## 3 Results and Discussion
### Identification of Functional Groups by FTIR-ATR  
Differentiation of the functional groups present in the biomaterial was then carried out between the pretreatment processes with ethylene glycol and ethanol (Figure 1). Similar bands were found in both pretreatment processes; however, differences could be identified in the transmission peaks of the functional groups. The main functional groups in the material (after pretreatment processes) are described in Table 1.  
The presence of carbonyl and hydroxyl groups confirmed by the bond vibrations observed at 1743 cm\({}^{-1}\) could be associated with carboxyl groups [43]. These functional groups can interact with metal ions by donating electron pairs from oxygen, allowing the formation of stable complexes. Similar results have been previously reported [101]. The strong peak found at 3299 cm\({}^{-1}\) for ethylene glycol pretreatment is related to the hydroxyl groups due to ethylene glycol having two hydroxyl radicals in the molecule. That is why its signal is higher than the material pretreated with ethanol, which has just one hydroxyl group. Thus, the residual solvent could have been present in the bio-adsorbent. The bands appearing between 1500-500 cm\({}^{-1}\) are the products of several types of bond vibrations, which is why it is often difficult to assign their origin to a particular band. In addition, these bond vibrations originate from a strong interaction between neighboring bonds. This complex region is often known as the fingerprint and can vary from one subsample to another [102]. However, characteristic bands of certain families of compounds can be identified. In this case, a high band between 1100 and 1000 cm\({}^{-1}\) is commonly associated with the presence of complex carbohydrates (cellulose) [102], which was observed in the samples from the present study (Figure 1). Figure 1: Infrared spectra (FTIR-ATR) of the biomass pretreated with ethanol (blue) and ethylene glycol (red) for the identification of present functional groups. Previous studies have used avocado seed powder [72; 103; 104] and applied several pretreatment processes to the bio-adsorption material using acids, bases, and salts [73; 100; 105]. However, to the best of our knowledge, pretreatment processes using ethanol or ethylene glycol have not been found. Both ethanol and ethylene glycol are industrially used as solvents. Both also have the same 2-carbon structure linked to one or two hydroxyl groups, respectively. Ethanol has degreasing properties [106], which would allow a partial elimination of the lipid content present on the surface of the biomaterial that hinders its interaction with metal ions. It could also act by dissolving other impurities that may be retained on the surface. On the other hand, although ethylene glycol does not have these degreasing properties, the presence of two hydroxyl ions could allow it to interact with the external molecular branches present in the biomaterial and, in this way, open or release them, preparing them to receive metal ions. Previous studies have used other compounds, such as alkalis and acids, to pretreat samples [43; 73; 76; 100; 105]. However, one purpose of this study was to find alternative reagents with lower toxicities and non-corrosiveness that are more easily accessible, particularly in developing countries. After the pretreatments, changes in the architecture of the material were visually evident (Figure 2). The untreated bio-adsorbent (Figure 2a) shows agglomerations and a low contact surface. In addition, the material's color has an orange tone, probably due to the presence of lipids in the material. In the case of the bio-adsorbent pretreated with ethanol, a change in the architecture is notable (Figure 2b): the bio-adsorbent particles are looser, and their color is less orange. The degreasing ethanol's capacity may allow a change in the structural conformation of the individual particles.