Research Article
SN Applied Sciences (2020) 2:1577 | https://doi.org/10.1007/s42452-020-03313-w
PCC depends on various synthesis routes or due to the interaction of some organic additives as dispersing agents during the manufacturing process [30–32]. In comparison to the unmodified one, those modified with chitosan dis- playing a slight displacement of peak around 1494 cm −1 toward lower absorption band at 1458 cm −1 is due to the differences in the crystal structure of calcite and aragonite. Rather it is assigned to the specific band for calcite [28]. The spectra display a broader peak at 3431 cm −1 mainly assigned to the OH or NH stretching vibrations related to chitosan. Moreover, the absorption band at 2919 and 2874 cm −1 designated for C–H stretch has become broad- ened due to the overlapping C–H bond associated with chitosan [33, 34]. A weak intensified peak at 1150 cm −1 and a shoulder at 1030 cm −1 could be assigned to an asymmetric stretch of C–O–C bridge and C–O stretch, respectively, revealing the presence of the polymer in the modified samples [34]. Another interesting feature in the modified PCC spectrum observed is that the peaks at 1082 and 856 cm −1 have sufficiently decreased. In fact, a more pronounced peak at 873 cm −1 and disappearance of peak at 700 cm −1 confirms the transformation of aragonite polymorphs to calcite phases. Note that peaks relayed to calcite phases are more intensified when chitosan was dis- solved in acetic acid for the modification of the PCC sur- face. The crystal structure of unmodified and modified PCC was further studied by XRD and shows a good correlation with FTIR data. The coexistence of both calcite and arago- nite phases can be detected from the diffraction pattern of pure PCC as noted from Fig. 2. The diffraction peaks positioned at 2 θ of 23.0° (112), 29.4° (104), 35.9° (110), 47.5° (018), and 28.5° (116) are asso- ciated with calcite polymorph whereas 2θ of 26.2° (111), 27.2° (021), 33.1° (012), 37.9° (112), 38.4° (130), 45.8° (221), 50.3° (132), and 52.5° (023) are related to aragonite phase [35]. After introducing chitosan in PCC dispersion, the intensity of peaks at 2 θ = 26.2° and 27.2° corresponding to the planes (111) and (021) of aragonite has been suf- ficiently reduced with an increase in the peak intensity at 2 θ = 29.4° corresponding to (104) planes of calcite. Further- more, the calcite and aragonite content quantified from respective intensities based on previously reported meth- ods were found to be 21% and 79%, respectively, in the ini- tial PCC sample [36]. On the other hand, chitosan-modified PCC is indexed as a mixture of the majority of calcite with small traces of aragonite. In particular, the dissolution of chitosan in acetic acid and sorption of the polymer on PCC surfaces produce a larger quantity of calcite crystals (97%) compared to hydrochloric acid (92%). The respec- tive aragonite quantity estimated was 3% and 8% [36]. However, traces of chitosan could not be detected from XRD patterns of the modified samples. This can be sup- ported with the earlier reports of XRD where authors did
Fig. 2 XRD patterns of A PCC, B PCC + chitosan (HCl) and C PCC + chitosan (acetic acid) black filled circle calcite, red filled cir- cle aragonite
not define the presence of chitosan acting as oriented sup- port for the growth of calcium carbonate polymorphs [37]. It was observed that when an acidic solution of chitosan (pH 5.5 in case of both acids) was subjected to the alkaline environment of PCC (pH 10.4), chitosan started precipitat- ing out of the solution resulting in co-aggregation of PCC and polymeric macromolecules with a final solution pH of 7.8–7.9 at equilibrium. The principle of chitosan regenera- tion in alkaline medium from acidic solution is governed by the screening of electrostatic repulsion between the protonated amine groups above its pKa value [38]. At this stage, the stiff rod-like structure of chitosan conforms more into a random coil followed by deposition on PCC surfaces [39]. Therefore, we have discussed the probable mechanism behind this polymorph transformation by analyzing the data from FTIR and XRD. Generally, calcium carbonate exists in the form of three anhydrous crystal polymorphs among which vaterite is the least stable and calcite being the most thermodynamically stable [40]. The findings from the recent studies suggest that nucleation and growth of resultant crystalline polymorphs (vaterite, aragonite and calcite) occur via dissolution–re-precipita- tion mechanism through an intermediate hydrated amor- phous calcium carbonate (ACC) precursors. [21, 35, 40, 41]. It was inferred that controlling of critical process variables like change of solution pH by acids and organic additives affect the yield of particular crystal polymorphs and its sta- bilization. pH-dependent dissolution rates of ACC lead to different crystallization pathways that ultimately define a particular calcium carbonate polymorph [42–46]. Ca 2+ and
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