Plumbojarosite (Trans)Formation in Lead Contaminated Soil

Plumbojarosite [PbFe₆(SO₄)₄(OH)₁₂] is a mineral occurring in oxidized zones of galena [PbS] deposits and in precipitates associated with sulfate-rich acid mine drainage.  Plumbojarosite is not commonly found in aerobic soils because of prevailing unfavorable soil conditions; i.e., low dissolved ferric iron and sulfate concentrations and pH above 4.  Thermodynamic calculation reveals plumbojarosite destabilization as sulfate leaches out over time and pH increases with regards to lead carbonate [PbCO₃] and soil ferric hydroxides [FeO(OH)]; e.g. goethite.  Despite this calculation we identified plumbojarosite by synchrotron-based micro-XRD to be a common lead-bearing mineral in a soil with near neutral pH that was contaminated during recycling of lead acid batteries.  We hypothesize that plumbojarosite was formed pedogenically as dynamic soil equilibrium was re-established in sulfate rich, acidic pH (< 2) environment between goethite and battery-derived lead phases; i.e., lead oxide [PbO] and sulfate [PbSO₄].  To test this hypothesis, we equilibrated goethite and one of the following lead chemicals (PbO, PbSO₄, and PbCO₃) at 6:1 molar ratio in ~0.02 M H₂SO₄.  Solids were washed repeatedly in nano-pure water to remove excess sulfate, while pH was measured for each supernatant.  Precipitates were freeze-dried and homogenized in an agate mortar.  Elemental analysis of supernatants by atomic absorption spectrometry revealed an inverse trend between dissolved iron and lead with only minute differences between individual treatments and plumbojarosite was identified by XRD only in PbO/goethite mixture.  Based on these results we determined that lead phase solubility is the limiting factor in pedogenic formation of plumbojarosite because PbO is the most soluble lead source in the experimental system.  This research improves not only our understanding of dynamics of plumbojarosite formation and transformation, but promises to improve our current reclamation strategies of contaminated soils.  Such information is vital in our effort to lower lead exposure and toxicity to humans.