IUPAC divides the adsorption isotherms into different categories depending on their properties . The adsorption isotherms shown here have clear characteristics of so-called type III isotherms. This is a quasiexponential curve lacking a knee-like formation in the lower range of relative humidity . This knee-like pattern would indicate the formation of a monolayer, which is not present here. In type III isotherms, adsorption on the lignin surface takes place at the most favorable sites i.e., at the sites where mainly hydroxyl or carbonyl functionalities are found, without the formation of a monolayer. In addition, steps and plateaus are absent from the isotherms, indicating nonporous or macroporous structures.