Arsenic oxidation state

For the XPS work, reference materials were examined to establish binding energies for the various arsenic oxidation states. Arsenic metal, AS2O3, As2S2> A82S3, all from Ventron, and AS2O3 from J. 

Since crushed basalt has been recommended as a major backfill component (1), experiments were completed to evaluate the rate of dissolved oxygen consumption and the redox conditions that develop in basalt-water systems under conditions similar to those expected in the near-field environment of a waste package. Two approaches to this problem were used in this study (l)the As(III)/As(V) redox couple as an indirect method of monitoring Eh and (2) the measurement of dissolved oxygen levels in solutions from hydrothermal experiments as a function of time. The first approach involves oxidation state determinations on trace levels of arsenic in solution (4-5) and provides an estimate of redox conditions over restricted intervals of time, depending on reaction rates and sensitivities of the analyses. The arsenic oxidation state approach also provides data at conditions that are more reducing than in solutions with detectable levels of dissolved oxygen. 

Analytical. Arsenic oxidation state determinations were per-formed by hydride generation-flame atomic absorption spectroscopy (AAS) at the University of Arizona Analytical Center. The analytical procedures are discussed in Brown, et al. (12). 

Arsenic Oxidation States. A solution sample was taken 257 hr after initiation of the 300°C basalt + arsenic-doped deionized water experiment (Run D2-8, Table II). The data from arsenic oxidation state AAS analysis of the initial As(V)-doped water (0-hr sample) and of the 257-hr solution sample are given in Table HI. All detectable arsenic was in the +3 oxidation state [As(V) <15pg/L] in the 257-hr sample. Standard additions of AsGD) and As(V) to the 257-hr sample were quantitatively recovered. To desorb arsenic from particulates in this sample, an aliquot of the solution was treated with 5% hydrofluoric acid. The higher As(III) content of the treated 257-hr sample aliquot (110 vs. 61pg/L, Table HI) demonstrates that sorption occurred. Scanning transmission electron microscopic (STEM) analysis of the particulates indicated the presence of poorly crystallized high-iron illite . 

The arsenic oxidation state data and the calculated pH at 300°C (see Table H) allow an upper limit on the Eh of the solution in the basalt-water experiment to be estimated from Equation (2). Assuming aH,0 = 1 and As(V) = 15 pg/L, this upper limit Eh value is -400 100 mV. The basalt-fluid redox buffer mechanism of Jacobs and Apted (2) gives an Eh of about -600 mV at 300°C and pH 7.8 (19). This mechanism involves ferrous ironbearing basalt glass + water reacting to magnetite + silica. 

Geochemical analyses of the contaminated sediments in the root zone using sequential chemical extractions showed that greater than half of the arsenic is strongly adsorbed (Keon et al. 2000, 2001). A mixture of arsenic oxidation states and associations was observed and supported by bulk XANES and EXAFS data collected at the SSRL. Arsenic in the upper 40 cm of the wetland, which contains the peak corresponding to maximum deposition, appears to be controlled by iron phases, with a small contribution from sulfidic phases. The results suggest that iron oxide phases may be present in the otherwise reducing wetland sediments as a substrate onto which arsenic can adsorb, perhaps due to cattail root plaque formation. 

Bromine has a lower electron affinity and electrode potential than chlorine but is still a very reactive element. It combines violently with alkali metals and reacts spontaneously with phosphorus, arsenic and antimony. When heated it reacts with many other elements, including gold, but it does not attack platinum, and silver forms a protective film of silver bromide. Because of the strong oxidising properties, bromine, like fluorine and chlorine, tends to form compounds with the electropositive element in a high oxidation state. 

Inorganic ar senic normally occurs in two oxidation states As(V) and As(III). Arsenic (V) gives a significantly lower response than ar senic (III). For pre-reduction As(V) to the As(III) concentrated hydrochloric acid and potassium iodide/ascorbic acid reagents were used. As organoarsenic compounds do not react with sodium tetrahydi oborate, they were decomposed with a mixture of HNO and on a hot plate. 

Molybdenum blue method. When arsenic, as arsenate, is treated with ammonium molybdate solution and the resulting heteropolymolybdoarsenate (arseno-molybdate) is reduced with hydrazinium sulphate or with tin(II) chloride, a blue soluble complex molybdenum blue is formed. The constitution is uncertain, but it is evident that the molybdenum is present in a lower oxidation state. The stable blue colour has a maximum absorption at about 840 nm and shows no appreciable change in 24 hours. Various techniques for carrying out the determination are available, but only one can be given here. Phosphate reacts in the same manner as arsenate (and with about the same sensitivity) and must be absent. 

Procedure. The arsenic must be in the arsenic (III) state this may be secured by first distilling in an all-glass apparatus with concentrated hydrochloric acid and hydrazinium sulphate, preferably in a stream of carbon dioxide or nitrogen. Another method consists in reducing the arsenate (obtained by the wet oxidation of a sample) with potassium iodide and tin(II) chloride the acid concentration of the solution after dilution to 100 mL must not exceed 0.2-0.5M 1 mL of 50 per cent potassium iodide solution and 1 mL of a 40 per cent solution of tin(II) chloride in concentrated hydrochloric acid are added, and the mixture heated to boiling. 

This apparatus may also be adapted for what are termed hydride generation methods (which are strictly speaking flame-assisted methods). Elements such as arsenic, antimony, and selenium are difficult to analyse by flame A AS because it is difficult to reduce compounds of these elements (especially those in the higher oxidation states) to the gaseous atomic state. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

In aqueous geochemistry, the important distinguishing property of metals is that, in general, they have a positive oxidation state (donate electrons to form cations in solution), but nonmetals have a negative oxidation state (receive electrons to form anions in solution). In reality, there is no clear dividing line between metals and nonmetals. For example, arsenic, which is classified as a nonmetal, behaves like a metal in its commonest valence states and is commonly listed as such. Other nonmetals, such as selenium, behave more like nonmetals. 

Johnson and Pilson [229] have described a spectrophotometric molybdenum blue method for the determination of phosphate, arsenate, and arsenite in estuary water and sea water. A reducing reagent is used to lower the oxidation state of any arsenic present to +3, which eliminates any absorbance caused by molybdoarsenate, since arsenite will not form the molybdenum complex. This results in an absorbance value for phosphate only. 

With the exception of antimony (V), which requires the presence of iodide for its reduction, all species can be reduced in an acid medium at a pH of 1 -2. However, the reduction of some species, including antimony (III), arsenic (III), and all tin species, will also proceed at higher pH, where arsenic (V) and antimony (V) are not converted to their hydrides. This effect permits the selective determination of the various oxidation states of these elements [714, 716]. In the case of tin, reduction can be achieved at the pH of the Tris-HCl... 

Comparison of As K edge absorption spectra for tailings and reference compounds of known oxidation states shows that arsenate (As5+) is the dominant... 

The optimal reaction conditions for the generation of the hydrides can be quite different for the various elements. The type of acid and its concentration in the sample solution often have a marked effect on sensitivity. Additional complications arise because many of the hydrideforming elements exist in two oxidation states which are not equally amenable to borohydride reduction. For example, potassium iodide is often used to pre-reduce AsV and SbV to the 3+ oxidation state for maximum sensitivity, but this can also cause reduction of Se IV to elemental selenium from which no hydride is formed. For this and other reasons Thompson et al. [132] found it necessary to develop a separate procedure for the determination of selenium in soils and sediments although arsenic, antimony and bismuth could be determined simultaneously [133]. A method for simultaneous determination of As III, Sb III and Se IV has been reported in which the problem of reduction of Se IV to Se O by potassium iodide was circumvented by adding the potassium iodide after the addition of sodium borohydride [134], Goulden et al. [123] have reported the simultaneous determination of arsenic, antimony, selenium, tin and bismuth, but it appears that in this case the generation of arsine and stibene occurs from the 5+ oxidation state. 

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