Synthesis of robust insoluble anhydrite whiskers from desulphurization gypsum containing organic matter via improved hydrothermal‐calcination process for the removal of lead and zinc ions

Abstract

Background: An improved hydrothermal-calcination method was used to convert desulphurization gypsum containing organic matter (OM-gypsum) to insoluble anhydrite (II-CaSO 4 ) whiskers, where OM-gypsum was directly transformed to soluble anhydrite (γ-CaSO 4 ) whiskers via the hydrothermal method followed by calcination. The synthesized II-CaSO 4 whiskers were applied for the removal of lead ion (Pb(II)) and zinc ion (Zn(II)).Results: The synthesized II-CaSO 4 whiskers possessed a smooth surface, large aspect ratio, and high whiteness of above 90%. The mother liquor after synthesis could be recycled. The synthesized II-CaSO 4 whiskers exhibited maximum adsorption capacities of 641.03 mg/g and 14.00 mg/g for Pb(II) and Zn(II), respectively, under optimal adsorption conditions. The pseudo-second-order model and Langmuir isotherm model were appropriate for describing the adsorption process of II-CaSO 4 whiskers. Compared with II-CaSO 4 whiskers calcined from hemihydrate gypsum and the short and columnar II-CaSO 4 crystals, the synthesized II-CaSO 4 whiskers in this work possessed a superior adsorption performance. Conclusion:The improved hydrothermal-calcination process could completely remove the organic matter from OM-gypsum, and the direct formation of γ-CaSO 4 intermediate helped avoid the lattice collapse caused by the removal of water molecules during calcination. The excellent adsorption performance of the synthesized II-CaSO 4 whiskers in this work could be attributed to their large negatively charged surface areas, which were formed by the large aspect ratio and excellent stability. This work provides a practical method for the comprehensive utilization of chemical gypsum containing organic impurities.