Trace element redox-controlled

Concentrations of trace elements in soil solution may be controlled by the solubility of certain solid phases via dissolution/(co-)precipitation or by other physicochemical and biological processes such as adsorption-desorption, complexation, and redox reactions. 

Reduction-oxidation is one of the most important processes controlling solubility and speciation of trace elements in soils, especially for those elements with changeable values, such as Cr, As and Se. Within normal ranges of redox potentials and pH commonly found in soils, the two most important oxidation states for Cr are Cr(III) and Cr(VI). Cr(III) is the most stable form of chromium and less soluble and nontoxic, but Cr(VI) is mobile, soluble and toxic. The main aqueous species of Cr(III) are Cr3+, Cr(OH)2+, Cr(OH)3° and Cr(OH)4" and the major aqueous species of Cr(VI)... 

The oxidation state of redox-sensitive trace elements such as As(III)/ As(V) and Cr(III)/Cr(VI) is thus affected by the redox conditions, as indicated by the occurrence of major reduced species. Kinetic control of the redox reactions plays an important role. As(III) appears in the anoxic hypolimnion in agreement with the thermodynamic redox sequence together with Fe(II) and sulfide, although the reduction of As(V) is incomplete under these conditions. Whereas the reduced As(III) species can clearly be observed in the... 

Although trace elements typically occur at concentrations of less than 1 ppb (part per billion) (or pg L-1, also reported in molar units), these elements are important in estuaries because of their toxic effects, as well as their importance as micronutrients for many organisms. The fate and transport of trace elements in estuaries are controlled by a variety of factors ranging from redox, ionic strength, abundance of adsorbing surfaces, and pH, just to name a few (Wen et al., 1999). The highly dynamic nature of... 

Trace elements may be present in solution with positive or negative charges and in different redox states. They occur predominantly in cationic form [Pb, Cu, Zn, Ni, Cd, Hg, Cr(III), and Co], but some trace elements are present in anionic form [As, Se, Cr(Vl), Mo, and B]. Redox reactions, both biotic and abiotic, are of paramount importance in controlling the oxidation state, and thus mobility, phytoavailability, and toxicity of many trace elements, including Cr, Se, Co, Pb, As, Ni, and Cu (Huang and Germida, 2002 Sparks, 2003). 

The impact of soil microorganisms and enzymes on all redox and dissolution and/ or precipitation processes is very crucial, and can exert major control over the behavior of trace elements (Burns and Dick 2002, Naidu et al. 2001, Kostyuk and Bun-nerberg 1999). The basic microbial phenomena in cycling processes of trace elements in... 

The role of Cu as an essential trace element has focused attention on possible roles for copper chelation of biologically active ligands, with subsequent interference of normal transport and distribution, as well as the role of the metal in redox reactions due to the accessible oxidation states of (I) and (II). Similarly, the physiological response of copper levels in disease conditions [50] and the overall role of trace metals in health and disease [51, 52] are relevant and of considerable importance. The increase in serum copper content in infections, arthritic diseases, and certain neoplasms is well documented and, in fact, the subsequent decrease in level upon treatment has been used successfully as an indicator of cancer remission [50]. Copper complexes may be effective in therapy due in part to their ability to mimic this physiological response of elevated copper [53] and, clearly, the interplay of introduced copper with pre-existent bound copper and effects on copper—protein mediated processes will affect the ultimate biological fate of the complex. Likewise, while the excess accumulation of free Cu, and indeed Fe and Zn, caused by malfunction or absence of normal metabolic pathways is extremely damaging to the body, the controlled release of such metals may be beneficially cytotoxic. The widespread pharmacological effects of copper complexes have been briefly reviewed [54]. 

Redox potential (oxidation-reduction) is considered a master variable with respect to controls on the concentration and speciation of many trace elements in natural waters (Stumm and Morgan 1981). Shifts between oxic, suboxic and anoxic conditions represent one of nature s most dramatic chemical variations. The response of lanthanides to variations in redox conditions has been studied in many of the world s classic anoxic and suboxic basins. These include (1) the Black Sea (German et al. 1991, Schijf et al. 1991, 1994, Schijf and De Baar 1995), (2) Saanich Inlet (Canada) (German and Elderfield 1989), (3) Chesapeake Bay (Sholkovitz and Elderfield 1988, Sholkovitz et al. 1992), (4) the Cariaco Trench (De Baar et al. 1988), (5) the Mediterranean Sea (Schijf et al. 1995) and (6) the northwest Indian Ocean (German and Elderfield 1990). The latter two regions are located on ocean shelves while the first three basins are estuarine and coastal. Data from the papers cited above are compiled in table A12. 

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