How to produce biochars with interesting characteristics for agricultural soils and pollutant capture?

Biochar is a carbon-rich material which can be prepared from various organic feedstock, including wastes from circular economy such as agricultural wastes and sewage sludge. In recent decades, biochar has been promoted as a natural-based solution for carbon sequestration to offset anthropogenic greenhouse gas emissions, and also to improve soil physicochemical and hydrostructural properties in various types of natural and cultivated ecosystems. Feedstock characteristics and pyrolysis conditions, including biochar activation, largely control the physicochemical properties of biochars such as composition of ashes, surface chemistry, surface specific area and pore size distribution. Understanding the link between the feedstock characteristics, the pyrolysis conditions and the resulting biochar properties is therefore a key step to improve the biochar quality, and to offer a wide range of biochars adapted to various needs. The work presents a comparison of the properties of non-activated and CO₂-activated biochars produced from different biomasses with contrasted fiber content, collected from the regional circular economy (Occitanie- France). The results showed that the CO₂-activation significantly increase the surface specific area of biochars independently of the biomass fiber content, and that this increase in surface specific area is strictly related to an increase in micropore distribution. Moreover, we showed that this increase in surface specific area, resulting from biochar CO₂-activation, is associated with higher pH and conductivity of biochars, but not with water retention capacity, which seems to be more related to the biomass fiber content. Finally, we showed that the biomass characteristics seem to be more related to the metallic element adsorption capacity of biochars than the surface specific area. Overall, this work shows the importance of understanding the entire biochar manufacturing process (i.e. from the characteristics of biomasses to the biochar activation methods) in order to produce high-quality biochars covering a wide range of needs in soil health. This work has been funded by the region Occitanie and ANR (Plan France Relance).