A review on waste biomass-derived biochar: Production, characterisation, and advanced analytical techniques for pollutants assessment in water and wastewater

Biochar has played a crucial role in economic activities and environmental sustainability, and its significance is expected to grow in the future. The current focus is on non-polluting, green production processes to enhance biochar applications across various fields, aligning with sustainable development goals. The thermochemical processes have been optimised for biochar production from cellulose, hemicellulose, and lignin-containing biomass. The widely used methods include pyrolysis (300–900°C), gasification (>700°C), hydrothermal liquefaction (200–400°C at 10–25 MPa), torrefaction (200–300°C), and carbonisation (300–600°C), each influencing the physicochemical properties of biochar. The key operating conditions, such as heating rates (5–50°C/min), residence times (5–120 min), and carrier gases (N₂, CO₂, or steam and other inert gases), significantly affect biochar yield (20–80 wt%) and compositions. The structural, physical and chemical characterization of biochar is essential for assessing its application potential. BET surface area analysis (50–1500 m²/g), porosity measurements (0.1–1.5 cm³/g), XRD, FTIR, XPS, elemental analysis (C: 50–90 wt%, H: 1–8 wt%, O: 5–40 wt%), and thermal stability studies (TGA, DTG) at 200–900°C are commonly employed. Furthermore, the detection of environmental pollutants, including heavy metals (Pb, Cd, As, Hg) at trace levels and emerging contaminants (ECs) such as pharmaceuticals, pesticides, and microplastics, requires advanced analytical methods. Techniques like GC-MS, HPLC, ICP-MS, and XPS are used to detect organic pollutants with precision down to the ng/L range, improving environmental risk assessments. A detailed discussion on combining these broad aspects of biochar characterization, pollutant detection technique in recent years and mechanism during pollutant removal using biochar has not been attempted. This review provides a comprehensive overview of biochar production technologies, characterization methodologies, and pollutant detection techniques. These insights contribute to enhancing the role of biochar in soil remediation, carbon sequestration, and pollutant adsorption.