300
D. Ferri· C. Manfredi· E. Vasca . C. Fontanella· V. Caruso
and frequently suggest that the authors complete the studies submitted with spectroscopic data. We have so far discussed our views vs. the care one uses in following and
performing the experimental stage. But the severe approach invoked must often come
to a compromise:
a With the many difficulties which characterize the real systems and thus make their
modelling problematic. The network of all the interactions within their reagents
requests extremely versatile computer programmes able to refine many thermodynamic parameters contemporarily. Computer facilities for the speciation of natural
fluids have been reviewed in 1997 by De Stefano et al. (see also references therein).
b With the fact that interactions between the components of natural fluids also give
rise to weak complexes, involving alkaline and alkaline earth ions, which are generally neglected in defining chemical models of natural aqueous systems. The interpretation of data concerning the formation of complexes is a delicate and controversial subject, but necessary if we consider the high concentration of alkaline, alkaline earth and chloride ions in sea water or, even worse, in salt brines. An interesting
review on weak complex formation has been carried out by Daniele et al. (1994).
c With the use of the available literature. We do not intend to absolve the bad data, but
bad data sometimes may tell one how to plan an investigation to collect good data.
We mean that there are grades of "bad." In the absence of other resources, partial,
inaccurate, or badly designed experiments may give some primary information and
help one plan a rigorous study.
As an application of what has been so far discussed, we propose two typical examples
of our research, carried out following the guidelines exposed above. The examples are
chosen among the studies carried out, either by the Royal Institute of Technology of
Stockholm or by the University of Naples, to realize a thermodynamic database, within
the Swedish project for the realization of a mathematical model for the migration of
radionuclides, from permanent repositories to the biosphere.
12.3.1
1 st Example: Uranium-Carbonate System
A series of model studies (Aberg et al. 1983a; Aberg et al.1983b; Biedermann et al.1982;
Bruno et al. 1986; Ciavatta et al. 1979; Ciavatta et al. 1981; Ciavatta et al. 1983; Ferri et al.
1981; Ferri et al. 1983; Ferri et al. 1993; Ferri et al. 1994; Ferri et al. 1988; Grenthe and
Ferri 1981; Grenthe et al. 1984), on the complex formation between U, in the oxidation
states +6, +5 and +4, and the carbonate ion was undertaken in the range 2 < pH < 12,
in oxidizing as well as in reducing solutions, using potentiometric and spectrophotometric methods. A number of species were identified, which form the thermodynamic
cycle of uranium in carbonate solutions, in function of pH and pE as it is shown in
Fig. 12.2.
Particular attention was devoted to a species having a concentration ratio of carbonate to uranyl equal 2 (which was known and had traditionally been assigned the
composition U02(C03)~-) that proved to have stoichiometry (U02h
characterized it further by X-ray diffraction (Aberg et al. 1983a) in concentrated solu-
D. Ferri· C. Manfredi· E. Vasca . C. Fontanella· V. Caruso
and frequently suggest that the authors complete the studies submitted with spectroscopic data. We have so far discussed our views vs. the care one uses in following and
performing the experimental stage. But the severe approach invoked must often come
to a compromise:
a With the many difficulties which characterize the real systems and thus make their
modelling problematic. The network of all the interactions within their reagents
requests extremely versatile computer programmes able to refine many thermodynamic parameters contemporarily. Computer facilities for the speciation of natural
fluids have been reviewed in 1997 by De Stefano et al. (see also references therein).
b With the fact that interactions between the components of natural fluids also give
rise to weak complexes, involving alkaline and alkaline earth ions, which are generally neglected in defining chemical models of natural aqueous systems. The interpretation of data concerning the formation of complexes is a delicate and controversial subject, but necessary if we consider the high concentration of alkaline, alkaline earth and chloride ions in sea water or, even worse, in salt brines. An interesting
review on weak complex formation has been carried out by Daniele et al. (1994).
c With the use of the available literature. We do not intend to absolve the bad data, but
bad data sometimes may tell one how to plan an investigation to collect good data.
We mean that there are grades of "bad." In the absence of other resources, partial,
inaccurate, or badly designed experiments may give some primary information and
help one plan a rigorous study.
As an application of what has been so far discussed, we propose two typical examples
of our research, carried out following the guidelines exposed above. The examples are
chosen among the studies carried out, either by the Royal Institute of Technology of
Stockholm or by the University of Naples, to realize a thermodynamic database, within
the Swedish project for the realization of a mathematical model for the migration of
radionuclides, from permanent repositories to the biosphere.
12.3.1
1 st Example: Uranium-Carbonate System
A series of model studies (Aberg et al. 1983a; Aberg et al.1983b; Biedermann et al.1982;
Bruno et al. 1986; Ciavatta et al. 1979; Ciavatta et al. 1981; Ciavatta et al. 1983; Ferri et al.
1981; Ferri et al. 1983; Ferri et al. 1993; Ferri et al. 1994; Ferri et al. 1988; Grenthe and
Ferri 1981; Grenthe et al. 1984), on the complex formation between U, in the oxidation
states +6, +5 and +4, and the carbonate ion was undertaken in the range 2 < pH < 12,
in oxidizing as well as in reducing solutions, using potentiometric and spectrophotometric methods. A number of species were identified, which form the thermodynamic
cycle of uranium in carbonate solutions, in function of pH and pE as it is shown in
Fig. 12.2.
Particular attention was devoted to a species having a concentration ratio of carbonate to uranyl equal 2 (which was known and had traditionally been assigned the
composition U02(C03)~-) that proved to have stoichiometry (U02h
characterized it further by X-ray diffraction (Aberg et al. 1983a) in concentrated solu-