Preparation And Modification Of Biochar-based Adsorbents And Capture Mechanism Of CO₂

The intensive rise in atmospheric carbon dioxide(CO2)due to rapid urbanization and industrialization has caused severe global issues such as global warming,climate change,and other environmental problems.This change in the global atmosphere has triggered the melting of glaciers,sea level to rise,acidification of sea,floods,desertification,and many other issues.The CO2 alone is responsible for almost 75%of the total greenhouse effect caused by man-made activities.The carbon capture and storage(CCS)is gaining worldwide attention as a promising technology to capture CO2 from point sources before releasing it into the atmosphere by various sorbents.Several technologies have been used and proposed in CCS to capture and separate CO2 from point sources;however,these technologies showed several limitations.In light of these limitations,porous sorbents are found the most adaptable for CO2 due to their remarkable properties.Nevertheless,these adsorbents show low adsorption capacity,stability,and recycling ability.Biochar-based carbonaceous adsorbents are gaining interest due to their high availability,hydrophobic nature,and low cost;however,they show limited adsorption of gas pollutants due to low textural properties.Modification of biochar-based carbonaceous adsorbents can produce adsorbents with high adsorption capacity and selectivity of the target gas.This scenario inspired us to use various engineered approaches to improve the physicochemical properties of biochar-based carbonaceous adsorbents.A series of experiments were carried out in this work,including carbonization,activation,and modification of chitosan-based biochar by using various chemical treatments,characterization techniques(scanning electron microscopy(SEM),X-ray powder diffraction(XRD),Raman spectroscopy,Elemental analyzer,Fourier-transform infrared spectroscopy(FT-IR),X-ray photoelectron spectroscopy(XPS),thermo-gravimetric analysis(TGA),Brunauer,Emmet and Teller(BET),and vibrating-sample magnetometer(VSM)and their CO2 adsorption capacity.The following are the key findings of this study:1.Highly microporous adsorbents rich with nitrogen(N)were produced by pyrolysis of renewable biomass(natural polysaccharide)modified with potassium hydroxide(KOH)and urea.The influence of temperature(600-900?)on N functionalities and porous structure development was examined.It was found that modified adsorbent prepared at 700? adsorbed more CO2 and maximum adsorption occurred at 0?(5.92 mmol g-1),whereas,4.45 mmolg-1 at 25?.Owing to cumulative impact of N,structure of micro-pores(0.68 cm3g-1)and large surface area(1404 m2g-1),the adsorption of CO2 increased.There was a strong adsorbent-adsorbate association due to heterogeneous nature of adsorbent,pyrrolic-N,and abundance of micro-pores,resulting in enhanced adsorption of CO2.2.Magnetically separable modified chitosan-based adsorbents were synthesized by pyrolysis at 800? in order to attain high sorption of CO2 using ferric chloride(FeCl3),magnesium oxide(MgO)nano-particles and melamine.The characterization techniques revealed that MgO nano-particles and melamine integrated on adsorbent surface enhanced the basic sites on adsorbent pores.The use of FeCl3 played a dual role,i.e.,created magnetic properties and acted as an activating agent that enhanced the porous structure of adsorbents.The CO2 adsorption capacity of as-synthesized adsorbent at 0? was 3.52 mmol g-1,while showed 2.7 mmol g-1 at 25?.The study suggests that adsorbents can perform well at different temperatures and be easily separated by an external magnetic field for regeneration.Moreover,isosteric heat of adsorption showed that the mode occurred mainly through the physisorption process due to higher interaction of the electro-tetrapolar nature of CO2 molecules and micro-pores.3.Chitosan,MgO,urea,and environment friendly alkali activating agent potassium carbonate(K2CO3)were used to produce modified carbonaceous adsorbents through single-step carbonization for enhanced stability and CO2 capturing.The maximum CO2 sorption capacity by B-A-N-MgO-0.5 recorded was 2.62 and 1.98 mmol g-1 at 0 and 25?,respectively.Adsorbent exhibited good adsorption performance for CO2 primarily due to improved physicochemical properties that contributed a crucial role.Moreover,MgO nano-particles,N-functionalities and oxygen(O)containing functional groups also contributed in CO2 sorption by providing binding sites for CO2 molecules.