Cycles, arsenic

Start the vapour generator cycle so that the absorption cell is flushed with argon gas and the pre-set volume of NaBH4 (1 mL) is pumped into the sample vessel. After the pre-selected reaction time (0.5 minute), AsH3 vapour is flushed into the absorption tube. Record the value of each arsenic signal as a peak height measurement. Read off the arsenic concentration of the sample, which is displayed on the instrument video screen. 

The range of processes that must be considered in the cycle of metals is described in Fig. 15-10 (Nelson et al., 1977). Both the complexity of metal cycle analysis in a real system and the importance of speciation are well-stated by Andreae (1979) in his overview of the arsenic cycle in seawater ... 

The biological cycle of arsenic in the surface ocean involves the uptake of arsenate by plankton, the conversion of arsenate to a number of as yet unidentified organic compounds, and the release of arsenite and methylated species into the seawater. Biological demethylation of the methyl-arsenicals and the oxidation of arsenite by as yet... 

The initiation step in the chain oxidation, reaction (43), is not affected by the presence of oxygen. SO4 radicals formed in (43) give arsenic(fV), reaction (45), initiating the following propagation cycle which leads to the reformation of As(IV)... 

The As (arsenic) concentration of seawater is controlled by input of rivers, sedimentation on the seafloor, weathering of the seafloor, exchange between atmosphere and seawater, volcanic gas input, and hydrothermal input. Previous studies on the geochemical cycle of As have not taken into account the hydrothermal flux of As. Therefore, hydrothermal flux of As from back-arc, island arc and midoceanic ridges to ocean is considered below. 

Chilvers D.C., Peterson P.J. Global cycling of arsenic. In Lead, Mercury and Arsenic in the Environment, T.C. Hutchinson, K.M. Meema, eds. New York John Wiley. 1987. 

Hsu PH (1989) Aluminum hydroxides and oxyhydroxides. In Dixon JB, Weed SB (ed) Minerals in soil environments, 2nd edn, pp 331-378 Inskeep WP, McDermott TR, Fendorf S (2002) Arsenic (V)/(III) cycling in soils and natural waters chemical and microbiological processes. In Frankenberger WT Jr (ed) Environmental chemistry of arsenic. Marcel Dekker, New York, pp 183-215... 

The application of these techniques has led to the discovery of a number of organometallic species of arsenic, tin, and antimony in the marine environment. Germanium has not been observed to form organometallic compounds in nature. Some aspects of the geochemical cycles of these elements which have been elucidated by the use of these methods are discussed. 

As mentioned above (see Scheme 1), three main directions of the decomposition of intermediates that formed are possible when phosphorus and arsenic ylides react with compounds bearing C=X bonds 5,6,19,63,64,88 (i) elimination of R3E15=X to form olefins (Wittig type reaction) (ii) retro-Wittig type decomposition and (iii) elimination of R3E15 and formation of three-membered cycles (Corey-Chaykovsky type reaction). According to the data of Erker and coworkers,12,13,51 under kinetic control, the reaction of phosphorus ylides with thiocarbonyl compounds also affords phosphines and thiiranes, whose further transformations lead to olefins and R3PS under thermodynamic control. 

Crecelius, E. A., Arsenic geochemical cycle in Lake Washington, Limnol. Oceanog.. 20, 441-451 (1975). 

The most important concept with respect to arsenic cycling in the environment is constant change. Arsenic is ubiquitous in living tissue and is constantly being oxidized, reduced, or otherwise... 

Developing appropriate models of arsenic cycling and budgets in natural ecosystems. 

Lee, T.C., K.C.C. Lee, C. Chang, and W.L. Jwo. 1986a. Cell-cycle dependence of the cytotoxicity and clastogenicity of sodium arsenate in Chinese hamster ovary cells. Bull. Inst. Zool. Acad. Sinica 25 91-97. 

Sanders, J.G. 1980. Arsenic cycling in marine systems. Mar. Environ. Res. 3 257-266. 

We can see that the soluble and exchange forms of these metals are present in small amounts accounting merely for a few percent of the total metal content in soil. The content of organometal species is relatively high in the upper profile rich in humic species, whereas it drops sharply in the mineral horizons. Copper is extensively involved in the biogeochemical cycle in the Forest ecosystems and this is less profound for cobalt. It is noteworthy that a large part of metals (in particular, of copper) become bound to iron hydroxides. This is typical for various trace elements, including arsenic, zinc and other elements with variable valence. 

Arsenic and nitrogen compete for the electrons participating in the natural electron-relay cycles in the body. The number of electrons transferred by nitrogen depends on the number of arsenic atoms competing for them. 

Figure 8.16 Arsenic and nitrogen compete for electrons both for and from the natural relay cycles in the body. The overall rate at which electrons are transferred by the nitrogen will alter when arsenic competes for them. The arsenic is poisonous, since these two pathways yield different products...

The first time the body realizes that arsenic has been incorporated is when the redox activity (as above) proceeds at potentials when nitrogen or phosphorus are inert. By the time we detect the arsenic poisoning (i.e. we feel unwell), it is generally too late, since atoms of arsenic are covalently bound within body tissue and cannot just be flushed out or treated with an antidote. The arsenic sequesters electrons that might otherwise be involved in other relay cycles, which is a concurrent kinetic process see Figure 8.16. 

Figure 8. Regeneration of ODA-clinoptilolite column loaded with arsenate by means of 2% NaCl aqueous solution and breakthrough curves for ODA-clinoptilolite in arsenate solution of co = 25 mg/L repeated cycle after regeneration, first cycle breakthrough curve on Pb-clinoptilolite (from the left).

Since properties of both phosphate and arsenate are very similar each other, the adsorption of phosphate was examined prior to the adsorption of arsenic species. Here, the feeding solution in the adsorption operation was 1 mM phosphate solution of pH3. Table 1 summarizes detailed experimental conditions and column performances during repeated adsorption-elution-regeneration cycles. Since supplied volumes of the feed are not constant (101 - 193 BV), it is not easy to judge the efficiency of the adsorption from total uptake of phosphate. Thus, removal of phosphate until 100 BV is listed at the last column of Table 1 as an index of the column performances. 

Similar studies of the formation of the first monolayer of GaAs have also been performed [157,252,253]. The chemistry of arsenic facilitates the ECALE cycle, as it is easily reduced to HsAs at negative potentials (Fig. 42) ... 

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