Study of H2S Removal Capability from Simulated Biogas by Using Waste-Derived Adsorbent Materials

Three waste-derived adsorbent materials (wood-derived biochar, sludge-derived activated carbon and activated ash) were pre-activated at the laboratory scale to apply them for the removal of H2S from a biogas stream. The H2S removal capabilities of each material were measured by a mass spectrometer, to detect the H2S concentration after the adsorption in an ambient environment. The activated ash adsorbent has the highest removal capacity at 3.22 mgH2S g−1, while wood-derived biochar has slightly lower H2S removal capability (2.2 mgH2S g−1). The physicochemical properties of pristine and spent materials were characterized by the thermogravimetric analyzer, elemental analysis, X-ray fluorescence spectroscopy and N2 adsorption and desorption. Wood-derived biochar is a highly porous material that adsorbs H2S by physical adsorption of the mesoporous structure. Activated ash is a non-porous material which adsorbs H2S by the reaction between the alkaline compositions and H2S. This study shows the great potential to apply waste-derived adsorbent materials to purify a biogas stream by removing H2S.


Introduction
Facing significant environmental degradation, with continuous increases in natural resource consumption, the new attention on green growth, but especially on low carbon development, turns out to be crucial to the circular economy (CE) approach. In this field, pyrolysis and gasification of carbonaceous materials such as waste tires, waste plastics and crude glycerol from the biodiesel industry have potential to become profitable liquid, gaseous and solid products [1,2]. Liquid products containing naphtha, tars and phenols can be applied straight away or upgraded as fuels [3]. Gas products containing syngas (H 2 and CO), methane and others can also be used as fuels depending on their purity [4,5] in several energy generators, such as: internal combustion engines (ICE) [6], microturbines (MT) [7] and highly efficient solid oxide fuel cell (SOFC) systems [8]. Solid products, on the other hand, are mainly chars that can be upgraded to activated carbons through chemical and/or physical processes [9]. Biogas or in general gas products play the vital role of fulfilling the energy demand worldwide, which could be applied in heat and power generation and vehicle fuel with proper samples were heated to 700 • C with a ramp rate at 7 • C min −1 . Carbon dioxide was the activating agent with a constant flow rate of 100 mL min −1 for 60 min under isothermal conditions.
The sludge-derived activated carbon (SDC) was prepared from an anaerobically digested and dried sewage sludge collected from the SMAT wastewater treatment plant (WWTP) site (Castiglione Torinese, Italy). Physical activation of milled sludge (~1 mm) was carried out in a stainless-steel reactor with diameter~34 mm and~570 mm in length, which was externally heated by a tubular furnace to 600 • C. These low-temperature values were selected to simulate a direct coupling with SOFC engine gas exhausts [39]. Nitrogen was the purge gas during the pyrolysis of the raw sludge sample for physical activation, and carbon dioxide and air were the activating agents. The activated samples were ground to obtain grain size~400 µm using a vibratory sieve shaker (Fritsch, Germany).

Experimental Set-Up
The schematic diagram for testing the H 2 S removal capability of the three selected samples is shown in Figure 1. A mixture of gases (CH 4 -to-CO 2 ratio at 1.5 and H 2 S at 200 ppm(v)) was used as simulated biogas. The mixture of the gas inlet was controlled by three individual specific mass flow controllers (Bronkhorst EL-FLOW, NL), to simulate an average biogas mixture deriving from the anaerobic digestion of organic waste. This concentration is representative of real biogas as reported in the literature [40][41][42]. These sorbent materials were placed in Teflon tubes with an internal diameter of 25 mm. The aspect ratio between the sorbent height and plastic tube inner diameter is~8.33. To remove residual gases inside the pores, the samples were pre-treated using 200 NmL min −1 N 2 flow for 30 min at 120 • C. A gas flow of 919.9 NmL min −1 was used for the H 2 S removal performance testing. The gas outlet was analyzed by a mass spectrometer (MS) (QGA, Hiden Analytical Ltd., UK) to monitor the online H 2 S concentration. The H 2 S removal tests were repeated for each sample to prove the reliability of the H 2 S removal capability. reactor with diameter ~34 mm and ~570 mm in length, which was externally heated by a tubular furnace to 600 °C. These low-temperature values were selected to simulate a direct coupling with SOFC engine gas exhausts [39]. Nitrogen was the purge gas during the pyrolysis of the raw sludge sample for physical activation, and carbon dioxide and air were the activating agents. The activated samples were ground to obtain grain size ~400 µm using a vibratory sieve shaker (Fritsch, Germany).

Experimental Set-Up
The schematic diagram for testing the H2S removal capability of the three selected samples is shown in Figure 1. A mixture of gases (CH4-to-CO2 ratio at 1.5 and H2S at 200 ppm(v)) was used as simulated biogas. The mixture of the gas inlet was controlled by three individual specific mass flow controllers (Bronkhorst EL-FLOW, NL), to simulate an average biogas mixture deriving from the anaerobic digestion of organic waste. This concentration is representative of real biogas as reported in the literature [40][41][42]. These sorbent materials were placed in Teflon tubes with an internal diameter of 25 mm. The aspect ratio between the sorbent height and plastic tube inner diameter is ~8.33. To remove residual gases inside the pores, the samples were pre-treated using 200 Nml min −1 N2 flow for 30 min at 120 °C. A gas flow of 919.9 Nml min −1 was used for the H2S removal performance testing. The gas outlet was analyzed by a mass spectrometer (MS) (QGA, Hiden Analytical Ltd., UK) to monitor the online H2S concentration. The H2S removal tests were repeated for each sample to prove the reliability of the H2S removal capability.

Methodology
The H2S adsorption capacity in terms of mg g −1 for H2S removal was calculated based on Equation (1) [43,44], which was corrected considering that the area enclosed by the breakthrough curve and the saturation line up to t1 is estimated by the difference between the rectangle (t1 (h) Cin (ppmv)) and the triangle (0.5 (t1-t0) (h) 1 (ppm(v)) areas: where:

Methodology
The H 2 S adsorption capacity in terms of mg g −1 for H 2 S removal was calculated based on Equation (1) [43,44], which was corrected considering that the area enclosed by the breakthrough curve and the saturation line up to t 1 is estimated by the difference between the rectangle (t 1 (h) C in (ppmv)) and the triangle (0.5 (t 1 − t 0 ) (h) 1 (ppmv)) areas: where: • t 0 , breakthrough time at the last detection of 0-0.5 ppmv (h); • t 1 , breakthrough time when the outlet H 2 S concentration is 1 ppmv (h); • Q tot = total gas flow rate (NL h −1 ); • MW = molecular weight of the trace compound removed (g mol −1 );

Characterization Methods
Elemental analysis: The elemental analysis for virgin and spent samples was obtained from SOCOTEC, UK. The oxygen content was calculated by the difference. The chemical composition of each pristine and spent adsorbent material was analyzed by a Panalytical X-ray fluorescence spectroscopy (XRF) Epsilon 3XL.
Scanning Electron Microscopy: A Zeiss EVO 10 scanning electron microscope (SEM) was used to characterize both the virgin adsorbents.
Thermogravimetric analysis: The ash contents of the virgin samples were estimated by using a Netzsch STA 2500 Regulus thermogravimetric analyzer (TGA). The heating program started with 10 • C min −1 to 120 • C and holding for 30 min to remove the moisture content in the virgin samples in the presence of nitrogen. Then, the sample was heated to 800 • C with 10 • C min −1 ramp rate to combust the organic compound. The air flow was constantly at 40 mL min −1 . The same TGA coupled with a Hiden Analytical QGA Mass spectrometer was used to study the sulfur compounds' decomposition behaviors with nitrogen as a constantly purged gas.
pH measurement: The pH values of the three virgin samples and the spent samples were measured to compare the pH evaluation of the three different samples after the H 2 S adsorption. Approximately 0.4 g of each of dry sample was added in 20 mL distilled water to make the homogenous suspension for pH measurement, by using a Hanna Instruments pH meter.
Physical adsorption and desorption: The porous properties of the virgin char and spent samples were determined by measuring the nitrogen adsorption and desorption isotherms from the samples at equilibrium vapor pressure using the static volumetric method. Nitrogen adsorption-desorption isotherms were recorded at 77 K using a Micromeritics 3Flex surface characterization analyzer. The samples were degassed at 300 • C overnight. Specific surface areas were determined according to the Brunauer, Emmett and Teller (BET) model [45,46], with pore diameters, volumes and distributions determined through Barrett-Joyner-Halenda (BJH) analysis [47].

H 2 S Removal Capacity
The H 2 S removal efficiency tests of three adsorbents were carried out in the simulated biogas stream. The H 2 S removal capacity of each adsorbent is listed in Table 1, which shows the AA with the highest removal capacity at 3.22 mg H2S g −1 and SDC with the lowest removal capacity at 0.1 mg H2S g −1 .
Each of the adsorbent materials has been repeated twice for the H 2 S removal test to confirm the experimental reproducibility.  Figure 2 is the SEM micrographs of virgin adsorbents which show the morphology of the adsorbent particles. Figure 2a,b are the SEM micrographs of virgin WDC adsorbents, the particles have cracking features. Figure 2c,d are the SEM micrographs of virgin SDC that present as smaller particles supported on the larger particles. Figure 2e,f are the SEM micrographs of virgin AA which demonstrate the large cluster of fine power. Figure 3 is the thermogravimetric analysis of the three raw adsorbents WDC, SDC and AA. The residue left after temperature-programmed oxidation is ash, whereas the weight loss is caused by the oxidation of organic composition. The results show the ash contents ranking of the three adsorbents is AA > SDC > WDC and the ranking of organic components is in a reverse direction. The results are consistent with the elemental analysis results listed in Table 2, where the carbon compositions for three raw adsorbents is ranked as WDC > SDC > AA. This result could explain why the H 2 S removal capacity of WDC is higher than SDC in Table 1, because the porous carbon content in WDC is 88.93% which is much higher than the content in SDC, which is 33.94%. The porosity of carbon could also be supported by the nitrogen adsorption and desorption analysis shown in Table 4, where the WDC has the highest surface area and total pore volume that are 210.18 m 2 g −1 and 0.5 cm 3 g −1 , respectively.  Although SDC contains a higher amount of carbon (33.94 wt.%) compared with AA (7.42 wt.%) in Table 2, the H 2 S removal capacity of SDC (0.1 mg H2S g −1 ) is lower than AA (3.22 mg H2S g −1 ) and this is related to the high ash content in AA (73.21 wt.%), as shown in Table 2. It has been reported that the incineration residues that contain many compounds would form sulfides as H 2 S-trap materials which can react with H 2 S at ambient temperature, such as manganese, zinc, copper and iron [37]. The porosity analysis results shown in Table 4 can also prove there is no significant difference between SDC and AA in terms of surface area and total pore volume.  Table 2 lists the elemental composition and pH values for the virgin and spent adsorbent materials. The CHNO compositions for virgin and spent WDC adsorbents are similar, and the pH value was slightly increased from 7.2 to 7.3 which indicates the chemical reaction has mostly not happened during the adsorption of H 2 S by WDC. The pH values for spent SDC and AA are slightly lower than the virgin adsorbents that indicate the chemical reaction occurred between the H 2 S and the alkaline compositions. In other words, the chemical adsorption of H 2 S by SDC and AA is more dominated than WDC. Table 3 presents the ash composition of the pristine and spent adsorbent materials by XRF which prove the existence of alkaline compositions, such as Fe, Cu, Mn and Ca. Calcium is the main mineral species in WDC and AA adsorbents, and iron is the main mineral species in SDC adsorbents. Juarez et al. [37] reported that alkaline or pH-neutral waste contain sulfites or a mixture of Fe-II, Fe-III, Mn-II, Cu-I, Cu-II and Zn-II which play the role of potential H 2 S-trap materials. The possible reaction for the H 2 S removal by SDC and AA adsorbents could relate to the "primary" and "secondary" reactions [37]. The primary dissociation reactions include Reactions (2) and (3), and the suggested secondary sulfidation reactions could be (3)- (11) [37,48,49].

Characterizations of Adsorbents
MnO + H 2 S = MnS + H 2 O (Same with Cy, Ca and Ni) Ducom et al. [50] reported the primary reactions (Equations (2) and (3)) are acid-base reactions since H 2 S is hydrophilic and dissociates in aqueous solution. The primary reactions occur very fast which are not the dominant reaction for H 2 S removal. Sarperi et al. explained the possible Fe sulfidation Reactions (4) and (5) [37]. The ash adsorbents may also contain hydrous ferrous oxide (HFO) and goethite (FeOOH) with a high specific surface area which will react with H 2 S as shown in Equations (6) and (7) [51]. Juarez et al. [37] suggested that Reactions (8)-(10) between H 2 S and sulfites will form polysulfides and elemental S 8 based on others' research [52,53].  Table 4 shows the porosity of the virgin and spent waste-derived adsorbent materials. The surface area of WDC decreased significantly from 210.18 to 96.95 m 2 g −1 after the H 2 S removal. This could explain the certain amount of H 2 S that was adsorbed by the porosity of WDC. However, there is no significant change of surface area and total pore volume for SDC and AA would also support the conclusion drawn earlier that the H 2 S removal capacity of WDC and AA was dominated by the chemical reactions rather than the physical adsorption. Although the AA adsorbent is not as porous as WDC, the surface area and total pore volume also decreased slightly which indicates that physical adsorption also happened. In Table 4, the average pore diameter for virgin WDC is 19.64 Å, which indicates the presence of mesopores. Figure 4 illustrates the N 2 adsorption and desorption isotherms of (a) virgin WDC, (b) spent WDC, (c) virgin SDC, (d) spent WDC, (c) virgin AA and (f) spent AA adsorbent materials. According to the BJH pore size distribution illustrated in Figure 5a, the presence of mesopore distributions in the WDC sample can be confirmed. The mesopore has been reported as the active site for H 2 S adsorption [33]. Table 4. Surface area, total pore volume and average pore diameter of virgin and spent adsorbents.

Sample
Surface Area (m 2 g −1 ) * Total Pore Volume (cm 3 g −1 ) Average  Figure 4a,b,e,f exhibit kinds of type IV or V isotherms according to the IUPAC classification, indicative of the presence of mesopores within the virgin and spent WDC and AA samples [54,55]. The hysteresis for virgin and spent WDC and AA samples adsorption isotherms shown in Figure 4a,b,e,f could relate to the morphological properties of the sample. The morphological properties have been summarized by other researchers which include the pore size distribution, pore geometry and connectivity of the porous materials [55,56]. The shape of the adsorption hysteresis loop exhibited similar as H 3 shape indicates the porous material could have slit-shaped pores with two ends open, which can be supported by other researchers' findings [55,57,58]. The slope associated with the hysteresis loop of the adsorption/desorption curves is related to the tensile strength effect [54,55,59]. The highlighted hysteresis curves that are shown in Figure 4b,f indicate the low-pressure hysteresis due to the volume change of the adsorbent. It could be caused by the swelling of the non-rigid pores in the spent SDC and AA adsorbent materials, or the pores have been occupied irreversibly by the similarly sized molecules [55,60].  Figure 5a,b illustrate the pore size distributions of the virgin and spent WDC adsorbents. According to the peak, peak A diminished significantly for the spent WDC in Figure 5b compared with the virgin WDC shown in Figure 5a, and the total pore volume decreased from 0.50 to 0.14 cm 3 g −1 as shown in Table 4, indicating that the mesopore size~38 Å contributes to H 2 S adsorption. The peak A in Figure 5c,d has no obvious change after H 2 S adsorption which could support the assumption made earlier that the chemical reaction is more dominated by the H 2 S removal from the simulated biogas stream by using the AA adsorbent material.  Figure 5a,b illustrate the pore size distributions of the virgin and spent WDC adsorbents. According to the peak, peak A diminished significantly for the spent WDC in Figure 5b compared with the virgin WDC shown in Figure 5a, and the total pore volume decreased from 0.50 to 0.14 cm 3 g −1 as shown in Table 4, indicating that the mesopore size ~38 Å contributes to H2S adsorption. The peak A in Figure 5c,d has no obvious change after H2S adsorption which could support the assumption made earlier that the chemical reaction is more dominated by the H2S removal from the simulated biogas stream by using the AA adsorbent material. In Figure 6a-d, peak A in the derivative thermal gravimetric (DTG) curves at temperature~100 • C indicates the removal of moisture in the samples. The results reveal that WDC and SDC contain such an amount of moisture, but not the AA sample, according to the DTG and mass spectrometry results in Figure 6. In Figure 6a-d, peak B at~620 • C and peak C at temperature~780 • C, and in Figure 6e,f, peak A at temperature~750 • C and peak B at temperature~780 • C are associated with the emission of CO 2 from the adsorbent materials. The results are also supported by Hervy et al.'s research [33], that the CO 2 emitted from adsorbents under the same TGA program at temperatures~200, 620 and 750 • C; the H 2 S emitted from adsorbents at temperature~370 • C; and the SO 2 emitted at temperatures~285 and 370 • C. There is no obvious SO 2 or H 2 S emitted from the virgin and spent adsorbent materials could cause by the relatively small amount that the equipment is not sensitive enough.

Conclusions
This work compared the H 2 S removal capability of three types of adsorbent material from waste (wood-derived biochar, sludge-derived activated carbon and activated ash) in ambient temperature. The raw adsorbent materials were pre-activated under certain activation methods. A simulated biogas stream with the CH 4 -to-CO 2 ratio at 1.5 and H 2 S concentration at 200 ppm(v) was used to test the H 2 S removal efficiency of each adsorbent material. The concentration of H 2 S after the adsorption was measured by an MS. The MS result shows that the AA adsorbent has the highest removal capacity at 3.22 mg H2S g −1 and WDC has slightly lower H 2 S removal capability at 2.2 mg H2S g −1 . The TGA result shows that the ash residues composition of each adsorbent materials is ranked as AA (73.21 wt.%) > SDC (37.68 wt.%) > WDC (7.94 wt.%). The ash content results are also supported by the elemental analysis in which the carbon content of each adsorbent material is ranked as WDC (88.93 wt.%) > SDC (33.97 wt.%) > AA (7.42 wt.%). The ash composition was measured by XRF which shows the existence of alkaline mineral species in all adsorbent materials, such as Fe, Cu and Mn. The main mineral species in the WDC and AA adsorbents is Ca, and the main mineral species in the SDC adsorbent is Fe. WDC has relatively higher H 2 S removal capability (2.2 mg H2S g −1 ) compared with SDC (0.1 mg H2S g −1 ), which is due to the physical adsorption by its high porosity, so that the surface area of WDC is 210.18 m 2 g −1 . The mesopores play the most important role in adsorbing H 2 S. AA has the highest capability to remove H 2 S, which is due to the high content of ash residues which contain an alkaline