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 exchan ge with a synthetic Na-Y zeolite has been studied under competitive and non competitive 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 attrib uted to an exchange site heterogeneity. The solution pH and the ratio of so rbate to sorbent are identified for which minimal imbibition of metal hydro xide and maintenance of zeolite structural integrity are ensured. An increa se in Fe and Co concentration over the range 0.005-0.05 mol dm-3 lowered th e removal efficiency but the external Fe was preferred to the indigenous so dium 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 Cu2/Na+ and Ni2+/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 h as been considered where the maximal Fe exchange was temperature independen t while Co exchange was promoted with increasing temperature. A Co/Fe/Na-Y ternary exchange isotherm was constructed from 20 pairs of experimental poi nts and is treated quantitatively in terms of ternary and pseudo-binary sep aration factors. The preference of the zeolite for exchange with iron over cobalt under noncompetitive conditions also extended to solutions containin g both metals. (C) 2000 Academic Press.