Sediments interaction with trace element

Carbonates, organic matter, Fe and Mn oxides, and clay minerals play important roles in controlling overall reactivity of trace elements in soils and sediments. This chapter addresses the interaction of trace elements with carbonates, organic matter, Fe and Mn oxides and clay minerals. Analytical techniques for trace element speciation in solid-phase and their distribution among various solid-phase components in arid and semi-arid soils are reviewed. Solubilities of trace elements in solid phases and their mineralogical characteristics in arid and semi-arid soils also are discussed. 

The determination of surface properties of particles is an important key to understanding interactions of trace elements and organic compounds between particulate and dissolved phases in estuarine and coastal systems. Specific surface area (SSA), cationic exchange capacity (CEC) and heat of immersion (AH) have been measured on native and treated suspended sediment and after oxidation with 15% H202- SSA and A H have also been measured on samples leached with NaOH and Na-dithionite in order to remove amorphous aluminosilicates. 

Trace element retention on earth materials is illustrated in several case studies, where selected contaminants (e.g., fluoride, cesium, mercury) interact with rocks, clays, soils, and sediments under different environmental conditions (e.g., pH, presence of organic ligands, salinity). 

This chapter considers methods of trace element speciation, and their application to soils, that involve selective chemical extraction techniques. It will be concerned firstly with extraction by single selective reagents and secondly with the development and application of sequential extraction procedures for soils and related materials. Sequential extraction procedures for sediments are discussed in depth in Chapter 11. Speciation in the soil solution and modelling aspects of its interaction with soil solid phases are comprehensively covered in Chapter 9 and will not be considered here. 

In the environmental field, specific attention is needed for the well known heavy metals, e.g. Pb, Cd, Hg, As, Tl. Another major interest is the interactions of these elements with the essential trace elements in the organism. Therefore not only the pure pollution view is in the foreground, but also the biochemistry of these elements which may have a toxic action in low concentration ranges. But the main point in the environmental field is the control of the environmental load of different substances. In this connection a lot of different matrices - soil, sediments, water, air, aquatic and terrestrial plants, animals and also human tissues and body fluids - are of interest. 

Fig. 2 shows the different pathways in which chemical elements contained in rocks are released to the different environmental compartments. Five main processes are responsible for their dispersion into the different ecosystems (1) Weathering, either directly by rain water on rock outcrops, by soil percolation water or by root exsu-dates, which interact with rock fragments, contained in the soil cover (2) Down hill mechanical transport of weathered rock particles, such as creep and erosion and subsequent sedimentation as till material or alluvial river and lake sediments (3) Transport in dissolved or low size colloidal form by surface and groundwater (4) Terrestrial and aquatic plants growing in undisturbed natural situations will take up whatever chemical elements they need and which are available in the surface and shallow groundwater. Trace elements taken up from the soil will accumulate in the leaves and will possibly enrich the soil by litterfall (5) Diffuse atmospheric input by aerosols and rain rock particles from volcanic eruptions, desertic areas (Chester et al., 1996), seaspray and their reaction with rain water. A considerable part of this can be anthropogenic. 

The degree of toxicity of a pollutant thus depends on the quantity and also, to a considerable extent, on the form in which it is present. The measurement of the total concentration of a trace element provides little or no information about its bioavailability or its interactions with sediments and suspended particles. Questions central to inorganic environmental analysis were described as follows by Taylor (1979) " One of the major errors in attempts to relate data obtained from laboratory experiments to the situation in the field is the failure to take into account the chemical form in which the material is present. The questions which must be asked in all cases are What proportion of the material present in the test solution is harmful Will this biologically active material exist in the natural environment If so, for how long ... 

Co-precipitation on carbonates was observed to represent an environmentally highly relevant process, based, in fact, on the formation of mixed carbonate phases. Besides zinc and cadmium, as already mentioned, carbonates are also a significant substrate of strontium [42], with accumulation factors up to 40x. Moreover, for sorption processes it is not absolutely necessary for the carbonate to be soUd - colloids of CaCOj also take part in sorption processes [43]. Carbonates, besides being important substrates of lead in aquatic systems [44], have the same role in soils, too [45]. Elements shown to be easily precipitated as carbonates in soil include Ca, Sr, Ba, Fe(II), Zn, Cd and Pb [46]. Siderite, FeC03, can represent an important trace element substrate in marine sediments, as well as in interactions which occur in ground waters [47, 48]. 

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