Controls on trace element distribution

Most modem quantitative trace element geochemistry assumes that trace elements are present in a mineral in solid solution through substitution and that their concentrations can be described in terms of equilibrium thermodynamics. Trace elements may mix in either an ideal or a non-ideal way in their host mineral. Their very low concentrations, however, lead to relatively simple relationships between composition and activity. When mixing is ideal the relationship between activity and composition is given by RaoulFs Law, i.e. 

If the trace element interacts with the major components of the host mineral, the activity will depart from the ideal mixing relationship and at low concentrations the activity composition relations obey Henry s Law. This states that at equilibrium the activity of a trace element is directly proportional to its composition  

The relatively simple mixing relationships between trace elements and major elements in their host minerals mean that the distribution of trace elements between minerals and melt can be quantified in a simple way, as outlined below. 

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Controls on trace element distribution Table 4.2 Mmeral/melt partition coefficients for and itic liquids... 

The concentration of free metal species in soil solution is controlled by several factors, the most significant of which are thermody-namic/kinetic parameters. Mathematical approaches to modeling soil solution -solid-phase equilibria - are broadly described in numerous publications (Lindsay 1979, Sposito etal. 1984, Waite 1991, Wolt 1994, Sparks 1995, Suarez 1999), and several models for calculating activity coefficients for trace metals are overviewed and discussed. Waite (1991) concluded that mathematical modeling clearly has a place in extending the information that can be obtained on trace element species distributed by other methods and will be of practical use in systems for which determination of concentrations of all species of interest is impossible because of sensitivity constrains or other analytical difficulties . 

Within the limitations imposed by the determination of trace elements in the total crude oil, when their concentration is essentially confined to a variable portion of the total crude oil, we may conclude that the contents of the trace elements S, V, Cl, Na, Ni, Zn, Co, Mn, Se, Br and As are controlled by maturation processes rather than migration processes, though this may simply reflect maturation of the asphaltene fraction of the crude oils. S, V, Se and Ni account for more than one quarter of the cumulative variance and represent metallo-organic compounds. The factor analyses further demonstrate not only the -efficacy of our separation systems between crude oil and entrained formation water, but show that it is most unlikely that the Fe present in the crude oils originates from corrosion products incorporated during production. Four elements, Fe, Mn, As and Co represent essentially unique factors. Finally, it is clear that considerably more work.is justified on trace elements in crude oils, particularly in relation to their distribution within the various fractions and in relation to interactions between organic and inorganic components. 

This chapter is organized as follows We first attempt to discuss, in terms of simplified models, how particles carrying functional groups behave in solutions whose variables are known or controlled. This is followed by observations and interpretations on the concentration of trace elements in rivers and how these trace elements are distributed between particulate and dissolved phase. Then, we will consider the regulation of metal ions and of other reactive elements in lakes above all, it will be shown that the interaction of these trace elements with biotic and non-biotic particles and the subsequent settling of these particles will be of utmost importance for their removal from the water/column. Finally considerations will be given to inquire to what extent similar interpretations can be given to oceans. 

On a relative basis, i.e. residues per 1000, there is virtually no one species like the other. In contrast, different shell samples from the same species and obtained from the same natural habitat yield identical amino acid patterns. It is of interest that (1) the structure of carbonates (aragonite-calcite-vaterite), (2) the content in trace elements, and (3) the stable isotope distribution are markedly effected by fluctuations in salinity, water temperature, Eh/pH conditions, and some anthropogenic factors. The same environmental parameters determine to a certain degree the chemical composition of the shell organic matrix. This feature suggests a cause-effect relationship between mineralogy and organic chemistry of a shell. In the final analysis, however, it is simply a reflection of the environmentally-controlled dynamics of the cell. 

Complicated biological systems (bioassays) at trace element concentration levels typical for offshore waters, are subject to serious danger of contamination. Without extreme precautions e.g. Carpenter and Lively (1980) and Fitzwater et al. (1982) found the toxic effect (inhibition of primary production) of contamination by the incubation bottles. Effects of adsorption to walls and particulate matter (sediment) should not be underestimated. Use of clean lab techniques and regular check of the trace element concentrations throughout the (biological) experiments is necessary to get an indication of the actual concentration and possible distribution of the different elements. Depending on the type of experiments it could be possible that other parametes should be known or even controlled pE, PO2, ionic strength, temperature, DOC etc. 

Along with the matrix RMs, a series of other quality control tools such as calibrants and standard solutions was also produced. On the occasion of the first set of reference materials, calibrants for trace elements and major components were also prepared and sent to the laboratories in order to minimize the differences in results produced by the use of different calibrants. For the other two sets of materials, multi component standard solutions for PAHs and pesticides were prepared and distributed in order to check the analytical performance of the chromatographic methods used. 

Monaci et al. (1997) performed a lichen-biomonitoring study in Siena by means of two different methods. The pattern of air quality in the study area was examined on the basis of the in situ frequency of different species of epiphytic lichens, i.e. using their species-specific sensitivity to the complex mixture of phytotoxic pollutants in the urban environment. The distribution of trace elements was evaluated quantitatively by an analysis of thalli of a tolerant species, P. caperata, known to be a reliable bioaccumulator of persistent atmospheric pollutants. The values obtained for Al, Ba, Cr, Cu, Fe, Pb and S were significantly higher in Sienese lichens over and above controls. Traffic was found to be the major source of atmospheric pollution. The pattern of trace-elemental deposition did not always coincide with air quality. lAP values were found to reflect essentially the emission of gaseous phytotoxic pollutants in the urban environment. 

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