## 4 Conclusions  
This work systematically examined the potential of producing high-quality amorphous silica from African biomass fuels such as cassava, yam, coconut, corncob, and cornhusk. The results showed that the leaching of the biomass fuels effectively removed a significant fraction of metallic impurities, especially K\({}_{2}\)O and carbon compounds, from the organic matrix. The relative reductions of K\({}_{2}\)O by the citric acid in yam peel was the lowest (79 wt.% db) in comparison to 92, 97, 98 and 97 wt.% db calculated for corncob, cassava peel, coconut husk and cornhusk, respectively. Acid leaching of the biomass in 5 \(w/v\)% citric acid resulted in producing ashes with larger surface areas, pore volume and completely white (no traces of carbon). The colour, surface area and pore volume of the resulting ashes after combustion were highly influenced by the initial composition of the biomass fuels and the concentration of citric acid used in the pre-treatment step. Cornhusk ash obtained the highest surface area (91 m\({}^{2}\)/g) and the highest silica content (>90 wt.% db), which was completely amorphous along with corncob (85 wt.% db SiO\({}_{2}\)). However, crystalline phases such as arcanic (K\({}_{2}\)SO\({}_{4}\)), sylvite (KCl) and calcite (CaCO\({}_{3}\)) were found in the ashes of cassava, yam, and coconut due to the influence of potassium and calcium ions. Consequently, the existing pre-treatment and combustion methods should be optimized and extensively investigated to avoid phase formations. After these modifications, the production of biogenic silica products, which has already been realized in rice husk ash should be possible from cassava, yam, and coconut residues. The results of this study widen the fundamental knowledge for the preparation of value-added materials from primary but less-known African residues opening up new value chains for the recovery of silica from cassava, yam, coconut, corncob, and cornhusk.  
Conceptualization, C.O.P., S.F. and M.N.; methodology, C.O.P.; formal analysis, C.O.P.; investigation, C.O.P.; resources, C.O.P. and S.F.; data curation, C.O.P. and H.B.D.; writing--original draft preparation, C.O.P.; writing--review and editing, C.O.P., S.F., T.S. and H.B.D.; visualization, C.O.P., S.F., H.B.D. and T.S.; supervision, S.F. and M.N.; project administration, S.F. and M.N. All authors have read and agreed to the published version of the manuscript.  
Figure 9: Comparison of the FTIR patterns of CorH-A-1% and CorH-A-5% with commercially available silica (CWK Köstropur® 021012) in the market (purity > 99.9%).  
**Funding:** This research was funded by the Federal Ministry of Food and Agriculture (BMEL) under the Grant Number 2819 DOKA05.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** Special gratitude to the analytical laboratory and technical team members of DBFZ for their invaluable support during this study. Special appreciation to [PERSON] and [PERSON] for their commitments to the analyses of the biomass fuels.  
**Conflicts of Interest:** The authors declare no conflict of interest.