title: Characterizing and quantilying controls on arsenic solubility over a pH range of 1-11 in a uranium mill-scale experiment.
abstract: A mill-scale hydrometallurgical experiment (2700 m3 of effluent treated/day) was conducted for three months at the Rabbit Lake uranium mine site located in northern Saskatchewan, Canada, to determine the controls on the solubility of dissolved arsenic over a pH range of 1-11 and to develop a thermodynamic database for the dominant mineralogical controls on arsenic in the mill and the resulting mill tailings. The arsenic concentrations in the mill ranged from 526 mg/L at pH 1.0 (initial) to 1.34 mg/L at pH 10.8 (final discharge). Geochemical modeling of the chemistry data shows that arsenic solubility is controlled by the formation of scorodite (FeAsO4-2H2O) from pH 2.4 to pH 3.1, with 99.8% of dissolved arsenic precipitated as scorodite. Model results show that scorodite is unstable (releasing arsenic back in to solution) above pH 3.1 and arsenic adsorption to the surface of 2-line ferrihydrite is the dominant controlling factor in the solubility of arsenic from pH 3.2 to pH 11.0, with 99.8% of dissolved arsenic removed from solution via this mechanism. Finally, model results show -0.2% of the total dissolved arsenic adsorbs to the surface of amorphous aluminum hydroxide from pH 5.0 to pH 8.0. Minor alterations to the thermodynamic properties of arsenite and arsenate adsorption to 2-line ferrihydrite allowed the fit between measured mill-scale and modeled concentrations for the pH range of 3.2-11.0 to be optimized.
authors.affiliation: Cameco Corporation, Saskatoon, SK, Canada. bjm328@mail.usask.ca
authors.forename: Brett I
authors.initials: BI
authors.lastname: Moldovan
authors.forename: M Jim
authors.initials: MJ
authors.lastname: Hendry
mesh_terms: D000327:Adsorption; D001151:Arsenic / Q000032:analysis* / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008906:Mining; D012995:Solubility; D013696:Temperature; D014501:Uranium; D014873:Water Pollutants / Q000032:analysis*
chemical_list: D014873:Water Pollutants; D014501:Uranium; D001151:Arsenic