Characterization of the solid third phases that precipitate from the organic solutions of Aliquat® 336 after extraction of molybdenum(VI) and vanadium(V)

Abstract

In this work, it is shown that solid third phases precipitate from certain organic solutions of Aliquat® 336 after extraction of molybdenum(VI) and/or vanadium(V). Both elemental analysis and IR and Raman spectroscopies have been used to characterize the isolated precipitates and elucidate the mechanism of precipitation. In the case of molybdenum(VI), various heptamolybdate and octamolybdate anions are rapidly extracted by anion exchange from acidic media (i.e., pH 1–4) by Aliquat® 336 in kerosene or n-heptane modified by n-decanol, but such extracted species then transform within several hours into hexamolybdate which precipitates as the greenish-yellow compound (R 3R′N) 2Mo 6O 19, when Aliquat® 336 is highly loaded in molybdenum. Mixed-valence species such as (Mo 5 VIMo VO 19) 3−, (Mo 4 VIMo 2 VO 19) 4− or (Mo 3 VMo 3 VIO 18H) 2− also precipitate in very small quantities as a red-garnet solid, as a result of the partial reduction of molybdenum(VI). In the case of vanadium(V), acidic decavanadate species are rapidly extracted at pH 1.5–2 by Aliquat® 336 in kerosene modified by n-decanol according to a mechanism of anion exchange, but the reddish extracted species turn olive green within a few days at room temperature while a black-greenish solid precipitates. This solid third phase is composed of mixed-valence decavanadate compounds such as (R 3R′N) 4(HV 3 IVV 7 VO 26) and/or (R 3R′N) 4(HV 7 IVV 3 VO 24). It is of interest that when cyclohexanol and ethyl-4-phenol are used as phase modifiers instead of n-decanol, the precipitation of mixed-valence decavanadates is no longer observed, but a compound mainly composed of an oxide of vanadium separates from the loaded organic phase after several months aging at room temperature. Finally, mixed V(V)–Mo(VI) and V(V)–Mo(VI)–P(V) compounds were found to slowly separate from the organic phase after the simultaneous extraction of vanadium(V) and molybdenum(VI), in the absence and in the presence of phosphate, respectively.