Pyromorphite growth on lead-sulfide surfaces

Electrochemical Scanning Tunneling Microscopy (EC-STM) and electron microscopies have been used to follow the nucleation and growth of ∼10-15 nm pyromorphite (Pb₅(PO₄)₃Cl,OH) particles on a galena (PbS) substrate under oxidative conditions. The particle sizes and crystal morphologies are found to be strongly affected by solution and oxidation potential, and in the earliest stages the particles are generally sufficiently small to be mobilized in a soil. It is clear that the particles grow epitaxially under these conditions, based on observations of the particles' adherence to the surface during imaging, their preferred crystallographic orientation, their growth along surface features on the galena, and commensurate atomic structures. Through cyclic voltammetry, we show that the presence of phosphate also partially passivates the surface of the galena to oxidation. We propose two possibilities for the mechanism of passivation, one is that pyromorphite nucleation inhibits the retreat of steps, and the second is that adsorbed phosphate stabilizes a lead-terminated surface structure by coordinating lead and slowing its dissolution.