# Removal of Metals and Dyes in Water Using Low-Cost Agro-Industrial Waste Materials
## 3 Results and Discussion  
### Characterization of the Biosorbent  
Figure 2A shows the SEM analysis of the unmodified wheat bran biochar (WBC), and Figure 2B shows the KOH-modified biochar (MWBC). Figure 2A presents a rather rough laminar morphology with small lines that may be characteristic of the different elements found in a lignocellulosic material such as wheat; among them, Mg, K, P, S, N, O, and C can be found as the most predominant in the coal structure thanks to the thermal process to which they were subjected (Table 1). Regarding Figure 2B, the effects of basic hydrolysis are observed when the biochar is impregnated with KOH. The surface evidences tubular-type cavities characteristic of cellulose-based materials; also, its porous structure denotes a greater surface area and traits that can be used in adsorption processes [17; 33]. In accordance with the energy-dispersive X-ray spectroscopy, the elements with the highest presence in the structure of the MWBC are carbon (59.87% w), oxygen (24.89% w), and potassium (15.25% w), confirming the successful modification process with KOH [17]. Considering the physical properties and the elemental content shown in Table 2, the modified biochar changes after alkali treatment. In particular, the KOH-modified wheat biochar shows a porous structure that denotes a higher specific surface area. This fact is also related to the fact that the basic hydrolysis effect helps to eliminate the inorganic matter present in the biochar during the activation process, which is reflected in the reduction of elements such as sulfur (S), phosphorus (P), nitrogen (N), and magnesium (Mg) [17]. Figure 3 presents the results of the determination of pH\({}_{\text{PZC}}\) for MWBC. This study is meaningful since the electric charges of sorbent materials will depend on the pH of the solution, which relates to the ionization of polar functional groups on the surface of the sorbent [34; 35]. The MWBC exhibits a pH\({}_{\text{PZC}}\) of 9.25, which implies that at this value concerning the conditions of the environment, the surface load density of the adsorbent would be zero [36]. Solutions with pH lower than pHZC will enhance the adsorption of anionic species by electrostatic interaction, and when pH > pHPZC, it will favor the adsorption of cationic compounds by covalent interactions [37]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{Cr (VI) isotherms} & \multicolumn{2}{c}{MB concentration, mg/L} & \multicolumn{3}{c}{100} \\ \cline{2-7}  & Cr (VI) concentration, mg/L & 3 & 7 & 10 & 13 & 16 \\ \hline \multirow{2}{*}{MB isotherms} & Cr (VI) concentration, mg/L & & & 100 & & \\ \cline{2-7}  & MB concentration, mg/L & 20 & 40 & 60 & 80 & 100 \\ \hline \end{tabular}
\end{table} 
Table 1: Experimental conditions for multicomponent assays. Figure 3: pH point of zero charge for WMBC. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Component** & **Unmodified Biochar (Wt.\%)** & **Biochar Modified with KOH (Wt.\%)** \\ \hline C & 64.11 & 59.87 \\ \hline O & 19.48 & 24.89 \\ \hline S & 0.71 & - \\ \hline K & 3.46 & 15.25 \\ \hline P & 1.01 & - \\ \hline N & 10.99 & - \\ \hline Mg & 0.24 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: EDS analysis of coals prepared from wheat residues. Figure 2: SEM-EDS analysis of the prepared biochar. (**A**) WBC and (**B**) MWBC. From the FTIR analysis of MWBC before the adsorption process (Figure 4a), a peak is observed at 3122.36 cm\({}^{-1}\) caused by the asymmetric stretching of the -NH\({}_{3}\) group. The deformation of the -NH group present in secondary amides is observed at 1556.85 cm\({}^{-1}\). A strong asymmetric stretching is noticed in 1360.16 cm\({}^{-1}\) due to the -COO- present in carboxylic acid salts [38]. The FTIR of the MWBC after MB adsorption (Figure 4b) shows an absorbance peak in 3183.30 cm\({}^{-1}\) because of the strong symmetric stretching -NH\({}_{2}\) and primary amides.