The Effect of Activation on the Structure of Biochars Prepared from Wood and from Posidonia Oceanica: A Spectroscopic Study

The structure of two biochars and of their activated carbons was investigated by ElectronParamagnetic Resonance, Solid State Nuclear Magnetic Resonance, and Fourier Transform Infraredspectroscopies, together with X-ray diffraction and nitrogen adsorption/desorption isotherm measurements.The biochars were obtained from wood and Posidonia Oceanica by slow pyrolysis up to600 C, whereas the activated carbons were prepared from the biochars by impregnation with KOH,heating up to 800 C. Two different KOH:biochar mass ratios were tested in the case of Posidonia,namely 4:1 and 2:1, while only the 4:1 ratio was used for wood. When the larger ratio was used,activation significantly increased the microporosity of the starting biochar, also creating bottle-neckpores not accessible to water molecules, and induced the formation of larger condensed aromaticnetworks arranged in interconnected conducting domains. In the case of Posidonia, activation usingthe 2:1 ratio mainly created mesopores and induced an increase in organic radical content byalmost four orders of magnitude. This huge increase was related to the presence of minerals in thestarting biochar.