Barrios et al. Biotechnology for Biofuels and Bioproducts
(2025) 18:48
Page 12 of 23
indexes than untreated fibers. For instance, the mean kink index decreased from 1.57 1/mm in the control sample to 1.48 1/mm and 1.43 1/mm in the 0.5 wt.% and 1.0 wt.% enzyme-treated samples, respectively. Similarly, these samples reduced the curl index from 0.081 to 0.077. The reduction in kink and curl indexes is associated with increased fiber flexibility, enhancing the bonding potential between fibers during paper formation and improving paper strength and quality [69]. Increased flexibility, as indicated by lower kink and curl indexes, is desirable as it allows for better fiber-to-fiber contact during the papermaking process, which is critical for developing strong paper products [70]. A significant reduction in fines content was observed for the enzymatically treated samples with increasing addition of cationic starch. For the 0.5 wt.% enzyme- treated condition, the fines content was reduced from 9.93% in the control sample to 3.56%, representing a reduction of approximately 64%. This significant decrease in fines content can be attributed to the coagulating and flocculating effects of the cationic starch, which facilitates the agglomeration of fine particles within the pulp suspension [71]. Reducing fines is beneficial, since it improves drainage during papermaking, leading to more efficient water removal and faster production times. In addition, the fines content analysis of the white water collected after drainage during standard handsheet formation (data not shown) confirmed that no loss of fines occurred during the drainage process, suggesting that all fines were effectively retained within the paper handsheet, further supporting the efficacy of the cationic starch in fines retention. Electrostatic properties Maximum dewatering rates are closely associated with the neutralization of fiber surface charges, which is thought to be more influential than neutralizing charges buried within the cell wall or under layers of fibrillation. The charges on fiber surfaces are crucial, because they dominate the colloidal interactions between fibers, cellulosic fines, and water molecules. This assumption holds if dewatering effects are indeed dominated by colloidal forces between individual fibrils on fibers and fines [72]. Effective dewatering, therefore, can be optimized by targeting the surface charge neutralization of fibers. A reliable technique for determining the charge of a fiber suspension is measuring the cationic/anionic demand using a streaming potential device. This volu- metric method is based on the reaction between posi- tively and negatively charged polyelectrolytes. The start potential obtained from this instrument represents the
Table 7 Effect of cell-free enzyme pretreatment on the charge of fiber surfaces Enzyme dose (%) Cationic starch dose (%) Start potential (mV) Cationic demand (μeq/L)
Refined to 1000 revs 0 0
− 1649
7 3 1 7 6 3 7 6 3
0.5
− 687 − 317 − 977 − 672 − 362 − 826 − 690 − 513
1 0
0.5
0.5
1 0
1
0.5
1
initial voltage or potential applied to the system before titration begins, and it varies depending on the experi- mental setup and the nature of the sample. In this study, the start potential for fiber suspensions after cell-free enzyme pretreatment is detailed in Table 7. All start potential values for the suspensions, without adding cati- onic starch, were negative and became less negative with increasing dosage of cationic polymer. The charge analy- sis revealed that cell-free enzyme pretreatments signifi- cantly altered the surface charge of the fibers. Refining and enzymatic pretreatment increased the negative surface charge of the fibers. The enzyme cocktail, rich in xylanases, likely reduced the presence of hydrophilic xylan groups on the fibers, contributing to the increased negative charge. As negatively charged polyelectrolytes, these fibers interact with cationic starch, a positively charged polyelectrolyte. This interaction likely reduces the affinity between water molecules and hydrophilic hydroxyl groups on the fiber surfaces, enhancing dewatering rates. In addition, the cationic starch promotes fiber bonding and fines retention, further increasing paper strength. The results showed that close to the neutralization point of the fiber suspension, where the fiber surface is close to saturation with cationic starch, the best conditions for dewatering (lowest EMC) and paper strength (highest tensile strength) were achieved. All treated systems exhibited a cationic demand, as shown in Table 7, indicating that polyDADMAC was used as the titrant for neutralization. Previous studies have shown that polyDADMAC, with a molecular mass higher than 100 kg/mol, adsorbs primarily on the external surface of cellulosic fibers without penetrating the micropores of the cell wall [73]. Given that the polyDADMAC used in this study has a molecular mass between 200 and 350 kg/mol, the
Made with FlippingBook. PDF to flipbook with ease