Selective separation of mercury (II) and cadmium (II) from aqueous solutions using complexation-ultrafiltration process

摘要

Complexation-ultrafiltration process was investigated to separate selectively mercury (II) and cadmium (II) from aqueous solutions by using poly (acrylic acid) sodium salt (PAASS) as a complexing agent. Polysulfone hollow fiber ultrafiltration membranes were employed in this process. Binary solutions containing the both metal ions were used to study the effects of loading ratio and pH on selective separation factor (β Cd/Hg) in the total recirculation mode. Results showed that when loading ratio varies from 0.01 to 2, β Cd/Hg increases with the loading ratio, and reaches a maximum value, then decreases rapidly. β Cd/Hg decreases gradually with the increase of pH. Further, the concentration experiment of selective separation was carried out at pH 5. Under the condition of initial mercury (II), cadmium (II) and PAASS concentrations 30, 30 and 40 mg/L respectively, there is a linear growth of metal concentration ratio (MCR) for mercury (II) with increasing volume concentration factor (VCF).MCR of mercury (II) reaches 14.83 and mercury concentration (Cr, Hg) in the retentate is 444.9 mg/L at VCF =15.However, cadmium concentration (Cr, Cd) in the retentate grows very slowly, final MCR for cadmium (II) is 1.18 and Cr, Cd is only 35.4 mg/L. This means that mercury is retained effectively while almost all cadmium passes through the membrane. A diafiltration technique was applied to separate further mercury (II) and cadmium (II) in the retentate. At the beginning stage, Cr, Cd declines sharply with increasing the diafiltration volume. When the diafiltration volume reaches 1.75 times that of the treated retentate, Cr, Cd is 5.24 mg/L and does not decrease significantly. Compared to the cadmium case, Cr, Hg does not change during this process.