A critical evaluation of tangential flow ultrafiltration for trace metal studies in freshwater systems. 1. Organic carbon

The utility of tangential flow ultrafiltration (TFUF) for size fractionatio n of natural organic matter(NOM) in freshwater streams was investigated, fo cusing on characterization or elimination of potential artifacts. Spiral-wo und polyethersulfone (PES) and regenerated cellulose (RCL) membranes with n ominal molecular weight limits of 10 kilodalton (kDa) and 100 kDa were comp ared as part of a large project assessing the utility of large volume (>5 L ) ultrafiltration for determining trace metal speciation in freshwaters. Wi th careful cleaning, reliable fractionations of carbon and trace metals in freshwater can be obtained, and a detailed protocol necessary to avoid pote ntially significant biases is presented. Both PES and RCL membranes can be cleaned efficiently to provide low carbon blanks (<0.03 mg C L-1) and recle aned at least 10 times without compromising membrane integrity. For RCL, re sidual acid from the cleaning procedure can lower the permeate pH to below 4 in low-alkalinity river samples, but this acid can be removed prior to sa mple filtration with a simple treatment. Mass balance experiments with stre am NOM show good recovery (99.7% +/- 9.5%, n = 67). Sorption losses are ver y low ton average, <2%) to RCL membranes hut are higher for PES. Replicate precision is typically better than +/-5% for carbon. There are significant differences between PES and RCL membranes: permeate carbon levels are simil ar, but RCL predicts a larger >10 kDa fraction and carbon sorption is more significant on PES. These findings could be explained by greater hydrophobi c interactions of dissolved organic carbon with PES and charge interactions with RCL. A strong negative correlation is observed between the percent of colloidal carbon (10 kDa - 0.4 mu m) and the log specific conductance of s treamwaters (R-2 = 0.71).