ABSTRACT
Chemical weathering is a natural soil process that occurs under the prevailing environmental conditions and results in the transfer of matter from unstable mineral phases to more stable mineral phases or soluble species. Weathering is not limited to indigenous soil minerals. Indeed, any material that is generated under a set of conditions that differs from those of the soil environment will undergo transformation if introduced into the soil. These transformations include the dissolution or alteration of the introduced or indigenous materials and the precipitation of relatively more stable mineral phases. For example, metallic Pb from spent ammunition rounds or lead pipe will oxidize in soil to form litharge (PbO), which is transformed further to cerussite (PbCO3) in alkaline soil. The metallic Pb is unstable in the presence of exceeding low levels of O2(g), and is oxidized to the lead oxide:
Pb0(s) + O2(g) → PbO(s) (3.1)
In this reaction, metallic lead (Pb) is the primary phase of lead, while litharge is a secondary phase. However, litharge is also unstable relative to a host of other lead-bearing minerals, including lead silicates, phosphates, sulfates, and carbonates. In an alkaline soil environment, the most stable leadbearing mineral is the carbonate, PbCO3, which is also a secondary phase:
PbO(s) + CO2(g) → PbCO3(s) (3.2)
Because cerussite is the stable mineral phase for chemical conditions in an alkaline soil environment, additional weathering can only result in the dissolution of cerussite and the release of Pb2+ to be mobilized or to participate in other retention processes (e.g., adsorption, plant uptake):
PbCO3(s) + 2H+(aq) → Pb2+(aq) + CO2(g) + H2O(l) (3.3)
The transfer of lead from the unstable metallic Pb phase, through the progressively more stable litharge and cerussite phases, to the soluble divalent cation, Pb2+, is an example of a weathering sequence. Typically, however, weathering is a term and phenomenon that is specific to the transformation of naturally occurring soil minerals, particularly the silicates.
