Some Physico-Chemical Properties of Cyanex 272 and Extraction of Some Metal Ions

Abstract

Using a reported technique for purifying organophosphorous extractants, the as-received Cyanex 272 extractant containing 85% bis-2,4,4-trimethylpentyl- phosphinic acid [BTMPPA] has been purified to a purity of about 99% BTMPPA. The purified reagent has been characterized and compared to the literature values. A technique for the colorimetric estimation of purified Cyanex 272 has been developed. It consists of the digestion of pure sample or its aqueous solution (after evaporating out most of water) with concentrated HNO3 (70%) - HCIO (70%) mixture at 2:1 ratio for 1 h. The oxidizing mixed acid quantitatively converts Cyanex 272 to a clear solution of orthophosphate which can be easily estimated by the molybdenum blue colorimetric method at 830 nm. The method is sensitive with a molar extinction coefficient of 2.6 x 10 and reproducible within ± 2%. Applying this technique of analysis, the dimerization constant (K2), distribution or partition coefficient (K) and the ionization constant (K) of the purified Cyanex 272 ie. BTMPPA have been estimated to be 190, 53 and 5.52 x 10, respectively. Interactions of BTMPPA+ diluent (n-hexane cyclohexane / benzene / toluene / chloroform / carbon tetrachloride / 1,2-dichloroethane (DCE) / 1-heptanol) binary mixtures have been studied in terms of density and viscosity measurements and also in terms of derived properties such as excess molar volumes (V), excess viscosities (n) and excess Gibbs free energy changes of activation of flow (AGE). Variations of these values together with the variations of the Grunberg-Nissan interaction parameter (d) for BTMPPA diluent binary systems show that BTMPPA interacts with the diluents in varying degrees. However, no correlation between d, AGE, nor V and the Cu2+/ Fe3+ - extraction characteristics with BTMPPA could be noticed. - The interfacial adsorption property of Cyanex 272 (purified) at the 0.50 mol/dm3 H2SO4 (pH = 0.8) / BTMPPA - diluent (carbon tetrachloride / cyclohexane/ DCE/ n-hexane / toluene / kerosene / benzene / chloroform / 1-heptanol/ 1-hexanol) interface have been investigated. Applying the Gibbs adsorption isotherm to the y vs. log [BTMPPA] plots for each of the diluent used (y being the interfacial tension, mN/m), the apparent cross-sectional areas (A in A2) of BTMPPA molecules adsorbed at the interfaces have been estimated. A (A3) is found to vary in the order: CCl4 (157 A2) <cy-C6H12 (166 A3) < DCE (174 A3) <n-C6H14 (176 A2) < C6H5-CH3 (211 A2) = kerosene (211 A2) < C6H6 (262 A2) < CHCl3 (335 A2) < 1-CH1SOH (532 A2) < 1. C6H3OH (734 Å). When the measurements are done in identical conditions of interfaces, then the cross-sectional areas can be correlated with the metal extraction data. The extraction characteristics of Fe3+ from 0.10 mol/dm3 (H, Na*) SO,2 medium by BTMPPA in kerosene have been investigatied. The equilibration time is less than 25 min. The [Fe3] dependence curve at a constant equilibrium pH and concentrations of extractant and sulphate has a slope of -0.5 and zero at lower and higher concentration regions of Fe3. The pH dependence at lower pH region is 2; whereas, the extractant and sulphate ion dependences are 1 and -1, respectively. The pH dependence is decreased at higher pH regions due to hydrolysis of Fe3*. The temperature dependence of distribution ratio (D) indicates that the extraction process is exothermic with an average AH value of 30 kJ/mol. The extraction equilibrium reaction is suggested to be: Fe (HSO2)2+H2A2(0) Fe.HSO4-A2(0) + HSO4 + 2H* at the lower pH region of investigation. At higher pH region, the hydrolytic Fe3* species is involved. The extraction equilibrium constant (Ke) has been evaluated to be 10°23 The extracted complex can be stripped by 0.50 mol/dm3 H2SO4 solution; but a three- stage stripping (O/A = 1 in each stage) is needed for 99% Fe3 removal from the organic phase. The solvent extraction of Cu2 from 0.10 mol/dm3 sulphate medium by BTMPPA dissolved in distilled kerosene has been investigated. The equilibration time is less than 15 min. The distribution ratio is found to decrease appreciably with equilibrium Cu concentration in the aqueous phase, particularly, at its higher concentration region indicating some sort of association of Cu2+ species in either of the phases. The pH and the extractant concentration dependences on D are 1 and 1.2, respectively. D is almost independent of sulphate ion concentrations in the aqueous phase. The AH value is 27.3 ± 0.5 kJ/mol. The 60% extraction equilibrium reaction is suggested to be: CuHSO4 + H2A20) CuHSO4.A.0.5 H2A2(0)+H" and some 40% reaction is likely to occur via the reaction: CuHSO4+1.5 H2A210) CuHSO4.A.HzAzo) + H satisfying the extractant dependence of 1.2. The apparent extraction equilibrium constant (K) is estimated to be 10315. The loading capacity of BTMPPA is 13.47 g Cu2+/100 g BTMPPA. Among H2SO4, HCI and HNO3 solutions as stripping agent, 1 mol/dm3 H2SO4 solution is found to be the best; 99.6% Cu2+ in the organic phase can be stripped off in three stages (O/A = 1 in each stage). The Cu2* distribution ratio measured at pH = 4 in different diluents does not show good correlation with the cross sectional area of BTMPPA molecules at interfaces formed by different diluents at pH 0.80.