Assessment and comparison of Anopore and cross flow filtration methods for the determination of dissolved iron size fractionation into soluble and colloidal phases in seawater

Jessica N. Fitzsimmons, Edward A. Boyle

Limnol. Oceanogr. Methods 12:246-263 (2014) | DOI: 10.4319/lom.2014.12.246

ABSTRACT: The two most frequently used methods for determining the size fractionation of dissolved iron (dFe) in seawater, 0.02 μm pore size Anopore membrane filtration and cross flow filtration using a 10 kDa regenerated cellulose filter, were evaluated and compared. Anopore filtration was found to produce consistent soluble Fe (sFe) concentrations in the filtrate even after > 1 L unfiltered seawater was filtered, indicating that clogging is not an issue for typical open ocean filtration volumes. Cross flow filtration (CFF) only achieved a 70% to 75% mass balance regardless of flow rate and seawater preconditioning; however, Fe losses arose largely from the colloidal (not soluble) size fraction, and Fe loss was constrained to Fe clogging in/on the CFF membrane. Both Anopore membrane and cross flow filtration methods reproducibly size fractionated dFe in seawater samples. Additionally, sFe arising from these two filtration methods were compared for the first time using seawater samples from the North Pacific and Atlantic Oceans. sFe from CFF was always lower than sFe from Anopore filtration, with the ratio of sFe in CFF/Anopore filtration averaging 68 ± 23% (n = 23). This sFe difference is attributed to a combination of the smaller effective pore size of the regenerated cellulose CFF system and the spatial variability in the concentration and composition of dFe in the 10 kDa - 0.02 μm size fraction. Finally, the advantages and disadvantages of each method were reviewed to offer future users a suite of factors with which to choose their ideal ultrafiltration method.