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Solubility of Symplesite (Ferrous Arsenate): Implications for Reduced Groundwaters and Other Geochemical Environments

United Nations Children's Fund (UNICEF), Water & Environmental Sanitation Section Dhaka, Bangladesh

Philip C. Singer

Dep. of Environ. Sciences and Eng. Univ. of North Carolina Chapel Hill, NC 27599

View larger version (14K): [in this window] [in a new window]   Fig. 1. Precipitation of ferrous arsenate. Initial conditions: 1000 uM Fe(II), 500 uM As(V), 100 mM NaNO3. Solid lines represent model predictions.

View larger version (12K): [in this window] [in a new window]   Fig. 2. Ferrous arsenate stoichiometry. Open symbols represent conditions of oversaturation with respect to Fe(OH)2(s).

View larger version (12K): [in this window] [in a new window]   Fig. 3. Log ion activity products (IAP) for ferrous arsenate (squares, left axis) and ferrous hydroxide (triangles, right axis). Dashed and solid lines represent equilibrium solubility products for ferrous arsenate and ferrous hydroxide, respectively. Filled symbols denote samples considered to be at equilibrium with ferrous arsenate or ferrous hydroxide phases.

View larger version (16K): [in this window] [in a new window]   Fig. 4. Stability field for symplesite. Isopleths represent total arsenate concentration in micrograms per liter. Solid lines were calculated with an ionic strength of 0.01, while dotted lines have an ionic strength of 0.05. Areas to the right of the isopleths are oversaturated with respect to symplesite.

View larger version (23K): [in this window] [in a new window]   Fig. 5. Cumulative frequency distributions of saturation indices for various minerals. Geochemical data from Kinniburgh and Smedley (2001).