## 3 Results and Discussion  
### Characterization of the Adsorbent Biomaterials  
The removal of heavy metals from an aqueous solution in lignocellulosic materials is related to the presence of several functional groups on its surface (hydroxyl, carbonyl, phosphate, amino, thiol). Although their presence does not guarantee an efficient removal of metals, because this process is affected by several factors such as the number of active sites, their accessibility, chemical state, and affinity for the metal, it does depend significantly on the type of compounds and elements present, being cellulose important in lignocellulosic compounds due to its chemical stability and mechanical strength. However, it has limited free hydroxyl groups since most of them are already forming hydrogen bonds [42]. The chemical analysis shown in Table 3 corroborates the high presence of carbons, oxygens, and hydrogens in the biomasses due to the high cellulose content present in the biomasses, and to a lesser degree, lignin, and hemicellulose. It is observed that the highest lignin content occurs in the plantain residues (PSR), so it would be expected that the removal would be higher in this material [43]. Figures 1 and 2 show the IR spectra of plantain and yam starch residues, before and after the adsorption process of Cr(VI) and Ni(II), where the records between wavelengths between 3000 and 3500 cm\({}^{-1}\) of O-H functional groups are initially highlighted, as well as between 3200 and 3500 cm\({}^{-1}\) may appear amines and small signals of an overtone of the carbonyl by stretching vibrations of the O-H bond [44]. The peak at 2927.94 cm\({}^{-1}\) is attributed to possible vibrations of C-H methyl, methylene, and methoxy groups [45], and further on between 2000 and 2500 cm\({}^{-1}\) signals of alkynes and carboxylic acids can be observed as a broad signal by hydrogen bridges with overtone pattern of the carboxylic acids by O-H stretching [46]. Following this, FTIR records peaks near 1650 cm\({}^{-1}\), indicating the presence of cellulose, hemicellulose, and lignin functional groups due to C=O and C-O stretching by the vibrations of carboxyl groups of hemicellulose and lignin [32]; it can also indicate the presence of aromatic rings due to hydrogen vibrations that represent the stretching of C=C. On the other hand, between 1000 and 1200 cm\({}^{-1}\), the presence of primary, secondary and tertiary alcohols is evidenced; the product of the stretching vibrations of the C-OH bond and the bending between the plane (1225-950 cm\({}^{-1}\)) are only complementary signals since the C-C, C-N, and C-O stretches fall in the same region, and several signals appear as a function of the number of hydrogens, which would affect the interactions that administered the adsorption when the adsorbate interacted with the carbonyl groups or surface alkenes of the bio sorbent [47]. \begin{table}
\begin{tabular}{c c c c} \hline
**Parameters (\%)** & **PSR** & **YSR** & **Methods** \\ \hline Lignin & 26.22 & 19.53 & TAPPI T 222 om-83 \\ Cellulose & 41.80 & 39.24 & TAPPI T 203 os-74 \\ Hemicellulose & 15.81 & 27.41 & Difference between holocellose and cellulose \\ \hline \end{tabular}
\end{table} 
Table 3: Bromatological analysis of residual plantain and yam biomasses. The presence of Cr (VI) and Ni (II) metal ions within the surface structure of the biomass is evidenced in Figures 1 and 2, the increase in the width of the spectral bands and the slight decrease in intensity for each of the adsorption processes, the infrared spectroscopy after the adsorption process of Ni (II) being the one that shows a slightly more noticeable decrease in intensity. For both metals, such decrease provided by the variation of the adsorption frequency can be attributed to the binding of Cr (VI) and Ni (II) ions to the different functional groups present in the biomass, as corroborated by the change in the intensity and width of the adsorption peak at 2341 cm\({}^{-1}\), due to the interaction of hydrogen  
Figure 1: IR spectrum of PSR before and after Cr (VI) and Ni (II) removal.