Synthesis and environmental applications of biochar-supported nano-zero-valent iron composites: a review

In the context of increasing pollution of almost all environmental media, there is a need for efficient and sustainable techniques to remove contaminants. Here, we review the biochar-supported nano-zero-valent iron composites with focus on their synthesis from wood, sludges and agricultural waste, and on their application to the removal of heavy metals in waters and soils. We also discuss removal mechanisms. We observe that metal removal rates of 90% and above can be achieved. The composite performance is influenced by the biochar feedstock, the pyrolysis temperature, generally within 300–1000 °C, and the iron/carbon ratio, e.g. 4:1, 2:1, and 1:2. Removal mechanisms include adsorption, reduction, and co-precipitation. We also discuss the optimization of preparation methods, their application in real environments, and the removal mechanisms for complex pollutants.