Biochar total surface area and total pore volume determined by N2 and CO2 physisorption are strongly influenced by degassing temperature.

The surface area and pore volume of carbonaceous materials, which are commonly determined by N2 and/or CO2 gas-physisorption, are important parameters when describing environmental processes such as adsorption. Their measurement requires prior degassing of samples, which can change the nature of the material. Current guidelines for biochar characterization recommend different degassing temperatures. To investigate how degassing temperatures affect gas-physisorption we systematically degassed a range of materials (four biochars, carbon nanotubes, and Al2O3 reference material) at different temperatures (105, 150, 200, 250 and 300°C; for ≥14h each). Degassing temperatures had no effect on Al2O3 or carbon nanotubes but the measured surface areas and pore volumes of biochars increased by up to 300% with degassing temperature. An equation is presented for predicting surface area obtained at different degassing temperatures. Elemental analysis and results from sorption batch experiments suggest that surface area and pore volume may increase as biochar components volatilize during degassing. Our results showed that degassing temperatures change material properties and influence gas-physisorption measurements, and therefore need to be standardized. These results may also apply to the characterization of other complex materials, including carbon nanotubes coated with natural organic matter and fouled activated carbon.