Chemical Engineering Journal 166 (2011) 249–255 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Efficiency, stoichiometry and structural studies of Cu(II) removal from aqueous solutions using di-2-ethylhexylphosphoric acid and tributylphosphate diluted in soybean oil Siu Hua Chang a,b , Tjoon Tow Teng b,∗ , Norli Ismail b a Faculty of Chemical Engineering, Universiti Teknologi MARA (UiTM), 13500 Permatang Pauh, Penang, Malaysia b School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia article info Article history: Received 7 September 2010 Received in revised form 26 October 2010 Accepted 26 October 2010 Keywords: Cu(II) Extraction Soybean oil-based organic solvent (SOS) Di-2-ethylhexylphosphoric acid Stripping abstract Solvent extraction of Cu(II) from aqueous solutions by a soybean oil-based organic solvent (SOS) compos- ing 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 (H 2 SO 4 ), hydrochloric acid (HCl) and nitric acid (HNO 3 )) 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 H 2 SO 4 > HCl > HNO 3 throughout different concentrations studied, with H 2 SO 4 attaining the highest %S (>99%) at 1.5 M. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Industrial effluents containing Cu(II) from numerous anthro- pogenic activities such as mining, electroplating, printed circuit board manufacturing, metal processing and metal finishing [1,2] are threatening the environment due to the toxicity of the Cu(II) discharged [3]. Conventional techniques used to remove Cu(II) from aqueous solutions include chemical precipitation [4], coagulation–flocculation, flotation, ion exchange, electro- oxidation, membrane filtration and biosorption [5–7]. Among all techniques, solvent extraction emerges as one of the well- established techniques applicable to Cu(II) removal from aqueous solutions [8]. The organic solvents used to remove solutes from aqueous solutions are usually composed of an extractant and a diluent. The former extracts solutes selectively while the latter controls the solvent conditions [9]. Sometimes, a phase modifier is added to overcome the emulsion formation and to improve the phase disengagement process in the aqueous–organic sys- ∗ Corresponding author. Tel.: +60 46532215; fax: +60 46573678. E-mail address: ttteng@usm.my (T.T. Teng). tem [10]. A wide variety of organic solvents have been applied to remove heavy metals from aqueous solutions, but they are mostly of petroleum-based which are not eco-friendly, non-renewable and could be inordinately expensive due to the limited resources [11]. Recently, greener solvents such as the vegetable oil-based organic solvents [12,13] were used as the alternatives to replace the classi- cal petroleum-based organic solvents in heavy metal removal from aqueous solutions. A soybean oil-based organic solvent (SOS) containing soybean oil (diluent), di-2-ethylhexylphosphoric acid (D2EHPA) (extrac- tant) and tributylphosphate (TBP) (phase modifier) was found to extract Cu(II) effectively from aqueous solutions in our previ- ous work [13]. Optimization of various process variables in Cu(II) extraction with SOS using the response surface methodology was also conducted [14]. In the present work, effects of initial Cu(II) concentration in the aqueous phase and temperature on the Cu(II) extraction with SOS were investigated under the optimized oper- ating conditions determined earlier [14]. The loading capacity of SOS, as well as the stoichiometry and structure of Cu(II)–organic complexes (extracted species) in SOS were also determined. Lastly, stripping or back-extraction of Cu(II) from the loaded SOS using various types and concentrations of mineral acids was investigated. 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.10.069