Minerals arsenic-containing

In Bangladesh, 15-25% of the population is exposed to unsafe levels of arsenic in drinking water from aquifers in contact with arsenic-containing minerals. The analytical problem is to reliably and cheaply identify wells in which arsenic is above 50 parts per billion (ppb). Arsenic at this level causes vascular and skin diseases and cancer. 

Many other arsenic-containing minerals are known and have some thermochemical measurements but they are beyond the scope of this paper. 

II. ENVIRONMENTAL ARSENIC COMPOUNDS A. Arsenic-Containing Minerals... 

Dissolved arsenic concentrations can be limited either by the solubility of minerals containing arsenic as a constituent element (or in solid solution) or by sorption of arsenic onto various mineral phases. For both the precipitation-dissolution of arsenic-containing minerals and sorption-desorption of arsenic onto solid phases, equilibrium calculations can indicate the level of control over dissolved arsenic concentrations that can be exerted by these processes. However, neither of these types of reactions is necessarily at equilibrium in natural waters. The kinetics of these reactions can be very sensitive to a variety of environmental parameters and to the level of microbial activity. In particular, a pronounced effect of the prevailing redox conditions is expected because potentially important sorbents (e.g., Fe(III) oxyhydroxides) are unstable under reducing conditions and because of the differing solubilities of As(V) and As(III) solids. 

This chapter summarizes the present state of knowledge concerning the role of bacteria in the oxidation of dissolved arsenite to arsenate, and in generating dissolved arsenite and arsenate from arsenic-containing minerals. 

Arsenic is widely distributed about the earth and has a terrestrial abundance of approximately 5 g/t (4). Over 150 arsenic-bearing minerals are known (1). Table 2 fists the most common minerals. The most important commercial source of arsenic, however, is as a by-product from the treatment of copper, lead, cobalt, and gold ores. The quantity of arsenic usually associated with lead and copper ores may range from a trace to 2 —3%, whereas the gold ores found in Sweden contain 7—11% arsenic. Small quantities of elemental arsenic have been found in a number of localities. 

Only one phosphide mineral appears on the list. The tabulation contains twenty-six arsenate species but not a single arsenide. The fibrous minerals recorded reflect the limitations imposed by natural circumstances, and especially the opportunities for elements to be juxtaposed geochemically and able to produce a particular mineral compound. Oxygen containing compounds form by far the most prevalent natural fibers, but simple hydroxide and carbonate compounds are also prominent on the list. There are more oxyhalides and water-containing minerals than pure chloride minerals, al-... 

Many metals, including magnesium, aluminium,13 zinc, tin and iron,14 precipitate arsenic and liberate arsine from aqueous arsenic acid. When copper is placed in such a solution containing mineral acid, copper arsenide is formed on the metal 15 this reaction is employed under the... 

Interferences with arsenic adsorption and ion exchange Dissolved organics and anions may interfere with arsenic adsorption and ion exchange in both natural environments and water treatment systems. In some cases, chemical species directly compete with arsenic for adsorption sites. They may also desorb and replace arsenic. Vanadium is one element that could interfere with the adsorption of arsenic onto mineral surfaces. In most cases, vanadium is not abundant in water. However, alkaline (pH 7.0-8.8) groundwaters in the loess aquifers of La Pampa, Argentina contain up to 12mgL 1 of vanadium (Smedley et al., 2005). The vanadium readily hinders the sorption of As(V) onto iron (III) (oxy)(hydr)oxides (Chapter 3). 

As discussed earlier in Section 3.17, the excessive application of arsenic-bearing pesticides and phosphate fertilizers on agricultural lands, golf courses, and lawns may locally contaminate surface waters and ground-waters (Welch et al., 2000), (Lewis et al., 2002), 590. Phosphates desorb arsenic from mineral surfaces and readily interfere with the sorption and coprecipitation of arsenic onto iron (oxy)(hydr)oxides (Campos, 2002). Commercial phosphate fertilizers also frequently contain >13 mg kg-1 of arsenic impurities (Campos, 2002), which may further contribute to groundwater contamination. 

Sedimentary rocks with the highest arsenic concentrations largely consist of materials that readily sorb or contain arsenic, such as organic matter, iron (oxy)(hydr)oxides, clay minerals, and sulfide compounds. Arsenian pyrite and arsenic-sorbing organic matter are especially common in coals and shales. Ironstones and iron formations are mainly composed of hematite and other iron (oxy)(hydr)oxides that readily sorb or coprecipitate arsenic. Iron compounds also occur as cements in some sandstones. Although almost any type of sedimentary rock could contain arsenic-rich minerals precipitated by subsurface fluids (Section 3.6.4), many sandstones and carbonates consist almost entirely of minerals that by themselves retain very little arsenic namely, quartz in sandstones and dolomite and calcite in limestones. 

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). 

The application of lead isotope analyses to provenancing almost pure copper artifacts seems relatively straightforward in most instances, but what of arsenical copper and tin bronze alloys In principle, the arsenic in arsenical copper may have been derived from high-arsenic minerals (such as tennantite or basic copper arsenates) containing lead of a different isotopic composition from that of the copper ore used to produce the copper. However, absolutely no archaeological or other evidence of proves that this was ever done in the Bronze Age Aegean. On the contrary, at Kythnos definite evidence shows that arsenical copper was produced in EBII times from arsenical copper ores of variable (sometimes zero) arsenic content but of uniform lead isotope composition. 

Arsenic. More than 300 arsenate and associated minerals have been identified (Escobar-Gonzalez and Monhemius 1988). Inevitably, some of the arsenic contained in these minerals enters any industrial circuit, and concentrations of As in soils and waters can become elevated due to mineral dissolution. The original National Priority List (USA) identified approximately 1000 sites in the United States (USA) that posed environmental health risks (Nriagu 1994 Allen et al. 1995) with arsenic cited as the second most common inorganic constituent after lead (Database 2001). The more common oxidation states of arsenic are III and V, and the predominant form is influenced by pH and redox potential. In aqueous solutions of neutral pH, arsenate is present... 

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