Efficient removal of fluoride from neutral wastewater by green synthesized Zr/calcium sulfate whiskers: An experimental and theoretical study

Insights into the Role of Na+ on the Transformation of Gypsum into α-Hemihydrate Whiskers in Alcohol-Water Systems

Alcohol-water solution-mediated transformation of gypsum into α-hemihydrate (α-HH) whiskers provides a green alternative for the high-value-added recycling of flue gas desulfurization (FGD) gypsum. However, the role of non-lattice cations during the transformation is still unclear. We report an evolution from "boosting-retarding" to "boosting-retarding-boosting" and finally to "boosting only" effect of non-lattice Na⁺ functioned by the concentration of ethylene glycol (EG) in water solutions. The driving force increased almost linearly upon the introduction of Na⁺ through the formation of ion pairs, and a higher slope was obtained at a higher EG concentration. Adsorption of Na⁺ ions and solidification of eugsterite on gypsum surfaces blocked the nucleation sites of α-HH. The retarding effect first rapidly increased and gradually approached a limit, following a parabolic trend after Na⁺ ions were introduced. Pentasalt, with a structure similar to that of α-HH, precipitated on the gypsum surface at higher c(Na⁺). The interaction of the driving force and the structural evolution of calcium sulfate ionic clusters accounts for the evolution of transformation kinetics. The retardation zone was compressed with the increase in EG volume ratios, and a monotonic boosting effect upon Na⁺ was observed at a 35.0 vol % of EG. Nucleation kinetics dominates the aspect ratio of α-HH whiskers. This study may provide a significant guidance for the utilization of FGD gypsum.