Recent Advances in H₂S Removal from Gas Streams

Abstract

Hydrogen sulfide is a toxic and corrosive gas; thus, in order to mitigate its environmental impact, its capture and removal from various emitting sources, natural and anthropogenic, is of a necessity. In this work, recent advances (2020–2022) proposed by a series of investigations are reviewed. Adsorption using metal-oxide-based adsorbents appeared to be the most popular technology, whereas solvent absorption is used to co-absorb both toxic H₂S and CO₂. The uses of the various membrane technologies for H₂S removal are also described.

1. Introduction

Hydrogen sulfide is a chemical in a close relationship with mankind, though this relationship is not connected only to its benefits but to the high grade of toxicity that it presents against humans and the environment. Hydrogen sulfide can be emitted both by natural and anthropogenic activities. This gas occurs naturally in the oceans, as well as other locations such as other bodies of water, volcanoes, etc., and gas and petroleum deposits.

Anthropogenic causes produce H₂S via releasing naturally occurring H₂S from petroleum and natural gas processing; other causes which release this toxic gas include localizations of municipal wastes, biogas manufacturing, and sewerage. Though important, the generation of H₂S is not a major part of the total emission of natural sulfur gas emissions, which relapsed in dimethyl sulfide and SO₂. Other anthropogenic causes of H₂S generation include wastewater processing, which yields undesirable products such as activated sludge. This waste is organic matter, and from this an important part is protein. Both reductive bacteria and methane bacteria are also responsible for the generation of H₂S [1]. The unpleasant odor of the sludge is caused from the above compounds that originated in the sewage sludge [2]. Pigs and cattle farms also release ammonia and organic sulfur into the environment; moreover, the immunity of some of these animals is achieved due to the inclusion of methionine into the animals’ regular feed. However, this releases undesirable excretions [3,4]. Natural gas is replaced in the steel industry by the use of coke oven gas [5]. Due to the presence of sulfur in the coal, H₂S is formed, and this species is undesirable since it promotes corrosion and formation of the also undesirable SOx volatile species. Kraft mills also contribute to the generation of H₂S and other sulfur-based gases.

From the above, it is obvious that there is a need to capture and treat both anthropogenic and naturally occurring H₂S when necessary and due to the toxic characteristics of this gas. The present manuscript describes recent advances (2020–2022) in the technologies used for H₂S removal from various sources. Moreover, due to its practical importance, some investigations related to the separation CO₂/H₂S are described.

2. The Technologies

2.1. Absorption Processes to Remove Hazardous H₂S from Gas Streams

This technology uses both chemical and physical solvents and is one of the most preferred to remove H₂S from these gas streams. The use of chemical solvents mitigates the presence of H₂S in the corresponding phase.

Among these chemicals, alkanolamine solutions know a wide usefulness because, together with toxic H₂S, the solvent removes CO₂ [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 H₂S removal [7]. The use of mixtures (AAILs) of tertiary amines and ionic liquids (amino acid) to remove H₂S was investigated in [8]. Among these compounds, tetramethylammonium arginine and tetramethylammonium glycine, [N111][Arg] and [N1111][Gly], respectively, presented H₂S 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 H₂S 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% H₂S removal efficiencies after four consecutive absorption–regeneration cycles.

Nanofluids also emerged as potential absorbents for H₂S. These absorbents are formed by dispersion of several inorganic compounds (SiO₂, Al₂O₃) in the form of nanoparticles or graphene oxide and carbon nanotubes into organic solvents (monoethanolamine, diethanolamine, etc.) [11]. Removal of H₂S via the use of Fe-Monoethanolamine-BmimCl solution with inert nanoparticles (SiO₂) presented good perspectives, though H₂S removal efficiency decreased after continuous use (cycles) [12].

Table 1. Removal by absorption technologies of H₂S and other gases.

Solvent	Gas Stream	Contactor	Temperature	% Capture	Reference
NaCl	H2S and NH3 in N2	Column	25 °C	Near 99	[13]
Fe-EDTA and others	H2S and CO2 in CH4	Column	Ambient	100	[14]
ChCl and others	H2S in N2	Bubbled stirrer reactor	30–70 °C	100	[10]
MDEA	H2S and CO2 in N2	Rotating packed bed	30–45 °C	Near 100	[15]
Modified lye	H2S in N2	reactor	No data	98	[16]
Yellow phosphorous and phosphate rock	H2S in N2	Bubble reactor	55–80 °C	88	[17]

presents some references about the use of various absorbents on H₂S 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 triethylenetetramine functionalized ionic liquids (TETAH-ILs) are mixed with ethylene glycol (EG) to investigate their performance on H₂S absorption [20]. The results showed that the mixture 10% [TETAH][BF4]-EG presented the best absorption capacities (1.128 mol H₂S/mol IL) at 30 °C and 100 mL/min. Removal of H₂S is attributed to the formation of H₂S-IL compounds via hydrogen bonds.

A bubble absorption column is used to investigate hydrodynamics of CO₂ and H₂S removal from pure water and water containing nanofluids dispersed with neat and OH- and NH₂-functionalized multiwalled carbon nanotubes [21]. Sodium dodecyl sulfate is used as surfactant and stabilizer. Maximum CO₂ removal and H₂S removal are found to be 0.0038 mmol/m²·s and 0.056 mmol/m²·s using NH₂-MWCNTs/nanofluid, respectively.

The improvement in the absorption properties of diethanolamine with respect to the removal of H₂S from sour industrial off gas was investigated in [22]. The influence of lean amine H₂S impurity (LAHI), lean amine temperature (LAT), and column pressure (CP) on H₂S removal was studied. The increase of LAHI and LAT is detrimental for H₂S removal from the gas stream and LAT, LAHI (83%) being the key factor on H₂S removal, whereas LAT (15%) and CP (2%) have a minor impact on this efficiency.

The next investigation used machine-learning operations to study the solubility of H₂S in fifteen ionic liquids [23]. No less than six machine-learning operations were used, and the respective results were compared. The conclusions showed that the least-squares support vector machine predicted H₂S solubility into the ionic liquids well with R² (0.99798), RMSE (0.01079), MSE (0.00012), RRSE (6.35%), RAE (4.35%), MAE (0.0060), and AARD (4.03). It was found that H₂S solubility decreased with temperature and has a direct dependence with the pressure. Ionic liquids such as [OMIM][Tf2N] are the best choice for H₂S capture.

The next investigation [24] explored various aspects of the processing and technologies used in acid gas removal (AGR). The work summarized processing by chemical absorption and mechanisms involved in the removal process; it also showed the main amine-based solvents currently used in such tasks. Absorption by physical methods is also discussed, summarizing pros and cons of the most used absorbents. Industrial applications of AGR processes were considered.

The removal of H₂S from industrial gas streams using an iron/copper bimetallic catalytic oxidation desulfurization system was investigated in [25]. The absorbent is formed by adding N-methyl pyrrolidone (NMP) and CuCl₂ aqueous solution to an iron-based ionic liquid (Fe-IL). The acidity and viscosity of the system are greatly reduced by addition of NMP and water, improving gas–liquid mass transfer efficiency. The presence of the copper(II) salt increased the oxidative properties of the solution, allowing for the improvement of the Fe³⁺ catalytic influence on the oxidation of H₂S to the monomer form of sulfur.

2.2. Adsorption Processes to Remove Hazardous H₂S 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 H₂S 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. H₂S removal by adsorption processing.

Adsorbent	SBET, m2/g	Conditions	H2S Capacity	Reference
Modified biosolid adsorbent	110–180	[H2S] = 1000 ppm	89–221 mg/g	[27]
MoO2 nanoparticle	48–65	[H2S] = 38–73 ppm T = 65–89 °C	0.033–0.081 g/g	[28]
Modified zeolite	333–550	[H2S] = 30–120 ppm T = 100–300 °C	14.7–70 mg/g	[29]
DES supported on fumed silica	124–133	[H2S] = 800 ppm T = 20–60 °C	3.9–14 mg/g	[30]
Carbon adsorbents	229–3217	[H2S] = 20 ppm	6.3–25.7 mmol/g	[31]
Jute-derived nanoporous carbons	1065–2580	T = 25 °CT	6.5–50 mmol/g	[32]
Cu-modified activated carbon	981–1769	[H2S] = 0.46 mg/L T = 25–110 °C	22.4–76 mg/g	[33]

shows some of these investigations.

A mixture of biosolid (sewage) and surfactant (pluronic surfactant F127, heated at 950 °C) was used for the adsorption of H₂S in [27]. It was shown that the presence of the surfactant increased the mesopore volume and carbon content of the biosolid, whereas the treatment at 950 °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 pyrrolitic nature. Besides adsorption, the catalytic sites provide an environment to promote the oxidation of H₂S 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^® (Mumbai, India) (176 mg/g).

Metal oxides presented have good properties for adsorbing H₂S; however, there is a continuous effort to improve their characteristics in this important field. Adsorption of H₂S by molybdenum(IV) oxide nanoparticles is described in the literature [28]. The best results are achieved at 0.081 and 0.074 g H₂S/g, at a temperature of 85 °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 H₂S concentration of 43 ppm in the feed gas stream. MnO₂ is the most effective adsorbent compared with other composite nano-sized metal oxides such as NiO/TiO_2, CoO/TiO₂, graphite oxide/ZnO, and CuO/TiO₂.

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 H₂S. 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 H₂S removal, of both adsorbents at the temperatures in the 100–300 °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 cm³/g and a surface area of up to 2580 m²/g. Within this material, increasing the pressure increased the H₂S adsorption capacity by 1 bar (19.1 mmol/g), 10 bar (32.6 mmol/g), and 35 bar (45.0 mmol/g). Again, the presence of pyrrolic-N atoms in the material helps with H₂S 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 H₂S [34]. The best sample of the activated carbon has 1523.2 m²/g of surface area, with 97.6 mg/g of gas uptake. The non-linear Langmuir isotherm model best fits the experimental results:

$${\left[{\mathrm{H}}_2\mathrm{S}\right]}_{\mathrm{e},\mathrm{a}}=\frac{{\left[{\mathrm{H}}_2\mathrm{S}\right]}_{\mathrm{m},\mathrm{a}}{\mathrm{K}}_{\mathrm{L}}{\left[{\mathrm{H}}_2\mathrm{S}\right]}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\left[{\mathrm{H}}_2\mathrm{S}\right]}_{\mathrm{e}}}$$

(1)

where [H₂S]_e,a is the gas uptake at an elapsed time, [H₂S]_m,a is the maximum monolayer uptake capacity, K_L is Langmuir constant, which is related to the affinity, and [H₂S]_e is the gas uptake at the equilibrium. The kinetic displayed pseudo-first-order trend:

$${\left[{\mathrm{H}}_2\mathrm{S}\right]}_{\mathrm{t}}={\left[{\mathrm{H}}_2\mathrm{S}\right]}_{\mathrm{e}}\left(1-\exp \left({\mathrm{k}}_1\mathrm{t}\right)\right)$$

(2)

In the above equation, [H₂S]_t is the uptake at an elapsed time, [H₂S]_e is the gas uptake at the equilibrium, and k₁ is the rate constant of the pseudo-first-order model. Both film diffusion and intraparticle models are responsible for H₂S 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 H₂S 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/cm³ (35.6–78.2 mg/g), attributable to their porosity properties and the presence of mineral oxides responsible for the catalytic H₂S 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 H₂S 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 H₂S from municipal solid waste [39]. The order for removal efficiency of H₂S (>94%) was banana peel > saw dust > rice hull.

The use of a goethite-based material to adsorb H₂S from a mimic biogas mixture (H₂S and N₂) 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 H₂S 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.

An activated carbon-fiber mat containing both metal chlorides and metal oxides is used to investigate H₂S (and ammonia) removal [43]. The adsorbent, containing NiCl₂ and MgO, has a hierarchical porous structure, large surface area, and a good dispersion of bimetallic active sites. Metal oxides promote the chemical adsorption of H₂S. The use of these ternary hybrid materials increases gas uptake with breakthrough capacities up to 209.2 mg/g.

Adsorbent materials from banana empty fruit bunch biochar (BEFBB) and banana peel biochar (BPB) wastes are used to remove H₂S from biogas [44]. Both types of adsorbents have good characteristics to remove H₂S (low performance in the case of CO₂ and methane), with breakthrough capacities of 7.65 mg/g and 5.85 mg/g, respectively. Pellet-sized adsorbent (0.5 cm) has a better gas removal efficiency than larger pellets (1.5 cm), this being attributable to its larger surface area. Both hydroxide and carboxylic groups present in the BEFBB material are responsible for the removal of H₂S from the gas stream.

Spent Fenton-like reagents are another type of material to be considered as potential materials to remove H₂S. Rice straw biochar and a spent Fenton-like reagent are used to prepare an adsorbent to investigate the removal of toxic H₂S [45]. With the inclusion of the Fenton reagent, the number of active sites on the biochar is increased, and the adsorbent specific surface area is decreased, the maximum gas uptake being up to 1000.6 mg/g at 120 °C. The presence of copper oxide in the adsorbent is responsible for the gas adsorption, whereas cuprous sulfide, copper sulfide, and elemental sulfur appeared in the adsorbent after hydrogen sulfide removal.

The use of copper and biochar produced a low cost H₂S adsorbent (Cu2.56WRS300), which is used in the adsorption of H₂S [46]. Again, the presence of copper leads to the formation of copper oxide sites on the surface of the hybrid material, and these active sites increase H₂S uptake onto the adsorbent, with a gas uptake of 1191.1 mg/g. Chemisorption is the key step in the removal of the hazardous gas from the stream. The hybrid adsorbent presented convenient regeneration.

2.3. Membranes and Membrane Contactors to Remove Hazardous H₂S from Gas Streams

The membranes as a separation technology are widely used in a variety of industries, and this wide usefulness is attributable to the properties or characteristics of the technology: modular aspects, easiness of operation, low environmental impact, etc., that in many aspects surpassed the offer presented by other separation technologies, this being specially noted when the operation, i.e., H₂S removal, is accomplished in areas or locations where communications are not easy [47].

There are some recent publications reviewing the use of these membrane technologies on the treatment of gases [48,49,50,51], but surprisingly, they are dedicated to industrial gases instead of H₂S; some of the gases mentioned in these reviews are: nitrogen, hydrogen, oxygen, CO₂, etc. In the case of H₂S, polymeric membranes are the candidates to resolve this important industrial issue. The permeation of gas across these types of membranes is ruled by diffusion, and a series of investigations [52,53] have mentioned that plasticization of the membrane performs well in the removal of H₂S when the gas in the stream is present at high concentrations; moreover, this plasticization improves the selective separation of H₂S over methane. This selective separation is attributed to the transport mechanism, which in the case of H₂S is of sorptive in nature, and this separation is negatively influenced by diffusion.

The transport ruled by solubility is particularly important in the case of rubbery polymers, and this is a key rule in the separation of H₂S from methane. It is reported [54] that the variation of the cross-linking density tuned the usefulness of polyethylene glycol-based membranes. This change in the membrane properties significantly improves the selectivity in the pairs H₂S/methane and CO₂/methane.

Table 3. H₂S removal by membrane technologies.

Membrane	Feed Gas	Working Conditions	H2S Capture	Reference
Crosslinked poly(ethylene glycol membrane	5% H2S	T = 25 °C P = 800 psi	0.08–25 barrier	[54]
Vinyl-poly(norborene) membrane	5–20% H2S	T = 25 °C P = 800 psi	Depending on feed gas composition	[55]
Cellulose triacetate HFM	20 mol% H2S	T = 35–50 °C P = 6.9–31.9 bar	140 GPU	[52]
Dense polymer membrane	0.5–20% H2S	T = 35 °C P = 7–46 bar	Depending on membrane type	[53]
Copolymide membranes	20% H2S	T = 22 °C P = 24–46 bar	Depending on membrane type	[56]

summarizes some of the recent literature about the use of membranes on H₂S removal.

Computational methods have developed a series of tools to improve the knowledge and optimize the use of these membrane technologies in the capture of gases [57]. Another implication of membrane technologies on the removal of gases is their use to act as an interface between the different streams, liquid and gases, feeding the removal process. These membranes, preferably with high porosity, increased the gas flow through the membrane structure. In practice, hollow fiber modules are the most used devices; under operation, the gas flowing in the shell side diffuses across the membrane’s pores to the tube side; here, the gas is conveniently absorbed by a solution, whereas these modules can be operated in co-current and counter-current form, and the latter is usually preferred since it gives a better contact between both operating phases. This counter-current configuration is shown in Figure 1.

In these membrane modules, the gas flux is controlled by its concentration gradient [58]. Mass transfer in the module is conveniently represented in Figure 2. Some parameters to be controlled in these hollow fiber operations are pore size and its distribution [59], and these assured the best transfer across the module fibers. Some improvements in the technology are described in the literature [60].

The use of an ionomer material (PFSA) as a membrane material in the removal of H₂S from a gas stream is described in [61]. Experimental work has considered a series of variables in the investigation, including humidity, temperature, and pressure. It has been demonstrated that the ionomer material has a great dependence on temperature and water activity and is an excellent material to separate gases from mixtures of them, i.e., H₂S is conveniently separate from CO₂ and CH₄ from a ternary gas mixture.

Hollow fiber membrane bioreactors (HFMBs) are used to investigate the removal of H₂S at various bed residence times [62] and other operational variables: pH, biomass types, etc. H₂S removal is higher when biotic bioreactors are used, sulfate being the principal species encountered in the reactor after the end of the operation. Data about other operational results are included in the reference.

A biogas containing ammonia and H₂S is cleaned in a hollow fiber membrane bioreactor [63]. Under different operational conditions, both gases are eliminated (up to 99%) from the biogas, H₂S having a greater critical loading rate than ammonia. After operation, different sulfur-bearing species (SO and sulfate) together with nitrogen are found in the bioreactor. This device is filled by several biomasses, including Sulfuricurvum sp., Smithella sp., Sulfuritales sp., Rhodanobacter sp., and Thiobacillus sp.

The removal of toxic H₂S gas has also been investigated using a lignin carbon-based membrane [64]. From this material, and after carbonization, an alkali lignocarbon is derived, and from this several hybrid materials are also derived using casting methodology (

Table 4. Different alkali lignocarbon-based materials used in the removal of H₂S.

Raw Material	Plasticizers	Dopants	Acronym
Alkali lignocarbon and polivynil alcohol	Glycerol and water	nano-CuO and Cu2+	CLA/PVA CuO-CLA/PVA-1 Cu-CLA/PVA-2

).

Results indicated that the inclusion of copper species into the raw material clearly improves the deodorization properties of these Cu-bearing composites if compared with those of the pristine raw material. It was also demonstrated that in the copper-doped hybrid materials, nano-CuO is the key active site in the H₂S removal from the gas stream.

3. Simultaneous and Selective H₂S–CO₂ Removal: A Case Study

In practice, it is normal that both H₂S and CO₂ appeared together in gas streams; thus, the removal of both from the stream can be accomplished by two different approaches: (i) selective H₂S-CO₂ removal and (ii) simultaneous CO₂ and H₂S removal. Some proposals about the two styles of processing are summarized in

Table 5. Technologies for the selective H₂S-CO₂ removal.

Technology	Characteristics	Industrial Use	Reference
Physical absorption	Dimethyl ether of polyethylene glycol	Yes	[65]
	N-methyl-2-pyrrolidene	Yes
Ionic liquid absorption	Ionic liquids	No	[66]
Hybrid membrane	Supported liquid membranes	No	[67]
Adsorption on pristine materials	Iron-oxide-based materials	Yes	[26,68]
	Molecular sieves-based materials	Yes
Adsorption on composite materials	Metal oxide/silica	No	[26,68]
	Metal oxide/activated carbon	No

and Table 6.

Since both gases have similar affinities and characteristics, the simultaneous removal of both species may have a series of benefits, since this removal can be carried out using known technologies: absorption, adsorption, and membranes.

It is said in [69] that the removal of H₂S has a higher cost than that of CO₂, costs which are increased by the operating and capital expenses in relation with what is used in the removal process. Thus, the removal of H₂S and CO₂ is possible by the use of known carbon capture and storage technologies; also of consideration is the fact that CO₂ and H₂S competed in the transport, which on one hand points to the influence of CO₂ with respect to H₂S removal and, on the contrary, H₂S towards CO₂ removal [26]. The above implies that environmental laws were not fulfilled in relation to tolerable limits, which at the end resulted in an increase of the operational costs.

Some other investigations about the selective H₂S/CO₂ removal are summarized next. As is mentioned above, ionic liquids are a series of chemicals proposed to remove H₂S from gas streams. Some of these chemicals are investigated to separate CO₂ and H₂S from methane and water [70]. Density functional theory investigations demonstrated the selective separation of H₂S from CO₂. This gas bonds to the nitrogen of the anion of the ionic liquid and forms a new C[sbnd]N bond, and H₂S transfers a proton to one of the nitrogen atoms of the ionic liquid; thus, HS^- is formed. Other simulations show that methane has weaker interactions with this type of chemical than CO₂ and H₂S.

The selective separation of H₂S from an acid gas mixture using an acid gas enrichment unit (AGEU) was investigated in [71]. The operation uses absorption with NaOH solutions; however, the addition of Na₂CO₃ and NaHCO₃ to the alkali solution avoids CO₂ absorption and thus favored the selective separation of H₂S. Different operational results are included in the work.

H₂S is selectively separated using a column filled with 2-tertiarybutylamino-2-ethoxyethanol [72], and the results are compared to those derived by the use of methyldiethanolamine as an absorbent. In fact, with the latter, the experimental selective factor was found to be 3.88, whereas in the case of the former, the value increased to 15.81. Other relevant simultaneous H₂S/CO₂ removal investigations are resumed below.

Table 6. Technologies for the simultaneous H₂S-CO₂ removal.

Technology	Characteristics	Industrial Use	Reference
Ionic liquid absorption	Azole-based protic ionic liquids	No	[66]
Inorganic membranes	Ceramic-based	No	[73]
	Carbon molecular sieve	No
Hybrid membrane	Metal organic framework-polyimide mixed	No	[67]
Adsorption on pristine materials	Molecular-sieve bases materials	Yes	[26,68]
Adsorption on composite materials	Metal oxide/silica	No	[26,68]
	Metal oxide/activated carbon	No

Table 6. Technologies for the simultaneous H₂S-CO₂ removal.

Technology	Characteristics	Industrial Use	Reference
Ionic liquid absorption	Azole-based protic ionic liquids	No	[66]
Inorganic membranes	Ceramic-based	No	[73]
	Carbon molecular sieve	No
Hybrid membrane	Metal organic framework-polyimide mixed	No	[67]
Adsorption on pristine materials	Molecular-sieve bases materials	Yes	[26,68]
Adsorption on composite materials	Metal oxide/silica	No	[26,68]
	Metal oxide/activated carbon	No

It is a normal practice in several waste locations that electrical power is produced by burning the biogas produced in the same sites. Next, reference [74] investigated the desulfurization of the biogas by coupling H₂S/CO₂ absorption units coupled to a photobioreactor. Total H₂S removal from the biogas was accomplished by the use of a bubble column or airlift and under various operational conditions. Some aspects in relation to the removal of CO₂ make the use of bacteria-microalgae biocultures on these systems of interest.

Amine-based reactions and an industrial acid gas removal process were used to design and simulate a gas cleaning process in [75]. The results indicated that the best results are given by a mixture of piperazine and methyldiethanolamine, saving operational cost and carbon emissions with respect to the use of the mixture of methyldiethanolamine and diethanolamine.

The generation of biogas has different sources, vinasse being one of them. Different alternatives are investigated for the elimination of water, CO₂, and H₂S from the biogas [76]. In practice, it has been demonstrated that the use of an anaerobic lagoon is preferred over a series of upflow anaerobic sludge blanket reactors, and from the various investigated alternatives to clean the biogas, water is best removed by silica gel adsorption, and after, both H₂S and CO₂ are absorbed on an Fe/EDTA solution.

Amine groups bearing ionic liquids supported on membranes have been used in the removal of CO₂ and H₂S and separating both from methane [77]. Facilitated transport is the mechanism responsible for CO₂ transport using the ionic liquid [DMPDAH][Tf2N] (containing a primary amine), the gas transport being greatly dependent on trans-membrane pressure.

In the simultaneous removal of both gases, apparently, absorption is the most used technology with the greatest technology readiness level, alkaline salts and amine being the most used absorbents; moreover, physical solvents are used in this absorption role. Adsorption processing continues investigations in this field in order to improve adsorbents’ performance and/or creating new ones, allowing higher efficiencies in the removal of H₂S and a complete regeneration to be used in further cycles. Both absorption and adsorption technologies have a complete rationalization in their use, i.e., absorption in the treatment of the most contaminated streams and adsorption to be used in the case of gases containing low concentrations of the contaminants. The above technologies need the regeneration of the adsorbent and the solvent; thus, they have intensive energy requirements [78].

The use of membranes needed a step ahead of pilot scale to show its potential in this industrial issue [79]. Membranes incorporating inorganic materials into the membrane’s polymeric structure are also of consideration to improve the removal of H₂S and CO₂. It is shown in [80] that the presence of H₂S in the gas stream is detrimental for the membrane’s performance, and this and other features make it so that, at the present time, membrane usefulness is limited to laboratory or small pilot plants; further developments are needed prior to use in commercial operations.

The use of a hybrid or intensified process used in other industries, i.e., membrane contactor supplemented by other technology needing lower energy requirements, within conventional separation processing is another possibility to benefit the separation of these undesirable gases from gas streams. Under this point of view, H₂S oxidation, over carbon-based catalysts, is a possibility to be used for the production of materials with a great capacity to store energy [81].

4. Conclusions

The removal of H₂S is of the utmost importance in order to avoid negative consequences such as exposure to humans and the overall environment. Though some of the technologies know commercial scales, they are far from being perfect; thus, there is still a wide margin for their improvement and optimization to gain further reduction in costs and increase in the separation characteristics of the technology. When H₂S is present in the gas stream, in the order of a few mg/L, adsorption with metal-oxide-based adsorbents is the most used separation technology in the removal of this toxic gas, whereas this technology together with absorption and membranes is used when there is a need to separate H₂S from natural gas and when H₂S is present in the gas stream in the order of mol% levels. Using membrane technologies, polymeric membranes are the most used material, whereas in absorption processing alkanolamines are the preferable solvents for practical use. Future investigations may be directed to the enhancement of the separation efficiency with respect to H₂S, and this goal is focused on in the development of materials and solvents with improved properties; the technologies to reach these expectations rely in better molecular design/grafting and the use of less expensive solvents and materials also fulfilling environmental laws, i.e., adsorbents produced from wastes.

The simultaneous recovery of both H₂Sand CO₂ is possible by the use of high-level technology readiness carbon-capture operations; however, the selective removal of both gases needed intensive investigations. This will help with the recovery and isolation of H₂S and its further processing to different valuable products, including hydrogen and sulfur, together with sulfur-containing composites/materials used in applications in relation to the concept of the circular economy.

Table 7. Advantages and disadvantages of the various H₂S removal technologies.

Technology	Advantages	Disadvantages
Absorption	Established technology, possibility to treat tail gas	Chemistry of alkanolamines, solvent regeneration seems difficult
Adsorption	Established technology, high removal capacity	Generation of toxic wastes, difficult to operate offshore, stability of the adsorbent
Membranes and membrane contactors	Modular configuration, large surface area per unit volume	Possible limitations due to permeability, resistance due to membrane, degradation of the membrane

summarizes advantages and disadvantages of the different H₂S removal technologies, though, in the opinion of this author, a series of potential readers of this manuscript will not agree with these and will add or void others.