The influencing mechanism of O2, H2O, and CO2 on the H2S removal of food waste digestate-derived biochar with abundant minerals

Hydrogen sulfide (H2S) removal has been a significant concern in various industries. In this study, food waste digestate-derived biochar (DFW-BC), a by-product of food waste treatment with abundant minerals, was assessed for removing H2S from different simulated biogas containing oxygen (O2) and carbon dioxide (CO2) and under different moisture (H2O) contents (0% and 20%) of biochar. The influencing mechanisms of the gas conditions combined with the moisture contents were also investigated. The results showed an H2S removal of 1.75 mg g−1 for dry biochar under pure H2S, 4.29 mg g−1 for dry biochar under H2S + O2, 5.29 mg g−1 for humid biochar under H2S, and 12.50 mg g−1 for humid biochar under H2S + O2. For dry DFW-BC, the high Fe content was responsible for the O2 enhancement. In contrast, O2 + H2O activated the catalytic H2S oxidation of the less reactive minerals (mainly Ca). The inhibition of CO2 on H2S adsorption was not obvious for dry DFW-BC; the specific pore structure may have provided a buffer against the physisorption competition of CO2. However, when H2O was present on DFW-BC, the changes in critical biochar properties and sulfur speciation as opposed to that without H2O implied an evident occurrence of CO2 chemisorption. This CO2 chemisorption partially hindered O2 + H2O enhancement, decreasing the H2S removal capacity from 12.50 to 8.88 mg g−1. The negative effect was ascribed to mineral carbonation of CO2, neutralizing the alkaline surface and immobilizing metal oxides, which thus reduced the acceleration in H2S dissociation and activation in catalytic H2S oxidation by O2 + H2O. Ash-dominated digestate-derived biochar (DFW-BC) was evaluated for H2S removal. O2 enhanced the H2S removal on dry DFW-BC and H2O + O2 achieved further improved performance. H2O-induced CO2 chemisorption reduced H2O + O2 promotion due to mineral carbonation. Ash-dominated digestate-derived biochar (DFW-BC) was evaluated for H2S removal. O2 enhanced the H2S removal on dry DFW-BC and H2O + O2 achieved further improved performance. H2O-induced CO2 chemisorption reduced H2O + O2 promotion due to mineral carbonation.


Introduction
Hydrogen sulfide (H 2 S) is an odorous and toxic gas emitted from various industries. It is corrosive to pipeline equipment during biogas utilization ) and can cause acute and chronic health problems if directly emitted into the environment (Ro et al. 2021). Besides, the odor threshold of H 2 S is within a very low range of 0.008-0.13 ppm (NRC 2010), making the H 2 S removal very desirable even when it is at low concentrations. Among various biogas purification or deodorization strategies, H 2 S adsorption onto solid media is one of the most commonly used methods (Velasco et al. 2019). Hydrogen sulfide adsorption onto activated carbons has been widely reported, as well as its excellent adsorption capacity (Elsayed et al. 2009;Bazan-Wozniak et al. 2017). However, activated carbon production requires an activation process, which increases the cost and may have adverse effects (Shang et al. 2013). Metal oxide and carbon nanomaterials (e.g., carbon nanotubes, graphene, and graphene oxide) also present significant effects in capturing H 2 S (Kailasa et al. 2020;Revabhai et al. 2022;Patel and Kailasa 2022), but they need special fabrication similar to activated carbon production. As a precursor of activated carbons, biochar is produced through a one-step biomass waste pyrolysis at relatively low temperatures with less sophisticated equipment. In this perspective, biochar is more eco-friendly and cost-effective than activated carbons or nanomaterials (Sahota et al. 2018;Sun et al. 2014;Creamer et al. 2014). Therefore, using biochar absorbents for H 2 S treatment has gained increasing attention in recent years (Bamdad et al. 2018).
The reaction mechanism during the H 2 S removal on carbons can be very complex with a variety of sulfur products (Elsay et al. 2009). Generally, the H 2 S removal on carbons involves two steps (Adib et al. 2000;Choudhury and Lansing 2021: (1) physical adsorption. Gaseous H 2 S is adsorbed on carbon surface (Eq. 1), dissolved in water (H 2 O) film (Eq. 2), and dissociated in an adsorbed state (Eq. 3); (2) oxidation step. The adsorbed H 2 S reacts with oxygen (O 2 ) to form ending products of elemental sulfur (S 0 ; Eq. 4), sulfur dioxide (SO 2 ; Eq. 5), and sulfate ( SO 2− 4 ; Eq. 6) in the presence of water and metal impurities. The basic reactions are as follows: ( where H 2 S gas , H 2 S ads-liq , and H 2 S ads are H 2 S in gas, liquid, and adsorbed phases, respectively; K H , K S , K a , and K R are equilibrium constants for processes of adsorption, gas solubility, dissociation, and surface reaction constant; O * ads is dissociatively adsorbed O 2 . Other researchers also expanded the reaction mechanism by studying various biochars. Particularly, rich minerals on biochars were emphasized as they could significantly promote the H 2 S removal capacity and affect the final sulfur products. Metal minerals can react with dissolved H 2 S in the water film to form metal sulfides (Choudhury and Lansing 2021), change the sulfate product from sulfuric acid to soluble or insoluble metal sulfates that would cause less acidification on biochar surface (Xu et al. 2014), and carry out catalytic H 2 S oxidation with the products of S 0 and metal sulfides (Bagreev and Bandosz 2005). Under dry and O 2 -absent conditions, gaseous H 2 S could also directly react with reactive metals in the ash (Eq. 8), or is removed by substitution reaction (Eq. 9) and oxidation reaction (Eq. 10) with O-containing groups for food waste and sludge-derived chars (Hervy et al. 2018): where C(O) is an active site; C(S) is a sulfur site; C free is a free carbon site. The performance of various types of biochars in H 2 S removal has been reported, and the effects of pH, surface chemistry, pore structure, and metal minerals were all examined to understand the adsorption mechanism. Most previous related studies highlight the importance of the alkaline biochar surface in H 2 S removal. As the rate-determining step is regarded as the dissociation of H 2 S into HS − as expressed by Eq. (3) (Sun et al. 2017), a highly alkaline nature of the adsorbent can facilitate H 2 S dissociation for further oxidation reactions (Xu et al. 2014). A neutral or acidic pH could significantly inhibit the dissociation of H 2 S and consequently suppress the following oxidative reactions (Sun et al. 2016). A water film would help dissolve H 2 S (Eq. 2), promoting the further dissociation (Xu et al. 2014). Caustic impregnants (sodium, potassium, etc.) on carbon materials also play a significant role in the dissociation of H 2 S into HS − as they are often strongly basic (Zhang et al. 2016a). Elsayed et al. (2009) reported that the amount of adsorbed H 2 S depended on the number of basic groups on the char surface. Additionally, a power function relationship between surface area and biochar desulfurization efficiency was proposed (Su et al. 2021). Su et al. (2021) reported that H 2 S removal performance was also closely related to the pore size distribution. While researchers usually emphasize the micropores as critical places for H 2 S oxidation (Choudhury and Lansing 2021;Ma et al. 2021;Surra et al. 2019), the mesopores have also been reported as the active site for H 2 S adsorption (Zhu et al. 2020). Reactive oxides in biochar minerals, such as iron (Fe) oxides and copper oxide, can directly react with gaseous H 2 S because of their high affinity for sulfur (Ciahotný et al. 2019).
The function of these inherent biochar properties was also influenced by extrinsic factors, including H 2 O and O 2 . For example, in dry conditions, H 2 S physisorption in super-and ultra-micropores is the most likely mechanism of H 2 S retention. In contrast, in the presence of H 2 O, the combination of local pH and pore size distribution is predominant (Scheufele et al. 2021). A significant presence of O 2 may increase active sites in the form of oxygenated groups (Scheufele et al. 2021). The reactivity of non-reactive metal oxides, including calcium oxide and magnesium oxide to H 2 S, is very low in dry conditions, but their catalytic role is activated in humid aerobic conditions (Bagreev and Bandosz 2005). Choudhury and Lansing (2021) suggested that the dissociation of H 2 S can be facilitated by a pH higher than its pKa (7.2), but further catalytic oxidation of H 2 S and increased removal efficiency are likely driven by reactive oxygen and metal oxides.
Additionally, there is limited information available regarding the influence of carbon dioxide (CO 2 ), which is acidic and can co-exist in biogas, on the adsorption capacity and mechanism of H 2 S, although individual CO 2 adsorption to chars-based adsorbents has been widely reported (Goel et al. 2021). In a few related studies, inconsistent CO 2 impact has been found, including (1) competitive adsorption for perilla, soybean stover, Korean oak, and Japanese oak biochars with 20% humidity in purifying a simulated biogas containing H 2 S (0.3%), CO 2 (40%), and CH 4 (59.7%) (Sethupathi et al. 2017), (2) cooperative effect in the 8.9 Å pores of commercial activated carbons when CO 2 concentration is greater than 5% in adsorbing 200 ppm of H 2 S (Gonçalves et al. 2018), and (3) no evident effect for neither wood pallets nor food waste and sludge-derived biochars using a simulated syngas consisting of 30% H 2 , 39.98% CO, 15% CH 4 , 15% CO 2 , and 200 ppm of H 2 S (Hervy et al. 2018). More research is required to gain a full understanding of the CO 2 impact. Regarding carbon element-dominated biochars, the role of CO 2 physisorption was especially highlighted in elucidating its impact on H 2 S removal (Sethupathi et al. 2017). In addition to physisorption, the chemisorption of CO 2 on chars has also been reported by their functional groups (Creamer et al. 2014) or the introduced metal oxides ( Goel et al. 2021). Xu et al. (2020) discovered that physisorption was the dominant CO 2 adsorption mechanism for the biochar composite with low iron content, but as the Fe content increased, chemical reactions became dominant. Moreover, the influence of CO 2 on H 2 S removal of biochars could also be affected by the presence of O 2 and H 2 O, related to the activated complex reactions with metal minerals (Huang et al. 2022a). Nevertheless, the influence of CO 2 chemisorption on H 2 S removal of biochars, particularly mineral-rich biochars, has rarely been explored. The information could be essential for predicting or optimizing H 2 S removal of biochars in scenarios with considerable CO 2 .
Compared to wood or crop waste-derived chars, biochars or activated chars made from anaerobically digested fiber have been much less reported (Pelaez-Samaniego et al. 2018). In our previous study, we found that the pyrolytic solid digestate from food waste (DFW) was characterized by a highly alkaline char surface, considerable pore volume, large surface area, and especially high ash content (> 70%) consisting of abundant minerals , which were all favorable for H 2 S adsorption and oxidation. Thus, DFW-derived biochar (DFW-BC) has a great potential to be an excellent H 2 S adsorbent; however, it could have CO 2 chemisorption effects owing to its high ash content. To the best of our knowledge, no research has been carried out to examine the H 2 S adsorption performance of the DFW-BC and the influencing mechanisms of extrinsic factors, such as O 2 , H 2 O, and CO 2 . The utilization of DFW-BC for gas pollutant removal would also expand the resource utilization options of DFW since its treatment is still a challenge .
Given the research needs, we employed DFW-BC to conduct H 2 S adsorption tests simulating different adsorption scenarios. The two major objectives of the experiment were as follows: (1) to reveal the performance and mechanism of DFW-BC in H 2 S removal under O 2 and H 2 O influence; and (2) to examine a potential interaction between CO 2 chemisorption and H 2 S removal to mineral-dominated biochars using DFW-BC as an example.

Preparation and characterization of DFW-BC
The feedstock (DFW) for producing DFW-BC was sourced from a two-stage food waste anaerobic digestion plant in Shenzhen, China. The detailed characteristics of the feedstock were reported by Wang et al. (2021). The moisture content of raw DFW was about 60%. Before pyrolysis, the DFW was freeze-dried for 48 h using a freeze dryer (FreeZone 2.5, Labconco, USA) and was adjusted to a moisture content of 5%. The pyrolysis procedure followed the previous study (Wang et al. 2022a, b). First, a vertical chamber furnace (SLGL-1200L, Siomm, China) was heated to reach a center temperature of 600℃ using a K-type thermocouple. Then, the furnace was purged with nitrogen (N 2 ) (purity > 99.999%) for 20 min at a flow rate of 1 L min −1 to acquire an O 2 -free condition. Next, the pre-treated DFW was placed into the center of furnace and went through fast pyrolysis that lasted for 20 min. This rapid pyrolysis process favors the DFW treatment as it is time-saving. The pyrolysis experiment was in triplicate, from which the average yield of biochar was 31.86%.
The following properties were measured for the raw DFW-BC. The water-extractable fraction was acquired from a 1:50 (w/v) mixture of DFW-BC and water. Then the fraction was directly measured for the pH value (FiveEasy Plus, Mettler Toledo, Switzerland). For the concentrations of soluble SO 2− 4 , an ion chromatography (Dionex Aquion, Thermo Fisher, USA) was applied to analyze the filtrate's composition (by a 0.45 μm filter membrane). The microstructure of DFW-BC was observed by scanning electron microscopy (SEM) (ZEISS SUPRA ® 55, Carl Zeiss, German). Nitrogen adsorption and the Brunauer-Emmett-Teller (BET) method were used to estimate the specific surface area while the Barret-Joyner-Halenda (BJH) desorption method was adopted to obtain the pore distribution and average pore volume. The surface functional groups were determined using Fourier transform infrared (FTIR) spectroscopy (IRTrancer-100, Shimadzu, Japan). An elemental analyzer (Perkin Elmer 2400 II, USA) was applied to measure the organic element (C, H, S & N) composition and it automatically output the averages from triplicate measurements. The ash content was obtained via a specific standard method in China (GB/T 212-2008) for proximate analysis of coal, i.e., weighing the residue of biochar after it was combusted to a constant mass in a muffle furnace at a high temperature (around 800℃). The content of O 2 was then calculated by a simple formula, i.e., O = (1 -C -H -N -S -Ash) × 100% . For measuring the metal concentrations, samples were prepared following the procedures of Wang et al. (2022a). First, biochar samples of 100 mg were digested with 10 mL of acid solution consisting of HCl, HNO 3 , and HF (3:1:1). The liquid from the last step was filtered by a 0.22 μm membrane filter. And then the filtered liquid was analyzed for metal concentrations by inductively coupled plasma mass spectrometry (iCAP RQ, Ther-moFisher, USA). Additionally, an analysis of X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5500 multi-technique system (Physical Electronics) to identify the chemical states of sulfur on DFW-BC.

Design of H 2 S adsorption tests
The H 2 S adsorption tests were all conducted at lab temperature. For each test, a fixed amount of 1 g DFW-BC was packed in a vertical plexiglass column that was 26 cm tall with an interior diameter of 15 mm. The gas inlet and outlet were at the bottom and top of the column, respectively. H 2 S concentration in biogas can be as low as hundreds of ppb in landfill gas (Ding et al. 2012) and as high as 3.5% for syngas produced from sulfur-rich coal gasification (Hervy et al. 2018). The adsorption kinetics and mass transfer rate can also be significantly different between low and high H 2 S concentrations (e.g., 10 vs. 3000 ppm) (Elsay et al. 2009). In this study, we chose a low level of H 2 S concentration (i.e., 150 ppm) to evaluate the H 2 S removal performance of DFW-BC. The inlet flow rate was controlled at 100 mL min −1 using a pre-calibrated mass flow controller. To continuously monitor the outlet H 2 S concentrations, an infrared biogas analyzer (Gasboard-3200plus, Hubei Cubic-Ruiyi Instrument Co., Ltd, China) was employed. Zero gas (99.999% N 2 ) and standard gas (150 ppm H 2 S) calibrations were performed on the analyzer before each adsorption test. A schematic diagram of the experimental setup and the demonstration of its reliability and repeatability can be found in our previous study (Huang et al. 2022a).
To investigate the individual impact of O 2 , H 2 O, and CO 2 as well as their combined impact, eight adsorption scenarios were created using two DFW-BC moisture contents (MCs) (0% and 20%), two O 2 contents (0% and 21% simulating the air), and two CO 2 contents (0% and 40%) in the syngas. The raw biochars were oven-dried to obtain dry DFW-BC. Thereafter, a calculated mass of water was added to the dried DFW-BC for the DFW-BC with 20% MC. Note that special efforts were taken during this procedure, i.e., instilling the water as slowly and evenly as we can for the most MC homogeneity of the samples.

Calculations
The H 2 S removal capacity q (mg g −1 ) was calculated using Eq. (11): where FR is the inlet H 2 S flow rate (mL min −1 ); ρ H 2 S is the H 2 S density (mg mL −1 ); t is the time (min); C i is the inlet H 2 S concentration (ppm); C t is the outlet concentration (ppm); m is the biochar mass (g).

Adsorption kinetics
Pseudo-first-order equation and Pseudo-second-order equation were derived following Eqs. (12) and (13): where q t and q e (mg g −1 ), respectively, are adsorbed H 2 S mass at time t (min) and at equilibrium; k 1 (min −1 ) and k 2 (g mg −1 min −1 ), respectively, are the rate constants for the pseudo-first-order and pseudo-second-order adsorption kinetics.

Properties of DFW-BC
The biochar had a strong alkaline surface proved by its high pH value (11.47), which is beneficial for H 2 S dissociation (Xu et al. 2014). High BET surface area and large micropore volume have been reported to promote the reaction rate of H 2 S oxidation (Ma et al. 2021). The DFW-BC is superior to most biochars made from other biomass wastes in terms of pore volume and surface area according to Additional file 1: Table S1 listing the critical properties of biochars from different feedstocks.
Micropores have been widely identified as a critical area where H 2 S oxidation takes place on carbons (Choudhury and Lansing 2021;Surra et al. 2019). Hervy et al. (2018) concluded that mesopores lower than 7 nm were also important adsorption sites for H 2 S oxidation on food waste and sludge-derived biochar in dry gas conditions. Additionally, a relatively larger pore size distribution (i.e., averaged 2.34 nm) could provide support for the adsorbates to reach the adsorption site on municipal sludge-derived biochar easily (Lin et al. 2021). The biochar exhibited an average pore diameter of 4.05 nm, with the majority of the pore size ranging from 2 to 6 nm (as shown in Fig. 1), suggesting a possible favorable pore structure for H 2 S adsorption. Compared to wood or crop waste-derived biochars with regular porous carbon skeletons, the DFW-BC presented a more heterogeneous surface and agglomerated morphology (Fig. 2), which is attributed to the high mineral content in the feedstock (Xu et al. 2020) (Table 1).
The ash content of the DFW-BC was particularly high (70.07%) than that of the wood and straw wastederived biochars (Weber and Quicker 2018;Gwenzi et al. 2021). This high ash content of biochar is attributed to the high ash content (24.30%) of the parent biomass (DFW). High ash content implied abundant mineral constituents in the biochar, which were anticipated to improve the sorption of acidic gases (Xu et al. 2017). The deposition of the metal minerals on the surface of DFW-BC was evidenced by SEM-EDS mapping (Fig. 2). Ca, Fe, and Al were the most abundant metal elements, followed by Na, Mg, and K. The high amount of Ca in the DFW-BC originated from food waste and the added CaO for dewatering the DFW (Yu et al. 2020). XRD analysis results showed that CaCO 3 was the major metal mineral in the DFW ) and the thermal decomposition of CaCO 3 commences at 550-600 ℃, with a loss of CO 2 and a product of CaO on the biochar (Rodriguez-Navarro et al. 2009;Singh et al. 2010). Furthermore, our previous study revealed that the main portion of Fe was reducible, while over 65% of Ca, Na, K, and Mg were acid extractable and exchangeable on the DFW-BC ). The existence and abundance of the above metal minerals supported the proposed reactions in the discussion section.

Removal capacities
The adsorption curves are presented in Fig. 3. Under the dry condition, the H 2 S adsorption capacity was only 1.75 mg g −1 . In contrast, the adsorption capacity of the biochar was significantly improved to 5.29 mg g −1 in the presence of 20% MC. The influence of O 2 was positive for the DFW-BC even under dry conditions, with a removal capacity of 4.29 mg g −1 . When the DFW-BC was humid, the combination of O 2 and H 2 O enhanced the H 2 S removal up to 12.50 mg g −1 , indicating the need for Fig. 1 The BET surface area and pore volume distribution for the DFW-BC. a is N 2 sorption and desorption curves. b is pore size distribution. c is dV/ dlog(D) pore volume ratio; and d is pore volume ratio both O 2 and H 2 O to reach an optimal performance of the DFW-BC in removing H 2 S.
The role of CO 2 was also examined. However, no evident impact on the H 2 S removal for DFW-BC under dry conditions was observed. In contrast, the promotion of H 2 O in the H 2 S removal capacity was partially disrupted when CO 2 was present. This conclusion was drawn from the fact that the H 2 S removal capacity was only 2.97 mg g −1 under 20% CO 2 + 20% MC as opposed to 5.29 mg g −1 under 20% MC. Similarly, the presence of CO 2 in the syngas only slightly inhibited the stimulation of O 2 in H 2 S removal on dry DFW-BC; the removal capacity was 3.37 mg g −1 under H 2 S + CO 2 + O 2 and 4.29 mg g −1 under H 2 S + O 2 . However, this CO 2 inhibition was induced to a more significant degree in the presence of H 2 O, reducing the removal capacity from 12.50 mg g −1 under H 2 S + O 2 + 20% MC to 8.88 mg g −1 under H 2 S + O 2 + CO 2 + 20% MC. In conclusion, the critical roles of O 2 and H 2 O in activating more efficient H 2 S removal on DFW-BC while an adverse influence of CO 2 on their promotion was demonstrated (Table 2).
Studies on H 2 S adsorption of digestate-derived biochars are very limited (Table 3). Comparing those materials to commercial activated chars, they showed

Items Values
Conductivity (    probably explained by the high biochar mass and moisture content (80-85%) but low inlet flow rate that were used in their adsorption system. Appropriate height of fixed-bed char and gas velocity are both necessary to avoid significant pressure drop and for a sufficient residence time for the gas molecules in the fixed-bed (Hervy et al. 2018). For example, Han et al. (2020) indicated that a higher gas velocity led to a reduced contact time that decreased the H 2 S removal of biochar derived from rice husk. In contrast, increasing the adsorption bed height would benefit the H 2 S removal due to the prolonged contact time (Meri et al. 2018;Papurello et al. 2020;Sawalha et al. 2020). However, the resistance to the flow also would become higher (i.e., higher head loss or pressure drop) because of the strong mechanical energy dissipation (Scheufele et al. 2021). The concentration level is another factor influencing the H 2 S adsorption kinetics and mass transfer rate (Elsay et al. 2009). Han et al. (2020) reported that the increase in H 2 S concentrations shortened the breakthrough time by decreasing the number ratio of adsorption sites to H 2 S molecules, but eventually increased the H 2 S removal capacity probably due to the increased partial pressure in the biochar bed.

Adsorption kinetics
Without regard to the presence of H 2 O, the adsorption process more closely matched the Pseudo-first-order equation (Table 4), indicating a dominant role of physisorption in controlling the adsorption process (Gwenzi et al. 2021). In contrast, the adsorption process was more aligned with the Pseudo-second-order equation when other gas components existed in the syngas, implying that chemisorption mainly determined the adsorption rate (Goel et al. 2021). It was interesting to notice that the major rate controlling process was once again physisorption with the introduction of H 2 O on the DFW-BC as suggested by the higher R 2 values for the Pseudo-firstorder equations.

Influence of feedstock type on biochar properties
The factors affecting the biochar properties can be categorized into feedstock and pyrolysis conditions, including temperature, heating rate, residence time, etc. Temperature is claimed to be the most significant factor while residence time and heating rate are less important (Weber and Quicker 2018;Guo et al. 2021). To elaborate how the feedstock combining temperature would affect critical biochar properties that determine H 2 S removal capacity, Additional file 1: Table S1 summarizes the results from previous studies. For most of the feedstocks, an increased temperature leads to increased carbon content, pH value, ash content, surface area, and pore volume. However, a higher temperature would cause decreased mass yield and functional groups, and extremely high pyrolysis temperatures (e.g., 800-1000 ℃) may result in a decrease in surface area and pore volume likely due to the shrinking solid matrix (Weber and Quicker 2018). Additionally, feedstock containing abundant inorganic nutrients would produce biochars with higher ash but lower carbon content than plant-based biomass mainly consisting of cellulose, hemicelluloses, lignin, etc. As listed in Additional file 1: Table S1, woody waste-derived biochars exhibit carbon content of 53.99-83.60% and even ≥ 95% at high pyrolysis temperatures (Weber and Quicker 2018), while manure-and sewage sludge-derived biochars own a relatively lower carbon content but high ash content of > 60% and > 80%, respectively. The higher the ash content, the higher the concentration of the mineral components on the biochar (Ayiania et al. 2019). Mineral components (e.g., carbonates) contributed to the alkalinity of biochar in an aqueous medium (Masebinu . This explains the relatively higher pH values for ash-dominated biochars than carbon-dominated biochars. For example, under a pyrolysis temperature of 600℃, the ash content and pH value were 3.59-4.38% and 9.60-9.82 for straw-derived biochars (Huang et al. 2022a) but were 70.07% and 11.47 for DFW-BC in this study. Feedstock type is also one crucial factor determining the functional group compositions besides the pyrolytic temperature. Higher fixed carbon and lignin in the feedstock led to a high amount of surface functional groups (Kumar et al. 2021). However, manure and sewage sludge-derived biochars present more N-and S-functional groups than woody biomass-based materials (Masebinu et al. 2019).
In brief, understanding the influence of feedstock type and pyrolysis conditions (mainly temperature) on biochar properties can help select or produce a promising H 2 S adsorbent.

Promotion by H 2 O in H 2 S removal on DFW-BC
H 2 S adsorption on carbons can result from several interactions, including physisorption by van der Waals forces or a more intense bond with O-containing groups and chemisorption through which H 2 S is oxidized by the O-containing groups (generating elemental sulfur S 0 ) or minerals (to metal sulfides, sulfites, or sulfides) (Han et al. 2020;Elsayed et al. 2009;Guo et al. 2006;Moreno-Castilla 2004). In humid conditions, a water film would be created on the biochar surface (Sahota et al. 2018). According to the aforementioned gas-liquid-solid reaction mechanism on carbon-based adsorbents (Li et al. , this water film should have enhanced H 2 S dissolution and ionization, improving H 2 S adsorption on biochars (Xu et al. 2014;Sitthikhankaew et al. 2014). The product S 0 is highly active atom and can self-assembly to small S clusters, which could diffuse along the carbon surface, covering or blocking the pores (Sun et al. 2022). Water can then promote sulfur deposition at different carbon sites and mechanically remove sulfur from active sites, decelerating the biochar deactivation process (Zhang et al. 2016b). Additionally, the reactivity of metal minerals in oxidizing H 2 S is inert under dry biochar conditions but stimulated under humid conditions (Bagreev and Bandosz 2005). These reasons were responsible for the improved H 2 S removal when DFW-BC became humid.
The results in Fig. 4a suggest that greater consumption of those basic O-containing groups occurred (including C=O, −COO, and −OH) after H 2 S adsorption on humid DFW-BC as opposed to dry DFW-BC. Given the improved H 2 S removal capacity in the presence of H 2 O (from 1.75 to 5.29 mg g −1 ), enhanced participation of these O-containing functional groups was expected for removing H 2 S. However, no such decreasing stretching vibration in those O-containing groups was observed for humid DFW-BC after H 2 S removal when O 2 or CO 2 was present, indicating their negligible roles in the further O 2 -induced promotion or CO 2 -induced inhibition.
Sulfur speciation and proportions on DFW-BC after H 2 S removal can be found in Fig. 5, which is supplemented by Additional file 1: Fig. S1 to show a more visible variation of sulfur composition. The XPS results in Fig. 5a, b show that H 2 O mainly stimulated the conversion of H 2 S to an end product of S 0 to improve the H 2 S removal on the DFW-BC. This explains why its pH value was not any lower than that of the dry DFW-BC. Additionally, the content of sulfate increases from 16.65% to 23.53% in Fig. 5a, b, among which soluble SO 2− 4 content displays an increase from 0.6 to 1.81 mg g −1 . This soluble SO 2− 4 content is attributed to the reactions with metal minerals including Ca, Na, K, and Mg, which are the major metal constituents in the ash and are mainly present in an extractable form as mentioned above. Similarly, in the study of Xu et al. (2014) on pig manure and sewage sludge-derived biochars, SO 2− 4 was the dominant sulfur form mainly present as soluble (K,Na) 2 SO 4 and CaSO 4 precipitate, respectively, evidencing the significance of abundant minerals in removing H 2 S. Hervy et al. (2018) ascribed the high H 2 S removal capacity of food waste/sludge-based chars to their high amount of ash and active mineral species, and Ca-containing particles was proved to contribute to its high H 2 S with the formation of CaSO 4 . In brief, H 2 O improved the H 2 S removal on DFW-BC with no generation of sulfides, but it increased S 0 and sulfates with no further decrease in pH.

Promotion by O 2 in H 2 S removal on DFW-BC
In dry biochar conditions, the presence of O 2 significantly improved the H 2 S removal on the DFW-BC from 1.75 to 4.29 mg g −1 . Along with the improved H 2 S removal capacity for DFW-BC, a less reduced pH (9.39) than that without O 2 (pH = 8.65) was observed. The result contradicted the finding of Hervy et al. (2018), which revealed that O 2 in the dry syngas decreased the H 2 S adsorption capacity of wood pallets and food waste-derived chars owning to the formation of acidic sulfur species. In our previous study, more surface acidification and inhibited H 2 S removal were also observed for dry straw biochars when O 2 was present (Huang et al. 2022a).
The reasons for the enhancement of H 2 S removal by O 2 in this study include the following: (1) direct oxidation of H 2 S by O 2 (He et al. 2011;Wu et al. 2018), (2) formation of more active sites (Sitthikhankaew et al. 2014); and (3) catalytic oxidation of H 2 S by metal oxides in a basic carbon surface environment (Bagreev and Bandosz 2005). Considering the more reactive nature of Fe and its high content, the major reactions explaining the O 2 improvement in H 2 S removal on dry DFW-BC should be dominated by iron oxides: The XPS analysis confirmed the newly generated sulfur and sulfides in the presence of O 2 (Fig. 5a vs. c). The results explained why H 2 S removal was improved while surface acidification on dry DFW-BC was reduced since the formation of sulfur and metal sulfides was expected to cause less influence than sulfuric acids (Bagreev and Bandosz 2005).
When O 2 was further mixed with 20% MC, a remarkable promotion in H 2 S removal capacity was observed for the DFW-BC; the removal capacity increased to the highest value among all adsorption scenarios. From Table 2 and Fig. 5c vs. d, there is no much further decrease in pH; however, sulfates, sulfur, and sulfides contents for the DFW-BC are observed to be higher under O 2 + H 2 O than under O 2 . The added H 2 O helped generate a water film that accelerated the dissociation of H 2 S, as mentioned above. As a result, the significantly increased content of soluble sulfate (from 1.06 to 6.39 mg g −1 ) could (14) be ascribed to the following reactions (17)-(18) (Xu et al. 2014): As discussed in Sect. 4.2, the generation of sulfides from direct reactions between metal minerals and H 2 S was not observed under either dry or humid DFW-BC conditions. Therefore, the more generated sulfides should be attributed to the enhanced catalytic H 2 S oxidation by O 2 and metal minerals in the presence of H 2 O (Choudhury and Lansing 2021

Effect and mechanism of CO 2 in interfering with the O 2 and H 2 O promotion
Under dry DFW-BC conditions, CO 2 did not significantly inhibit H 2 S adsorption (via competition for adsorption sites) (Sethupathi et al. 2017). Additionally, no evident difference in the speciation of sulfur products between Fig. 5a, e was observed. When H 2 O was present, the presence of CO 2 in the syngas had an inhibitory effect on H 2 S removal, which decreased by 44% from that without CO 2 . When comparing the sulfur speciation between Fig. 5b, f, CO 2 induced 3.58% of sulfides that were newly generated while reduced the proportion of S 0 from 12.37% to 5.58%. These results indicated that the co-existing CO 2 modified (17) the mechanism of H 2 S removal on humid DFW-BC but not dry DFW-BC.
Generally, CO 2 removal on biochar is believed to be physisorption, owing to its textural property (surface area and micropore volume) (Chen et al. 2017;Bamdad et al. 2018;Sethupathi et al. 2017). This process involves gas diffusion in the macropore, mass transfer of gas-solid interface reaction in the mesopore, and CO 2 deposition in the micropore. As for increased CO 2 physisorption, previous studies demonstrated the benefits of large surface area and micropore volume but smaller pore size via producing biochar at higher pyrolysis temperatures (Creamer et al. 2014), or biochar modification through air activation (Plaza et al. 2014), CO 2 -ammonia treatment , impregnation with sodium lignosulfonate , pre-carbonization + KOH impregnation (Song et al. 2014), and so on. The insignificant effect of CO 2 presence on H 2 S adsorption to dry DFW-BC suggested that its pore structure (mainly small mesopores) had advantages over other wood-derived or crop wastederived biochars in buffering the CO 2 competition for adsorption sites (Sethupathi et al. 2017;Huang et al. 2022a). However, surface chemistry, which includes chemisorption through functional groups ) and mineral carbonation of CO 2 (Xu et al. 2020), could play an important role in capturing more CO 2 . According to Xu et al. (2020), the governing CO 2 sorption mechanism was switched from physical adsorption to chemical reaction when the metal content of biochars increased. Since the DFW-BC has a strong alkaline surface with abundant minerals, an alkaline water film could have formed on the carbon surface, leading to a chemisorption competition between H 2 S and CO 2 . Mineral carbonation of CO 2 occurs in the presence of H 2 O, with Ca and K as examples through the following reactions (Sitthikhankaew et al. 2014):  The reactions were supported by the decreased sulfur generation from the catalytic effects of metal minerals, but newly produced sulfides, as shown in Fig. 5b vs. f. When comparing the decreased H 2 S removal under O 2 + CO 2 + H 2 O to that under O 2 + H 2 O, the notably increased S 0 and sulfite contents were associated with decreased sulfide and sulfate fractions in Fig. 5 (among which soluble sulfate reduced from 6.39 to 1.88 mg g −1 ). As elaborated in Sect. 4.3, the three mechanisms for O 2 enhancing the H 2 S removal comprised direct oxidation of H 2 S, generation of more active sites, and catalytic oxidation by Fe oxides. Based on O 2 enhancement, the further H 2 O improvement was attributed to the promoted dissociation of H 2 S in the alkaline water film and catalytic oxidation by Ca, Na, K, and Mg oxides. However, in the presence of CO 2 + H 2 O, owing to the consumption of metal oxides to form carbonates and bicarbonate by the aforementioned mineral carbonation of CO 2 , the alkaline surface (OH − ) for dissociating H 2 S was neutralized and the catalytic oxidation of H 2 S by metal oxides was reduced. Consequently, the further H 2 O improvement based on the O 2 enhancement was partially hindered by the presence of CO 2 in the syngas.

Biochar and sulfur recovery
As for the recovery of active carbons, chemical treatment, solvent washing, microwave, and thermal treatment have all been adopted as regeneration methods (Song et al. 2014). However, given the relatively low cost, wide sources of feedstock, and the environmentfriendly nature of biochar, reuse of the spent biochar may be more appealing than its regeneration. Firstly, the spent biochar can be directly applied to land as a source of micro-nutrient in sulfur deficient soils. Zhang et al. (2017) demonstrated that biochar after H 2 S adsorption significantly increased corn plant biomass by 31-49% and soybean biomass by 4-14% through a 90-day greenhouse study. Additionally, several studies have reported the multiple merits of biochar as a soil amendment for enhanced microbial activities, including methanotrophic activities in landfill cover soil that can mitigate fugitive methane emission from landfills (Reddy et al. 2014). In our previous study, H 2 S-saturated biochar could still maintain its capacity for improving (26) the methane oxidation efficiency of landfill cover soil ). The spent biochar can also be secondly utilized for the removal of other pollutants, such as mercury from condensate oil. It is well known that sulfur owns a high affinity for mercury. Thus, H 2 S sulfurized biochar was investigated for removing ionic mercury from gasoline by Wang et al. (2020). They found that the removal efficiency of H 2 S-treated tobacco biochar was increased by 87% based on that without H 2 S treatment, and the inorganic sulfides played a major role through ion exchange with metals. Apparently, these are only a few examples depicting the recovery potential of the spent biochar and its sulfur, and more diverse utilization approaches can be expected.

Practical applications and future research prospects
Exploring DFW-BC as an H 2 S adsorbent is a win-win strategy as it expands the resource utilization options of DFW with less environmental impact and offers an alternative low-cost adsorbent for H 2 S removal. This study demonstrates DFW-BC as a promising H 2 S adsorbent. In the future, modification of DFW-BC can be conducted for assessing the improvement in the H 2 S removal efficacy. For example, physical activation (e.g., through the introduction of steam or CO 2 ) during the pyrolysis process or chemical activation (e.g., through Na 2 CO 3 impregnation of DFW-BC) is worth investigating investigated (Hervy et al. 2018;Pelaez-Samaniego et al. 2018). In scaling up the H 2 S adsorption system for practical industrial applications, a deep understanding of the operational parameters (H 2 S concentration, temperature, gas velocity, fixed-bed height, etc.) would also be required for optimized design.

Conclusions
In this study, the removal capacities of H 2 S on DFW-BC with high ash content (70.07%) and abundant metal minerals were investigated. this buffering capacity was attributed to its specific pore structure. For humid DFW-BC, the presence of CO 2 partially neutralized the H 2 O and O 2 + H 2 O enhancement in the H 2 S removal. This negative effect was due to the CO 2 chemisorption through the mineral carbonation on the humid DFW-BC, which consequently hindered the acceleration of H 2 S dissociation and reduced the activated catalytic oxidation by metal minerals. This study provides new insights into potential CO 2 interaction with H 2 S removal on biochars with high ash content and abundant minerals as opposed to the previously emphasized physisorption competition. The information would also assist in optimizing the desulfuration or deodorization performance of digestate-derived biochars in different scenarios, especially when considerable amounts of CO 2 are present in the biogas.

Additional file 1: Table S1
Comparisons of biochars produced from different feedstocks and pyrolysis temperatures. Fig. S1 Proportions of sulfur products after H 2 S adsorption for DFW-BC.