Microbial activity in waterlogged sediments causes the development of anaerobic conditions. If sulphate is present, this can be exploited by sulphate-reducing bacteria and converted into sulphide, while iron-reducing bacteria can reduce insoluble iron(III) to soluble iron(II) [1]. The iron(II) reacts with sulphide to form mackinawite (a form of FeS with an open, layered structure; Figure 47), which has a very reactive surface and a large surface area. In such sediments, the fate of trace elements is greatly affected by their potential for reaction with mackinawite, particularly where they can be reduced by and taken up on the mineral surface.


Fig. 47: The layered structure of mackinawite. Sulphur atoms are shown in yellow and iron atoms in red.

Actinide elements, such as uranium, neptunium and plutonium, are released into the environment as a result of nuclear weapons testing, nuclear fuel reprocessing and radioactive waste disposal, and their environmental mobility is therefore of great interest. The uranyl ion, [UO2]2+, is known to be removed effectively from solution by mackinawite, through partial reduction and precipitation on the mineral surface as an oxide containing both U(IV) and [UO2]2+ [2]. Neptunium is particularly interesting since it has a long half life (2.16 x 106 years) and is expected generally to be present as the relatively soluble [NpO2]+ ion, although, like uranium, in anaerobic conditions it may be reduced to Np(IV), a much less soluble form.

We therefore prepared mackinawite in the laboratory and allowed [NpO2]+ to react with it. Because mackinawite is very air-sensitive, and the isotope 237Np is a high specific activity alpha emitter, all the samples had to be prepared in inert atmosphere conditions in a specialised radiochemistry laboratory, then shipped to Grenoble for XAS analysis on the beamline BM20 (ROBL CRG), the only facility in Europe where such measurements can be made.

Neptunium is very different from uranium in several ways. First, less than 10% of the [NpO2]+ in solution is taken up by the surface, compared with over 98% of [UO2]2+. Second, the XANES spectra for the neptunium samples show that it has been reduced to Np(IV) on reaction with mackinawite and, third, EXAFS analysis shows that the Np is coordinated directly to the surface sulphur atoms on the mackinawite (Figure 48).

Fig. 48: Possible coordination environment of neptunium on the mackinawite surface.

This experiment illustrates clearly the power of molecular-level spectroscopic analysis of model environmental systems. In the absence of such data, we would have only the empirical observation that [UO2]2+ was removed from solution much more efficiently than [NpO2]+ and would have had to speculate on the reasons for this effect. This XAS experiment allows us to make predictions on the environmental behaviour of radioactive elements with increased confidence.

[1] D.E. Canfield, B. Thamdrup and J. Hansen, Geochim. Cosmochim. Acta 57, 3867-3889 (1993).
[2] L.N. Moyes, R.H. Parkman, J.M. Charnock, D.J. Vaughan, F.R. Livens, C.R. Hughes and A. Braithwaite, Environ. Sci. Technol. 34, 1062-1068 (2000).

Principal publication and Authors
L.N. Moyes (a), M.J. Jones (a), W.A. Reed (a), F.R. Livens (a), J.M. Charnock (b), J.F.W. Mosselmans (b), C. Hennig (c), D.J. Vaughan (d) and R.A.D. Pattrick (d), Environ. Sci. Technol. 36, 179-183 (2002).
(a) Centre for Radiochemistry Research, Manchester (UK)
(b) CLRC Daresbury Laboratory, Warrington (UK)
(c) Institut für Radiochemie, FZ Rossendorf, Dresden (Germany) and ROBL, ESRF
(d) Williamson Centre for Molecular Environmental Science, Manchester (UK)