Redox environment

Broadly speaking the classification of meteorites follows the geological mineral classification and with 275 mineral species reported so far this quickly becomes complex some classes of meteorite have only one member. The mineral structure does convey essential information about the temperature at which the meteorite formed as well as the reduction-oxidation (redox) environment was the environment in which it formed rich in oxygen Meteorites have been classified into three broad classes ... 

Fuerstenau (1980) found that sulphide minerals are naturally floatable in the absence of oxygen. Yoon (1981) ever attributed the natural floatability of some sulphide minerals to their very low solubility. Finkelstein et al. (1975) considered that the natural floatability of sulphide minerals are due to the formation of elemental sulphur and related to the thickness of formation of elemental sulphur at the surface. Some authors reported that the hydrophobic entity in collectorless flotation of sulphide minerals were the metal-deficient poly sulphide (Buckley et al., 1985). No matter whichever mechanism, investigators increasingly concluded that most sulphide minerals are not naturally floatable and floated only under some suitable redox environment. Some authors considered that the natural floatability of sulphide minerals was restricted to some special sulphide minerals such as molybdenite, stibnite, orpiment etc. owing to the effects of crystal structure and the collectorless floatability of most sulphide minerals could be classified into self-induced and sulphur-induced floatability (Trahar, 1984 Heyes and Trahar, 1984 Hayes et al., 1987 Wang et al., 1991b, c Hu et al, 2000). 

Microzone A small volume of water or sohd matter in which the redox environment is considerably different from that in the surrounding sediment or seawater. 

Based on the pE value, redox environments are classified as follows (a) pE > 7 indicates an oxic environment, (b) at pE values between 2 and 7, the environment is considered suboxic, and (c) pE < 2 indicates an environment considered anoxic. The occurrence of redox reactions in the subsurface environment is limited by the decomposition and reduction of water ... 

Gulens, J Champ, D.R. and Jackson, R.E. (1979) Influence of redox environments on the mobility of arsenic in groundwater, in Chemical Modeling in Aqueous Systems (ed. E.A. Jenne), American Chemical Society. 

Just choosing the most widely applied procedure (namely that of Tessier et al., 1979) could yield data of doubtful reliability for a particular matrix or objective, but may nevertheless allow comparison with results of many other studies. In practice, there is always an optimisation necessary between compatibility and reliability. The limitations reported here and elsewhere lead to the conclusion that results given by sequential sediment extraction experiments can be used for an assessment of specific release scenarios particularly related to changing pH, complexing ligand availability and redox environments rather than for true metal speciation in sediments. The latter can be achieved only by using intrumental speciation techniques, either alone or in combination with sequential extraction. It is in this area of research that new developments have appeared since the first edition of this volume. Particularly... 

Further in vivo experiments demonstrated that AS exhibits a unique nitrosation signature which differs from that of DEA-NO inasmuch as substantial amounts of a mercury-resistant nitroso species are generated in the heart, whereas Y-nitrosothiols are the major reaction products in plasma. These data are consistent with the notion that the generation of nitroxyl in vivo gives rise to formation of nitrosative post-translational protein modifications in the form of either S- or Wnitroso products, depending on the redox environment. 

The redox environment can also determine some of the properties of metallic and non-metallic species. For example, the toxicity of arsenic when present in oxic (oxidizing) environments such as As( V) is very low, whereas its reduced form, As(III), is highly poisonous. The opposite occurs with Cr(VI) that is much more toxic than its reduced counterpart, Cr(III). 

Mobility can also be severely affected by the redox environment. For example, Fe(II) and Mn(II) species are ordinarily soluble in natural waters deficient in oxygen, but their oxidized forms precipitate quite easily. The stability regions of insoluble Mn oxides can be seen in the Pourbaix diagram depicted in Figure 6.8. 

Brault, S., Gobeil, F., Fortier, A., Honore, J.C., Joyal, J.S., Sapieha, P.S., Kooli, A., Martin, E., Hardy, P., Riheiro-da-Silva, A., Peri, K., Lachapelle, P., Varma, D.R., Chemtob, S. (2007). Lysophosphatidic acid induces endothelial cell death by modulating redox environment. Am. J. Physiol. 292 R1174-83. 

Influence of Redox Environments on the Mobility of Arsenic in Ground Water... 

Hydrous oxide surfaces of sand immobilize As by adsorption processes ( 3). The results of our studies show that the extent of adsorption varies with the oxidation state of the As, the redox environment and/or the pH of the eluting water. The influence of these parameters on the mobility of As was studied by eluting As through sand columns waters of different redox... 

The adsorption and retention characteristics of arsenic are also influenced by the amount of As loaded on the column and by the nature of the column materials. Much stronger retention of both arsenic species is observed. Figure 4, when the amount loaded is decreased from 180 yg to 0.01 yg, but the influence of the initial oxidation state of the arsenic and of the redox environment on mobility are still evident. The change in mobility of both arsenic species as a result of the decrease in the amount loaded reflects the limited adsorption capacity of these sands. The influence of the column material on mobility is illustrated in Figure 5 increased adsorption of both As(III) and As(V) species is observed when medium-grained sands (5-35 mesh size and with Fe and Mn content 0.8% and 0.013% (w/w) respectively) are used as compared to the fine-grained sands (both sands were loaded with 180 yg As). 

On the basis of this study, it is not possible to decide whether pH or the redox environment control the mobility each has an important role. Nonetheless, a marked difference is observed in the mobility between As(V) and As(III), and the relative concentration of each species is governed primarily by the redox environment. 

The influence of redox environments and/or pH on the mobility of arsenic through sand columns was studied by using waters of different redox characteristics for elution and by comparing the elution profiles of As(V) to that of As(III). The mobility of arsenic was affected by each of the above parameters, by the amount of arsenic loads onto the columns and the nature of the column materials. Solution studies have shown that both As(III) and As(V) from complexes with Fe(III) the solubility of the complexes being dependent upon the oxidation state of the arsenic and the solution pH. 

Change in the cellular redox environment can lead to several biological effects ranging from altered signal transduction pathways, gene expression, mutagenesis and cell death (apoptosis). Oxidative stress has now been implicated in many diseases such as atherosclerosis, Parkinson s disease, Alzheimer s disease, cancer, etc. For the protection of cells from oxidative stress, supplementation with exogenous antioxidants becomes necessary. 

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