On the integrity of cross-flow filtration for collecting marine organic colloids
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The application of cross-flow filtration (CFF) to separate organic colloids with their sorbed load from dissolved components in seawater has been tested. Investigations have aimed at deconvoluting and quantifying several aspects of seawater CFF integrity: organic carbon blanks, membrane interactions (for both organic colloids and trace organic constituents), and the effective filter cut-off. A dynamic feed, permeate, and retentate sampling protocol during regular CFF fractionation of coastal seawater organic carbon and macromolecular standards was used to gain diagnostic information about sampling integrity. The organic carbon blank of the membrane module was significant. Large amounts of low-C water were required to flush the system to attain acceptable levels in what appears to be a diffusion-limited conditioning process. Observed decreasing permeate flux as a function of concentration factor (CF) was shown to be directly correlated with CFF colloid-fouling. A series of experiments were performed with a set of standard macromolecules, added at realistic seawater concentrations, to distinguish colloidal losses to the membrane from breakthrough. It was found that large fractions of carbohydrate, proteinaceous, and synthetic amphiphilic macromolecules became associated with the Osmonics PT1 (polysulfone, nominal cut-off 1.0 kD; 1 kD = 1000 daltons) CFF all-glass-and-stainless-steel system while partitioning between the seawater suspension and the membrane slowly reached steady state. Without further losses to the membrane, the retention coefficients for the colloid standards were determined, and an inherent seawater colloidal suspension cut-off at similar to 50 kD was found. Comparisons with the membrane science literature and theoretical calculations an presented to rationalize the observed artifacts and to direct optimization schemes.