Adsorption of CO ₂ from Flue Gas Using Biochars Modified via a Photooxidative Method

Biochar is regarded as one of the most promising adsorbing materials for CO2 capture owing to its inexpensive raw materials and adjustable physicochemical characteristics. However, the lack of efficient and clean modified methods limits the application of the biochar adsorption technique in the CO2 capture field. Here, a series of rice straw-based biochars modified by a clean photooxidative method were prepared to adsorb CO2 in simulated flue gas, and several issues regarding CO2 adsorption were explored. Research reveals that biochar porosity and active groups/sites on biochar have been improved via photooxidative modification, which are the main factors of improvement in CO2 adsorption performance after the photooxidative modification. A rise in H2O2 concentration possesses a dual impact on CO2 adsorption on the RSWUV4 sample (i.e., 4% is the optimal H2O2 concentration for biochar). Raising the temperature results in a lower adsorption capacity of RSWUV4 for CO2. The optimized adsorption capacity of CO2 reaches 3.5 mmol/g (12% CO2) at 25 °C, and the CO2/N2 selectivity is 23. The CO2 adsorption process on the RSWUV4 involves both physical adsorption (leading) and chemical adsorption. Oxygen-containing active sites such as C–O and nitrogen-containing active sites such as N-5 were consumed, showing that C–O and N-5 may be the main active sites for the chemical adsorption of CO2. The proposed regenerating technique can regenerate the saturated samples (CO2 adsorption capacity of only 2.9% is reduced after ten cycles). This research offers new perspectives and guidance for the efficient and green modification of biochar for CO2 adsorption.