Ferric arsenite

Ferric arsenite is reduced to a slight extent when heated in an inert atmosphere 1 if heated in hydrogen under pressure it decomposes, forming iron arsenide, iron oxide and arsenious oxide.2... 

On adding freshly precipitated ferric arsenite to potassium hydroxide solution until no more dissolves, and subsequently evaporating, the soluble potassium salt, 0K2O.5Fe2Os.9As2O,.24H26, is obtained as a reddish-brown amorphous substance, which dissolves in water, yielding an alkaline solution. ... 

Ferric orthoarsenite cannot be prepared directly from ferric hydroxide and arsenious oxide.4 The brown product obtained by shaking freshly precipitated ferric hydroxide with an aqueous solution of arsenious oxide has been described 5 as a basic ferric arsenite of composition 4Fe203.As203.5H20. A similar substance is obtained by adding aqueous arsenious oxide or sodium arsenite to ferric acetate solution. If ferric chloride, sulphate or nitrate is used, the ferric salt is not completely precipitated. The product is oxidised in moist air, and decomposes when heated. It is very doubtful whether this is a chemical individual, however, for it has been shown that the removal of arsenious oxide from the solution by the ferric hydroxide is due to adsorption, the amount removed depending upon the conditions and the age of the adsorbent. This subject is discussed more fully on p. 154. 

Ferric arsenite, 4Fe203.As203.5H20, may be prepared by shaking freshly precipitated ferric hydroxide with an aqueous solution of arsenious oxide, or by adding sodium arsenite (or an aqueous solution of arsenious oxide) to ferric acetate. It is brown in colour, and oxidised by the air when moist.2... 

SYNS FERRIC ARSENITE,BASIC FERRIC ARSENITE, soUd (DOT)... 

FERRATE(4-), HEX.YKIS(CYANO-C)-, TETRAPOTASSIUM, (OC-6-11)- seeTECSOO FERRIAMICIDE see MQW500 FERRIC ARSENATE, soUd (DOT) see IGNOOO FERRIC ARSENITE, soUd (DOT) see IGOOOO FERRIC ARSENITE, BASIC see IGOOOO FERRIC CHLORIDE see FAUOOO FERRIC CHLORIDE (UN 1733) (DOT) see FAUOOO FERRIC CHLORIDE, soludon (UN 2582) (DOT) see FAUOOO... 

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. 

Figure 3 - Mixed Ferric Arsenite and Arsenate Precipitation...

An interesting feature of the Russian process is the two-step method employed for the complete recovery of arsenic from solution waste-streams. In the first step, which is similar to the recovery method used in the Thylox process, the solution is heated to 70°C (158 F), and arsenic sulfide is precipitated by the addition of 75% sulfuric acid. The precipitate is separated from the liquid by filtration, dissolved in aqueous sodium carbonate, and returned to the circulating solution-stream. The clear liquid is then passed to the second step where it is made alkaline with sodium carbonate solution and treated with a solution of ferric sulfate. In this operation the small amount of arsenic remaining in the solution after the first step is fixed and precipitated as ferric arsenite and arsenate. The precipitate is finally removed by filtration, and the filtrate, which contains about 10 to 20 ppm of arsenic, is either discarded or processed for recovery of thiosulfate. Wooden tanks lined with acid-resistant materials are used in both steps of the arsenic-recovery operation. Each tank is sized for a solution residence time of 4 hr and provided with a mechanical agitator. 

The hberated iodine, as the complex triiodide ion, may be titrated with standard thiosulfate solution. A general iodometric assay method for organic peroxides has been pubUshed (253). Some peroxyesters may be determined by ferric ion-catalyzed iodometric analysis or by cupric ion catalysis. The latter has become an ASTM Standard procedure (254). Other reducing agents are ferrous, titanous, chromous, staimous, and arsenite ions triphenylphosphine diphenyl sulfide and triphenjiarsine (255,256). 

Pigna M, Colombo C, Violante A (2003) Competitive sorption of arsenate and phosphate on synthetic hematites (in Italian). Proceedings XXI Congress of Societa Italiana Chimica Agraria SICA (Ancona), pp 70-76 Quirk JP (1955) Significance of surface area calculated from water vapour sorption isotherms by use of the B. E. T. equation. Soil Sci 80 423-430 Rancourt DG, Fortin D, Pichler T, Lamarche G (2001) Mineralogical characterization of a natural As-rich hydrous ferric oxide coprecipitate formed by mining hydrothermal fluids and seawater. Am Mineral 86 834-851 Raven K, Jain A, Loeppert, RH (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environ Sci Technol 32 344-349... 

Haywood and Riley [14] have described a spectrophotometric method for the determination of arsenic in seawater. Adsorption colloid flotation has been employed to separate phosphate and arsenate from seawater [15]. These two anions, in 500 ml filtered seawater, are brought to the surface in less than 5 min, by use of ferric hydroxide (added as 0.1 M FeC 2 ml) as collector, at pH 4, in the presence of sodium dodecyl sulfate [added as 0.05% ethanolic solution (4 ml)] and a stream of nitrogen (15 ml/minutes). The foam is then removed and phosphate and arsenate are determined spectrophotometrically [16]. Recoveries of arsenate and arsenite exceeding 90% were obtained by this procedure. 

In REACT, we prepare the calculation by disenabling the redox couple between trivalent and pentavalent arsenic (arsenite and arsenate, respectively). As well, we disenable the couples for ferric iron and cupric copper, since we will not consider either ferrous or cupric species. We load dataset FeOH+.dat , which contains the reactions from the Dzombak and Morel (1990) surface complexation model, including those for which binding constants have only been estimated. The procedure is... 

Fig. 31.5. Minerals formed during reaction at 25 °C of a hypothetical acid drainage water with calcite (top), and fractions of the amounts of arsenite, arsenate, copper, lead, and zinc present initially in solution that sorb onto ferric hydroxide over the course of the reaction path (bottom). Bottom figure is plotted against pH, which increases as the water reacts with calcite.

Chanda, M., O Driscoll, K. F., Rempel, G. L., Ligand exchange sorption of arsenate and arsenite anions by chelating resins in ferric ion form I. Weak-base chieating resin Dow XFS-4195. Reactive Polym. 7,1988, 251-261. 

Both arsenious acid and sodium arsenite are strongly adsorbed by ferric hydroxide. The amount adsorbed from a given volume of solution... 

Colloidal saccharated iron is sometimes used in place of ferric hydroxide as an antidote in arsenical poisoning, but its adsorptive capacity depends on the alkalinity of the medium.4 Thus a commercial preparation containing 0-75 per cent, of sodium hydroxide was found to adsorb 12-57 per cent, of arsenious oxide (reckoned on the amount of iron present) addition of alkali increased the adsorption until, with 1-28 per cent, of sodium hydroxide present, there was a maximum adsorption of 27 per cent. The addition of acid correspondingly diminished the adsorption. A gel of ferric magnesium hydroxide, if prepared without boiling, also adsorbs arsenic from sodium arsenite solutions.5... 

After an extensive study of the adsorption of arsenious oxide by metallic hydroxides,3 Sen concluded that this type of adsorption resembles that of cations by manganese dioxide, and that the chemical affinity between the adsorbent and the substance adsorbed plays an important part, thus differing from adsorption by charcoal. It has been observed that soils having a high absorption capacity for bases also absorb the arsenite ion from solutions of 0-001 to 0-01X concentration.4 The absorption increases with time, without reaching an end-point, and the process follows the normal adsorption equation C1=kC1Jn. The addition of ferric oxide or calcium carbonate to the soil considerably increases the capacity for absorption, but such salts as calcium sulphate or copper sulphate have no effect. 

Duquesne, K., Lebrun, S., Casiot, C. et al. (2003) Immobilization of arsenite and ferric iron by Acidithiobacillus ferrooxidans and its relevance to acid mine drainage. Applied and Environmental Microbiology, 69(10), 6165— 73. 

Jang, M., Min, S.-H., Kim, T.-H. and Park, J.K. (2006) Removal of arsenite and arsenate using hydrous ferric oxide incorporated into naturally occurring porous diatomite. Environmental Science and Technology, 40(5), 1636-43. 

Reductive Dissolution. Many substances in nature contain the same metal or metalloid, but under different oxidation states. For example, the metalloid arsenic may exist as arsenite (AsIII, As03) or arsenate (AsIV, As04) in the forms of ferrous-arsenite or ferric-arsenate, respectively. Ferrous-arsenite is more soluble than ferric-arsenate for this reason, one may be interested in studying the kinetics of arsenate reduction to arsenite. Similar chemistry applies to all elements present in soil-water systems with more than one oxidation state (e.g., iron, manganese, selenium, and chromium). 

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