MODELING OF EXTRACTION EQUILIBRIUM FOR ZINC(II) EXTRACTION BY A BIBENZIMIDAZOLE TYPE REAGENT (ACORGA ZNX 50) FROM CHLORIDE SOLUTIONS

The extraction of zinc(II) by ACORGA ZNX 50 (a bibenzimidazole-based e xtractant) from chloride ion solutions was investigated at constant wa ter activity (a(w)) and constant total concentration (sigma) of aqueou s species, It is shown that zinc(II) is extracted as a binuclear compl ex according to the following equation: 2Zn(2+) + 4Cl(-) + 2 (L) over bar double left right arrow <((ZnCl(2)L)(2))over bar> with K-ex = [<(( ZnCl(2)L)(2))over bar>][Zn2+](-2)[Cl-](-4)[(L) over bar](-2). It is al so shown that the chloride ions favour the extraction of zinc at low c oncentrations (i.e., [Cl-]<1 M), but have a depressing influence at hi gher concentrations, as a result of the formation of ZnCl3- and ZnCl42 - species in the aqueous phase. At constant a(w) and sigma, the plot o f zinc concentration in the organic phase versus chloride ion concentr ation exhibits a maximum at about 1 M Cl-. The distribution of zinc(II ) can be satisfactorily simulated with the following series of constan t values: (1) log beta(1) = 0.1, log beta(2) = 0.5, log beta(3) = 0.7, log beta(4) = -0.2 and log K-ex = 4.05 +/- 0.10, at a(w) = 0.83 and s igma = 8 M; (2) log beta(1) = 0.4, log beta(2) = 1.1, log beta(3) = 1. 4, log beta(4) = 1.25 and log K-ex = 7.35 +/- 0.10, at a(w) = 0.62 and sigma = 12 M; with beta(i) = [ZnCli2-i][Zn2+](-1)[Cl-](-i). The water activity of the aqueous phase has a strong influence on the value of the extraction constant (K-ex).