A green method for extracting molybdenum (VI) from aqueous solution with aqueous two-phase system without any extractant

Abstract

Abstract An aqueous two-phase system (ATPS) of TX-100 + (NH4)2SO4 + H2O without using any extractant was reported for the selective extraction and separation of molybdenum (VI) from aqueous media. The effects of aqueous pH, initial molybdenum concentration, phase-forming ammonium sulfate and TX-100 concentration, extracting temperature on the extraction of molybdenum were investigated. The experimental results indicate that the extraction rate and distribution coefficient of molybdenum have remarkable relationship with molybdenum species in aqueous phase and reach the maximum of 97% and 75 at pH = 3.00. The extraction rate of molybdenum decrease with increasing temperature within 298.15–343.15 K, and its distribution coefficient decrease with increasing temperature within 303.15–343.15 K, and the thermodynamic analysis of extracting process indicates that polymolybdate anion transferring from salt-rich phase to TX-100-rich micellar phase is a spontaneous and exothermic process. The high distribution coefficient and extraction rate of molybdenum are obtained in the broad initial molybdenum concentration range of 0.1–30 g L−1 in aqueous phase. The extraction mechanism reveals that there is no significant interaction between polymolybdate anion and ethylene oxide unit in TX-100 molecule by Fourier Transform Infrared Spectroscopy (FT-IR) and zeta potential of TX-100 micellar phase slightly changes from positive to negative after extracting polymolybdate anion, and from negative to positive again after adding charge modifier making for separation of molybdenum and metal cations by Dynamic Light Scattering (DLS), which can be deduced that polymolybdate anion is extracted into TX-100 micellar core depending on salting-out effect of phase-forming ammonium sulfate and relatively high hydrophobic nature of polymolybdate anion by micelle solubilization. The stripping rate of molybdenum can be easily achieved to 95% in single stage stripping. Some metal cations (Fe(III), Co(II), Ni(II), Cu(II), Zn(II)) are remained in salt-rich phase and separated from molybdenum under the suitable conditions by adding charge modifiers.