Arsenate inorganic As V precipitation

The addition of lime to control acid drainage from mining wastes typically produces calcium arsenates (Pichler, Hendry and Hall, 2001). Bothe and Brown (1999) further concluded that lime precipitates As(V) as a number of hydroxyl and hydrated calcium arsenates (Ca4(OH)2(AsC 4)2 4H2O, CaslAsCLLOH (arsenate apatite), and/or Ca3(AsC 4)2 3H2O) rather than anhydrous tricalcium orthoarsenate (Ca3(As04)2). Calcium arsenates also occur in coal combustion byproducts (Chapter 7). In the flue gas treatment systems of coal combustion facilities, volatile arsenic can readily react with calcium to form the arsenates on the surfaces of flyash and injected lime (Seames and Wendt, 2000 Yudovich and Ketris, 2005, 175). 

Although calcium arsenates may readily precipitate in acid mine drainage and form in flue gas treatment systems, Robins and Tozawa (1982) warn that the compounds may not have long-term stability, which could lead to disposal and environmental problems. Besides dissolving under acidic conditions, the presence of carbonate, bicarbonate, or CO2 may decompose calcium arsenates. When calcium arsenates react with CO2, calcium carbonate forms and the arsenic could be released into the environment (Ghimire et al., 2003, 4946 Jing, Korfiatis and Meng, 2003, 5055-5056). 

The solubilities of aluminum, manganese, Fe(II), and Fe(III) arsenates in water are very different than the values for calcium arsenates (Chapter 2). While calcium arsenates are less soluble under alkaline conditions, the solubility of the other arsenates generally increases with increasing pH (Sadiq et al., 2002, 306-307). Under pH 2-12 conditions, Mn(II) and Fe(II) arsenates are also less soluble than aluminum and Fe(III) arsenates (Sadiq et al., 2002, 307). 

Geochemical modeling is often used to identify the compounds that primarily control the chemistry of arsenic in aqueous solutions. Modeling studies indicate that the arsenic concentrations of Kelly Lake, Ontario, Canada, are controlled by the precipitation and dissolution of Fe(II) arsenates rather than calcium or Fe(III) arsenates (Sadiq et al., 2002). The arsenic in the lake originated from runoff from the nearby Sudbury mining district and airborne particles from local ore smelters (Sadiq et al., 2002). 

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