Alkaline, Neutral and Acidic Extracts from Rubber Tree ( <i>Hevea brasiliensis</i> ) Seed Shells and Their Application as Biodiesel Antioxidants

Biodiesel is a viable alternative to petroleum diesel, with several advantages, including lower pollutant emissions. However, biodiesel possesses a low oxidation stability due to its composition, which complicates its transport, storage, and consumption. Thus, the addition of antioxidants is mandatory, and most of the antioxidants used are synthetic, creating a need for the development of new natural antioxidant feedstocks. In this context, the rubber tree (Hevea brasiliensis) seed is a coproduct of the rubber tree cultivation, which is widely used for the extraction of latex. These seeds do not currently possess large-scale applications. Therefore, the objective of this study was to produce acidic, alkaline, and neutral medium extracts from rubber tree seed shells to be used as antioxidant additives to soybean oil biodiesel. The extraction processes were optimized via mathematical modeling, the antioxidant activity of the extracts was quantified with the DPPH (2,2-diphenyl-1-picrylhydrazyl) capture method, and total phenolic compounds were quantified via the Folin–Ciocalteau method. Furthermore, the extracts were added to biodiesel, and its oxidation stability was quantified using a Rancimat 873 device. In accordance, the acid medium extract reached 79.664% antioxidant activity and increased the induction time to 16.29 ± 0.14 h when added in combination with 100 ppm of tertiary butylhydroquinone (TBHQ), indicating synergistic effects. Conversely, in conjunction with 100 ppm of TBHQ, the alkaline and neutral extracts reduced the induction time from 5.00 ± 0.10 h (control) to 3.11 ± 0.02 h and 2.31 ± 0.06 h, respectively. Moreover, the biodiesel with acidic extract possessed characteristics in accordance with the Brazilian National Petroleum, Natural Gas, and Biofuels Agency reference values. Accordingly, it was concluded that the rubber tree seed shell acidic medium extract possesses strong synergy with TBHQ, and mixes of the two are viable as antioxidants for soybean-based biodiesel.