Biochar from pyrolysis of rice husk biomass—characteristics, modification and environmental application

Biochar produced from various biomass has been widely used in environmental applications owing to its ability to immobilize or remove the contaminants from soil, water and air. The present work summarizes various pyrolysis variants used for biochar production from rice husk, modification of biochar and its environmental application. The high volatile matter content (70.2–78.5%) and carbon content (35.2–44.7%) favoured production of biochar from rice husk through pyrolysis. Microwave-assisted hydrothermal carbonization showed highest biochar yield from rice husk (57.9%) compared to other process variants, whereas wet pyrolysis produced biochar with the highest carbon content (71.2%). Steam activation of rice husk biochar resulted in a broader pore size distribution with the presence of significant micropores compared to CO2 activation. A substantial improvement in surface area and microporous volume was observed with alkali activation compared to that of acid activation, whereas metal impregnation caused a reduction in surface area. Rice husk biochar with/without modification has been employed for adsorption of pollutants such as cations, dyes, nutrients and tetracycline. The nutrient-loaded rice husk biochar improved the soil fertility and cation exchange capacity. The studies indicated that the choice of suitable pyrolysis variant and biochar modification method is vital to improve the adsorption capacity and nutrient release potential of the rice husk biochar.