In this study, the effects of Na+ and Ca2+ on the apparent MWt distribution and removal efficiency of NOM by deadend ultrafiltration were investigated. NOM samples from Singapore (reservoir-based), IHSS (river-based standard) and Biohumic (soil-based) were used. The effective particle sizes and MWt distributions of the NOM, with and without ions, were analyzed by light scattering and HPSEC, respectively. The effective size of the NOM was significantly reduced by increasing the Na+ concentration. This was robably due to coil-to-globule conversion and polyelectrolyte interaction, changing the NOM conformation from linear to spherical. However, increasing the concentration of Ca2+ showed competing effects. At relatively low concentrations of Ca2+, the size of the NOM was reduced, presumably by the mechanism attributed to Na+. As the concentration of Ca2+ was further increased the NOM size increased, due to aggregation or chelation between humic acid in the NOM and Ca2+ becoming the dominant factor. Ion interactions with NOM also had a significant impact on removal by ultrafiltration. With addition of Na+, the Singapore NOM retention decreased from 52% to 29%. In contrast, the retention of Biohumic NOM increased from 45% to around 70% with an increase in Ca2+ concentration.

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