Variation in the properties of biochars produced by mixing agricultural residues and mineral soils for agricultural application

The research and application of biochars enriched with minerals have increased in recent years; however, the mineral fraction used consists of specific minerals, such as clay minerals and synthesized compounds. In this work, the effects of adding two specific soil types (sandy and clayey) to rice and coffee husks in order to generate biochars via pyrolysis was investigated. Chemical, physical–chemical, thermal, spectroscopic and crystallographic analyses were conducted on the produced biochars. The study confirmed that the presence of mineral soils during the pyrolysis process increases the yield, C retention ratio, and specific surface area. It also decreases the pH, cation exchange capacity (CEC), nutrient content, and carbon-to-nitrogen ratio of biochars. However, the biochars produced by mixing coffee husks and mineral soils still demonstrate a capacity to increase the pH and the CEC of tropical soils. In addition, increased C retention demonstrates an environmental benefit of this biochar production method. Biomass pyrolysis combined with clayey soil results in a biochar with a higher degree of aromaticity and higher thermal stability when compared to biomass pyrolysis alone. These characteristics give the biochar a recalcitrant character, without the necessity for steps related to the synthesis of specific mineral compounds, which reduces the economic and energy cost of the process.