Selective leaching techniques

In the case of solid environmental materials such as soils, sediments or indeed suspended particulates the use of selective chemical leaching methods can provide some basic information regarding the nature of the radionuclide species bound to the solid matrix. A typical sequential leaching might follow a procedure similar to that suggested for the study of trace metal binding to sediments by Gupta and Chen (1975) as shown in Fig. 13.2. 

Readily reducible species, manganese oxide and amorphous iron oxide 

The successive chemical extractions are designed to indicate the type of component phase to which the trace component is bound and from this the nature of the bound species may theoretically be inferred. But as Cremers and Henrion (1985) have pointed out, the whole basis of this type of approach is controversial and the literature contains many examples of conflicting conclusions regarding the relative contribution of the various phases. For example, in some cases plutonium and americium are reported to be solely associated with hydrous oxides (Edgington et al., 1976 Aston and Stanners, 1981) whilst, in others, these radionuclides are claimed to be fairly uniformly distributed throughout the various phase components of what appear to be similar sediments (Hetherington, 1978). 

Part of the difficulty in applying such techniques is that the extraction processes operationally assign a given fraction (or species) of the radionuclide to a particular component phase but it has been shown that extractants are not as selective as sometimes claimed. 

Indeed the overall effect of such chemicals on the surface chemistry of soils and sediments remains poorly understood. Despite the difficulties this type of technique is still widely used. As with many operationally defined methods, it is difficult to make direct comparisons between independent studies however, for individual studies sequential extraction can play a useful role in an overall programme of study regarding the chemical forms of radionuclides bound to the solid phase under different environmental conditions so long as it is realised that in this type of study the species are themselves really defined by the nature of the extraction system employed (see Chapter 10). 

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Selective leach techniques have become popular in mineral exploration for the treatment of geochemical soil samples. Their popularity stems from the fact that they are considered to extract selectively a particular hydromorphically-transported component of metals in the sample and, as such, show better anomaly-to-background contrasts than do conventional strong acid digestions which dissolve most of the chemical matrix of the soil. 

Rapin, F. and Forstner, U. (1983) Sequential leaching techniques for particulate metal speciation the selectivity of various extractants. In Proceedings of the 4th International Conference on Heavy Metals in the Environment, Vol. 2, Heidelberg, CEP Consultants, Edinburgh, pp. 1074-1077. 

The importance of the total sample dissolution technique mentioned earlier becomes apparent because the detrital phase contains Th and U that is both lattice-bound and adsorbed but in unknown proportions. Bischoff and Fitzpatrick (1991) demonstrated that it is not possible to quantitatively separate these two components by selective leaching. 

Techniques of Hydrothermal Synthesis. In every run, the materials were contained in small sealed silver tubes to prevent selective leaching. A small amount of distilled water was added before the tube was sealed. The type of pressure vessel used was the test tube (6). 

The selection of a mining method to use for a particular uranium ore deposit is governed only by safety and economic considerations. Underground, open-pit mining, and in situ leaching techniques are used to recover uranium. 

In spite of the development and evolution of new techniques, such as microprobe analysis and advances in the electrochemical methods, it is not clearly known which mechanism is operative. It, however, appears on the basis of the hterature published that both mechanisms may operate, i.e. selective leaching or dissolution of both components and re-plating of one of the... 

Skeletal (Raney ) catalysts are made by a very simple technique. An alloy of two metals in roughly equal proportions, where one metal is the desired catalytic material, and the other is dissolvable in hydroxide, is first made. This alloy is crashed and leached in concentrated hydroxide solution. The soluble metal selectively dissolves, leaving behind a highly porous spongelike structure of the desired catalytic metal. Catalysts formed by this technique show high activity and selectivity, and have found wide use in industry, particularly for hydrogenation and dehydrogenation reactions. 

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