Dissolution of D2EHPA in liquid–liquid extraction process: Implication on metal removal and organic content of the treated water

Effects of pH, extractant/diluent ratios, and metal concentrations on the extent of extractant dissolution during liquid–liquid extraction were investigated. Experimental result shows that D2EHPA dissolution increases dramatically at pH above 4, leveling off at pH 6–7. The phenomenon is consistent with deprotonation of D2EHPA and the domination of negatively charged D2EHPA species at pH of higher than 4. Concentration of D2EHPA in the aqueous phase, i.e., the extent of extractant dissolution, drops after addition of metal and decreases with increasing metal concentration. The amount of D2EHPA ‘re-entering’ the organic phase is calculated to be 2.04 mol per mol of Cd added, which is quite closed to the stoichiometric molar ratio of 2 between D2EHPA and Cd via ion exchange reaction. The effect of metal species on the extent of extractant/metal complexes re-entering is in the order of Cd≈≈Zn ＞ Ag, which might be coincident to the complexation stability of these metals with D2EHPA. The extent of extractant dissolution in liquid–liquid extraction process depends on the type and concentration of metal to be removed, pH of aqueous phase, and extractant/diluent ratios.

► D2EHPA dissolution and metal removal efficiency are investigated.
► A model with only one fitting parameter was developed to fit D2EHPA dissolution data.
► The amount of D2EHPA ‘re-entering’ the organic phase is 2.04 mol/mol of Cd added.
► Extent of extractant/metal complexes re-entering depends on metal species.