Introduction

Nowadays, the extensive consumption of fossil fuels often causes severe environmental issues, such as greenhouse effect, acidic rain, and haze, due to the generation of several gaseous pollutants (Xu and Zhang 2016; Liu and Wang 2019; Liu et al. 2019b; Wang and Liu 2019a; Yang et al. 2018c, d). Beyond the environmental pollution problem, the limited reserves also lead to the search for alternative fuel sources which are cheap, abundant, renewable, and environmentally benign. Biogas, mainly a mixture of methane and carbon dioxide, is recognized as a sustainable alternative. Biogas can be generated by anaerobic decomposition of diverse types of biomass, such as animal wastes, crop residues, household wastes, and so forth (Walsh et al. 1988). The major nature of biogas is close to natural gas, making it a suitable alternative for fuel synthesis and electricity generation. However, raw biogas commonly contains hydrogen sulfide, which is one of the most toxic, hazardous, and corrosive gases with foul odor (Stirling 2000; Weiland 2010). A few tens to 20,000 ppmv H2S will be produced in the biogas derived from versatile biomass feedstock (Allegue and Hinge 2012). H2S even in trace amount is extremely toxic to the human respiratory and nerve system and can corrode the instruments and piping systems as well as poison downstream catalysts, leading to the loss of device serving lives and deactivation of the catalysts (Kaur et al. 2008; Sun et al. 2018; Liu et al. 2019a). To avoid ‘sulfur poisoning,’ the sulfur content in hydrogen-enriched gaseous reformate supplied for solid oxide fuel cells should be less than 1 ppmv (Meng et al. 2010). In some emerging technologies, the H2S concentration in syngas or reformate gas cannot exceed 0.1 ppmv, such as proton exchange membrane fuel cells or Fischer–Tropsch synthesis processes (Bezverkhyy et al. 2012). Therefore, it is indispensable to capture H2S from raw biogas prior to its combustion or conversion.

The methods for hydrogen sulfide capture from raw biogas can be primarily classified into wet desulfurization technology and dry desulfurization technology (Wang and Wang 2018, 2019). The wet desulfurization technology mainly utilizes amine solution to absorb hydrogen sulfide via gas–liquid acid–base reaction, which can meet the requirements of high desulfurization loads (Wang and Liu 2019b). However, it suffers from low absorption rate, volatile solvent, high energy consumption, equipment corrosion, and secondary pollution (Liu et al. 2016a, b). Dry desulfurization technology has been extensively investigated owing to its merits of higher desulfurization accuracy, simpler equipment, more convenient operation, less pollution, and lower energy consumption. Dry desulfurization is virtually a chemical reaction process in which the active components in sorbents react with hydrogen sulfide and then transform into stable sulfides. At present, most researching works on dry desulfurization technology primarily focused on medium- or high-temperature desulfurization at temperatures of ~ 500–800 °C, which was mainly proposed in conjunction with integrated gasification combined cycle power generation technology (Liu et al. 2016c, d, e). Though medium- and high-temperature dry desulfurization can remove hydrogen sulfide effectively, it still suffers from high energy consumption, easy sintering of desulfurizer, and complex reaction. Dry desulfurization operating at low temperatures ranging from room temperature to 200 °C can avoid the above-mentioned issues, which is an appealing route for capturing hydrogen sulfide from raw biogas.

Over the last decade, enormous efforts have been devoted to developing cost-effective low-temperature sorbents for raw biogas desulfurization. Samokhvalov and Tatarchuk (2011) critically commented the developments on experimental determination of the nature of active sites and the mechanism of H2S, COS, CS2 adsorption on ZnO at low temperature. Ozekmekci et al. (2015) shortly reviewed the application of natural or commercial zeolites in H2S removal. Peluso et al. (2019) discussed the recent progress on H2S removal by nanoporous materials, in particular, activated carbons, zeolites, ordered mesoporous silica, and metal–organic frameworks. Although a couple of reviews on low-temperature H2S removal were available in studies, these reviews primarily concentrated on one specific sorbent or few types of sorbents. In this review, the full picture of the recent progress of all kinds of conventional and emerging sorbents for hydrogen sulfide capture from biogas at low temperature in the last decade is elaborated, which aims to provide inspiration and guidance for developing novel and affordable solid sorbents for biogas purification at ambient conditions efficiently.

Carbon materials

Carbon materials, such as biochar, semi-coke, and activated carbon, have been widely employed as low-temperature desulfurizers at early years due to the advantages of well-developed porous structures and abundant surface functional groups. Besides, biochars and activated carbons are inexpensive, because they can be attained from plants, crops, food wastes, and industrial wastes (Klasson et al. 2013; Zou et al. 2018; Yang and Wang 2017; Kosheleva et al. 2019; Yang et al. 2019; Xu and Hussain 2018). The wealthy pore structures contribute to their strong physical adsorption abilities, while the rich surface oxygen-, hydrogen-, and nitrogen-contained heterocycle groups are dedicated to their good chemical adsorption abilities (Bansal and Goyal 2000; Yang and Wang 2017; Xu and Adewuyi 2018; Liu et al. 2018, 2019c).

Xu et al. (2014) prepared biochars from pig manure and sewage sludge for hydrogen sulfide removal at room temperature. The pig manure biochar showed higher sulfur capacity than the sewage sludge biochar. Moisture and high alkalinity are favorable for H2S adsorption over the biochar. Moreover, mineral elements in biochar play crucial roles in the form of final sulfur products. Sun et al. (2016) obtained a porous biochar with big surface area of ~ 60 m2/g from black liquor. The as-produced biochar is alkaline and possesses microporous and mesoporous structures. The degree of heterogeneity on the biochar surface is far more than that of commercial activated carbon, which contributes to a higher sulfur capacity of ~ 70 mg/g at room temperature and ambient pressure by using column dynamic adsorption. Hervy et al. (2018) developed two kinds of chars by pyrolysis of food waste and sludge for H2S removal at ambient temperature in various dry gas matrices. Inherent mineral species, especially Ca and Fe, increased H2S removal efficiency by promoting the formation of metal sulfide and metal sulfate species on char surface. CO2 had little impact on the H2S removal performance, while O2 in air matrix decreased sulfur capacity because of the formation of sulfur acid species. This char could be completely regenerated using a thermal treatment at 750 °C in nitrogen stream.

Xiao et al. (2008) adopted Na2CO3-impregnated activated carbon for low-concentration H2S adsorption at 30 or 60 °C under anaerobic conditions in a fixed bed. The Na2CO3-modified activated carbon exhibited above three times more sulfur capacity than virgin activated carbon under dry condition. The sulfur capacity of virgin activated carbon and Na2CO3-modified activated carbon both increased with the incremental relative humidity but decreased slightly with increasing temperature. Nowicki et al. (2014) obtained chars and activated carbons by pyrolysis of coffee industry waste. They studied the effects of pyrolysis temperature, activation method, mineral matter content, porous structure, and acid–base character on H2S removal efficiency. The impacts of mineral matter content and acid–base character on sulfur capacity are much bigger than the type of porous structure and the degree of surface area development. A low-grade activated carbon was first upgraded by steam activation and further modified with KOH for H2S adsorption at ambient temperature under various operating conditions. KOH-impregnated activated carbons presented significantly higher sulfur capacity and longer breakthrough time than those of non-modified one. O2 greatly lengthened the breakthrough time, while steam significantly increased the sulfur capacity. In contrast, CO2 strongly suppressed H2S adsorption due to competitive adsorption and the reaction between CO2 and H2S (Sitthikhankaew et al. 2014).

Barelli et al. (2017) also used KOH–KI-treated activated carbon for deep hydrogen sulfide removal from biogas at 45 °C under gas mixture with composition similar to a real biogas produced in a wastewater treatment plant. Water had a positive effect on desulfurization process, which was enhanced by the simultaneous presence of small percentages of O2. CO2 exhibited marked negative effect on desulfurization process, whereas high methane content could significantly increase the sulfur capacity. Aslam et al. (2015) reported that waste oil fly ash (OFA) was suitable for producing activated carbon. The surface area of OFA was significantly increased from 4 to 375 m2/g after acidic treatment and subsequent CO2 activation at 990 °C. The OFA-derived activated carbon modified with HNO3 and ammonia solution was used for H2S removal from natural gas at 22 °C. The maximal sulfur capacity of ~ 0.3 mg/g was reached over ammonia-functionalized activated carbon associated with ~ 86% regeneration rate. The ammonia-treated activated carbon performed more effectively toward H2S adsorption than the acid-treated one.

Menezes et al. (2018) investigated the mechanisms of H2S retention at low concentration on impregnated activated carbon at low temperature. The chemical modification of activated carbons with NaOH or Fe2O3 resulted in a considerable rise in sulfur capacity. Despite the decrease in surface area and pore volume resulting from the incorporation of chemical species, Na or Fe, into the internal pores of the activated carbon, sodium and iron modification intensified the interaction between sorbent surface and H2S molecules, leading to the increase in sulfur capacity. The mechanism of H2S adsorption was related to chemisorption. Boutillara et al. (2019) prepared copper-modified activated carbons from Algerian olive stones via one-step chemical activation with CuCl2. Though the produced activated carbons possessed rather low surface areas of ~ 400 m2/g and micropore volumes of ~ 0.15 cm3/g, they offered good hydrogen sulfide adsorption performance due to the interaction of hydrogen sulfide with copper compounds and surface functional groups. They also performed better than the activated carbon prepared with ZnCl2 activation.

Surra et al. (2019) prepared two activated carbons by physical activation of maize cob waste with CO2 and two other activated carbons by chemical activation. The textural properties, i.e., surface area and microporosity, might play a more important role than mineral content in H2S adsorption. High oxygen content in sorbent favored the catalytic oxidation reaction of H2S, facilitating H2S removal. Qi et al. (2018) developed porous carbon spheres (PCSs) via phase inversion route followed by carbonization using graphitic carbon nitride and polyvinylidene fluoride as precursors. The as-prepared porous carbon spheres were employed for simultaneous removal of hydrogen sulfide and benzene at 25 °C. The structure and chemical property of the porous carbon spheres could be tailored via controlling g-C3N4 content. The promotion effect on the H2S adsorption performance was attributed to the change of electron distribution on nitrogen atom. The enhancement on mass transfer caused by meso/macrohierarchical porous structure was certified through adsorption kinetic. The different adsorption mechanism of hydrogen sulfide and benzene on porous carbon spheres is depicted in Fig. 1.

Fig. 1
figure 1

Scheme for simultaneous removal of benzene and hydrogen sulfide over porous carbon spheres (PCSs). Reproduced with permission from Qi et al. (2018). The polluted air consisted of volatile organic compounds (VOCs) and toxic industrial chemicals (TICs). Due to the low polarity of benzene, the interaction force between adsorbate and PCSs was unable to affect the benzene adsorption performance. The C6H6 adsorption on PCSs was primarily dominated by physisorption which highly relied in micropores (Lillo-Ródenas et al. 2005; Li et al. 2011; Gil et al. 2014). Different from benzene, H2S adsorption process encompassed physisorption and chemisorption determined by porous structure and chemical polarity, respectively. The chemical polarity determined by functional groups, especially basic groups, played a key role in hydrogen sulfide adsorption (Bandosz et al. 2000; Yan et al. 2004)

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Zeolites

Zeolites usually employed to capture the molecules are known as molecular sieves (Ozekmekci et al. 2015). Zeolites are kinds of crystalline aluminosilicates with tunable porous structure, acid density, and shape selectivity (Liu et al. 2016d). In recent decade, zeolites had been extensively applied for hydrogen sulfide removal as well.

Lee et al. (2009) incorporated iron into Na–A-type zeolite via melting slag and ferric chloride solution for adsorption removal of H2S and NH3. Iron content and calcination temperature had significant impacts on gas adsorption capacity. Binder types had little influence on the gas adsorption capacity. The H2S adsorption capacity was proportional to iron content and inversely proportional to calcination temperature. Rezaei et al. (2012) loaded copper on many titanosilicate supports for low-content hydrogen sulfide removal at room temperature. Copper supported on Engelhard Titanosilicate-2 (Cu-ETS-2) was identified as a superior hydrogen sulfide scavenger for maintaining H2S content below 0.5 ppmv, probably owing to its high cation exchange capacity and copper dispersion. Rezaei et al. (2015) reported that ETS-2 appealed to be a promising substrate to load active sites for deep hydrogen sulfide removal for gas purification applications at room temperature. The hydrogen sulfide adsorption performances of Ag-, Ca-, Cu-, and Zn-exchanged ETS-2 were tested at room temperature. The trend of sulfur capacity was as follows: Cu-ETS-2 > Ag-ETS-2 > Zn-ETS-2 > Ca-ETS-2 > Na-ETS-2. The Cu-exchanged ETS-2 performed the best with the highest sulfur capacity of ~ 30 mg/g.

Micoli et al. (2014) adopted Cu- and Zn-modified 13X zeolites obtained by ion exchange or impregnation route as H2S sorbents for biogas purification for fueling molten carbonate fuel cells. Addition of Cu or Zn ions significantly enhanced the H2S adsorption property of the 13X zeolite, especially for Cu-exchanged zeolite presenting the best performance with a breakthrough time of 580 min at 0.5 ppm H2S, which was 12 times longer than the parent zeolite. Barelli et al. (2018) prepared a Cu-exchanged 13X zeolite sorbent for hydrogen sulfide capture from biogas to obtain a desulfurized fuel suitable for molten carbonate fuel cell systems. The Cu-exchanged 13X zeolite also presented high potentiality for hydrogen sulfide adsorption, specifically at lower space velocity and reaction temperature and in the presence of higher methane concentration in the gas mixture. The enhanced desulfurization performance was plausibly attributed to the presence of numerous Cu2+ ions, leading to an efficient physical–chemical adsorption.

Metal oxides

Hydrogen sulfide has a high chemical affinity with metal cations and surface metal sites. In recent decade, hydrogen sulfide adsorption using metal oxides at low temperature has attracted increasing attentions. Among them, iron oxide, copper oxide, and zinc oxide are the most commonly used low-temperature sorbents for hydrogen sulfide removal.

Iron oxide

Iron oxide, Fe2O3, is a potential candidate for low-temperature desulfurization owing to its low cost, abundant sources, and fairly favorable thermodynamics in reaction with hydrogen sulfide at low temperature.

Sahu et al. (2011) employed red mud for hydrogen sulfide removal at ambient condition. The color of some red mud varied from red to black, indicating that iron oxide was converted to iron sulfide. The hydrogen sulfide was captured in the form of FeS2, FeS, CaSO4·2H2O, sulfur, sulfide, and bisulfide of Na. Long and Loc (2016) prepared a bentonite-containing iron oxide sorbent by hydrothermal precipitation approach for hydrogen sulfide removal in a continuous fixed-bed column at room temperature. The total sulfur uptake slightly increased with incremental bed depth and initial H2S concentration, whereas it decreased with increasing flow rate.

Huang et al. (2015) successfully synthesized γ-Fe2O3/SiO2 sorbents with three-dimensionally ordered macropore (3DOM) structure using a colloidal crystal templating method for hydrogen sulfide removal at low temperatures of 20–80 °C. The γ-Fe2O3 showed an enhanced activity compared to commercial sorbent for hydrogen sulfide capture at temperatures over 60 °C, whereas α-Fe2O3 exhibited little activity. 3DOM γ-Fe2O3/SiO2 performed the best in terms of sulfur capacity and utilization rate due to its large surface area, high porosity, and nanosized active particles. Furthermore, moisture was favorable for hydrogen sulfide removal at low temperature. Since partial γ-Fe2O3 transferred into α-Fe2O3, the conventional regeneration method with air at high temperature was not suitable for the regeneration of γ-Fe2O3/SiO2 composites. Fauteux-Lefebvre et al. (2015) embedded iron nanoparticles in carbon nanofilaments for in-depth hydrogen sulfide adsorption at 100 and 300 °C. The resultant sorbent could decrease hydrogen sulfide concentration from 500 to below 1.5 ppmv. Sulfur capacity greatly augmented with the elevation of reaction temperature under all space velocity conditions. Flow rate had nearly no impact on adsorption rate, indicating that desulfurization process was not controlled by mass diffusion phenomena.

Arcibar-Orozco et al. (2015) studied the role of surface chemistry and morphology in the reactive adsorption of hydrogen sulfide on iron (hydr)oxide/graphite oxide composites. The addition of graphite oxide enlarged the surface area attributed to the formation of new micropores and increased the number of acidic functional groups, but had no influence on the crystal structure. The sulfur adsorption capacity of magnetite/graphite oxide composites increased, whereas ferrihydrite/graphite oxide composites decreased after adding graphite oxide. Raabe et al. (2019) investigated the performances of 35 different iron oxides/hydroxides toward hydrogen sulfide capture from natural gas and biogas at 35 °C. Fe(OH)3 and α-FeOOH presented the highest efficiency for hydrogen sulfide removal, whereas α-Fe2O3 performed fairly worse. Textural properties and acidic site amounts had great impacts on the desulfurization performance. The adsorption processes of hydrogen sulfide over FeOOH are represented in Fig. 2.

Fig. 2
figure 2

Reactive adsorption of hydrogen sulfide on FeOOH. The hydrogen sulfide reacting with FeOOH yields FeS and S8. The produced S8 can react again with FeOOH, producing FeS and oxygen. Reproduced with permission from Raabe et al. (2019)

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Copper oxide

Because of excellent catalytic oxidation performance, copper oxide has been widely used as a catalyst for removal of many gaseous pollutants (Xu and Adewuyi 2018). From the perspective of thermodynamic equilibrium, the reaction equilibrium constant of CuO and H2S is significantly higher than that of Fe2O3 and ZnO. Furthermore, CuO possesses good desulfurization accuracy and high sulfur capacity, making it an attractive low-temperature sorbent for hydrogen sulfide adsorption (Slimane and Abbasian 2000).

Jiang et al. (2010) found that adding few Al into copper oxide could reduce CuO crystalline size, which thereby increased the sulfur capacity in the temperature range of 25–100 °C. The smaller crystalline size and bigger specific surface area of CuO could reduce the thickness of potential sulfide shell on the sorbent exterior layer and increase the area of the interface for the exchange of sulfur ions and oxygen ions. Montes et al. (2013) supported CuO onto MSU-1 (Michigan State University-1) for hydrogen sulfide capture from natural gas at room temperature. 20Cu/MSU-1 with 20 wt% Cu displayed the highest sulfur capacity of ~ 19 mg/g. The pore system and uniform distribution of CuO in MSU-1 had big influence on sulfur capacity.

Liu et al. (2015) employed CuO-based sorbents for hydrogen sulfide removal at low temperature. The CuO-based sorbents could reduce hydrogen sulfide to less than 0.1 ppmv at 40 °C. The Fe–Al co-doped CuO performed the best with the biggest sulfur capacity of ~ 114 mg/g. The CuO-based sorbents could be regenerated with air at 100–200 °C accompanied by a stable sulfur capacity during four sulfidation–regeneration cycles. Wang et al. (2017) applied CuO-based sorbents with 3DOM structures for hydrogen sulfide removal at ambient temperature. The sulfur capacity of CuO/3DOM was nearly six times bigger than that of the CuO-based sorbent without 3DOM structures. Moisture played a crucial role in sustaining high sulfidation activity, but leading to a higher H2S concentration before breakthrough. The regeneration temperature had to exceed 600 °C due to the decomposition of CuSO4. The scanning electron microscope (SEM) and transmission electron microscope (TEM) images of fresh and regenerated CuO/3DOM sorbents are shown in Fig. 3.

Fig. 3
figure 3

Micromorphologies: scanning electron microscope (SEM) image of fresh CuO/3DOM (three-dimensionally ordered macropores) (a); transmission electron microscope (TEM) images of fresh CuO/3DOM (b, c); regenerated CuO/3DOM (e, f); and high-resolution TEM image of fresh CuO/3DOM (d). Reproduced with permission from Wang et al. (2017). The fresh CuO/3DOM possessed uniform and highly ordered inverse opal structure on a large scale associated with interconnected macropores (Fig. 1a). The magnified TEM image of fresh CuO/3DOM (Fig. 1b, c) revealed that CuO nanoparticles were uniform in size of ~ 19 nm. The TEM images of regenerated CuO/3DOM (Fig. 1e, f) clearly showed that 3DOM structures were well maintained, but the grains grew slightly larger (~ 30 nm). The high-resolution TEM image of fresh CuO/3DOM (Fig. 1d) exhibited clear lattice fringes with spacing of 0.2324 and 0.2532 nm corresponding to the (111) and (002) crystal planes of CuO, respectively

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Zhang et al. (2017) conducted a mechanistic and kinetic study concerning the adsorption and dissociation of H2S molecule on versatile CuO (111) surfaces based on density functional theory. The S-containing species primarily interacted with surface Cu via S atom. H atoms were predominantly adsorbed at the outermost surface O site. The activity of H2S adsorption and dissociation on CuO (111) surfaces followed the order of stoichiometric > reduced > sulfurized surface. Pola-Albores et al. (2018) studied the effect of chemical character, morphology, and microstructure on the desulfurization performance of CuO-based sorbent at room temperatures, which was regarded as a potential application in biogas cleaning. The CuO-based sorbent was prepared by calcination of metallic copper. Three copper phases (Cu0, Cu2O, and CuO) were identified in the sorbent. The H2S adsorption testing results revealed that CuO could be a feasible sorbent for hydrogen sulfide capture from biogas. Though CuO-based materials accompanied by apparently big particles and low surface area, they presented fast saturation rates and higher metallic sulfide contents, which were probably resulted from the heterogeneous composition of the CuO-based sorbents.

Zinc oxide

Zinc oxide, ZnO, has attracted numerous researching interests in recent decade because of its great potential in reducing H2S concentrations to below 100 ppb at room temperature (Neveux et al. 2012).

Wang et al. (2008c) supported ZnO nanoparticles on SBA-15 (Santa Barbara amorphous material-15) via incipient wetness impregnation and ultrasonic method followed by in situ activation at 250 °C for hydrogen sulfide removal from gas streams at ambient conditions. The as-obtained ZnO/SBA-15 sorbent presented an outstanding ability to reduce H2S concentration down to ppb level from gas stream. The maximal breakthrough sulfur capacity of ~ 436 mg/g was achieved over the sorbent with 3.04 wt% Zn. The increase in sulfur capacity could be attributed to the combination of the high surface area of SBA-15 and the promising desulfurization properties of ZnO nanoparticles. Wang et al. (2008a) loaded ZnO on an aluminum-substituted SBA-15 (Al-SBA-15) by post-synthesis and immobilization method via microwave-assisted route for low-content H2S removal from a gas mixture at ambient condition. The as-synthesized sorbent had well-ordered hexagonal mesopores and was rich in micropores. ZnO nanoparticles were well dispersed and anchored both in the channel and the wall of the Al-SBA-15. The ZnO/Al-SBA-15 sorbent with 2.1 wt% Zn performed the best. Both micropores and mesopores were active sites for H2S capture, especially micropores.

Hernández et al. (2011) developed an activated carbon-supported ZnO sorbent for simultaneous desulfurization and dehalogenation of landfill biogas at ambient temperature. The principal goal was to identify a multifunctional adsorption bed that would be able to purify the landfill biogas to sulfur and chlorine concentrations of below 1 ppmv associated with a high removal efficiency of > 99%. The sorbent with 10% ZnO showed a higher sulfur capacity than commercial activated carbon toward H2S adsorption owing to the presence of well-dispersed ZnO nanoparticles on activated carbon surface. Liu et al. (2012) synthesized ZnO/SiO2 gel composites via sol–gel route combined with ambient drying process and subsequent thermal treatment for hydrogen sulfide removal at room temperature. The as-synthesized sorbents displayed high surface area with multimodal pore size distributions in micropore and mesopore region. The gel composites with 30 wt% ZnO and heating treatment at 400 °C exhibited high desulfurization performances with maximal sulfur capacity of ~ 96 mg/g. Physical adsorption and active phase reactivation both governed the desulfurization processes.

Tajizadegan et al. (2013) prepared ZnO–Al2O3 composite particles by heterogeneous precipitation method using bayerite seed particles for hydrogen sulfide adsorption at low temperature. The as-prepared ZnO–Al2O3 composite presented a greater sulfur capacity of ~ 52 mg/g than pure ZnO of ~ 28 mg/g mainly due to the higher surface area, more pore volume, and unique morphology in nanoscale. The micromorphologies of ZnO–Al2O3 composite together with pure ZnO were examined by field emission scanning electron microscopy (FESEM) analysis as presented in Fig. 4.

Fig. 4
figure 4

Field emission scanning electron microscopy (FESEM) micrographs of a pure ZnO and b ZnO–Al2O3 composite. Reproduced with permission from Tajizadegan et al. (2013). The pure ZnO consisted of massive agglomerated ZnO nanoparticles with spherical shapes and a size distribution of 40–60 nm. After combination of ZnO and Al2O3, ZnO particles displayed flat morphologies by forming ZnO nanosheets

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Geng et al. (2019) developed a novel melt infiltration method for synthesizing molecular sieves (MCM-41, SBA-15, and MCM-48)-supported ZnO sorbents for hydrogen sulfide removal at room temperature. The melted zinc nitrate hexahydrate precursor penetrated into the pores of the molecular sieves by capillary force, resulting in more evenly dispersed ZnO particles in the pores as compared to impregnation method. The optimal ZnO loading values for MCM-41, SBA-15, and MCM-48 were 20, 20, and 30%, respectively. Their corresponding utilization ratios were 69.8, 52.2, and 45.1%, correspondingly. With the same ZnO loading, the utilization ratios of the sorbents obtained by impregnation approach were only 26.8, 38.2, and 28.3%, respectively. Larger specific surface area, larger pore size, and higher strength were favorable for desulfurization. However, molecular sieves with a smaller pore size and higher strength could benefit much more from the melt infiltration method.

Wang et al. (2014a) developed a sequence of ZnO–SiO2 composites with 3DOM structure via colloidal crystal template method for hydrogen sulfide removal at room temperature. The as-synthesized composites presented remarkable desulfurization performances at room temperature probably resulted from the unique structure features of 3DOM composites, big surface areas, nanocrystalline ZnO, and well-ordered interconnected macropores with abundant mesopores. The addition of silica was conducive to sustain the 3DOM structure and the high dispersion of ZnO. The highest ZnO content was up to 73 wt%, and the optimum calcination temperature was 500 °C. The as-prepared ZnO–SiO2 sorbent was reusable with 67.4% sulfur capacity of the first value after four sulfidation–regeneration cycles. Moisture in gas phase had a positive effect on hydrogen sulfide adsorption. The action mode of water on sulfidation reaction is illustrated in Fig. 5.

Fig. 5
figure 5

Role of water on hydrogen sulfide adsorption over ZnO–SiO2 composites with three-dimensionally ordered macropore structure. Water vapor exerted its influence by condensing as a film on ZnO surface. First, H2S could be absorbed by water film (He et al. 2011). Second, hydroxylation occurred on the surface of ZnO in contact with the liquid water film, which in turn resulted in gradual alkalinification of the water film (Noei et al. 2008; Raymand et al. 2011). Basic conditions could trigger the dissociation of the dissolved hydrogen sulfide into HS−and S2− anions (Mabayoje et al. 2012; Montes-Morán et al. 2012). The liquid water film thinned and even disappeared with the elevation of reaction temperature, thereby suppressing the absorption and dissociation of gaseous hydrogen sulfide. Reproduced with permission from Wang et al. (2014a)

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Awume et al. (2017) applied ZnO nanoparticles for hydrogen sulfide removal from gas and swine manure gas at low temperature of 1–41 °C using laboratory and semi-pilot scale system. The breakthrough and equilibrium sulfur capacity augmented with incremental H2S concentration. The sulfur capacities of ZnO nanoparticles with diameter of 18 nm were higher than those of 80–200 nm. The equilibrium sulfur capacity was not affected by temperature at 1–22 °C but increased at 41 °C. Manure gas treatment in semi-pilot scale adsorption system reduced H2S concentration from an average inlet value of ~ 236 ppmv to a negligible level. Singh et al. (2019) developed scrubber-like ZnO-loaded multiwall carbon nanotubes (MWCNT) sorbents for hydrogen sulfide capture at ambient atmospheric condition in a laboratory-scale scrubber apparatus. Surprisingly, ZnO and MWCNTs exhibited excellent synergistic effect toward hydrogen sulfide adsorption at ambient temperature. In addition, ZnO-MWCNTs presented the highest H2S removal efficiency of 98% even in a realistic condition, which could be efficiently used for fifty cycles without any loss in desulfurization activity.

Yang et al. (2018a, b) fabricated a series of mesoporous ZnO/SiO2 composites by sol–gel route with the aid of activated carbon. The as-prepared sorbents exhibited excellent performance for hydrogen sulfide capture. The highest sulfur capacity of ~ 161 mg/g was achieved at ~ 30 °C with corresponding ZnO utilization up to 69%. Activated carbon addition played double roles in the enhancement of the desulfurization performance: (1) anchoring and confining ZnO grains for producing highly dispersed small nanoparticles and (2) generating more oxygen vacancies in ZnO for facilitating lattice diffusion. The formation mechanism of well-dispersed ZnO nanoparticles with the aid of activated carbon is depicted in Fig. 6.

Fig. 6
figure 6

ZnO nanoparticle growth with or without activated carbon (AC) addition. Without AC addition, Zn species tended to agglomerate in the silica matrix and grew into larger ZnO nanoparticles after heating at 500 °C. Introducing few AC into the precursors would promote ZnO dispersion in four ways: (1) some Zn species might enter into the pores of AC, reducing Zn stacking issue in the silica matrix; (2) Zn2+ ions could be anchored onto AC surface attributed to the strong coordination interaction between the enriched functional oxygen group and metal ions, resulting in evenly dispersion of ZnO nanoparticles; (3) the limited pore size of AC could efficiently prevent the formed ZnO nanoparticles from growing to large particles; and (4) several cavities could be generated when AC combusted, in which the highly dispersed ZnO nanoparticles deposited. However, the total number of silica decreased with further addition of AC, and many cavities formed by AC annealing might collapse and lead to the reagglomeration of the ZnO nanoparticles. Hence, the adding amount of AC played a pivotal role in achieving a high dispersion of ZnO nanoparticles in the silica matrix. Reproduced with permission from Yang et al. (2018a, b)

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Composite oxide

Although the desulfurization performance of single Fe2O3, ZnO, and CuO has been improved to a certain extent by loading onto a carrier with large surface area or reducing particle size with nanosynthesis approaches, the sulfidation potential of single metal oxide is still restrained by its instinctive physical and chemical properties. Therefore, increasing research interests has been dedicated to the development of hybrid oxides.

Dhage et al. (2010) doped Cu into ZnO/SiO2 sorbent and applied for ultradeep adsorptive removal of hydrogen sulfide from gaseous fuel reformates for fuel cells at room temperature. Cu addition greatly enhanced the desulfurization capacity of the ZnO/SiO2 associated with ~ 92% ZnO utilization. The as-prepared sorbent sustained a high sulfur uptake capacity over ten sulfidation–regeneration cycles. Electron spin resonance (ESR) revealed that fresh and spent Cu–ZnO/SiO2 contained Cu2+ in single dispersion and coordination state. During desulfurization, partial Cu2+ ions were reduced into low-valence Cu+ ions. The higher the copper content in the sorbent was, the lower the reduction yield of Cu2+ to Cu+, which was related to the sulfur uptake capacity.

Later, Dhage et al. (2013) studied the active adsorption sites of Cu–ZnO/SiO2 sorbent by using experiment and simulation method. X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, electron spin resonance (ESR), ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy (DRS) techniques, and calculations by density functional theory (DFT) were employed to identify the active sites for hydrogen sulfide adsorption over Cu–ZnO/SiO2. The ZnO phase and Cu promoter sites in Cu–ZnO/SiO2 sorbent were nano-dispersed evidenced by XRD. The Cu promoter was presented in the form of Cu2+ sites of octahedral geometry identified by the complementary ESR and UV–Vis DRS analysis. DFT calculations revealed that there was significant energy discrimination of H2S binding toward a Cu2+ site relative to binding to a Zn2+ site, as compared to H2O adsorbing on the same two sites. On the ZnO surface with some Zn2+ sites substituted with Cu2+, a displacement of H2O with H2S at the surface Cu2+ sites occurred, contributing to the promotion effect of Cu2+ sites.

Dhage et al. (2011) further studied the effect of Fe/Mn doping on the desulfurization performance of ZnO/SiO2 at room temperature by using Operando ESR technique. The sulfur capacities of Fe/Mn doped ZnO/SiO2 prominently higher than those of commercial pure ZnO and un-doped ZnO/SiO2. The sulfur capacity and breakthrough characteristics remained satisfactory after ten sulfidation–regeneration cycles. Operando ESR disclosed that Fe3+ cations occupied the surface of ZnO, while Mn3+ cations were distributed within ZnO.

Balsamo et al. (2016) prepared a sequence of activated carbon-supported Cu–Zn mixed oxide sorbents with Cu/Zn atomic ratios ranging from 0:1 to 1:1 for reactive adsorption of hydrogen sulfide at 30 °C. The utilization factor of the active phase augmented along with Cu content up to 76%, suggesting a prominent promoting effect of Cu addition on the reactivity of ZnO. Temperature programmed desorption (TPD) of hydrogen sulfide and sulfur dioxide from saturated sorbents revealed that hydrogen sulfide adsorption was coupled with oxidation phenomena, resulting in the formation of metal sulfates apart from metal sulfides and/or elemental sulfur. De Falco et al. (2018) further studied the cooperative effect of Zn and Cu toward hydrogen sulfide adsorption at 30 °C. Adding little Cu into zinc oxide could improve the degree of ZnO conversion, while doping a few Cu into ZnO could assist in preventing micropores from severe clogging. CuO likely functioned as both oxygen donor and structural promoter.

Cimino et al. (2018) prepared a γ-alumina sphere-supported Zn–Cu mixed oxide sorbent for reactive adsorption of hydrogen sulfide from gaseous streams at room temperature. The effect of support on desulfurization process was highlighted as well. The as-prepared Zn–Cu sorbent showed a significant lower breakthrough time and sulfur capacity at saturation than the activated carbon-supported one. The probable reason was that the surface area of the γ-alumina sphere was much smaller than that of activated carbon, which was unfavorable for the dispersion of Zn–Cu oxides and stabilization of metal hydroxynitrates.

Sandra et al. (2017) incorporated Bi into ZnO via solvothermal method for hydrogen sulfide capture at room temperature. The sulfur capacity of ZnO was drastically increased by doping 25 at.% Bi. The breakthrough sulfur capacity of the Zn-Bi mixed oxide reached ~ 80 mg/g which was 12 times higher than that of commercially available and synthesized ZnO nanoparticles. The Bi-doped ZnO sorbent could maintain 30% of its initial activity by using quick regeneration process during ten sulfidation–regeneration cycles.

Yang et al. (2018c, d) added Ni into ZnO/Al2O3 via sol–gel approach for hydrogen sulfide removal from a wet gas stream. The sulfur capacity gradually increased at first and then decreased with incremental Ni content. The concentration of oxygen vacancies augmented after adding Ni species, which strongly contributed to the enhancement in breakthrough sulfur capacity. Oxygen vacancies played dual roles in hydrogen sulfide adsorption. On the one hand, they could efficiently facilitate the dissociation of H2S molecules by offering high hydroxy concentration on ZnO surface. On the other hand, oxygen vacancies could efficiently promote the counter diffusion of HS− or S2− and O2− in the bulk phase of ZnO.

Li et al. (2017) loaded Cu–Fe nanocomposites on diatomite for simultaneous adsorption/oxidation removal of hydrogen sulfide and phosphine from crude acetylene gas at 80 °C. When Cu and Fe loading amounts were 10 wt% and 6 wt% correspondingly, the Cu–Fe sorbent reached the best removal efficiency and the largest breakthrough sulfur capacity. It sustained an impressive efficiency of 100% for 150 min and 60 min for H2S and PH3, respectively. The Cu–Fe sorbent modified with hydrochloric acid could offer more O–H, C–O, and C=C functional groups, generating more unsaturated sites to enhance the removal performance for H2S and PH3.

Ordered mesoporous silica

Due to the relatively large and uniform pore size, large surface area, and controlled surface chemistry, ordered mesoporous silicas are recognized as fascinating materials for adsorption process (Gargiulo et al. 2014). However, virgin silica has limited affinity toward hydrogen sulfide; thus, the ordered mesoporous silica has to be modified with appropriate functional groups. Amines are the most widely studied functional groups for this application. Among diverse amines, polyethylenimine (PEI) was considered as one of the most effective functionalizing agents for ordered mesoporous silica due to its high density of amino groups capable of interacting with hydrogen sulfide (Xu et al. 2005).

Wang et al. (2008b) synthesized a sequence of PEI-functionalized ordered mesoporous silicas, such as MCM-41, MCM-48, and SBA-15, for hydrogen sulfide removal from a model gas stream at ambient conditions. The H2S concentration could be effectively reduced from 4000 ppmv to below 2 ppmv. The PEI content and reaction temperature had significant influences on the desulfurization performance. The decrease in reaction temperature and space velocity could enhance the sulfur capacity. The SBA-15 loaded with 50 wt% PEI gave the optimal breakthrough sulfur capacity, while loading 65 wt% PEI on SBA-15 had the highest saturation sulfur capacity. Moreover, the developed sorbents could be easily regenerated at mild conditions and showed excellent regenerability and stability.

Chen et al. (2010) developed a recyclable PEI-loaded hierarchical porous silica monolith sorbent for hydrogen sulfide removal at low temperature. The as-prepared sorbent displayed a large breakthrough sulfur capacity of ~ 43 mg/g at 22 °C, which was ~ 60% larger than that of PEI-loaded SBA-15 or MCM-41. The optimal PEI loading and PEI molecular weight were 65 wt% and 600, respectively. The desulfurization performance greatly increased with the decrease in reaction temperature. The as-prepared sorbent could be easily regenerated at 75 °C and exhibited excellent regenerability and stability as well.

Jaiboon et al. (2014) prepared PEI-modified silica xerogels of high porosity for hydrogen sulfide capture from biogas at low temperature. The H2S removal efficiency could be enhanced by decreasing reaction temperature and increasing amine loading values. The silica xerogel loaded with 50 wt% PEI presented the longest breakthrough time as well as breakthrough and saturation sulfur capacity at 30 °C. The optimal sorbent could be easily regenerated at a mild temperature of 50 °C for at least ten successive adsorption–desorption cycles without any detectable loss of the desulfurization performance or regeneration ability.

Similarly, Yoosuk et al. (2016) employed low-cost fumed silica grafted with PEI for simultaneously removing carbon dioxide and hydrogen sulfide at low temperature in a single- and double-stage system. The 40 wt% PEI/fumed silica gave the best compromise between the highest CO2/H2S sorption capacity and minimal amine leaching. The number of the available amine groups and the interaction between the amine groups and CO2 or H2S molecule primarily affected the sorbent sulfur capacity and amine efficiency.

As for non-polymeric amines, triamine, tetramethylammonium (TMA), and methyl-diethyl-amine (MDEA) had also been applied to functionalize the ordered mesoporous silica for hydrogen sulfide adsorption. Belmabkhout et al. (2009) reported that triamine-grafted pore-expanded mesoporous silica showed high CO2 and H2S adsorption capacity as well as high selectivity toward acid gases versus CH4. Moisture could enhance the CO2 removal capability without altering the H2S adsorption capacity. Thus, CO2 and H2S might be removed simultaneously or sequentially, beneficial for natural gas and biogas purification. Xue and Liu (2012) supported MDEA on SBA-15 for H2S removal from gas stream. The MDEA-grafted SBA-15 displayed a good desulfurization performance. The breakthrough sulfur capacity and saturation sulfur capacity both increased with the incremental initial H2S concentration. Moisture could also improve the desulfurization performance.

Bal’zhinimaev et al. (2017) synthesized a quaternary ammonium base-modified silicate fiberglass sorbent for hydrogen sulfide removal from natural gas at 25 °C. A new gel-like film was produced on fiberglass surface under hydrothermal treatment with tetramethylammonium hydroxide aqueous solution. This film was regarded as a phase with density less than pristine fiberglass, in which the TMA species, as the H2S adsorption sites, are confined. The N-modified fiberglass sorbents exhibited good regenerability in the presence of water.

Metal–organic frameworks

Metal–organic frameworks (MOFs) are porous hybrid materials in which metal ions or small metal nanoclusters are linked with multifunctional organic ligands (Yaghi and Li 1995; Li et al. 1999; Rowsell and Yaghi 2004). MOFs possess advantages over activated carbons and zeolites because of their high surface area, unique pore structure, and tunable pore size (Liu et al. 2010). Therefore, MOFs display great potential in the field of gas adsorption, methane or hydrogen storage, and catalysis (Zhou et al. 2012). By far, MOFs have been widely applied in the field of environmental remediation and energy production (Kumar et al. 2018), such as CO2 capture for biogas upgrading (Cavenati et al. 2008), dyeing wastewater treatment (Sun et al. 2019), and so forth.

Hamon et al. (2009) conducted hydrogen sulfide gravimetric isotherm adsorption measurements for six different MOFs. The MOFs were labeled as MIL (Matériaux Institut Lavoisier). All the materials presented fairly high sulfur capacity. However, the MIL-101 with the best performance could not be fully regenerated. The structures of MIL-47(V) and MIL-53(Al, Cr, Fe) as well as view of the structure of MIL-100(Cr) and MIL-101(Cr) are depicted in Fig. 7. The three-dimensional frameworks of MIL-47(V) and MIL-53(Al, Cr, Fe) were built up from corner-sharing chains of MIVO6 or MIIIO4(OH)2 (M = Al, Cr, V) octahedra connected through terephthalate moieties, which delimited a one-dimensional diamond-shaped pore system with specific surface area of ~ 1000 m2/g. The frameworks of MIL-100(Cr) and MIL-101(Cr) were composed of trimers of chromium octahedra linked with trimesate and terephthalate, respectively; which formed supertetrahedral motifs that further assembled to generate crystallized mesoporous hybrid solids with large specific surface areas above ~ 2000 m2/g.

Fig. 7
figure 7

Structure of (top left) MIL-47(V) and MIL-53(Al, Cr, Fe) and (top right) view of the structure of MIL-100(Cr) and MIL-101(Cr). MIL represented Matériaux Institut Lavoisier. The corresponding inorganic subunits are shown below each structure. Metal, oxygen, and carbon atoms were presented in green, red, and black, color, respectively, while terminal water molecules and fluorine are shown in gray. Reproduced with permission from Hamon et al. (2009)

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On the contrary, Peluso et al. (2014) reported that MIL-101 had a high sulfur capacity at near-ambient temperature and low heat release during hydrogen sulfide adsorption. The H2S adsorption on MIL-101 was fully reversible, suggesting a potential application of the MIL-101 in hydrogen sulfide removal from industrial gas streams with low H2S partial pressure, such as biogas.

Wang et al. (2014b) employed zinc-based MOFs for adsorption removal of dimethyl sulfide, ethyl mercaptan, and hydrogen sulfide at ambient temperature. The zinc-based MOFs performed the best toward hydrogen sulfide removal, followed by ethyl mercaptan and dimethyl sulfide. In the case of hydrogen sulfide, the interaction with sulfur atoms originated from the amino group and Zn site in the zinc-based MOFs. The former was more like an acid–base interaction, whereas the latter would yield ZnS and H2O and lead to serious structure destruction of the zinc-based MOFs.

Similarly, Li et al. (2015) synthesized copper-based MOFs by a hydrothermal method for adsorption removal of hydrogen sulfide, ethyl mercaptan, and dimethyl sulfide at low temperature. The H2S breakthrough capacity gradually augmented with incremental reaction temperature in the range of 30–80 °C. The highest H2S breakthrough capacity of ~ 57 mg/g was obtained at 80 °C. A strong interaction was existed between MOFs and H2S molecules, leading to the formation of various amounts of CuS and severe structure collapse.

Vaesen et al. (2013) synthesized amino-functionalized Ti-based MOFs for simultaneous removal of hydrogen sulfide and carbon dioxide from natural gas and biogas. The separation of H2S and CO2 from CH4 was performed by means of both macroscopic and molecular simulation methods. The amino-functionalized Ti-based MOFs presented a high H2S adsorption capacity just as the H2S–CH4 selectivity. Besides, it could tolerate H2S interaction without structure destruction, thus envisaging fully reversible adsorption operations.

MOFs are newly developed sorbents for hydrogen sulfide adsorption. By far, limited works had been dedicated to this topic with few encouraging results. However, recent in-depth molecular modeling studies revealed that MOFs are superior to zeolites for desulfurization of gas mixtures containing high sulfide concentration. This is because MOFs with larger pore volume results in a greater sulfide uptake (Peng and Cao 2013). In addition, more promising results in the field of selective capture of sulfur species were suggested based on molecular simulation studies concerning the use of novel covalent organic frameworks (Wang et al. 2015).

Conclusion

Adsorption technology with solid sorbents is recognized as one of the most effective routes to remove hydrogen sulfide from biogas at low temperature. The challenge for H2S capture from biogas is to find a non-toxic, inexpensive, and regenerable sorbent with a high activity at low temperature or even room temperature. This review summarized the recent progress of solid sorbents for biogas desulfurization over the last decade. Up to now, a variety of solid sorbents have been applied for adsorption removal of hydrogen sulfide at ambient conditions, such as biomass-derived chars, activated carbons, metal oxides, zeolites, ordered mesoporous silicas, and metal–organic frameworks. Different types of sorbents displayed versatile properties toward hydrogen sulfide adsorption. Some conclusions and prospects are highlighted as follows.

Carbon materials, particularly activated carbons, are used for hydrogen sulfide removal from industrial gas streams at low temperature in the early years due to their well-developed porous structures and abundant surface functional groups. Diverse activated carbons can be facilely obtained via high-temperature pyrolysis of different kinds of biomass. However, the low adsorption selectivity and regeneration difficulty of activated carbons hindered their industrial applications in desulfurization. Zeolites are another fully studied sorbents for hydrogen sulfide removal for biogas purification process. Zeolites possess high sulfur capacity increasing with modification of metals or metal oxides. They are promising materials due to their high surface–volume ratios, one of the most important factors in adsorption. Among various zeolites, ETS-2 can be a good choice to remove H2S. Excellent metal exchange forms of ETS-2 rather than copper form should be tried for the removal of H2S both experimentally and theoretically.

Metal oxides, including single metal oxides (iron oxide, copper oxide, and zinc oxide) and composite metal oxides, are still the most researched low-temperature sorbents for hydrogen sulfide capture from biogas or natural gas owing to their good reactivity and high sulfur capacity. Nevertheless, spent metal oxides usually have to be regenerated at elevated temperatures, which often results in aggregation of metal oxide grains and loss of reactivity. Constructing metal oxide nanoparticle sorbents with three-dimensionally ordered macropore structures or supporting metal oxides on carriers with large surface area could be feasible pathways to overcome the above-mentioned issues.

Ordered mesoporous silicas and metal–organic frameworks are two new types of low-temperature sorbents for hydrogen sulfide capture from biogas or natural gas. The amine-grafted ordered mesoporous silicas of good desulfurization activity can be regenerated at lower temperatures. However, the large-scale application of ordered mesoporous silica sorbents for biogas desulfurization is limited by their relatively lower sulfur capacities and higher capital costs as compared to conventional sorbents. Ongoing and future efforts should pay more attention on how to further increase the adsorption capacity and reduce the capital cost.

Metal–organic frameworks have just been employed for hydrogen sulfide removal at low temperature in recent years. Some encouraging results are obtained during last decade, indicating that metal–organic frameworks might be an efficient low-temperature sorbents for biogas desulfurization. However, the activity and stability of metal–organic frameworks need to be ensured prior to its large-scale application in hydrogen sulfide removal for biogas cleaning. The pore size and porosity, central metal, functional linkers, loaded active species, and especially, the unsaturated sites used in coordination can significantly affect the interactions between sulfur species and metal–organic frameworks that are needed for efficient adsorptive removal of hydrogen sulfide. Thus, more efforts have to be dedicated to the development of efficient and stable metal–organic frameworks for biogas desulfurization. Besides, covalent organic frameworks might also be good candidates for effective removal of hydrogen sulfide at low temperature.