Nanomaterials 2023 , 13 , 2536
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3. Results and Discussion 3.1. Pretreatment Effects on Fiber Morphology and Carboxyl Content of the Pulp
Table 4 shows the effect of the different pretreatments on the chemical composition of the pulp. The washing stage followed the enzymatic pretreatments to remove part of the ashes, which explains the reduction in ash content. This reduction in ashes could not be detected by Sanchez-Salvador et al. (2022) because of the low ash content of the virgin pulps [27]. Park et al. (2022) observed that ash in recycled paper has a high affinity for cellulase enzymes and attached to them, reducing enzyme efficiency [43]. Thus, ash attached to the enzyme is removed during pulp washing. In the case of TEMPO pretreat- ments, the increase in ash content is attributed to the production of NaCl as byproduct during the oxidation process and the addition of NaBr, both of which are not completely removed during washing [44]. Part of the extractives could be degraded or solubilized by the enzymes, as shown in Table 4. Consequently, this led to an increase in the percentage of cellulose. However, excessive amounts of enzymes or NaClO, particularly at a high TEMPO-mediated oxidation degree (TEMPO_15), could result in the excessive degradation of cellulose chains, leading to the formation of soluble oligosaccharides or sugars, which were subsequently removed during the washing stage. Hemicellulose seems to remain intact after the pretreatment process. Figure 1 shows the carboxyl content of the pretreated pulps. The type of pretreatment and its severity determine the final quantity of carboxyl groups. Mechanical and enzymatic pretreatments do not contribute to the generation of carboxyl groups. In contrast, the number of carboxyl groups generated through TEMPO-mediated oxidation increases with the dosage of the oxidant, although not to the same extent as the NaClO dosage. This discrepancy arises because a portion of the NaClO is consumed in the oxidation of other components found in recovered papers, such as dissolved lignin and colloidal material [45]. Therefore, only a fraction of the NaClO is available to oxidize hydroxyl groups to carboxyl groups. Furthermore, there is a saturation phenomenon; when the dosage of NaClO surpasses a threshold, the overdosed NaClO remains in the medium and causes secondary reactions instead of effectively oxidizing cellulose.
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Figure 1. Carboxylic content of the recycled fibers after each pretreatment.
Table 5 and Figure 2 present the morphological parameters of the fibers and the optical images of the pretreated fibers produced by different pretreatments. Table 5 shows fibers and fines which are detectable by the MorFi analyzer, whose detection limit is around 5 μ m (for length).
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