Ferric arsenate precipitation

As seen in the above equations, the aqueous oxidation processes convert sulfur in the feed to dissolved sulfate, while arsenic is oxidized and precipitated as ferric arsenate compounds. So, problems of the emission of sulfur and arsenic oxides caused by roasting are avoided in the aqueous oxidation processes. The two different industrial methods which achieve the oxidation reactions are pressure oxidation and biological oxidation. 

The precipitated hexahydrate gradually undergoes oxidation on exposure to moist air, yielding ferric arsenate and ferric oxide.3 It is sparingly soluble in aqueous ammonia,4 but is insoluble in the presence of ammonium salts. 

The monohydrate, FeAs04.H20, is formed when anhydrous ferric arsenate, arsenic acid, hydrogen peroxide and water are heated in a sealed tube for 14 days at 170° C. a hemihydrate may be obtained in a similar manner by substituting precipitated ferric dihydrogen arsenate for the normal salt. 

Liu, R., Qu, J., Xia, S. et al. (2007a) Silicate hindering in situ formed ferric hydroxide precipitation inhibiting arsenic removal from water. Environmental Engineering Science, 24(5), 707-15. 

Similar rate equations can be developed for the parallel leaching of other minerals (see Eq. 3). The oxidation of intermediate sulfur compounds to sulfates such as Eq. 2 is a consecutive reaction to mineral leaching. Other reactions are virtually instantaneous, e.g., precipitation of insoluble ferric arsenates from soluble NH4H2ASO4 which is consecutive to Eq. 3. On the other hand, the instantaneous shift in the amonia -ammonium equilibrium,... 

Figure 4. Normalized, kl-weighted As-EXAFS spectra (a) and uncorrected Fourier transforms (FTs) (b) of scorodite (a crystalline ferric arsenate), an x-ray amorphous analog, and As(V) sorbed to amorphous hydrous ferric oxide (HFO), The EXAFS spectra can clearly be used to distinguish the coordination environment of arsenic in each of the materials. The highly symmetric local environment of arsenic in scorodite is shown in (c) each arsenic is surrounded by 4 Fe neighbors at a distance of 3.34 A. (see Table 2 for details of precipitate fits, and Table 3 for details of sorption sample fit). Reprinted from Foster (1999). The arrow highlights the region of particular difference among the three spectra. Peak positions in FTs are not corrected for phase-shift effects, and are therefore approximately 0.5 A shorter than the true distance.

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. 

Ferric iron solution is now continuously added to both acid and neutral leaching. This has resulted in higher iron utilization and enhanced impurity precipitation efficiency. The addition of soluble iron to the acid leach promotes the precipitation of ferric arsenate, enhancing arsenic rejection to the residues and dramatically lowering the arsenic levels in the plant electrolytes. Previously, the arsenic concentration in the acid leach electrolyte was as high as 5 g/L. The arsenic concentration is now below one gram per liter. 

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...

The use of hydrothermal precipitation with iron to remove oxyanions such as those of arsenic, antimony, vanadium and titanium, particularly ferric arsenate has been proposed (Swash Monhemius 1995, 1996). This disposal option is based on geochemical considerations in that these minerals are stable, for example over 60% of arsenic minerals are arsenates of which... 

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

Chemical precipitation is used in porcelain enameling to precipitate dissolved metals and phosphates. Chemical precipitation can be utilized to permit removal of metal ions such as iron, lead, tin, copper, zinc, cadmium, aluminum, mercury, manganese, cobalt, antimony, arsenic, beryllium, molybdenum, and trivalent chromium. Removal efficiency can approach 100% for the reduction of heavy metal ions. Porcelain enameling plants commonly use lime, caustic, and carbonate for chemical precipitation and pH adjustment. Coagulants used in the industry include alum, ferric chloride, ferric sulfate, and polymers.10-12... 

Some innovating treatment technologies may be introduced in the treatment of wastewater generated in the aluminum fluoride industry to make its effluent safer. The ion exchange process can be applied to the clarified solution to remove copper and chromium. At a very low concentration, these two pollutants can be removed by xanthate precipitation.24 A combination of lime and ferric sulfate coagulation will effectively reduce arsenic concentration in the wastewater. 

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. 

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... 

The neutron activation method for the determination of arsenic and antimony in seawater has been described by Ryabin et al. [66]. After coprecipitation of arsenic acid and antimony in a 100 ml sample of water by adding a solution of ferric iron (10 mg iron per litre) followed by aqueous ammonia to give a pH of 8.4, the precipitate is filtered off and, together with the filter paper, is wrapped in a polyethylene and aluminium foil. It is then irradiated in a silica ampoule in a neutron flux of 1.8 x 1013 neutrons cm-2 s 1 for 1 - 2 h. Two days after irradiation, the y-ray activity at 0.56 MeV is measured with use of a Nal (Tl) spectrometer coupled with a multichannel pulse-height analyser, and compared with that of standards. 

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. 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

A third plant uses a chemical precipitation step for removing arsenic and zinc from contaminated surface water runoff. Ferric sulfate and hme are alternately added while the wastewater is vacuum-filtered and sludge is contract-hauled. The entire treatment system consists of dual-media filtration, carbon adsorption, ion exchange, chemical precipitation, and vacuum hltration. Sampling results across the entire treatment system indicated that arsenic was reduced from 6.9 to 0.2 mg/L and zinc from 0.34 to 0.11 mg/L. 

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