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Minerals 2017, 7(8), 140;

Physicochemical and Additive Controls on the Multistep Precipitation Pathway of Gypsum

LEC, IACT, CSIC, Universidad de Granada, 18100 Granada, Spain
GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
Department of Geology, Museum Building, Trinity College Dublin, 2 Dublin, Ireland
Structural Biology Brussels, Flanders Interuniversity Institute for Biotechnology and Vrije Universiteit Brussel, 1050 Elsene, Belgium
Department of Earth Sciences, Free University of Berlin, 12249 Berlin, Germany
School of Earth and Environment, University of Leeds, LS2 9JT Leeds, UK
University Grenoble-Alpes, CNRS, ISTerre, F-38000 Grenoble, France
Authors to whom correspondence should be addressed.
Received: 6 July 2017 / Revised: 2 August 2017 / Accepted: 3 August 2017 / Published: 9 August 2017
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Synchrotron-based small- and wide-angle X-ray scattering (SAXS/WAXS) was used to examine in situ the precipitation of gypsum (CaSO4·2H2O) from solution. We determined the role of (I) supersaturation, (II) temperature and (III) additives (Mg2+ and citric acid) on the precipitation mechanism and rate of gypsum. Detailed analysis of the SAXS data showed that for all tested supersaturations and temperatures the same nucleation pathway was maintained, i.e., formation of primary particles that aggregate and transform/re-organize into gypsum. In the presence of Mg2+ more primary particle are formed compared to the pure experiment, but the onset of their transformation/reorganization was slowed down. Citrate reduces the formation of primary particles resulting in a longer induction time of gypsum formation. Based on the WAXS data we determined that the precipitation rate of gypsum increased 5-fold from 4 to 40 °C, which results in an effective activation energy of ~30 kJ·mol−1. Mg2+ reduces the precipitation rate of gypsum by more than half, most likely by blocking the attachment sites of the growth units, while citric acid only weakly hampers the growth of gypsum by lowering the effective supersaturation. In short, our results show that the nucleation mechanism is independent of the solution conditions and that Mg2+ and citric acid influence differently the nucleation pathway and growth kinetics of gypsum. These insights are key for further improving our ability to control the crystallization process of calcium sulphate. View Full-Text
Keywords: nucleation; multistep pathway; crystal growth; gypsum; additives nucleation; multistep pathway; crystal growth; gypsum; additives

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Ossorio, M.; Stawski, T.M.; Rodríguez-Blanco, J.D.; Sleutel, M.; García-Ruiz, J.M.; Benning, L.G.; Van Driessche, A.E.S. Physicochemical and Additive Controls on the Multistep Precipitation Pathway of Gypsum. Minerals 2017, 7, 140.

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