Surfactants for Separation Processes: Enhanced Ultrafiltration

Pollution by toxic metal compounds is a deep concern in all industrial countries. A process based on enhanced ultrafiltration is proposed in order to separate cadmium and chromate ions from wastewater. Inorganic membranes (zirconium oxide coated on carbon) are used in the separation cell, and ionic surfactants (NaDS or CTABr) are added in the effluent. The surfactants, which entrap metal ions present in the feed, are retained by the membrane barrier and thus the permeate is clear of metal compounds. This paper is devoted to precise experimental fields in which the permeate is lower than 0.2 g$\dot{s}$m⁻³ for cadmium and 0.1 g$\dot{s}$m⁻³ for chromate, these concentrations being the standards for European countries. The paper presents only experimental results describing the influence of operating conditions on the efficiency of the separation; a theory of rejection of pure surfactant by the same membranes has recently been published. As long as the feed concentration is less than or equal to 150 g$\dot{s}$m⁻³ for cadmium or 20 g$\dot{s}$m⁻³ for hexavalent chromium (respectively, 750 or 200 times the norm), a satisfactory permeate is obtained in a single stage process at 25 and 30°C; more than 99% of metallic ions is retained.