Arsenopyrite arsenous acid

Because arsenopyrite is floatable in acidic conditions and non-floatable in basic conditions (see Fig. 2.16), it seems reasonable to assume that reactions (2-63) or (2-28) and (2-29) are dominant oxidation in acidic solutions. Elemental sulphur is responsible for the hydrophobicity of arsenopyrite in acidic media. In alkaline solutions, reactions (2-64) and (2-65) may be dominant resulting in the formation of oxy-sulfur species and arsenate species with minor sulphur. 

Ores of arsenic orpiment, realgar, arsenolite, arsenopyrite. Compounds of arsenic arsine, arsenic trioxide, arsenious acid, cupric hydrogen arsenite, arsenic pentoxide, arsenic acid, sodium arsenate. The Marsh test for arsenic. U.ses of arsenic and its compounds lead shot, insecticides, weed killers, chemotherapy. 

Arsenic trioxide may be made by burning arsenic in air or by the hydrolysis of an arsenic trihaUde. Commercially, it is obtained by roasting arsenopyrite [1303-18-0] FeAsS. It dissolves in water to a slight extent (1.7 g/100 g water at 25°C) to form a weaMy acidic solution which probably contains the species H AsO, orthoarsenous acid [36465-76-6]. The oxide is amphoteric and hence soluble in acids and bases. It is frequendy used as a primary analytical standard in oxidimetry because it is readily attainable in a high state of purity and is quantitatively oxidized by many reagents commonly used in volumetric analysis, eg, dichromate, nitric acid, hypochlorite, and inon(III). 

Flotation of arsenical gold ores associated with base metals is accomplished using a sequential flotation technique, with flotation of base metals followed by flotation of gold-containing pyrite/arsenopyrite. The pyrite/arsenopyrite is floated at a weakly acid pH with a xanthate collector. 

If arsenopyrite is allowed to stand in aqueous hydrochloric acid for some time, the formation of arsenic trisulphide may be observed.5... 

Arsenopyrite is the most common mineral where arsenic is a major component (Welch et al., 2000, 594, 597). Ideally, unaltered arsenopyrite contains about 85 % As(I-) and 15 % As(0) (Nesbitt, Muir and Pratt, 1995). The oxidation of arsenopyrite is primarily responsible for arsenic-rich acid mine drainage in many gold and other ore mines (Welch et al., 2000, 594, 597). 

A variety of bacteria and other microorganisms, such as the archaeum Ferriplasma acidarmanus, may be actively involved in the oxidation of arsenopyrite (Gihring et al., 1999 Cruz et al., 2005 Barrett et al., 1993). Specifically, (Gihring et al., 1999) collected Thiobacillus caldus and Ferriplasma acidarmanus from acid mine drainage at Iron Mountain, California, USA. The mine drainage had a temperature of approximately 42 °C, a pH of 0.7, and contained about 50 mg L 1 of arsenic. T. caldus growths on the surfaces of arsenopyrite actually hindered the oxidation of the mineral, whereas F. acidarmanus was very tolerant of arsenic and accelerated the dissolution of arsenopyrite (Gihring et al., 1999). 

Although iron, manganese, magnesium, calcium, and aluminum arsenates are usually too water soluble to control arsenic mobility in soils (Inskeep, McDermott and Fendorf, 2002), 187, iron, aluminum, or manganese arsenates occur in some acidic soils. In particular, scorodite may form from the partial weathering of arsenian pyrite or arsenopyrite (Inskeep, McDermott and Fendorf, 2002), 187. Calcium arsenates may be present in alkaline calcium-rich soils (Matschullat, 2000), 303 (Mandal and Suzuki, 2002), 204. 

The oxidation of arsenopyrite [FeAsS] releases both sulfur and arsenic. Buckley and Walker (1988) studied the oxidation of arsenopyrite in alkaline and in acidic aqueous solutions. In air, the mineral reacted rapidly, and the oxidation of arsenic to As(III) was more rapid than the oxidation of iron on the same surface. Only a small amount of sulfur oxidation occurred. Under acidic conditions, the mineral formed sulfur-rich surfaces. 

An investigation of the surface composition and chemical state of three naturally weathered arsenopyrite samples exposed for periods ranging from 14 d to 25 yr showed that the arsenopyrite surface has an effective passivating layer that protects the mineral from further oxidation (Nesbitt and Muir, 1998). The same samples were then reacted with mine-waste waters, which caused extensive leaching of the arsenopyrite surface below the oxidized overlayers. The acidic nature of the solution caused dissolution of the previously accumulated ferric arsenite and arsenate salts. 

The growth of the bacterium is inhibited by benzoic acid, sorbate, and sodium laurylate (Onysko et al., 1984), and nitrate at 50 mM inhibits completely the oxidation of ferrous ion by the bacterium (Eccleston et al., 1985). Although the bacterium is sensitive to chloride ion, it becomes resistant to 140 pM chloride ion by training (Shiratori and Sonta, 1993). The bacterium is fairly resistant to heavy metal ions its activity to oxidize ferrous ion is scarcely inhibited in the presence of 65 mM cupric ion, 100 mM nickel ion, 100 mM cobalt ion, 100 mM zinc ion, 100 mM cadmium ion, and 0.1 mM silver ion (Eccleston et al., 1985). The bacterium acquires the ability to grow even in the presence of 2 mM uranyl ion (Martin et al., 1983). Furthermore, it becomes resistant to arsenate and arse-nite by training a strain of the bacterium has been obtained which oxidizes ferrous ion in the presence of 80 mM arsenite and 287 mM arsenate (Collinet and Morin, 1990 Leduc and Ferroni, 1994). The resistant ability of the bacterium to arsenite and arsenate is important when they are applied for the solubilization of arsenopyrite (FeAsS) [reactions (5.8) and (5.9)]. Leptospirillum ferrooxidans is generally more sensitive to heavy metal ions than A. ferrooxidans (Eccleston et al., 1985). 

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