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

Orthophosphate fixation fixation of metal contaminated soils in environments that undergo freeze thaw cycles is understudied. Freeze thaw cycling potentially influences the reaction rate, mineral chemical stability and physical breakdown of particles during fixation. 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 freeze-thaw cycles. Systems were subjected to multiple freeze-thaw cycles from +10 degrees C to -20 degrees C and then analysed by X-ray diffractometry. Pyromorphite formed in all systems and was stable over multiple freeze-thaw cycles. Low temperature lead orthophosphate reaction efficiency varied according to both phosphate and lead source; the most time-efficient pyromorphite formation was observed when PbSO4 and Na3PO4 were present together. These findings have implications for the manner in which metal contaminated materials in freezing ground can be treated with phosphate. (C) 2011 Elsevier Ltd. All rights reserved.