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Comparison of two commonly used techniques for molecular weight determination of natural organics, ultrafiltration (UF) fractionation and high-performance size exclusion chromatography (SEC), shows that neither technique gives absolute measures of molecular weight. Investigations of International Humic Substances Society standard humic and fulvic acids as well as natural organic matter concentrated from surface freshwaters show that charge effects and solution conditions are important in both SEC and UF fractionation with various components of the natural organics being affected differently. Membranes with a smaller molecular weight cutoff (MWCO) produce permeates with a lower UV/DOC ratio, suggesting that the more aromatic components of natural organics are removed by the lower molecular weight cutoff membranes. Variation in ionic strength has little effect on the rejection of humic acid fractions but does significantly influence the rejection of low molecular weight acids. pH and organic concentration do not affect DOC rejection significantly over the pH range of 4.5-10 and the DOC concentration range of 15-60 mgL(-1). These results indicate that UF should not be applied for quantitative "size" analysis unless performed under well-defined conditions. If performed under conditions appropriate to water treatment, UF fractionation can give information of direct applicability to treatment such as the MWCO required to achieve significant organics removal.

Charge Effects in the Fractionation of Natural Organics Using Ultrafiltration