Research Article
SN Applied Sciences (2020) 2:1577 | https://doi.org/10.1007/s42452-020-03313-w
exhibited in Fig. 4A. There was a significant increment in TOC in the presence of acetic acid as the concentration of chitosan was raised. This initial examination aided us to eliminate the usage of acetic acid since it would be dif- ficult to evaluate the actual concentration of deposited chitosan from the chitosan-acetic acid standard curve. Figure 4B describes the deposition of chitosan onto PCC. Depending on the degree of deacetylation, the intrinsic pKa of chitosan from shrimp is almost close to 6.5 [33]. Hence, the polymer is completely soluble at pH below its pKa value with 90% of its amine group protonated in the glucosamine units [58]. The polymer possesses a cationic behavior in the acidic medium with electrostatic repul- sion between its chains [33]. This is in agreement with our result showing that the charge density of chitosan in HCl (pH 5.5) is + 7.3 meq/g. In our study, the high alkalinity of PCC makes the amine group in chitosan deprotonated. The deprotonation means that at pH greater than neutrality chitosan starts becoming insoluble as discussed in previ- ous sections with much of a coiled entangled structure rather than a stiff rod-like structure resulting in signifi- cantly higher adsorption on PCC [55]. Similar conclusions were drawn for the adsorption of chitosan on quartz at pH 9 [38]. It is evident from the graph in Fig. 4B that the deposition increases with an increasing amount of chi- tosan and the maximum value of the deposited amount is found to be 10.7% to reach the saturation level at an added dose of 11% of PCC. When subjected to alkaline pH 10.4 exerted by PCC in CO 2 free water, the chitosan might have lost its enough cationic charge to neutralize the colloidal anionic charge on PCC [20]. This hypothesis can be related to our charge density measurement data
and CO 2 [32]. In comparison, it is seen that the thermal degradation profile for modified PCCs has changed with two consecutive degradation steps. Besides weight loss due to PCC, an additional weight loss is seen to take place between 210 and 400 °C. This is due to the dehydration of the saccharide ring accompanied by a random chain breakdown favoring complete decomposition of the chi- tosan molecule [55]. There is no evidence of accompany- ing curves of OH ions in the DTG graph (Fig. 3B) which further ensures that water is not present in the samples. Moreover, the mass loss for samples PCC + Chitosan (Acetic acid) and PCC + Chitosan (HCl) is around 47% which is relatively higher than the unmodified one whose mass loss was 44% indicating the existence of sorbed bio modifier on PCC surface [56]. Additionally, it is notice- able that though T onset for calcium carbonate decompo- sition is almost analogous in all the samples. Maximum weight loss temperature T max of modified PCCs deviates from the actual T max of the unmodified one as observed from the DTG plot. T max for sample PCC, PCC + Chitosan (Acetic acid) and PCC + Chitosan (HCl) is 657 °C, 647 °C and 662 °C, respectively, implying that chitosan precipitated from HCl on PCC surface makes the filler more thermal stable compared to acetic acid. It is possible that acetic acid-mediated less thermal stable composites are attrib- uted to the occurrence of chitosonium acetate originated from residual acids between chitosan chains in the dried composites [57]. The sorption mechanism of chitosan on the PCC sur- face was then illustrated through the TOC analysis, and the dissolving medium considered was HCl since organic acid from acetic acid interfered with TOC of chitosan as
ing medium for chitosan. A Macroscopic optical photographs of unmodified and modified filler slurries taken with a digital camera
Fig. 4 A Total organic carbon (TOC) versus chitosan concentration in 0.1% HCl and 1% acetic acid. B Sorption of chitosan onto PCC at ambient temperature for 30 min when HCl was used as dissolv-
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