Synergistic Effect of Tartaric Acid in the Extraction of Iron(III) from Aqueous Nitrate by Di(2-ethyl hexyl)phosphoric Acid in Hexane

Abstract

The addition of tartaric acid to aqueous nitrate solutions containing Fe(III) produces an increase in the solvent extraction of Fe(III) by di(2-ethyl hexyl)phosphoric acid, DEHPA, in hexane solutions. This effect has been studied and it is explained in terms of chemical equilibrium between the two liquid phases by applying the following reactions: Fe3++L2-= FeL+; eq. const. log 13=7. Iron(III) is known to be extractable by alkylphosphoric acids (HA).' Information concerning the speciation of this system is wide and varies according to different authors.2-' Thus, while Baes2,3 suggests the formation of FeA3(HA)3 and Fe3A8C104 species, Sato and coworkers4'5 report species FeA3(HA)3 and Fe2Al0H4 regarding the extraction of Fe(III) from hydrochloric acid solutions. In a recent study Fe(III) was determined to form different metal extracted species (FeA3(HA)3, Fe2A3(HA)s and Fe3A3(HA)3) in the organic phase.8The extraction efficiency may be enhanced by using an appropriate mixture of organic extractants.9 Also, the complexation of metal ions in the aqueous phase contributes to a modification of the extraction process which, in general, is reduced by such aqueous reactions. On the other hand, it is known that metal citrates and tartrates can be extracted as anionic chelate compounds. 10 In our case, Fe(III) forms stable aqueous complexes with tartrate 1 which prevents the precipitation of Fe(III) hydroxide, even at high pH values. Surprisingly, extraction enhancement of Fe(III) by di(2-ethyl hexyl)-phosphoric acid-hexane solutions has been observed in presence of aqueous tartaric acid. In the present work, this phenomenon was studied in order to elucidate the chemical reactions involved. ExperimentalReagents, solutions and apparatus Iron(III) nitrate (Merck, analytical reagent grade (A.R.)) was used to prepare stock solutions in 0.5 M N03-. The Fe3+ content and free acid were determined by standard procedures.12"3 NaNO3 (Probus, A.R.) was purified and stored (described in ref. 14) and was used as to maintain a constant ionic strength at 0.5 M. An 59Fe(N03)3 radioactive stock solution was prepared from the corresponding iron(III) citrate salt (supplied by General Radioquimica S.A.) of 0.2 mCi/ml activity and was used in radiotracer analysis.Tartaric acid (Industrias Quimicas y Tartaricas S.A., A.R.) was used to prepare stock solutions in 0.5 M NaNO3; its purity was determined by acid-base titrations.Solutions of HNO3 in 0.5 M N03-were employed to modify the pH of the aqueous solutions. The pH of aqueous solutions was measured by a pH-meter (Crison) and a calibrated combined glass electrode.Solutions of di(2-ethyl hexyl)phosphoric acid, DEHPA (HA), were prepared from DEHPA (BDH) in hexane (Probus, A.R.).The y-activities of radioactive solutions were measured with a well-type NaI scintillation counter (Berthold BF 2306).UV measurements were carried out with a Shimadzu spectrophotometer.