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.