## 4 Discussion  
The main advantage of using waste lignocellulosic materials like the ones tested here is that they do not need to be reused since they are very cheap materials. Any process to reuse these materials would be far more expensive than using new ones. The specific surface area determined by BET (Table 1) for the different materials was very similar for walnut shell, chestnut shell and burnt pine wood, being around 4 m\({}^{2}\)g\({}^{-1}\), higher than the 1.74 m\({}^{2}\)g\({}^{-1}\) found in pine wood. The partial burning of pine wood increased the specific surface area to more than double, which showed some effect on the adsorption phenomenon. The specific area of burnt pine wood is similar to that of biochars prepared from rice straw at 400 \({}^{\circ}\)C (4.4 m\({}^{2}\)g\({}^{-1}\)) but lower than the ones obtained at 700 \({}^{\circ}\)C (116.2 m\({}^{2}\)g\({}^{-1}\)) [38]. The specific area of 3.97 m\({}^{2}\)g\({}^{-1}\) was obtained for waste-apricot-activated carbon prepared by chemical activation at lower temperature (400 \({}^{\circ}\)C) which is similar to the materials tested here. Of course, when comparing the same material chemically activated at 900 \({}^{\circ}\)C with specific area of 1214 m\({}^{2}\)g\({}^{-1}\) the values are much lower [39]. The lower adsorption area (Table 1) of pine wood might be one of the main reasons for the lower adsorption observed for pine wood in Figure 6 compared to the remaining materials such as walnut and chestnut shells. On the other hand, the pore size varied between 5.40 and 7.33 nm for the remaining materials and was higher at 26.54 nm for pine wood. Higher-size micropores generally lead to a transition towards a surface or multilayer adsorption mechanism [40,41]. The physical adsorption mechanism in small micropores is mainly pore filling because the overlapping of pore wall potentials results in stronger binding of the adsorbate, or enhanced adsorption. The diffractograms witness some polycrystalline domain for most of the materials with quite broad bands. In the case of the burnt pine wood, it was completely amorphous (Figure S1). The metal adsorption on lignocellulosic materials is mainly due to the high diversity and number of functional groups. FTIR-ATR tests were made to characterize the initial materials' functional groups and to determine possible changes in the spectra after nickel adsorption. All lignocellulosic materials showed high diversity of functional groups except for burnt pine wood which might justify the lower adsorption presented in Figure 7 by burnt pine wood. This functional group diversity allows, in accordance with [PERSON] et al. [21], positively charged heavy metal ions to bind with these groups, under suitable pH conditions which have been found in Figure 6 via charge interaction, to be adsorbed onto the biosorbent particles. In accordance with [PERSON] et al. [42], many functional groups were affected by the adsorption of nickel on _Bacillus Interosporus_. The reported mechanisms were ion exchange, adsorption and complexation. The best adsorption rates with pH (Figure 6) were obtained by chestnut shell with adsorption around 75%, followed by walnut shell attaining an adsorption of 71%, pine wood and burnt pine wood that presented ca. 44% and ca. 64%, respectively. Although walnut shell had the highest surface area, at pH 5 the adsorption is slightly higher for chestnut shell. Nevertheless, the difference is not significative, and adsorption is not only dependent on the specific surface area but also on the pore size and pore volume and on the chemical composition of the material. The higher pore volume and pore size obtained for chestnut shell in relation to walnut shell might explain this difference. Similarly to other studies, optimal conditions were demonstrated to be attained at pH 5.0, for example for activated carbon prepared from almond husk [43]. Nevertheless, pH 6.0 was found to be the optimal pH to remove nickel by rice husk ash [9].