NEXAFS and XPS characterisation of carbon functional groups of fresh and aged biochars

[Display omitted] •We analysed the light fractions of biochar after ageing in soils for up to 2years.•NEXAFS spectroscopy and XPS were used to measure the proportion of aromatic C.•Aromatic C was substantially increased with biochar addition to soils.•C functional groups in biochars did not change after ageing in soil for 1year.•XPS data showed an increased in carboxyl groups in biochars after 2years ageing. The oxidation of surface functional groups on biochar increases its reactivity and may contribute to the cation exchange capacity of soil. In this study, two Eucalyptus wood biochars, produced at 450°C (B450) and 550°C (B550), were incubated separately in each of the four contrasting soils for up to 2years at 20°C, 40°C and 60°C. Carbon functional groups of the light fraction (< 1.8g/cm3) of the control and biochar amended soils (fresh and aged for 1 and 2years at 20°C, 40°C and 60°C) were investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The spectra of biochar and light fractions of the control and biochar amended soils showed two distinct peaks at ∼285.1eV and 288.5eV, which were attributed to the C1s-π∗CC transitions of aromatic C and C1s-π∗CO transitions of carboxylic C, carboxyamide C and carbonyl C. The proportion of aromatic C was substantially greater in the light fraction of the biochar amended soils than the corresponding light fraction of the control soils. Also, the proportion of aromatic C was much higher in the light fraction of the B550 amended soils than in the corresponding B450 amended soils. Neither NEXAFS nor XPS results show any consistent change in the proportion of aromatic C of biochar amended soils after 1year ageing. However, XPS analysis of hand-picked biochar samples showed an increase in the proportion of carboxyl groups after ageing for 2years, with an average value of 8.9% in the 2year aged samples compared with 3.0% in the original biochar and 6.4% in the control soil. Our data suggest that much longer ageing time will be needed for the development of a significant amount of carboxyl groups on biochar surfaces.