Micellar enhanced ultrafiltration (MEUF) of metal cations with oleylethoxycarboxylate

•We apply RO90, a pH responsive surfactant, for metal ion removal via MEUF.•The surfactant has a low cmc value.•Removal efficiency for several cations is >95% for [RO90]/[Me2+]=10.•Simultaneous ion removal is controlled by the complex formation with RO90.

The technical grade nonaoxyethylene oleylether carboxylic acid RO90 is investigated as surfactant for heavy metal ion removal via micellar enhanced ultrafiltration (MEUF). In contrast to sodium dodecylsulfate (SDS), which is a standard surfactant for this purpose, RO90 has a much lower cmc (~0.014 g L−1) and forms larger aggregates (~8 nm). These properties render RO90 a much more efficient surfactant for MEUF than SDS. The surfactant can be filtered out very efficiently using a membrane with a MWCO ≤10 kDa and the gel-layer concentration is about 170 g L−1. For a molar [RO90]/[Me2+] ratio of about 10, metal ion removal efficiency is >95% for the investigated divalent cations Mg2+, Ni2+, Cd2+, Cu2+, Fe2+, and Zn2+. The simultaneous removal results in the following order: Fe2+~Cu2+>Cd2+>Zn2+>Ni2+>Mg2+, which is a result of the different complex binding constants as shown earlier by Bunting and Thong for different alkylcarboxylates. This shows that RO90 in MEUF is a selectively binding surfactant with superior properties compared to so far employed surfactants.