Natural systems, trace metal complexation

While these calculations provide information about the ultimate equilibrium conditions, redox reactions are often slow on human time scales, and sometimes even on geological time scales. Furthermore, the reactions in natural systems are complex and may be catalyzed or inhibited by the solids or trace constituents present. There is a dearth of information on the kinetics of redox reactions in such systems, but it is clear that many chemical species commonly found in environmental samples would not be present if equilibrium were attained. Furthermore, the conditions at equilibrium depend on the concentration of other species in the system, many of which are difficult or impossible to determine analytically. Morgan and Stone (1985) reviewed the kinetics of many environmentally important reactions and pointed out that determination of whether an equilibrium model is appropriate in a given situation depends on the relative time constants of the chemical reactions of interest and the physical processes governing the movement of material through the system. This point is discussed in some detail in Section 15.3.8. In the absence of detailed information with which to evaluate these time constants, chemical analysis for metals in each of their oxidation states, rather than equilibrium calculations, must be conducted to evaluate the current state of a system and the biological or geochemical importance of the metals it contains. 

The enhanced chemiluminescence associated with the autoxidation of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) in the presence of trace amounts of iron(II) is being used extensively for selective determination of Fe(II) under natural conditions (149-152). The specificity of the reaction is that iron(II) induces chemiluminescence with 02, but not with H202, which was utilized as an oxidizing agent in the determination of other trace metals. The oxidation of luminol by 02 is often referred to as an iron(II)-catalyzed process but it is not a catalytic reaction in reality because iron(II) is not involved in a redox cycle, rather it is oxidized to iron(III). In other words, the lower oxidation state metal ion should be regarded as a co-substrate in this system. Nevertheless, the reaction deserves attention because it is one of the few cases where a metal ion significantly affects the autoxidation kinetics of a substrate without actually forming a complex with it. 

The solid-water interface, mostly established by the particles in natural waters and soils, plays a commanding role in regulating the concentrations of most dissolved reactive trace elements in soil and natural water systems and in the coupling of various hydrogeochemical cycles (Fig. 1.1). Usually the concentrations of most trace elements (M or mol kg-1) are much larger in solid or surface phases than in the water phase. Thus, the capacity of particles to bind trace elements (ion exchange, adsorption) must be considered in addition to the effect of solute complex formers in influencing the speciation of the trace metals. 

Figure 8.12 Schematic representation of trace metal interactions in a system containing an inorganic surface, micro-organisms and micro-organism exopolymers (adapted from Lion eta/., 1988). In a natural aquatic system other complexing substances will be present, namely fulvic-type compounds, which will interact with the metals, the solid surface and the biopolymers (Buffi eetal., 1998).

Nowadays, not only Fe but other trace metals as well, for example, Mn, Co, or Cu, are thought to limit primary production. It is thus a real challenge for oceanographers not just to assess correctly the very low levels of Fe and Mn in the oceans but also to carry out the speciation of these elements (total dissolved concentrations are at the nM level, labile forms oxidation states in natural aquatic systems Fe(II), which is readily soluble, and Fe(III), which is almost insoluble. Flowever, both Fe ions can form diverse complexes with organic ligands with different labilities and solubilities, and colloidal particles, which are also considered part of the dissolved phase. Manganese also exists in two oxidation states in aquatic systems soluble Mn(II) and insoluble Mn(IV) both are present in a dynamic cycle in seawater. The nonlabile Mn pool consists of oxidized Mn(IV) species, but these can be photochemically reduced and thus solubilized.23... 

Morel, F., McDuff, R. E. and Morgan, J. J. Interactions and chemostasis in aquatic chemical systems Role of pH, pE, solubility and complexation, p. 157-200, jn Singer, P.C. ed., "Trace Metals and Metal-Organic Interactions in Natural Waters," Ann Arbor Science Publishers, Ann Arbor, Michigan, 1973. 

The adsorption-isotherm and ion-exchange models are of limited applicability when modeling complex and variable natural systems, particularly when the sorbates of interest are minor or trace ionic species (<10 " to 10" mol/kg), and the sorbents exhibit pH-dependent surface charge. For such conditions, the adsorption of trace ionic species often takes place against the net surface charge of the sorbent. This is the behavior of most toxic trace metal cations, including those of the heavy metals and radionuclides when adsorbed by positively charged metal oxyhydroxides, for example. 

Equilibrium models are widely used in assessments of trace metal bioavailability, toxicity, and transport through the environment. Properly applied, equilibrium models are powerful tools in such assessments. Due to a variety of factors, however, equilibrium modeling often falls short of its full potential. One problem, of special importance in equilibrium characterizations, is simplistic modeling. The use of simplistic chemical models is particularly important because it affects not only the modeling of complex natural systems, but also modeling of relatively simple chemical media used to generate primary thermodynamic data. 

Sciences de TUnivers. His research interests focus on environmental mineralogy and biogeochemistry of metal contaminants and trace elements using X-ray structural techniques. In the mid-80s, he initiated a new research program on the structure and surface reactivity of poorly crystallized Fe oxides. In the early 90s, this program was extended to Mn oxides, and specifically to minerals of the bimessite family. In the mid-90s, he pioneered the application of synchrotron techniques to determination of the speciation of heavy metals in natural systems. In the last two years, he was a key developer of an X-ray microprobe at the Advanced Light Source of the Lawrence Berkeley National Laboratory dedicated to the study of complex environmental materials. He is also co-lead PI of the French Absorption spectroscopy beamline in Material and Environmental sciences (FAME) at the European Synchrotron Radiation Facility (ESRF) in Grenoble. 

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