## 3 Results  
### Materials Characterization  
The BET analysis (Table 1) shows the specific surface area of the different materials which is similar for walnut shell, chestnut shell and burnt pine wood, between 3.97 and 4.85 m\({}^{2}\)g\({}^{-1}\). For these three materials the pore size varies between 5.40 and 7.33 nm. For pine wood the specific surface area and the pore size are completely different, 1.74 m\({}^{2}\)g\({}^{-1}\) and 26.54 nm, respectively. The XRD patterns (Figure 1 and Supplementary Figures S1-S4) do not show sharp diffraction patterns. Two broad diffraction bands at ca. 16\({}^{\circ}\) and 22\({}^{\circ}\) 20 are observed and assigned to cellulose [30]. The diffractograms witness some polycrystalline domain, but the bands are quite broad. In the case of the burnt pine wood, it is completely amorphous (Figure S1). The results obtained from FTIR-ATR analysis are presented in the Figures 2-5, for walnut shell, chestnut shell, pine wood and burnt pine wood, respectively. Similar spectra are found for walnut shell and chestnut shell. Burnt pine wood presented a much different spectrum from the other materials. The FTIR spectra of walnut shells, chestnut shells and pine wood exhibit similar bands. The band at around 3430 cm\({}^{-1}\) corresponds to O-H stretching vibration which is present either in polysaccharides and lignin. The main differences are that for pine wood the maximum is around 3330 cm\({}^{-1}\) and for walnut and chestnut shells it is around 3320 cm\({}^{-1}\). The two bands at 2900-2800 cm\({}^{-1}\) are composed by the overlapping of the stretch asymmetric vibrations of -CH\({}_{2}\)- (generally around 2935-2915 cm\({}^{-1}\)) and -CH\({}_{3}\) (2970-2950 cm\({}^{-1}\)) and by the overlapping of stretch symmetric vibrations of -CH\({}_{2}\)- (2865-2845 cm\({}^{-1}\)) and -CH\({}_{3}\) (2880-2860 cm\({}^{-1}\)) [31]. Generally, the asymmetric band presents higher absorptivity which in this case is only true for walnut shells and is somewhat true for pine wood. The region between 1750 and 1700 cm\({}^{-1}\) is due to C=O linkage that exhibits strong absorption which can be at smaller or higher wavelength according to the functional group [32]. Walnut shells presented the highest absorption at these wavelengths. The band at around 1595 cm\({}^{-1}\) has been attributed to vibrations in the aromatic ring of lignin plus C=O stretching. Aromatic rings are known to exhibit a band at approximately 1500 cm\({}^{-1}\), corresponding to benzene ring stretching vibrations. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Materials** & **BET Surface Area (m\({}^{2}\)g\({}^{-1}\))** & **Pore Volume (cm\({}^{3}\) g\({}^{-1}\))** & **Pore Size (nm)** \\ \hline Walnut shell & 4.855 & \(1.195\times 10^{-3}\) & 5.41 \\ Chestnut shell & 4.210 & \(4.202\times 10^{-3}\) & 7.33 \\ Pine wood & 1.738 & \(4.920\times 10^{-4}\) & 26.54 \\ Burnt pine wood & 3.968 & \(3.388\times 10^{-3}\) & 5.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: BET analysis of the several materials. Figure 1: Powder X-ray diffraction of the different materials. This band was higher in the pine wood spectrum. The peak at around 1025-1035 cm\({}^{-1}\) assigned to C-O-C deformation mostly present in carbohydrates was higher in the chestnut shells spectrum. The walnut shell spectrum after nickel adsorption was similar to the initial spectrum with the exception of the two bands at 2900-2800 cm\({}^{-1}\) which decrease slightly, and the maximum seems to shift from asymmetric to symmetric vibrations and the peak at around 1730 cm\({}^{-1}\) also decreases. The chestnut shell spectrum after nickel adsorption was very similar to the initial spectrum except for the region between 900 and 1400 cm\({}^{-1}\) where there seems to be a decrease in the 1144 cm\({}^{-1}\) and 1010 cm\({}^{-1}\) peak heights.