Efficiency, stoichiometry and structural studies of Cu(II) removal from aqueous solutions using di-2-ethylhexylphosphoric acid and tributylphosphate diluted in soybean oil

Solvent extraction of Cu(II) from aqueous solutions by a soybean oil-based organic solvent (SOS) composing of soybean oil (diluent), di-2-ethylhexylphosphoric acid (extractant) and tributylphosphate (phase modifier) was investigated. Effects of initial Cu(II) concentration in the aqueous phase (25–500 mg/L (0.39–7.88 mM)) and temperature (298–323 K) on the percentage extraction (%E) of Cu(II) were studied. It was found that the initial Cu(II) concentration did not influence %E appreciably and high %E (>98%) was achieved throughout the experimental ranges studied. The %E at various temperatures, on the other hand, decreased consistently with temperature. The loading capacity of SOS (2400 mg/L (37.82 mM)), as well as the stoichiometry (4:1) and structure (inner sphere) of Cu(II)–organic complexes (extracted species) in SOS were also determined. Stripping of Cu(II) from the loaded SOS was investigated with various types (sulfuric acid (H2SO4), hydrochloric acid (HCl) and nitric acid (HNO3)) and concentrations (0.05–2.00 M) of mineral acids. The decreasing order of percentage stripping (%S) of Cu(II) by various types of acids was found to be H2SO4 > HCl > HNO3 throughout different concentrations studied, with H2SO4 attaining the highest %S (>99%) at 1.5 M.