Use of Electrodialysis to Remove Heavy Metals from Water

The removal of heavy metals from water by using electrodialysis is discussed. Parameters studied include current efficiency, stack resistance, and osmotic water transfer. Four single-electrolyte systems are investigated: CdCl₂and CdS0₄are used as representative heavy metal salts; NaCl and CaCl₂are studied in order to enhance the understanding of physical electrodialytic processes in general and to provide a basis of comparison. The variables of electrolyte type, electrolyte concentration, pH, temperature, and elapsed time of membrane usage are examined. Results indicate that the purification of cadmium-laden waters can be achieved while maintaining high current efficiencies and reasonable stack resistances. The osmotic water transfers (in units of liter/mole) of the above cadmium salts are small enough to allow a high percentage of a wastewater stream to be reclaimed as purified water, but are higher than the water transfers of NaCl and CaCl₂ due to increased hydration effects.