Green Pretreatment of Tropical Fruit Peels Using Triethylammonium Hydrogen Sulfate: A Route Toward Sustainable Biomass Valorization

The search for biofuels has recently intensified because of the urgent need to replace fossil fuels with renewable alternatives. Investigation of biomass, especially waste, presents an excellent option for biofuel production, including second-generation (2G) ethanol, which can be produced from lignocellulosic waste. 2G Ethanol production requires pretreatment and hydrolysis of biomass to break down cellulose, generate higher amounts of sugars, and consequently increase production yields. Ionic liquids (ILs), composed of organic and inorganic ions, have low melting points, low vapor pressures, and the ability to solubilize cellulose, making them effective in breaking down cellulose and thus emerging as an efficient alternative to acid pretreatment. Therefore, this study aimed to evaluate the efficiency of triethylammonium hydrogen sulfate IL in the pretreatment of tropical fruit peel residues such as bananas, oranges, and mangoes. For this purpose, the biomass was characterized through sugar quantification and determination of ash, moisture, extractives, holocellulose, α-cellulose, and hemicellulose content. Two pretreatment processes were conducted for lignocellulosic biomass: one in an oil bath and the other in an oven. Additionally, yield analyses, scanning electron microscopy (SEM), and infrared spectroscopy (IR) were performed on the products obtained from the pretreatments. Based on characterization analyses of the raw materials, mango residue was identified as the biomass with the highest potential for bioethanol production, followed by orange and banana residues owing to its high sugar content, low ash and moisture content, and favorable cellulosic composition. Among the evaluated pretreatments, the oven method showed the best results in weakening the lignin-hemicellulose-cellulose complex and lignin precipitation, also indicating mango residues as being the most promising in terms of cellulose pulp production and lignin removal. This study adds value by demonstrating a low-cost and practical approach to pretreating abundant tropical fruit peel residues using triethylammonium hydrogen sulfate, an accessible and more affordable ionic liquid. Additionally, it provides significant scientific value by addressing two major global challenges: the need for renewable energy alternatives and the growing demand for sustainable waste valorization methods.