Arsenic in precipitation

Eventually, atmospheric arsenic falls on the Earth s surface through wet and dry deposition. Sweet, Weiss and Vermette (1998) evaluated the deposition of arsenic over lakes Erie, Michigan, and Superior in the Great Lakes of North America. Overall, the wet and dry deposition rates for the lakes were very similar. Specifically, they were 72-94 and 66-91 pgm 2year 1, respectively. 

Arsenic in precipitation from unpolluted ocean air averages about 0.019 pg L 1 (Hering and Kneebone, 2002), 157 and terrestrial rainwater concentrations (at least over the USA) also have similar averages of around 0.013-0.032 pgL-1 ((Smedley and Kinniburgh, 2002), 522 Table 3.17). As the precipitation infiltrates into the subsurface, its chemistry changes as it reacts with sediments, soils, and rocks. Therefore, the arsenic chemistry of the groundwater of an area may be very different than its precipitation chemistry. 

Unless contaminated by coal combustion facilities, ore smelters, or other arsenic emitters, melted snow tends to have much 1 pgL-1 of arsenic (Table 3.17). The arsenic concentrations in the precipitation of an area may also change over time. Specifically, snowpacks in Colorado and New Mexico, USA, had less arsenic in 1999-2000 ( 0.01-0.02 pgL-1 in meltwater) than averages from nearby sampling stations in 1993-1999 (0.05-0.14 jag U1 in meltwater) ((Ingersoll, 2000) Table 3.17). The origin(s) of the arsenic is unknown, but may be related to emissions from nearby coal-fired power plants (Ingersoll, 2000), 2. 

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It is recommended that the eompound be fused with a mixture of sodium carbonate (2 parts) and sodium peroxide (1 part) as in the test for Plvoaphoms. Extract the fused mass with water, filter, and acidify with dilute hydrochloric acid. Pass hydrogen sulphide through the hot solution arsenic is precipitated as yellow arsenic sulphide. If antimony is present, it will be precipitated as orange antimony trisulphide. 

Arsenic tniodide (arsenic(III) iodide), Asl, can be precipitated from a hot solution of trivalent arsenic in hydrochloric acid by the addition of potassium iodide, or it can be formed by treating elemental arsenic with a solution of iodine in carbon disulfide. It is not as easily hydrolyzed as the other arsenic haUdes, but it decomposes slowly in air at 100 °C (rapidly at 200°C) to give a mixture of iodine, arsenic trioxide, and elemental arsenic. Solutions of Asl are unstable, particularly in the presence of moisture. 

The Structure of AS2S5 is unknown. It is said to be formed as a yellow solid by passing a rapid stream of H2S gas into an ice-cold solution of an arsenate in cone HCl slower passage of H2S at room temperature results in reduction of arsenate to arsenite and consequent precipitation of AS2S3. It decomposes in air above 95° to give AS2S3 and sulfur. 

Discussion. Arsenates in solution are precipitated as silver arsenate, Ag3 As04, by the addition of neutral silver nitrate solution the solution must be neutral, or if slightly acid, an excess of sodium acetate must be present to reduce the acidity if strongly acid, most of the acid should be neutralised by aqueous sodium hydroxide. The silver arsenate is dissolved in dilute nitric acid, and the silver titrated with standard thiocyanate solution. The silver arsenate has nearly six times the weight of the arsenic, hence quite small amounts of arsenic may be determined by this procedure. 

Arsenic. The presence of arsenic in an organic compound is generally revealed by the formation of a dull grey mirror of arsenic on the walls of the test-tube when the compound is fusM with sodium in the Lassaigne test. Usually sufficient arsenic is found in the fusion solution to give a yellow precipitate of arsenic trisulphide when the solution is acidified with hydrochloric acid and treated with hydrogen sulphide. 

Reduction Hexavalent chromium Ferrous sulfate arsenic, and results in precipitation of arsenic-iron-manganese compounds. Reduces Cr(VI) to Cr(III)... 

Wastewater treatment in the copper sulfate industry can further be improved, particularly the removal of the toxic metals, through sulfide precipitation, ion exchange, and xanthate processes. Addition of ferric chloride alongside alkaline precipitation can improve the removal of arsenic in the wastewater. 

Ashokkumar et al. [128] have reported the sonochemical conversion of As(III) to As(V) in an aqueous solution as a process for the removal of arsenic in contaminated water. A very interesting pH independent sonochemical conversion of arsenic (III) to arsenic (V), besides its precipitation as sulphide in aqueous solutions, in the pH range 5 to 9, and subsequent adsorption on coagulants such as Fe(OH)3 and Al(OH)3 has also been reported [129] from this laboratory. 

The precipitation of arsenic with H2S gas in the normal condition could occur only in strongly acidic medium whereas another cation of the second group Cd(II), precipitates only in faintly acidic medium, therefore, the precipitation of both cadmium and arsenic with H2S gas in the same solution was not easily possible. To precipitate both in the same solution, the H2S gas is conventionally first passed into the strongly acidic original solution of basic radicals followed by its bubbling into the diluted solution. To examine the role of ultrasound on the precipitation of arsenic in faintly acidic or neutral medium, few experiments were carried out. The results obtained showed effective precipitation of arsenic even in mild reaction solutions, with their pH ranging from 5.1 to 8.8. under ultrasonic field. Hence Cd2+ and As3+/5+ both could be precipitated in the same solution at low pH under the... 

A scheme could thus be proposed for the precipitation and removal of arsenic in the ultrasonic field in the ppm and ppb range at normal pH as provided in Fig. 9.3. 

The mobility of arsenic compounds in soils is affected by sorp-tion/desorption on/from soil components or co-precipitation with metal ions. The importance of oxides (mainly Fe-oxides) in controlling the mobility and concentration of arsenic in natural environments has been studied for a long time (Livesey and Huang 1981 Frankenberger 2002 and references there in Smedley and Kinniburgh 2002). Because the elements which correlate best with arsenic in soils and sediments are iron, aluminum and manganese, the use of Fe salts (as well as Al and Mn salts) is a common practice in water treatment for the removal of arsenic. The coprecipitation of arsenic with ferric or aluminum hydroxide has been a practical and effective technique to remove this toxic element from polluted waters... 

Passive oxidation of mine water from the Maude Mine removes up to 98% of the contained As through precipitation of ferrihydrite and scavenging of As from solution. The remaining arsenic in the water can be removed by the use of the coagulating agents poly-aluminium chloride or ferric chloride. 

Moldovan, B.J., Jiang, D.-T., Hendry, M.J. 2003. Mineralogical characterization of arsenic in uranium mine tailings precipitated from iron-rich hydrometallurgical solutions. Environmental Science and Technology, 37, 873-879. 

The aim of this work was to investigate the arsenic mobilization from the tailings material (200 - 500 pg/g As) into the seepage water (up to 3.5 mg/L As) and the process of seepage water effluent forming an immobilized precipitate (up to 8 % As) in the creek. Different analytical methods for the determination of total concentrations and different sequential extraction methods as well as hyphenated techniques for speciation analysis were applied to follow the way of the arsenic in this environment. 

The mobilization of arsenic from the tailings material seems to be a slow and continuos process attributed to reduction of iron phases. The seepage water of the middle source contains arsenite as well as arsenate in high concentrations and seems to be the only water source in contact with the tailings material. The concentrations of arsenic downstream are still high and the immobilization process by precipitation of iron hydroxide and coprecipitation or sorption of arsenic is incomplete. A reason for this may be the slow kinetics of the oxidation process and the transport of fine grained hydroxide particles. These particles are mobile and can bind the arsenic (mainly as arsenate) too. 

The immobilized arsenic in the precipitate is bound only by sorption onto the amorphous iron hydroxides. A sustainable immobilization would need additional action. 

Iron and aluminum precipitate out when treated with ammonia and are removed by filtration. Other metals, such as copper, zinc, lead and arsenic are precipitated and removed as sulfides upon passing hydrogen sufide through the solution. Colloidal particles of metaUic sulfides and sulfur are removed by treatment with iron(ll) sulfide. The purified solution of manganese(ll) sulfate is then electrolyzed in an electrolytic cell using lead anode and HasteUoy or Type 316 stainless steel cathode, both of which are resistant to acid. Manganese is deposited on the cathode as a thin film. 

Antimony(III) halides are chemically reactive, but less so than their phosphorus or arsenic analogues. Antimony(III) chloride forms a clear solution with water, and there is no evidence for Sb3+ ions dilution results in precipitation of insoluble oxychlorides of various compositions, e.g. SbOCl, Sb405a2, SbsOuCl2. Some reactions of SbCl3 are shown in Scheme 3. Antimony(III) fluoride is an important fluorinating agent. 

The irradiated ash sample, with arsenic carrier added, is digested under reflux with hydrochloric, nitric, and perchloric acids. Arsenic (III) is then distilled from the mixture with hydrobromic acid and collected in water. Elemental arsenic is precipitated with sodium hypophosphite, and the activity of the precipitate is counted. The 0.559 MeV y-ray photopeak of 26.5-hr 76As is measured. Radiochemical yields are quantitative. 

A general method for the separation of vanadium from arsenic, molybdenum, phosphorus, chromium, uranium, tungsten, and silicon, consists in precipitating these metals as their respective lead salts and digesting the precipitate with potassium carbonate, whereupon all the lead salts are decomposed with the exception of the lead vanadate.5... 

In 1869 Bettendorff recorded 1 the formation of a voluminous brown precipitate when stannous chloride was added to a solution of arsenious oxide, or of magnesium ammonium arsenate, in hydrochloric acid. The precipitate proved to be arsenic (96 to 99 per cent.) with traces of tin which were irremovable. The speed of precipitation depends upon the amount of arsenic present and the temperature. With solutions containing little arsenic, Bettendorff observed, on warming, a yellow colour before the precipitate appeared, but he was unable to prove that the colour was due to arsenic. The reaction involved may be represented thus—... 

Arsenic does not combine directly with molecular hydrogen,9 and the element may be purified by sublimation in that gas. Hydrides, however, may be obtained by indirect methods (see pp. 79-84). Arsenic may be displaced by the gas from solutions of its salts at high temperatures and pressures. Thus arsenic separates in large well-defined crystals when a solution of sodium arsenate is subjected to the action of hydrogen at 25 atm. pressure 10 the action commences at 300° C., 15 per cent, of the arsenic being precipitated at this temperature, but it increases rapidly with rising temperature and at 350° C. 77 per cent, of the arsenic is liberated. Arsine is not produced in the reaction. 

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