Revealing maleic acid role in the preparation of α-hemihydrate gypsum from titanium gypsum through experiments and DFT calculations

• Published in: The Science of the total environment Vol. 897; p. 166405
• Main Authors: Du, Jingwei, Tian, Lin, Qi, Manfu, Zhang, Chen, Di, Hongfeng, Zhi, Xiao, Zhu, Jianping
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-11-2023
• Subjects: CaCl2; DFT calculations; Fe3; Maleic acid; Titanium gypsum; α-Hemihydrate gypsum (α-HH); Titanium gypsum; Fe3; CaCl2; DFT calculations; Maleic acid; α-Hemihydrate gypsum (α-HH)
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2023.166405

Titanium gypsum (TG) is rarely used to produce α-hemihydrate gypsum (α-HH) because of its poor crystallinity and high impurity and moisture contents. Here, a method is proposed to prepare α - HH by adjusting the reaction temperature, CaCl2 solution concentration and maleic acid dosage based on acid leaching and heat-treated TG as raw material. The effect of maleic acid and Fe3+ ions on the preparation of α-HH were systematically analyzed using density functional theory (DFT) and typical materials characterization methods, X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). Under the optimal conditions (CaCl2 concentration of 23 % and reaction temperature of 95 °C), the maleic acid is chemically adsorbed on the crystal surfaces of α-HH, the strongest adsorption is in the (111) surface. Increasing the maleic acid concentration from 0 to 0.15 % decreased the aspect ratio of the α-HH crystals from 8.26 to 0.96, respectively, where the optimal dosage was 0.1 %. The theoretical results proved that the substitution energy of Fe3+ was greater than that of Ca2+, and Fe3+ ions can spontaneously enter the α-HH lattice to replace Ca2+ ions. Furthermore, the adsorption energy of maleic acid on the (111) surface increased after the substitution of Fe3+ to generate a synergistic effect that hinders α-HH growth along the c-axis, resulting in the preferred morphology. The results of this study provide a new method for using waste TG to produce a high-value-added product.
1.Clean titanium gypsum by low concentration acid leaching + heat treatment process.2.Preparation of hemihydrate gypsum by regulating reaction temperature and concentration of reaction medium in atmospheric pressure calcium chloride solution with titanium gypsum as raw material.3.Maleic acid was used as modifier crystal to control the crystal morphology and make it have the characteristics of α-hemihydrate gypsum.4.Analysis of the synergistic modifier mechanism between maleic acid and Fe3+, an intrinsic impurity in titanium gypsum, using a combination of DFT calculations and experiments. [Display omitted]
•Titanium gypsum (TG)-prepared α-hemihydrate gypsum often has poor characteristics.•α-hemihydrate gypsum was successfully prepared using cleaner TG.•Solution concentration, reaction temperature, and maleic acid dosage were optimized.•Effects of maleic acid and ferric (Fe3+) ions evaluated by simulation and experiment.•Inherent Fe3+ ions present in the TG are crucial to α-hemihydrate gypsum formation.