Iron-modified hydrotalcite-like materials as highly efficient phosphate sorbents

Abstract

Highly efficient sorbents for phosphate removal from aqueous solutions based on the calcined forms of Fe(III)-substituted Layered Double Hydroxides (LDH) materials have been developed in this study. Hydrotalcite-like materials with Mg/M 3+ ∼3 (where M = Al 3+, Fe 3+ or combined) have been synthesized following simple co-precipitation method and were subsequently calcined in air at 450 °C. Both as-synthesized and calcined materials were characterized by means of X-ray Diffraction (XRD), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), elemental (C) analysis, N 2 porosimetry, Scanning Electron Microscopy (SEM). All the materials were evaluated for the sorption of phosphates by batch equilibrium sorption experiments and kinetic measurements (effect of contact time). It was shown that chlorides or nitrates, being the charge-balancing anions in the LDH structure, are more easily exchanged by phosphates compared to carbonates. In the Fe(III)-modified LDHs, an increase of the Fe loading led to the decrease of the sorption efficiency. The maximum uptake of phosphates for both the Mg–Al LDH and Mg–Fe LDH samples containing mainly carbonates as charge-balancing anions was relatively low (ca. ⩽25 mg P/g sorbent) while it was higher for the LDH samples containing mainly chlorides (∼80 mg P/g). On the other hand, the maximum sorption capacity for the calcined Mg–Al LDHs and the calcined Fe(III)-substituted sorbents were very high, ca. ∼250 and ∼350 mg P/g, respectively. The sorption data of both the as-synthesized and calcined LDHs was best fitted by the Freundlich model. Both the Mg–Al and Fe-substituted LDH sorbents were regenerated with mixed aqueous solution of NaCl and NaOH and were reused with a small loss of removal efficiency.