ABSTRACT
Co extraction of Zn and Fe was also observed. Figure 16b shows the evolution of cadmium concentration at the outlet of the extraction and stripping modules in a long-term experiment. It can be seen that the concentration of Cd was reduced from 50 mg/L at the inlet of the module to values below 2 mg/L at its outlet. The percentage of cadmium extraction remained constant over the whole period, proving that the efficiency of Aliquat 336 as an extractant re mained unaltered. The concentration of Cd in the stripping phase followed a different pattern, showing an increase with time until a steady state was reached. The apparent reason was that mass transport rate of cadmium in the stripping step was lower than that of the extraction step, resulting in the accumulation of cadmium in the organic phase. Also, the flow rate of the stripping phase had no influence on the extraction percentage, whereas the variation of the flow rate of the feed phase introduced a remarkable effect that was due to the influ ence on the extraction process of the cadmium concentration in the organic phase. Analysis of the steady state results of Cd transport from the acidic feed phase to the stripping phase led to the membrane mass transport coefficient, K m = 1.76 X 10-8 m/s.
