## 2 The Technologies
### Adsorption Processes to Remove Hazardous H\({}_{2}\)S from Gas Streams
Again, the presence of pyrrolic-N atoms in the material helps with H\({}_{2}\)S uptake onto it, this being attributable to hydrogen bonds; at the same time, negatively charged sulfur atoms around the pyrrolic-N atoms are responsible for a physisorption process. This material presented an 84% of adsorption regeneration after five cycles. Using a peanut shell as precursor, impregnated with copper and activated by KOH, an activated carbon was formed and used to investigate the removal of H\({}_{2}\)S [34]. The best sample of the activated carbon has 1523.2 m\({}^{2}\)/g of surface area, with 97.6 mg/g of gas uptake. The non-linear Langmuir isotherm model best fits the experimental results:  
\[\left[\text{H}_{2}\text{S}\right]_{\text{e,a}}=\frac{\left[\text{H}_{2}\text{ S}\right]_{\text{m,a}}\text{K}_{\text{L}}\left[\text{H}_{2}\text{S}\right]_{ \text{e}}}{1+\text{K}_{\text{L}}\left[\text{H}_{2}\text{S}\right]_{\text{e}}} \tag{1}\]  
where [H\({}_{2}\)S]\({}_{\text{e,a}}\) is the gas uptake at an elapsed time, [H\({}_{2}\)S]\({}_{\text{m,a}}\) is the maximum monolayer uptake capacity, K\({}_{\text{L}}\) is Langmuir constant, which is related to the affinity, and [H\({}_{2}\)S]\({}_{\text{e}}\) is the gas uptake at the equilibrium. The kinetic displayed pseudo-first-order trend:  
\[\left[\text{H}_{2}\text{S}\right]_{\text{t}}=\left[\text{H}_{2}\text{S}\right] _{\text{e}}(1-\exp(\text{k}_{\text{t}}\text{t})) \tag{2}\]  
In the above equation, [H\({}_{2}\)S]\({}_{\text{t}}\) is the uptake at an elapsed time, [H\({}_{2}\)S]\({}_{\text{e}}\) is the gas uptake at the equilibrium, and k\({}_{\text{t}}\) is the rate constant of the pseudo-first-order model. Both film diffusion and intraparticle models are responsible for H\({}_{2}\)S uptake onto the adsorbent. After five cycles, the adsorbent maintains a gas removal rate of 90%. Biochar, biomass ashes, and sewage sludge (incinerated) are used to compare their performance on H\({}_{2}\)S from a biogas [35]. Sewage sludge is discarded due to its surface characteristics and low porosity. Biochar exhibited the highest adsorption capacity of 171.8 mg/g, its low density translated into low volumetric adsorption capacity, limiting its scale-up. Biomass ashes have the highest volumetric adsorption uptakes, 22.2-38.3 mg/cm(35.6-78.2 mg/g), attributable to their porosity properties and the presence of mineral oxides responsible for the catalytic H\({}_{2}\)S oxidation. Biochar has the highest adsorption uptake (171.8 mg/g), but its use at a greater scale is limited by its low density. It is known that activated carbons incorporating metal or metal oxides have enhanced their H\({}_{2}\)S removal efficiencies [36]. Several investigations [37; 38] have surveyed the life-cycle cost and the environmental footprints/impact of these hybrid materials. Biochars derived from banana peel, rice hull, and sawdust by pyrolysis are used as adsorbents of H\({}_{2}\)S from municipal solid waste [39]. The order for removal efficiency of H\({}_{2}\)S (>94%) was banana peel > saw dust > rice hull. The use of a goethite-based material to adsorb H\({}_{2}\)S from a mimic biogas mixture (H\({}_{2}\)S and N\({}_{2}\)) was described in [40]. In the process, crystalline FeS was formed. With respect to pristine s-C3N6 (\(-0.33\) eV to \(-0.45\) eV), Fe-, Pt-, and Ti-modified s-C3N6 structures (\(-1.06\) eV to \(-2.66\) eV) presented larger adsorption energies for the removal of H\({}_{2}\)S on their surfaces [41]. s-C3N6-Fe has semiconducting properties after adsorbing this harmful gas. Pristine s-C3N6 and its modified derivatives represented a series of smart adsorbents and sensing media for toxic gases. The defective GaSe monolayer material having selenium vacancies and being doped with oxygen and nitrogen is another material to be considered in the removal of toxic gases from gaseous streams [42]. After gas adsorption, the distance between the defective GaSe monolayer and four toxic molecules is reduced.