The utilisation of layered hydroxysalts in the separation, immobilisation and long term storage of long-lived radio-anions of nuclear power legacy waste origin.
thesisposted on 22.11.2013, 13:39 by Andrew D. Butterworth
Long lived radioactive species such as 129I‾ and 99TcO4‾ are formed as by-products of nuclear fission. These species have extremely long half-lives (1.5 million and 211,000 years respectively), are biologically assimilating and due to their anionic nature, can move freely within a geosphere. Historical disposal methods for active species are no longer considered acceptable to the general public and other ways in which to treat the waste are being explored. One remove and concentrate method to remove radioactive species from the biosphere is using alkaline-resistant materials which trap active species that can be encapsulated in a high pH concrete matrix. Layered hydroxides and layered double salts which consist of positively charged layers between which exchangeable anions and water molecules lie are good candidates for these materials due to the basic conditions in which they form. The synthesis, anion exchange properties and stability of copper, lanthanum, nickel and zinc hydroxysalts have been investigated. The structures consist of layers of edge-sharing metal hydroxide octahedra together with an interlayer space containing the anion and in some cases water molecules. Products were characterised by powder X-ray diffraction and vibrational spectroscopy to confirm the identity of structure of the material formed and the anion incorporated. Only Cu2(OH)3(NO3), Cu2(OH)3(OAc).H2O, Ni2(OH)3(NO3) and Zn5(OH)3(NO3).2H2O consistently exhibited exchange capabilities with iodide as the target anion. In terms of exchange rate and efficiency, copper hydroxyacetate is a more suitable precursor as equilibrium is achieved in 10 mins; whereas other LHS containing nitrate as the occupying require longer than 1 day to reach equilibrium. Cu2(OH)3(Ac).H2O has been shown to easily exchange acetate for monovalent anions X- (X = Ha-, NO3-, ClO4¬-, IO4¬-, SbO3-, OH-). Exchange reactions with ReO4- (used as a surrogate to TcO4-) and trigonal pyramidal monovalent anions (such as IO3-) were unsuccessful. Exposure to divalent anions (CO32-, Cr2O42-, SeO3-) resulted in no interaction whereas exposure to trivalent PO43- forms Cu3(PO4)2. Quantitative analysis has shown that, contrary to XRPD and FTIR data, full exchange of acetate for an equimolar amount of iodide within a Cu2(OH)3+ framework does not occur with 100% efficiency. Activity counting, gravimetric analysis and ion specific probe analysis suggested that only ~92-93% exchange occurs. The stability of TcO4- and I¬- analogues with respect to pH has been investigated. Activity counting has shown that even in pH 9.5 solution, 57-73% of 125I- and 99TcO4- immobilised with a Cu2(OH)3+ framework is leached into solution after 16 days. Exposure of Cu2(OH)3I to high carbonate, nitrate and chloride environments shows a progressive loss of iodide into solution as the anionic radius of the incoming anion decreases and the concentration of the incoming anion increases. In the case of chloride and nitrate incoming anions, only a 2:1 chloride to iodide ratio is need for full exchange whereas a ratio of 10:1 nitrate to iodide is required. In situ ion exchange experiments at Diamond allowed the exchange of the hydroxyacetate material to be investigated in flow experiments showing similar facile exchange as demonstrated under batch conditions. Rietveld refinements on deuterated samples of halide analogues of the materials have allowed accurate structure determinations for the first time (Cu2(OD)3Cl - a = 5.726Å, b = 6.125 Å, c = 5.634 Å, β = 93.100°, Cu2(OD)3Br a = 6.085 Å, b = 6.144 Å, c = 5.650 Å, β = 93.593°, Cu2(OD)3I a = 6.587 Å, b = 6.179 Å, c = 5.680 Å, β = 95.044°). As the size of the halide increases, the hydroxide coordination alters reflecting to changing sigma/pi donor capability of the halide.
EPSRC via the DIAMOND university consortium