Vermiculite Modified with Fe3+ Polyhydroxy Cations Is a Low-Cost and Highly Available Adsorbent for the Removal of Phosphate Ions

Abstract

This paper demonstrates that intercalating Na+ homoionic vermiculite with Fe3+ polyhydroxy cations (1:1 molar ratio OH− to Fe3+) significantly improved the affinity of the clay mineral-based sorbent toward phosphate. Kinetic experiments revealed that adsorption is fast, approaching an equilibrium within about 200 min of contact time, and that the rate-limiting step is the intraparticle diffusion. Adsorption isotherms fitted to the Freundlich equation and a two-site Langmuir model, consistent with the heterogeneity of adsorption sites. The separation factor derived from the Langmuir constant revealed that the adsorption was favorable and even irreversible for high-affinity minor adsorption sites. The adsorption capacity was 299 ± 63 μmol g−1 (9.3 ± 2.1 mg P g−1), a value similar to several other clay-based phosphate adsorbents. Application to reservoir water spiked with 10 mg L−1 in P removed about 71% of the available phosphate.