More on Mechanism and Some Important Properties of Chromate Ion Exchange

Sufficient experimental data are presented here to verify the chromate ion-exchange mechanism proposed earlier by Sengupta and Clifford (1986a). Most of the experiments were tailored to prove the validity of the mechanism, and also to highlight the unusual effects associated with it. One significant finding of this study is that, in spite of very gradual breakthrough during fixed-bed column runs, chromate (VI) concentration at the exit of the column remains almost independent of the influent Cr(VI) concentration for a long time after the start of the run. Mass-transfer limited kinetics are not responsible for this conspicuous column characteristic. Such an anomalous behavior is explained with the proposed ion-exchange mechanism, which suggests dimerization of HCrO4- into Cr2O72- inside the commercial organic-base anionexchange resins. A more theoretically sound attempt can now be developed for designing fixed bed chromate-exchange processes at acidic pH in the presence of other competing ions.