**4. Conclusions**

Olive tree pruning biomass (OTPB) was identified as a lignocellulosic source for the production of cellulose nanofibers from cellulosic pulps obtained by a sustainable pulping process. The cellulose nanofibers were produced by two different pretreatments, mechanical and TEMPO-mediated oxidation, followed by high-pressure homogenization. The influence of the residual lignin content on the effectiveness of the different pretreatments was analyzed by the thorough characterization of the cellulose nanofibers produced. All the cellulose nanofibers produced in this work were in the nanometric range; however, important differences were observed. TEMPO-mediated oxidation is more effective with bleached pulp; however, mechanical pretreatment was favored by the presence of lignin. The presence of lignin results in cellulose nanofibers with low crystallinity indices following mechanical pretreatment and TEMPO-mediated oxidation (24.69% and 39.13%) in comparison with those produced from bleached nanofibers (39.13% and 48.99%). The thermal stability of the cellulose nanofibers produced by mechanical treatment shows similar values regardless of the presence of lignin (343 ◦C); however, in the TEMPO-mediated oxidation, the lignin content produces a greater thermal stability (325 ◦C) in comparison with that obtained with bleached nanofibers (298 ◦C). The characteristics of the cellulose nanofibers obtained are of great interest for their application in different sectors.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4395/10/5/696/s1, Figure S1: Cellulose pulp images.

**Author Contributions:** The article was conceptualized by A.R. and F.P.-R. The experimental work was realized by M.S.-G., I.B.-V. and E.C. The article was written by E.E. The article was reviewer by A.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors are grateful to Spain's DGICyT, MICINN for funding this research within the framework of the Project CTQ2016-78729-R.

**Acknowledgments:** The authors wish to acknowledge the support of the Central Service for Research Support (SCAI) at the university of Córdoba.

**Conflicts of Interest:** The authors declare no conflict of interest
