Carbon dioxide (CO 2 ) is the main anthropogenic greenhouse gas contributing to global 29 warming, causing tremendous impacts on the global ecosystem. Fossil fuel combustion is the 30 main anthropogenic source of CO 2 emissions. Biochar, a porous carbonaceous material 31 produced through the thermochemical conversion of organic materials in oxygen-depleted 32 conditions, is emerging as a cost-effective green sorbent to maintain environmental quality by 33 capturing CO 2 . Currently, the modification of biochar using different physico-chemical 34 processes, as well as the synthesis of biochar composites to enhance the contaminant sorption 35 capacity, has drawn significant interest from the scientific community, which could also be 36 used for capturing CO 2 . This review summarizes and evaluates the potential of using pristine 37 and engineered biochar as CO 2 capturing media, as well as the factors influencing the CO 2 38 adsorption capacity of biochar and issues related to the synthesis of biochar-based CO 2 39 adsorbents. The CO 2 adsorption capacity of biochar is greatly governed by physico-chemical 40 properties of biochar such as specific surface area, microporosity, aromaticity, 41 hydrophobicity and the presence of basic functional groups which are influenced by 42 feedstock type and production conditions of biochar. Micropore area (R 2 = 0.9032, n=32) and 43 micropore volume (R 2 = 0.8793, n=32) showed a significant positive relationship with CO 2 44 adsorption capacity of biochar. These properties of biochar are closely related to the type of 45 feedstock and the thermochemical conditions of biochar production. Engineered biochar 46 significantly increases CO 2 adsorption capacity of pristine biochar due to modification of 47 surface properties. Despite the progress in biochar development, further studies should be 48 conducted to develop cost-effective, sustainable biochar-based composites for use in large-49 scale CO 2 capture.

Biochar is produced as a charred material with high surface area and abundant functional groups by pyrolysis, which refers to the process of thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. The carbon component in biochar is relatively stable, and, hence, biochar was originally proposed as a soil amendment to store carbon in the soil. Biochar has multifunctional values that include the use of it for the following purposes: soil amendment to improve soil health, nutrient and microbial carrier, immobilising agent for remediation of toxic metals and organic contaminants in soil and water, catalyst for industrial applications, porous material for mitigating greenhouse gas emissions and odorous compounds, and feed supplement to improve animal health and nutrient intake efficiency and, thus, productivity. This article provides for the first time an overview of the multifunctional values and unintended consequences of biochar applications.

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