Ligand exchange sorption of arsenate and arsenite anions by chelating resins in ferric ion form I. Weak-base chelating resin dow XFS-4195

Abstract

The weak-base chelating resin Dow XFS-4195 has been used in ferric ion form for removal and recovery of arsenate and arsenite anions from dilute aqueous solutions by ligand-exchange sorption. Arsenate and arsenite anions are effectively sorbed by the ferric-form resin in preference to other common anions at low solution concentrations, the saturation sorption capacities being 46 and 51 mg As/g wet resin for arsenate and arsenite, respectively. However, at low concentrations (< 1 mmol/1) the equilibrium sorption of arsenate is several times higher and the rate of sorption is also an order of magnitude greater than those for arsenite. The sorbed species are readily stripped with dilute sodium hydroxide into a concentrated form for recovery. An enrichment of more than 100-fold relative to the influent concentration (<100 ppm As) is obtained in ligand-exchange column operation for arsenate, as compared to less than 20-fold enrichment for arsenite. Reactivation of the stripped resin is performed in one step by treatment with an acidified solution (pH 0.8) of ferric chloride which reconverts the resin to the chelated-Fe 3+ form. The regenerated ligand exchanger exhibits a higher sorption capacity due to additional ion-exchange sorption by the residual ferric hydroxide gel in the resin pores. The ligand sorption capacity of the polymer-chelated Fe 3+ ion is, however, several-fold higher than the anion exchange capacity of the ferric hydroxide gel, with respect to both arsenate and arsenite anions. In ligand sorption, the useful pH range for arsenate anions is 4–7 with the sorption maximum at pH ⋍ 5 , while arsenite anions are effectively sorbed at pH 8–11, the sorption being maximum at pH ⋍ 10 .