Structural and elemental analysis of biochars in the search of a synthetic path to mimetize anthropic Amazon soils

In this work, chemical and structural properties of various biochars were analyzed and compared with those from a highly stable anthropic soil, Terra Preta de Índio (TPI). TPI is believed to be responsible for the fertility of Amazonian soils and their stability; therefore, the production of a synthetic TPI would be of great interest for agricultural applications. Biochar produced from different raw biomasses were comprehensively characterized and, based on the obtained results, a preliminary study was performed testing three different routes of chemical activation using nitric acid, phosphoric acid, and potassium hydroxide as activating agents. After chemical activations, metal contents in the biochars decreased, as expected, and high degrees of carbonization were observed. In the case of the activation performed with HNO₃, intense signals related to carboxylic groups in TG-MS analysis and in potentiometric titrations point out to a highly oxygenated biochar. Structural analysis showed that activations generated point defects in sp²-carbon structures of biochar, with the material obtained after KOH activation showing a high surface area (569 m² g^-1), an important feature for the use as soil amendment.