar to 50. Therefore, the retenti on of LMW molecules can give rise to an overestimate of the colloidal fract ion, especially under lower CFs. The percentage of HMW molecules passing th rough the I kDa membrane decreases rapidly with increasing size or MW. On a verage, similar to 15% of vitamin B-12 (1.3 kDa) and similar to 3% of a 3 k Da 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 h igh 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, rhoda mine 6G and natural DOC are recovered at 92-95%, while losses of glutathion e and vitamin B-12 to the membrane can be significant. Possible sorptive lo sses and overall mass balance of molecules are thus dominated by their phys icochemical properties but not by their MWs. Most retained LMW DOC can be f urther removed during diafiltration whereas loss of HMW DOC during diafiltr ation is minimal. Since retention of LMW molecules is the main problem rath er than the permeation of HMW molecules, a high CF (> 40) is recommended fo r isolating marine colloids by ultrafiltration. (C) 2000 Elsevier Science B .V. All rights reserved.