Arsenic surface layer

An explosive variety (or compound) of arsenic was produced as a surface layer on the exposed iron surfaces of a corroded lead-lined vessel which contained 35% sulfuric... 

Martin et al. (2007a) investigated the accumulation and potential release of arsenic in a paddy field in Bangladesh irrigated with arsenic contaminated groundwater. The oxalate-extractable fraction related to amorphous hydrous oxide-bound arsenic represented the dominant arsenic form in the surface layer (47%). A high percentage of arsenic was removed by phosphate (22%). 

The later observation of Morita and Shibata (19) is relevant here. They examined the distribution of the various arsenic compounds in Hizikia fusiforme (this alga is unusual in that it contains about 50% of its total arsenic as arsenate). The total arsenic concentrations were higher at the surface layers than in the center of the alga. Arsenate... 

An explosive variety (or compound) of arsenic was produced as a surface layer on the exposed iron surfaces of a corroded lead-lined vessel which contained 35% sulfuric acid with a high arsenic content. It exploded on friction or ignition, and contained no hydrogen, but variable small amount of iron and lead. It may have been analogous to explosive antimony. 

The surface chemistry of Li in ethereal solutions has been studied rigorously by several groups. Koch et al. [72-74] studied the surface layers formed on Li in THF and 2Me-THF solutions of LiAsF6, which appear as brown films covering the electrodes. They concluded that these films are polymers of lithiated arsenic oxide that contain additional functional groups such as F, RCT, etc. However, these conclusions did not result from direct spectroscopic studies of Li surfaces. Important, indirect information on the composition of the surface films formed... 

These experiments pointed out that respiratory reduction of As(V) sorbed to solid phases can indeed occur in nature, but its extent and the degree of mobilization of the As(III) product is constrained by the type of minerals present in a given system. What remains unclear is whether micro-organisms can actually reduce As(V) while it is attached to the mineral surface, or if they attack a mono-layer of aqueous As(V) that is in equilibrium with the As(V) adsorbed onto the surface layer. If, as is the case for dissimilatory metal-reducing bacteria such as Geobacter sulfurreducens and Shewanella oneidensis (44,45), components of the electron transport chain are localized to the outer-membrane of some arsenate-respiring bacteria, direct reductive dissolution of insoluble arsenate minerals may be possible by attached bacteria. Too little is known at present about the topology... 

In gallium arsenide, relaxation affects mainly the surface layer The surface arsenic atoms of the (110) face are raised slightly with respect to the surface plane, whereas the gallium atoms are shifted to the inside. The same conclusions have been drawn for ZnO and ZnS ... 

Known reaction thermoc ramics and kinetics show the oxidizer distribution between alloy corrponents at the contact of liquid phases and the external diffusion process are conditioned by the sulphate concentrations in the surface layer of rretal melt and the equilibrium constants for interactions of metals with sulphate. In such cases the oxidation of liquid alloys enables the selective concentration of a target corrponent in one of the phases - slag or alloy. In particular, it has been found experimentally that in processing of iron-tin-arsenic rrelts by sodium and calcium sulphates the rate of oxidation of iron allowed the concentration up to 90% Sn and up to 90% As in alloy with the loss of not less than 60% Fe [3]. For short short... 

For a reconstmcted surface, the effect of an adsorbate can be to provide a more bulk-like enviromnent for the outemiost layer of substrate atoms, thereby lifting the reconstmction. An example of this is As adsorbed onto Si(l 11)-(7 X 7) [37]. Arsenic atoms have one less valence electron than Si. Thus, if an As atom were to replace each outemiost Si atom in the bulk-temiinated stmcture, a smooth surface with no impaired electrons would be produced, with a second layer consisting of Si atoms in their bulk positions. Arsenic adsorption has, in fact, been found to remove the reconstmction and fomi a Si(l 11)-(1 x l)-As stmcture. This surface has a particularly high stability due to the absence of dangling bonds. 

The use of a heavy arsenal of surface science (XPS, UPS, STM, AES, TPD) and electrochemical (cyclic voltammetry, AC Impedance) techniques (Chapter 5) showed that Equations (12.2) and (12.3) simply reflect the formation of an overall neutral backspillover formed double layer at the metal/gas interface. It thus became obvious that electrochemical promotion is just catalysis in presence of a controllable double layer which affects the bonding strength, Eb, of reactants and intermediates frequently in the simple form ... 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

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