## 2 The Technologies  
### Absorption Processes to Remove Hazardous H\({}_{2}\)S from Gas Streams  
This technology uses both chemical and physical solvents and is one of the most preferred to remove H\({}_{2}\)S from these gas streams. The use of chemical solvents mitigates the presence of H\({}_{2}\)S in the corresponding phase. Among these chemicals, alkanolamine solutions know a wide usefulness because, together with toxic H\({}_{2}\)S, the solvent removes CO\({}_{2}\)[6]. One step ahead the use of conventional solvents, ionic liquids are considered as their reliable alternatives. The well-known singularities or properties that these chemicals make are gaining positions on H\({}_{2}\)S removal [7]. The use of mixtures (AAILs) of tertiary amines and ionic liquids (amino acid) to remove H\({}_{2}\)S was investigated in [8]. Among these compounds, tetramethylammonium arginine and tetramethylammonium glycine, [N111][Arg] and [N1111][Gly], respectively, presented H\({}_{2}\)S removal rates of 100%. The rate of gas removal is increased when transferring protons between the ionic liquid and the tertiary amine. Larger-scale applications of the method seemed to be unpractical due to the costs and complex synthesis of the ionic liquids. In term of costs, deep eutectic solvents (DESs) are an option to ionic liquids. Interaction between H\({}_{2}\)S and DES (basic) does not imply chemical reaction. Functionalizing these deep eutectic solvents with some chemicals, including amines, and oxidizing reagents produced an increase in the absorptive and regenerative properties of the compounds [9]. The addition of polyethyleneimine to deep eutectic solvents was investigated [10], the mixture having 90% H\({}_{2}\)S removal efficiencies after four consecutive absorption-regeneration cycles. Nanofluids also emerged as potential absorbents for H\({}_{2}\)S. These absorbents are formed by dispersion of several inorganic compounds (SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\)) in the form of nanoparticles or graphene oxide and carbon nanotubes into organic solvents (monoethanolamine, diethanolamine, etc.) [11]. Removal of H\({}_{2}\)S via the use of Fe-Monoethanolamine-BmimCl solution with inert nanoparticles (SiO\({}_{2}\)) presented good perspectives, though H\({}_{2}\)S removal efficiency decreased after continuous use (cycles) [12]. Table 1 presents some references about the use of various absorbents on H\({}_{2}\)S removal. Some of the difficulties presented by conventional solvents appeared to be resolved with the use of these ionic liquids and deep eutectic solvents; however, some of the properties of these compounds, i.e., high viscosity, can be considered a drawback since it reduces mass transfer coefficients and thus causes an increase in energy demand [18]. The use of mixtures of these compounds can improve the removal of the toxic gas; however, the greatest negative impact of these mixtures is the lack of maintaining the removal properties after continuous use [19]. A new perspective has been raised in terms of improving the regeneration step to avoid this being lost in the absorption properties. Various triethylenetramine functionalized ionic liquids (TETAH-ILs) are mixed with ethylene glycol (EG) to investigate their performance on H\({}_{2}\)S absorption [20]. The results showed that the mixture 10% [TETAH][BF4]-EG presented the best absorption capacities (1.128 mol H\({}_{2}\)S/mol IL) at 30 \({}^{\circ}\)C and 100 mL/min. Removal of H\({}_{2}\)S is attributed to the formation of H\({}_{2}\)S-IL compounds via hydrogen bonds. A bubble absorption column is used to investigate hydrodynamics of CO\({}_{2}\) and H\({}_{2}\)S removal from pure water and water containing nanofluids dispersed with neat and OH- and NH\({}_{2}\)-functionalized multiwalled carbon nanotubes [21]. Sodium dodecyl sulfate is used as surfactant and stabilizer. Maximum CO\({}_{2}\) removal and H\({}_{2}\)S removal are found to be 0.0038 mmol/m\({}^{2}\)-s and 0.056 mmol/m\({}^{2}\)-s using NH\({}_{2}\)-MWCNTs/nanofluid, respectively.