Chromate ion-exchange study for cooling water

In spite of high chromate selectivity, the ion-exchange process for Cr(VI) recovery from cooling tower blowdown is yet to be commercially popular. Possible degradation of the ion-exchange resin by the oxidative action of Cr(VI) during ion exchange has, for long, been considered as the prime obstacle. In recent years, resins have been manufactured with fairly acceptable properties to withstand both physical attrition and chemical oxidation. Another conspicuous shortcoming which has been observed by all the previous investigators and demonstrated during the course of this research is early, gradual Cr(VI) breakthrough during fixed- bed column runs at acidic pH in the presence of competing sulfate and chloride anions. The advantage of high chromate selectivity is essentially lost due to the early Cr(VI) breakthrough because the column runs are always terminated after a pre-determined level of Cr(VI) has appeared in the treated water. Experimental results provide sufficient evidence that such early, gradual breakthrough is not due to poor column kinetics or electrolyte penetration. Furthermore, chromate/chloride and chromate/sulfate isotherms at acidic pH show positive curvature (unfavorable type) which is again unusual for a preferred component. The chromate ion-exchange mechanism has been investigated in order to explain the foregoing anomalies for the chromate-exchange process. Chromate may exist in different ionic forms in the aqueous phase and their relative predominance is governed by pH, total Cr(VI) concentration and the ionic strength of the aqueous phase. [...]