Trace element fractions reducible

Davis A. M., Hashimoto A., and Parsad N. M. (1999) Trace element fractionation during evaporation in reducing atmospheres. Lunar Planet. Sci. XXX, 2023. Lunar and Planetary Institute, Houston (CD-ROM). 

Two parameters, the redistribution index (Uts) and the reduced partitioning parameter (IR), are used to describe the redistribution processes of trace elements in contaminated arid soils (Figs. 6.5-6.6) (Han et al., 2003a). The redistribution index depicts the removal or attainment of element-contaminated soils from or to the fractional distribution pattern characteristic of non-amended soils. However, the reduced partitioning parameter quantifies the relative binding intensity of trace elements in soils. 

Of the major solids formed from melts, many, but not all, at equilibrium, the overwhelming influence is of cooperative interaction between ionic units of similar shape and size as we see in crystals. Trace elements apart from forming isolated minerals are fractioned in bulk oxides, for example, in particular orders as the melt solidifies, and this reduces the relative availability of some elements such as Cr and Ni (see Williams, and Williams and Frausto da Silva (1999) in Further Reading). Again the interaction of selective molten minerals and water creates extremely reactive environments and such environments still exist, especially in the deep sea black smokers (hydrothermal vents), around which particular mixed minerals form, which could also have been involved in prebiotic chemistry and are still involved in the peculiarities of life in these smokers . In Figure 1.6 we summarise... 

The results in Table 5 show a large increase in proportion of Mn and Ti, and increases for Mg and Ca from coal to extract. Fe stays about the same and all the others decrease. Previous work Q) has also shown that the elements which increase in proportion also deposit readily and steadily on the hydrocracking catalyst. Thus, any method which reduces the ash level in the filtered extract solution needs to be, if at all posible, selective towards Mn, Ti, Ca, Mg, and Fe. In the case of digestion pressure, as shown by the results above, most trace elements do not significantly change their proportion as the ash level decreases, except for Mn where the situation becomes worse and Mg where there is some improvement. Liquefaction of different-sized coal fractions has little effect on the distribution of elements in the ash as the overall ash level decreases. However, with the use of HAO with different saturates levels, there is a distinct increase in the levels of Ti with decreasing ash level, but a significant decrease in Mn. 

Several options have been outlined for the reduction of mineral matter to low levels in filtered extract solution. These are mainly applicable to the British Coal process, but have implications for other 2-stage processes. By running the digestion at a pressure above about 30 bar and reducing the filtration temperature as required, low levels of trace elements have been obtained with a series of different coals and HAO solvents. Discarding the fine fraction of the coal from the feed to the digestor will also result in lower levels of trace elements in the extract solution, as will increasing the saturates content of the HAO. However, in the latter case this may also lead to reduced overall conversion. 

Conditions that are conducive to the accumulation and decay of plant residues, and therefore conducive to the formation of coal, are typically associated with water-saturated and reducing environments. Consequently, a large portion of the trace elements associated with the mineral fraction of coal are expected to occur in reduced forms, primarily as sulphides or carbonates. However, because of its abundance compared to sulphur, it is unlikely that the complete reduction of iron oxides to iron sulphide would ever occur. Therefore, the presence of Fe oxides, and trace... 

Various workers have questioned the ability of sequential extraction to provide accurate information on the mineralogical phases with which trace elements are associated in soil or sediments (e.g. Nirel and Morel, 1990). Problems, including non-selectivity of reagents and readsorption of analytes following release, are frequently reported. Hence, nowadays, most environmental analytical chemists accept that sequential extraction should be considered an operational form of speciation, in which the fractions isolated are defined purely by the sequence of reagents used, and not as a means to determine information on the specific mineralogical phases to which trace elements are bound. Modern sequential extraction procedures label the fractions obtained in terms of the type of chemical reaction used to isolate them, in order to emphasise this, e.g. reducible or oxidisable species. Unfortunately, this distinction is not always made clear in the wider environmental literature. 

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