『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℃, and oxygen-saturated
atmosphere using both raw galena and a pyritic sludge from the
Aznalcollar（oの頭に´） 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℃ in a saturated O2
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 aH+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 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 H2S(aq) is converted into H2S(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 HNO3
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, PbSO4
and S precipitate on the PbS surface.
The XPS surface analysis and the XRD pattern of the reacted {100}
PbS surface yield further insight into the existence of lead-sulphur
phases such as anglesite (PbSO4) and elemental sulphur on the
PbS surface.
Keywords: Galena reactivity; Dissolution; Precipitation; In situ
AFM 』
1. Introduction
2. Experimental methodology
2.1. Sample characterization
2.2. Flow-through experiments
2.3. In situ AFM experimental setup
2.4. XPS
2.5. Solutions
3. Calculations
4. Results
4.1. Nonstirred flow-through experiments
4.2. Solution chemistry in in situ and ex situ AFM experiments
4.3. Galena surface in in situ AFM and ex situ AFM experiments
4.4. The Fe(III) effect on galena dissolution in the AFM and
nonstirred flow-through experiments
5. Discussion
5.1. Galena dissolution based on the bulk solution chemistry
5.1.1. Nonstirred flow-through experiments
5.1.2. Galena dissolution in the in situ AFM experiments
5.1.3. Comparison of dissolution rates obtained by in situ,
ex situ, and flow-through experiments
5.2. Galena dissolution based on the surface topography variation
5.2.1. The dissolution rate of the {100} cleavage surface
5.2.2. The dissolution features of the {100} cleavage surface
5.3. Fe(III) effect on the galena dissolution mechanism
6. Conclusions
Acknowledgements
References