CHEMICALLY FACILITATED DONNAN DIALYSIS AND ITS APPLICATION IN A FIBER OPTIC HEAVY-METAL SENSOR

Chemically facilitated Donnan dialysis (CFDD) of heavy metal ions in combination with continuous reagent flow has been studied. A simplified model to describe this process has been established. The model relates the diffusion behavior of metal ions through a Nafion cation-exchange membrane with the stability constant of complexation, the ionic strength of the receiving and sample solutions, the flow rate of the receiving solution, and the area-to-volume ratio of the membrane dialysis cell. A novel fiber optic heavy metal sensor has been fabricated by directly interfacing the dialysis device with a fiber optic colorimetric detection mechanism. The sensor is specific for the measurement of Pb(II) and Cd(II) in aqueous solutions, utilizing sodium thiosulfate in the receiving solution to selectively enhance the mass transport of these two ions. In a stopped-flow operation mode, the sensor detection limit is 3 X 10(-9) M Pb(II) or Cd(II) (S/N = 3) at 20 min accumulation. On the basis of the theoretical model, the factors affecting the sensor selectivity and dynamic range have been identified and discussed.