The behavior of Pb, a toxic heavy metal that commonly exists in flue gas desulfurization (FGD) gypsum, requires a systematic study for FGD gypsum recycling. In this study, we focus on the role and fate of Pb2+ during the conversion of calcium sulfate dihydrate (DH, the leading component of FGD gypsum) to alpha-hemihydrate (alpha-HH) whiskers, in Pb2+ ion-containing ethylene glycol (EG)-water solutions. The results demonstrate that Pb2+ retards DH-alpha-HH conversion by decreasing the driving force and simultaneously reducing the aspect ratios by shortening the lengths of the whiskers. Four major Pb2+ species, including free Pb2+ ions, Pb2+ in anglesite precipitate, weakly adsorbed Pb2+, and Pb2+ doped onto/into calcium sulfate surfaces are determined to be present. Most Pb2+ is distributed in the solids, while a small amount of Pb2+ moves freely in the solutions. During the induction period of conversion, the majority of Pb2+ is weakly adsorbed on DH, and the remaining Pb2+ exists in anglesite. Upon the initiation of conversion, the anglesite phase starts accumulating, and the adsorbed Pb2+ transfers into the surface lattice of alpha-HH (denoted as doping Pb2+). The adsorbed Pb2+ on alpha-HH increases significantly from 0.055 to 1.63 wt% and the doping Pb2+ decreases slightly from 0.82 to 0.76 wt% within a c(Pb2+)(initial) value of 500-1500 mg/L. Kinetically controlled lattice incorporation accounts for the variation in doping Pb2+. These findings provide new insights into the synthesis of alpha-HH whiskers, which is influenced by heavy metals and may be useful in FGD gypsum utilization.