As shown by Eq. (4), %Retained is nothing more than a ratio calculated from the actual observations. However, when the pH of the system is lower than that required for hydroxide precipitation, the amount of retained is usually interpreted as the amount adsorbed to the mineral surfaces. In Fig. 5 the theoretical solubility of copper hydroxide (Cu(OH)2) is shown for comparison. The solubility isotherm, referring to an initial copper concentration of 15 mg/l, is calculated using the MinteqA2 v.3.0 computer model (Allison et al., 1991) and is based on thermodynamic data given by Allison and Brown (1987). As seen in Fig. 5, the experimentally obtained retention isotherms are located approximately 1–1.5 pH units to the left of the solubility isotherm, thereby suggesting adsorption/desorption as the predominant mechanisms controlling retention.