Comparison of separation methods of heavy metal from surfactant micellar solutions for the recovery of surfactant

A surfactant-based membrane process, micellar-enhanced ultrafiltration (MEUF), was studied in order to remove heavy metal ions from groundwater or wastewater. Retentate is the concentrated solution of surfactants and heavy metals retained by the membrane. Because surfactant accounts for a large portion of operating costs, it is essential to recover and reuse the surfactant from the retentate stream as economically as possible. Three methods were tested for the separation of metal ions from simulated retentate of MEUF: (1) acidification followed by ultrafiltration (UF), (2) use of a chelating agent followed by UF, and (3) precipitation by ferric- and ferrocyanide followed by centrifugation. Copper and cadmium were completely separated from the surfactant solution by acidification (lowering pH to strong acidic region) followed by UF. By adding ethylenediaminetetraacetic acid (EDTA) followed by UF, the separation efficiency of copper and cadmium was 100% and 75.5%, respectively. As pH increased, iminodiacetic acid and citric acid also showed high separation efficiency due to the deprotonation of the carboxylic group. Ferricyanide precipitated with copper and cadmium, and most of surfactants were recovered after centrifugation. In terms of the recovery of surfactant, at least 84% was in acidification and addition of EDTA. In the case of centrifugation after complexation, almost 100% of surfactant recovery was achieved. In general use, it is considered that acidification is the simplest and most efficient process for surfactant recovery. However, ferricyanide has a potential applicability because of the rapid precipitation with metal ions and selectivity.