## 3 Results and Discussion
### Ash Analysis
Nonetheless, the amounts of some metallic oxide (such as Al\({}_{2}\)O\({}_{3}\)) in 5 \(w\)/\(v\)% CA treated ashes of yam and cassava residues were more than those of 1 \(w\)/\(v\)% citric acid samples. This confirmed that the treatment of yam and cassava biomass fuels with citric acid for silica extraction or removal of Al\({}_{2}\)O\({}_{3}\) might not be suitable at higher citric acid concentrations. Probably, the use of inorganic acids including HCl and HNO\({}_{3}\) or inorganic bases such as NH\({}_{4}\)OH and NaOH as adopted by several researchers ([PERSON] et al. [94]; [PERSON] [95]; [PERSON] et al. [96]) could be used in pre-treating cassava and yam biofuels. [PERSON] et al. [15] reported a 61.5% silica yield from cassava periderm after leaching with HCl. This study found higher silica content in the ashes of the acid-treated biomass fuels compared to the other ash-forming elements such as K\({}^{+}\) ions. The silica contents of all the studied samples under each pre-treatment (1 and 5 \(w\)/\(v\)% citric acid) increased in the following order: CorH-A-5% (90.87 wt.% db) > CorC-A-5% (85.12 wt.% db) > CorH-A-1% (81.45) wt.% db) > CochA-5% (73.97 wt.% db) > CorC-A-1% (66.13 wt.% db) > YamP-A-1% (58.94 wt.% db) > YamP-A-5% (53.47 wt.% db) > CasP-A-1% (42.15 wt.% db) > CasP-A-5% (39.72 wt.% db). The observed variability of silica contents across the ashes of the various biomass fuels might be due to the diverse Si-accumulation mechanism used by plants [40]. The role of silica in plants is to offer some protection against certain biotic and abiotic stressors [41]. As a result, depending on their requirements and the quantity of silica in the soil, various plants uptake varying concentrations of silica particles in the form of silicic acid [Si(OH)\({}_{4}\)] from the soil [97]. [PERSON] et al. [98] reported that the SiO\({}_{2}\) concentrations in plants vary owing to unequal silica deposition in tissues, variations in origin and growth circumstances (soil, humidity, sun exposure), and genotypic variations. On a dry weight basis, the silica in plant shoots can naturally fluctuate between 0.1 and 10% dry weight depending on the plant species [98, 99]. In rice (_O. sativa_), cultivars usually showed a reduced silica concentration than japonica cultivars [98]. [PERSON] et al. [100] found above 7 wt.% db deviation in the silica concentration of various batches of raw rice husk from different regions in Egypt. In addition, 5 wt.% db SiO\({}_{2}\) variations have been reported in the results of rice husk ash in India [101]. According to [PERSON] et al. [102] and [PERSON] et al. [103], silica is less deposited in most _Angiosperms_, _Gymnospermae_, _Filicopsidae_, and _Peterolophyta_. However, some plants,  
Figure 4: Chemical composition of the inorganic fraction (=100 wt.% db) of ashes obtained from the thermal treatment of the untreated and acid-leached samples with citric acid. such as rice (_Oryza sativa_) and sugarcane (_Saccharum oficinarum_), may accumulate as much as 39 mg [104] and 10.2 mg [105] of silica per gram, respectively. Thus, the affinity for silica by the plants may influence the total concentration stored inside their shoots and roots [97], which might explain the differences in silica contents as observed in the case of _Zea mays_ examined in this study. Two different parts of the plant (cob and husk) were examined, explaining the variability in silica contents. For the rest of the examined biomass fuels, the variability in silica contents may be due to differences in origin or genotypic differences, as explained above. The variations of carbon content (not directly determined but indirectly estimated from the LOI values of the ashes) decreased from 7.8, 5.83, 4.67, 53 and 34.47 wt.% db in the untreated ashes of cassava, yam, coconut, corncob, and cornhusk, respectively to undetectable levels in the ashes of the 5 wt.% acid-leached biomass fuels. Comparative Study of Physical Morphology, Textural Properties, and Phase Analysis of Ashes 3.5.1.