Removal of atmospheric pollutants using biochar: preparation, application, regeneration and its future research

Air quality management is critical for achieving Sustainable Development Goal 3. Pollutants such as VOCs, NOx, and particulate matter contribute to over 3 million premature deaths each year. Annually, 140 Gt of biomass waste is produced mainly in the EU, Brazil, the USA, India, and China, with crop residue burning contributing to 18% of global CO₂ emissions and releasing harmful pollutants like PM and VOCs. This review highlights biochar as a viable solution for air pollution remediation, showcasing its strong adsorption capabilities for gases like CO₂ and NOx. Biochar can be produced from agricultural waste using methods such as pyrolysis as well as gasification and hydrothermal carbonization. These production methods create biochar with specific physicochemical properties that vary based on the type of feedstock used and the processing conditions. Activation techniques enhance adsorption capacity, achieving an 86% microporous structure with a surface area of 151 m²/g, with eucalyptus-activated biochar showing a 99.76% pollutant removal efficiency. Biochar has shown significant removal capabilities for various air pollutants, with miscanthus capturing MEK at 2.5 to 43 mg/g, bamboo-activated biochar achieving 89.19% removal of PM_2.5, and rice husk biochar demonstrating a 95.7 mg/g capacity for NO and 100.181 mg/g for SO₂. Indoor pollution mitigation is enhanced as micro-gasification cookstoves reduce CO, CO₂, and PM_2.5 emissions by 79%, while finer biochar particles achieve 6% to 75% removal for VOCs like formaldehyde. Its porous structure allows for effective pollutant adsorption via physisorption and chemisorption. Reactivation methods, both thermal and non-thermal, enhance its adsorption capacity while preserving its integrity. Despite its benefits for air quality and carbon sequestration, biochar faces challenges, including greenhouse gas emissions during production and costly regeneration. However, converting biomass to biochar could sequester 0.3 to 2 Gt of CO₂ annually by 2050, supporting carbon market initiatives and circular economy goals.