## 3 Results and Discussion
### Ash Analysis
#### 3.5.2 Nitrogen Gas Adsorption-Desorption Measurements  
The isotherms for the CorH-U-A, CorH-A-1% and CorH-A-5% are given in Figure 6 as examples since the other samples show similar behaviour; the corresponding textural properties data along with the different samples are summarized in Table 4. For the treated samples (CorH-A-1% and CorH-A-5%), the isotherms exhibited a common characteristic of type IV(a) isotherm according to the IUPAC classifications [115], with a small knee at \(p/p_{0}=0.05\). They showed an H3 apparent hysteresis loop with closure points around \(p/p_{0}=0.40\), which is often associated with the effects of capillary condensation in non-uniform pore arrangements [116; 117]. In addition, the presence of hysteresis loop in isotherms has been attributed to the presence of larger mesopores in the ash, which can only be accessed through the smaller mesopores [115]. Furthermore, the isotherms followed a similar path at the lower regions, and the amount of nitrogen adsorbed increases when approaching the saturation vapour pressure (\(p/p_{0}=1\)). The absence of an apparent plateau close to \(p/p_{0}=1\) implies the presence of larger meso and macropores, which cannot be fully filled with the nitrogen condensate [47] and/or the presence of interstices between particles. Conversely, the isotherm for the untreated ash sample (CorH-U-A) exhibited a much lower nitrogen adsorption capacity with no visible hysteresis loop, indicating no or much smaller porosity than the acid-leached ash samples. Agglomeration of ash particles has been reported to be accelerated by the presence of alkali metals [118], which were high in CorH-U-A according to the elemental analysis in Figure 4, resulting in diminished porosity [119]. CorH-U-A might contain some microporosity, which might have been formed by possible agglomeration of ash particles [35]. The textural properties, including the specific surface area (BET) and pore volume (NLDFT) from the nitrogen adsorption-desorption isotherms of the untreated and acid leached ashes are given in Table 4.  
Ashes/biogenic silica from cassava and yam biomass fuels exhibited the lowest surface area and pore volume compared to those of the other biomass fuels. According to [PERSON] et al. [120], a chelate reaction occurs between the carboxyl group (-COOH) of the citric acid and the metal impurities in the biomass fuels. Consequently, hydrolysis of the organic matter cleaves the macromolecules (monosaccharides), thereby removing the embedded impurities and opening the pores within the biomass fuels [121]. Table 4 shows that the surface area and pore volume of the ashes (biogenic silica) increase with \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **S\({}_{\text{BET}}\) (m\({}^{2}\)/g)** & **V\({}_{\text{P}}\) (cm\({}^{3}\)/g)** \\ \hline \multicolumn{3}{c}{Untreated} \\ \hline CasP-U-A & 3 & 0 \\ YamP-U-A & 2 & 0 \\ CoCH-U-A & 0 & 0 \\ CorC-U-A & 2 & 0.01 \\ CorH-U-A & 9 & 0.04 \\ \hline \multicolumn{3}{c}{\(1\)\(w/v\) \% CA} \\ \hline CasP-A-1\% & 16 & 0.06 \\ YamP-A-1\% & 7 & 0.16 \\ CoCH-A-1\% & 20 & 0.05 \\ CorC-A-1\% & 47 & 0.09 \\ CorH-A-1\% & 67 & 0.17 \\ \hline \multicolumn{3}{c}{\(5\)\(w/v\) \% CA} \\ \hline CasP-A-5\% & 26 & 0.10 \\ YamP-A-5\% & 5 & 0.02 \\ CoCH-A-5\% & 56 & 0.14 \\ CorC-A-5\% & 70 & 0.14 \\ CorH-A-5\% & 91 & 0.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of the structural characteristics (S\({}_{\text{BET}}\) and pore volume V\({}_{\text{P}}\)) of ashes derived from biomass fuels leached in \(1\)\(w/v\)% CA and \(5\)\(w/v\)% CA at a combustion temperature of 600 \({}^{\circ}\)C and residence time of 2 h.  
Figure 6: Adsorption–desorption isotherms of ashes from untreated and treated comhusk. increasing citric acid concentration for all biomass fuels investigated. For example, the specific area of the CorH-U-A was 9 m\({}^{2}\)/g; however, after being treated with 1 \(w\)/\(v\)% citric acid, it increases to 67 m\({}^{2}\)/g and continued up to 91 m\({}^{2}\)/g at 5 \(w\)/\(v\)% CA concentration.