The effect of the presence of Zn is solutions supersaturated with respect to calcium carbonate was investigated at 25°C, pH 8.50. The stability domain of the calcium carbonate solutions was measured in the supersaturation ratio (SRcalcite) values between 20.89-32.36. Below the least SRcalcite value the solutions were stable. Above it, precipitation was spontaneous past induction time, inversely proportional to SRcalcite according to the classical nucleation theory (CNT). In the presence of 1.3 ppm of Zn in the supersaturated solutions, the stability domain was shifted to higher SRcalcite values (47.86<SRcalcite<112.2). The presence of Zn stabilized all three calcium carbonate polymorphs. In the absence of Zn only vaterite and calcite were identified in the precipitated. In all cases no Zn oxides were formed. The rate of spontaneous precipitation of calcium carbonate in the presence of Zn, was reduced by as much as 98% at the least SRcalcite values tested. Adsorption of Zn at the active sites of the supercritical nuclei of vaterite is responsible for the reduction of the rates of CaCO3 precipitation and for the delay of conversion of unstable polymorphs. Finally, induction times in the presence of Zn were significantly longer for the same SRcalcite values without Zn, suggesting that Zn is a nucleation inhibitor as well.
Calcium carbonate precipitation in the presence of pollutants is a carrier of importance for their transport to the sediments and for their subsequent release depending on the local microenvironment conditions1,2. Zinc is often present in industrial waters (heat exchangers and boilers) mainly for corrosion protection. Depending on the alkalinity and calcium concentration of waters used in water intensive processes, calcium carbonate fouling is common. Moreover, among other metals, is present in natural waters together with iron and copper, where the concentration of Zn(II) may reach at levels of several ppm. Zinc has been reported to retard precipitation of calcium carbonate scale in heat exchangers and in oil production wells even at concentration of 2 ppm3,4. Similar calcium carbonate precipitation inhibition was attributed to the presence of zinc species released from the surface of the equipment involved with physical water treatment5,6. It is interesting to note that the presence of just 50 ppb of zinc in solutions of rather high supersaturation with respect to all calcium carbonate polymorphs, resulted to dramatic reduction of both nucleation and subsequent crystal growth6. The presence of zinc in the solutions in which calcium carbonate precipitated, favored the stabilization of the metastable aragonite polymorph6. Interestingly, it has also been reported that zinc acted as an accelerator of heterogeneous nucleation in the bulk solution and the effect was more pronounced at higher concentrations7. In general, the addition of chemical inhibitors to control scale formation by crystallization is the practice used most often. Commonly used antiscalants include condensed polyphosphates, organophosphates, and polyelectrolytes containing carboxylic and sulfonate functional groups. The mechanism of inhibition is quite diverse and includes chelation, dispersion and retardation of the growth of inorganic minerals due to poisoning of the active growth sites8,9. Metallic cations such as iron, copper and zinc are very important, because of their natural origin in water and the investigation of their role in the kinetics and the mechanism of inorganic fouling deserves further and deeper exploration. Despite research interest for the role of the presence of zinc in calcium carbonate supersaturated solutions, the related literature is short of investigations of the spontaneous precipitation of calcium carbonate polymorphs and their subsequent growth in aqueous solutions, supersaturated with respect to the calcium carbonate polymorphs containing zinc at concentration levels precluding the formation of sparingly soluble salts of zinc carbonate (Zn(OH)2, ZnCO3 or Zn5 (OH) 6 (CO3)2).
