Chromate ion exchange mechanism for cooling water

Abstract

The chromate ion exchange recovery process for cooling tower blowdown is unique due to the early, gradual breakthrough of highly preferred Cr(VI) from fixed-bed columns for all types of anion-exchange resins. It is shown that the early Cr(VI) breakthrough is not due to poor column kinetics but is predictable from an equilibrium model by using the appropriate exchange reaction involving both HCrO/sub 4//sup -/ and Cr/sub 2/O/sub 7//sup 2 -/. At acidic pH, HCrO/sub 4//sup -/ is practically the only Cr(VI) species in the aqueous phase, while in the exchanger phase both HCrO/sub 4//sup -/ and Cr/sub 2/O/sub 7//sup 2 -/ exist. The presence of Cr/sub 2/O/sub 7//sup 2 -/ in the exchanger's solid phase causes a positively cured (concave upward) isotherm at relatively low Cr(VI) loading of the resin, and this equilibrium property is primarily responsible for the unusual, gradual breakthrough of Cr(VI). The presence of Cr/sub 2/O/sub 7//sup -/ in the solid phase may be viewed as the dimerization of HCrO/sub 4//sup -/, according to the Donnan equilibrium principle.