## 3 Results
### Production of Nanocellulose from Recycled Paper Sludge (RPS)  
First, the hydrolysis of RPS was examined without a pretreatment. As assumed, signals indicating NC were not obtained, indicating that without a pretreatment, NC is not produced due to the lignin that surrounds the cellulosic content of the biomass and prevents access to it. Figure 10 in Appendix A demonstrates the results after a 30-min ozonation pretreatment (Section 2.4) and then hydrolysis of RPS. Unlike Whatman filter paper, the RPS solution gradually darkened during hydrolysis as the biomass was broken down. In addition, as only partial lignin removal was necessary, traces of lignin remained in the pretreated RPS. Along with RPS being naturally brown, lignin also imparts a dark color [71; 72]. After dilution and centrifugation (Section 2.7), a slightly turbid white supernatant was obtained, indicating NC suspension, similar to samples obtained from the hydrolysis of Whatman filter paper.  
An important hypothesis worth mentioning is that the hydrothermal conversion of biomass derivatives under exothermic reactions triggered by dehydrating agents with acidic properties (e.g., H\({}_{2}\)SO\({}_{4}\)) may lead to the formation of a carbon nanoparticle (CNP). This process is carried out at atmospheric pressure and exhibits a color change toward yellow, red, or brown [73]. Having fluorescence properties, a CNP may interfere with fluorescence signals of different origins. However, obtaining no color change during Whatman paper hydrolysis, yet similar turbid white solutions from both biomass after centrifugation, serves evidence that the dark color originates from RPS breakdown only. Moreover, obtaining no fluorescence signal when testing these solutions alone, strengthens the belief that NC fluorescent detection requires a fluorescence agent. Consequently, CFW significantly increased the fluorescence signals, detecting NC.  
Figure 7 demonstrates the last three data points in the increasing range (15-25 min) of each hydrolysis time and their slope. These specific data points were chosen as they exhibit the most considerable change in the signal. The yellow curve, representing a hydrolysis duration of 4 h, exhibits the largest slope; hence, a 4-h hydrolysis produced the highest NC amount among all other tested durations. To quantify the NC yield of the 4-h sample for its further use as a dye sorbent, the COD test was used; a 10% yield was noted.  
When further comparing the results obtained from Whatman filter paper and RPS, the signals' increase was obtained at the same range (30,000-50,000 RFU). This may indicate similar solution behavior for NC produced from pretreated RPS and a pure source of cellulose (i.e., Whatman filter paper). Although the conditions of the entire procedure were not fully optimized for obtaining maximum NC yield, this initial result constitutes a promising step in the method development process. These results suggest that after a pretreatment (i.e., delignification), RPS demonstrates a similar behavior as a pure cellulosic source, confirming that ozonation exposes the cellulosic content of the biomass.  
Figure 6: Total phenol content released from RPS during its ozonation pretreatment.  
SEM imaging of the obtained NC is presented in Figure 8. Rod-shaped particles were observed with an average diameter and length of 165 nm and 2431 nm, respectively, suggesting cellulose nanofibrils. Understanding NC's morphology is essential for its efficient application; [PERSON] et al. (2015) reported that NC with a high diameter to length ratio is suitable for filters and sorbents [74], similar to the application proposed in the current study.