A critical review of biochar versus hydrochar and their application for H₂S removal from biogas

Abstract

Biogas contains significant quantities of undesirable and toxic compounds, such as hydrogen sulfide (H₂S), posing severe concerns when used in energy production-related applications. Therefore, biogas needs to be upgraded by removing H₂S to increase their bioenergy application attractiveness and lower negative environmental impacts. Commercially available biogas upgradation processes can be expensive for small and medium-scale biogas production plants, such as wastewater treatment facilities via anaerobic digestion process. In addition, an all-inclusive review detailing a comparison of biochar and hydrochar for H₂S removal is currently unavailable. Therefore, the current study aimed to critically and systematically review the application of biochar/hydrochar for H₂S removal from biogas. To achieve this, the first part of the review critically discussed the production technologies and properties of biochar vs. hydrochar. In addition, exisiting technologies for H₂S removal and adsorption mechanisms, namely physical adsorption, reactive adsorption, and chemisorption, responsible for H₂S removal with char materials were discussed. Also, the factors, including feedstock type, activation strategies, reaction temperature, moisture content, and other process parameters that could influence the adsorption behaviour are critically summarised. Finally, synergy and trade-offs between char and biogas production sectors and the techno-economic feasibility of using char for the adsorption of H₂S are presented. Biochar’s excellent structural properties coupled with alkaline pH and high metal content, facilitate physisorption and chemisorption as pathways for H₂S removal. In the case of hydrochar, H₂S removal occurs mainly via chemisorption, which can be attributed to well-preserved surface functional groups. Challenges of using biochar/hydrochar as commercial adsorbents for H₂S removal from biogas stream were highlighted and perspectives for future research were provided.

Graphical abstract

1 Introduction

Since the Industrial Revolution in the early 1760s, extensive use of fossil fuels for industrial activities resulted in several environmental issues, such as the emission of greenhouse gases and other toxic gases into the environment. Given the finite nature of fossil reserves and challenges, such as environmental pollution and climate change, associated with the use of fossil fuels, efforts are made by the scientific community to identify an alternative source of fuel for energy generation that is cheap, readily available, and renewable. Biogas is considered as an alternative energy source that could levy the dependence on fossil resources and, at the same time, reduce the carbon footprint of energy production and consumption (Fonseca-Bermúdez et al. 2023). Hydrogen sulfide (H₂S) is one of the traditional acidic gas contaminants in many streams, such as natural gas, refinery gas, landfill gas, fuel gas, and biogas. Hence, removing H₂S from the carrier gas stream is crucial due to their acute toxicity and the prevention of corrosion in piping and downstream processing facilities. Biogas, largely a mixture of methane (CH₄) and carbon dioxide (CO₂), is a product of anaerobic digestion (AD), a process that involves the biological degradation of organic matter in an oxygen-limited environment. Alongside CH₄ (53–70%), and CO₂ (30–47%), biogas also contains trace concentration of N₂ (0–3%), H₂O (5–10%), O₂ (0–1%), H₂S (0–10000 ppm), NH₃ (0–100 ppm), hydrocarbons (0–200 mg m⁻³) and siloxanes (0–41 mg m⁻³), depending on the type of feedstock and process conditions (Katariya and Patolia 2023).

The concentration of these impurities can be concerning depending on the end-use of the biogas. For example, H₂S is a potential biogas contaminant and the presence of H₂S in biogas could create several operational and technical issues, such as corrosion of mechanical parts and breakdown of the energy conversion system (Ahmad et al. 2021b). The tolerance limit of H₂S in biogas varies depending on the technology considered and applications. For instance, H₂S concentrations should not exceed 1 ppm for fuel cell applications (Ghimire et al. 2021). Similarly, when biogas is considered for combustion applications, the recommended levels of H₂S are between 200 and 300 ppm (Cattaneo et al. 2023). During combustion, H₂S is oxidized into acidic sulfur dioxide (SO₂), highly corrosive to metal surfaces. Furthermore, the corrosion process speeds up at higher temperatures and leads to operational failures. In addition to being corrosive, H₂S causes catalyst poisoning during steam reforming applications. H₂S is highly toxic, and according to the National Institute of Occupational Safety and Health, the recommended exposure limit of H₂S is 10 ppm, which is a 10 min ceiling, and 20 ppm is the general industrial ceiling limit. Therefore, it is necessary to remove H₂S from biogas before it is considered for any downstream application, particularly for energy generation. H₂S-free, biomethane/hydrogen-rich biogas is an important renewable resource in the energy sector for clean thermal energy and power production via direct combustion in gas engines (Bui et al. 2021).

The methods or techniques for capturing H₂S from biogas have been classified into ex-situ and in-situ removal techniques. The selection of an appropriate technique depends on various factors, such as biogas composition, degree of removal required, intended application of the treated biogas, and operational conditions (Allegue et al. 2012). Figure 1 summarizes various in-situ and ex-situ H₂S removal techniques reported in the literature. In the last decade, significant efforts have been made in the removal of H₂S through the use of various techniques, such as wet oxidation (Tian et al. 2022), scrubbing (Liu and Wang 2019), adsorption (Cheng et al. 2024), and biological oxidation (Peluso et al. 2019). However, a common drawback associated with most of the above techniques, particularly wet scrubbing (liquid absorption) is that they are dependent on expensive catalysts or chemicals that may lead to secondary pollution or even equipment corrosion. On the other hand, biological processes are slower and often require strict control of operational conditions. In this regard, adsorption techniques have gained a great deal of attention due to their low cost and energy requirement, ease of operation and excellent removal capacity under low pollutant concentrations (Zhao et al. 2022b).

Fig. 1

Ex-situ and in-situ techniques for the removal of H₂S from biogas

Several adsorbents, including activated carbon, carbon fibre, graphene, carbon nanotubes, polymers, metal–organic frameworks (MOFs), nanomaterials, and biochar, have been demonstrated for H₂S adsorption. Among the various adsorbents, activated carbon is the most widely used, however, the high cost (1100–1700 USD/ton) can limit their large-scale application (Fdez-Sanromán et al. 2020). Cost-effective adsorbents such as biochar and hydrochar are carbon-rich materials obtained from the pyrolysis and hydrothermal processing of biomass, respectively (Vuppaladadiyam et al. 2022), (Mumtaz et al. 2023). Char (in the context of this review refers to both biochar and hydrochar) can be low in cost (90–100 USD/ton) (Fdez-Sanromán et al. 2020). At the same time, char possesses desirable properties, such as high surface area, excellent porosity, and surface functional group, favouring their adsorption performance. Furthermore, char produced from wastes, such as biosolids (stabilised sewage sludge) have high inherent metals/mineral contents (Hakeem et al. 2022) that could further enhance their H₂S adsorption capacity (Fdez-Sanromán et al. 2020). Therefore, char is widely used as an adsorbent in various environmental applications such as nutrient recovery (Koulouri et al. 2024) and heavy metal removal from wastewater (Yuan et al. 2024).

In recent times, the application of biochar/hydrochar for gas phase adsorption has gained attention. Several studies have reported the application of char for the removal of various gas components, such as CO₂, SO₂, H₂S, and other gaseous pollutants. However, the removal capacity of char varies with gaseous pollutants and their performance depends on the properties of char and the experimental conditions employed. While some studies elucidated the correlation between biochar’s physiochemical properties and H₂S adsorption mechanism, such understanding of hydrochar for H₂S adsorption is limited. Specifically, the in-depth understanding of hydrochar efficacy for H₂S adsorption and associated mechanisms requires investigations. Therefore, to fill this gap, we present an all-inclusive state-of-art progress on biochar and hydrochar, particularly for the adsorption of H₂S from biogas. The production processes, physiochemical properties, mechanisms, removal conditions, and adsorption performances of biochar vs hydrochar are discussed in-depth. In the later part of this review, an attempt was made to emphasize the possibility of creating a sustainable circular economy approach by integrating thermochemical processes for char production from wastewater biosolids and using the produced biochar/hydrochar for biogas upgradation all within wastewater treatment facilities. Additionally, a detailed technical comparison of various H₂S removal technologies is conducted based on the understanding gained from this review. Finally, challenges and research perspectives for developing efficient biochar/hydrochar-based adsorbents are highlighted.

2 Overview of H₂S removal techniques

2.1 Biological techniques

This technique employs an acidophilic oxidizing group of bacteria naturally occurring in the environment to decompose H₂S. The gram-negative bacteria oxidise H₂S/sulfides to elemental sulfur and sulfates through two domains (photoautotrophs and chemolithotrophs) via the sulfur oxidising reactions shown in Eqs. 1 and 2 (Rana et al. 2020).

$$C{O}_{2}+2{H}_{2}S \to C{H}_{2}O+ {H}_{2}O+2S$$

(1)

$$2S+3{O}_{2}+2{H}_{2}O \to 2{H}_{2}S{O}_{4}+2{H}^{+}+S{O}_{4}$$

(2)

Photoautotrophs bacteria use carbon sources for the photosynthetic reaction where CO₂ acts as a terminal electron acceptor and H₂S acts as the electron donor. This redox reaction converts H₂S to sulfur and sulfate (Nguyen et al. 2022a). H₂S oxidation can also be achieved by chemolithotroph bacteria that use O₂ (aerobic) or nitrate or nitrate (anaerobic) as an electron acceptor, grow on H₂S as an energy source and use CO₂ as a carbon source [1]. The oxidation of H₂S by biological technique is a well-known process for the abatement of H₂S emissions in industrial-scale biogas plants (Cheng et al. 2018; Ho et al. 2013; Lin et al. 2013). The most common approach of microbiological techniques for H₂S capture is based on different types of equipment such as biofilter, biotrickling filter, and bioscrubber. For instance, in a biofilter system, the filter bed is packed with organic media that has bacterial supplementing nutrients and a diverse microbial community developing a biofilm (Lestari et al. 2016). The packing media and biofilm formation are responsible for the range of processes (absorption, adsorption, phase transfer) required for the biodegradation of H₂S (Ho et al. 2013). A comprehensive review summarising the process mechanisms, operation efficiency, and limitations of H₂S removal using biological techniques is provided elsewhere (Barbusiński et al. 2021; Huynh Nhut et al. 2020; Khoshnevisan et al. 2017; Pudi et al. 2022; Syed et al. 2006). Examples of commercial installations of biofilter and bioscrubber for H₂S removal from biogas are Sulfothane, DMT Clear Gas Solution Sulfurex®BR and Sulfurex®BF, and THIOPAQ (a reference technology for low-pressure biogas treatment).

2.2 Absorption

This is a traditional industrial technique of scrubbing acidic gases, including H₂S, from gas streams such as biogas, hydrocarbons, fuel gas using liquid solvents. Commonly used chemical media for H₂S absorption are amine-based solvents, alkalis, ionic liquids, and deep eutectic solvents. In this method, the H₂S-containing gas stream is contacted with the chemical solution which selectively removes H₂S from the carrier gas and the exiting gas stream is lean in H₂S while the exiting solvent is rich in H₂S. The mechanism of H₂S absorption using chemical solvents can be either a physical or chemical process and may involve the dissolution of trace H₂S components by the solvent and a chemical reaction of the dissolved solute with the solvent. The saturated solvent is regenerated by chemical and thermal desorption and reused in the process. Absorption using alkanolamine solvents such as monoethanolamine (MEA) and diethanolamine (DEA) remains one of the most traditional approaches for gas sweetening and is well-advanced in the relevant industry. However, for biogas which typically contains more CO₂ than H₂S by vol, amine-based absorption might be limited for selective H₂S removal, the use of Fe(SO4)₃, FeCl₂, NaOH, and Fe(OH)₃, has been suggested for high H₂S removal efficiency from biogas; albeit at the optimum operating conditions (Zulkefli et al. 2016). Nevertheless, the simultaneous removal of CO₂ and H₂S in biogas using MEA and other aqueous alkaline solvents has been widely demonstrated to improve biogas quality with respect to calorific content and H₂S contamination levels (Marín et al. 2020; Tahir et al. 2015; Tippayawong and Thanompongchart 2010). Detailed information about the principle of operations, summaries of lab-scale results and limitations of the H₂S absorption method using different solvents can be found in a recent review (Qayyum et al. 2020). Typical commercial installations of absorption technologies for H₂S removal from biogas streams include DMT Clean Gas Solutions Sulfurex®CR, ExxonMobil and BASF OASE® sulfex™ and Merichem LO-CAT® sulfur recovery technology.

2.3 Adsorption

Adsorption is a gas–solid separation of H₂S from carrier gas stream using solid adsorbent. The H₂S-containing gas is passed over a bed of solid adsorbent and by a range of interactions which can be physical or chemical, H₂S is selectively removed on the surface or in the bulk of the adsorbent (Chan et al. 2022). A range of adsorbent materials have been demonstrated for removing H₂S from biogas – commonly used adsorbents are zeolites, metal oxides, carbon-based (nanomaterials, biochar, activated char), MOFs and porous organic polymers, and composite (Pudi et al. 2022). A review of the most promising adsorbent materials for the removal of H₂S from industrial gases has been documented in a recent paper (Georgiadis et al. 2020). The desired features of adsorbent for H₂S include a large surface area per unit weight and pores having a larger diameter than the molecular diameter of H₂S. The most commonly used commercial adsorbent materials for H₂S removal from biogas are activated carbon and metal oxides. Metal oxides catalytically oxidise H₂S to elemental sulfur and strongly bind the sulfur in form of metal sulfide. The abundance of porous structure and specific surface area enables activated carbon to capture H₂S onto their pores, similar to zeolites. More recently, biochar (and activated char) has been demonstrated as efficient material for removing H₂S from biogas with performance comparable to active carbon materials (Choudhury and Lansing 2021; Shang et al. 2016). Biochar adsorbents are considered cheap and sustainable alternatives to expensive conventional materials (such as zeolites, MOFs, and polymers) as they can be sourced from waste biomass, and they do not require tedious synthesis before use for H₂S adsorption. Biochar can combine the features of metal oxides and activated carbon with respect to their inorganic compositions and surface properties, therefore, they can offer viable H₂S removal capacity. However, biochar use as an efficient commercial adsorbent for H₂S removal is still under development and biosolids biochar can be an attractive option for real circular economy solutions for wastewater-generated biogas as detailed in Sect. 5. Adsorption is generally well-suited for treating dilute H₂S gas streams. The efficiency of H₂S removal by adsorption (gas–solid contact) is highly influenced by the properties of the adsorbent (surface chemistry), as well as the operating parameters (temperature, relative humidity, gas hourly space velocity, and the concentration of H₂S, O₂ and other contaminants (Sitthikhankaew et al. 2014; Vaziri and Babler 2019). A comprehensive review of H₂S adsorption from biogas using waste biomass-derived adsorbent has been reported in the literature (Sitthikhankaew et al. 2014). Commercial H₂S adsorption installations based on iron oxide and mixed metal oxide adsorbents are SULFATREAT® and SELECT FAMILY®, respectively.

2.4 Electrochemical

The electrochemical technique for H₂S removal uses electrical potential to induce electron flow in an electrochemical cell via a series of oxidation and reduction reactions, which ionises dissolved H₂S into 2H⁺ and S⁻. H₂S removal in electrochemical systems can occur under two mechanisms – oxidation and precipitation (Pudi et al. 2022). The direct oxidation of H₂S can produce S⁰, S₂O₃²⁻, SO₃²⁻, or SO₄²⁻, depending on the anode material and applied potential – low applied voltage will result in partial oxidation of sulfide to S⁰ while at high potential, sulfate and/or thiosulfate are generated (Dutta et al. 2008). However, the formation of S⁰ is the preferred reaction from an economic viewpoint as it needs less electrical energy, and the solid sulfur deposit on the anode can be easily separated from the system (Pudi et al. 2022). For the electrochemical precipitation process, also referred to as electrocoagulation, a sacrificial anode is made of a metal, such as iron, and releases ferrous/ferric ions into the electrolyte, which react with dissolved H₂S to produce insoluble iron sulfide precipitates. The solid residue is then removed from the aqueous solution. The reaction of metal ions with aqueous sulfide to produce metal sulfides at the anode simultaneously cause H₂ gas bubbles production at the cathode due to the dissociation of water molecules (Ahmad et al. 2021b; Pikaar et al. 2015). The H₂ will aid the electro-floatation of the formed metal sulfide on the surface for easy removal. The interest in electrochemical methods for sulfide removal is due to the simultaneous removal of organic compounds/pollutants from industrial waste streams. While a high electrochemical conversion of gaseous H₂S to sulfur can be achieved, a liquid absorber is usually needed to solubilise the sulfide in an aqueous electrolyte before its oxidation in an electrochemical cell (Kang et al. 2019). Elemental sulfur is deposited onto the electrode and deactivates it over time, which is considered a significant limitation of the electrochemical process (Dutta et al. 2009).

2.5 Membrane

Membrane separation is an advanced technology in a variety of industrial processes. Compared to adsorption and absorption, separation of acidic gases by membrane can be accomplished cost-effectively in a decentralised location due to the smaller footprint, high modularity (high membrane area per unit module) and ease of operation (Chan et al. 2022). In membrane separation, physical porous barriers are used for selective gas transport (permeation) driven by transmembrane pressure. The porous membrane materials are diverse and must be permeable, selective, mechanically stable, and chemically inert to separate the gas component. In industrial H₂S removal, polymeric membranes, including glassy polymer, rubbery polymer, and hybrid membranes, have been widely used in various configurations (Shi et al. 2020). The development of various polymer-based membranes and membrane processes for H₂S separation from natural gas, biogas, and coal gas have been reported in a recent review (Ma et al. 2021b). To improve the simultaneous removal of H₂S and CO₂ using the membrane process, membrane contactors involving the addition of liquid/solid adsorbent to polymeric membranes have been found to enhance the selective H₂S separation from CH₄ and CO₂-rich gas stream (Ma et al. 2021b). In this type of process using ePTFE-HFM membrane module, H₂S removal efficiency ranges from 70 to 85% using water, 90–100% using NaOH, and 94–100% using amine solution contactors (Marzouk et al. 2012). However it should be noted that the use of liquid contactors for enhanced H₂S removal using membrane can cause fouling issues and reduce gas flux (Ahmed et al. 2022). Nevertheless, membrane techniques have commercial applications for natural gas sweetening and biogas upgrading for several decades, using materials such as cellulose acetate-based Cynara®/Cameron (SLB 2023) and Separex™/UOP, and polyimide-based MEDAL™/Air Liquide.

3 Biochar vs. hydrochar: production technologies and properties

3.1 Biochar production

Thermochemical processes are commonly used to produce biochar from various biomass feedstocks. While pyrolysis, gasification, and torrefaction are used primarily for producing biochar, hydrothermal processes such as hydrothermal liquefaction and hydrothermal carbonization are used to produce hydrochar. Enormous reviews on char production technologies are published elsewhere (Adeniyi et al. 2023; Fang et al. 2023; Ighalo et al. 2023; Li et al. 2023; Masud et al. 2023; Patel and Panwar 2023; Rathnayake et al. 2023; Roshan et al. 2023; Safarian 2023; Sharma et al. 2023; Supraja et al. 2023; Tawfik et al. 2023; Venkatachalam et al. 2023; Wang et al. 2023; Wu et al. 2023; Yuan et al. 2023; Zakaria et al. 2023). Biochar refers to the solid product obtained from the thermochemical decomposition of biomass resources and generally has stable carbon structures. Pyrolysis is widely acknowledged as one of the primary routes for producing biochar under an inert atmosphere at temperatures typically between 300 and 700 °C. The oxygen-free inert atmosphere prevents carbon combustion in biomass, facilitating a higher degree of carbonisation (Malyan et al. 2021). Pyrolysis is operationally categorized into three types: (i) slow, (ii) fast, and (iii) flash pyrolysis, depending on the process conditions, such as temperature, heating rate, residence time, and feed particle sizes (Sharma et al. 2024). However, other various forms of pyrolysis techniques, such as microwave-assisted pyrolysis, hydro pyrolysis, autothermal pyrolysis, catalytic pyrolysis and co-pyrolysis have emerged with the aim of efficient feed conversion and enhanced product recovery (Vuppaladadiyam et al. 2022). Product distribution during pyrolysis largely depends on the operational conditions and feed materials in all these techniques. For instance, the typical slow heating rate and longer residence time in the slow pyrolysis process favours biochar production, while fast and flash pyrolysis, characterised by a fast heating rate and short residence time favours bio-oil production.

Dry torrefaction, or mild pyrolysis, occurs in the temperature range of 200–300 °C and residence time varies from 30 min to 4 h (Table 1). During dry torrefaction, only 10% of energy contained in biomass is lost in the form of gases and overall, a mass loss of ca. 30% occurs. It is worth noting that torrefaction improves biomass's physical and chemical properties and leads to improved energy density, better ignition, less moisture, better grindability, high C/H and C/O ratios and hydrophobicity (Mamvura and Danha 2020). Dry torrefaction can be done under oxidative (in the presence of air/oxygen) or non-oxidative (inert conditions). Although solids yield is low in oxidative environments, higher reaction rates are expected, resulting in a shorter torrefaction duration (Chen et al. 2021b). Gasification is a high-temperature thermochemical conversion process in a partly oxidizing environment created by adding air, oxygen, steam, carbon dioxide, or other oxidising agents. Gasification is mainly employed to generate syngas as the main product; however, a small fraction of char is produced (Libra et al. 2011; Patel et al. 2020). Table 1 summarises the operational parameters and typical product yields for the various thermochemical techniques for biochar production.

Table 1 Operational parameters and yields of biochar production technologies (Kambo and Dutta 2015; Lee et al. 2020; Marcilla et al. 2013; Patel et al. 2020; Raheem et al. 2018)

3.2 Hydrochar production

Hydrothermal technologies employ liquid media to convert organic substrate, and in the process, four product streams are generated (hydrochar, biocrude, syngas, and aqueous product). Hydrothermal techniques are promising for processing wet feedstock, as energy-intensive feedstock drying is unnecessary and could be eliminated. Hydrothermal technologies are broadly categorized into hydrothermal carbonization, hydrothermal liquefaction, and supercritical water gasification based on operating conditions (Masoumi et al. 2021). While hydrothermal carbonization is primarily used to produce hydrochar, hydrothermal liquefaction and supercritical water gasification are used to produce biocrude and syngas as primary products, respectively, and hydrochar as a co-product (Marzbali et al. 2021).

Hydrothermal carbonization (HTC) is a low-temperature process, usually carried out below water critical temperatures between 180 and 250 °C under autogenous pressure, and residence time can vary from minutes to several hours (Gupta et al. 2020). Depending on the operational conditions, the product distribution in HTC is 2–5% gas, 40–70% hydrochar and 10–20% biocrude oil (Shyam et al. 2022). HTC comprises three stages, dehydration, decarboxylation, and decarbonylation, and can be used to treat diverse wet organic wastes such as food waste (Periyavaram et al. 2023), sewage sludge (Liu et al. 2021b), fruit waste (Chen et al. 2017a) etc. Product formation, properties, and composition depend on the process parameters and initial feedstock composition (Funke and Ziegler 2010; Libra et al. 2011). Carefully controlling operational parameters and feed may result in highly functionalized carbon materials (Biller and Ross 2016). By decreasing the H/C and O/C molar ratio, the HTC process promotes dehydration and decarboxylation reactions, producing hydrochar with characteristics close to coal (Fang et al. 2018). The mechanism of hydrochar formation from the thermal decomposition of complex organic biopolymers (carbohydrate, proteins, and lipid) into simple intermediate compounds (sugar, amino acid, and fatty acid) followed by series of chemical reactions of the fragements via amidation, hydrolysis, deamination, Millard reaction, dehydration, aromatisation, retro-aldol condensation, among others, during HTC has been summarised elsewhere (Le et al. 2022).

Hydrothermal liquefaction (HTL) is usually carried out in temperatures between 250 and 360 °C, under autogenous water pressures between 10 and 25 MPa to produce solid, liquid, and gaseous product streams. Though HTL produces liquid fuel called biocrude as a major product fraction, hydrochar with high ash content and gases is also produced (Gollakota et al. 2018). Different chemical compounds, including aromatic, aliphatic, phenolics, esters, carboxylic acids, and nitrogenous structures, are available and are distributed in the liquid, solid, and gas phase products. Hydrochar from the HTL process results from recombination reactions in the aqueous phase, and the formation of hydrochar is highly influenced by the partial carbonization reaction of organic molecules released during hydrolysis (Kassim et al. 2022). Supercritical water gasification (SWG) occurs under supercritical conditions at temperatures greater than 375 °C to produce mainly syngas with a higher H₂ concentration than conventional gasification (He et al. 2014). SWG is a high-temperature steam reforming process that involves two steps; (i) the hydrolysis of feedstock in supercritical water and (ii) further gasification of aqueous oligomers produced in the previous step. Since the solubility of water under supercritical conditions is high, the intermediate products produced during biomass decomposition are dissolved, resulting in a remarkable reduction in the tar and coke content (He et al. 2014). Unlike conventional thermal gasification, which is carried out at elevated temperatures (> 800 °C), SWG is carried out at relatively low temperatures (< 600 °C). Table 2 summarises the operational parameters and product yields for the various hydrothermal techniques for hydrochar production.

Table 2 Operational parameters and product yields of various hydrothermal technologies (Chen et al. 2017a; Gollakota et al. 2018; Masoumi et al. 2021; Patel et al. 2020)

3.3 Properties of biochar vs. hydrochar

3.3.1 Physical properties

Although processes such as gasification, HTL and SWG can produce char, the yields of the processes are not significant, as shown in Tables 1 and 2. Therefore, this section focused on the properties of biochar and hydrochar produced via slow pyrolysis and HTC processes, respectively. The fibre and biochemical compositions of the feedstock directly influence the properties of char produced from the thermal treatment processes. In thermal and hydrothermal processes, with the increase in temperature, the organic components of the feedstock decompose via a set of chemical reactions, leaving a solid residue product with modified surface properties. Feedstock type and operational parameters, such as temperature, feed particle size, heating rate, residence time, and reactor configuration, significantly influence the char properties (Vuppaladadiyam et al. 2023a, b). The higher heating value, carbon content, and specific surface area (SSA) of biochar increase at high temperatures and heating rates at the expense of biochar yield (Ahmad et al. 2021a). Similarly, increased carbon content, fixed carbon content and energy density were reported for hydrochar with increased HTC temperature (Zhang et al. 2019b).

The surface properties of char include SSA, pore volume and pore size. While the SSA is the main indicator of the sorption interphase on the char, pore size and volume could influence effective solid–gas interactions and exchange between the active sites and reactants over the biochar surface (You et al. 2017). In the case of slow pyrolysis, treatment temperature and residence time are the most influential parameters on biochar yield and properties. With the increase in temperature, the volatile content of the biochar decreases and the fixed carbon content increases. The pyrolysis temperature also plays an important role in defining biochar's microstructure and surface properties. Zhao et al. investigated the influence of temperature on apple tree branches-derived biochar properties. The authors noted that the SSA and pore volume of biochar increased with the increase in pyrolysis temperature and had the highest SSA of 108.59 m² g⁻¹ and pore volume of 0.059 cm³ g⁻¹ at 600 °C (Zhao et al. 2017). Even though an increase in residence time is known to increase the SSA, the effect of residence time on enhancing SSA is less pronounced than the influence of temperature. Results reported in recent studies confirm an increase in the SSA of biochar with an increase in pyrolysis temperature (Jin et al. 2020; Kim et al. 2012; Weber and Quicker 2018). Conversely, hydrochar obtained via HTC generally has a very poor surface area and porosity (Ighalo et al. 2022b). Similar to observations for pyrolysis-derived biochar, the SSA and pore properties of hydrochar also increase with an increase in the reaction temperature (Ighalo et al. 2022a). In addition to reaction temperature, residence time also influences the SSA and pore properties of hydrochar. According to Garlapalli et al., higher residence time would facilitate the devolatilization and polymerization of organic components in the feed and improve the pore properties (Garlapalli et al. 2016). Table 3 presents the structural properties of biochar and hydrochar derived from various biomass sources. It can be noticed from Table 3 that the SSA of biochar and hydrochar vary over a wide range. While for biochar, the SSA varied from 6 to 1495 m² g⁻¹, for hydrochar, it varied from 3 to 169 m² g⁻¹, indicating that the SSA is higher in the case of the former than the latter.

Table 3 Summary of structural properties of biochar and hydrochar derived from various biomass feedstocks

The hydrologic properties of char are crucial when it is used as an adsorbent to remove acidic gases, such as H₂S. The availability of surface functional groups and a change in porosity during production could define the hydrologic properties of char. While the decrease in functional groups could alter the affinity of biochar to water, an increase in porosity may influence the amount of water that can be adsorbed. Increasing temperature could increase the aromaticity and hydrophobicity of the biochar due to the removal of polar surface functional groups, which is reflected in the low O/C ratio (Kharel et al. 2019). In a study by Pimchuai et al. the authors reported that an increase in the temperature leads to the production of more hydrophobic char. The moisture content of the biochar was significantly lower compared to the raw feedstock. For instance, when sawdust and sawdust-derived biochar were immersed in water for 2 h, the moisture rise with the former was 150.33%, while the moisture rise in the latter was a meagre 2.16% (Pimchuai et al. 2010). However, contradicting results have been reported on the influence of temperature on the hydrophobicity of biochar.

Zornoza et al. produced biochar at 300 °C from cotton crop residue and swine manure, which displayed high hydrophobic characteristics. When the pyrolysis temperature was raised to more than 500 °C, the resulting biochar did not display any hydrophobicity (Zornoza et al. 2016), adhering to results reported in the literature (de Jesus Duarte et al., 2023; Suansa et al. 2021). However, this does not indicate that biochar is hydrophilic when produced at higher temperatures. They can be considered less hydrophobic due to the absence of non-polar functional groups, such as aliphatic, that may be destroyed at temperatures above 400 °C. On the other hand, biochar’s water-holding capacity depends on the porosity and interconnectedness of the pores. Therefore, biochar produced at a higher temperature, though hydrophobic, are expected to hold more moisture in its pore structure. On the other side, hydrochar is slightly less hydrophobic when compared to biochar and the surface hydrophobicity of hydrochar decreases with increasing temperature. The destruction of the surface functional groups with rising temperature could be recognized as a plausible reason for the decreasing hydrophobicity (Cheng et al. 2022; Zhang et al. 2021). However, when compared to biochar, hydrochar retains significant oxygenated functional groups after the hydrothermal process, and the oxygenated functional groups being hydrophilic, hydrochar is relatively less hydrophobic than biochar.

3.3.2 Chemical properties

In addition to the breakdown of the fibrous structure in biomass leading to changes in physical properties, carbonization of biomass could also lead to changes in chemical properties in char. It is worth noting that the physical properties may influence the chemical properties of char. For instance, polycyclic aromatic hydrocarbons (PAHs) can be correlated with the biochar's micropore volume and large surface area. Similarly, minerals' dispersion could influence the biochar morphology (Yip et al. 2010). Biochar and hydrochar are carbon-rich materials, and their main organic contents can typically be assessed using C, H, N, S, and O compositions. Alongside organic elements, biochar contains inorganic elements such as Ca, Mg, P, Si, Al, Fe, and K as well as trace metals (Cu, Ni, Zn, Co, Mn). While the high carbon content defines the quality of char material, the N/C, H/C and O/C are considered a measure of the char's recalcitrancy, aromaticity and biochemical stability, respectively (Rangabhashiyam et al. 2022). Several studies reported the elemental compositions of biochar and hydrochar. Mineral content in the char could influence its pH, surface chemistry and porosity. Biochar produced from high ash-containing feedstocks and at high temperatures generally has a pH above 7 (alkaline). It is believed that the alkaline earth (Ca, Mg) and alkali (Na, K) minerals are mainly responsible for the high pH of such biochar (Bamdad et al. 2018). However, in a recent study, Xu et al. noticed that sludge-derived biochar had a higher mineral content than wheat-straw-derived biochar, but the pH was lower for sludge biochar. The authors attributed the low pH to the abundant availability of Fe in sludge biochar, which may have caused hydrolysis to produce H⁺ in the solution (Xu et al. 2016). Therefore, it could be understood that the mineral composition rather than the total ash content influences the pH of biochar. In addition to the mineral content, surface functional groups like carboxyl and hydroxyl groups contribute to low biochar pH, particularly when produced at low temperatures. At higher temperatures, acidic functional groups, such as formyl, carboxyl, or hydroxyl groups, get destroyed, promoting the separation of minerals such as NaOH, CaCO₃, KOH and MgCO₃ from the solid carbon matrix, leading to higher pH values (Xu et al. 2017). Generally, the ash content in the feedstock is directly connected to the percentage of metal species in the char. Demineralization of inorganic composition is common in HTC, as the process is carried out in the presence of a solvent that solubilises the mineral matter into the aqueous phase product. Hydrochar produced via HTC is generally acidic due to the availability of more acidic functional groups than that present in biochar (Fei et al. 2019). In a recent study by Liu et al., the authors compared the oxygenated functional groups of pinewood-derived hydro/biochar. They noticed that the oxygenated functional groups in hydrochar were 340% higher than in biochar (Liu et al. 2020a). In addition, the pH of hydrochar depends on the feedstock. For instance, the pH of hydrochar obtained at 180 °C via HTC of pure cellulose was higher than hydrochar from woody biomass (Saha et al. 2019). Organic content in HTC's effluents may get deposited on the surface of the hydrochar, which could influence its pH when the solids are not properly washed.

The surface functional groups play an essential role in removing gaseous pollutants as they can facilitate the chemical adsorption (in most cases) of gaseous pollutants and stabilize them in biochar. Oxygen and nitrogen-containing functional groups are considered major active sites for the adsorption process (Sajjadi et al. 2019). Oxygenated functional groups include carbonyl, carboxyl, chromene, hydroxyl, ketone, lactone, phenol, and pyrone functional groups (Saha and Kienbaum 2019). The surface oxides may be derived from the feedstock or formed during the conversion process or following an activation process that involves oxidation. While chromenes, ketones and pyrones contribute to the basicity of the char, carbonyl, carboxyl and hydroxyl contribute to the acidity. However, the carboxyl and hydroxyl groups react with metallic cations to form salts, contributing to the alkalinity (Sajjadi et al. 2019). Nitrogen-containing functional groups are usually added to the biochar by treating biochar with agents such as amine, ammonia, nitric acid, and urea. Most common N-containing functional groups include amine-N, pyridinic-N, pyridinic-N-oxide, pyrrolic-N, etc. (Feng et al. 2021). Both biochar and hydrochar are composed of aromatic and aliphatic structures with different surface functional groups.

4 H₂S adsorption: mechanisms and processes

4.1 Adsorption mechanism: biochar vs. hydrochar

As highlighted in the previous section, biochar/hydrochar have highly heterogeneous compositions and physicochemical properties largely driven by process conditions and feed type. Therefore, the H₂S removal mechanism on char materials is a complex process that can involve the formation of various sulfur products. It is worth noting that adsorptive desulfurization on carbon material undergoes physical and weak chemical adsorption and, therefore, is more suitable at low temperatures. Generally, H₂S removal using carbon materials involves two steps; (i) physisorption step: Gaseous H₂S is adsorbed on the surface of the carbon material, dissolved in the H₂O film, and gets dissociated in the adsorbed state, as shown in Eq. 5. (ii) Oxidation step: In the presence of water and metal impurities, the adsorbed H₂S reacts with O₂ to form elemental sulfur (S⁰) and sulfur dioxide and sulfates. Adib et al. (Adib et al. 2000) proposed a mechanism for H₂S oxidation (Eqs. 3–6) at low temperatures, which was later extended to alkaline carbons (Eqs. 6–9) by Yan et al. (2002). The basic reactions are as given below.

$$H_{2} S_{gas} \mathop{\longrightarrow}\limits^{{K_{H} }}\,H_{2} S_{ads}$$

(3)

$$H_{2} S_{ads} \mathop{\longrightarrow}\limits^{{K_{s} }}\,H_{2} S_{ads - liq}$$

(4)

$$H_{2} S_{ads - liq} \mathop{\longrightarrow}\limits^{{K_{a} }}\,HS_{ads}^{ - } \, + \,H^{ + }$$

(5)

$$2HS_{ads}^{ - } \, + \,O_{ads}^{*} \mathop{\longrightarrow}\limits^{{K_{R} }}\,2S_{ads} \, + \,OH^{ - }$$

(6)

$$HS_{ads}^{ - } \, + \,3O_{ads}^{*} \mathop{\longrightarrow}\limits^{{K_{R} }}\,SO_{2ads} \, + \,OH^{ - }$$

(7)

$$SO_{2ads} \, + \,O_{ads}^{*} \, + \,\,H_{2} O\,\,\mathop{\longrightarrow}\limits^{{K_{R} }}\,SO_{4ads}^{2 - } \, + \,2H^{ + }$$

(8)

$$H^{ + } \, + \,OH^{ - } \mathop{\longrightarrow}\limits^{{K_{R} }}\,H_{2} O$$

(9)

where $H_{2} S_{gas}$, $H_{2} S_{ads}$ and $H_{2} S_{ads - liq}$ are H₂S in gas adsorbed and liquid phases, respectively; K_H, K_a, K_s and K_R are equilibrium constants for adsorption, gas dissociation, solubility, and surface reaction process, respectively. $O_{ads}^{*}$ represents dissociative adsorbed O₂.

Several researchers expanded the reaction mechanism by studying different biochar under different conditions (Fig. 2). Leuch et al. extended the adsorption mechanism proposed by Adib et al. under dry conditions, which can be found elsewhere (Yan et al. 2002). Similarly, Bagreev and Bandosz suggested a mechanism for the oxidation of H₂S on carbon material and according to the authors, H₂S is not adsorbed on the carbon surface, instead the carbon surface is expected to play an important role in the oxidation of H₂S. According to Chowdary and Lansing, biochar rich in mineral contents can promote H₂S adsorption, influence the final sulfur products, and change the products from sulfuric acid to soluble and insoluble metal oxides (Choudhury and Lansing 2021). In addition, these minerals act as catalysts in the catalytic oxidation of H₂S to elemental sulfur and sulfur oxides (Bagreev and Bandosz 2005). Hervy et al. (2018a) found that the adsorption of H₂S is closely related to the properties of biochar, which includes surface area, basic O-containing functional groups, pore volume, mineral species, and disorganised carbon structure (Hervy et al. 2018a). Under dry and oxygen-deprived conditions, H₂S may directly react with the metals available in the ash (Eq. 10) to form metal sulfides. Further oxidation of metal sulfur with organic and inorganic species results in the formation of metal sulphate.

$$xH_{2} S_{gas} \, + \,MO_{x} \, \rightleftharpoons MS_{x} \, + \,xH_{2} O$$

(10)

Fig. 2

Physical and chemical adsorption mechanisms involved during the adsorption of H₂S onto biochar

The availability of surface functional groups, particularly O-containing functional groups, could promote H₂S adsorption by substitution (Eq. 11) and oxidation reactions (Eq. 12).

$$C\,(O)\, + \,H_{2} S_{gas} \, \rightleftharpoons \,C(S)\, + \,H_{2} O$$

(11)

$$C\,(O)\, + \,H_{2} S_{gas} \, \rightleftharpoons \,C_{free} \, + \,\,S_{sol} \, + \,SO_{x} \, + \,H_{2} O$$

(12)

where C(O) and (C_free) are the active and free carbon sites. Further, the defects in carbon structure can provide free carbon sites that could react with H₂S according to Eqs. (13) and (14).

$$C_{free} + \,H_{2} S_{gas} \,\, \rightleftharpoons \,C\left( {H_{2} S} \right)_{ads}$$

(13)

$$\,C\left( {H_{2} S} \right)_{ads} \, + \,H_{2} S_{gas} \, \rightleftharpoons C\left( {S_{x} H_{y} } \right)_{ads} \,$$

(14)

The mechanism of adsorption of H₂S with hydrochar is presented in Fig. 3. It is worth noting that literature demonstrating the H₂S adsorption mechanism in the case of hydrochar is sparsely reported. According to Hu et al. (2024) elemental sulfur is the primary product of the desulfurization process, followed by sulfuric acid and sulfonic acid as by-products, as shown in Fig. 3.

Fig. 3

Illustration of H₂S removal mechanism of activated hydrochar. Adapted with permissions from (Hu et al. 2024)

Under wet conditions, the authors noted that a water film is formed on the surface of the hydrochar, and the following reactions were reported:

$$H_{2} S_{gas} \to \,H_{2} S_{ads}$$

(15)

$$H_{2} S_{ads} \to \,\,H_{2} S_{ads - liq}$$

(16)

$$H_{2} S_{ads - liq} \to \,\,HS_{ads}^{ - } \, + \,H^{ + }$$

(17)

$$O_{2gas} \to \,\,O_{2ads} \to \,O_{2ads - aq}$$

(18)

$$O_{2ads} \, + \,\,e^{ - } \, \to \,\,O^{* - }$$

(19)

$$HS_{ads}^{ - } \, + \,\,O^{* - } \,\, \to \,\,S_{ads} \, + \,OH^{ - } \, + e^{ - }$$

(20)

$$S_{ads} \, + \,O^{* - } \, \to \,\,\,SO_{3}^{2 - } \, + e^{ - }$$

(21)

$$SO_{3}^{2 - } + \,O^{* - } \, \to \,\,\,SO_{4}^{2 - } \, + e^{ - }$$

(22)

$$H^{ + } \, + \,OH^{ - } \to \,H_{2} O$$

(23)

In summary, chemical adsorption occurs on the surface of the char, while physical adsorption mainly occurs in the pore structure of the char matrix. A summary of recent studies that proposed a mechanism for H₂S adsorption using biochar and hydrochar is given in Table 4.

Table 4 Recent studies elucidating the mechanism of H₂S adsorption with biochar and hydrochar

Unlike biochar, hydrochar does not have excellent structural properties, such as high surface area and porosity. The dissociation of H₂S in the water film is the rate-limiting step in adsorption. It should be noted that an acidic environment hinders the dissociation of H₂S, which leads to the availability of H₂S ions at low concentrations. In an alkaline environment, the degree of dissociation is high, which favours the formation of elemental sulfur species resistant to further oxidation. While biochar is alkaline in nature, hydrochar is acidic. Therefore, it can be expected that using biochar for adsorption may favour the formation of elemental sulfur, while using hydrochar may favor the formation of sulfur oxides and sulfuric acid (Georgiadis et al. 2020). As biochar has excellent structural properties coupled with alkaline nature and high metal content, both physisorption and chemisorption can be seen as pathways for H₂S removal. However, in the case of hydrochar, H₂S removal occurs mainly via chemisorption, which can be attributed to well-preserved surface functional groups and high metal content. However, it is worth noting that, H₂S adsorption also relies on other influential parameters, such as feedstock properties, type of metals/metal oxides and removal conditions, which are discussed in detail in the following sections.

4.2 Factors influencing H ₂ S adsorption

The potential of char in H₂S removal is closely related to feedstock type, production conditions, functionalization strategies and removal conditions. Different factors such as feedstock type and preparation factors, activation and functionalization strategies and operational conditions, including reaction temperature, co-existing gases, relative humidity and corresponding adsorption efficiency reported in the literature are presented in Table 5.

Table 5 Reported studies on H₂S removal efficiency using biochar and hydrochar under different conditions

4.2.1 Feedstock type and preparation factors

The type of feedstock considered for char production, the conversion process, and production conditions can influence the char properties and, therefore, the performance and removal mechanism of the H₂S. Lignocellulosic, aquatic, and sludge biomass are the most commonly used feedstock reported in the literature. Generally, biochar produced from lignocellulosic biomass has a well-developed pore structure compared to biochar produced from sludge (Dissanayake et al. 2020; Zielińska et al. 2015). A study done by Dieguez-Alonso et al. (2018) compared the characteristics of biochar and hydrochar generated from pine wastes. The authors noticed that hydrochar had lower SSA than biochar. However, the pore volume of hydrochar was higher than the biochar produced from both feedstocks. While micropores contribute to the majority of pores in biochar, mesopores and macropores majorly contribute to the pore volume in the case of hydrochar. In the same study, the authors compared the characteristics of hydrochar produced from different feedstocks. They noticed that hydrochar produced at 240 °C with a retention time of 360 min from pine wastes had a significantly higher SSA and pore volume than corn digestate-derived hydrochar (Dieguez-Alonso et al. 2018). In another study done by Juntarchat and Onthong on comparing biochar produced from banana peel and banana empty fruit bunches, the biochar from the latter possessed a higher fixed carbon (ca. 38%), pH (ca. 9) and SSA (3.18 m² g⁻¹) than the former. In addition, the authors investigated the impact of biochar type and pellet size on the adsorption of H₂S using synthetic biochar. They concluded that the smaller the size of the pellet, the higher the removal efficiency. The authors attributed the availability of carboxylic and hydroxide groups to the removal of H₂S (Juntarachat and Onthong 2022).

4.2.2 Activation and functionalization strategies

Activation is generally performed to improve the physicochemical properties of char and, thus, the H₂S removal efficiency. These methods involve physical activation and chemical activation. The thermal treatment of biomass results in the release of non-carbon elements, mainly hydrogen and oxygen, forming a solid carbon structure with a rudimentary pore structure, incipient pore structure and sparsely distributed functional groups. These attributes of raw bio/hydrochar could limit its potential as an adsorbent (Wen et al. 2023). However, as shown in Fig. 4, activation techniques can be used to enhance char's physical and chemical properties (such as pH, SSA, charge, porosity, and functional groups).

Fig. 4

Various widely used biochar activation and functionalization strategies

Physical activation techniques include subjecting solid adsorbent material to high temperatures under a controlled flow of reactive activating agents such as CO₂, steam, oxygen, air and/or their mixture. Physical activation improves the porosity of the material and introduces a few surface functional groups onto the char. For instance, oxygenated functional groups will be introduced using H₂O, CO₂, and O₂ activation, while N-containing functional groups may be introduced with NH₃ activation (Zhao et al. 2022b). In a recent study by Chang et al., the authors reported an increase in the activation temperature and residence time from 800 to 900 °C and 20 to 80 min, respectively, increased the SSA of corn cobs-derived biochar from 405 to 1705 m² g⁻¹ under CO₂ activation. In addition, under similar conditions, when the activation media was changed from CO₂ to steam, the SSA increased from 568 to 1063 m² g⁻¹ (Chang et al. 2000). However, it is worth noting that an increase in temperature may have a negative effect on the structural properties, mainly due to the transformation of micropores into meso- and macropores. On the other hand, as discussed previously, hydrochar generally has a poor porous structure and low SSA when compared to biochar and hence needs activation to enhance structural properties. The physical activation of hydrochar derived from various wastes, including horse manure, grass, sludge and beer wastes, was investigated by Hao et al. The authors utilized CO₂ as an activating agent and noticed a rich cluster of ultra micropores post-activation (Hao et al. 2013).

Similar to the physical activation of char, chemical activation is also recognized to enhance the structural properties of char material. Chemical activation with alkalis (KOH, NaOH), acids (HNO₃, H₃PO₄, H₂SO₄), and salts (ZnCl₂, MgCl₂, K₂CO₃) is widely reported (Wang and Wang 2019; Zhao et al. 2022b) and has been used to enhance the formation of micropores in biochar. In addition to enhancing pore structure and SSA, acid or alkali activation increases the acidic and basic groups on the surface of the biochar (Rajapaksha et al. 2016). Dissanayake et al. compared the effectiveness of sole chemical activation and chemical followed by physical activation for biochar produced from pure wood chips, wood chips, and chicken manure. The authors noticed that activation with KOH and KOH + CO₂ resulted in a ten-fold increase in the SSA of biochar, from 125.7 to 1281.6 and 1403.9 m² g⁻¹, respectively. In general, KOH activation results in biochar with a large SSA and high porosity but accompanied with high chemical consumption. While using H₃PO₄ as an activation agent results in biochar with microporous characteristics (Zhang et al. 2019a) and typical acidic functional groups, such as carboxylic and phenolic (Guo and Lua 2003), HNO₃ activation may enhance the carbon and oxygen functional groups via oxidation (Opatokun et al. 2017). Chatir et al. investigated the influence of H₃PO₄ on the structural properties of argan nut shell-derived hydrochar. The authors noticed that at an impregnation ratio of 3, hydrochar had a remarkable increase in the SSA from 6.45 to 1880 m² g⁻¹. Also, the micropore, mesopore and total pore volumes of hydrochar increased from 0.002, 0.017 and 0.019 cm³ g⁻¹ to 0.67, 0.69 and 1.36 cm³ g⁻¹, respectively, probably due to the increase in the contact area between H₃PO₄ and hydrochar (El Hadrami et al. 2022).

Plasma, ultrasonic and microwave radiation treatments were also reported to substantially enhance the pore texture and active sites in bio/hydrochar. Plasma treatment can induce new functional groups under different gas environments, thanks to the oxidative and reductive reactions that occur during the activation process (Xu et al. 2018a). Gupta et al. investigated the impact of low-temperature plasma treatment on the pore structure of pine biochar. The authors reported that plasma treatment improved the porous structure of the biochar and noticed that the SSA, pore volume and pore size increased from 440, 0.455 and 4.8 to 654 m² g⁻¹, 0.936 cm³ g⁻¹ and 4.9 nm, respectively. The authors compared plasma treatment against chemical activation and mentioned that the plasma-activated biochar contained significantly higher mesopores than its counterpart (Gupta et al. 2015). Shao et al. compared the properties of raw hydrochar and microwave-activated hydrochar produced via HTC of a feedstock comprised of green leaves and wood. The authors noticed that the structural properties, including SSA, pore volume and diameter, increased post-microwave activation. The SSA and pore volume increased by ⁓10 folds from 1.62 m² g⁻¹ SSA and 0.003 cm³ g⁻¹ pore volume (Shao et al. 2020). However, according to Yan et al., the reaction temperature could play an important role in tuning the properties of hydrochar. The authors noticed that the SSA of the bamboo-derived hydrochar at 220 °C was 5.14 m² g⁻¹, which increased to 7.81 m² g⁻¹ at 260 °C. However, with a further increase in operational temperature to 300 °C, the SSA reduced to 3.61 m² g⁻¹ (Yan et al. 2017).

Alongside modification techniques, a few functionalization strategies are also widely used to activate biochar/hydrochar and make them suitable for engineering applications. Transition metals and noble metals (Co, Ce, Cu, Fe, Mn, Ni and Ti) doping onto char was reported to greatly enhance the adsorption of H₂S through catalytic reactions. Biochar serves mainly as a catalyst support or carrier and, in some cases, participates in the removal process as an active catalyst material (Chen et al. 2017b). In a recent study by Wang et al., the authors synthesized Cu-loaded hydrochar to understand the impact of Cu valence on H₂S removal capacity. The results of the study indicated that Cu⁰-activated hydrochar had the highest H₂S breakthrough capacity (ca. 348 mg g⁻¹). Catalytic oxidation and reactive adsorption were explained as the major reasons for the high H₂S removal performance. In addition, the authors mentioned that the availability of moisture substantially enhanced the catalytic oxidation of H₂S to elemental S (Wang et al. 2022). In addition, as mentioned earlier, surface alkaline properties are generally suitable for the removal of acidic gases, such as H₂S. Functional modifications to incorporate basic sites onto char to strongly interact with acidic gases could benefit the adsorption of H₂S. In this context, ammonia modification to introduce basic nitrogen functionalities appears to be a promising approach. In a recent study by Surra et al., an attempt was made to activate biochar using anaerobic liquid digestate (ALD) and compared the H₂S removal efficiency of the ALD activation process pre- and post-pyrolysis of maize cob waste-derived char. The authors considered two cases: (i) ALD as the activation agent for raw maize cob waste, followed by pyrolysis and (ii) pyrolysis of maize cob waste followed by activation of char with ALD. The biochar produced in the former case displayed a higher H₂S removal capacity (0.47 mg g⁻¹) than the latter (0.25 mg g⁻¹) (Surra et al. 2019).

4.2.3 Operating conditions during adsorption

In addition to feedstock type, preparation conditions, activation strategies, and functionalization of char, the removal conditions, including reaction temperature, co-existence of other gases such as oxygen, carbon dioxide, and relative humidity, also play a major role in the H₂S removal process. It is worth noting that a low reaction temperature favours physical adsorption (Han et al. 2020), whereas a high reaction temperature favours chemical adsorption (Liu et al. 2020a). However, extremely high temperatures may destroy active sites and pore structures in the biochar (Zhao et al. 2019). In addition, the increasing temperature intensifies the thermal movement of gas molecules, lowering their adhesion on the surface of biochar, which in turn lowers the chemical adsorption efficiency and adsorption capacity (Ding and Liu 2020; Xu et al. 2018b). Han et al. investigated the influence of reaction temperature on the H₂S removal efficiency of two macroalgae-derived biochar (MC1 and MC2). The authors tested the effect of different reaction temperatures (25, 50, 75 and 200 °C) on H₂S removal efficiency and noticed that the breakthrough time was reduced from 10 to 6 min for MC1 and 2 to 1 min for MC2. The authors mentioned that H₂S adsorption was favoured at lower temperatures, indicating physical adsorption as a major adsorption mechanism (Han et al. 2020).

For acidic gases, low relative humidity is considered beneficial. For instance, H₂S can react with moisture to form sulfuric acid. However, a high relative humidity may facilitate the formation of a water film on the biochar surface and, impede the diffusion of gas molecules, and inhibit H₂S removal (Sun et al. 2018). Sitthikhankaew et al. studied the effect of humidity on H₂S removal efficiency of activated char. The authors tested activated carbon (AC) at 70% relative humidity and reported that the material displayed excellent H₂S removal capacity (0.284 L g⁻¹). In the same study, the authors tested the influence of the existence of co-gases, such as CO₂ and O₂ with H₂S on the removal efficiency of AC. At 70% relative humidity, while the addition of 2% O₂ increased the H₂S adsorption capacity from 0.01 to 0.059 L g⁻¹, the addition of 40% CO₂ increased the adsorption capacity from 0.01 to 0.091 L g⁻¹ (Sitthikhankaew et al. 2014).

The other crucial operational condition that could influence the removal capacity of biochar/hydrochar is the co-existence of H₂S with other gases such as O₂, CO₂, SO₂ etc. In a study by Sethupati et al., the authors reported that the H₂S removal capacity of biochar in the presence of CO₂ was low when compared to H₂S alone. They also mentioned that the similar removal mechanism of H₂S and CO₂ may have resulted in competitive adsorption (Sethupathi et al. 2017). Qin et al. used sludge-derived biochar to remove H₂S from MSW landfill-generated biogas, which contained volatile organic compounds (VOCs), and NH₃. The authors observed a removal efficiency of 95–100%. Huang et al. (2022) studied the influence of the co-existence of O₂, H₂O, and CO₂ with H₂S on H₂S removal capacity of food digestate-derived biochar. The authors reported a removal capacity of 1.75 mg g⁻¹ for dry biochar in a pure H₂S environment. However, the removal capacity increased to 4.29, 5.29 and 12.50 mg g⁻¹ for dry biochar in H₂S + O₂, humid biochar in pure H₂S and humid biochar in H₂S + O₂ environments, respectively. The authors reported that the removal of H₂S under dry conditions was due to the high mineral content in the biochar. However, with the co-existence of CO₂ in the gas stream, the H₂S adsorption decreased, possibly due to the physisorption competition with CO₂.

5 Potential integration options and circular economy solutions for wastewater treatment plants

A possible trade-off between biogas and char sectors conceptualized from the literature survey conducted in this study is presented in Fig. 5. The system can be divided into three sectors: wastewater treatment plants (WWTPs), thermal processes (pyrolysis and HTC), and biogas (including anaerobic digestion and other subsidiaries). As discussed in previous sections, adsorption using biochar/hydrochar could be considered as one of the potential routes to remove H₂S from biogas. From an economic point of view, the production of biochar/hydrochar from waste feedstock can be cost-effective compared to commercially available adsorbents, such as activated carbon or MOFs. Integration of wastewater treatment/anaerobic digestion with thermal processes, including pyrolysis, gasification and HTC, can offer an efficient approach to cost-effective management of waste, such as sewage sludge, biosolids, and alum sludge (Cho et al. 2023). As shown in Fig. 5, char can be produced from biosolids in three ways. Case (i) demonstrates the production of hydrochar, where sewage sludge or undigested biosolids from WWTP can be directly used as feedstock or direct use of the digestate from the AD process to produce hydrochar. Since HTC requires wet organic feedstocks, case (i) can handle unstabilised and non-dewatered solid streams to convert into hydrochar at 180–250 °C. Case (ii) involves the production of biochar via gasification of digested/stabilised AD residues at 600–900  °C under partly oxidising condition, and case (iii) involves the production of biochar from the pyrolysis of digested/stabilised AD residues at 400–700 °C under oxygen-limited environment. It is worth noting that the production of biochar in case (ii) and case (iii) require an energy intensive drying step before the digestate is feed into process. This is because the solids content and calorific value of the feestock is critical in pyrolysis and gasification, particularly from a thermal energy balance perspective.

Fig. 5

Possible concept for a sustainable circular economy between biochar/hydrochar production, anaerobic digestion, and biogas upgradation within water utility

Once the char is produced, it could be further activated chemically or physically and/or impregnated with metals to produce activated biochar/hydrochar with enhanced structural properties. Both raw and modified char can be added to an anaerobic digestion unit as additives to boost biogas production (Ahmad et al. 2021a; Masebinu et al. 2019; Zhang et al. 2019b) or can be used as an adsorbent for the continuous removal of pollutants such as CO₂, H₂S and siloxanes from the biogas stream, termed the biogas upgradation process. The pollutant-free clean biogas is then stored/bottled and used for different mobile and stationary applications. Integrating char production with wastewater treatment process and the onsite application of the produced char as additives in AD systems, adsorbent for wastewater purification, and biogas desulfurisation can offer many environmental and economic benefits, such as:

• i. thermal treatment of sewage sludge and final biosolids to produce biochar/hydrochar provides huge potential for contaminant destruction, volume reduction, and odour elimination (Hakeem et al. 2024; Patel et al. 2020).

• ii. use of biosolids for char production can allow for maximum recovery of nutrients (N, P, K) and organic carbon, reduce GHG emissions associated with biosolids stockpiling and composting, and reduce risks of diffuse contaminants entry into the environment (air, soil and water) through the traditional land application of biosolids (Nahar et al. 2024; Elgarahy et al. 2024).

• iii. use of char materials as additives in AD process can improve the system stability through pH buffering, microbial sheltering, ammonia inhibition, micronutrient supplements, etc., thereby increasing the overall biomethane yield and improving sludge dewaterability (Hassan et al. 2023; Hoang et al. 2022).

• iv. adding biochar to AD systems can facilitate in-situ adsorption of H₂S and other trace gas pollutants, control odour, and produce contaminants-free and biomethane-rich biogas stream (Choudhury et al. 2020; Liu et al. 2021c).

• v. char produced (raw or modified) from wastewater treatment residues can be used as partial replacement for costly activated carbon in wastewater and biogas purification for micropollutant (H₂S, siloxanes, CO₂, PFAS, heavy metals, dyes, pharmaceuticals, etc.) removal (Patel et al. 2023; Enaime et al. 2020).

6 Comparison of techno-economic feasibility and technology readiness level for selective H₂S removal

The technologies for the removal of H₂S are industrially advanced, with several commercial installations traditionally used for gas sweetening. Broadly, the method for H₂S capture can be classified into physicochemical techniques and biological techniques, which can involve dry or wet desulfurisation methods. Each of these technologies uses a range of materials, has different removal efficiency, and presents different advantages and disadvantages (Pudi et al. 2022), as shown in Table 6. The technological performance of the various H₂S removal technologies from biogas is summarised in Table 6. From Table 6, each technique has its strengths and limitations concerning H₂S removal efficiency, operating conditions, process selectivity, materials/solvents requirements and commercial application.

Table 6 Advantages and disadvantages of H₂S removal techniques

For example, adsorption using metal oxide adsorbents is the most established technology on a commercial scale for the fine removal of H₂S, while chemical absorption using amine solvent is most industrially matured for bulk removal of CO₂ and H₂S simultaneously. However, the performance and techno-commercial feasibility of H₂S adsorption and absorption can change significantly when using different solvents and materials, biogas compositions, and operating conditions. All technologies have their own specific merits and demerits. No technology offers optimal solutions to every biogas upgrading situation and requirement. The choice of an economically optimal technology will strongly depend on the volume and quality of the raw biogas to be treated, the desired biogas purity and the final utilisation of the gas. The configuration and operation of the anaerobic digestion process can aid the easy integration of a particular H₂S removal technology. For instance, biological and membrane techniques might be easy to adopt as in-situ H₂S removal technology within the existing wastewater treatment plants as these types of technology are common with wastewater and sludge treatment processes. New WWTP installations might consider adsorption and absorption technology for onsite biogas upgrading. However, the type and management of generated spent substrates/residues and local environmental regulations can guide the appropriate choice of an H₂S removal technology. An attempt has been made to provide a relative comparison of the H₂S removal technology across various indicators as shown in Table 7.

Table 7 General relative comparison of the performance of common techniques for H₂S removal (Chuang et al. 2000; Ghimire et al. 2021; Pikaar et al. 2015; Pudi et al. 2022; Sun et al. 2015)

There are no comprehensive comparative studies on the various technologies for H₂S removal under the same/similar conditions. As such, there are discrepancies about the specific capital, operation and maintenance costs, biomethane loss, solvents, and energy requirements for the different technologies at varying treatment scales. In a previous review conducted by Sun et al. (2015), where the energy efficiency and techno-economic analysis of five different biogas upgrading technologies were reported, there was no significant difference in the capital cost of technology investment. The capital expenditures are closely related to the plant capacity, and the larger the plant, the lower the CAPEX for treating a unit volume of biogas. However, energy requirements were observed to vary across the five technologies. For example, the energy consumption in kWh per Nm³ of treated biogas was reported as: 0.45–0.90 for water scrubbing, 0.8–1.54 for cryogenic separation, 0.49–0.67 for physical absorption, 0.12–0.44 for chemical adsorption, 0.3–1.0 for pressure swing adsorption, and 0.25–0.43 for membrane technology. In all these technologies, the energy efficiency ranges from 82 to 98%, the lowest for pressure swing adsorption and the highest for water scrubbing (plus regeneration). However, the operating and maintenance expenditures associated with chemical absorption and cryogenic separation were comparatively higher due to solvent cost and loss on regeneration in the case of chemical adsorption, and low energy efficiency in the case of cryogenic technology. The data presented in Table 7 is for guidance only based on qualitative assessment, detailed comparative studies are required to validate the data. The technology readiness level is based on the best optimum technology using commercially tested materials/solvents, and it can differ if adsorbent/solvents are changed. For instance, while the TRL of adsorption techniques using activated carbon and metal oxides is high, as they have been proven industrially, the TRL of the same adsorption technique using biochar materials will range from low to medium. A similar trend will be observed for the absorption technique using liquid amine against alkali and ionic liquid solvents.

7 Challenges and future research perspectives

7.1 Feedstock availability and quality

Biochar and hydrochar have excellent potential as adsorbents, however, several research gaps still need to be addressed. The first and most significant challenge is the availability of suitable feedstock for char production, which can be later used as an adsorbent for H₂S removal. Based on the extensive literature analysis conducted in this study, it is worth mentioning that the feedstock type and quality could significantly influence the properties and effectiveness of char. Ensuring a reliable and stable supply of feedstock is challenging, particularly in areas with limited biomass availability. This presents an opportunity for water utilities and waste treatment facilities to use wastes, such as municipal solid wastes or biosolids, as feedstock for biochar production. In addition, biochar production is an energy-intensive process, which may require significant investment in infrastructure and equipment. This further hinders biochar production in regions with limited resources. Biochar is generally produced from residual biomass such as woodchips, crop residues, biosolids, and municipal solid wastes. Conventional kiln-derived biochar produced from impure feedstock may contain harmful contaminants, such as heavy metals, polycyclic aromatic hydrocarbons, per- and polyfluorinated compounds, and volatile organic compounds. Improper disposal of conventionally produced char post adsorption would negatively impact the ecosystem. Therefore, careful selection of feedstock or use of sophisticated reactors is recommended to avoid unanticipated entry of pollutants into the natural environment.

7.2 Understanding of mechanisms and co-existence of other pollutants

Although H₂S adsorption with biochar has been extensively investigated and reported in the literature, there is still a research gap for hydrochar adsorption mechanism. The H₂S adsorption mechanism for hydrochar is sparsely available in the literature and research efforts are warranted in this area. For both hydrochar and biochar, the co-existence of other gases or pollutants could increase the complexity of the adsorption process. It is worth noting that the co-existence of multiple organic and inorganic pollutants may compete for the active spots on the biochar and therefore, would enhance/limit the adsorption process. Under instances where H₂S is selectively adsorbed, tailoring biochar properties for selective H₂S adsorption can be challenging. In addition, most of the literature on using char/activated carbon for H₂S adsorption is mainly conducted using feed gas that contains H₂S mixed with N₂ or air. However, these conditions do not represent the real case scenario. For instance, it is relatively easier to separate H₂S from air than from biogas as the former contains O₂, which assists in the oxidation reaction (Ahmad et al. 2021b). Even though, biochar is reported to have high adsorption efficiencies, it cannot be concluded in general that similar high efficiencies can be noticed with real biogas stream. Efforts need to be made in this regard to understand the true potential of biochar in real-time applications for biogas desulfurization. In addition, molecular simulation could play an important role in understanding the fundamentals of adsorption on carbon materials. However, much research is needed in developing novel kinetic models to investigate adsorption mechanisms and to model adsorption systems.

7.3 Commercialisation of technology

The major concern during the development of biochar-based adsorption technology is the commercial viability of the technology. Predominantly, the commercial sustainability of using biomass/waste-derived adsorbent for H₂S adsorption is still under research. Lifecycle analysis (LCA), together with environmental impact assessment (EIA) and techno-economic analysis (TEA), could provide a detailed understanding of the commercial feasibility of biochar-based H₂S adsorption technology. It is worth noting that a decision made after taking into consideration the understandings from LCA, TEA, and EIA would be a strong indicator for investors to transition from commercial adsorbents to biochar/hydrochar. Using biochar for H₂S adsorption could support sustainable development by recycling and reusing waste resources. The spent biochar sorbent obtained from the H₂S removal process can be used as a soil fertilizer as it is rich in elemental sulfur (Zhang et al. 2017). Cost analysis for using biomass/waste-derived biochar/hydrochar as a replacement for commercially available adsorbents is not reported in the literature. Most of the studies reported in the literature are done at a lab-scale and the char is used without cost justification for modification or activation. It is worth noting that cost is an important parameter for commercialization of any technology and detailed cost-based comparisons between commercial and waste-based adsorbents are currently a research gap that needs to be addressed. Furthermore, the link between biomass/waste-derived char for environmental applications, such as adsorption of H₂S, circular economy and sustainable development goals (SDGs) lies in their collective response to promote sustainable practices and contribute to SDGs. Using char for H₂S adsorption aligns with several SDGs listed below:

• SDG 7- Affordable and Clean Energy: Using biomass or waste as a renewable resource to produce biochar could contribute to the production of clean energy.

• SDG 12-Responsible Consumption and Production: Converting biomass waste to valuable products such as biochar promotes responsible production and encourages sustainable consumption and production.

• SDG 13-Climate Action: Biochar’s carbon sequestration potential assists in mitigating climate change and attaining carbon reduction targets.

• SDG 15-Life on Land: Application of biochar to soil could promote biodiversity, combat land degradation, and safeguard ecosystems.

7.4 Regeneration of biochar

Besides the adsorption capacity, the feasibility of using biochar at a large scale also depends on its regenerability (Bamdad et al. 2019). Spent biochar can be recycled several times via various desorption processes before being finally disposed. Several studies have reported the regeneration of biochar; however, most of them are focused on thermal desorption methods. A few studies reported the regeneration of spent sorbents after use in gas phase adsorption. However, only a handful of studies were identified in our literature analysis that reported the regeneration of spent char after the adsorption of H₂S (Chen et al. 2021a; Yuan et al. 2021). However, contradicting results were reported in the literature regarding the adsorption capacity of biochar after repetitive regeneration of spent sorbent (Fig. 6). Yuan et al. evaluated the H₂S adsorption potential of biochar produced from leftover rice. In their study, the authors used the calcination process to regenerate spent sorbent and subsequently evaluated its regeneration potential. They noticed that the regenerated biochar attained 91.6% of initial breakthrough capacity after five regeneration cycles (Yuan et al. 2021). On the contrary, in a study done by Chen et al. using activated biochar derived from microalgae biomass for H₂S adsorption, the authors reported that the performance of biochar declined rapidly after two to three cycles of regeneration (Fig. 6). The authors simply purged air for 12 h at room temperature to regenerate the spent sorbent. The difference in the breakthrough capacity, as seen in Fig. 6, indicates the importance of choosing proper regeneration techniques. 6

Fig. 6

Breakthrough capacity and regeneration cycles of (a) and (b) microalgae-derived biochar regenerated via purging air at room temperature and (c) leftover rice-derived biochar regenerated via calcination

In the studies done on using biochar for the adsorption of other acidic gases, such as CO₂ and SO₂, the authors noticed that thermal regeneration processes conducted between 400 and 500 °C resulted in good regeneration potential (Bamdad et al. 2018, 2019; Shen and Zhang 2019; Xu et al. 2019).

8 Conclusions

H₂S from biogas can be removed using commercially available technologies; however, these technologies can be expensive and energy-intensive. Therefore, it is extremely essential to identify low-cost and efficient adsorbents that can be used to upgrade biogas for commercial applications. This review promotes the application of biochar and hydrochar obtained from the pyrolysis and hydrothermal conversion of biomass in upgrading biogas. This study provided a critical analysis of biochar and hydrochar for H₂S adsorption from biogas. It was found that the well-developed porous structure, high surface area, and abundant chemical functional groups on char materials make them excellent alternatives to commercially available activated carbons. In addition, biochar has excellent structural properties coupled with an alkaline nature and high mineral matter, which facilitates both physisorption and chemisorption phenomena for H₂S removal. However, in the case of hydrochar, H₂S removal occurs mainly via chemisorption, which can be attributed to well-preserved surface chemical functional groups and high metal content. Alongside char heterogenous properties, other factors such as production methods, adsorption conditions, and biogas compositions critically affect the H₂S adsorption efficiency of biochar/hydrochar. It was also observed that biochar/hydrochar production from wastewater treatment residues (sewage sludge/biosolids) and the subsequent use of the char materials as additives in anaerobic digestion systems to improve biogas production and for further biogas desulfurisation can offer real circular economy opportunities for the water sector. Some of the benefit of integrating char production and onsite utilisation within WWTPs includes increased biomethane yield, in-situ adsorption of H₂S in anaerobic digestion unit, and use of clean biogas for combined heat and power generation, albeit, studies are warranted to investigate these potential outcomes.

Despite the promising physicochemical and structural properties of char materials, their industrial application for biogas cleaning are hindered by low adsorption capacity and limited selectivity for H₂S as well as regeneration stability. These limitations arose from the diverse feedstock compositions and thermochemical conversion methods for biochar/hydrochar production which strongly affect the yield and properties of the char materials for contaminant adsorption in gas streams. This underscores the need to develop efficient functionalisation strategy to tailor char properties for the desired application as H₂S adsorbents. Furthermore, there are still far limited studies reporting the use of biochar/hydrochar for H₂S adsorption from simulated or real biogas stream, the observed performance of biochar/hydrochar in the gas phase adsorption of pure H₂S might be impacted in multicomponents gas stream typical of biogas. Bench-scale and sub-pilot studies are needed to demonstrate the application beyond lab-scale experiment which are mostly reported. Results from large-scale experiment can provide a more realistic assessment of the overall techno-commercial feasibility of biochar/hydrochar for H₂S adsorption.