Other redox-sensitive elements

Kneebone, P.E. and Hering, J.G. (2000) Behavior of arsenic and other redox-sensitive elements in Crowley Lake, CA a reservoir in the Los Angeles aqueduct system. Environmental Science and Technology, 34(20), 4307-12. 

Smedley, P. L, Zhang, M., Zhang, G. Y., and Luo, Z. D., 2001c, Arsenic and other redox-sensitive elements in groundwater from the Huhhot Basin, Inner Mongoha in Cidu, R., ed., Water Rock Interaction Lisse, A.A. Balkema, p. 581-584. 

Other examples of redox-sensitive elements include heavy elements such as uranium, plutonium, and neptunium, all of which can exist in multiple oxidation states in natural waters. Redox conditions in natural waters are also indirectly important for solute species associated with redox-sensitive elements. For example, dissolution of iron (hydr)oxides under reducing conditions may lead to the solubilization and hence mobilization of associated solid phase species, e.g. arsenate, phosphate (see Sections 3.3.2.1, 3.3.3.2, and 3.3.4.1). 

Particle reactive metals such as lead and trace metals with nutrient-like behavior (e.g.. Cd, Kremling and Pohl, 1989) are mostly removed from surface waters by adsorption or incorporation into particles. They accumulate at horizontal interfaces such as the pycnocline or the redox boundary because of slow sinking rates. On their way through the water column, trace metals adsorbed to particles can be influenced by the change of important variables such as salinity and pH and several other processes including agglomeration and modification of redox-sensitive elements. 

Redox-sensitive elements such as As, Mo, Se, and V, are soluble along with U(VI) in oxidized groundwaters where they occur as oxyanions. However, in the reduced waters at the redox interface of a roll front or other sedimentary U deposit, they are precipitated nearby along with U(IV) in insoluble minerals (cf. Wanty et al. 1987). 

Two metals (Zn and Mn) have been chosen as proxies of sapropel formation because they show distinct concentration features during the sapropel formation (Fig. 2). Zn in contrast to Mn is not a redox sensitive element and therefore its concentrations merely reflect the overall mobilization and accumulation of trace metals. Zn on the other hand exhibits almost 20% higher concentrations in the sapropel compared to the ambient sediment above and below the sapropel. A ratio, lO Zn/Mn, mirrors the sapropel formation to a very high degree (Fig. 3). It increases very sharply from 0.6 at a sediment depth close to 42 cm to 1.1 at the beginning of the sapropel at a sediment depth of 39 cm. This initial stage is usually referred to as a protosapropel. The metal ratio then stays between 1 and 1.1... 

It now seems that an overdose of paracetamol causes a massive chemical stress, which causes an immediate adaptive defense response in the liver cell, which senses danger via redox-sensitive transcription factors. A number of mechanisms are involved, including the release, as a result of the stress, of a transcription factor Nrf-2 from its binding with Keap 1, a cytoplasmic inhibitor. Nrf-2 translocates to the nucleus and with other activators binds to an antioxidant-response element. This leads to transcription of a number of genes, so producing a... 

Arsenic and selenium demonstrate many similarities in their behavior in the environment. Both are redox sensitive and occur in several oxidation states under different environmental conditions. Both partition preferentially into sulfide minerals and metal oxides and are concentrated naturally in areas of mineralization and geothermal activity. Both elements occur as oxyanions in solution and, depending on redox status, are potentially mobile in the near-neutral to alkaline pH conditions that typify many natural waters. However, there are also some major differences. Selenium is immobile under reducing conditions while the mobility of arsenic is less predictable and depends on a range of other factors. Selenium also appears to partition more strongly with organic matter than arsenic. 

As a complication to Ip concepts, several of the above elements are redox-sensitive, and thus can occur in more than one valence state. These include As, Cr, Fe, Mn, Mo, and U among many others. The solubilities of these elements compounds depend on which oxidation state of the element is stable in the compound and in solution for the given conditions. Thus, minerals of reduced Fe and Mn +, and of the oxidized form of uranium, are comparatively soluble, and the.se. species occur as soluble aquocations. In contrast, Fe and Mn (oxidized) and (reduced) most often occur in insoluble oxides and hydroxides. 

The light actinide elements differ from their 4f analogues, the lanthanides, in their ability to exist in common solutions in oxidation states III through VI. The potential appearance of multiple oxidation states is of particular importance for the transport characteristics of redox-sensitive actinides, such as Np and Pu. Figure 2-2 shows the reported redox potentials for U, Np, Pu, and Am ions at acidic, neutral, and basic pH. For U, Np, and Am the redox potentials between the oxidation states differ sufficiently enough so that one or two states are usually favored over all others. The redox potentials of Pu in the oxidation states III, FV, V, and VI are all remarkably similar around approximately 1.0 V. Therefore, under certain conditions Pu can coexist in up to four oxidation states in the same solution. Table 2-1 summarizes the oxidation states of the actinides and highlights the cnvironmentully most relevant ones. Some of the oxidation states listed,. such as Pat III) or Put VII), can be synthesized only under extreme conditions, far from those round in natural. systems. In the III and IV oxidation. states, the actinides loini hydiated An and An ... 

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