The equilibrium distribution of cadmium ions between the treated biosolids and solution is important in determining the maximum sorption capacity. Two isotherm models are technically applied to describe the equilibrium sorption: Langmuir and Freundlich models. The Langmuir model assumes that the uptake of metal ions on a homogeneous surface by monolayer adsorption without any interaction between adsorbed ions. The linearised Langmuir isotherm is given as follows (Wang et al., 2006).