The Behaviour of Arsenic During Jarosite Precipitation: Reactions at 150°C and the Mechanism of Arsenic Precipitation

Abstract

The behaviour of arsenate (As5+) during jarosite precipitation at 150°C has been elucidated as a function of the solution composition and the initial solution pH. Arsenate is preferentially precipitated during jarosite formation, and nearly complete arsenic precipitation occurs from solutions containing > 5 g/l As5+. Arsenic precipitation is retarded at initial solution pHs < 1.0, but is nearly independent of the concentration of a given alkali sulphate, copper sulphate or zinc sulphate. Low ferric sulphate concentrations result in the selective formation of scorodite (FeAsO4·2H2O), but the mass of precipitate is small. The chemical and X-ray diffraction results suggest a low level of AsO4 substitution in jarositetype compounds: ≍ 1.5 wt % AsO4 is structurally incorporated in sodium jarosite or lead jarosite, and up to 4 wt % AsO4 substitution occurs in potassium jarosite. Large amounts of arsenate are present as a separate well crystallized scorodite phase. It is thought that AsO43− substitutes for SO42− in the jarosite structure, with charge neutrality maintained by the conversion of appropriate amounts of OH− to H2O. Substitution of the larger AsO4 anion does not noticeably affect the jarosite cell parameters. Lastly, it is demonstrated that not only do many jarosite-family minerals contain arsenate, but the amounts are consistent with the results obtained in the synthesis experiments.