De-acidification of waste cooking oils by adsorption on industrial waste: Kinetic analysis of a green pretreatment for biodiesel production

This study presents an innovative approach to biodiesel production, examining the potential use of various adsorbents to reduce the acidity of waste cooking oils (WCO). The production of biodiesel from WCO via alkaline transesterification is an established process. However, the quality of this raw material can vary based on its origins, uses, and storage conditions. This work utilised two waste oil samples, WCO1 and WCO2, with free fatty acid (FFA) contents of 3.48 % and 1.05 %, respectively, exceeding the current standards’ limits. This could hinder the productivity of the transesterification process, thereby necessitating deacidification. Adsorption is a straightforward and low-cost potential pretreatment method. Bentonite, biochar, zeolites, and industrial waste serve as potential adsorbents. The adsorbents were then evaluated in FFA adsorption trials. Industrial waste demonstrated the best performance, resulting in acidity reductions of 89.7 % and 66.7 % for WCO1 and WCO2, respectively. A more detailed study explored the influence of temperature (25–80 °C) and the oil/adsorbent ratio (2–8) on deacidification performance. The maximum adsorbent capacity was achieved after 6 h at 40 °C for an O/A ratio of 8, resulting in 34.3 mg/g. Finally, a kinetic study was conducted using the experimental data obtained, considering the main kinetic models in linearised and non-linearised forms to optimise the pretreatment process.