The calculated isotherm of amoxicillin adsorbed onto virgin GAC is shown in Fig. S3, Supplementary Information. The experimental data better fit the Langmuir isotherm (Equation (7)) than the Freundlich isotherm (Equation (8), as can be seen in Fig. S3, Supplementary Information. Thus, AMX molecules were evenly distributed in the mono-layer form on the adsorption sites. Nonetheless, the isotherm obeyed the Freundlich isotherm with good accuracy. This isotherm indicates multi-layer adsorption with interactions between molecules on a heterogenous surface. Additionally, Dubinin-Radushkevich isotherm shows the distribution of Gaussian energy on a heterogenous surface. In this model, E represents free adsorption energy. If E is in the 1–8 kJ/mol range, the adsorption will be physical while values higher than 8 kJ/mol indicate a chemical adsorption (Foo & Hameed 2010; Meneghel et al. 2013; Yang et al. 2015). According to Table 3, E = 3.089 kJ/mol was obtained in this study showing the physical adsorption of AMX.