For redox-sensitive radionuclides such as uranium, technetium, neptunium and plutonium, oxidation state is one of the primary controls on mobility, affecting precipitation, complexation, sorption and colloid formation behaviour. The dominant oxidation states for some key radionuclides at pH 7 are shown in Figure 4. Microbial metabolism can drive a wide range of redox transformations, utilising a succession of terminal electron acceptors (TEA), including some redox-active radionuclides, for the oxidation of organic matter. The amount of

energy gained from the use of each TEA influences the rate and sequence of TEA utilisation. The classical TEA sequence is reduction of: O2, NO3 Mn(IV), Fe(iii), SO42 , followed finally by methanogenesis,17 but radionuclides such as uranium and technetium can also be used as TEAs. Under Fe(iii) and SO42 reducing conditions, U(vi) (as UO221) and Tc(vii) (as TcO4 ) can be reduced by a wide range of microorganisms to less mobile U(iv) and Tc(iv), respectively.1,82,83 There have also have been a few limited studies reporting microbial reduction of neptunium and Pu.84 85 The Fe(iii)-reducing bacteria Geobacter sulfurreducens and Shewanella oneidensis have been reported to slowly reduce Pu(iv), as amorphous Pu(OH)4, to Pu(iii); for S. oneidensis, the rate of reduction was increased by the presence of riboflavin as an endogenous redox mediator.84 Shewanella oneidensis and a mixed consortium of sulfate-reducing bacteria have been found to reduced soluble NpO2+ to insoluble Np(iv).85,86

in addition to direct microbial reduction, the oxidation state of redox-active radionuclides will also be influenced by presence of microbially-generated redox-active species, reactive mineral phases and microbial alteration of mineral phases. Redox-active ions exposed at the surface of a mineral, such as sulfur or iron in mackinawite (FeS) can reduce an adsorbed radionuclide such as U(vi) and Tc (vii).87 This changes the controls on subsequent remobilization processes, and makes oxidation the dominant re-suspension pathway rather than the presence of competing cations in solution or pH fluctuations. Livens et al.87 found that the reduced uranium was readily reoxidised and desorbed upon introduction of oxygen. Reduced iron sediments within a soil profile can also immobilise Tc(vii) by similar surface-mediated reduction to Tc(iv).88 in contrast to uranium, however, technetium does not remobilise as readily with oxygen when it is in association with mackinawite.89

Bacteria can reduce transition metals (notably iron and manganese) locked within mineral structures90,91 and this could alter the characteristics of the reactive surface, generate new reactive mineral phases or release redox-active species into solution.15,74 in particular, iron-bearing minerals can play a crucial role as mediators between microbial anaerobic respiration and redox sensitive radionuclides. Ferrous iron released by microbial reduction of iron-bearing phases can react with a number of ligands and phases present in solution to form a range of new iron phases: oxyhydroxides (magnetite, goethite),92 car­bonates (siderite) or phosphates (vivianite)93 and others. The mineral phases formed in the environment can be hard to predict, but are likely to be domi­nated by carbonates and hydroxides due to the abundance of those ligands in solution. Wildung et al.88 investigated technetium reduction in shallow aquifer sediments from the US Atlantic Coastal Plane. The primary control on the reduction of Tc(vii) was the amount of readily extractable (and so more reac­tive) Fe(ii) present in the sediments. Other studies have also found that Tc(Vii) can be reduced to Tc(iV), as TcO2 by biogenic Fe(ii) , with TcO2 associated with the biogenic Fe(ii) mineral phase.94,95 Upon reoxidation of reduced sediments there can be significant reoxidation and remobilization of technetium, but it is dependent on the nature of the oxidant. When sediments are reoxidised with air, a significant (50-80%) fraction of the reduced and immobilised technetium can be reoxidised and remobilised; however, when the oxidant present is nitrate, there is much more limited (o 10%) reoxidation of technetium.96,97

Bioreduction may also lead to dissolution of the mineral phase and loss of sorption sites.98 Bacteria can reduce iron within a number of iron oxyhydr — oxides of varying crystallinity: hematite, goethite, lepidocrocite and schwert — mannite,99,100 and even micas such as biotite,101 smectite102,103 and illite.99,104 This can cause release and remobilization of radionuclides which have been adsorbed or incorporated into the mineral phase. Langley et al. (2009)105 investigated the impact of microbes on strontium sorbed to bacteriogenic iron oxides. Microbial reduction of the ferric iron within the iron oxides remobilised strontium (increased its concentration in solution), most likely due to loss of sorption sites. The authors suggest, however, that in a natural system remo­bilised strontium would be transported upwards by advection and recaptured within newly-formed bacteriogenic iron oxides near the surface of the water body, once again retarding its transport.

Under circumneutral conditions, abiotic or biotic oxidation of Fe(n) or Mn(n) leads to the formation of new oxyhydroxide phases.106,107 These sec­ondary mineral phases are characterised by large surface area and small crystal size, and have very high sorption capacity.107,106 At low concentrations, oxi­dised uranium has been reported to form an inner sphere complex on biomi­neralising manganese oxides.108 At high concentrations U(vi) was sequestered very efficiently and was incorporated into the oxide structure. The large cation caused distortion of the manganese oxide lattice and the formation of a mineral with tunnel-like structures. The results highlight the significance of the solution chemistry during mineral formation and the sequence of biomineralisation processes and the presence of radionuclides in solution.