Abstract
Citrate ion possesses the right structure to be strongly bound on specific crystal faces of gypsum, CaSO4·2H2O, which results in an increase in the induction time of precipitation, and changes the structure and morphology of the solid. The inhibition of the precipitation kinetics of gypsum, in the stoichiometric reaction of Na2SO4 and CaCl2 was investigated in aqueous solutions with high supersaturation (0.1 M initial reactant concentrations). The induction time was found to be the longest when the inhibitor was present in fully deprotonated form in the solution (pH > 5). The inhibition effect levels off with the increasing inhibitor concentrations (ca. 3 mM). As a result of citrate binding, on the basis of SEM measurements, the morphology of the crystals change from rod-like to plate-like. Addition of high concentration of NaCl (mimicking seawater conditions) was found to facilitate the inhibition, but from the morphological studies, the inhibition mechanism of citrate appears to change under hypersaline conditions (relative to systems without added electrolyte.) From UV and IR spectroscopic measurements, it was found that the inhibitor ended up in the mother liquor after the completion of the precipitation reaction.
Highlights
Additives of various kinds have been frequently used to manipulate the crystallization process of gypsum, CaSO4Á2H2O Even in the first reports published in this field in 1958, the possibility of inhibiting crystallization reaction with using different additives was already discussed [1,2], and it was suggested that the most adequate types of inhibitors were the ones which were capable of binding onto the surface of the crystals forming, altering the growth process of the crystals
We have shown earlier [41] that for our systems conductometry is a suitable method even for fast reactions, but only in the absence of supporting electrolyte; on the contrary, Ca ion-selective electrode (Ca-ISE) works well in, e.g., hypersaline solutions, but the precipitation reaction needs to be slow enough for the establishment of reliable potential readings
In close-to-industrial conditions and at high gypsum supersaturation, the inhibiting effect of citric acid is still largely unexplored, our test reactions were performed under conditions, where the gypsum precipitation is favored in inhibitor-free systems
Summary
Additives of various kinds have been frequently used to manipulate the crystallization process of gypsum, CaSO4Á2H2O Even in the first reports published in this field in 1958, the possibility of inhibiting crystallization reaction with using different additives was already discussed [1,2], and it was suggested that the most adequate types of inhibitors were the ones which were capable of binding onto the surface of the crystals forming, altering the growth process of the crystals. It was found that at elevated temperature and in acidic media, they were still capable of slowing down the process [10] The extent of this effect depends on the experimental parameters employed and the concentration of the additive [28,29,30] If their individual contributions are determined, the information can be useful for various processes, because with varying morphology and crystal size the filtering properties of the solid change. We embarked on studying the crystallization kinetics of gypsum in the presence of citric acid/citrate with the aim of contributing to the description of these missing points listed above
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