Role and fate of the lead during the conversion of calcium sulfate dihydrate to α-hemihydrate whiskers in ethylene glycol-water solutions

Abstract 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 Pb 2+ during the conversion of calcium sulfate dihydrate (DH, the leading component of FGD gypsum) to α-hemihydrate (α-HH) whiskers, in Pb 2+ ion-containing ethylene glycol (EG)-water solutions. The results demonstrate that Pb 2+ retards DH–α-HH conversion by decreasing the driving force and simultaneously reducing the aspect ratios by shortening the lengths of the whiskers. Four major Pb 2+ species, including free Pb 2+ ions, Pb 2+ in anglesite precipitate, weakly adsorbed Pb 2+ , and Pb 2+ doped onto/into calcium sulfate surfaces are determined to be present. Most Pb 2+ is distributed in the solids, while a small amount of Pb 2+ moves freely in the solutions. During the induction period of conversion, the majority of Pb 2+ is weakly adsorbed on DH, and the remaining Pb 2+ exists in anglesite. Upon the initiation of conversion, the anglesite phase starts accumulating, and the adsorbed Pb 2+ transfers into the surface lattice of α-HH (denoted as doping Pb 2+ ). The adsorbed Pb 2+ on α-HH increases significantly from 0.055 to 1.63 wt% and the doping Pb 2+ decreases slightly from 0.82 to 0.76 wt% within a c (Pb 2+ ) initial value of 500–1500 mg/L. Kinetically controlled lattice incorporation accounts for the variation in doping Pb 2+ . These findings provide new insights into the synthesis of α-HH whiskers, which is influenced by heavy metals and may be useful in FGD gypsum utilization.