where Ce is the equilibrium concentration of the Au(III) ions in mg L−1, qe refers to the amount of Au(III) ions adsorbed per unit mass of O-MWCNTs in mg g−1, Q0 and b symbols correspond to Langmuir constants for O-MWCNTs and are related to the maximum Au(III) ions adsorption capacity (mg g−1) and the adsorption equilibrium constant (L mg−1), respectively.