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Although CO2 is prominent as the most important greenhouse gas, responsible for 64% of anthropogenic global warming, it is also a viable indicator for indoor air quality (IAQ). Due to the incessant increase in the human population and residence time indoors, the need to control indoor CO2 levels has become exigent. To this effect, dryphase removal technology via adsorption with zeolites, activated carbons (AC) and activated carbon fibers (ACFs) had sufficed. Chemically modified AC and ACF surfaces through alkali impregnation have been used to improve their selectivity toward CO2 at room temperature. Here we appraise the various methods in the literature and carry out performance evaluation based on the physical and chemical modification induced by the chemical agents and experimental conditions. This study reviews the improved adsorption of low concentration (0.3%) via surface reformation of commercial carbon-based adsorbents, and the highest adsorption capacity was 2.2 mmol/g CO2 at the indoor level, which was achieved by AC pellets doped with ammine functionalities.

Direct disposal of flue gas scrubbing-derived waste water with a high level (9000–10,000 mg/L) of ammonium ion (NH4 +) into aquatic systems has contributed to environmental depreciation. Here, we report a feasibility study on NH4 + recovery and conversion to struvite (NH4MgPO4·6H2O), which is a slow-release fertilizer. Such conversion also aids in compliance with the discharge limits for nitrogen-based compounds. Lab-scale experiments were performed to determine the optimum pH and molar ratio (Mg2+:NH4 +:PO4 3−) for struvite formation. A chemical equilibrium model (Visual Minteq) was also employed to corroborate the experimental results. The optimum pH for struvite precipitation was found to be pH 9 with a molar ratio of Mg2+:NH4 +:PO4³− = 1:1:1. At this pH, more than 93, 92.3 and 100% of the NH4 +, Mg2+ and PO4 3−, respectively, were removed from the scrubbing waste water Visual Minteq simulation also demonstrated optimum struvite formation at pH 9–10. Both X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) analysis revealed that the synthesized struvite was comparable to that of a commercial struvite. Thus, our findings confirmed the possibility of synthesizing struvite from de-NOx scrubbing wastewater utilizing the residual ammonium ions (NH4 +).