Liquid-liquid extraction of pertechnetic acid (TcVII) by tri-n-butyl phosphate: where is the proton? A molecular dynamics investigation.
We report a molecular dynamics study on pertechnetic acid (PTA) extraction from water to an oil phase containing either pure TBP (tri-n-butyl phosphate) or a TBP/n-hexane mixture, with the main aim to understand the state of the acid (associated TcO(4)H vs dissociated TcO(4)(-)H(+)) and its "complexation" by TBP. Experimentally, Tc(VII) is extracted from acidic water to TBP:alkane solutions in 1:3 or 1:4 Tc:TBP ratio, suggesting that three or four TBPs coordinate to TcO(4)H or TcO(4)(-). According to simulations in TBP solution, however, neither TcO(4)H nor TcO(4)(-) species displays specific coordination to TBP. We thus investigated several hypothetical states of the proton of the dissociated pertechnetic acid in organic phases and at their aqueous interfaces, comparing "pH neutral" to nitric acid containing systems. Proton hydrates are shown to coordinate 3-4 TBPs, in the form of H(3)O(+)(TBP)(3) and H(5)O(2)(+)(TBP)(4) hydrogen-bonded adducts, whereas TBPH(+) binds 1 TBP. The MD and PMF results complemented by those of QM investigations suggest that Tc(VII) is extracted as TcO(4)(-)(H(3)O)(+)(TBP)(3) or TcO(4)(-)(H(5)O(2))(+)(TBP)(4) contact ion pairs instead of the neutral form TcO(4)H of the acid. They explain why nitric acid promotes the Tc(VII) extraction. Comparison between nitric acid (mainly extracted via its neutral form NO(3)H) and pertechnetic acid is discussed.