Research Article
SN Applied Sciences (2020) 2:1577 | https://doi.org/10.1007/s42452-020-03313-w
CO 3− binding strength and equilibrium carbonate species CO 3− /, HCO3 − at different pH are the determining factors for ACC structures that mimic certain local polymorphs which further transform into calcite, aragonite or vaterite. The driving force for the conversion of one polymorph to other was postulated to be the HCO 3− ions as calcium car- bonate growth rate was proportional to the ion activity product of Ca 2+ and HCO 3− [47]. On the other hand, chi- tosan as an additive has been studied as a template for the nucleation and growth of different polymorphic structures based on the charge density and solution pH [21, 22]. This near-neutral macromolecule at alkaline pH was thought to gather Ca 2+ ions on its surface providing an interface between this organic–inorganic composite and the sur- rounding liquid for active nucleation site of calcite crys- tallization [48]. Therefore, as per our system concerned, the transformation of aragonite crystals to polycrystalline calcite aggregates can be explained on the principle of (1) dissolution–recrystallization mechanism and (2) sorption of chitosan on the crystal surface. In our opinion, the fast dissolution of metastable aragonite at this pH might have released dissolved inorganic HCO 3− and Ca 2+ ions with co- precipitation of chitosan on aragonite surface [49]. At this point, interaction of chitosan with precipitated calcium carbonate would be difficult to explain since electrostatic forces would not be very strong because of deprotonated amine groups of chitosan in weak alkaline condition [48, 50]. However, chitosan described as a hydro- gel in near neutral pH solution could be able to promote calcium carbonate polymorphism. Therefore, we can con- clude that chitosan with its amine and OH groups depos- ited on the crystal surface and served as a template for the recrystallization of calcite aggregates [48]. Inhibition to the
growth of free single crystal could be due to the polymeric chains of the macromolecule since it has the ability to form bridges between calcium carbonate particles. Moreover, from the work of Busenberg and Plummer 1987 percent- age transformation of more soluble aragonite to the less soluble calcite in contact with water depends on solu- tion pH, solution chemistry, temperature and precipita- tion time. Various models established for this mechanistic behavior state that the rate of dissolution of primary crys- tals is equal to the growth of secondary mineral crystals [51]. Several authors have explained that electronegative charge on specific anionic additives likely to adsorb Ca 2+ ions which further attracts the HCO 3 / – CO 3− ions to facili- tate nucleation of calcium carbonate species [42, 52]. So, in our case, the appearance of a larger number of calcite crystals in the presence of chitosan/acetic acid system may be expected due to an additional effect of this organic acid adsorption on the polymorph rearrangements. Pre- viously, it was observed that carboxylated organic acids with their deprotonated negatively charged sites (COO–) at 7–8 adsorbed on the surface of aragonite crystals inducing their recrystallization to calcite [42]. Although further investigations are necessary to better understand the mechanism of active nucleation pathway to control of rearrangements of PCC polymorphs. The presence of chitosan in the modified PCC was further confirmed by the thermal behavior as observed from TGA-DTG curves in Fig. 3A). Unmodified PCC exhibits single-step degrada- tion profile when the temperature is increased from 40 to 900 °C. This is in agreement with the reported literature [53, 54]. The degradation begins at T onset = 511 °C and the decomposition completes at temperature 683 °C since this temperature range accelerates the formation of CaO
Fig. 3 A TGA and B DTG curves of PCC and chitosan-modified PCC
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