• The upcycling of polymeric waste to nanoporous activated carbon for potential CO 2 capture. • Engineering the pores and surface functionality by chemical carbonization followed by activation. • The ultra-micro porosity, C-O and C-N functionality ascribed due to 1:2 KOH impregnation contributing maximum CO 2 capture. • The proposed framework offers tuning of the desired attributes of nanoporous activated carbon for CO 2 capture. The efficient CO 2 capture requires engineering a low-cost, highly efficient adsorbent. Herein, the upcycling of waste floral foam into chemically activated nanoporous carbon (CANC) is reported. The implications of the impregnation ratio of KOH on the porosity, surface functionality of CANC, and its role in CO 2 capture are examined and discussed. The optimized sample, CANC-2 (SSA 1043 m 2 /g), with a large ultra-micropore volume and higher oxygen and nitrogen content, demonstrates 3.71 mmol/g CO 2 capture capacity at 15 ℃ and 1 atm. The framework provided here offers a technique for tuning the attributes of nanoporous carbon favorable for CO 2 capture. Ultimately, the pollution control of solid polymeric waste can be done by upcycling it into value-added products which further utilized in environmental applications.