Some researchers computing the solubility product of scorodite from dissolution experiments have incorrectly assumed that all ion activity coefficients equaled unity (cf. Chukhlantsev, 1956; Krause and Ettel, 1989). Others have not corrected for ferric hydroxide and/or ferric sulfate complexing (Chukhlantsev, 1956). Further, most workers have not corrected for the presence of ferric arsenate complexes (cf. Dove and Rimstidt, 1985; Krause and Ettel, 1989). The FeH2AsO4+2 and FeHAsO4+1 complexes have log K(assoc) constants of 4.04 and 9.86, respectively (Robins, 1990; Whiting, 1992).

In this study published raw solubility data measured under conditions of congruent dissolution of scorodite (generally pH below 2.5) has been reevaluated using the geochemical code PHREEQC (Parkhurst and Appelo, 1999) with a modified thermodynamic data base for arsenic species and considering all of the above corrections. Results indicate that the solubility products (as log Ksp values) for amorphous and crystalline scorodite are -22.89 and -25.87, respectively, at 25oC and 1 bar pressure.

In surface and subsurface waters affected by mill tailings disposal, the solubility of scorodite may limit maximum As concentrations to less than 1-2 mg/L. Most observers have argued that scorodite will dissolve incongruently to form ferrihydrite (HFO) or goethite in buried mill tailings. However, under such conditions the stability field of scorodite relative to that of ferrihydrite (HFO) may be enlarged up to pH = 7-8 because of the poor crystallinity and high solubility of the HFO precipitated from such high arsenic, high sulfate solutions.