Assessment of the surface chemistry of wood-derived biochars using wet chemistry, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy

In order to understand the reactivity of biochar in soil, we thoroughly examined the carbonaceous surface of different biochars, paying particular attention to the distribution of oxygen-containing functional groups. Biochar was produced from pine, poplar and willow at two different temperatures (400 and 550°C) and characterised using elemental analysis and wet chemistry (Boehm and potentiometric titrations, cation-exchange capacity (CEC) measurement). In addition, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses were performed on both untreated and acid-washed biochar samples. Qualitative relationships were observed between data generated from the titration methods under investigation and XPS analyses, both describing the general distribution of oxygen-containing surface functional groups. Total acidity of biochar ranged widely, between 32 and 1067 mmol kg–1, and was mostly attributed to the presence of hydroxyl or phenol groups. The number of functional groups containing oxygen decreased with increasing pyrolysis temperature, mainly because of a decrease in the content of phenol groups. A quantitative comparison of titrations and CEC (i.e. biochar’s ion-exchange capacity) measurements was compromised by a masking effect caused by the biochar’s inorganic fraction (<8%). An acid-washing step with nitric acid was shown not to alter the biochar surface systematically. The use of potentiometric titrations with an acid-washing pretreatment proved to be suitable to quantify biochar acidic functional groups, and hence biochar acidity.