Arsenate precipitation

The background of this study is the investigation at the site Bielatal (see Daus et al. this volume). The arsenic concentrations in the seepage water of this tailings pond are high (up to 4 mg/1) and natural arsenic precipitation processes are incomplete. The neutral pH, the presence of both arsenite and arsenate (sum 1 mg/L), and the oxygen saturation of the water are the boundary conditions of the described experiments. 

Phosphorus pentachloride also reacts to yield arsenic trichloride.7 Arsenious oxide in aqueous solution is reduced by hypophosphorous acid, especially on boiling, when phosphine is liberated and brown arsenic precipitated.8 The reduction is readily brought about by calcium hypophosphite dissolved in 10 parts of hydrochloric acid (dens. 1-126), this salt being preferable in use to the sodium salt and providing an extremely sensitive reagent, although in the presence of slight traces of arsenic the brown colour may appear only after the lapse of 20 to 30 minutes.9 Arsenious oxide dissolves in arsenic trichloride to form an oxychloride.10... 

Disodium hydrogen arsentate reacts in an analogous way the product is the white bismuth arsenate precipitate ... 

Gold arsenides.—Arsenic precipitates from auric-chloride solution an arsenide, AugAs.6 Fusion of thus substance with potassium cyanide converts it into a yellow arsenide, Au4As3, of density 16-2. Both substances have the character of alloys. 

Arsenic in soilds has been fractionated by Jackson s T28) procedure for soil phosphorus (15. 27). In this laboratory, a modification of Jackson s procedure is being used on sediment solids. A series of 1 molar solutions of NH4CI, NH4OH, acid ammonium oxalate (29) and HCl are used in sequence. The chloride fraction, or exchangeable fraction, contains weakly adsorbed, coulombically bound arsenic. The hydroxide fraction, contains iron and aluminum arsenate precipitates and surface precipitates (i.e. adsorbed arsenic species having both chemical and coulombic bonding to oxide surfaces). The oxalate, or reductant soluble fraction, contains arsenic occluded in amorphous weathering products. The acid, or calcium, fraction contains arseno-apatites. 

Moon, D. H., Dermatas, D. and Menounou, N. (2004). Arsenic immobilization by calcium-arsenic precipitates in lime treated soils. Sci. Total Environ. 330(1-3), 171-185. 

Amendments of ferrous salts, which oxidize to Fe(OH)3 in soils, and calcite (lime), which sorbs arsenate and supplies Ca to form Ca arsenate precipitates, seem to diminish the level of toxicity of As in soils. 

There are several methods for the evaluation of chemical compositions of polyphosphates produced by the hydrolysis of sodium meta-arsenate-phosphates. The first step of these methods is based on the isolation of polyphosphates as the silver salts. Crystals of sodium meta-arsenate-phosphate are dissolved by the above procedure. After the pH of the resulting solution is adjusted to 7.5, a 10% excess of IM silver nitrate is added. In the course of this treatment, silver arsenate precipitates, while the quantity of hydrogen ion is released according to the equation. 

B Escobar, E Huenupi, I Godoy, J Wiertz. Arsenic precipitation in the bioleaching of enargite by Sulfolobus BC. Biotechnol Lett 22 205-209, 2000. 

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. 

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

Initial laboratory tests aimed at stabilizing the arsenic at pH 5 and 8 showed a lead solubility of 30 an 32 ppm. These tests were carried out without excess lime addition. During the TCLP test, acetic acid is added so that the pH of the leach solution is not controlled at any particular value. This explains the relatively high Pb solubility. Phosphate was then added during the arsenic precipitation stage, again without excess alkaline material. The results of the TCLP tests with phosphate additions showed a Pb leachate concentration of 40 to 60 mg/1. These data indicate that phosphate alone is not able to control the leaching of the lead. 

Figure 5.2. Schematic representation of the integrated concept for combined transport and treatment of arsenic species (right polluted water entrance and outlet of treated water left a re-circulation vessel to which FeCls or AICI3 is periodically added for arsenic precipitation).

Arsenic species. Arsenic acid and soluble arsenates precipitate Zr from strongly acidic solutions. 

Arsenic species. Soluble arsenites and arsenates precipitate Co forming the corresponding cobalt arsenite or arsenate, bluish white, soluble in NH3 or in adds, including arsenic acid. 

Arsenite precipitates Ag3As03, bright yellow, insoluble in H2O, readily dissolved or transposed by both acids and bases. Arsenate precipitates Ag3As04, brown, insoluble in H2O, soluble in H3O and NH3. 

Arsenites precipitate from Pb ", bulky, white Pb3(As03)2 aq, difficultly soluble in water, readily soluble in dilute acids and OH . Arsenate precipitates white lead arsenate from neutral or shghtly alkaline Pb°, soluble in OH and HNO3 insoluble in CH3CO2H. It may be a mixture of Pb3(As04>2 and PbHAs04, depending on conditions. 

Heavy metals (except lead and arsenic) (precipitated as sulfides) trace 0.003... 

These facts led to a process design with two special characteristics a preliminary evaporation step to volatilize inherently volatile CW agents and other components, and a final arsenic precipitation process. These steps are followed by flue gas scrubbing. 

Arsenic Precipitation / Filtration Ferric Arsenate Special Waste)... 

Arsenates. Arsenic acid and ammonium, sodium and potassium arsenate precipitate uranium as uranyl metal arsenate. Silver, titanium, zlrconliun, thorium and lead Interfere. Separation Is made from the alkali metals, alkaline earths, aluminum. Iron (ll), and rare earths. Including trlvalent cerium.-2 ... 

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