Evaluation of two cross-flow ultrafiltration membranes for isolating marine organic colloids

Laboratory and field studies were performed to evaluate two 1 kilo-Dalton (kD) cross-flow ultrafiltration (CFF) membranes (a Millipore Prep-scale CFF membrane constructed primarily from regenerated cellulose and an Amicon CFF polysulfone membrane) to isolate colloids (operationally defined as particles or macromolecules between 1 kD and 0.2–1 μm) from sea water. We focused on three crucial aspects when applying the CFF technique: retention characteristics, sorptive potential and ultrafilter breakthrough. Lab results showed that both CFF systems retained ≥91% of a 3000 nominal molecular weight (NMW) dextran standard, consistent with the manufacturerʹs rated cutoff. The Millipore membrane showed essentially no loss of a dextran standard, while 33% was lost for the same molecule onto the Amicon CFF membrane. Both membranes showed higher losses of a protein standard (Lactalbumin) added to sea water. For bulk organic carbon (OC), both membranes usually had reasonable recovery (100±10%) as long as the membranes were preconditioned. This was true for both lab experiments and field investigations in open ocean water off Bermuda. However, data from 234Th and 230Th analysis of samples from a station off Bermuda showed very large losses and hence low recovery from CFF. Results of these fractionated OC and 234Th distributions are also discussed in the context of prior studies. Ultrafilter breakthrough of both high molecular weight (HMW) and low molecular weight (LMW) compounds may occur throughout the CFF process, especially when processing coastal sea water where COC is relatively enriched. A permeation coefficient model provides an overall reasonable fit to the data characterizing the permeation behaviour of CFF; the retentate prediction based on the model indicates that breakthrough becomes more significant after the concentration factor (cf) is higher than 5, which implies that fractionation of organic components increases at higher cf.