Emulsion liquid membranes for wastewater treatment: Equilibrium models for some typical metal-extractant systems

Extraction and recovery of heavy metals from wastewater is more attractive than methods such as precipitation that result in sludges that have to be disposed in landfills. Emulsion liquid membranes are capable of extracting metals from dilute waste streams to levels much below those possible by equilibrium-limited solvent extraction. Binary equilibrium data are reported for copper, nickel, and zinc with extractants that can be used in emulsion liquid membrane formulations. Predictive models that incorporate aqueous-phase nonidealities and all aqueous-phase ionic reactions have been developed. For the Cu-LIX and Zn-D2EHPA systems, a single value for the equilibrium constant (K) is accurate over a large range of pH and ionic strength. For the Ni-D2EHPA system, an average single K value was obtained for loadings (fraction of D2EHPA dimers complexed to the metal in the organic phase) less than 0.1. At higher loadings, the organic-phase nonidealities become significant, and the equilibrium constant was fitted as an exponential function of the loading. Emulsion liquid membrane extractions of Cu, Ni, and Zn from aqueous phases were successfully carried out. The effects of various parameters such as mixing speed, stripping reagent concentration, and extractant concentration on the extraction process are reported.