Enhanced Carbon Sequestration of Sustainable Biochar via Metal Salt Regulation: Insight into Reaction Mechanism and Carbon Footprint

Sustainable biochar can sequester carbon and therefore, mitigate climate change. However, only a small fraction of biomass carbon is retained during biochar synthesis, greatly restricting its carbon-sequestration capacity. A significant boost of the carbon-sequestration potential of biochar has so far been a challenge. This study reveals that when biochar is modified by FeCl3, its carbon-sequestration capacity is boosted to 247.73% of that of pristine biochar derived at 500 °C. Meanwhile, pristine biochar retains only 43.18% of its biomass carbon, while FeCl3-modified biochar retains 75.20% of its carbon by forming complexes between the iron salts and the carboxyl- and hydroxyl-rich organic compounds derived from biomass pyrolysis. As react proceeds, the complexes are further converted into ferrites and organic carbon. The resulting minerals provide physical barriers against carbon decomposition, further enhancing the long-term stability of biochar. Life cycle assessment results further show that ferric salt can markedly enhance the greenhouse gas─reduction potential of biomass-to-biochar-to-soil systems. The more cycles from biomass to upgraded biochar, the more potent the carbon-negative effect is. Undoubtedly, such discoveries hold significant implications for accelerating carbon neutrality.