Polymers 2022 , 14 , 3309
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Figure 6. Median flocs size as a function of flocculation time for several concentrations of the best flocculants. The evolution of the size of the flocs obtained with CPAM, for the various dosages tested, shows a very distinct behavior compared to both CCs. Independently of the dosage of CPAM, the flocculation curves are very similar, with the flocs reaching a size of ca. 31 μ m for a dosage of 1 mg/g of PCC and a size of ca. 43 μ m for the highest dosage of 10 mg/g of PCC. Although the highest dosage ultimately resulted in the largest floc sizes, it is possible to verify a decline in the flocculation rate, at the initial minutes of flocculation (1 to 7 min), compared to the lower dosages. Such behavior can be an indication of an excess of polymer, resulting in slower flocculation kinetics due to the changed diffusion barriers and repulsive forces [55]. With the CCs, the size of the final flocs seems to be very dependent on the initial dosage of polymer. Sample CH0.13_F presented an initial flocculation peak at around the 2-minute mark (similar to CPAM). At a dosage of 1 and 2 mg/g of PCC, the flocculation was slower than with CPAM, whereas, at a dosage of 4 mg/g of PCC or above, the opposite occurred. With an addition of 1 to 4 mg/g of PCC, there was an initial increase in the size of the flocs until the 2-minute point, with the floc sizes stabilizing afterward until the end of the test (7 to 38 μ m). For a higher dosage (8 and 10 mg/g of PCC) the flocs continued to increase in size until the end of the test (ca. 154 and 321 μ m, respectively). Sample GT0.16_F showed the slowest flocculation kinetics among the three samples. While both CPAM and CH0.13_F presented an initial burst in flocculation for all the dosages tested, the sample GT 0.16_F presented a slow and steady increase in the size of the flocs at a lower dosage (1 and 2 mg/g of PCC), with the flocculation kinetics increasing for higher dosages. For the tested dosages of 1 to 10 mg/g of PCC, a final floc size of between 8.5 and 17.5 μ m was obtained. In the literature, CPAM of high Mw and low to medium CD have been utilized to promote flocculation via the bridging mechanism (which is typically associated with a fast flocculation rate) [32]. Thus, due to the low CD and long fibrils, bridging is probably the dominant mech- anism for sample CH0.13_F. The existence of residual carboxyl groups (ionized at pH 9) in the cellulose fibers’ structure, and their possible attractive interaction with quaternary ammonium groups (and also with the slightly positively charged PCC), may favor the entanglement of the long fibrils between adjacent flocs; this trend may explain the extreme size of the flocs obtained for 8 and 10 mg/g of PCC. On the other hand, sample GT0.16_F presented slower flocculation kinetics. This reduction in the average size of the flocs, comparatively to CH0.13_F, is a result of the higher level of degradation of the fibrils (shorter fibrils) and the partial solubilization of the sample (SF of 40%). Observing the various flocculation curves, it is possible to verify that not all the systems were able to achieve a steady state. Nevertheless, since the focus of the work is to explore
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