Abstract
ABSTRACTCombining computational modeling with experimental measurements has revealed the self-assembly of nano-aggregate structures in the transfer of HCl and PtCl62– from an aqueous phase into toluene by the common industrial extractant tributyl phosphate (TBP). Molecular dynamics simulations have been coupled to analytical measurements to provide an atomistic interpretation of the mode of action of TBP under 6 M and 10 M HCl conditions. The structures conform to reverse micelles, where the Cl– or PtCl62– core is encapsulated by a hydration shell that acts as a mediating bridge to the electronegative oxygen atom in the TBP phosphate groups. For the 6 M HCl extraction model, the data support stable aggregates forming from 2–3 TBP molecules around one chloride anion if the number of water molecules encapsulating the chloride anion is no more than five; increasing the water content to 10 molecules allows a fourth TBP molecule to coordinate. For the 10 M HCl extraction model, stable structures are obtained that conform to the empirical formula (TBP.HCl.H2O)3–5. At 6 M HCl, extraction of PtCl62– is achieved by encapsulation by four TBP molecules; the data for extraction at 10 M HCl indicate larger aggregates containing multiple PtCl62– anions are likely to be forming. In all cases, the hydrated core regions of the reverse micelles are considerably exposed. The diameters of the self-assembled structures around chloride ions agree well with available literature data from small-angle neutron-scattering experiments.
Highlights
Tri-n-butyl phosphate (TBP, Figure 1) is a well-known compound employed in the recovery of inorganic acids and metals by solvent extraction from acidic media.[1,2,3,4] It is used extensively in the nuclear industry for the reprocessing of uranium, plutonium and thorium in the PUREX process,[5] and finds application as a modifier and solvent in many processes.[6,7] This paper concerns the mode of action of tri-butyl phosphate (TBP) in the recovery of PtCl62- from aqueous HCl solutions of the type obtained by oxidative leaching of platinum group metal (PGM) concentrates.[8]
Classical molecular dynamics simulations performed on models constructed from experimentally derived ratios of HCl and H2O resulted in the spontaneous formation of assemblies that can be described as reverse micelles
The diameter of chloride-containing micelles (16-18 Å for the 6 M HCl extraction model, rising to ca. 20-22 Å for the models explored for the 10 M HCl extraction model) match well with data reported from small angle neutron scattering (SANS) experiments, giving validity to the structures described
Summary
Tri-n-butyl phosphate (TBP, Figure 1) is a well-known compound employed in the recovery of inorganic acids (notably HCl, HClO4 and HNO3) and metals by solvent extraction from acidic media.[1,2,3,4] It is used extensively in the nuclear industry for the reprocessing of uranium, plutonium and thorium in the PUREX process,[5] and finds application as a modifier and solvent in many processes.[6,7] This paper concerns the mode of action of TBP in the recovery of PtCl62- from aqueous HCl solutions of the type obtained by oxidative leaching of platinum group metal (PGM) concentrates.[8]. This study began with an analytical investigation to determine, as far as possible, the relative proportions of TBP, H2O, HCl and PtCl62- that comprise the reverse micelles as a function of HCl concentration These results were used to guide computational modelling work, where self-assembly of reverse micelle structures were observed from components placed randomly in a toluene solvent box. Due to the high excess of chloride (2-12 M HCl) present in the (0.01 M) PtCl62 extractions, and as TBP will co-extract both these species, it was not possible to determine the levels of H2O and H+ associated with transport of just PtCl62 into the organic phase because the values are swamped by those involved in the extraction of HCl. water and acid content analysis are only reported in the main text for the HCl extractions; data obtained in the presence of 0.01 M Na2PtCl6 can be found in the electronic supplement S1.2. Simulations were run under NVT ensemble conditions to achieve equilibrium for approximately 0.05 ns (in integration time steps of 0.1 fs using the standard Velocity-Verlet algorithm), followed by simulations using the NPT ensemble for a minimum of 9 ns (integration time steps of either 0.5 or 1 fs), thermostated at room temperature and pressure using the Nosé-Hoover thermonstat/barostat system.[40,41] Further details can be found in the electronic supplement
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