The relative distribution of Pb() species (i.e. Pb2+, Pb(OH)+ and Pb(OH)2) in the aqueous solution is calculated using a MINEQL software [37], and the result is shown in Fig. 5. It can be seen that Pb2+ is the dominant species at pH lower than 4.0, and the hydrolysis of Pb2+ to form Pb(OH)+ and Pb(OH)2 starts at about pH 3.6. The hydrolysis proceeds very rapidly over a narrow pH range from 5.0 to 7.0, and Pb(OH)2 (Ksp = 1.2 × 10−15) [27] becomes the dominant species in the solution at pH higher than 6.0.