Solubility and stability of lead arsenates at 25°C

Abstract

Lead arsenates were the most extensively used arsenical insecticides in the past. Their residues, especially the water-soluble fractions in environment may pose a potential human health risk. The solubility products (K _sp) and free energies of formation (Δ G _f °) of PbHAsO₄·2H₂O(c), Pb₅(AsO₄)₃OH·H₂O(c), Pb₃(AsO₄)₂(c) and Pb₈As₂O₁₃(8PbO·As₂O₅)(c) at 25°C were determined by precipitation and dissolution experiments. The composition of precipitates depended on the Pb/As molar ratios and pH values. The solid precipitates were lamellar PbHAsO₄·2H₂O(c) at pH = 3 ∼ 5 and Pb/As = 1.0, columnar Pb₅(AsO₄)₃OH·H₂O(c) at pH = 12–13 and Pb/As = 1.5, Pb₅(AsO₄)₃OH·H₂O(c) with little amounts of Pb₈As₂O₁₃(c) at pH = 12–13, Pb/As = 1.67 and 2.0. The mixture of PbHAsO₄·2H₂O(c) and Pb₃(AsO₄)₂(c) formed at pH = 6.51 and Pb/As = 1.5. Based on the analytical results, the solubility products (K _sp) of PbHAsO₄·2H₂O(c), Pb₅(AsO₄)₃OH·H₂O(c), Pb₃(AsO₄)₂(c) and Pb₈As₂O₁₃(c) were calculated by the computer program PHREEQC as 10^{− 10.70}(10^{− 7.81}∼ 10^{− 14.32}), 10^{− 81.75}(10^{− 74.37}∼ 10^{− 84.83}), 10^{− 33.83}(10^{− 32.05}∼ 10^{− 34.49}) and 10^{− 174.24}(10^{− 156.64}∼ 10^{− 178.35}), respectively. Correspondingly, the free energies of formation (Δ G _f °) of these lead arsenates were calculated to be −1274.36kJ/mol, −2928.33 kJ/mol, −1563.12 kJ/mol and −3909.69 kJ/mol.

Keywords:

• Lead arsenate
• solubility product
• free energy of formation
• stability

Acknowledgments

The authors thank the Guangxi Key Laboratory of Environmental Engineering, Protection and Assessment for the research assistance and financial supports from the National Natural Science Foundation of China (NSFC 40773059 and 40263001) and the Provincial Natural Science Foundation of Guangxi (GuiKeZi 0728222 and GuiKeGong 0816003-1-1). Financial assistance for this research was also provided through the Program to Sponsor Teams for Innovation in the Construction of Talent Highlands in Guangxi Institutions of Higher Learning and the Special Research Program for Excellent Talents in Guangxi (GuiKeRen 2007-71, No.2004217).

Notes

*Calculated from original concentration.

*Calculated from original concentration.

*Calculated from original concentration.

*Calculated from original concentration.