Solubilities and stabilities of ferric arsenate compounds
Abstract
Basic ferric arsenates precipitated from hydrometallurgical liquors by hydrolysis are shown to be very insoluble over a range of pHs. The stability range (solubilities <1 mg l−1) becomes wider with increasing Fe/As ratios. Precipitates with Fe/As ratio=8 have solubilities <1 mg l−1 between Ph=3 and 8. The As solubility did not increase during aging over a 2–3.7 yr period. Naturally occurring iron oxide samples with Fe/As ratios=3–4 show even lower As solubilities than precipitates with the same Ar/As ratios. The solubility of crystalline scorodite, FeAsO42H2O, was found to be about two orders of magnitude lower than reported in the literature for apparently amorphous FeAsO4H2O. The solubility and the free energy of formation were recalculated for crystalline scorodite. This investigation shows that basic ferric arsenates appear to be suitable for safe disposal of arsenic.
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