## 2 The Technologies
### Adsorption Processes to Remove Hazardous H\({}_{2}\)S from Gas Streams  
Various materials: metal oxides, zeolites, activated carbon, metal organic frameworks (MOFs), biochar, mesoporous silica, ash, and composite materials have been investigated in the removal of H\({}_{2}\)S from gas streams. Selection of these materials is based in some of their properties, such as elevated uptake capacity, selectivity, and thermal and mechanical stability with respect to the removal of this toxic gas [26]. Table 2 shows some of these investigations. A mixture of biosolid (sewage) and surfactant (pluronic surfactant F127, heated at 950 \({}^{\circ}\)C) was used for the adsorption of H\({}_{2}\)S in [27]. It was shown that the presence of the surfactant increased the mesopore volume and carbon content of the biosolid, whereas the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**adsorbent** & **S\({}_{\text{BET}}\), m\({}^{2}\)/g** & **Conditions** & **H\({}_{2}\)S Capacity** & **Reference** \\ \hline Modified biosolid & & & & \\ adsorbent & & & & \\  & & & & \\ MoO\({}_{2}\) nanoparticle & 48–65 & & & & \\  & & & & \\  & & & & \\ Modified zeolite & 333–550 & & & & \\  & & & & \\ DES supported on & & & & \\  & & & & & \\  & & & & \\  & & & & \\  & & & & \\ \end{tabular}
\end{table} 
Table 2: H\({}_{2}\)S removal by adsorption processing. treatment at 950 \({}^{\circ}\)C developed the micropores, leading to the dispersal of the catalytic sites (Ca and Fe oxides) responsible for the gas adsorption, and increased the nitrogen atoms of pyrrolic nature. Besides adsorption, the catalytic sites provide an environment to promote the oxidation of H\({}_{2}\)S to elemental sulfur. As a consequence, the breakthrough capacity is greatly improved (250% better than that presented by the biosolid alone); moreover, this capacity (221.2 mg/g) is better than those derived from other adsorbents such as STIX(North Tonawanda, NY, USA) (201 mg/g), S-208 (36 mg/g), or Centaur(r) (Mumbai, India) (176 mg/g). Metal oxides presented have good properties for adsorbing H\({}_{2}\)S; however, there is a continuous effort to improve their characteristics in this important field. Adsorption of H\({}_{2}\)S by molybdenum(IV) oxide nanoparticles is described in the literature [28]. The best results are achieved at 0.081 and 0.074 g H\({}_{2}\)S/g, at a temperature of 85 \({}^{\circ}\)C, pressure of 16 bar, and superficial velocity of 0.018 m/s. The above capacities are obtained when the adsorbent presents non-spherical and spherical shapes and using an initial H\({}_{2}\)S concentration of 43 ppm in the feed gas stream. MnO\({}_{2}\) is the most effective adsorbent compared with other composite nano-sized metal oxides such as NiO/TiO\({}_{2}\), CoO/TiO\({}_{2}\), graphite oxide/ZnO, and CuO/TiO\({}_{2}\). Zeolites are other group of adsorbent materials of a wide use in several fields; some of the properties of these materials, i.e., elevated porosity and surface areas and porosities, are responsible for the wide use of these materials, including the removal of H\({}_{2}\)S. Y and ZSM-5 zeolites' properties (including surface and pore structure) have been modified by the use of magnetite nanoparticles [29], being the performance, with respect to the H\({}_{2}\)S removal, of both adsorbents at the temperatures in the 100-300 \({}^{\circ}\)C range at a pilot scale. Activated carbons and biochars are other types of adsorbents with active adsorption sites. Jute thread waste (a cellulosic biomass) activated with KOH served to yield a N-S-rich nanoporous carbon [34]. The carbon has a pore volume of up to 1.50 cm\({}^{3}\)/g and a surface area of up to 2580 m\({}^{2}\)/g. Within this material, increasing the pressure increased the H\({}_{2}\)S adsorption capacity by 1 bar (19.1 mmol/g), 10 bar (32.6 mmol/g), and 35 bar (45.0 mmol/g).