Abstract
The primary nucleation mechanism of the gypsum in a bulk aqueous medium was identified as a heterogeneous one for 0.05 and 0.03 mol·L−1 CaSO4·2H2O solutions and 25 °C. By means of a particle counter and dynamic light scattering (DLS) technique, solid nano/microimpurities were found, and controlled in stock brines for gypsum supersaturated solutions preparation. It is demonstrated that the common procedure of reagent grade 0.10 mol·L‒1 CaCl2 and Na2SO4 aqueous solutions filtration via 200 nm membranes is capable to reduce the foreign solid microimpurities content (size > 100 nm) from 106 to 103 units in 1 mL, but fails to affect the more numerous nanofraction (size < 100 nm). Thus, the gypsum nucleation takes place in presence of a significant amount of “nano/microcodust” templates, and has a heterogenous character. The induction time, measured by conductivity for the similar supersaturation levels, reveals a well detectable dependence on nano/microdust concentent: an increasing background particle concentration substantially decreases the induction period at a constant saturation state and temperature, and thus increases the nucleation rate. Therefore, the gypsum nucleation reaction starts tentatively through the fast heterogeneous formation of well-defined, primary nuclei via [Ca2+], [SO42‒], and [CaSO4]o species sorption on the surface of “nano/microdust” particles. Thus, the “nano/microdust”, naturally occurring in any high purity chemical, plays a key role in sparingly soluble salts nucleation in the bulk aqueous medium.
Highlights
Gypsum crystallization has been a matter of intensive research since the end of XIX Century [1].From the systematic studies of van’t Hoff [2] to more recent reports [3,4,5,6], there are over 6000 publications in this field, accounted for by Chemical Abstracts
The stock 0.10 mol·L−1 solutions of high purity chemicals Na2 SO4 and CaCl2 in a deionized water reveal a high content of solid microimpurities larger than 100 nm in 1 mL: 370,000 and 480,000 respectively, Table 3
It is reasonable to suppose that the fraction sized below 100 nm is rather significant. This fraction is not detectable by a particle counter used in this study
Summary
Gypsum crystallization has been a matter of intensive research since the end of XIX Century [1].From the systematic studies of van’t Hoff [2] to more recent reports [3,4,5,6], there are over 6000 publications in this field, accounted for by Chemical Abstracts. It is not surprising that the mechanisms of gypsum nucleation and growth in supersaturated aqueous solutions became a matter of intensive research during the past century [3,4,5,6,9,10,11,12,13,14,15,16,17,18,19,20,21]. These efforts resulted in a Minerals 2020, 10, 1083; doi:10.3390/min10121083 www.mdpi.com/journal/minerals
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