In situ amidation of NH3 in biogas on sludge-derived carbon to promote coexisting CO2 adsorption.

CO2 adsorption is a highly promising and economically viable method for biogas upgrading. The use of sludge as a precursor for carbon adsorbents is a sustainable solution that helps minimize secondary environmental pollution. Herein, we prepared acid-modified sludge-derived carbon (S-SC) via the KOH activation of SC, followed by the confinement of concentrated H2SO4 within its pores. This modification increased the CO2 adsorption capacity of SC by facilitating the conversion of NH3, commonly present in biogas, into amides on the carbon surface. Dynamic adsorption tests revealed that S-SC achieved a CO2 uptake of 5.47 mmol·g-1 at 40 vol.% CO2 and 1000 ppm NH3, which is 1.67 times higher than that of unmodified SC. Physicochemical characterization results showed that KOH activation introduced numerous oxygen-containing functional groups on the surface of adsorbents and concentrated H2SO4 was successfully confined within the pores of S-SC. In situ infrared spectroscopy confirmed the in situ conversion of NH3 into amides on the surface of S-SC, enhancing its CO2 adsorption capacity. Based on the Freundlich adsorption model parameters, S-SC exhibited an increased isosteric heat of CO2 adsorption. Density functional theory calculations showed that S-SC interacted with CO2via hydrogen bonding in a bidentate configuration. Moreover, the S-SC adsorbent displayed superior cyclic stability after 20 adsorption-regeneration cycles. The proposed simple and effective strategy, involving the use of S-SC as an adsorbent, has promising potential in biogas upgrading and various CO2 separation processes.