Colloidal aluminum and iron in seawater: An intercomparison between various cross-flow ultrafiltration systems

Aluminum (Al) and iron (Fe) were measured on samples from 13 different cross-flow ultrafiltration (CFF) systems representing five manufacturers at both Colloid Intercomparison experiments conducted in August 1994, at Woods Hole Oceanographic Institute (WHOI), Massachusetts, and January 1995 in Kona, Hawaii. Most systems displayed characteristics of contamination for Al and scavenging of Fe. No correlation was found between these artifacts and filter type. The percent of the colloidal fraction associated with Al and Fe was calculated, but showed that, for Al, Amicons tended to retain more of this fraction than did the Filtrons, Osmonics or Membrex systems tested. The percentage of colloidal Al identified in the Pacific Ocean sample ranged from < 1% to ˜ 11%. There was also substantial variability between the colloidal fractions retained by individual filter types of the same manufacturer; this apparent inconsistency between the same type of filter and manufacturer is attributed to differences in system composition and system clean-up. Conclusions are made based on the need for future work concerning the monitoring of mass balances in all fractions during an ultrafiltration run, and the understanding and reduction of trace-metal sources or sinks in these systems.