Investigation into the influencing factors and adsorption characteristics in the effective capture of carbon dioxide in flue gas by chitosan grafted Leca biocomposite

ABSTRACT The main goal of this research was to evaluate the capability of chitosan (CS)-lightweight expanded clay aggregate (Leca) composite as a green adsorbent for efficient capture of carbon dioxide (CO2). The adsorption process was modelled by response surface methodology (RSM). The composite was recognised by BET, SEM, EDX, XRF, XRD, TGA, XPS and FTIR analysis. It was evidenced the CS-Leca properties attributing to its adsorption ability were eminently improved as compared to pristine Leca. Maximum removal efficiency of CO2 (91.59%) was achieved at 40°C, 40 mL/min gas flow rate and 5% moisture content. Under aforementioned provisions, the actual CO2 removal percentage was found to be 90.4%. According to uptake isotherm studies, the Toth model indicated good affinity between the sorbent and sorbate. This model declares adsorption of CO2 onto CS-Leca involves a heterogeneous and uneven adsorption behaviour. The monolayer adsorption capacity (qm) was determined to be 351.24 mg/g. The outcomes of thermodynamic studies displayed that the adsorption was physical and exothermic in nature. The exothermic isosteric heat of adsorption (IHA) was found as 37.1–48.3 kJ/mol which represented weak chemical interactions between CS-Leca and CO2 molecules. Adsorption mechanism studies revealed that multiple mechanisms are involved in the adsorption reaction of CO2. Regeneration tests of CS-Leca exhibited multiple potential applications. All these results enlightened that the CS-Leca, due to its grafting polymer, can be applied as a potential sorbent to the recycle of CO2 and other acidic gases.


Introduction
Global warming has become a drastic environmental challenge and has created world community concern in recent years [1].Greenhouse gases (GHGs) are the primary concern behind this global issue.Carbon dioxide (CO 2 ), as a primary GHG, is believed to be the main contributor to the evolution of this crisis, contributing around 60% [2].Based on the evidence, the atmospheric levels of CO 2 have risen from about 270 ppm in the preindustrial revolution to 400 ppm [3].The growth trend of CO 2 emissions that related to the rising global population along with human activities and their energy demands [4] have caused mostly adverse effects in the last decades and would lead to diverse irreversible impacts for natural and human systems in the near future [5].Therefore, it is most important to mitigate atmospheric CO 2 concentrations using novel physicochemical techniques.Since power plants are the maximum consumers of fossil fuels and produce 40% of global emissions [6]; Post-combustion CO 2 removal from the power plant flue gases can be a potential approach to reduce CO 2 emissions [7].
Currently, the most widely used technology for CO 2 reduction is absorption by liquid amines, but it has several major drawbacks such as the huge equipment requirements, highly corrosive nature and desorption problems [8].Thus, the eco-friendly, simple, efficient, low energy consumption and low-cost alternative strategies must be considered.The adsorption process is a befitting method that has the aforementioned features [9,10].To date, the various types of solid materials (including zeolites [11], polymer composites [12,13], metal oxides [14], amine-functionalised mesoporous silica [15], and metal organic frameworks (MOFs), to name a few [16]) have been tested for CO 2 adsorption/capture.However, each of these sorbents faces certain privileges and critical defects.Also, their synthesis method, usage conditions, production cost, adsorption efficiency, regeneration capability, and toxicity for the environment and human health are considered as the key factors for choosing a suitable sorbent for viable applications.In the recent years, biopolymers, due to regenerable, efficient, and low cost material [13], have attracted the interest of researchers.
Biopolymers prepared from natural sources, are often available, green, eco-friend and low cytotoxic [17].More recently, significant efforts have been made to improve the CO 2 capture/uptake capacity of the adsorbents by modifying them through impregnating or compounding the polymers (with each other or with other materials).Because the adsorption process mainly depends on surface basicity, which is attributed the functional groups, specific surface area and porosity of adsorbent [8,18].Chitosan (CS), a natural polysaccharide, mainly comprises nitrogen heteroatoms and contains two hydroxyl (-OH) groups and one amino (-NH 2 ) group in each repeating dimer which serve as the adsorptive sites for acidic CO 2 molecules through a series of mechanisms of electrostatic interaction, hydrogen-bonding interaction, and acid-based interactions [19].Chitosan is the second most biopolymer found in nature (e.g. in the exoskeletons of shrimp, lobster and crabs) which has been widely used in a variety of knowledge fields due to its superior features like recyclability, nontoxicity, good biocompatibility, and biodegradability [20].Nevertheless, raw chitosan has revealed some barriers such as poor mechanical strength, low chemical stability, and severe thermal degradation.To overcome these downsides, the researchers used different modifiers such as metal-based particles [21,22], polymer materials [23][24][25], clay-like materials [20,[26][27][28][29], carbon-based material [30][31][32], etc., among which, clay-like materials are desired [33].This reinforcement action has been revealed to be able to significantly elevate the stability of CS and its adsorption capability towards pollutants [34,35].In this study, lightweight expanded clay aggregate (Leca) as a key member of clay-like materials family was considered to be supporter for chitosan due to its remarkable features, viz., high porosity, good thermal stability, costeffectiveness, universal accessibility, and environmentally friendly [36].Leca not only carries chitosan powders, but also upgrades its mechanical strength.It should be noted the chitosan composites have been extensively investigated in aqueous environments [37,38], but records about its use in gas adsorption are limited.
This method, is a critical approach optimisation from the viewpoint of operational cost management as well as ensure efficiency of flue gas treatment on an industrial scale.In this process, optimum points of independent variables determined in laboratory, and analysing of corresponding data is done.Then, obtained conditions can be applied to achieve maximum efficiency, even by manipulating the variables (according to the flue gas condition of each industry).In the recent years, researchers have proposed the use of experimental designing methods to achieve this goal [5,37].The response surface methodology (RSM) is an empiric statistical technique which used to designing, analysis, and optimisation of process variables, and their responses.Likewise, from the viewpoint of practical applications, it is necessary to investigate the adsorption performance experiments by using a continuous flow reactor.
To the best of our knowledge, upon the published literature, no relevant study has yet been reported on the utilising of CS-Leca composite beads for the capture of CO 2 ; and considering the interesting characteristics of CS and Leca, the present research, was aimed at exploring the prepared CS-Leca composite for efficient adsorption of CO 2 under the optimised influence of the process parameters including gas flow rate (40-100 mL/min), temperature (40-70°C), and moisture content (5-25%) based on the simulated flue gas conditions in a fixed-bed reactor.Besides, the structure and morphology of CS-Leca composite were systematically examined by various advanced instrumental techniques, viz., BET, SEM, EDX, XRF, XRD, TGA, XPS and FTIR.In addition, equilibrium, thermodynamic, and mechanisms of adsorption process were thoroughly evaluated.Moreover, its reusability properties also was studied.

Leca beads preparation
The raw Leca beads utilised in this research were provided from Saveh Leca factory, located in Iran.Since the sizes of the beads are <10 mm, they were sieved through a related mesh to obtain the desired bead sizes (3-6 mm) served as the carrier of chitosan.Then, these beads were rinsed with UPW to eliminate impurities after dried at 80°C for 8 h.

Preparation of crosslinked Leca beads
As regards the higher chemical resistance of Leca beads, above all, they were immersed in HCl solution (6 mol/L) for 24 h, to achieve activated Leca beads by activating their surface hydroxyl groups.Then, these activated beads were washed by ultrapure water and put inside of an oven to dry (at 100°C for 12 h).Besides, in this research, we used APTES, as a cross-linking agent, aiming to facilitate the bonding between Leca and chitosan.In this framework, a certain amount of activated beads were refluxed in H 2 SO 4 /H 2 O 2 solution (1:1 v/v) for 25 min, and then separated by filtration.After that, they were transferred to an Erlenmeyer flask containing 300 mL of 85% (v/v) APTES reagent solution and stirred vigorously for 15 min.At last, after being washed the functionalised Leca beads with ultrapure water, they were dried at 80°C in vacuum for 1 h.These modified beads were donated CS-Leca composite production.

Chitosan polymer grafting on APTES-crosslinked Leca beads
The CS nanoparticles were grafted on the surface of Leca beads [39].The involved mechanism was a dissolution-deposition reaction, in which 5 g of CS nanopowders was dissolved in 300 mL of acetic acid solution (5% v/v, pH of 3.0) with continuous stirring for 3 h under ambient conditions.Then, it was ultrasonicated for 30 min to get a homogeneous polymer solution.In the next step, 20 gr of the above crosslinked beads were put in the polymer solution and were mixed for 2 h.At this point, to adjust the pH value to ~7.0, the required volume of aqueous NaOH solution (1 M) drop-wise under slow stirring was added to the system.Over the stage, the CS-grafted Leca beads were successfully produced and filtered.Subsequently, they were immersed in ultrapure water for 24 h to enhance their adsorption performance [40].Eventually, the resulting wet beads were placed inside an oven to get dried and obtain CS-grafted Leca beads.The product obtained called CS-Leca composite, was applied as an adsorptive medium in fixed-bed minicolumn to evaluate its potential for adsorbing CO 2 .Schematic 1 schematically illustrates the synthetic route of CS-Leca composite.

Adsorption experiments
In the current research, adsorption process of CO 2 on CS-Leca composites was examined in a lab-scale continuous flow system.Adsorption capacity of the CS-Leca for CO 2 and the effects of operating parameters on this process were explored according to an experimental design developed using RSM.The experimental set-up of used system is schematically presented in Figure 1, in which a glass-made column of internal diameter 2 cm and height 20 cm was used to perform the fixed bed experiments.Before inserting this column in the system, a known quantity of CS-Leca composites was loaded in the column and both ends of it were supported by steel mesh, followed by glass wool to avoid the adsorbent losses and produce an even flow during the filtration process.
The procedure for performing adsorption experiments begins with adjusting the gas flow rates of CO 2 and N 2 to selected flow rates using separately mass flow controllers, which range (40-100 mL/min) was selected based on some preliminary tests.Thereafter, these two gases were mixed together in a mixing chamber.Subsequently, a water bath was employed to evaluate the effects of humidity on adsorption within a range of humidity from 5% to 25%, similar to combined-cycle power plant (CCPP) flue gas, that its control was carried out using an indoor air quality metre.An adjustable gas heater was used to explore the influence of temperature on CO 2 removal and to maintain the temperature of the gas at predefined value (40-70°C) which is that of the flue gas after the desulphurisation process.A temperature sensor was utilised to measure the temperature.At this stage, after adjusting the system's pressure at 1 bar, the simulated flue gas passed upwardly through the adsorption column.Finally, the initial and residual concentrations of CO 2 were recorded at regular time intervals until the bed saturation.The uptake capacity of the bed is calculated by Equations ( 1) and ( 2).
Here, q eq (mL/min) is the volumetric flow rate; C 0 (vol %) and C (vol %) index the initial and residual concentrations of CO 2 , respectively; m (g) reports the adsorbent weight or the mass of the bed; and t total (min) shows the operation time.A (min) is related to the pollution concentrations changes in the column over a period of time which its values can be obtained by the plot of C/C 0 versus time (the area above the breakthrough curve).
The mass transfer zone (MTZ, cm), as an important portion of fixed bed adsorption zones, represents a region inside the column where adsorption occurs and is calculated via Equation (3) [41]: In this equation, the z (cm) indicates the height of bed; t b (min) and t s (min) refer to breakthrough time at 5% of the initial CO 2 concentration and exhaustion/saturation time at 90% of the initial CO 2 concentration, respectively.Also, Equation ( 4) is used to determine the quantity of CO 2 adsorbed in the column (q total , mg).In addition, the CO 2 % removal efficiency, can be calculated by the following relationship.
In Eq. ( 5), all symbols are same as aforementioned.

Modelling of adsorption isotherms
To design adsorption systems, the equilibrium adsorption isotherm should be established based on the factors governing the distribution of pollutant molecules across the gas/ solid phase.Therefore, in this study, adsorption isotherms were expressed in terms of adsorbent capacity, surface properties, and affinity.In this framework, the isotherm experimental runs were carried out under optimal conditions and different CO 2 pressures (0.5-5 bar).The operation method was the same as presented in section 2.4.Then, the experimental isotherm data were simulated with four of the traditional isotherm models (i.e. the Langmuir, Freundlich, Toth and Sips models).The nonlinear expressions of the isotherm models are described by the following equations (Equations ( 7) to ( 10)): The Langmuir isotherm is essentially based on cover of adsorbent surface with a complete monolayer (homogeneous), adsorbent layers of uniform and independent adsorption of a molecule to a given site.In the Langmuir formula, q m,L (mg/g) indexes the maximum adsorption capacity, P (bar) designates the pressure, b L (L/mg) is the Langmuir coefficient representing the free energy and q L (mg/g) defines the amount of adsorbed CO 2 .Furthermore, to better characterise the reversible and/or irreversible nature and favourability of adsorption mechanism, a non-dimensional parameter, R L , was applied (R L = 1/[1+ bC 0 ]).The values of R L = 0, 0 < R L <1, R L = 1, and R L >1 reflect irreversible, favourable, linear, and unfavourable sorption, respectively.On the other hand, the Freundlich isotherm is based on the multi-molecular layer adsorption (heterogeneous), uneven adsorption, and lack of uniform distribution of energy.In the Freundlich formula, q F (mg/g) defines the amount of adsorbed CO 2 ; k F (mg/g) (L/mg)1/n symbolises the Freundlich capacity variable, and n F (dimensionless) is the Freundlich exponent and indicates the adsorption strength/intensity.The values of n F >1, n F <1, and 1< n F < 10 signify physical, chemical and favourable (linear) sorption, respectively.The Toth model can also be used to explore non-ideal adsorption on heterogeneous surfaces.In the Toth formula, q T (mg/g) denotes the maximum adsorption capacity and q m,T (mg/g) refers to the amount of adsorbed CO 2 ; m T is the Toth model constant (known as heterogeneity parameter); and k t (L/g) shows the Toth coefficient that associated with Henry's law.But, the Sips model is a combination of Langmuir and Freundlich isotherms that predicts the multi-layer adsorption systems.In the Sips formula, q m,s (mg/g) and q s (mg/g) indicate the theoretical maximum adsorption uptake and the amount of adsorbed CO 2 , respectively; k s (L/mg) is the Sips model constant and m s is the Sips exponent.

Nonlinear regression analysis
< See Supporting File>

The reusability of CS-Leca bio-composite beads
In this study, while employing CS-Leca as adsorbent, a continuous adsorption experiment was carried out and related breakthrough curve was constructed.Then, exhausted bed was regenerated to evacuate the guest molecules and physically adsorbed moisture using continuous N 2 purge at temperature of 100°C for 1.5 h; and reused to the following experiment.The mentioned process was performed for seven consecutive times.Finally, based on breakthrough curves, the adsorption rates (mg/g) are calculated by using Equation (4).

Characteristics
Texture, surface morphology and functional groups of produced CS/Leca composite beads were characterised systematically with BET, SEM, EDX, XRF, XRD, TGA, FTIR and XPS analysis.The N 2 adsorption/desorption technique was used to inspect specific areas and pore volume of unmodified Leca and CS-Leca composite.Figures S1(a,b) display N 2sorption isotherms.As seen in Figure S1, native Leca has a type II isotherm model, indicative of a multilayer adsorption and macropore fillings.After chitosan grafting, the isotherm was shifted to type IV implying that CS-Leca composite could be considered as a mesoporous material based on the IUPAC classification [42].CS-Leca's isotherm also exhibits the H3 hysteresis loop at high pressure indicating the hierarchical pore structures and further confirms the mesoporous nature of that [43].Based on the results of BET analysis, the BET surface area was obtained as 95.5 m 2 /g, 849.4 m 2 /g, and 780.9 m 2 /g for CS, Leca, and CS-Leca, respectively.According to the obtained results of BJH method (Figures S1(c,d)), the total pore volume values for CS, Leca and CS-Leca were estimated to be 0.34 cm 3 /g, 1.23 cm 3 /g, and 0.92 cm 3 /g, respectively.The reduction of surface area and pore volume is ascribed to the impregnation of functionalised chitosan on the surface of Leca and filling up its pore mouths.
To identify the surface morphology and structure arrangements of the studied material, the SEM technique was employed [44].Representative SEM images of Leca, and CS-Leca are illustrated in Figure 2 (A, left).Leca beads display uneven/rough morphology with porous structure as well as some white deposits on them, which were attributed to the oxides of silicon, aluminium, iron and, etc., as reported elsewhere [45].Comparing Leca and CS-Leca, the surface of Leca has become smoother and irregular after modification.It suggested that the CS biopolymer is uniformly grafted on the Leca beads to form a large number of clusters.To demonstrate the elemental compositions of materials, EDX was used.Figure 2 (A, right) shows the EDX spectrum of the samples.As can be seen, the peaks observed in Leca are also observed in CS-Leca, except two peaks appeared at 0.28, 2.01 which are associated to C and N atoms originated from natural biopolymer.Also, there was an increase in the O peak intensity.But the intensity of other elements is reduced after grafting.Noteworthy, the Si peak intensity remained almost constant which may be related to the cross-linked APTES.These results have provide clear idea about attachment of CS to the surface of Leca beads.These results approve by the XRF results (Table S3 in the supporting file).
The crystallographic structure of the products were systematically analysed by XRD.X-Ray diffractograms of as-prepared Leca materials, CS polymer and CS-Leca composites can be seen in Figure 2B.The diffractogram of Leca exhibits diffraction peaks at 22.23, 23.4,24.25, 27.74, 29.5, 31, 37.2, 43.16, 43.98, and 62.35 at 2θ scale, indicating similar to other clay materials, Leca in its amorphous phase, is naturally rich in the crystalline phases including anorthite, dolomite, calcite, and quartz.These results are quite consistent not only with that of other researchers [46,47], but also with the database in JCPDS.In native chitosan, the intense characteristic peak emerged at 2θ = 19.98.Noteworthy, in the diffractogram of CS-Leca, the mentioned earmark peak was detected for chitosan, which associated with the amorphous-nature of this polymer.As well as, the diffraction peaks corresponding to the crystalline phases of Leca remained entirely unchanged confirming that the overall crystallographic structure of Leca are well retained.These results provided the mineralogical composition of studied materials and are in good agreement with the EDX results.
In order to examine the thermal stability of Leca beads, pure CS and CS-Leca composite and provide the appropriate processing temperature for CS-Leca adsorbent manufacturing, thermogravimetric analysis (TGA) was conducted by heating samples under nitrogen environment up to 800 °C at a steady heating rate of 10 °C/min.Figure 2(C) shows the TGA thermograms of samples.As presented in Figure 2(C), the mass loss of natural Leca in the studied temperature range was only 3%, which most likely related to the vaporisation of the physically absorbed water molecules.In the TGA thermogram of chitosan, three steps of mass loss were observed.In the first step (below 100°C), the occurred mass decrease was about 10%, which is attributed to the dehydration process.In the second step, in the temperature range 300-400 °C, about 40% of CS mass is lost which is due to the breakdown of the saccharide rings (volatile fraction of chitosan); in the third step (above 400°C), about 14% mass loss was occurred, that may be correlated to the complete breakdown of chitosan residues [28].The TGA curve phase transition of CS-Leca resembles to the TGA of CS, both having three steps of mass losses.At an early step, a mass loss of <3% up to 100°C was accounted (for the loss of hydration water), in the subsequent steps, mass decrease of 30.4% that happened up to 570°C, which represent the breakdown of volatile and residual fraction of grafted/deposited chitosan on the surface of composite, respectively.The TGA thermogram for CS-Leca showed several shifts in the thermal decomposition temperatures as compare to the pristine CS, which is due to the cross-linking reaction between chitosan and Leca in the composite.Therefore, it can be calculated that 30.4% CS-Leca weight loss corresponds to 54% CS content in CS-Leca.Accordingly, CS-Leca composite can be regarded as a thermally stable material applied to environmental remediation objectives at high temperatures with good performance.
To further recognise the elemental compositions of materials, XPS spectroscopic characterisation was used.Figure 2 [48].The XPS spectra of CS-Leca exhibit several shifts in binding energies of new peaks; besides, the emerged peaks at 397.1 eV and 283.2 eV, which is relating to N 1s and C 1s, respectively, indicate that chitosan and Leca appropriately compounded with each other [25].
The data of XRF analysis for the identification of Leca composition are reported in Table S3 in the supporting file.According to Table S3 in the supporting file, natural Leca contained a significant quantity of SiO 2 , Al 2 O 3 , and Fe 2 O 3 .Therefore, the natural Leca sample is a typical aluminosilicate mineral.Interestingly, these outcomes confirm the EDX and XRF results.
The FT-IR spectra of CS, Leca, and CS-Leca composite prior and post CO 2 capture were scrutinised and shown in Figure 2E.Considering the Figure 2E(a), the pristine Leca displays presence of one strong/broad band at 1066 cm −1 which assigns to the bending and stretching modes of Si-O functional group; in this spectra, the band located at 3451 cm −1 belongs to the bending of interlayer water O-H and its stretching.The strong band at 785 cm −1 affirms the presence of quartz phase in the structure of Leca.As shown in Figure 2E(b), the band at 1095 cm −1 is skeletal vibration of C-O stretch.The 1251 cm −1 band relates to amide III and two bands (1379 cm −1 and 1423 cm −1 ) relate to the C-CH 3 stretch.Also in spectrum of 1579 cm −1 and 1647 cm −1 , CS has a medium peaks associated respectively with NH 2 bending vibration and deformation of C = O stretching.Due to the polysaccharide property of the CS, weak bands at 2885 cm −1 (C-H asymmetric stretching) is visible.The strong and broad band at 3421 cm −1 is related to the O-H and amino groups [49].Comparison between three spectrums (Figure 2E(a-c) together with emerging similar peaks in modified composite (Figure 2E(c)) as well as shifting of  the peaks, affirmed that the CS polymer imprinted on the Leca particle surface.After CO 2 capture (Figure 2E(d)), the peaks at 1088 cm −1 and 1424 cm −1 shifted to a lowerfrequency band at 1029 cm −1 and 1389 cm −1 , respectively, but two peaks at 1639 cm −1 and 3436 cm −1 slightly shifted to a higher frequency band at 1655 cm −1 and 3460 cm −1 .Besides, the peak at 1179 cm −1 , 1603 cm −1 and 2902 cm −1 wholly disappeared.Moreover, two new absorption bands appeared at 2324 cm −1 and 2396 cm −1 caused by CO 2 adsorption on the surface of the CS-Leca.These changes confirm the chemical nature of adsorption reaction and evident that the functional groups of composite are involve in the sorption reaction.

Effect of the experimental parameters based on contour plots and breakthrough curves
The independent and interactive effects of the process parameters on the recovery/ removal of CO 2 were investigated by contour plotting.The effects of temperature (40-70°C) and gas flow rate (40-100 mL/min) on the recovery performance were simultaneously monitored.Changes in the flue gas temperature can exert significant control on various conditions, including the sorption capacity of CO 2 molecules with a changing surface energy, carbonation reaction, and molecular diffusion rate.As portrayed in Figure S4(a,b) in the supporting file, the adsorption rate at lower temperature is high and gets diminished with increasing the temperature value; thus, temperature = 40°C, which was near the temperature of CCPP flue gas, was selected as an optimum temperature.Also, it is worth of notice that at higher temperatures (>80°C) the carbonation reaction might stop [50].The reduction of removal percentage with increasing temperature is ascribed to the reversible and exothermic nature of the adsorption process.With increasing temperature, the vibration of CO 2 molecules becomes more intense and decreases the contact between adsorbent/adsorbate, as a result the removal percentage decreased [51].In addition, it can be observed from the breakthrough curves (Figure S3) that all the curves were shifted towards the left with raising temperature and the adsorption capacity declined.As presented in Table S4, by increase of the gas temperature from 46°C to 64°C the adsorption capacity declines from 323.15 mg/g to 284.98 mg/g.With a deep look, investigation of temperature effect on adsorbent performance is important, as it can supply valuable facts about the thermal naturality, spontaneity and feasibility of the adsorption reaction.The above results suggested that the adsorption reaction is exothermic; in addition the decreasing of uptake capacities also indicated the antagonistic effect of temperature on the 'favorability'.In other studies, similar outcomes have been observed [52].
Figure S5 in the supporting file portrays the combined influences between gas flow rate and moisture content on the CO 2 removal at optimal temperature.As presented in Figure S5(a,b) in the supporting file, the removal of CO 2 reduced from 87% to 77% when the gas flow rate enhanced from 52 mL/min to 88 mL/min.Furthermore, the breakthrough period decreased at higher flow rates (Figure S3).The reduction of uptake capacity with increasing CO 2 flow rate is ascribed to decrease the contact and retention times of CO 2 molecules with sorbent, which leads to a decrease in the quantity of CO 2 adsorbed in the column [5].
It was also studied the effect of the interaction between moisture/humidity and temperature on the removal efficiency (Figure S6 in the supporting file).As clearly observed in Figure S6(a,b) in the supporting file, the role of moisture content on the removal efficiency is inverse.For example, the removal rate decreased from 91% to 83% when the moisture enhanced from 10% to 20%.This imply the fact that the influence of humidity on the CO 2 removal rate is more significant at low temperature, as reported previously by Zhao et al. [53].Also, adsorption effectiveness dropped from 89.5% to 81.5% when the humidity enhanced from 5% to 25%.The moisture content of 5% (which occurred the excellent efficiency), was selected as an optimum humidity.At the lower humidity, the predominant CO 2 species is bicarbonate; in these condition there is a high ratio of surface binding sites which makes it easy to adsorb CO 2 molecules, whereas, increase in humidity causes the formation of carbonate ions, which leads to saturation of the sorbent active sites with them.It was resulted to a decrease in the removal percentage.Furthermore, based on the breakthrough curves (Figure S3), with a decline in moisture content from 25% to 15%, the adsorption capacity improved from 240.31 mg/g to 251.54 mg/g.Also, it is worth of notice that we performed experimental adsorption studies for bare Leca and CS-Leca composite.The removal efficiencies obtained for bare Leca were much lower than that of CS-Leca composite.

The adsorption process optimisation
As aforementioned, the reduced model due to satisfactory agreement with the coded data, was selected as a well-fitted model.Therefore, optimisation process was done based on the designed model (reduced model); therefore, this model was employed for the optimisation process.Stationary points of response surface are some confirmatory data near optimal conditions.Based on stationary point in original units obtained from the full reduced model, the maximum removal performance (91.59%) with involving the all parameters simultaneously, were achieved at temperature of 40°C, gas flow rate of 40 mL/min and moisture content of 5%.Under these conditions, a series of breakthrough experiments were carried out and the average amount of the actual removal efficiency attained approximately 90.4%.The closeness of theoretical removal performance (91.59%) to the actual removal efficiency (90.4%) testified the reliability/high adequacy of the selected model.Note that the aims of optimisation study were segregated as I) to determine the optimum values of influential parameters to get the maximum performance of adsorbent, and II) to provide required CO 2 removal efficiencies from industrial flue gases, which their outlet concentrations change regard to local environmental legislation requirements, and III) to minimise operational costs through control of practical conditions.

Uptake isotherm investigation
To characterise the experimental data of CO 2 adsorption on fabricated composite, adsorption results were analysed and fitted onto the aforementioned isotherm models (section 2.6); and the optimum values of the models' parameters were achieved by nonlinear fitting algorithms with Solver in Excel.The (q e )-P curves for the models are plotted in Figure 3.As represented in Figure 3, the role of gas pressure on the removal efficiency is positive.Besides, isotherm parameters along with their corresponding R 2 , RMSE and ARE % values for each model are presented in Table 1.According to models outputs, all of the models are valid within the range of the experimental conditions.Nevertheless, the Toth model with greater R lower RMSE and smaller ARE %, indicated the best affinity with the experimental data.This model with m 0.52 declares adsorption of CO CS-Leca involves a heterogeneous adsorption behaviour with uneven adsorption, is also reaffirming the good agreement between characterisation results and experimental outcomes.
According to Table 1, the maximum adsorption capacity of CO mass unit of CS-Leca was determined as 351.24 mg/g for Langmuir model, which enlightened that the CS-Leca, due to its grafting polymer, can be applied as a potential sorbent to the recycle of CO other acidic gases.Comparison of the maximum uptake between current composite and previous adsorbents for the capture of CO depicted in  Table 2.The low b L value implies that the sorption process is exothermic, which is in accordance with our findings in the RSM, the related contour plotting and the thermodynamic studies results.Also, the value of R L (0.88) achieved from the Langmuir model, and n F (4.46) achieved from the Freundlich model indicated that adsorption of CO 2 onto CS-Leca is favourable and physical type (within the pores), respectively [54].Furthermore, a suitable was achieved from the Sips model (q m,S = 358.78mg/g), which is close to the achieved value from Langmuir model (q m,L ).The high CO 2 efficient capture of CS-Leca composite is attributed to its porosity as well as electron-rich N species in the amide and amine groups, which increased the basicity of its surfaces.Moreover, the congruence between the outputs of Toth (m T < 1) and Sips (m S > 1) models emphasised that CS-Leca composite surface has a multilayer structure and CO 2 chemisorption (onto the nitrogen surface groups) might be a heterogeneous process [13].

Thermodynamic properties
Investigation of temperature effect on the adsorbent performance is important, as it can supply valuable facts about the thermal naturality, feasibility and spontaneity of the adsorption reaction [60,61].Thermodynamic studies were investigated in the temperature range 313-348 °K under the optimal circumstances (moisture content = 5%, flow rate = 40 mL/min, and pressure = 5 bar).The rate of CO 2 diffusion onto the CS-Leca were found to decrease linearly with increasing temperature.This can be related to the intensive vibration of CO 2 molecules and decelerated useful contact between gas/solid phase resulting in heighten the temperature [51].
Actually, with increasing temperature, the bonds CO 2 molecules and CS-Leca disintegrates and as a result adsorbed molecules are separated from composite and return to the flow, as a result the removal percentage decreased.To calculate the major thermodynamic parameters (ΔG°, ΔΗ°, and ΔS°), the following equations were used: Where, q e (mg/g) is related to the amount of adsorbed CO 2 and C e (mg/L) is the concentration of CO 2 at the saturation time, T (°K) denotes the flow temperature and R (8.314 J/mol.K) refers to the ideal gas constant, ΔG° (kJ/mol (corresponds to Gibbs free energy change, ΔS° (J/mol.K) indicates entropy change, and ΔH° (kJ/mol) represents standard enthalpy change.In brief, K D was obtained from the van't Hoff equation (Eq.( 14)) and its plot (ln K D versus 1/T).ΔG° was calculated based on Equation (15).The ΔΗ° and ΔS° values are slope and intercept of van't Hoff plot (Table S8 in the supporting file).The negative values of ΔG° revealed the feasibility and spontaneity of CO 2 molecules sorption on the CS-Leca composite.The negative value of ΔH° proved that the adsorption of CO 2 molecules onto the CS-Leca have an exothermic nature [62].Additionally, the enthalpy value demonstrated that the interactions were dominantly from contribution of weak intermolecular forces and physisorption is the main process.The negative value of entropy indicated a consequently decrease randomness as a result of CO 2 sorption on the surface of CS-Leca composite [63].Furthermore, the isosteric heat of adsorption (IHA) as another important thermodynamics parameters is conveniently applied to recognise the energetic heterogeneity of composite surfaces.This parameter can provide a lot of information about deep understanding the strength of interaction between CS-Leca and CO 2 .In this framework, to determine the value of IHA, adsorption isotherm experiments were performed at various temperatures.Then, its values were calculated with ln (P) versus (1/T) plot and Clausis-Clapeyron equation that can be epitomised as: In Equation ( 18), all the symbols are same as aforementioned and θ indicates the thickness of adsorption layer.The calculated IHA values are summarised in Table S8 in the supporting file.As seen in Table S8 in the supporting file, the values of IHA are within the range of 37.1-48.3kJ/mol, authenticates a weak chemical interactions between adsorbate and adsorbent [64].Besides, the negative values of IHA further affirmed the exothermic process of the adsorption reaction.In the other studies, analogue results refer to the obtained values of IHA and type of contributed interaction in the adsorption reaction of CO 2 on various similar adsorbents were reported [65,66].Overall, small adsorption energy of CS-Leca for CO 2 along with its great adsorption performance assure its cost-effectivity owing to the low energy requirement for regeneration.

Possible mechanisms of CO 2 sorption
The Lewis acid-base reaction between the reactant molecules (CS-Leca, H 2 O and CO 2 ) can be predicted from the ionic charge of CS amine groups (as basic groups) and acidic CO 2 molecules, resulting in the formation of surface ammonium carbamate (Figure S7, Scheme A) [67].Furthermore, Lee et al. have reported dipole-quadrupole interaction between electron-rich N species in the amine (-NH 2 ) functional group of dimer chitosan and electron-deficient carbon atom of CO 2 molecules, can be another main mechanism in adsorption process (Figure S7, Scheme B) [68].In addition, the adsorption isotherm and thermodynamic data confirmed that the physisorption, in which pore diffusion and weak hydrogen bonds play dominant role in the adsorption process, involved in the interaction between CO 2 and CS-Leca.Besides, the hydrogen bonding between hydroxyl and amino groups of chitosan and lone pair containing oxygen atoms of CO 2 may be another dominant mechanism involved in this process (Figure S7, Scheme C) [69,70].Moreover, the (-OH) group owing to the higher electron density around (O) could enhance the CO 2 adsorption rate by CS-Leca composite; which is an agreement with the previous reports [71].All these results revealed that the CS-Leca, due to its high porosity and large surface area as well as its adsorptive functional groups has a promising application potential for fast and efficient treatment of CO 2containing gases.Figure S7 shows the potential mechanisms might be involved in the interaction between CS-Leca and CO 2 .

Reusability investigations
The material regeneration and recyclability study in practical applications is important because it helps to better understand the stability and cost-effectivity of adsorbent as well as its nature of environmental-friendly. Figure 4 portrays the regeneration of CS-Leca for CO 2 capture from simulated flue gas.As observed in Figure 4(a), the performance of tested composite is fairly stable.Also, the sorption capacities were found to be 252.02mg/g and 246.01 mg/g, respectively, for the 1st run and the 7th run (Figure 4(b)).Therefore, the CS-Leca could be reused up to seven consecutive cycles with a small deterioration (~2.5%) in its initial performances.To confirm the stability of tested composite, its morphology was explored by SEM image after seven runs (data not shown) and it displayed an analogous morphology.This results affirm a remarkable balance between ecological impact of CS-Leca and its affordability.

Conclusions
The purpose of this study was to optimize/predict the adsorption potential of CS-Leca bio-adsorbent towards CO 2 capture by deploying the RSM optimisation technique.The physico-chemical characteristics of the synthesised composite were comprehensively evaluated by various analytical methods.The results confirmed that the CS polymer successfully grafted on the Leca bead surface.The influence of operating parameters on the absorbent performance was examined in a fixed-bed reactor and their breakthrough curves were designed.The obtained data from breakthrough studies were entered to RSM and its outputs suggested gas flow rate as the dominant factor affecting the capture of CO 2 .The modelling findings demonstrated the adsorption reaction is favourable and exothermic.Regarding optimisation outcomes, the maximum CO 2 removal percentage was found to be 91.59% at 40°C temperature, 40 mL/min gas flow rate and 5% moisture content.Under the aforementioned provisions, the actual CO 2 removal percentage was achieved 90.4%.The adsorption isotherm data were well fitted by the Freundlich model indicating that adsorption of CO 2 molecules occurred on a multi-layered surface which support the data obtained from characteristics analyses.Also, the monolayer adsorption capacity (q m ) reached 351.24 mg/g.Excellent adsorptive performance, well-regenerability, availability, low-cost, and environmentally friendly properties of CS-Leca biocomposite make it a candidate material towards CO 2 capture from flue gas.

Figure 1 .
Figure 1.Schematic diagram of the experimental set-up for CO 2 adsorption.

Table 1 .
Adsorption isotherm constants together with determination coefficients.

Table 2 .
Comparison of q m of CS-Leca bio-composite towards CO 2 capture with other materials.