Charge Effects in the Fractionation of Natural Organics using Ultrafiltration

Abstract

Comparison of two commonly used techniques for molecular weight determination of natural organics, UF fractionation and high performance size exclusion chromatography (SEC), show that neither technique gives absolute measures of molecular weight. Investigations of both 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 cut-off produce permeates with a lower UV/DOC ratio suggesting that the more aromatic components of natural organics are removed by the lower molecular weight cut-off 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 4.5 to 10 and the DOC concentration range of 15 to 60 mgL-1. These results indicate that UF should not be applied for quantitative “size” analysis unless operated under welldefined conditions. If operated 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.