Products and stability of phosphate reactions with lead under freeze-thaw cycling in simple systems.

Orthophosphate �xation of metal contaminated soils in environments that undergo freezeethaw cycles is understudied. Freezeethaw cycling potentially in�uences the reaction rate, mineral chemical stability and physical breakdown of particles during �xation. This study determines what products form when phosphate (triple superphosphate [Ca(H2PO4)2] or sodium phosphate [Na3PO4]) reacts with lead (PbSO4 or PbCl2) in simple chemical systems in vitro, and assesses potential changes in formation during freezee thaw cycles. Systems were subjected to multiple freezeethaw cycles from �10 C to �20 C and then analysed by X-ray diffractometry. Pyromorphite formed in all systems and was stable over multiple freezeethaw cycles. Low temperature lead orthophosphate reaction ef�ciency varied according to both phosphate and lead source; the most time-ef�cient pyromorphite formation was observed when PbSO4 and Na3PO4 were present together. These �ndings have implications for the manner in which metal contaminated materials in freezing ground can be treated with phosphate.