Characterization of Poorly-Crystalline Ferric Arsenate Precipitated from Equimolar Fe(III)-As(V) Solutions in the pH Ran
- PDF / 514,305 Bytes
- 12 Pages / 593.972 x 792 pts Page_size
- 71 Downloads / 160 Views
INDUSTRIAL metallurgical operations may release arsenic in effluents or process solutions. Due to its high toxicity, arsenic has to be removed and immobilized in environmentally stable forms. The standard method of arsenic removal from industrial mineral processing effluents is through coprecipitation with Fe(III), through neutralization in the form of poorly-crystalline Fe(III)AsO4 solids. The Fe(III)-to-As(V) molar ratio in these precipitates is at least 3, in order to have better control of the arsenic solubility.[1] The predominant mechanistic view for arsenic retention in the Fe(III)-AsO4 coprecipitates is the adsorption of arsenate ions on the surface of the ferrihydrite. Hence, names such as arsenical ferrihydrite, arsenicbearing ferrihydrite, and arsenic-bearing hydrous iron (or ferric) oxide[1–4] have been given to these solids. Using the extended X-ray absorption fine structure (EXAFS) technique, among other tools, several investigators have determined that inner-sphere complexation is the type of bonding between adsorbed arsenate ions and the surface of ferrihydrite.[5] Different kinds of inner-sphere complexes have been proposed, such as bidentate corner sharing, bidentate edge sharing, and monodentate corner sharing. Thus, Waychunas et al.[6] determined the bidentate corner sharing to be the main J.F. LE BERRE, Postdoctoral Student, and R. GAUVIN and G.P. DEMOPOULOS, Professors, are with the Department of Mining and Materials Engineering, McGill University, H3A 2B2, Montreal, PQ, Canada. Contact e-mail: [email protected] Manuscript submitted December 22, 2006. Article published online September 11, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
type of complex between arsenate ions and iron hydroxide groups. They further suggested the presence of monodentate corner sharing as a second type of complex. Later, Manceau[7] suggested, instead, that the formation of bidentate edge sharing was the dominant complex, which was contested by Waychunas et al.[8,9] Fendorf et al.[10] noted, as well, the presence of the three complexes; they have further reported that the abundance of each complex is related to the degree of surface loading. More recently, Sherman and Randall confirmed the findings of Waychunas et al., that the bidentate corner sharing complex was the complex of choice for the linkage of AsO4 tetrahedra and FeO6 octahedra. It is interesting to note, however, that the determination of the inner-sphere surface complexation mechanism for arsenate retention on ferrihydrite in coprecipitation systems has been done almost exclusively with materials prepared at an elevated pH (higher than 7) and/or from relatively low As(V)-concentration solutions (typically in the range of 10-6 to 10-3 M or less than 65 mg/L arsenic[5,6,9]). Recent work involving adsorption and coprecipitation studies (with an Fe(III)-to-As(V) molar ratio higher or equal to 2) in a wide pH range (from 3 to 8) and relatively elevated As(V) concentrations (higher than 0.01 M or 700 mg/L)* has found evidence for the *Such elevated arseni
Data Loading...
