Ion Exchange of Divalent Cobalt and Iron with Na-Y Zeolite: Binary and Ternary Exchange Equilibria

Divalent cobalt and iron removal from aqueous solutions by batch ion exchange with a synthetic Na-Y zeolite has been studied under competitive and noncompetitive conditions. The binary Co/Na and Fe/Na ion-exchange equilibrium isotherms, constructed at 291+/-2 K and a total solution positive charge concentration of 0.1 eq dm(-3), exhibited sigmoidal shapes that are attributed to an exchange site heterogeneity. The solution pH and the ratio of sorbate to sorbent are identified for which minimal imbibition of metal hydroxide and maintenance of zeolite structural integrity are ensured. An increase in Fe and Co concentration over the range 0.005-0.05 mol dm(-3) lowered the removal efficiency but the external Fe was preferred to the indigenous sodium over the entire concentration range; there was a switch in preference from Co to Na at [Co] in excess of 0.034 mol dm(-3). Exchange data for Cu(2+)/Na(+) and Ni(2+)/Na(+) binary systems are included for comparative purposes, and ion exchange affinity and Na-Y exchange capacity are discussed in terms of metal ion hydration and ion location. The effect of exchange temperature has been considered where the maximal Fe exchange was temperature independent while Co exchange was promoted with increasing temperature. A Co/Fe/Na-Y ternary exchange isotherm was constructed from 20 pairs of experimental points and is treated quantitatively in terms of ternary and pseudo-binary separation factors. The preference of the zeolite for exchange with iron over cobalt under noncompetitive conditions also extended to solutions containing both metals. Copyright 2000 Academic Press.