Abstract
The pH-stat technique has been used to study the influence of several natural and synthetic additives containing carboxyl, phenolic, and sulfonic groups on the precipitation of calcium carbonate from supersaturated solutions (Ca=HCO3 = 5.50 mM, NaCl=0.10 M, pH 8.50, 35°C). The additives investigated were natural i.e., green tea extract, GTE; sodium lignosulfonate, SLS and synthetic i.e., poly(acrylic acid), P-AA; poly(2-acrylamido-2-methyl-propane sulfonic acid), P-SA. It has been found that addition of low concentrations of additive has three fold effects: a) it delays the precipitation of calcium carbonate, b) it inhibits the rate of precipitation, and c) it influences the calcium carbonate polymorph. Based on the results obtained under the test conditions, the ranking in terms of decreasing effectiveness is: P-AA ≫ GTE>SLS ∼ P-SA. The results also indicate that the delay in precipitation increases with increasing additive concentration. SEM and X-ray diffraction studies of calcium carbonate precipitated in the presence of additives reveal that whereas P-AA and P-SA favor the formation of vaterite, GTE and SLS influence the formation of calcite. By contrast, the predominant polymorph formed in the absence of additive is vaterite. Kinetic analysis suggests Langmuir-type adsorption of added additive ions on the active growth sites of CaCO3 crystals.
Kurzfassung
Zur Untersuchung des Einflusses von mehreren natürlichen und synthetischen Additiven, die Carboxyl-, Phenol- und Sulfongruppen enthalten, auf die Ausfällung von Kalziumcarbonat aus übersättigten Lösungen (Ca=HCO3 = 5,50 mM, NaCl=0,10 M, pH 8,50, 35°C) wurde das pH-stat-Verfahren eingesetzt. Die untersuchten Additive waren natürliche, d.h. grüner Tee-Extrakt, GTE, Natriumligninsulfonat, SLS und synthetische, d.h. Polyacrylsäure, P-AA, Poly(2-acrylamido-2-methyl-propansulfonsäure), P-SA. Es wurde gefunden, dass die Zugabe niedriger Konzentrationen von Additiven einen dreifachen Effekt bewirkt: a) es verzögert die Ausfällung von Kalziumcarbonat, b) es inhibiert den Grad der Ausfällung und c) es beeinflusst die Polymorphie von Kalziumcarbonat. Basierend auf den Ergebnissen, die unter den Testbedingungen erhalten wurden, ergibt sich folgende Reihenfolge mit abnehmender Effektivität: P-AA ≫ GTE>SLS ∼ P-SA. Die Ergebnisse zeigen außerdem, dass die Verzögerung der Ausfällung mit steigender Additivkonzentration zunimmt. SEM- und Röntgenbeugungsuntersuchungen von Kalziumcarbonatausfällungen in Gegenwart von Additiven lassen erkennen, dass P-AA und P-SA die Ausbildung von Vaterit begünstigen, während GTE und SLS die Ausbildung von Calcit beeinflussen. In Abwesenheit von Additiven ist im Gegensatz dazu das Vaterit die überwiegende polymorphe Form. Kinetikanalysen weisen auf eine Adsorption der Additiv-Ionen an den aktiven Wachstumsstellen der CaCO3-Kristalle nach Langmuir hin.
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