Dissolved arsenic species

In sulfide-rich and anoxic waters, dissolved thioarsenic species commonly occur. Thioarsenic species form when sulfur substitutes for one or more of the oxygens in the dissolved forms of arsenious or arsenic acids (e.g. fbAsCbS-). Traditionally, aqueous thioarsenic species have been identified as thioarsenites 

The formation of thioarsenates under sulfide-rich and anoxic conditions is not understood. Stauder, Raue and Sacher (2005) suggested that inorganic As(III) disproportionates into thioarsenates and elemental arsenic, perhaps through the following reaction  

However, Wallschlager and Stadey (2007), 3879 disagree. In their mass balance experiments, Wallschlager and Stadey (2007), 3879 did not detect any significant loss of soluble arsenic, which would be expected from the precipitation of elemental arsenic. 

The chemical properties of thioarsenates are largely unknown, but are expected to be different than the properties of thioarsenites or other arsenic species (Wallschlager and Stadey, 2007), 3880. If thioarsenates are prominent in sulfide-rich and anoxic environments, measurements of their sorption and other chemical properties are required to understand and predict their mobility in natural environments. 

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In natural waters, arsenic may exist as one or more dissolved species, whose chemistry would depend on the chemistry of the waters. Over time, arsenic species dissolved in water may (1) interact with biological organisms and possibly methylate or demethylate (Chapter 4), (2) undergo abiotic or biotic oxidation, reduction, or other reactions, (3) sorb onto solids, often through ion exchange, (4) precipitate, or (5) coprecipitate. This section discusses the dissolution of solid arsenic compounds in water, the chemistry of dissolved arsenic species in aqueous solutions, and how the chemistry of the dissolved species varies with water chemistry and, in particular, pH, redox conditions, and the presence of dissolved sulfides. Discussions also include introductions to sorption, ion exchange, precipitation, and coprecipitation, which have important applications with arsenic in natural environments (Chapters 3 and 6) and water treatment technologies (Chapter 7). 

H3ASO30 is the dominant dissolved arsenic species in most hydrothermal fluids because the fluids originate in reducing environments and mostly have pH values of 0-8 (Ballantyne and Moore, 1988, 478 Pokrovski et al., 1996 Chapter 2). The concentrations and compositions of dissolved arsenic species in hydrothermal... 

A. G. Howard, C. Salou, Cysteine enhancement of the cryogenic trap hydride AAS determination of dissolved arsenic species, Anal. Chim. Acta, 333 (1996), 89-96. 

Arsenic can exist in several oxidation states, as both inorganic and organometallic species, and in dissolved and gaseous phases (Table I). Dissolved arsenic species can adsorb to suspended solids and be carried down to the sediments in an aquatic system. Since gaseous arsenic compounds can form, arsenic can be removed from the sediments as dissolved gas or in gas bubbles (e.g. CH ). Thus, arsenic can cycle within aquatic ecosystems and this cyclic behavior has been reviewed by Ferguson and Gavis (1 ) and Woolson 2). In any given system, it is necessary to understand the behavior of a variety of different arsenic compounds as well as a variety of environmental compartments in order to totally characterize the cyclic behavior of this element. 

Kinetics and Adsorption. If microorganisms only take up dissolved arsenic species, then adsorption can affect rates of species transformations by lowering concentrations of reactants. In this section calculations of changes In concentrations of reactants and products of arsenic species transformations In sediments are presented. Rates of transformation are assumed to be proportional to species concentrations and the rate of another reaction, V, coupled to the transformation. That Is, the transformation reaction Is not assumed to be a source of energy or structural material for the microorganisms. Thus,... 

Andreae M. O. and Andreae T. W. (1989) Dissolved arsenic species in the Schelde estuary and watershed, Belgium. Estuar. Coast. Shelf Sci. 29, 421-433. 

Chiu, V. Q., and Hering, J. G., 2000, Arsenic adsorption and oxidation at manganite surfaces 1. Method for simultaneous determination of adsorbed and dissolved arsenic species Environmental Science Technology, v. 34, p. 2029-2034. 

Redox status, pH, and Fe presence affected the speciation and solubility of arsenic. The Yellow Latosol had a higher concentration of the As (V) than did the Red Latosol. At redox potentials of 200 and 400 mV, As (V) was the major dissolved arsenic species. The experimental data indicated that As solubility was mainly controlled by an iron phase. Under moderately reduced Red Latosol conditions (0 and —200 mV) in the Red Latosol, arsenite... 

In other situations, such as at mines in Ron Phibun (Thailand) and Globe and Phoenix (Zimbabwe), arsenopyrite may not completely oxidize and water-soluble arsenic species may form instead of less soluble scorodite (Williams, 2001, 274). The Ron Phibun and Globe and Phoenix waters contain up to 5.114 and 7.400mgL-1 of arsenic, respectively (Williams, 2001, 270). About 40% of the total dissolved inorganic... 

Another example of aqueous speciation that includes redox can be shown with the arsenic pe-pH diagram shown in Figure 1. Arsenic can exist in several oxidation states including As(-lll) as in arsine gas (ASH3), As(0) as in elemental arsenic, As(ll) as in realgar (AsS), As(lll) as in orpiment (AS2S3) and dissolved arsenite, and As(V) as in dissolved arsenate. Figure 1 shows the dominant dissolved species, arsenate and arsenite, and their hydrolysis products as a function of redox potential and pH based on the thermodynamic evaluation of Nordstrom and Archer (2003). These results show the dominance of hydrolysis for arsenate species, but it is of minor consequence for the arsenite species. 

Figure 1 Species predominance diagram for dissolved arsenic at 25 °C and 1 bar (source Nordstrom and...

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