Valorization of Raw and Calcined Chicken Eggshell for Sulfur Dioxide and Hydrogen Sulfide Removal at Low Temperature

Chicken eggshell (ES) is a waste from the food industry with a high calcium content produced in substantial quantity with very limited recycling. In this study, eco-friendly sorbents from raw ES and calcined ES were tested for sulfur dioxide (SO2) and hydrogen sulfide (H2S) removal. The raw ES was tested for SO2 and H2S adsorption at different particle size, with and without the ES membrane layer. Raw ES was then subjected to calcination at different temperatures (800 °C to 1100 °C) to produce calcium oxide. The effect of relative humidity and reaction temperature of the gases was also tested for raw and calcined ES. Characterization of the raw, calcinated and spent sorbents confirmed that calcined eggshell CES (900 °C) showed the best adsorption capacity for both SO2 (3.53 mg/g) and H2S (2.62 mg/g) gas. Moreover, in the presence of 40% of relative humidity in the inlet gas, the adsorption capacity of SO2 and H2S gases improved greatly to about 11.68 mg/g and 7.96 mg/g respectively. Characterization of the raw and spent sorbents confirmed that chemisorption plays an important role in the adsorption process for both pollutants. The results indicated that CES can be used as an alternative sorbent for SO2 and H2S removal.


Introduction
Sulfur dioxide (SO 2 ) and hydrogen sulfide (H 2 S) are both considered toxic gases. SO 2 is mainly part of the flue gases while H 2 S is naturally present in many fossil fuels and quickly oxidizes to SO 2 upon burning. Direct release of these acidic gases to the open air can cause serious environmental repercussions [1,2]. SO 2 can be removed from flue gas in many ways and this process is named as flue gas desulfurization (FGD). The adsorption processes are already used and are well known for SO 2 removal as well as for H 2 S removal. Common sorbents for SO 2 removal are mostly calcium-based oxide/hydroxide (CaO/Ca(OH) 2 ), zinc oxide-based (ZnO), sodium hydroxide based (NaOH) and ammonia-based [3]. Limestone, slaked lime or a mixture of slaked lime with fly ash is commercially used in FGD systems [4]. There is a lot of room for improvement in the traditional FGD technologies as they consume a large quantity of water and at times CO 2 leakage to the environment [5,6]. Therefore, recently, many alternative sorbents such as red mud and various modified carbonaceous catalysts, have been developed with the aim to reduce the cost, promote principles of circular economy, and improve energy efficiency [7,8].
The removal of H 2 S, on the other hand, is considered a crucial step in the biogas industry because of its toxic and corrosive nature [9]. Effective utilization of biogas as biomethane is a challenge because of its costly purification steps [10]. Many technologies such as adsorption, alkaline washing (absorption), membrane separation, and cryogenic distillation have been tested to efficiently removes H 2 S [11]. The most common type of sorbent used for adsorption process is impregnated activated carbon [10]. Activated carbon has been reported to have H 2 S removal capacities in the range of 150-650 mg/g [12].

Characterization of Raw and Calcined Eggshell before and after Adsorption Tests
The morphology of ES sorbents was examined with Field Emission Scanning Electron Microscope (FESEM) images. Figure 1a shows the outer shell of ES and its FESEM images at different magnification. It can be seen that the outer shell of ES has a smooth surface with cracks. Figure 1b is the inner ES with the membrane. FESEM images clearly show fibrous network morphology of protein which is very porous in nature. Figure 1c shows the actual image of powdered (<90 µm) raw eggshell (RES) and FESEM images at different magnification. The porous nature of RES can be clearly seen with the pore hole like structures on the particles. Pore structures of ES sorbents are categorized as Type II as per Brunauer, Deming and Teller classification, stating their characteristics belong to macroporous material, nonporous materials, or materials with open voids. Catalysts 2021, 11, x FOR PEER REVIEW 3 of 20 . Figure 1. Digital camera and FESEM images of raw eggshell. (a) Outer at 300× and 5000× magnification, (b) inner at 300× and 5000× magnification, and (c) particle size of <90 µm at 10,000× and 30,000× magnification. Figure 2a,b show FESEM images of (CES 900 °C) and (CES 1100 °C). The images for 900 °C show a stable and structured particle compared to the one calcined at 1100 °C. (CES 900 °C) shows well-arranged particles with smooth surfaces on each particle. (CES 1100 °C) was totally the opposite, particles lose their shapes, and each particle shows intensive surface cracks due to the sintering process.   Figure 2a,b show FESEM images of (CES 900 • C) and (CES 1100 • C). The images for 900 • C show a stable and structured particle compared to the one calcined at 1100 • C. (CES 900 • C) shows well-arranged particles with smooth surfaces on each particle. (CES 1100 • C) was totally the opposite, particles lose their shapes, and each particle shows intensive surface cracks due to the sintering process.  Figure 2a,b show FESEM images of (CES 900 °C) and (CES 1100 °C). The images for 900 °C show a stable and structured particle compared to the one calcined at 1100 °C. (CES 900 °C) shows well-arranged particles with smooth surfaces on each particle. (CES 1100 °C) was totally the opposite, particles lose their shapes, and each particle shows intensive surface cracks due to the sintering process.   BET surface area values of the CES sorbents in comparison to RES are shown in Table 1. BET surface area of RES was reported as 0.56 m 2 /g and the readings were increasing as RES was calcined. However, the values decrease when the temperature was further increased up to 1000 • C and 1100 • C. The highest BET surface area of 6.74 m 2 /g was recorded at 900 • C. The BET surface area of CES was low compared to commercial-grade CaO whose BET surface area is in the range of 11-25 m 2 /g [22].  Figure 3 shows the nitrogen-adsorption isotherm for CES. The isotherm is of Type IV. As per IUPAC standard, this kind of isotherm is obtained for a combination of microporous and mesoporous structure which is formed at a higher relative pressure. The low BET surface area of CES could be due to eggshell structure and impurities. BET surface area values of the CES sorbents in comparison to RES are shown in Table  1. BET surface area of RES was reported as 0.56 m 2 /g and the readings were increasing as RES was calcined. However, the values decrease when the temperature was further increased up to 1000 °C and 1100 °C. The highest BET surface area of 6.74 m 2 /g was recorded at 900 °C. The BET surface area of CES was low compared to commercial-grade CaO whose BET surface area is in the range of 11-25 m 2 /g [22]. 2.68 Figure 3 shows the nitrogen-adsorption isotherm for CES. The isotherm is of Type IV. As per IUPAC standard, this kind of isotherm is obtained for a combination of microporous and mesoporous structure which is formed at a higher relative pressure. The low BET surface area of CES could be due to eggshell structure and impurities.   The elemental content of RES, CES, and their respective spent sorbents are shown in Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorption of SO 2 and H 2 S and the occurrence of chemisorption. The elemental content of RES, CES, and their respective spent sorbents are shown in Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorption of SO2 and H2S and the occurrence of chemisorption. The elemental content of RES, CES, and their respective spent sorbents are shown in Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorption of SO2 and H2S and the occurrence of chemisorption. The elemental content of RES, CES, and their respective spent sorbents are shown in Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorption of SO2 and H2S and the occurrence of chemisorption. The elemental content of RES, CES, and their respective spent sorbents are shown in Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorption of SO2 and H2S and the occurrence of chemisorption. The elemental content of RES, CES, and their respective spent sorbents are shown in Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorption of SO2 and H2S and the occurrence of chemisorption. The Ca content in the CES is more compared to the one in raw eggshell (RES) because, during the calcination, CO2 and other volatile matters are released. It is also evident that the contents of other impurities in ES such as Zn, Mg, Al, and Cu remain almost the same in RES, CES and spent sorbents. This shows that these impurities were not involved in the sorption process. In the spent adsorbent, the content of Ca was reduced. This indicated the conversion of Ca into sulfite complex.
FTIR spectra of RES, (CES 900 °C) and the spent sorbents are presented in Figure 4. The wideband approximately at 3430 cm −1 in the RES is attributed to the stretching of the OH bond [23]. The two well-defined bands at 1413 cm −1 and 874 cm −1 are distinctive to the bending of C-O bond of CaCO3 while the band at 712 cm −1 is related to Ca-O bond [24]. These indicate that RES comprises of calcite [25]. For (CES 900 °C), the well-defined band at approximately 3630 cm −1 corresponds to the vibration of OH bonds probably attached The Ca content in the CES is more compared to the one in raw eggshell (RES) because, during the calcination, CO 2 and other volatile matters are released. It is also evident that the contents of other impurities in ES such as Zn, Mg, Al, and Cu remain almost the same in RES, CES and spent sorbents. This shows that these impurities were not involved in the sorption process. In the spent adsorbent, the content of Ca was reduced. This indicated the conversion of Ca into sulfite complex.
FTIR spectra of RES, (CES 900 • C) and the spent sorbents are presented in Figure 4. The wideband approximately at 3430 cm −1 in the RES is attributed to the stretching of the OH bond [23]. The two well-defined bands at 1413 cm −1 and 874 cm −1 are distinctive to the bending of C-O bond of CaCO 3 while the band at 712 cm −1 is related to Ca-O bond [24]. These indicate that RES comprises of calcite [25]. For (CES 900 • C), the well-defined band at approximately 3630 cm −1 corresponds to the vibration of OH bonds probably attached to the surface of CaO [24]. The peak at 1413 cm −1 is sharper in (CES 900 • C) showing a higher percentage of CaO and more prevailing than RES. New peaks were not detected in the spent RES sorbents. Nevertheless, there were changes in the intensity of the peaks. This could be due to the contact of the acidic gases. However, a new peak at 1080 cm −1 was visible for (CES 900 • C) spent sorbents. This affirms the presence of sulfite [26].
The Ca content in the CES is more compared to the one in raw eggshell (RES) because, during the calcination, CO2 and other volatile matters are released. It is also evident that the contents of other impurities in ES such as Zn, Mg, Al, and Cu remain almost the same in RES, CES and spent sorbents. This shows that these impurities were not involved in the sorption process. In the spent adsorbent, the content of Ca was reduced. This indicated the conversion of Ca into sulfite complex.
FTIR spectra of RES, (CES 900 °C) and the spent sorbents are presented in Figure 4. The wideband approximately at 3430 cm −1 in the RES is attributed to the stretching of the OH bond [23]. The two well-defined bands at 1413 cm −1 and 874 cm −1 are distinctive to the bending of C-O bond of CaCO3 while the band at 712 cm −1 is related to Ca-O bond [24]. These indicate that RES comprises of calcite [25]. For (CES 900 °C), the well-defined band at approximately 3630 cm −1 corresponds to the vibration of OH bonds probably attached to the surface of CaO [24]. The peak at 1413 cm −1 is sharper in (CES 900 °C) showing a higher percentage of CaO and more prevailing than RES. New peaks were not detected in the spent RES sorbents. Nevertheless, there were changes in the intensity of the peaks. This could be due to the contact of the acidic gases. However, a new peak at 1080 cm −1 was visible for (CES 900 °C) spent sorbents. This affirms the presence of sulfite [26].  The proximate analyses of RES and CES are listed in Table 3. The moisture content and volatile matter of CES were much lesser than RES due to the high-temperature calcination process. The residue for CES was far greater than RES which shows the stability of CES at high temperature and amount of CaO produced.  The proximate analyses of RES and CES are listed in Table 3. The moisture content and volatile matter of CES were much lesser than RES due to the high-temperature calcination process. The residue for CES was far greater than RES which shows the stability of CES at high temperature and amount of CaO produced.  Figure 5 show X-ray diffraction (XRD) of RES, (CES 900 • C) and the spent sorbents. RES showed a major peak at 2θ = 29.5 • which indicates that CaCO 3 is a major constituent of the waste ES. In the (CES 900 • C), regular peaks were obtained at 2θ = 32 • , 34 • , 37.5 • , and 54 • , showing the conversion of CaCO 3 to CaO [23]. It is noted that peak at 2θ = 29.5 • is no longer visible in the (CES 900 • C), which implies a complete conversion of CaCO 3 to CaO.  For the spent RES, it can be seen that there was no significant difference in the crystalline structure after the adsorption of SO2 and H2S. It shows no chemical interaction between the sorbent and the gases. Thus, it can be concluded that for RES the adsorption was merely physical. However, for spent (CES 900 °C), the initial peaks at 2θ = 32°, 34°, For the spent RES, it can be seen that there was no significant difference in the crystalline structure after the adsorption of SO 2 and H 2 S. It shows no chemical interaction between the sorbent and the gases. Thus, it can be concluded that for RES the adsorption was merely physical. However, for spent (CES 900 • C), the initial peaks at 2θ = 32 • , 34 • , 37.5 • , and 54 • have disappeared and new peaks were formed at 2θ = 17 • and 29 • . The peak at 34 • in the (CES 900 • C) corresponds to CaO. This peak reduced in the spent sorbent, indicating the presence of unreacted CaO in the spent sorbent. The small peaks at 2θ = 17 • , 28 • , 28 • , and 2θ = 34 • , 47 • , 52 • , and 54 • may correspond to CaSO 3 and Ca(OH) 2 respectively for the spent (CES 900 • C). These spread-out peaks show that the crystallinity of (CES 900 • C) after adsorption has dropped. Similar peaks were reported by others as well [27].
pH values of the RES and CES before and after adsorption are tabulated in Tables 4 and 5 to show the reactivity of the acidic gases on RES and CES. There was a clear increase in pH with the increase in calcination temperature because of the formation of CaO which is basic in nature. Whereas for RES, the sorbents with membrane have slightly higher pH compared to the one without membrane. This is due to the different types of protein in the membrane. It was noticed that for all cases, spent sorbent's pH values dropped one level, indicating successful adsorption of acidic gases.

Effect of the Eggshell Membrane and Particle Size
Three different particle sizes (<90 µm, 90-125 µm, and 125-180 µm) of RES with and without membrane were tested for the SO 2 and H 2 S removal. During this analysis, other parameters such as sorbent dosage (1 g), flow rate (300 mL/min), humidity (0%), gas inlet concentration (300 ppm), pressure (1 bar), and reaction temperature (ambient temperature, 29 • C) were all kept constant. Table 4 shows the adsorption capacity and saturation time of SO 2 and H 2 S adsorption by RES with and without membrane. It was noticed that for both H 2 S and SO 2 the adsorption capacities were less than 1.1 mg/g on dry basis. The breakthrough point was not detected which indirectly shows that there was no immediate chemical interaction between the gas and sorbent. It is known that CaCO 3 is a highly stable material at ambient conditions. Thus, RES with or without membrane could have similar behavior. RES with membrane recorded higher adsorption capacity and longer saturation time for both H 2 S and SO 2 gas compared to the one without membrane. According to Tsai et al. (2006), ES membrane comprises of a grid of fibrous proteins which contributes to its large surface areas and these fibrous proteins have higher BET surface area compared to the shell itself [28]. In the literature, the role of ES membrane in the removal of reactive dyes, heavy metals, phenols, and various other substances was reported and in most of the cases, it was reported that the adsorption capacity was better with membrane compared to one without membrane [29,30]. Thus, in this study, it was found that RES with membrane enhanced the sorption of H 2 S and SO 2 .
As for the effect of particle size, as anticipated, the smallest particle size i.e., <90 µm shows the best results for both RES with and without membrane. The calculated adsorption capacity values followed the following sequence for both gases: 90 µm > 125 µm > 180 µm. Witoon (2011) had stated that the smaller particle size of calcined ES had higher CO 2 capture capacity because it provides a greater exposed surface for the adsorption [23]. Similarly, in this study, smaller particles can offer a greater surface area for gas-solid interactions. The macro-pores and pits are irregularly dispersed over the surface of the RES, which could be one of the factors for low adsorption capacity as evident from the FESEM image. At high temperature, CaCO 3 breaks down to CaO (s) and release CO 2 . The Ca +2 of CaO is unstable and reacts with SO 2(g) replacing oxygen at high temperature. The reaction of SO 2 gas on CaCO 3 can be shown by the following chemical reaction [31]; However, calcium sulfate (CaSO 4 ) is only formed from the reaction between calcium carbonate (CaCO 3 ) and SO 2 gas when the temperature is more than 750 K in presence of oxygen (O 2 ) as illustrated below [31]; CaCO 3(s) + SO 2(g) + 1 2 O 2(g) → CaSO 4(s) + CO 2(g) .
As the experiments were carried out in room temperature and O 2 was not induced in this study, the only possible reaction would be the formation of calcium sulfite (CaSO 3 ) rather than CaSO 4 . For the reaction between CaCO 3 of RES and H 2 S, the following direct sulfidation reaction is expected [32]:

CaCO 3(s) + H 2 S (g) → CaS (s) + CO 2(g) + H 2 O (v) .
(3) However, at room temperature, CaCO 3 is very stable and the chances of the above reactions are very slim. However, for RES only physical adsorption has happened for both SO 2 and H 2 S which is the main reason for its low adsorption capacities. As physical adsorption is happening, so the particle size and surface porosity played an important role.

Effect of Calcination Temperature
The influence of various calcination temperature on RES and their effect on SO 2 and H 2 S adsorption were tested using dried and powdered (<90 µm) RES with membrane. Other adsorption parameters were kept the same as in section "Effect of the eggshell membrane and particle size". Figure 6a,b shows the breakthrough curves of SO 2 and H 2 S versus calcination temperature of ES. Among the calcination temperature, 900 • C shows a prominent outstanding curve. Breakthrough points were very short however, it has a longer saturation time (84 min and 77 min for SO 2 and H 2 S, respectively) for both gases. Table 5 tabulates the adsorption capacity of the CES at different temperatures. (CES 900 • C) shows the highest adsorption capacity i.e., 2.63 mg/g and 3.53 mg/g for H 2 S and SO 2 respectively. High adsorption capacity at 900 • C calcination could be due to the complete conversion of calcium carbonate (CaCO 3 ) to calcium oxide (CaO). It is noted that complete conversion takes places around 930 • C [32,33]. Thus, at temperature 800 • C, lower adsorption capacity was noticed for both gases. Moreover, the 800 • C calcined ES was visibly grayish in color compared to the ones calcined at a higher temperature which were whitish confirming the incomplete calcination. At higher temperature i.e., >900 • C, the adsorption capacities for both SO 2 and H 2 S decreased. Moreover, the saturation time was shorter and there was no breakthrough point. The decrease in adsorption capacity is due to the sintering effect. Similar work on SO 2 adsorption by calcined limestone reported that pore size distribution is greatly affected by the calcination temperature.
It was reported that 950 • C was the optimized calcination temperature for limestone and at this temperature, the pores of CaO have the least diffusion resistance and highest activity for SO 2 removal [34]. Another similar work mentioned that sintering of CaO derived from pure CaCO 3 starts at 800-900 • C and it becomes more severe after 950 • C. ES is less pure than the commercial limestone, where for each 100 g of air-dried of ES waste only 88 g are of CaCO 3 [35], which corresponds to an increase in the rate of sintering [36,37]. Also, it was proven that a breakdown of the pores occurs for 1100 • C. These statements can be confirmed using BET surface area readings, and FESEM images.
due to the sintering effect. Similar work on SO2 adsorption by calcined limestone reported that pore size distribution is greatly affected by the calcination temperature. It was reported that 950 °C was the optimized calcination temperature for limestone and at this temperature, the pores of CaO have the least diffusion resistance and highest activity for SO2 removal [34]. Another similar work mentioned that sintering of CaO derived from pure CaCO3 starts at 800-900 °C and it becomes more severe after 950 °C. ES is less pure than the commercial limestone, where for each 100 g of air-dried of ES waste only 88 g are of CaCO3 [35], which corresponds to an increase in the rate of sintering

Effect of Reaction Temperature and Humidity
The influence of reaction temperature on SO 2 and H 2 S adsorption by RES and (CES 900 • C) was evaluated using two different reactor temperatures (100 • C and 200 • C). Other parameters such as sorbent dosage (1 g), flow rate (300 mL/min), gas inlet concentration (300 ppm), and pressure (1 bar) were all kept constant. Table 6 shows the adsorption capacity for the effect of reaction temperature and humidity. Figure 7 shows the breakthrough curve for the effect of reaction temperature on H 2 S and SO 2 removal. The trend was comparatively better than the ones done at the room temperature (29 • C) earlier. However, the impact was very minimal for (CES 900 • C). Meanwhile, for RES, the adsorption capacity was doubled when the reactor temperature was increased from 30 • C to 200 • C. It was claimed that an increase in reaction temperature, increases the chemical interaction of SO 2 and H 2 S with limestone-based CaCO 3 or CaO or Ca(OH) 2 sorbents [38][39][40]. The effects of reactor temperature in the range of room temperature to 200 • C is considered low in magnitude. At a temperature below 200 • C, only calcium sulfite will be formed during the reaction of SO 2 with CaCO 3 and CaO [41]. Calcium sulfite, however, is stable at around 200 • C and only becomes unstable at a temperature above 727 • C decomposing to form calcium sulfate and calcium carbide [42]. Though, at a temperature above 100 • C the rate of calcium sulfite formation tends to increase [41]. There are limited data on SO 2 and H 2 S by CaCO 3 and CaO at temperature < 250 • C. Most of the studies have been done at elevated temperatures, i.e., >400 • C. These analyses illustrated that there are some interactions between SO 2 and H 2 S with RES and CES even at low temperature. A lower temperature will be favorable for SO 2 and H 2 S removal because the temperature of flue gas going to the stack is around 150 • C and the working temperature of is about 55 • C [43,44]. However, at a higher temperature, the reaction could be further enhanced. A study of SO 2 adsorption by lime (80% CaO) reported that the conversion rate gets double when the temperature increases from 400 • C to 800 • C [45]. Moreover, during the reaction of CaCO 3 with H 2 S, complete conversion of CaCO 3 to CaS is only feasible if sulfidation is carried out at a temperature above its calcination temperature [46]. Thus, it can be concluded that a more conducive environment is created for the sulfidation of both RES and (CES 900 • C) by increasing the reaction temperature up to 200 • C.   Figure 8 shows the breakthrough curves of RES and (CES 900 °C) with a response to the effect of humidity. All other parameters were kept constant. It was noticed that with the addition of 40% relative humidity (RH), the performance of both RES and (CES 900 °C) improved significantly. The adsorption capacity of SO2 and H2S by (CES 900 °C) increased almost triple with humidity. There were significant improvements in the breakthrough time as well as saturation time for both SO2 and H2S. The breakthrough point was clearly noticeable for (CES 900 °C). For RES, a great increase was noticed for SO2, however, for H2S, only small improvement was noticed. Results can be compared from Table 4-6. The presence of humidity in the inlet gas improved the adsorption capacity of SO2 more than the H2S for RES. This could be due to the solubility of SO2. The solubility of SO2 in water is about 16 times more than the solubility of H2 as per the data published by [47]. The chemical reaction of SO2 and CaCO3 in the presence of RH is as shown below; (b) Figure 7. Breakthrough curves of (a) SO 2 and (b) H 2 S at different reaction temperature. Figure 8 shows the breakthrough curves of RES and (CES 900 • C) with a response to the effect of humidity. All other parameters were kept constant. It was noticed that with the addition of 40% relative humidity (RH), the performance of both RES and (CES 900 • C) improved significantly. The adsorption capacity of SO 2 and H 2 S by (CES 900 • C) increased almost triple with humidity. There were significant improvements in the breakthrough time as well as saturation time for both SO 2 and H 2 S. The breakthrough point was clearly noticeable for (CES 900 • C). For RES, a great increase was noticed for SO 2 , however, for H 2 S, only small improvement was noticed. Results can be compared from Tables 4-6. The presence of humidity in the inlet gas improved the adsorption capacity of SO 2 more than the H 2 S for RES. This could be due to the solubility of SO 2. The solubility of SO 2 in water is about 16 times more than the solubility of H 2 as per the data published by [47]. The chemical reaction of SO 2 and CaCO 3 in the presence of RH is as shown below; Ca(OH)(CO 3 H) + SO 2 (g) → CaSO 3 + H 2 CO 3 .
Carbonic acid (H 2 CO 3 ) is a product of this surface reactions, not alike the one without RH which produces carbon dioxide [31]. Between 30% and 85% RH, SO 2 and CaCO 3 reaction are improved significantly, approximately by 5 to 10 fold for single crystal CaCO 3 (calcite) in the presence of moisture [48]. A slight improvement is noticed because of the humidity which made the contact possible between CaCO 3 and the acidic gases [49]. It is known that CaO reaction in the presence of water vapor will form Ca(OH) 2 . At low temperature, it is expected that only sulfite hemihydrate will be formed when SO 2 is present [41]. The following reaction could occur; The chemisorption process of SO 2 onto the sorbent surface as described Equation (6) chemical reaction increases with increasing RH [41]. It has been reported that moisture can enhance the adsorption capacity of carbonates and oxides for atmospheric gases [31]. For example, in a humid air condition, the deposition velocity of SO 2 gas onto calcite and dolomite increases [31]. Moreover, SO 2 can oxidize to SO 3 , to form sulfuric acid [50]. Similarly, for H 2 S, there was a small increase in the adsorption capacity although the gas is not readily soluble in water. This increase is attributed to the contact time between CaO sorbent and water vapor. H 2 S partly dissolves in water to form a weak acid and CaO would readily attract a water molecule to convert to a more stable form of calcium hydroxide (CaOH) 2 . Yet, the chances of additional reaction between CaOH and H 2 S to form CaS are low as this reaction could only happen at high temperature, i.e., above 900 • C [51]. chemical reaction increases with increasing RH [41]. It has been reported that moisture can enhance the adsorption capacity of carbonates and oxides for atmospheric gases [31]. For example, in a humid air condition, the deposition velocity of SO2 gas onto calcite and dolomite increases [31]. Moreover, SO2 can oxidize to SO3, to form sulfuric acid [50]. Similarly, for H2S, there was a small increase in the adsorption capacity although the gas is not readily soluble in water. This increase is attributed to the contact time between CaO sorbent and water vapor. H2S partly dissolves in water to form a weak acid and CaO would readily attract a water molecule to convert to a more stable form of calcium hydroxide (CaOH)2. Yet, the chances of additional reaction between CaOH and H2S to form CaS are low as this reaction could only happen at high temperature, i.e., above 900 °C [51].  Table 7 shows a comparison of various sources of Ca-based sorbents and its potential to adsorb SO2 and H2S respectively. It can be seen that the adsorption capacity of the CES is comparable to that of the Ca-based sorbents. Lower adsorption capacity could be due to the low BET surface area, impurities in ES, and the unstable nature of ES. Some of the Ca-sorbents reported in the literature were modified with chemicals which further enhanced the adsorption capacity. No one has reported ES in the form of CaCO3. If it has been tested in the raw form maybe the reported adsorption capacity readings would be much lower compared to ES. Thus, it is expected that if ES is further modified to calcined  Table 7 shows a comparison of various sources of Ca-based sorbents and its potential to adsorb SO 2 and H 2 S respectively. It can be seen that the adsorption capacity of the CES is comparable to that of the Ca-based sorbents. Lower adsorption capacity could be due to the low BET surface area, impurities in ES, and the unstable nature of ES. Some of the Ca-sorbents reported in the literature were modified with chemicals which further enhanced the adsorption capacity. No one has reported ES in the form of CaCO 3 . If it has been tested in the raw form maybe the reported adsorption capacity readings would be much lower compared to ES. Thus, it is expected that if ES is further modified to calcined ES, it could definitely perform better than the ones reported in the literature. The only one who has done similar work on ES is Witoon (2011) who tried for CO 2 capture at various temperatures by TGA method [23]. The carbonation rate was around 35% at 750 • C of calcination temperature. Thus, there is a big potential for calcined ES to be used for pollutant gases removal.

Sorbent Preparation
Chicken eggshell waste (brown in color classified as Grade B and C) was collected from the student's food court in the university campus. It was thoroughly soaked and washed with tap water until a clean eggshell were obtained. Two types of raw eggshell were prepared i.e., with the membrane (RES) and without the membrane. For the samples without the membrane, the membrane was carefully removed after soaking in water. The ES samples were then dried in an UF 110 oven (Memmert, Schwabach, Germany) at 105 • C for 24 h to remove the excessive moisture. Finally, the samples were ground to different particle sizes using a MX-GM1011H dry blender (Panasonic, Selangor, Malaysia). The powdered ES samples were then sieved using a WS TYLER RX29 vibrating sieve (Fisher Scientific, Pittsburgh, PA, USA). Three different particle sizes were isolated (<90 µm, 90-125 µm and 125-180 µm). Calcined ES (CES) samples were prepared by heating the powdered RES at different temperatures (800 • C, 900 • C, 950 • C, 1000 • C, 1100 • C) for 2 h using a LEF-103S model muffle furnace (LabTech, Debrecen, Hungary). The retention time for calcination was chosen based on preliminary studies.

Adsorption Tests
The adsorption tests were performed in a lab-scale adsorption reactor as shown in Figure S1 (Supplementary Material). SO 2 and H 2 S gas flow from the gas cylinders (2000 ppm, 99% purity) were controlled automatically using SDPROC mass flow controllers and flow meters (Aalborg, New York, NY, USA). The gas flow rates of both gases into the reactor were kept at 300 mL/min throughout the experiments. H 2 S and SO 2 concentrations were kept constant at 300 ppm by introducing nitrogen gas as balance. Low flowrate and concentration were chosen for the overall safety of the lab. A down-flow fixed bed reactor made of stainless steel with an internal diameter of 9 mm and a height of 180 mm was prepared to fill the sorbents. The reactor was fixed inside an oven (Memmert, Schwabach, Germany) for temperature study. In each run, 1 g of sorbent was placed inside the reactor. The initial and outlet concentrations of H 2 S and SO 2 gas were measured using Biogas 5000 Portable Gas Analyzer (Geotech, Chelmsford Essex, UK) and Vario-Plus Industrial Gas-Analyzer (MRU Instruments Inc., Humble, TX, USA) respectively. SO 2 and H 2 S analyzers recorded the gas concentrations every second. The adsorption capacity was calculated from the following equation [58] using the breakthrough curve generated during the experiment: where Q is adsorption capacity (mg/ g), c o is the initial inlet concentration (ppm), M w is the gas molecular weight (g/mol), q is the total flow rate (L/min), w is the weight of sorbent (g), V m is the molar volume (L/mol), c is the outlet concentration of the gas (ppm) at time t (min). The adsorption capacity calculation was based on an average of 3 repetitions of the adsorption breakthrough curve. The differences between the 3 readings were less than 3%.

Process Parameters Study
In the process study, the effects of the relative humidity in the inlet gas and reaction temperature were evaluated for both RES and CES sorbents. Only optimized sorbents (both RES and CES) were selected for the process study. The reactor was fixed inside the oven and the temperature of the oven was varied from room temperature (approximately 30 • C) to 200 • C to study the effect of temperature on adsorption tests. Whereas the humidity in the inlet gas was created by passing the inlet gas through an airtight conical flask which was submerged in a temperature-controlled water bath before it could enter the reactor at room temperature. Required humidity in the inlet gas was created as the gas passing through the water bath was saturated with water vapor at the set temperature. The temperature of the water bath was set based on the steam tables formulation to create 40% relative humidity in the inlet gas. 40% of relative humidity was selected as it can be generated at room temperature and also to protect the gas sensors in the analyzer.

Characterization of the Sorbents
Morphology of the RES and CES were analysed by field emission scanning electron microscope (FESEM), model JEOL JSM-6701F (JEOL, Akishima City, Japan). The energy-dispersive X-ray spectroscopy (EDS) (JEOL, Akishima City, Japan) was employed to detect the specific elements on the surface of the materials. Fourier Transform Infrared (FTIR) Lambda 35 (Perkin Elmer, Waltham, MA, USA) was used to determine the surface functional groups of the sorbents. The spectra were recorded in the spectral range of 400-4000 cm −1 with a resolution of 4 cm −1 by mixing a small quantity of the sorbent with potassium bromide. pH was measured by preparing a solution in the ratio of 0.1 g of sorbent in 20 mL deionized water and stirred for 1.5 hr. A digital pH meter was used (Hanna Instruments, Woonsocket, RI, USA). X-ray diffraction model Lab X XRD-6000 (Shimadzu, Tokyo, Japan) was used to identify the XRD patterns of the sorbents at room temperature at 2θ with a step size of 0.02. The Brunauer-Emmett-Teller (BET) surface area and pore size distribution were calculated using nitrogen adsorption and desorption isotherms conducted at 77 K with micromeritics, ASAP 2020 V4.02 (Micromeritis, Norcross, GA, USA) volumetric gas adsorption instrument. Thermogravimetric analysis (TGA) of RES and CES was done with TGA/DSC 3+ (Mettler Toledo, Ohio, OH, USA) for proximate analysis. Nitrogen gas was used at 20 mL/min at a heating rate of 10 • C /min until 900 • C. Original images of ES were taken with a smartphone.

Conclusions
RES and CES sorbents were tested for SO 2 and H 2 S adsorption. It was found that RES with membrane and having the smallest particle size i.e., <90 µm showed the best adsorption capacity for both SO 2 and H 2 S. (CES 900 • C) showed the best adsorption capacity among the other calcination temperature and RES. It can be concluded that physical adsorption was dominant over the chemical adsorption for both RES and CES sorbents. The characterization study shows the existences of sulfur element in the spent adsorbents which further verifies the adsorption of SO 2 and H 2 S by CES. The presence of the relative humidity in the inlet gas and increasing reaction temperature improved the performance of both RES and CES sorbents. (CES 900 • C) showed a greater adsorption capacity compared to RES with the addition of humidity. The best adsorption capacity of SO 2 and H 2 S was recorded as 11.68 mg/g and 7.96 mg/g respectively using (CES 900 • C) with 40% RH. These results indicate that chicken eggshell have great potential to be used as sorbents upon modification for the removal of pollutant gases such as SO 2 and H 2 S from contaminated air.