Galena surface reactivity at acidic pH and 25 °C based on flow-through and in situ AFM experiments

Abstract

A study of (PbS) galena dissolution at acidic conditions was carried out by means of nonstirred flow-through experiments , in situ and ex situ Atomic Force Microscopy (AFM) experiments, X-ray photoelectron spectroscopy (XPS) surface analysis, and X-ray diffraction (XRD) patterns of the reacted surfaces. The nonstirred flow-through experiments were performed at pH 3, 25 °C, and oxygen-saturated atmosphere using both raw galena and a pyritic sludge from the Aznalcóllar mine tailing (SW Spain) with 0.8 wt.% of galena. Based on the Pb release, the steady-state dissolution rate of galena, normalized to the initial specific surface area, is 1.2±0.18×10 −10 mol m −2 s −1. The in situ AFM experiments were carried out at the pH range from 1 to 3 at 20±3 °C in a saturated O 2 atmosphere using galena fragments of known dimensions. Also based on Pb release, the galena dissolution rates were estimated by normalizing to the geometric area. A derived empirical rate law describing the dissolution rate–pH dependency at 1<pH<3 can be expressed as R Pb = 10 − 5.16 a H + 0.41 where R Pb is the galena dissolution rate in mol m −2 s −1. Moreover, using the AFM images, galena dissolution rates were estimated by carrying out a systematic section analysis of the surface microtopography variation as dissolution of the {100} cleavage surface occurred. The AFM-estimated galena dissolution rates were slower than the dissolution rates based on the Pb release probably because of lower reactivity of the area scanned by the AFM probe compared with the entire surface reactivity. The dissolution of galena appeared to be noncongruent as aqueous sulphur depletion was observed, resulting in a Pb/S ratio higher than one. We suggest that through the overall dissolution reaction, a fraction of H 2S (aq) is converted into H 2S (g) as S is detached from the PbS surface, causing the aqueous S deficit. The overall dissolution mechanism observed on the {100} galena surface is similar to the one reported by De Giudici and Zuddas (De Giudici, G., Zuddas, P., 2001. In situ investigation of galena dissolution in oxygen saturated solution: Evolution of surface features and kinetic rate. Geochimica et Cosmochimica Acta 65, 9, 1381–1389) in which surface protrusions form over the PbS surface and dissolve continuously. Furthermore, the ex situ Tapping mode images show the growth of larger protrusions on galena substrate at acidic pH. A potential oxidative effect of the reacting solution on the galena dissolution mechanism at acidic pH was also studied: (1) As the HNO 3 solution is more oxidative than HCl solution, the protrusions formed faster over the PbS surface; (2) since the Fe(III) in solution reduces to Fe(II) to oxidize sulphur to sulphate, PbSO 4 and S precipitate on the PbS surface. The XPS surface analysis and the XRD pattern of the reacted {100} PbS surfaces yield further insight into the existence of lead–sulphur phases such as anglesite (PbSO 4) and elemental sulphur on the PbS surface.