## 1 Introduction  
Biodiesel, which contains fatty acid alkyl esters and generally exists in the form of fatty acid methyl esters (FAME), is an attractive alternative fuel due to its superior properties including biodegradability, renewability, and low toxicity. Biodiesel production and consumption have rapidly expanded in recent years because of its positive environmental effects [1, 2]. The most common method for obtaining biodiesel is the methanolysis of highly refined oil in the presence of homogeneous alkaline catalysts such as NaOH, KOH, CH\({}_{3}\)ONa, and CH\({}_{3}\)OK [3, 4]. These homogeneous-basic catalysts, on the other hand, cannot be recovered or regenerated after the reaction and should be neutralized with acid or removed with a substantial volume of hot water, raising overall biodiesel production costs and causing significant environmental damage. Compared with homogeneous-based catalysts, heterogeneous-based catalysts are easier to be separated from liquid products, have improved selectivity, and can be regenerated and reused, thus decreasing the challenges associated with their disposal and minimizing process costs [5]. In certain cases, heterogeneous catalysis allows for a switch in reactivity, enabling the development of new and more beneficial synthetic methods [6]. Therefore, it is imperative to develop promising methods for biodiesel production utilizing heterogeneous base catalysts. Salts or hydroxides of alkali metals (as active precursors) are generally more active than acids in transesterification [7], and frequently imported into suitable support materials such as zirconium dioxide [8], magnesia [9; 10], silica [11], and zeolites [12; 13] for the preparation of heterogeneous base catalysts. However, these catalysts require costly supports or high-temperature thermal treatments, which incur higher catalyst production costs and offset the economic and environmental advantages of biodiesel production. Therefore, the application of these heterogeneous-based catalysts in realistic large-scale biodiesel production is still limited. Fortunately, clay-based or modified clay-based catalysts have attracted the attention of researchers looking for low-cost raw materials. Some clay catalysts such as NaOH-activation kaolin clay [14], KOH-treated bentonite, and KF-modified smectite clay [15] have been reported for biodiesel production on the laboratory scale via the transesterification of vegetable oil in excess methanol. However, they also have some inherent shortcomings such as the requirement of high-temperature thermal treatment and the use of the toxic precursor (KF). Based on the above discussions, sufficient research focused on the preparation of an efficient and affordable catalyst for biodiesel production is very necessary. Herein, we report the synthesis of an economical and eco-friendly acid-activated clay (AC) catalyst by treating the local clay with a waste acid recycling method, and the as-prepared AC catalyst was further applied in biodiesel production under ambient reaction conditions. This can effectively and ingeniously maximize the resource utilization of waste and low-cost clay. Due to the main component of natural clay being montmorillonite and quartz contained in local clay, AC possessed the characteristics of acid-activated montmorillonite, such as high specific surface area, surface acidity, and mesoporous structure [16; 17; 18]. Therefore, AC has been widely reported in various catalytic reactions [19; 20; 21] and as a catalyst support [22; 23]. Notably, the information about solid base catalysts derived from AC in the production of biodiesel is scarce. In this study, AC was successively modified by calcium hydroxide and sodium hydroxide to prepare a novel Ca- and Na-modified AC-based clay (CaNa/AC) catalyst and then applied in biodiesel production from Jatropha oil. Na-modified AC (Na/AC) as the reference catalysts was also prepared for comparison.