## 5 Conclusions  
The material characterization showed that the materials have similar specific surface areas between 3.97 and 4.85 m\({}^{2}\)g\({}^{-1}\) except pine wood with 1.74 m\({}^{2}\)g\({}^{-1}\) that had higher pore size of 26.54 nm compared to the remaining materials (5.40-7.33 nm). The diffractograms analysis showed that all the lignocellulosic materials presented some crystalline domains except for burnt pine wood, which was completely amorphous. All materials showed similar lignocellulosic characteristic FTIR bands with the main difference observed in the burnt pine wood spectrum that had a lack of some functional groups compared to the other biosorbents. The best pH to remove nickel was found to be around 5.0. At this pH the adsorption was higher for chestnut shells, walnut shells, burnt pine wood and pine wood, respectively. The Langmuir model fitted the sorption process best. This means that the sorption mostly takes place at specific homogeneous sites of the biosorbent. Nevertheless, since Freundlich isotherm also gives good results, we cannot discard the possibility of adsorption happening in heterogeneous sites. The kinetics was well described by PFO and PSO, but pseudo-second order presented higher R\({}^{2}\) to all biosorbents tested. The Elovich and intraparticle diffusion models presented R\({}^{2}\) greater than 0.90, which means that the sorption process may occur in more than one way. The Elovich equation is known to describe chemisorption well; therefore, the occurrence of chemical reactions between the surface and the adsorbate cannot be discharged. The intraparticle diffusion model may not be involved in the whole adsorption process but might explain some part of nickel adsorption onto lignocellulosic materials. Finally, the results show that walnut shell, chestnut shell, pine wood and burnt pine wood can be applied as adsorbent materials and have the capability to remove nickel from aqueous solutions. The following are available online at [[https://www.mdpi.com/article/10.3390/app12020933/s1](https://www.mdpi.com/article/10.3390/app12020933/s1)]([https://www.mdpi.com/article/10.3390/app12020933/s1](https://www.mdpi.com/article/10.3390/app12020933/s1)), Figure S1: Powder X-ray diffraction of all the materials, Figure S2: Powder X-ray diffraction of wood before and after reaction, Figure S3: Powder X-ray diffraction of chestnut shell before and after reaction, Figure S4: Powder X-ray diffraction of walnut shell before and after reaction, Figure S5: SEM images from wood (a) before, (b) after 10 min of reaction and (c) after 25 min of reaction, Figure S6: SEM images from chestnut shell (a) before, (b) after 10 min of reaction and (c) after 25 min of reaction, Figure S7: SEM images from walnut shell (a) before, (b) after 10 min of reaction and (c) after 25 min of reaction, Figure S8: SEM images from burnt wood (a) before, (b) after 10 min of reaction and (c) after 25 min of reaction. Conceptualization by [PERSON]; investigation by [PERSON]; formal analysis by [PERSON] and [PERSON], helped by [PERSON]; writing, review and editing were performed by all authors; and funding acquisition by [PERSON] and [PERSON]. All authors have read and agreed to the published version of the manuscript. National Funds through the FCT--Foundation for Science and Technology, I.P., within the scope of the project Ref\({}^{\prime}\) UIDB/00681/2020. I.C.M.S.S.-V. (Ref. 197_97_ARH-2018) was financed by the National Funds (OE), through FCT, I.P., under the framework contract provided for in paragraphs 4, 5 and 6 of Article 23 of Decree-Law 57/2016 of August 29, as changed by Law No. 57/2017 of July 19. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable. **Acknowledgments:** Furthermore, we would like to thank the CERNAS Research Centre and the Polytechnic Institute of Viseu for their support. **Conflicts of Interest:** The authors declare no conflict of interest.