Ferric arsenite precipitation

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... 

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 arsenic and iron in solution did not reflect the full extent to which the arsenopyrite had been oxidized. Acidiflcation of the culture medium in each flask with 1 ml of concentrated HCl at the end of the experiment increased the arsenic concentration in solution 1.6-fold and the iron concentration 4.4-fold in uninoculated flasks and 1.6- and 7.2-fold, respectively, in inoculated flasks. The increase in dissolved As and Fe on acidification suggests that a portion of the mobilized iron and arsenic was precipitated as iron arsenate and arsenite in inoculated as well as uninoculated flasks. The weight ratios of Fe/As were always higher over 21 days in uninoculated flasks than in inoculated flasks, and in both types of flasks dropped in the first few days of incubation and then increased again. Precipitation of ferric arsenate (scorodite) as well as potassium jarosite [KFcs (804)2(011)6] in bacterial arsenical pyrite oxidation was reported by Carlson et al.(35). 

Cassity and Pesic (36) found that arsenate but not arsenite stimulated dissolved Fe + oxidation by T. ferrooxidans through precipitation of Fe + as ferric arsenate. 

Extensive work has been done over the last twenty years on the removal of arsenic from heavy metal effluent streams. The most recent paper by Donnelly and Anderson (2) summarizes these developments and references more than 25 papers on this subject. One of the problems with arsenic precipitation is that arsenic will occur in two valence states, arsenate and arsenite. Using ferric sulfate as a precipitant, the pH regime for optimum precipitation is different for the two species. Figure 3 presents a graph which outlines the best pH for a mixture of the species. 

Arsenic. Arsenic is a frequent contaminant in metal ores (especially sulphides) and therefore appears in hydrometallurgical processes applied to these ores. Because of its toxicity, it must be efficiently removed from aqueous process streams before discharge. Traditionally, this has been done with lime, but a large excess is required and the product (calcium arsenite and arsenate) can re-release some As under the influence of atmospheric CO2. A more reliable process is co-precipitation with iron salts under near-neutral conditions. This requires that both elements are in their higher oxidation states, and H2O2 can achieve this efficiently in dilute solution. Ferric iron can be added as such or generated in situ after arsenic oxidation ... 

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