Dissolution of lead- and lead–arsenic-jarosites at pH 2 and 8 and 20 °C: Insights from batch experiments

Lead- and Pb–As-jarosites are minerals common to acidic, sulphate-rich environments, including weathering zones of sulphide ore deposits and acid rock or acid mine drainage (ARD/AMD) sites, and often form on or near galena. The structures of these jarosites are based on linear tetrahedral–octahedral–tetrahedral (T–O–T) sheets, comprised of slightly distorted FeO6 octahedra and SO42− (–AsO43− in Pb–As-jarosites) tetrahedra. To better understand the dissolution mechanisms and products of the break down of Pb- and Pb–As-jarosite, preliminary batch dissolution experiments were conducted on synthetic Pb- and Pb–As-jarosite at pH 2 and 20 °C, to mimic environments affected by ARD/AMD, and at pH 8 and 20 °C, to simulate ARD/AMD environments recently remediated with slaked lime (Ca(OH)2). All four dissolutions are incongruent. Dissolution of Pb-jarosite at pH 2 yields aqueous Pb, Fe and SO42−. The pH 8 Pb-jarosite dissolution yields aqueous Pb, SO42− and poorly crystalline Fe(OH)3, which does not appear to resorb Pb or SO42−, possibly due to the low solution pH (3.44–3.54) at the end of the experiment. The pH 2 and 8 dissolutions of Pb–As-jarosite result in the formation of secondary compounds (poorly crystalline PbSO4 for pH 2 dissolution; poorly crystalline PbSO4 and Fe(OH)3 for pH 8 dissolution), which may act as dissolution inhibitors after 250 to 300 h of dissolution. In the pH 2 dissolution, aqueous Fe, SO42− and AsO43− also form, and in the pH 8 dissolution, Fe(OH)3 precipitates then subsequently resorbs aqueous AsO43−. The dissolutions probably proceed by preferred dissolution of the A- and T-sites, which contain Pb, and SO42− and AsO43−, respectively, rather than Fe, which is sterically remote, within the T–O–T Pb- and Pb–As-jarosite structures. These data provide the foundation necessary for further, more detailed investigations into the dissolution of Pb- and Pb–As-jarosites.