Selenium species removal by nanofiltration: Determination of retention mechanisms.

Selenium (Se) is a dissolved oxyanion drinking water contaminant requiring appropriate removal technologies. The removal of selenite (SeIV) and selenite (SeVI) with nanofiltration (NF) was investigated with an emphasis on the role of Se speciation and membrane charge screening on the retention mechanisms. The pH (2 to 12) showed strong pH dependence of Se retention, which was due to the speciation. No significant impact of salinity was observed by increasing NaCl concentration from 0.58 to 20 g/L. Application of the Donnan steric pore partitioning model with dielectric exclusion (DSPM-DE) showed that Donnan exclusion was the dominant retention mechanism for the oxyanions Se species. Nine different organic matter (OM) types were investigated at 10 mgC/L to determine if OM affects Se retention. Only OM characterised by negatively charged fractions, such as humic acid (HA), enhanced Se retention with NF270 of up to 20% for SeIV and 10% for SeVI. This was explained by enhanced Donnan exclusion. NF270 was effective in removing Se from real water (Gahard groundwater, Ille et Vilaine, France). The EU guideline (20 µg/L) of Se in drinking water was achieved with comparable performance to OM-free experiments using synthetic waters.