Impact of glutamic acid on the transformation of calcium sulfate hemihydrate to dihydrate: Characterization, equilibrium, kinetics, and thermodynamics

Abstract

In this study, the characteristics and mechanisms of glutamic acid adsorption on the transformation process of calcium sulfate hemihydrate to calcium sulfate dihydrate were investigated in a batch-type crystallizer. The phase transformation was followed by conductivity measurements in both the absence and the presence of glutamic acid used as additive. The structure of the samples obtained during this process was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Brunauer-Emmett-Teller, thermogravimetric analyzer-mass spectrometer, and ATR-FTIR analyses. From the results, it was shown that the uptake of glutamic acid caused a retarding effect in the transformation rate. It was also found that the thin- and weak-layered structure was replaced by a thick and compact plate-like structure. The adsorption isotherms, kinetics, and thermodynamics were also explored to describe the process. According to the results, the Langmuir isotherm model fit quite well, as demonstrated by the high R-2 value, calculated using a linear and nonlinear procedure, and the maximum adsorption amount of 169.49mg/g. The pseudo-first-order and pseudo-second-order kinetic models were applied to analyze the kinetic experimental data, and it was determined that the adsorption of glutamic acid followed the pseudo-second-order model, which indicates that the adsorption rate depended on chemisorption. The thermodynamic results illustrated that the adsorption of glutamic acid in the transformation process was spontaneous and endothermic.