Changes in the pore structure of modified sludge-activated carbon and its effect on the adsorption characteristics of CO2 under high pressure

Although sludge pyrolysis has been widely studied in sludge disposal, the utilization of sludge pyrolysis products is still challenging. In this study, a sludge-activated carbon adsorbent with CO2 adsorption capacity was prepared by pyrolysis and activation of municipal sewage sludge, and then the activated carbon was modified with hydrofluoric acid (HF), aminopropyl triethoxysilane-diethanol monoisopropanolamine (A-D) and sodium hydroxide (NaOH) to increase CO2 adsorption capacity of activated carbon. By characterizing the microscopic morphology, surface functional groups, and pore characteristics of the sludge-activated carbon before and after modification, and measuring the CO2 adsorption capacity under different pressures, the modification mechanism and high-pressure adsorption characteristics of the sludge-activated carbon under different modification conditions were studied. The results show that HF and NaOH modification can increase the specific surface area and pore volume while reducing the pore size, and the effect of alkali modification is more significant. Amine modification reduces its specific surface area, pore volume and pore size due to the loading of amino groups. Acid modification reduced the saturated adsorption capacity of sludge-activated carbon, and both amine modification and alkali modification increased the saturated adsorption capacity of sludge-activated carbon. The adsorption-promoting pressure thresholds of HF, NaOH, and A-D were determined to be 0.95 MPa, 2.55 MPa, and 3.85 MPa, respectively. When the equilibrium pressure exceeds the adsorption-promoting pressure threshold, the three modification methods will no longer promote the CO2 adsorption of sludge-activated carbon. Moreover, when the adsorption equilibrium pressure is greater than 0.95 MPa, increasing the adsorption equilibrium pressure no longer causes a significant change in the change rate of adsorption capacity of the sludge-activated carbon.