As was shown by Ma et al. [25, 26], the mechanism of Pb immobilization in the contaminated soils by
HA could be dissolution of HA (eq.1), and the precipitation of hydroxypyromorphite (eq.2). However,
Shaskova posed another reaction mechanism [27]. Pb may be adsorbed first on the surface of the HA and
then cation exchange would prompte Pb2+ ions substituting Ca2+ ions in soil solution, following reaction
(eq.3) [28]. Furthermore, as indicated by this equation, the mechanism is controlled by the amount of
exchangeable cation.