Arsenic valence states

Creed, J.T., Martin,T. D., and Brockhoff, C. A. (1995). Ultrasonic nebulization and arsenic valence state considerations prior to determination via inductively coupled plasma mass spectrometry.J.zlnu/./lr. Spectrom. 10(6), 443. 

The most common toxic metals in industrial use are cadmium, chromium, lead, silver, and mercury less commonly used are arsenic, selenium (both metalloids), and barium. Cadmium, a metal commonly used in alloys and myriads of other industrial uses, is fairly mobile in the environment and is responsible for many maladies including renal failure and a degenerative bone disease called "ITA ITA" disease. Chromium, most often found in plating wastes, is also environmentally mobile and is most toxic in the Cr valence state. Lead has been historically used as a component of an antiknock compound in gasoline and, along with chromium (as lead chromate), in paint and pigments. 

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. 

Fig. 1. X-ray absorption near-edge structure (XANES) of reference compounds with various As valence states and mine tailings samples. The As K-edge excitation potential for arsenic in the ground state (As0) is at 11868 eV. The As K-edge excitation potential increases with increasing valence state.

Nitric acid reacts with practically all common metals. Such reactions, however, can vary, forming different products depending on the position of the metal in electrochemical series, the concentration of nitric acid, temperature, and pH. Very weakly electropositive metals such as arsenic, antimony, or tin are oxidized to oxides in higher valence states e.g.,... 

The only stable and naturally occurring isotope of arsenic is 75 As, where each atom of this isotope has 33 protons and 42 neutrons. The most common valence states of arsenic are —3, 0, +3 and +5. Arsenic and its compounds include elemental forms, organoarsenicals, arsenides, arsenosulfides, arsenites and arsenates. Arsenic forms also partially substitute for sulfide, sulfate, and possibly carbonate in a variety of minerals (Chapter 2). In the presence of surface and near-surface aerated water, arsenide and arsenosulhde minerals oxidize to more water-soluble arsenates (Chapter 3). 

The size of an arsenic atom depends on its valence state and the number of surrounding atoms (its coordination number). When valence electrons are removed from an atom, the radius of the atom not only decreases because of the removal of the electrons, but also from the protons attracting the remaining electrons closer to the nucleus (Nebergall, Schmidt and Holtzclaw, 1976), 141. An increase in the number of surrounding atoms (coordination number) will deform the electron cloud of an ion and change its ionic radius (Faure, 1998), 91. Table 2.2 lists the radii in angstroms (A) for arsenic and its ions with their most common coordination numbers. 

The most common valence states of arsenic are —3, 0, +3, and +5 (Shih, 2005), 86. The —3 valence state forms through the addition of three more electrons to fill the 4p orbital. In the most common form of elemental arsenic (As(0)), which is the rhombohedral or gray form, each arsenic atom equally shares its 4p valence electrons with three neighboring arsenic atoms in a trigonal pyramid structure ((Klein, 2002), 336-337 Figure 2.1). The rhombohedral structure produces two sets of distances between closest arsenic atoms, which are 2.51 and 3.15 A (Baur and Onishi, 1978), 33-A-2. The +3 valence state results when the three electrons in the 4p orbital become more attracted to bonded nonmetals, which under natural conditions are usually sulfur or oxygen. When the electrons in both the 4s and 4p orbitals tend to be associated more with bonded nonmetals (such as oxygen or sulfur), the arsenic atom has a +5 valence state. 

Like sulfide in pyrite, arsenic in arsenic-rich (arsenian) pyrite (FeS2) and many arsenide and arseno-sulflde minerals has a valence state of — 1 or 0. These valence states result from arsenic forming covalent bonds with other arsenic atoms or sulfur (Klein, 2002), 340, 369 (Foster, 2003), 35 (O Day, 2006), 80. In the arsenide niccolite (also called nickeline, NiAs), every nickel atom is surrounded by six arsenic atoms, where arsenic has a valence state of —1 and nickel is +1 (Klein, 2002), 360 (Foster, 2003), 35. The... 

The oxidation of arsenic refers to an increase in its valence state to as high as +5 through chemical reactions that cause the arsenic to lose valence electrons. As examples, As(0) may oxidize to As(III), and As(III) to As(V). During the oxidation process, chemical oxidants receive the electrons from the arsenic and are reduced. 

The reduction of arsenic refers to a decrease in its valence state to as low as —3 through chemical reactions that cause the arsenic to gain valence electrons. During the reduction process, reductants are oxidized as they donate electrons to arsenic. In general, As(V) converts faster into As(III) in reducing environments than As(III) transforms into As(V) under oxidizing conditions (Stollenwerk, 2003), 71. 

Biomass may also sorb As(III) from water. Teixeira and Ciminelli (2005) removed considerable As(III) with ground chicken feathers treated with ammonium thioglycolate. X-ray absorption near edge structure (XANES) spectra indicate that the adsorbed arsenic is still in the +3 valence state and that each atom is bound to three sulfur atoms associated with reduced cysteine amino acids (HC>2CCH(NH2)CH2SH) in the feathers. At pH 5 and biomass dosages of 2.0gL 1, the sorption capacity of the material was as high as 0.265 mmol As(III) g-1 biomass (19.9 mg As(III) g-1 biomass Table 7.2). The presence of 0.01 mol L-1 of phosphate had only minor effects on the sorption capacity, which was 0.260 mmol As(III) g 1 biomass (19.5 mg As(III) g-1 biomass) (Teixeira and Ciminelli, 2005, 898). 

Arsenate A mineral, compound, or aqueous species containing AsC>43, where the valence state of the arsenic is pentavalent (As(V)). Arsenate is often abbreviated As(V) in the literature, especially in documents dealing with arsenic treatment. The inorganic arsenic acid species, EUAstTu and HAsC>42, are the most common dissolved forms of arsenate in near / H neutral aqueous solutions (compare with arsenite and thioarsenate). 

Arsenide An arsenic atom with a valence state of —3. A mineral or other compound where the major anion is As3-. 

Extended X-ray absorption fine structure (EXAFS) spectrum Part of an X-ray absorption spectrum that is used to identify the coordination of atoms, estimate bond lengths, and determine the adsorption complexes on the surfaces of adsorbents. EXAFS spectra may provide useful information on the speciation (valence state), surface complexes, and the coordination of arsenic on adsorbents (e.g. (Randall, Sherman and Ragnarsdottir, 2001 Ladeira, et al. (2001) Teixeira and Ciminelli (2005) Kober, et al. (2005)) (compare with X-ray absorption spectroscopy (XAS), X-ray absorption near edge structure (XANES) spectra, and X-ray absorption fine structure spectroscopy (XAFS)). 

Fourier transform infrared (FTIR) spectroscopy An analytical method that uses infrared radiation to investigate the chemical characteristics of a sample. This method may be used to identify the valence states of arsenic on adsorbents and bonds between arsenic and other elements (e.g. (Goldberg and Johnston, 2001)) (compare with Raman spectroscopy). 

Speciation (chemistry) The chemical species in a sample, which includes information on its valence state and specific chemistry. For example, the speciation of arsenic in a groundwater sample may include arsenic fluoride species, such as AsCbF2-. Also, an analytical method that identifies the valence state of a chemical species in a sample. 

Valence state Also called the oxidation number or oxidation state. An integer (positive, negative, or zero) that describes the number of electrons that must be added or removed from an atom to give it a neutral charge. Typical valence states for arsenic are -3, 0, +3, and +5. 

X-ray absorption near edge structure (XANES) spectrum An analysis from X-ray absorption spectroscopy (XAS) and, in particular, X-ray absorption fine structure (XAFS) spectroscopy. XANES can be used to identify the valence state of arsenic in solid samples (Teixeira and Ciminelli, 2005 Kober et al., 2005). 

Arsenic, chromium, mercury, selenium, and tin have been the object of numerous investigations. Because some of them are classified as probable human carcinogens23-25 (strictly speaking, some of their species), the accurate assessment of concentration and speciation in environmental matrices is enormously important. Unfortunately, such factors as chemical reactions between species, low concentration, microbial activity, redox conditions, as well as the presence of other dissolved metal ions, may cause the amounts and distributions of chemical species in a sample to vary. In response to these problems, analytical research efforts have focused on developing techniques enabling the original valence state of the metals to be preserved. Table 2.3 lists some of these stabilization methods. 

Another group of As-bearing minerals contains arsenic in the 5+ valence state as arsenate commonly substituting for the phosphate group. An unidentified As-bearing iron phosphate, usually associated with banded iron oxyhydroxide as veins or masses, has a P/As atomic ratio on the order of 4 (Belkin et al., 1997). Jarosite... 

The cyano- complex of univalent gold, Au(CN), has been mentioned in connection with recovery of the metal. Other stable derivatives of this valence state are complexes with gold-to-sulfur, gold-to-arsenic, and gold-to-phosphorus bonds. The structures pictured below may be regarded as representative ... 

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