Waste oleaster seed-derived activated carbon mixed with coarse particles of fluid catalytic cracking as a highly-efficient CO2 adsorbent at low temperatures

Process Safety and Environmental Protection 178 (2023) 580–594 Available online 23 August 2023 0957-5820/© 2023 The Authors. Published by Elsevier Ltd on behalf of Institution of Chemical Engineers. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Waste oleaster seed-derived activated carbon mixed with coarse particles of fluid catalytic cracking as a highly-efficient CO 2 adsorbent at low temperatures Mir-Jamaleddin Athari a , Maryam Tahmasebpoor a , * , 1 , Babak Azimi a , Mohammad Heidari a , Covadonga Pevida b , * a Faculty of Chemical & Petroleum Engineering, University of Tabriz, P.O. Box 51666–16471, Tabriz, Iran b Instituto de Ciencia y Tecnología del Carbono, INCAR-CSIC, c/ Francisco Pintado Fe, 26, 33011 Oviedo, Spain ARTICLE INFO Keywords: CO2 capture Activated carbon Oleaster seed Fluidization FCC particles SiO2 nanoparticles ABSTRACT Activated Carbon (AC) derived from low-priced biomass materials has been identified as the efficient and costeffective candidate for low-temperature CO 2 adsorption technique performed, though suffering from limited CO 2 capture capacity and heterogeneous fluidization in fluidized-bed systems have remained the main challenges for the industrialization of carbonaceous sorbents. Herein, for the first time, the novel, low-cost, and highly fluidizable carbonaceous sorbent derived from waste-based oleaster seed (OS) has been developed through mixing with cost-effective coarse fluid catalytic cracking (FCC) particles. Hydrophobic SiO 2 nanoparticles (NPs) which are well-known as highly efficient assistant materials for enhanced fluidization of hard-to-fluidize particles was also tested for the comparison. To determine the best activator and activator/precursor weight ratio, AC sorbent obtained from the pyrolysis of OS was prepared with two disparate activators of KOH and ZnCl 2 , and the activator/precursor weight ratios of 2, 3, 4, and the outcomes reveal the highest average CO 2 capture capacity of 2.78 mmol/g over three successive cycles for KOH-activated sorbent with activator/precursor weight ratio of 2. This great OS-derived sorbent was mixed with different wt% of coarse FCC and SiO 2 NPs to boost its fluidity; for which, the bed expansion reached from 1.6 to 3.5, at the gas velocity of 5 cm/s by mixing with 2.5 wt% FCC, similar to 2.5 wt% SiO 2 NPs-assisted sample. In addition to homogeneous fluidizability, a superior multicyclic stability of 95.5% during 25 multiple adsorption/desorption cycles was also assessed. The highly cost-effective FCC-mixed OS-derived activated carbon with both enhanced fluidity and multicyclic CO 2 capture activity is introduced as the low-temperature CO 2 capture candidate. 1. Introduction In addition to shortage of water, and intense drinking water pollution, global warming phenomenon, chiefly arising from the severe CO 2 emission, have widely created concerns for environmentalists and our society (Ashrafivala et al., 2022). Carbon dioxide (CO 2 ), as an anthropogenic gas stemmed mainly from excessive consumption of fossil fuels that allocates a large portion of greenhouse gases, plays a crucial role in absorbing infrared radiation, increment of the earth temperature, and its devastation consequently (Wang et al., 2021; Valeh-e-Sheyda et al., 2022; Gimeno et al., 2021; Soodmand et al., 2023). Hence, researchers and environmental activists have put effort into methods and technologies in the Carbon Capture, Utilization, and Storage (CCUS) framework, including pre-combustion, oxyfuel combustion, and post-combustion capture approaches, that eliminate CO 2 from the flue gas and from the aerosphere (Malekli and Aslani, 2022; Ren et al., 2023; Imani et al., 2023a). Regarding the engineering principles and also process efficiency, post-combustion capture strategy provides sensible benefits compared to the preand oxyfuel combustion technologies (Heidari et al., 2021, 2020). During the last decades, several potent and effective approaches have been scrutinized regarding the implementation of an integrated commercial technology in CCUs scope which physical and chemical absorption (Tiwari et al., 2022; Sultan et al., 2022; Azadpour et al., 2021), adsorption (Samaddoost et al., 2023; * Corresponding authors. E-mail addresses: [email protected] (M. Tahmasebpoor), [email protected] (C. Pevida). 1 3338497 Contents lists available at ScienceDirect Process Safety and Environmental Protection journal homepage: www.journals.elsevier.com/process-safety-and-environmental-protection https://doi.org/10.1016/j.psep.2023.08.062 Received 6 June 2023; Received in revised form 15 August 2023; Accepted 20 August 2023 Process Safety and Environmental Protection 178 (2023) 580–594 581 Zhang et al., 2022), membranes (Cao et al., 2021), and cryogenics separation (Babar et al., 2021) are noticed as the main classes among. Recently, CO 2 adsorption by means of solid-state sorbents has enticed focus of attentions owing to it’s potential of being substituted for near to medium term without encountering any significant technology risks or changes (Gu et al., 2022; Heidari et al., 2022). Various forms of sorbents such as Al 2 O 3 (Guo et al., 2023), zeolites (Kim et al., 2022), Metal- –organic framework (MOF) (Yulia et al., 2022; Nejatbakhsh et al., 2022), MgO (Gong et al., 2023), CaO-based sorbent (Mousavi et al., 2023; Ochedi et al., 2020; Sattari et al., 2021), and carbon-based sorbents (Ai et al., 2021) have been broadly exploited through different targets. Amidst the employed solid-state sorbents, environmentalists have specially considered the low-cost activated carbon sorbents due to their desirable specifications as follows: widely accessible, fast kinetics, high recyclability potential, supreme porosity, tailorable structure for being used in favorable fields, and extraordinary thermomechanical properties (Ligero et al., 2023; Mukherjee et al., 2022). Activated carbons (ACs) can be acquired within the pyrolytic process of carbonaceous sources containing high carbon and low ash content in an oxygen-depleted environment which causes to elimination of their volatile compounds leading to formation of porous biomass materials with broad range of applications (Malini et al., 2023; Lu et al., 2022a; Ma et al., 2022). The preparation approach, activation procedure either physical or chemical method, type of activating agent and various both the interior and exterior structures are some other main factors sensitively overshadow the CO 2 uptake efficiency of synthesized ACs (Malini et al., 2023; Tahmasebpoor et al., 2023). Acquired char from different bio sources within pyrolysis technique suffers from insufficient porosity, which hampers its practical applications. Aiming to improve their textural features, chars are exposed to either chemical or physical activation process, leading to the development of the highly micro-scale porous structure that plays the most imposing role in adsorbing CO 2 with high-performance (Ogungbenro et al., 2020; Iranvandi et al., 2023). The facile chemical activation benefits from proper weave of terminal product, and superior efficiency. According to literatures, sodium hydroxide (NaOH), potassium hydroxide (KOH), sulfuric acid (H 2 SO 4 ), phosphoric acid (H 3 PO 4 ), and some salts such as zinc chloride (ZnCl 2 ) and potassium carbonate (K 2 CO 3 ) are commonly considered as the most utilized chemical activators (Lu et al., 2022b). Ismail et al (Ismail et al., 2022). synthesized bamboo-based AC through H 3 PO 4 activation process at 500 ℃ for 120 min. Final product possessing suitable surface area up to 1490 m 2 /g showed promising performance toward CO 2 uptake around 1.45 mmol/g at 25 ℃ and 1 bar. In another work, simple K 2 CO 3 activation procedure was applied to fabricate AC from spent coffee by Kim et al. (2020). Prepared porous carbon activated at 700 ◦C for 5 h revealed the notable CO 2 uptake (4.54 mmol/g) at 25 ◦C and 1 atm. Iranvandi et al. (2023) indicated that activating date seeds by KOH led to a great promotion in resulted ACs characteristics and average CO 2 uptake (1.258 mmol/g) through 25 successive cycles under practical condition. Elaeagnus angustifolia which is the scientific name of Russian olive or oleaster, native to western and central Asia, Iran, from southern Russia and Kazakhstan to Turkey, parts of India and Pakistan, ordinarily grows in harsh conditions (relatively cold and arid regions) and offers great antiflamatory and antioxidant properties. Yagmur et al. (2020) appraised the characteristics of carbon-based sorbent obtained from activation of oleaster fruit and its fractions (peel, flesh and seed) via KOH and ZnCl 2 . Flesh originated ACs, indicated higher BET surface area and total micropore volume commensurate to 2021, 1816 m 2 /g and 1.293, 0.978 cm 3 /g for ZnCl 2 and KOH activated samples, respectively. Synthesized samples also revealed tolerable textural characteristics, indicating their great applicability toward carbon-based processes. In another work, the iodine adsorption performance of oleaster seed based AC was evaluated by S¸ahin et al. (2015). One-step activation of oleaster seeds via ZnCl 2 under 500 ℃ in N 2 atmosphere for 1 h was applied to achieve AC. It was demonstrated that the resulted sample represented proper textural properties with a preferable number of adsorbed iodine (1445 mg/g) suggesting oleaster seed could be an appropriate candidate for CO 2 uptake has yet to be tested. Improving the fluidization behavior of hard-to-fluidize nanoparticles under high gas velocity through the facile methods has still remained as a challenge for researchers (Raganati et al., 2018; Amjadi et al., 2019; Amjadi and Tahmasebpoor, 2018; Rahimvandi Noupoor and Tahmasebpoor, 2019). The carbon-based materials encounter the poor fluidizability stemming from the high interparticle adhesion forces due to their nature belonging to Geldart C (Raganati et al., 2018; Nobarzad et al., 2022). ACs are distinguished by outstanding features, especially their extraordinary surface area is suited them for CCUS technologies. Hence, enhancement of their fluidity in order to exposing their pores into the sorbent’ species such as CO 2 molecules should be seriously considered (Nejatbakhsh et al., 2023). Synthesized AC fine powders classified into the C-type Geldart particles suffer from the inopportune fluidization behavior attributed to their cohesive essence (Geldart, 1973). Multitudinous permanent dense agglomerates are appeared entire the bed through fluidization successively stems from powerful interaction among AC fine particles (Iranvandi et al., 2023). These strong interparticle adhesion forces lead to the heterogeneous fluidization conduct of fine powder particles, resulting in the formation of stable gas channels, bubbles, slugs and also the disorganized flow gas dispensation through the bed, inefficient particles-gas contact, and reduced CO 2 adsorption efficiency (Zhou and Zhu, 2020; Imani et al., 2022). The fluidity of hard-to-fluidize powder being homogenized, researchers have proposed several external methods, including acoustic fields (Raganati et al., 2015a, 2015b), sound-assisted methods (Raganati and Ammendola, 2021; Raganati et al., 2014), mechanical vibration (Nam et al., 2004). However, according to prohibitive methods internal approaches, the utilization of internal and low-cost methods was recently aimed to alleviate the fluidization difficulty of powders (Imani et al., 2023b; Nobarzad et al., 2021). Therefore, in order to boost the gas-solid contact efficiency through homogenization of the fluidity, mixing the hard-to-fluidized powders with easily fluidizable particles has been recognized as a reliable approach. Without implementing additional and expensive apparatus to reduce the interparticle cohesive forces between powders, mixing these powders with easily-fluidized materials, a considerably cost-effective method compared to external techniques, get to abate these intrusive forces and fluidize the powders under high gas velocities (Heidari et al., 2020; Nobarzad et al., 2022; Imani et al., 2022). As the operational efficiency of materials, in miscellaneous chemical aspects, being promoted through literature merging with NPs (Zeinali Heris et al., 2023), Azimi et al. (2019) studied the effect of different nanoparticles (NPs) on fluidity of a novel and hard-to-fluidize metal-- promoted CaO sorbent. It was shown the fluidization behavior of employed sorbent was positively affected by addition of hydrophobic SiO 2 and hydrophilic Al 2 O 3 and SiO 2 NPs, resulting in 3.73, 3.25, and 1.65 bed expansion ratio, respectively at gas velocity of 4 cm/s. Fluidization behavior of the novel synthesized carbon nanotubes (CNTs) before and after blending with low-cost fluidizable-SiO 2 auxiliary substance was evaluated by Javidi et al (Nobarzad et al., 2022). Researchers proved that addition of a minimum amount of 7.5 and 5 wt% SiO 2 NPs into pure and synthesized carbon nanotubes, respectively, greatly modified bed expansion ratio, minimum fluidization velocity, and Richardson-Zaki n index by stablishing agglomerate particulate fluidization (APF). In the other study, the influence of adding the cost-effective fluid catalytic cracking (FCC) coarse particles on fluidity of hard-to-fluidize Al 2 O 3 and TiO 2 nanopowders was investigated by Tahmasebpoor et al. (2019). They indicated that mixing an optimum amount of FCC course particles around 15–20 wt% with Al 2 O 3 and TiO 2 NPs developed their bed expansion ratio about 2 and 1.5 times, respectively. Based on the conducted literature review on carbon-based CO 2 sorbents, most investigations have stated that the biomass-based precursors M.-J. Athari et al. Process Safety and Environmental Protection 178 (2023) 580–594 582 are the most efficient, cost-effective, and eco-friendly candidates for preparing CO 2 sorbents; and then revealed their substantial CO 2 uptake capability. On the other hand, according to geographical investigations, oleaster seed (OS) is all-time waste biomass all over the world without any considerable utilization. To date, nor consideration has been paid to develop carbon-based CO 2 sorbents from the waste oleaster seeds. To the best of knowledge, the development of fluidizable ACs derived from waste materials with great CO 2 uptake capacity has not been achieved. Although the minor studies evaluated the addition of auxiliary and fluidizable materials to ACs to improve their fluidizability, no study has focused on exerting FCC as a promoting additive to develop highly fluidizable and sustainable AC-based CO 2 sorbent. Herein, for the first time, the environmentally-friendly, sustainable, low-priced, and fluidizable OS-derived ACs physically mixed with coarse FCC particles and SiO 2 NPs were prepared. To begin with, pyrolyzed OSs were activated with KOH and ZnCl 2 , considering three disparate activator/OS weight ratios of 2, 3, and 4 to select the most efficient CO 2 sorbent. The best candidate was mixed with 2.5 and 5 wt% of SiO 2 NPs and coarse FCC particles separately to boost the gas-solid contact efficiency through improving the fluidity in a lab-scale fluidized bed system. Ultimately, the stability of the best sample with better fluidity and higher CO 2 uptake performance was tested within 25 successive cycles. 2. Experimental 2.1. Materials The required waste OSs, employed as the carbon precursor, were gathered from the local shops in Iran. The carbon-derived sorbents were activated with two chemical activators: KOH (supplied by Merck) and ZnCl 2 (supplied by Merck). HCl (hydrochloric acid supplied by Arman Sina, Iran) was used to purify and solubilize the carbon-based samples. Two fluidizable additives were utilized, including Aerosil® 974 NPs (hydrophobic SiO 2 supplied by Evonik industry) and coarse FCC (supplied by Evonik industry) particles for improving the fluidity behavior of samples. The structural properties of SiO 2 NPs and FCC coarse particles are listed in Table 1. 2.2. Preparation 2.2.1. Pure and activated OSacquired carbon samples In the first step, all unpleasant components on the assembled OSs were washed with distilled water. Having been washed, they were completely desiccated at 120 ℃ for 12 h. Using the crushing and sieving methods, the OSs were gathered in particle sizes ranging between 250 and 300 µm. To activate the carbon particles, the 3 mol/L KOH and ZnCl 2 solutions were employed with various activator/OSacquired carbon weight ratios of 2, 3, and 4. The specific amount of sieved OSacquired carbon was soaked in the activation solution for 24 h at 25 ℃. Then, the wet solution was dried in an oven at 100 ℃ for 24 h. Having been desiccated, impregnated samples were processed in the furnace for 150 min at 600 ℃ as the optimized temperature to finalize the activation (Bai et al., 2023). The last step of preparation of activated OS-acquired carbons is devoted to washing them using 0.1 mol/L HCl and distilled water. The HCl-assisted purified sorbents were dried for 24 h at 90 ℃. Desiccated samples were crushed with a 1200-mess sieve. The production yields of the pyrolyzed and activated samples have been reckoned through the Eq. (1) (Tay et al., 2009): Yield of pyrolyzed or activated carbon(wt%) =Weight of pyrolyzed or activated carbon Weight of waste biomass ×100 (1) Table 2 specifies the abbreviation name and morphological, structural, and textural features as well as the production yields of developed OS-acquired sorbents. 2.2.2. Fluidizable additive-merged sorbents The coarse FCC particles and hydrophobic SiO 2 NPs (Tahmasebpoor et al., 2019), were used as the auxiliary additives to physically mixed with the OS-acquired activation carbons. Aiming at developing easily fluidizable sorbents, two concentrations, 2.5 and 5 wt%, of SiO 2 NPs and coarse FCC mixed with the most sustainable and efficient OS-acquired sorbent. The fluidizable SiO 2 NPs carry the non-fluidizable particles and contribute to decreasing the interparticle cohesive forces between sorbent particles. In addition, the FCC coarse particles act as the agglomerate breaker and decrease the size of formed aggregates during the fluidization (Tahmasebpoor et al., 2019). 2.3. Characterizations Aiming at investigating the porosity of developed sorbents, all samples were characterized with the Brunauer–Emmett–Teller (BET) method in the relative pressure (P/P 0 ) range between 0.02 and 0.22, measured by N 2 physisorption at 77 K in a BELSORP MINI II instrument after degassing in a vacuum at 120 ◦C for 2 h. The surface area (m 2 /g) and micropore volume (cm 3 /g) are measured through t-plot analysis. To recognize the attendance of functional groups on the surface of prepared sorbents, Fourier Transform Infrared (FTIR) measurement using a TENSOR 27 instrument from Brucker Co was conducted. Scanning electron microscopy (SEM) analysis by a 6301 F HOEL (United States) microscope was employed to assess the morphological alteration in surface properties of synthesized activated carbons. The SEM apparatus was connected with Oxford INCA sight X EDX experiment involved on SAMX detector to distinguish elemental dispersion. 2.4. CO 2 uptake assessment A TGA/DSC1 STARe apparatus from the Mettler Toledo instrument operating at atmospheric pressure was utilized to measure the CO 2 uptake capacity of developed sorbents. Prior to starting the 1st cycle, all samples were heated at 120 ◦C under N 2 (50 mL/min) for 60 min to eliminate physically adsorbed moisture and impurities. The CO 2 available in inlet gas was captured in the adsorption step at 25 ℃ under 90 vol% CO 2 balanced with N 2 for 120 min. In the 2nd step of each cycle, the sorbent was regenerated by desorbing the adsorbed CO 2 at 120 ◦C under N 2 with a flow rate of 50 mL/min for 60 min. Having the desorption step been terminated, the temperature of the TGA instrument decreased to 25 ℃ to embark on the new cycle. It should be noted that the cyclic stability tests conducted at 25 ℃ under 100 vol% CO 2 . The multicyclic CO 2 uptake stability of promising sorbents was measured through TGA over 25 consecutive carbonation/regeneration cycles under the abovementioned conditions. Using the weight variation of sorbents during the TGA tests, the CO 2 uptake capacity (mmol CO 2 / g sorbent) of developed sorbents was calculated through the following equation: CO2uptake capacity =Mn−M0 0.044 ×Mn (2) where M n refers to the mass of the sample after n th carbonation step, and M 0 addresses the primary mass of sample prior to starting the first cycle (Iranvandi et al., 2023). Table 1 Structural features of utilized fluidizable additives. Components Material name Particle diameter Particle density (kg/m 3 ) Tapped density (kg/m 3 ) Aerosil® 974 SiO 2 16 nm 1200 50 FCC coarse particles FCC 65 µm 993 828 M.-J. Athari et al. Process Safety and Environmental Protection 178 (2023) 580–594 583 2.5. Fluidization analysis The fluidization tests of the best sample mixed with fluidizable additives were conducted in the laboratory-scale fluidized bed system, containing an 80-cm glass column with an inner diameter of 26 mm, at the room condition (25 ◦C and atmospheric pressure) and under the CO 2 /N 2 mixture gas comprising 15 vol% CO 2 . The fluidity quality of samples was interpreted with the bed expansion ratio, measured as follows: Bed expansion ratio =Hf Hi (3) where H f and H i correspond the height of bed at the specific gas velocity and the initial bed height prior to inserting the inlet gas, respectively. The higher bed expansion ratio indicates the less formation of gas channels, the more efficient gas-solid contact; thus, the more homogenous fluidity behavior. To apply the highest accuracy and acquire exact data, all tests were conducted three times with 5-min intervals. 3. Results 3.1. Pure carbon sorbent acquired from OS The pure sorbent derived from pyrolyzing OS was first characterized to specify its sorption capability and textural features. The mass gain versus time during the TGA process for OS sorbent is presented in Fig. 1a. Having been subjected to inlet gas containing 90 vol%, the total mass of OS increases considerably versus time during the sorption step, ascribing to capturing the CO 2 molecules by OS sorbent. With the process situation change to desorption condition, the captured CO 2 molecules are desorbed and separated for sequestration. Derived from the TGA profile, CO 2 uptake capacity of OS over three successive sorption/ desorption cycles are listed in Table 3. As seen, CO 2 uptake capacity values of 1.53, 1.52, and 1.50 mmol/g are reported at 1st, 2nd and 3rd cycles, respectively, indicating the average uptake capacity of 1.52 mmol/g. The acquired uptake quantities corroborate the effectiveness of OSs to prepare the efficient carbon-based CO 2 sorbent performing at low-temperature CO 2 uptake process. The morphological and textural porosity of OS obtained from BET/ BJH analyses are reported in Table 2. The micropore surface area of 278.25 m 2 /g and micropore volume of 0.184 cm 3 /g indicate the significant porosity of OS structure, showing the capability of OSs for synthesizing highly sustainable CO 2 sorbent. Fig. 1b illustrates the SEM image of prepared OS sorbent. The representative image shows the presence of several pores in the OS surface, associated with compact zones and agglomerates. As a result, the OS sorbent possesses an irregular compact morphology concomitant with the minor formation of micropores. Surface chemistry compositions of the pyrolytic sample measured via FTIR analysis are illustrated in Fig. 1c. The broad peak that appeared in the range of 3900–3000 cm −1 is related to the presence of the O-H bonds stemmed from alcohol, phenol, carboxylic acid, and adsorbed moisture (S¸ahin et al., 2015; Kaouah et al., 2013; Naghash-Hamed et al., 2023a). The other two distinct peaks at 2923 and 2855 cm −1 with the non-sensitive shifts and varied intensities are seeable attributed to aliphatic C-H bonds due to the presence of methyl and methylene groups (Kaouah et al., 2013; Sarwar et al., 2021). A low-intensity peak at 2530 cm −1 is also detectable and related to C – – C or C – – – C bonds. An intense peak can be observed in 1543 cm −1 which appears because of C – – O bonds (Baytar et al., 2021). In addition, a relatively intense peak at 871 cm −1 corresponded to C-H bonds is detected (S¸ahin et al., 2015). To recognize the distribution of elements in the OS surface, the EDX diagram of OS is exhibited in Fig. 1d. As expected, the OS surface contains a high carbon concentration of 79.1 wt%, and minor contents of impurities, namely Si (0.32 wt%), K (0.4 wt%), Ca (1.39 wt%), and Zr (1.33 wt%). It can be acclaimed that EDX values confirm the carbonbased nature of the developed OS and its high purity. 3.2. KOH-activated carbons acquired from OS Aiming at boosting the morphological, textural, and adsorptive features of OS-derived carbon, this material was activated with KOH through the chemical activation methods. With three diverse KOH/OS weight ratios, including 2, 3, and 4, been employed, the KOH-activated carbons were analyzed through the TGA instrument to specify the most efficient concentration. The weight gain/loss profiles of KOH-activated samples over three successive adsorption/desorption cycles at 25 ℃/ 120 ◦C under 90 vol% CO 2 are plotted in Fig. 2a. As shown, during all three cycles, the more mass of CO 2 is adsorbed by OS-K2, followed by OS-K3 and OS-K4. The CO 2 capture capacity values for OS-K2, OS-K3 and OS-K4 derived from Fig. 2a is demonstrated in Fig. 2b. The modification of OS-derived carbon with KOH results in a considerable increase in its sorption capacity. Accordingly, having been modified with KOH with KOH/OS weight ratio of 2, the initial sorption capacity of OS reaches up to 2.78 mmol/g from 1.53 mmol/g, showing 81.7% improvement. It is worth noting that the chemical integration of KOH into OS-derived carbons plays a positive role in boosting its CO 2 uptake capacity at low temperatures. On the other hand, the reduction in sorption capability of OS-derived AC ensues from enhancing the content of utilized KOH. With the KOH/OS ratio increased to 3 and 4, the average sorption capacity of OS-K2 reduces from 2.78 mmol/g to 2.49 and 2.41 mmol/g, respectively. In conclusion, based on the reported TGA data, OS-K2 indicates the superior CO 2 uptake activity among KOHactivated OS-derived carbon samples at all cycles. It can be concluded that increasing the KOH content deteriorates the multicyclic uptake potential of ACs. Fig. 3a illustrates the FTIR spectra diagram of OS-K2 and OS-K4 in wavenumber ranging between 3000 and 4000 cm −1 . Wide peaks Table 2 The structural, morphological, textural features and production yields of developed sorbents. Sorbent name Activator Activator/OS-acquired carbon weight ratio Auxiliary additive Surface area (m 2 / g) Micropore volume (cm 3 / g) Yield (%) Type Composition (wt %) OS - - - - 278.25 0.184 41.2 OS-K2 KOH 2 - - 564.23 0.525 35.3 OS-K3 KOH 3 - - 502.81 0.486 32.6 OS-K4 KOH 4 - - 447.15 0.441 31.4 OS-Z2 ZnCl 2 2 - - 344.64 0.258 37.1 OS-Z3 ZnCl 2 3 - - 288.91 0.197 35.6 OS-Z4 ZnCl 2 4 - - 248.64 0.160 34.7 OS-K2-S2.5 KOH 2 SiO 2 2.5 517.45 0.482 - OS-K2-S5 KOH 2 SiO 2 5 497.13 0.455 - OS-K2-F2.5 KOH 2 FCC 2.5 509.37 0.461 - OS-K2-F5 KOH 2 FCC 5 461.73 0.439 - M.-J. Athari et al. Process Safety and Environmental Protection 178 (2023) 580–594 584 Fig. 1. (a) TGA profile at adsorption/desorption temperatures of 25 ℃/120 ◦C in atmospheric pressure and under 90 vol% CO 2 balanced with N 2 , (c) SEM image, (b) FTIR spectra, and (d) EDS spectra for OS sample. M.-J. Athari et al. Process Safety and Environmental Protection 178 (2023) 580–594 585 elongated from 2965 to 3900 cm −1 are determined in that O-H bonds originated from functional groups and adsorbed humid exist in the texture of samples (S ¸ahin et al., 2015; Kaouah et al., 2013; Naghash-Hamed et al., 2022a). Similar to the pyrolytic sample, two weak peaks at 2923 and 2855 cm −1 are attributed to the methyl and methylene-based C-H aliphatic bonds (Kaouah et al., 2013; Sarwar et al., 2021; Naghash-Hamed et al., 2022b). Other apparent peaks at 1550 cm −1 are exclusively detected for KOH-activated samples that correspond to oxygen-infused C – – O bonds, such as carbonyl. It points to the fact that the nature of the employed activator crucially affects the type of the generated functional groups on the surface of the synthesized AC. Broad peaks prorated in the range of 900–1300 cm −1 appear for these samples showing the maximum intensities at 1110 and 1120 cm −1 . Mentioned peaks appear because of C-O bonds in different functional groups such as alcohol, carboxylic acid, ether, and ester (Sarwar et al., 2021; Yousefi et al., 2023). Differences in the appeared peaks and their intensities indicate the proper engagement of the activator and raw substance. In order to conduct the visible assessment of the morphology of KOHbased modified ACs, SEM images of OS-K2, OS-K3, and OS-K4 are illustrated in Fig. 3b, c, and d, respectively. The KOH-based activated ACs possess more roughness morphology and higher micropores compared to OS, indicating the effectiveness of the KOH-based activation on OS-derived carbons. According to SEM images, increasing the KOH content not only smooths the external surface of carbon but also collapses the external micropores and wrecks the porosity (Wickramaratne and Jaroniec, 2013a; Adeniran et al., 2014; Coromina et al., 2016). The more well-developed pores are formed in the case of OS-K2 compared to OS-K3 and OS-K4 due to its higher degree of roughness. In addition to the external surface, the large holes of OS-K2 morphology contain numerous cracks, crevices, and nanograins, indicative of the porous texture. In addition, among KOH-activated OS-derived carbons, the highest surface area of OS-K2 is apparent. Conformed to surface area and pore volume data reported in Table 2, OS-K2 contains more micropores and accessible morphological sites in comparison with OS-K3 and OS-K4. Coupled with the SEM test, the derived EDS spectra of OS-K2 to verify the accuracy of the employed preparation route is depicted in Fig. 3e. The presented spectra indicate the presence of impurities, including Si, Ca and Zr, in minor concentrations. In addition, the surface of OS-K2 is roughly full of carbon according to the sample nature. Textural and morphological properties, namely pore volume and surface area values, of OS and KOH-activated carbons derived from OS are shown is Fig. 3f. As expected, the utilization of KOH activator in the preparation of OS-derived carbons serves to enhance their accessible surface area and consequently porosity. In light of this, the surface area of OS increases from 278.25 cm 2 /g to 564.23, 502.81, and 447.15 by KOH-based modification with KOH/OS weight ratio of 2, 3, and 4, respectively. Furthermore, the pore volume values of 0.525, 0.486, and 0.441 cm 3 /g for OS-K2, OS-K3, and OS-K4, sequentially, are recorded, indicating 85%, 64% and 39% increment compared to that for OS sample (0.184 cm 3 /g). In addition to quantitative features, the pore volume distribution (PVD) diagram of OS, OS-K2, and OS-K4 are depicted in Fig. 3g. These diagrams concede the reported pore volume data so that the more volume of micropores and mesopores with low diameters are formed in the case of OS-K2, followed by OS-K4 and OS. Apparently PVD diagrams indicating the greater contribute of micropores, OS, OS-K2, and OS-K4 can be classified as the microporous structures. Moreover, coupled with the PVD diagram in Fig. 3 g, N 2 adsorption-desorption isotherms of OS, OS-K2, and OS-K4 versus relative pressure ranging between 0 and 1 is indicative of the classification of type II mode for all samples, confirming their micro and mesoporous texture and unrestricted mono and multilayer sorption up to relative pressure of ~1 (Naghash-Hamed et al., 2022a). Specified by the condensation of N 2 molecules at the relative pressure ranging between 0.8 and 1 in micro and mesopores, the hysteresis loop type of H3 is characterized for developed samples (Mousavi et al., 2023). Based on the determined hysteresis loop type, it can be deduced that OS, OS-K2, and OS-K4 possess the micro and mesoporous structure in compliance with the PVD diagram and SEM images. The alkalinity of potassium which causes corrosive properties at high temperatures results in uniform distribution of KOH between carbon layers and the formation of micropores. In addition, it has been reported that the incorporation of KOH into carbon-based configurations widens carbon layers, resulting in improved porosity (Otowa et al., 1993). Moreover, Sudaryanto et al. (2006) stated that the KOH contributes to promoting carbon oxidation and expanding the pore width. The superior porosity of OS-K2 compared to OS-K3 and OS-K4 is probably due to the adverse effect of additional KOH content on the microporous structure. With more KOH being included, the formation of mesopores expedites Table 3 Multicyclic CO 2 uptake of developed sorbents at adsorption/desorption temperatures of 25 ℃/120 ◦C in atmospheric pressure and under 90 vol% CO 2 balanced with N 2 . Sorbent CO 2 uptake (mmol/g) 1 st cycle 2 nd cycle 3 rd cycle Average OS 1.53 1.52 1.50 1.52 OS-K2 2.78 2.77 2.80 2.78 OS-K3 2.52 2.49 2.47 2.49 OS-K4 2.41 2.42 2.41 2.41 OS-Z2 1.77 1.73 1.71 1.74 OS-Z3 1.66 1.65 1.63 1.65 OS-Z4 1.54 1.52 1.51 1.52 OS-K2-S2.5 2.63 2.6 2.6 2.61 OS-K2-S5 2.54 2.43 2.48 2.48 OS-K2-F2.5 2.62 2.57 2.6 2.6 OS-K2-F5 2.48 2.46 2.41 2.45 Fig. 2. (a) TGA profiles for OS-K2, OS-K3, and OS-K4 (b) CO 2 uptake of OS, OSK2, OS-K3, and OS-K4 within three cycles at adsorption/desorption temperatures of 25 ℃/120 ◦C and under 90 vol% CO 2 balanced with N 2 . M.-J. Athari et al. Process Safety and Environmental Protection 178 (2023) 580–594 586 and the volume of micropores drops considerably, arising from the carbon layers expansion in the presence of excess KOH and then the collapse of micropores during the formation of mesopores (Coromina et al., 2016; Saleh et al., 2013; Sangchoom and Mokaya, 2015; Wickramaratne and Jaroniec, 2013b). Among KOH-based ACs, the OS-K2 indicates the most surface area and micropore volumes, revealing its higher microand meso-porosity, as presented in Fig. 3g. There is a symmetric correspondence between textural properties and CO 2 uptake Fig. 3. (a) FTIR spectra of OS-K2 and OS-K4, SEM images of (b) OS-K2, (c) OS-K3, and (d) OS-K4, (e) EDS spectra of OS-K2, (f) micropore volume and surface area values of KOH-activated carbon derived from OS, and (g) pore volume distribution diagram merged N 2 adsorption-desorption isotherms of OS, OS-K2, and OS-K4. M.-J. Athari et al. Process Safety and Environmental Protection 178 (2023) 580–594 587 capacity of developed samples; based which, OS-K2 showing the best CO 2 capture capacity takes advantages of higher microand mesopores volume and surface area. 3.3. ZnCl 2 -activated carbons acquired from OS In addition to KOH, OS-derived carbon was also modified with ZnCl 2 to determine the more suitable activator and investigate the influence of ZnCl 2 on adsorptive and structural properties of OS sample. Fig. 4a shows the weight gain/loss profile for ZnCl 2 -activated carbon derived from OS precursor with ZnCl 2 /OS weight ratios of 2, 3, and 4. The depicted trends indicate the most weight gain in the case of OS-Z2 during all three cycles. According to the fact that the weight increase during the sorption step arises from the CO 2 adsorption by samples, OSZ2 adsorbs more mass of CO 2 molecules compared to OS-Z3 and OS-Z4. In conclusion, the higher CO 2 uptake capacity during the three-cycle process belongs to OS-Z2. The quantitative CO 2 uptake capacity of OS-derived ACs with ZnCl 2 during three adsorption/desorption cycles at 25 ℃/120 ◦C under 90 vol % CO 2 / 10 vol% N 2 is presented in Fig. 4b. Outcomes confirm the efficient role of ZnCl 2 in enhancing the CO 2 adsorption capability of OSderived carbons. To this point, with ZnCl 2 -activated OS being prepared by considering ZnCl 2 /OS weight ratio of 2, its initial and average uptake capacity increases from 1.53 and 1.52 mmol/g to 1.77 and 1.74 mmol/gr, leading to 15.7 and 14.5 improvements in its CO 2 uptake activity, respectively. Among all developed samples activated with ZnCl 2 , the higher amount of CO 2 is captured by OS-Z2 rather than that of OS-Z3 and OS-Z4. Identical to KOH-activated sorbents, the increase in ZnCl 2 concentration adversely affects the multicyclic activity of OSderived carbons in which the average CO 2 capture capacity of OS-Z2 decreases from 1.74 mmol/g to 1.65 and 1.52 mmol/g by raising the ZnCl 2 /OS weight ratio to 3 and 4, respectively. Based on the findings, the ZnCl 2 /OS weight ratio of 2 can be considered the most efficient ZnCl 2 concentration. It can be concluded that based on the multicyclic CO 2 uptake performance of KOHand ZnCl 2 -based ACs derived from OS, the usage of chemical activators with an activator/OS weight ratio of 2 remarkably boosts the capability of ACs to adsorb CO 2 molecules at low temperatures. Fig. 5a shows the FTIR results of ZnCl 2 -activated samples. O-H bond attributed to the present moisture and other functional groups such as alcohol, phenol, and carboxylic acid leads to the formation of a stretched peak in the range of 3900–3000 cm −1 (S¸ahin et al., 2015; Kaouah et al., 2013; Naghash-Hamed et al., 2023a). Observed peaks at 2925 and 2855 cm −1 owning low intensities as similar to other samples address C-H aliphatic bonds (Kaouah et al., 2013; Sarwar et al., 2021). The low-intensity peak at 2530 cm −1 related to C – – C or C – – – C bonds in the case of the raw sample disappears for activated samples. Relatively wide stretching in the range of 1700–1300 cm −1 with the summit at 1616 and 1552 cm −1 can be attributed to C-O bonds of the carbonyls, carboxylic acids, and lactones. Furthermore, formed intense peaks at 1045 cm −1 correspond to C-O bonds originating from functional groups, whereas these broad peaks vanish in the case of the raw sample (Naghash-Hamed et al., 2023b; S ¸ahin et al., 2015). It can be deduced that the activation agent offended these bonds to form the new bonds. Aiming at confirming the quantitative structural features, the SEM micrographs of OS-Z2, OS-Z3, and OS-Z4 are illustrated in Fig. 5b, c, and d, respectively. Comparing the images reveals more morphological porosity of OS-Z2 than that of OS-Z3, and OS-Z4. In addition, the decreased initial and average CO 2 uptake values of OS-Z3 and OS-Z4 ensue from their more compact morphology and less surface-located pores in comparison with OS-Z2. It is worth noting that the SEM pictures validate the morphological and textural values acquired by BET and BJH tests for ZnCl 2 -activated OS-derived carbon sorbents. The EDS spectra of OS-Z2 to recognize the nature of developed sorbent and the elements’ distribution in its morphology is depicted in Fig. 5e. Identical to EDS spectra of OS and OS-K2 represented in Fig. 1d and Fig. 3e, respectively, the morphology of OS-Z2 comprises the high concentration of carbon as expected and minor content of Si, K, Ca, and Zr. The high carbon concentration in surface-located zones of OS-Z2 authenticates the utilized preparation route for OS-Z2. Fig. 5f demonstrates the surface area and pore volume of OS, OS-Z2, OS-Z3, and OS-Z4 in order to investigate the structural changes in OS structure by using diverse ZnCl 2 concentrations and comparing the porosity of developed ZnCl 2 -based ACs. The utilization of ZnCl 2 with a ZnCl 2 /OS weight ratio of 2 causes 23.9% and 40.2% enhancement in surface area and micropore volume values of OS-derived sorbent, respectively. The PVD diagrams of OS, OS-Z2, and OS-Z4 to conduct the graphical evaluation of porosity and ZnCl 2 -based activation are exhibited in Fig. 5g. OS-Z2 and OS-Z4 textures mainly consist of micropores and minor mesopores. In light of this, N 2 adsorption/desorption isotherms of OS-Z2 and OS-Z4 are entirely in compliance with PVD trends that show type mode of II and the hysteresis loop type of H3, identical with OS, OS-K2, and OS-K4. The More formation of micropores and mesopores grows out of the ZnCl 2 -based activation of OS with a ZnCl 2 / OS weight ratio of 2, whereas increasing the mentioned weight ratio from 2 to 4 deteriorates the porosity of activated carbon significantly collapses the porous zones. The presence of adequate ZnCl 2 during the preparation of OS-derived carbon decreases the agglomeration and forms more internal pores, resulting in higher micropore volume, accessible surface area, and consequently better porosity. ZnCl 2 facilitates the decomposition of carbon-based material as a dehydrating regent during the pyrolysis technique, leading to charring and formation of the microporous structure (S¸ahin et al., 2015; Hsu and Teng, 2000). Having the ZnCl 2 /OS been raised, a considerable reduction is acquired in surface area and micropore volume values. The porosity of ZnCl 2 -activated carbons follows the same trend as for the KOH-activated counterparts. This detrimental impact of increasing loaded ZnCl 2 amount stems from the Fig. 4. (a) TGA profiles for OS-Z2, OS-Z3, and OS-Z4 and (b) CO 2 uptake capacity of OS, OS-Z2, OS-Z3, and OS-Z4 within three cycles at adsorption/ desorption temperatures of 25 ℃/120 ◦C and under 90 vol% CO 2 balanced with N 2 . M.-J. Athari et al. Process Safety and Environmental Protection 178 (2023) 580–594 588 Fig. 5. (a) FTIR spectra of OS-Z2 and OS-Z4, SEM images of (c) OS-Z2, (d) OS-Z3, and (e) OS-Z4, (f) EDS spectra of OS-Z2, (f) pore volume and surface area values of ZnCl 2 -activated carbon derived from OS, and (g) pore volume distribution diagrams merged with N 2 adsorption/desorption isotherms of OS-Z2, and OS, OS-Z4. M.-J. Athari et al.