Fig. 4 shows the retention of copper as a function of reaction time for three different combinations of solid/ solution ratio and initial iron concentration. As is apparent from these results, the presence of iron in the initial solution has marked effect on copper retention. The two isotherms representing experimental runs conducted with an initial iron concentration of 250 mg/l exhibit the same behaviour; after a rapid increase the retention reaches a maximum value before a slow but significant decrease is observed. Here, the maximum retention is found to be close to 79% for a solid/solution ratio of 10 g/l, whereas the 5 g/l isotherm peaks at approximately 33%. In contrast, the isotherm representing the initially iron-free system increases monotonically as a function of time until virtually all the copper is retained. The isotherms obtained at a solid/ solution ratio of 10 g/l and an initial iron concentration of 250 mg/l are hereby referred to as the 10:250’ isotherms, whereas the isotherms obtained at the same solid/solution ratio, but without iron initially present, are referred to as the 10:0’ isotherms.