LONG-TERM CHANGES IN THE ADSORPTIVE PROPERTIES OF FEP SEPARATING FUNNELS USED IN A MIXED DITHIOCARBAMATE FREON-TF EXTRACTION SYSTEM

Liquid-liquid extraction methods for concentrating trace metals from sea water often involve chelation by dithiocarbamates as the first step. When fluorinated ethylene-propylene (FEP) separating funnels are used, hydrophobic adsorption of the dithiocarbamates on the inner walls can lead to a small (ca. 2%) loss of metal if any time is allowed between addition of the chelating mixture and extraction with Freon-TF. When the FEP funnels have been used extensively (mixing periods totalling 200 h) over a period of years, the adsorption of metals present in the 0-20 nmol l-1 concentration range in sea water can be very significant (30-40% for Co, Ni, Cu; 60-70% for Zn, Mn, Fe). Under these conditions, the dependence of metal recovery on both initial metal concentration and concentration of chelating agent further suggests that metal adsorption is due to the presence of surface complexation sites involving the "aged" FEP surface and the dithiocarbamates. Ageing of the FEP surface thus limits the useful life of the FEP separating funnels used in a dithiocarbamate-Freon-TF extraction system.