Research Article
SN Applied Sciences (2020) 2:1577 | https://doi.org/10.1007/s42452-020-03313-w
where we could observe that, initially, PCC had an anionic charge density of − 5.49 μeq/g. Even PCC functionalized with 4.5% chitosan, anionic charge density decreased to − 4.4 μeq/g but no charge reversal or charge nullifica- tion could be detected at this point. We, therefore, pro- pose that the dominant mechanism was flocculation of PCC by the polymer rather than coagulation. Roussy and Van Vooren observed that solubility and insolubility of chitosan at different pH strongly affects the coagulation/ flocculation behavior of mineral colloids [58]. They con- cluded that at high pH the neutralized amine groups of polymer chains physically entrap the mineral colloids in its network by bridging mechanism leading to instability of the colloids. Similar trends are also observed in our study. The final pH measured at a chitosan dose of 11% of PCC after adsorption decreased to 7.2. The amount of chitosan on PCC remained almost constant with a further increase in the dose of chitosan. With 4.5% addition of chitosan to PCC, the deposited chitosan amount quantified was 4.3%. An excessive amount of chitosan might have led to re-sta- bilization of the suspension indicated by the high value of TOC in the supernatant. A similar explanation was given by Roussy and Van Vooren when residual turbidity increased due to excess addition of chitosan for decantation of col- loid particles at natural water pH [39]. The increase in the zeta potential of PCC from − 14.3 to − 11.3 mv indicates that adsorbed uncharged chitosan just moved the slip sur- face of the electrical double layer with some electrostatic interactions. Otherwise, the surface potential of PCC might have been close to zero or reversed [38]. The crystal morphology and surface morphology of the PCC particles were studied by optical microscope and FE-SEM images as depicted in Figs. 5 and 6, respec- tively. The micrographs obtained under different magni- fication revealed that aggregated mineral flocs in both modified and unmodified PCC have become dark due to light opaqueness. More aggregated polymorphism was observed with modified one representing surface treat- ment of PCC either by encapsulation, flocculation, and adsorption/precipitation. Recrystallization of aragonite particles in the presence of chitosan was further exam- ined by FE-SEM. The coexistence of aragonite and calcite is evident from Fig. 6A with a predominance of aragonite crystals in the native PCC. The rod-shaped aragonite crystals are arranged in bundles in a semi-circular structure [43]. As discussed previously in the above sections of the study there are pos- sibilities that interaction between NH 2 and OH groups in chitosan with carbonate ions can affect the ultimate crys- tal habit leading to the formation of a scalenohedral type of calcite in the presence of chitosan [3] [59]. The prev- alence of calcite with traces of aragonite is observed in Fig. 6G which can relate well with the increased intensity of
peak corresponding to (104) plane. The aggregation phe- nomenon can further be corroborated with optical micro- scope images revealing irregular shaped dense and big- ger flocculated particles in modified PCCs. Several other authors reported a morphological change of different grade of fillers preflocculating or modifying with different polymers [6, 14, 60]. The median particle size of native PCC was around 11 μm which increased dramatically to 104 μm after treatment with chitosan when HCl was considered as a dissolution medium. Individual PCC particles were being aggregated to larger flocs in modified PCC through a bridging mechanism by precipitated polymeric chains either as a result of weak electrostatic interaction between chitosan and Ca 2+ ions or by physical entrapment of the minerals in its coiled structure. Additionally, Fig. 7 shows that modified PCC exhibits much broader size distribution compared to the native one indicating nonuniform aggregation of filler particles as also observed in FE-SEM images. Similar particle size distribution is not only obtained by papermakers in pro- ducing pre-flocculated filler particles [61] but also can be observed in other industries where flocculation is an important mechanism to optimize a system. The encapsu- lation mechanism is proposed in Fig. 8. As prepared filler slurries were then employed to analyze the effect of chi- tosan modified filler particles on tensile strength and filler retention calculated from residual ash in handsheets. It is a well-known fact that incorporating filler at high ash level interferes with fiber–fiber bonding resulting in decreased paper strength. Paper strength generally depends on Fig. 5 Optical microscopic images (magnification 10X and 40X) of A unmodified PCC. B , C Chitosan-modified PCC. B Represents chi- tosan dissolved in acetic acid. C Represents chitosan dissolved in HCl
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