*3.2. Adsorption*

As mentioned, glycerol can be converted into a material that has promising properties for application as adsorbent materials. Their adsorbent capacity was examined to different adsorbates such as medicines (flumequine, tetracycline and paracetamol) [54,65], aqueous phase chromium Cr(VI), dyes (methylene blue and indigo carmine), VOCs (toluene and hexane), and ethene, ethylene. The adsorption studies (Figure 6) in aqueous solutions are different from the gas adsorption, which normally requires special equipment based on gravimetric or volumetric methods. The schematic representation of a volumetric apparatus is shown in Figure 7a, for the ethene/ethylene separation, and in Figure 7b, for H2S adsorption.

**Figure 6.** Representation of the adsorption studies.

**Figure 7.** Volumetric system used for ethane/ethylene separation (**a**) and H2S adsorption (**b**) studies. Reproduced with permission from [66], Wyley, 2022.

As already mentioned, to the best of our knowledge, the work published in 2016 by Álvarez-Torrellas et al. was the first to study the application of glycerol-based activated carbons as adsorbent materials [55]. The authors focused on the preparation of 3 activated carbon (GBCM200, GBSM300 and GBCM350) and their application as adsorbent materials for the removal of the antibiotic compounds (flumequine and tetracycline) from aqueous solution. The adsorption of flumequine was found to be dependent on the textural properties of the glycerol-based activated carbon materials. The maximum adsorption capacity (41.5 mg·g<sup>−</sup>1) was verified onto sample GBCM350. The sequence of flumequine adsorption capacity in the glycerol-activated carbon series was the following: GBCM350 > GBCM300 > GBCM200 with adsorption capacities of 41.5 mg·g<sup>−</sup>1, 33.7 mg·g<sup>−</sup><sup>1</sup> and 0.9 mg·g<sup>−</sup>1, respectively. For tetracycline this sequence was GBCM350 > GBCM200 > GBCM300 (58.1 mg·g<sup>−</sup>1, 53.9 mg·g<sup>−</sup><sup>1</sup> and 51.3 mg·g<sup>−</sup>1, respectively). The activated carbons showed a higher adsorption capacity for tetracycline and its adsorption was almost the same for all three activated carbons, showing that the adsorption of this antibiotic was not dependent on the structural differences obtained at the different activation temperatures used. Additionally, no relation between the antibiotic structure and activated carbon properties was found.

Cui et al. [54] investigated the obtained activated carbon from liquid glycerol as adsorbent for the removal of gas phase volatile organic compounds (VOCs) and aqueous phase chromium Cr(VI). The adsorption capacities reported for toluene, hexane, and Cr(VI) were 1.5 <sup>g</sup>·g<sup>−</sup>1, 1.1 <sup>g</sup>·g<sup>−</sup>1, and 56 mg·g<sup>−</sup>1, respectively. The adsorption of the compound

in aqueous solutions was much lower than the VOCs, probably due to the competing adsorption of water, making the comparison among them difficult.

Naverkar et al. [58] examined the adsorption of methylene blue by glycerol-based carbons (GBC-120 and GBC-350). The samples GBC-120 and GBC-350 presented BET surface areas of 21 <sup>m</sup>2·g<sup>−</sup><sup>1</sup> and 464 <sup>m</sup>2·g<sup>−</sup>1, respectively. The sample (GBC-120) exhibited maximum methylene blue adsorption of 1050 mg·g<sup>−</sup>1. According to the authors, the higher equilibrium adsorption of 1050 mg<sup>−</sup><sup>1</sup> on GBC-120 was attributed to the presence of a large amount of –SO3H groups compared with GBC-350, where several surface functionalities were lost upon thermal treatment.

Gonçalves et al. [53] also investigated the adsorption of organic contaminants from water: a dye (methylene blue) and a drug (paracetamol) on glycerol-based carbons. The activated carbons from glycerol were also tested as capacitor materials (described in Section 3.3). More recently, glycerol-based magnetic carbon composites were synthesized by Medeiros et al. [59]. The composites (GFe3-600 and GFe3-800) were tested as adsorbents of dyes (methylene blue and indigo carmine). The sample GFe3-800 showed a higher adsorption capacity than GFe3-600 for methylene blue, adsorbed up to 82% and 62% in 60 min, respectively.

In 2021, Batista et al. [52] prepared a series of glycerin-activated carbons from crude glycerin (82% glycerol) for gas separation by adsorption processes. The glycerin-activated carbons were evaluated as adsorbents for the adsorption of ethane and ethylene. All the adsorbents were shown to be ethane selective. The materials exhibited a higher adsorption capacity of ethane (8.92–14.81 mmol·g<sup>−</sup>1) than ethylene (8.27–12.63 mmol·g<sup>−</sup>1). The glycerin-activated carbons (except for the sample G@700/3) after two regeneration cycles presented ~100% of the adsorption capacity. In addition, in another work, M. Batista et al. [59] used the glycerin-based activated carbon (Gta@600) and its chitosan-based carbon (Gta@600Chi) as H2S adsorbents. The chitosan-based carbon (Gta@600Chi) presented a H2S insignificant release due to its chemical adsorption. However, the Gta@600 adsorbed a significant amount of H2S and it could be investigated for other applications such as natural gas purification.

The results already available in the literature clearly show activated carbons obtained from glycerol can adsorb compounds with different structures and properties (Table 2), and therefore indicate the importance to extend the research to the adsorption of other class of chemicals. Most of the studies may have applications in environmental problems. However, as was shown they may also be used for separation processes. Nevertheless, and despite their potential, more studies should be conducted, namely regeneration studies. At this moment no commercial products exist, and their development is dependent on more research to be possible obtain high effective products at low production cost. In Table 2 is presented the adsorption data on glycerol-based carbons.
