Biochar-based adsorbents for carbon dioxide capture: A critical review

Carbon dioxide (CO₂) is the main anthropogenic greenhouse gas contributing to global warming, causing tremendous impacts on the global ecosystem. Fossil fuel combustion is the main anthropogenic source of CO₂ emissions. Biochar, a porous carbonaceous material produced through the thermochemical conversion of organic materials in oxygen-depleted conditions, is emerging as a cost-effective green sorbent to maintain environmental quality by capturing CO₂. Currently, the modification of biochar using different physico-chemical processes, as well as the synthesis of biochar composites to enhance the contaminant sorption capacity, has drawn significant interest from the scientific community, which could also be used for capturing CO₂. This review summarizes and evaluates the potential of using pristine and engineered biochar as CO₂ capturing media, as well as the factors influencing the CO₂ adsorption capacity of biochar and issues related to the synthesis of biochar-based CO₂ adsorbents. The CO₂ adsorption capacity of biochar is greatly governed by physico-chemical properties of biochar such as specific surface area, microporosity, aromaticity, hydrophobicity and the presence of basic functional groups which are influenced by feedstock type and production conditions of biochar. Micropore area (R² = 0.9032, n = 32) and micropore volume (R² = 0.8793, n = 32) showed a significant positive relationship with CO₂ adsorption capacity of biochar. These properties of biochar are closely related to the type of feedstock and the thermochemical conditions of biochar production. Engineered biochar significantly increases CO₂ adsorption capacity of pristine biochar due to modification of surface properties. Despite the progress in biochar development, further studies should be conducted to develop cost-effective, sustainable biochar-based composites for use in large-scale CO₂ capture.