Introduction

In recent decades, humanity has been concerned with the continued instability of the international oil market. In addition, there has been increasing concern about environmental issues, and this involves the need for sustainable alternatives for the production of clean and renewable energy. Among these technological alternatives, biofuels are studied worldwide, and from this perspective, biodiesel plays a key role in replacing petrodiesel because it is biodegradable and fully renewable if it is made with bioethanol (Kucek et al., 2007; Aydin, 214).
Biodiesel is chemically defined as alkyl monoesters of long chain fatty acids derived from renewable raw materials, such as vegetable oils, animal fats and recycled cooking oil. The most well-known process is a chemical reaction in which the triacylglycerides (TAGs) found in these oily materials (e.g., soybean oil) combine with an alcohol (methanol or ethanol) in the presence of an alkali catalyst (usually sodium hydroxide, alkoxides) to produce alkyl (biodiesel) monoesters and glycerin. As a co-product, glycerol has little or no fuel value, but its various industrial applications are critical to supporting process economics (Kucek et al., 2007; Ferrari et al., 205; Lôbo et al., 2009).
However, in the transesterification process some undesirable by-products are generated which must be removed, such as unreacted tri, di and monoacylglycerols; methanol; catalyst; soap; glycerin and water (Faccini et al., 2011). The purification of biodiesel can occur in two ways: dry or wet. The wet method requires water or solvent for the removal of the by-products generated but produces large volumes of effluents due to the need to use a lot of water in the process. However, purification with dry adsorbents is an alternative method, as it uses chemical adsorbents that act to remove by-products without generating liquid effluent (Gomide, 1988).
In the last few decades, new natural and synthetic adsorbents have been studied to purify oils of different origins as well as frying oil (oil containing higher acidity and other contaminants that influence biodiesel production) (Alves et al., 2016). Among the various adsorbents used for the purification of biodiesel are some silicates and mixtures of silicates with magnesium and aluminum oxides as well as various silicates formed by the fusion of lime, magnesium and aluminum oxides with diatomaceous earth (Faccini et al., 2011; Araujo et al., 2010; Turan and Yalcuk, 2013).
Magnesol® is the most widely used commercial adsorbent in the biodiesel purification process. This salt is the amorphous form of hydrated magnesium silicate, MgO.nSiO2.xH2O, on the surface of which are active sites that adsorb the compounds based on their dielectric constant and acidic and basic properties (Alves et al., 2016). In biodiesel purification, it acts by removing free or even bound molecules of glycerol, soap, potassium, sulfur, residual methanol and traces of the catalyst. There is also evidence that the use of Magnesol® increases the stability of biodiesel in the oxidation process. The disadvantage of this is the high cost of the product (Sundus et al., 2017).
In the literature, there are few studies on the reuse of different types of adsorbents, thus this work intends to demonstrate the reusability of Magnesol® and, consequently, the reduction of this adsorbent as residue. For this, biodeisels from frying soybean oil (possibility of using frying oil for energy purposes and reduction of environmental impacts) and virgin were used to verify the efficacy in both cases, as well as the cultivar soybeans because soy is responsible for more than 90% of biodiesel production in Brazil and the USA.