REMOVAL OF IRON FROM AQUEOUS SOLUTIONS BY ION EXCHANGE WITH NA-Y ZEOLITE

Abstract

Iron removal from aqueous solutions by batch ion exchange with a solid Na-Y zeolite has been studied. The pH of the solution was monitored continuously during the ion exchange process and the impact of pH on iron hydroxide precipitation and zeolite structural stability is discussed. In the case of the Fe(II)/Na-Y exchange system, the pH of the iron solutions was low enough to prevent the oxidation of Fe(II) and subsequent hydroxide deposition. The Al and Si contents in the solution were negligible, indicating maintenance of structural integrity, while scanning electron microscopic analysis did not reveal any structural breakdown. The ion exchange equilibrium isotherm, constructed at 291 ± 2 K and a total solution positive charge concentration of 0.1 equiv. dm−3, exhibited a sigmoidal shape and a maximum exchange of 74% of the indigenous sodium content; maximum exchange was essentially independent of exchange temperature. An increase in the initial Fe(II) concentration, in the range 0.005–0.05 mol dm−3, lowered the removal efficiency, but the external Fe(II) was preferred to the indigenous sodium over the entire concentration range. A maximum Fe(II) recovery of 84% from the maximally exchanged zeolite was achieved using 2 mol dm−3 solutions of NaCl as regenerant while the regenerated Na-Y delivered 68% of the original Fe(II) exchange capacity. The Fe(II) recovery was lower from samples exchanged under reflux conditions, while drying the loaded zeolite at 383 K also suppressed the degree of recovery. The treatment of Fe(III) solutions with Na-Y was not feasible due to the acidity associated with the zeolite/salt slurries, which promotes excessive hydroxide deposition and structural disintegration of the zeolite.