Unique CO2 adsorption of pine needle biochar-based activated carbons by induction of functionality transition

[Display omitted] •The CO2 removal by the activated carbon derived from pine needles were investigated.•Transitions of surface groups and porosity were observed as activation progressed.•High porosity from severe activation caused physical adsorption and good cyclability.•N and Ca based groups from mild activation caused chemisorption and good selectivity.•Modulation of functionality and porosity let pine needles sorb CO2 in various cases. CO2 capture has become the world’s most urgent agenda nowadays. In this work, we induced functionality transition of heteroatom-rich pine needle biochar based activated carbon for CO2 adsorption by modulating activation conditions. The surface functionalities and porosity of the activated carbon derived from pine needles were investigated intensively according to the activation conditions. The transition of surface functional groups and development of porosity were observed as activation progressed. CO2 adsorption performances were determined under various conditions, and the adsorption capacities, adsorption selectivities, and cyclabilities were evaluated. From these results, different CO2 adsorption mechanisms based on the surface functionality and porosity were clearly defined. The pyridinic, pyrrolic(N- based), and Ca(OH)2(Ca-based) CO2 sorbing functional groups derived from mild activation enabled chemical sorption with great adsorption selectivity. The high porosity derived from the severe activation conditions resulted in physical adsorption with excellent cyclability at 298 K but the chemical sorption property was weakened by the shift of surface groups to graphitic-N and CaCO3 rich groups. The modulation of functional groups and porosity enabled utilizing pine needles for effective CO2 removal under various conditions.