DISSOLUTION OF PYROMORPHITE IN LACTIC ACID - AN ATOMIC FORCE MICROSCOPY STUDY

The aqueous concentrations of Pb in soils, sediments, and aquatic environments are mostly controlled by the dissolution and precipitation of discrete mineral phases of which pyromorphite Pb₁₀(PO₄)₆Cl₂ is the most stable. In-situ lead immobilization through phosphate amendments resulting in formation of pyromorphite provides a cost-effective method for reducing the leaching, migration, and bioavailability of lead from soils. However, microbial activities in soils affect dissolution and mobilization of toxic metals. Of particular concern is the microbial dissolution and remobilization of lead compounds. Anaerobic bacteria Clostridium sp. solubilizes lead due to production of lactic acid and the lowering of the pH.

In this study we used atomic force microscopy (AFM) to study pyromorphite dissolution in lactic acid at low pH (1.5 – 2). This technique offers the possibility to link microtopography-dependent mechanisms and rates of single-crystal dissolution to mechanisms determined by solution chemistry experiments. Changes in surface topography on oriented prismatic and hexagonal faces of natural pyromorphite monocrystals were observed.

Wide, flat, rectangular terraces with steps were apparent on unreacted prismatic faces. Exposure to lactic acid promoted development of etch pits and retreat of steps. Imaging of unreacted hexagonal faces revealed poorly defined hexagonal cleavage grooves in otherwise relatively flat surfaces. Neither terraces nor steps were observed. Exposure to lactic acid resulted in a rough appearance of the surface. Theses observations indicate that at the conditions of the experiment mechanism of pyromorphite dissolution depends on crystallographic orientation. On faces parallel to crystal elongation, dissolution occurs at step edges as a simple layer-by-layer removal. In contrast, on faces perpendicular to elongation, dissolution occurs relatively evenly at the entire surface.