Citric Acid Recovery and Methanol Production from a Waste Food Fruit Sample by Thermal Decomposition of a Reusable Zinc Citrate Complex

Citric acid (CA) is an important organic acid that is produced on a large scale by the fermentation process. The CA recovery in fermentation technology requires a large amount of CaSO4 as waste and involves a multi-step complex process. Similarly, the production of methanol by the conventional route requires harsh reaction conditions. Therefore, in the present investigation, zinc metal was extracted as zinc oxide from the used alkaline battery material and subsequently employed for CA recovery and methanol production in a more economical way. The phase formation and surface area of the metal oxides (Fe2O3, MnO2, TiO2, and ZnO) were confirmed by the X-ray diffraction and sorption analysis. In the series of materials used in this study, the ZnO exclusively reacted with CA in the waste fruit sample to produce the zinc citrate complex. The maximum uptake of CA was found to be 68% (933.3 mg/g) on ZnO material after 10 min of contact time, which is much higher uptake in the series of the studied materials. Furthermore, the upgrading strategy was developed to produce methanol by the dry distillation method from the recovered zinc citrate complex. The decomposed gaseous products (CH3OH, CO2, and ethylene) were confirmed by thermogravimetric analysis–mass spectrometry (TGA-MS) probe analysis. At first, a loss in mass of about 23% was seen between 100 and 300 °C on the zinc citrate complex's surface owing to the dehydration reaction of CA-containing hydroxyl groups. The decarboxylation process of the citrate molecule resulted in the second mass loss. During the decarboxylation reaction, the three-carboxylate anion readily breaks down into zinc oxide and CO2 at higher temperatures, as seen by the significant amount of CO2 production. Based on the TGA-MS analysis, we strongly suggest that the ketonization reaction does not occur between the ketones. The proposed green technology enables the use and recycling of electronic and food waste into value-added raw materials for the production of fine chemicals.