The results obtained for batch precipitation of copper and zinc
using synthetic sulphide and digester overflow after 120 min
are shown in Fig. 2. Similar results were reported by van Hille
et al. (2004). The results in Fig. 2(B) show that the amount of
zinc ions precipitated from solution was not affected by the
sulphide source for all metal to sulphide molar ratios tested.
However, the pH was slightly lower in the biological system.
This was attributed to the presence of residual organic acids,
mainly acetate, which originated from the growth medium.
For metal to sulphide molar ratios below 1, an excess amount
of sulphide was present in solution and the pH was significantly
higher in the synthetic system, given the absence of
organic acids.