Effects of freeze-thaw cycling on metal-phosphate formation and stability in single and multi-metal systems

Freeze-thaw cycling may influence the chemistry, mineral stability and reaction rate during metal orthophosphate fixation. This study assessed the formation and stability of Cu-, Pb-, and Zn-phosphates in chemically simple laboratory systems during 240 freeze-thaw cycles (120 days) from +10 to 20 degrees C, using X-ray diffractometry. In single heavy metal systems, chloro- and hydroxy-pyromorphite (Pb-5(PO4)(3)(Cl,OH)), sodalite (Na6Zn6(PO4)(6)center dot 8H(2)O), chiral zincophosphate (Na-12(Zn12P12O48)center dot 12H(2)O), and copper phosphate hydrate (Cu-3(PO4)(2)center dot 3H(2)O) were the primary phosphate minerals that formed, and were typically stable during the experiment. Zinc and Cu-phosphate formation was reduced in multi heavy metal systems, and was substantially lower in abundance than chloropyromorphite. Successful Cu-, Pb- and Zn-phosphate formation can be expected in cold and freezing environments like the polar regions. However, field implementation of orthophosphate fixation needs to consider competing ion effects, concentration of the phosphate source, and the amount of free-water. (C) 2013 Elsevier Ltd. All rights reserved.